What matters need to be paid attention to during wave soldering operations in PCBA processing?

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In PCBA processing and production, we often encounter situations where there are a lot of post-soldering materials. At this time, wave soldering is needed for post-soldering processing. So what should we pay attention to during wave soldering operations? Today I will talk to you about wave soldering:

We all know that the wave surface is covered by a layer of oxide scale, which remains almost static along the entire length of the solder wave. During the wave soldering process, the PCB contacts the front surface of the tin wave, and the oxide scale breaks. The tin wave is pushed forward without wrinkles? This shows that the entire oxide scale and PCB move at the same speed.

Generally, in order to avoid poor wave soldering, the following methods can be used: use components/PCBs with good solderability, increase the activity of the soldering flux, increase the preheating temperature of the PCB, increase the wetting performance of the pad, and increase the temperature of the solder. , Remove harmful impurities and reduce the cohesion of the solder to facilitate the separation of the solder between the two solder joints.

Common preheating methods in wave soldering machines: air convection heating, infrared heater heating, heating by a combination of hot air and radiation; wave soldering process curve analysis: the wetting time refers to the time when the wetting starts after the solder joint comes into contact with the solder Time and residence time refer to the time from when a certain solder joint on the PCB contacts the wave crest surface to leaving the wave crest surface. Preheating temperature refers to the temperature reached before the PCB contacts the wave crest surface. Welding temperature refers to the soldering temperature. Important welding parameters. Usually 50°C ~60°C higher than the melting point of the solder (183°C). In most cases, it refers to the temperature of the soldering furnace. The temperature of the PCB solder joints soldered during actual operation is lower than the furnace temperature. This is because the PCB absorbs heat.

Why choose high-precision HDI circuit board? Comprehensive analysis!

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HDI, or High Density Interconnector, is an advanced printed circuit board (PCB) manufacturing technology. Its design and manufacturing process undergoes precise calculations and fine processing, and has many advantages, so it is very popular. Next, we will comprehensively analyze the reasons for choosing high-precision HDI circuit boards.
1. High-density wiring: HDI circuit boards have higher circuit density and smaller size, allowing denser wiring and more compact designs. This makes them particularly suitable for small devices or electronic products that require a high degree of integration, such as smartphones, tablets, etc.
2. Higher performance: HDI circuit boards can provide higher signal transmission speeds and lower signal delays because they can use shorter signal paths and smaller wiring spacing. This makes them ideal for high-speed digital signal transmission or high-frequency applications.
3. Enhanced signal integrity: HDI circuit boards can reduce signal crosstalk and signal attenuation, and improve signal integrity and stability. This is very important for complex circuit designs and applications requiring high performance.
4. Higher reliability: HDI circuit boards usually use more advanced manufacturing processes and materials, such as microvia technology, blind and buried via technology, etc., making them more reliable and stable. This can reduce product failure rates and extend product life.
5. Reduced costs: Although the manufacturing cost of HDI circuit boards may be slightly higher than traditional circuit boards, the overall product cost can be reduced due to its more compact design and higher performance. For example, HDI circuit boards can reduce the use of external connectors and components, thereby saving space and material costs.
6. Increased design freedom: HDI circuit boards enable more flexible and innovative designs because they enable more complex wiring and more compact component layouts. This allows designers to better realize their design concepts and meet clients’ specific needs.
Choosing high-precision HDI circuit boards can bring higher performance, better reliability and greater design freedom to product design, especially for applications that require high-density wiring, high-speed signal transmission and high-performance requirements. It is a very ideal choice.

Is the circuit board covered with grid copper or solid copper?

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The so-called copper pouring is to use the idle space on the circuit board as the reference plane and then fill it with solid copper. These copper areas are also called copper filling.

The significance of copper coating is to: reduce the impedance of the ground wire and improve the anti-interference ability; reduce the voltage drop and improve the power efficiency; and connect to the ground wire to reduce the loop area.

Also for the purpose of keeping the PCB from deforming as much as possible during welding, most PCB manufacturers will also require PCB designers to fill the open areas of the PCB with copper or grid-like ground wires. If the copper is not handled properly, it will The gain outweighs the loss. Does copper coating have “more pros than cons” or “does more cons than pros”?

Everyone knows that at high frequencies, the distributed capacitance of the wiring on the printed circuit board will work. When the length is greater than 1/20 of the corresponding wavelength of the noise frequency, the antenna effect will occur, and the noise will be emitted outward through the wiring. If there is poorly grounded copper in the PCB, the copper becomes a tool for transmitting noise.

Therefore, in high-frequency circuits, do not think that the ground wire is connected to the ground somewhere. This is the “ground wire”. You must drill holes in the wiring with a spacing of less than λ/20, and connect it to the ground. The ground plane of the shelf is “well grounded”. If the copper coating is handled properly, the copper coating not only increases the current, but also plays the dual role of shielding interference.

There are generally two basic methods of copper pouring, namely large-area copper pouring and grid copper pouring. People often ask whether it is better to pour copper pouring over a large area or with grid copper pouring. It is difficult to generalize.

why? Covering a large area with copper has the dual functions of increasing current and shielding. However, if a large area of copper is covered with wave soldering, the board may warp or even blister. Therefore, when covering a large area with copper, several grooves are usually opened to alleviate blistering of the copper foil.

Simple grid copper coating mainly has a shielding effect, and the effect of increasing current is reduced. From the perspective of heat dissipation, the grid is beneficial (it reduces the heating surface of copper) and plays a certain role in electromagnetic shielding.

It should be pointed out that the grid is composed of traces in staggered directions. We know that for circuits, the width of the traces has its corresponding “electrical length” for the operating frequency of the circuit board (the actual size divided by The digital frequency corresponding to the working frequency is available, please refer to relevant books for details).

When the operating frequency is not very high, perhaps the role of the grid lines is not very obvious. Once the electrical length matches the operating frequency, it is very bad. You will find that the circuit cannot work properly at all, and interference is being emitted everywhere. signal of.

The suggestion is to choose based on the working conditions of the designed circuit board, and don’t stick to one thing. Therefore, high-frequency circuits have multi-purpose grids that require high interference resistance, and low-frequency circuits have high-current circuits and other circuits that commonly use complete copper laying.

Layout and routing of PCB components.

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PCB design is 90% in device layout and 10% in wiring. If the PCB is designed well, it can get twice the result with half the effort and also improve the electrical characteristics of the PCB.

For example, if you want to improve work efficiency, you need to pay attention to the space for wiring to prevent re-routing due to lack of space; or if you don’t want to find that it cannot be soldered when soldering the board, you need to pay attention to the placement of components and consider the edges of the board. Distance and other factors; and if you want a PCB board that looks good and is easy to debug, you need to pay more attention to the overall layout of the PCB. These must be planned in advance to make the PCB board symmetrical, clean and beautiful.

Of course, for the same circuit diagram, 100 electronic engineers will have 1,000 wiring schemes. Because designing a circuit board is also a process of artistic creation, and different people have different aesthetic standards, so we do not define fixed PCB layout wiring standards. But I will provide you with a basic idea, based on which designers can design the most beautiful PCB board in their mind.

PCB layout tips

1. Understand the physical limitations of the circuit board

Before placing components, determine the mounting holes of the circuit board, the location of edge connectors, and the mechanical size constraints of the circuit board.

2. Understand the circuit board manufacturing process

The assembly process and testing process of the circuit, whether it is necessary to reserve space for the PCB V-shaped groove, the component welding process, etc.

3. Leave breathing space for integrated chips

When placing components, try to leave at least 350mil between them. For chips with many pins, the space needs to be larger.

4. The same device has the same direction

For identical devices, keep the formation as consistent as possible. It facilitates the later assembly, inspection and testing of circuit boards and ensures consistent high solder joints.

5. Reduce lead crossings

Reduce lead crossings by adjusting device position and orientation. It can save a lot of effort for later wiring.

6. Place circuit edge components first

For devices that cannot be moved arbitrarily due to mechanical restrictions, they must be placed first, such as external connectors, switches, USB ports, etc. on the circuit board.

7. Avoid conflicts between devices

Absolutely avoid overlapping and sharing the pads of devices or overlapping the edges of devices in order to route them in a small circuit board. It is best to maintain a distance of 40mil between all devices.

8. Place the devices on the same side as much as possible

The devices on the circuit board are completed by an automatic device placement machine. The devices are only on one side. The PCB production process only needs one pass. Otherwise, the devices will be placed twice, which wastes production time and costs.

9. Keep the polarity of chip pins and devices consistent

If the polarity and orientation of the components on the circuit board are messy, it will hinder the successful soldering of the circuit board.

10. The device location is similar to that on the schematic diagram

When designing the schematic diagram, the positional relationship between the devices has been optimized (the shortest connections and the least crossovers), so it will be more reasonable to place the PCB devices according to the device positions on the schematic diagram.

PCB routing rules

1. Routing direction control rules

That is, the wiring directions of adjacent layers form an orthogonal structure. Avoid running different signal lines in the same direction on adjacent layers to reduce unnecessary inter-layer interference; when it is difficult to avoid this situation due to board structure limitations (such as some backplanes), especially when the signal rate is high, Consider using ground planes to isolate wiring layers and ground signal lines to isolate signal lines.

2. Open-loop inspection rules for wiring

Generally, wiring with one end floating (Dangling Line) is not allowed, mainly to avoid the “antenna effect” and reduce unnecessary interference radiation and reception, otherwise it may bring unpredictable results.

3. Impedance matching check rules

The wiring width of the same network should be consistent. Changes in line width will cause uneven line characteristic impedance. When the transmission speed is high, reflection will occur. This situation should be avoided in the design.

4. Trace length control rules

That is, the short line rule. When designing, the wiring length should be kept as short as possible to reduce interference problems caused by too long wiring. Especially for some important signal lines, such as clock lines, the oscillator must be placed very close to the device. The place.

5. Chamfering rules

Sharp angles and right angles should be avoided in PCB design, which will produce unnecessary radiation and poor process performance.

6. Device decoupling rules

Add necessary decoupling capacitors to the printed plate to filter out interference signals on the power supply and stabilize the power supply signal.

7. Ground loop rules

The minimum loop rule means that the loop area formed by the signal line and its loop should be as small as possible. The smaller the loop area, the less external radiation and the smaller the external interference received.

8. Integrity rules for power and ground layers

For areas with dense via holes, care should be taken to avoid the holes connecting to each other in the hollowed-out areas of the power supply and ground layers, forming a division of the plane layer, thereby destroying the integrity of the plane layer, and thereby causing an increase in the loop area of the signal line in the ground layer. .

9. Shielding protection

The corresponding ground loop rule is actually to minimize the loop area of the signal, which is more common in some more important signals, such as clock signals and synchronization signals.

10. Cabling closed-loop inspection rules

Prevent signal lines from forming self-loops between different layers. Such problems are prone to occur in multilayer board designs, and self-loops will cause radiated interference.

11. Isolated copper zone control rules

The emergence of isolated copper areas will bring about some unpredictable problems. Therefore, connecting the isolated copper areas with other signals will help improve the signal quality. Usually, the isolated copper areas are grounded or deleted.

PCB wiring rules: What are the connector connection methods?

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Everyone knows about PCB, but do you know about its wiring? As an integral part of the whole machine, a PCB generally cannot constitute an electronic product, and there must be problems with external connections. For example, electrical connections are required between PCBs, between PCBs and off-board components, and between PCBs and equipment panels. PCB wiring can be said to be a basic course for every electronic engineer. The following is a brief analysis of the wiring of each PCB component for your reference.

In more complex instruments and equipment, plug-in connectors are often used. This “building block” structure not only ensures the quality of product mass production, reduces the cost of the system, but also provides convenience for debugging and maintenance. When equipment fails, maintenance personnel do not have to check to the component level (that is, check the cause of the failure and trace it back to the specific components. This work takes a lot of time), as long as they determine which board is abnormal. It can be replaced immediately, troubleshooting in the shortest possible time, shortening downtime and improving equipment utilization. The replaced circuit board can be repaired in ample time and used as spare parts after repair.

1. Standard pin connection

This method can be used for external connection of PCB, especially in small instruments, pin connection is often used. Two PCBs are connected through standard pins. The two PCBs are generally parallel or vertical, making it easy to achieve mass production.

2. PCB socket

This method is to make a printed plug from the edge of the PCB. The plug part is designed according to the size of the socket, the number of contacts, the contact distance, the position of the positioning holes, etc., so that it matches the dedicated PCB socket.

When making boards, the plug part needs to be gold-plated to improve wear resistance and reduce contact resistance. This method has simple assembly, good interchangeability and maintenance performance, and is suitable for standardized mass production. The disadvantage is that the PCB cost increases, and the PCB manufacturing accuracy and process requirements are higher; the reliability is slightly poor, and poor contact is often caused by the oxidation of the plug part or the aging of the socket reed. In order to improve the reliability of external connections, the same lead wire is often led out in parallel through the contacts on the same side or both sides of the circuit board.

PCB socket connection methods are often used in products with multi-board structures. There are two types of sockets and PCB or base plates: reed type and pin type.

PCB wiring rules

1. Cross circuits are not allowed in printed circuits. For lines that may cross, “drilling” and “winding” can be used to solve them. That is, let a certain lead “drill” through the gap at the foot of other resistors, capacitors, and transistors, or “wind” through one end of a lead that may cross. In special cases, the circuit is very complicated. To simplify the design, Wire jumpers are allowed to solve cross-circuit problems.

2. Resistors, diodes, tubular capacitors and other components can be installed in two ways: “vertical” and “horizontal”. The vertical type means that the component body is installed and welded perpendicularly to the circuit board. Its advantage is that it saves space. The horizontal type means that the component body is installed and welded parallel and close to the circuit board. Its advantage is that the mechanical strength of the component installation is better. For these two different mounting components, the component hole spacing on the printed circuit board is different.

3. The grounding points of circuits of the same level should be as close as possible, and the power supply filter capacitor of this level of circuit should also be connected to the grounding point of this level. In particular, the grounding points of the base and emitter of the transistor of this level cannot be too far apart, otherwise interference and self-excitation will occur because the copper foil between the two grounding points is too long. A circuit using such a “one-point grounding method” will work more slowly. Stable and not prone to self-excitation.

4. The general ground wire must be strictly arranged according to the principle of high frequency-medium frequency-low frequency in the order of weak current to strong current. It must not be randomly connected over and over again. It is better to have longer wiring between levels. Follow this rule. In particular, the requirements for the grounding wire arrangement of the frequency converter head, regeneration head, and frequency modulation head are more stringent. If there is any improper grounding wire arrangement, self-excitation will occur and the grounding wire will not work. High-frequency circuits such as FM heads often use large-area surrounding ground wires to ensure good shielding effects.

5. High-current leads (public ground wires, power amplifier power leads, etc.) should be as wide as possible to reduce wiring resistance and voltage drop, and reduce self-excitation caused by parasitic coupling.

6. Keep high-impedance traces as short as possible, and low-impedance traces as long as possible, because high-impedance traces tend to emit and absorb signals, causing circuit instability. The power wire, ground wire, base wire of non-feedback components, emitter wire, etc. are all low-impedance wires. The base wire of the emitter follower and the ground wires of the two channels of the radio must be separated and formed into a separate path. , until the end of the function and then combined together. If the two ground wires are connected back and forth, it is easy to produce crosstalk and reduce the separation.

Explore the PCB rigid-flex board process: making electronic devices more flexible and adaptable!

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In today’s era of rapid technological advancement, the flexibility and adaptability of electronic devices have become people’s pursuit of products. As an advanced technology, PCB rigid-flex board technology is attracting more and more attention and applications. This article will analyze the PCB rigid-flex board process and explore its advantages in improving the flexibility and adaptability of electronic equipment.

PCB rigid-flex board technology is a manufacturing process that combines rigid boards and flexible boards. Through rigid-flex integration, electronic devices can have the stability of rigid boards and the bending of flexible boards. This process can realize the free change of electronic devices in different forms and environments, thereby better adapting to user needs.

PCB rigid-flex board technology can greatly reduce the size and weight of electronic equipment. Traditional rigid boards have certain design limitations, while flexible boards can bend and fold more flexibly. By combining the two, electronic devices can achieve smaller and lighter designs while maintaining stability, making them easy to carry and use.

The rigid-flex board process can also improve the durability and reliability of electronic equipment. Flexible boards have good earthquake resistance, impact resistance and high temperature resistance, and can effectively protect electronic components from the external environment. The rigid board can provide better support and fixation to prevent electronic components from loosening or being damaged. Through the combination of the two, electronic equipment can maintain stable performance and reliable working status in the face of various complex environments and working conditions.

In addition, the PCB rigid-flex board process can also improve the wiring density and performance of electronic equipment. Flexible boards have high bending ability and small thickness, which can achieve more compact wiring design and improve the connection efficiency between electronic components. Rigid boards, on the other hand, can provide better electrical performance and signal transmission quality. Through the combination of the two, electronic devices can achieve higher integration and better performance.

The emergence of PCB rigid-flex board technology provides a new solution for the flexibility and adaptability of electronic equipment. Through the combination of rigid and flexible, electronic devices can be further improved in terms of size, weight, durability and performance. In the future, with the continuous development and application of this technology, we have reason to believe that electronic devices will become more and more flexible and adaptable, bringing people a more convenient and comfortable technological experience!

Quickly understand the various types and uses of fpc!

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Flexible Printed Circuit Board (FPC) is a flexible electronic component that is widely used in various electronic devices. It is different from traditional rigid circuit boards. Its main feature is that it can be bent, folded and twisted to adapt to various complex shapes and space constraints. In the field of modern technology, FPC is used more and more widely. Today we will reveal the secrets of flexible circuit boards and quickly understand the various types and uses of FPC.
First, let us understand the basic structure of fpc. FPC consists of conductive layer, insulating layer and protective layer. The conductive layer is usually made of copper foil and is used to transmit current and signals. The insulating layer is made of materials such as flexible polyimide and serves to isolate the conductive layer. The protective layer is used to protect the FPC from the external environment, such as moisture, dust and mechanical damage.
According to different application requirements, FPC can be divided into many types.
Single-sided FPC is the simplest type, with only one conductive layer, so the wiring density is not high, but it has good flex resistance.
Double-sided FPC has conductive layers on both sides, which is suitable for some scenarios that require more complex circuit layouts, but its softness is worse than that of single-sided panels.
Multilayer FPC, which has multiple conductive layers embedded inside, is suitable for more complex electronic devices.
Rigid-flexible board combines the characteristics of rigid circuit board and flexible circuit board, which can not only meet the needs of rigid structure, but also adapt to certain bending.

 

So, what are the main applications of fpc? First of all, FPC is widely used in mobile devices, such as smartphones, tablets, etc. Because FPC has high flexibility and foldability, it can adapt to the needs of equipment of various shapes and sizes. Secondly, it is also widely used in the field of automotive electronics. In cars, FPC can connect various sensors, displays and control modules to realize vehicle intelligence and automation. In addition, it is also commonly used in medical equipment, aerospace, industrial control and other fields to provide reliable circuit connections for various electronic devices.
Flexible circuit boards are flexible and foldable electronic components that are widely used in various electronic devices. By understanding the types and uses of FPC, we can better understand and apply this advanced electronic component and promote the development and innovation of science and technology.

What is the science of plugging holes in PCB circuit boards?

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Conductive holes are also known as conductive holes. In order to meet customer requirements, circuit board via holes must be plugged. After a lot of practice, the traditional aluminum sheet plugging process was changed, and white mesh was used to complete the circuit board surface solder mask and plugging. hole. Stable production and reliable quality.

Via holes play the role of connecting circuits to each other. The development of the electronics industry also promotes the development of PCB and puts forward higher requirements for printed board manufacturing technology and surface mounting technology. Via hole plugging technology emerged as the times require, and it should meet the following requirements:

(1) There is only copper in the via hole, and the solder mask can be plugged or not;

(2) There must be tin-lead in the via hole, and there is a certain thickness requirement (4 microns). There must be no solder mask ink entering the hole, causing tin beads to be hidden in the hole;

(3) The via hole must be plugged with solder mask ink, opaque, and must not have tin rings, tin beads, and flatness.

As electronic products develop in the direction of “light, thin, short and small”, PCBs are also developing towards high density and difficulty. Therefore, a large number of SMT and BGA PCBs appear. Customers require plug holes when mounting components. The main ones are Five functions:

(1) Prevent tin from the via hole penetrating the component surface to cause a short circuit when the PCB is wave soldered; especially when we place the via hole on the BGA pad, we must first make a plug hole and then gold plating to facilitate the welding of the BGA.

(2) Avoid flux remaining in the via hole;

(3) After surface mounting and component assembly in the electronics factory are completed, the PCB must be vacuumed on the testing machine to form negative pressure before completion:

(4) Prevent surface solder paste from flowing into the holes, causing virtual soldering and affecting placement;

(5) Prevent tin beads from popping up during wave soldering and causing short circuits.

Implementation of conductive hole plugging process
For surface mount boards, especially the mounting of BGA and IC, the via holes must be flat, with a convexity of plus or minus 1 mil. There must be no red tinting on the edges of the via holes; tin beads are hidden in the via holes. In order to satisfy customers According to the requirements, the via hole plugging process can be described as diverse, the process flow is extremely long, and the process control is difficult. Problems such as oil leakage during hot air leveling and green oil solder resistance testing and oil explosion after curing often occur. Based on the actual production conditions, we will summarize various PCB hole plugging processes and make some comparisons and elaborations on the processes, advantages and disadvantages:

Note: The working principle of hot air leveling is to use hot air to remove excess solder from the surface and holes of the printed circuit board. The remaining solder is evenly covered on the pads, non-resisting solder lines and surface packaging points. It is a surface treatment method for printed circuit boards. one.

1. Hole plugging process after hot air leveling
This process flow is: board solder mask→HAL→hole plugging→curing. The non-hole plugging process is used for production. After hot air leveling, an aluminum sheet screen or an ink-blocking net is used to complete the via hole plugging of all fortresses required by the customer. The plugging ink can be photosensitive ink or thermosetting ink. Under the condition of ensuring that the color of the wet film is consistent, it is best to use the same ink as the board surface. This process can ensure that the via hole does not lose oil after hot air leveling, but it can easily cause the plug hole ink to contaminate the board surface and make it uneven. Customers can easily cause false soldering during mounting (especially in BGA). So many customers do not accept this method.

2. Hole plugging process before hot air leveling
2.1 Plug the holes with aluminum sheets, solidify, and grind the plate for pattern transfer.

In this process, a CNC drilling machine is used to drill out the aluminum sheet that needs to be plugged, and the holes are made into a screen to plug the holes to ensure that the via holes are filled with plugging ink. The plugging ink can also be thermosetting ink, which must have high hardness. , the resin shrinkage changes little, and the bonding force with the hole wall is good. The process flow is: pre-processing → hole plugging → plate grinding → pattern transfer → etching → solder mask on the board surface

This method can ensure that the via hole plug hole is flat, and hot air leveling will not cause quality problems such as oil explosion and oil loss on the edge of the hole. However, this process requires one-time thickening of copper to make the hole wall copper thickness meet the customer’s standards. Therefore, there are very high requirements for copper plating on the entire board, and there are also high requirements for the performance of the plate grinding machine to ensure that the resin on the copper surface is completely removed and the copper surface is clean and not contaminated. Many PCB factories do not have a one-time copper thickening process, and the performance of the equipment does not meet the requirements, resulting in this process being rarely used in PCB factories.

2.2 Plug the holes with aluminum sheets and directly screen-print the solder mask on the board.

In this process, a CNC drilling machine is used to drill out the aluminum sheet that needs to be plugged, and the screen is made into a screen. It is installed on the screen printing machine to plug the holes. After the plugging is completed, it should not be parked for more than 30 minutes. A 36T screen is used to directly screen the board surface to resist soldering. The process flow is: pre-treatment – hole plugging – screen printing – pre-baking – exposure – development – curing

Using this process can ensure that the via holes are well covered, the plug holes are flat, and the wet film has the same color. After hot air leveling, it can ensure that the via holes are not tinned and there are no tin beads hidden in the holes. However, it is easy to cause the ink in the holes to be stained after curing. pad, resulting in poor solderability; after hot air leveling, the edges of the via holes bubble and oil is removed. Production control using this process is difficult, and process engineers must adopt special processes and parameters to ensure the quality of the plug holes.

2.3 After the aluminum sheet is plugged, developed, pre-cured, and ground, the board surface is soldered.

Use a CNC drilling machine to drill out the aluminum sheet that requires plugging holes, make a screen, install it on a shift screen printing machine to plug the holes, the plug holes must be full, preferably protruding on both sides, and then solidify and grind the plate for surface treatment. The process flow is: pre-processing – hole plugging – pre-baking – development – pre-curing – solder mask on the board

Since this process uses plug hole curing, it can ensure that the via holes do not lose oil or explode after HAL. However, after HAL, it is difficult to completely solve the problem of tin beads hiding in via holes and tin application on via holes, so many customers do not accept it.

2.4 The solder mask and plug holes on the board are completed at the same time.

This method uses a 36T (43T) screen, installed on the screen printing machine, using a backing plate or nail bed to plug all the via holes while completing the board. The process flow is: pre-processing – screen printing – -Pre-baking–exposure–development–curing.

This process takes a short time and has a high utilization rate of the equipment. It can ensure that no oil will fall out of the via holes after hot air leveling, and no tin will be applied to the conductive holes. However, due to the use of silk screen printing for plugging, there is a large amount of air in the via holes, which will cause a large amount of air in the via holes during curing. , the air expands and breaks through the solder mask, causing holes and unevenness. Hot air leveling will leave a small amount of tin hidden in the via holes. At present, after a lot of experiments, our company has selected different types of inks and viscosities, adjusted the pressure of screen printing, etc., and basically solved the voids and unevenness of the vias, and has adopted this process for mass production.

Why does the PCB board warp? Why does it cause so much harm after deformation?

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The dangers of PCB board deformation

In automated surface mounting lines, if the circuit board is not flat, it will cause inaccurate positioning, components cannot be inserted or mounted into the board’s holes and surface mounting pads, and the automatic insertion machine may even be damaged.

The circuit board with the components on it is bent after soldering, and it is difficult to cut the component pins neatly. The board cannot be installed into the chassis or the socket inside the machine, so it is also very troublesome for the assembly factory to encounter a warped board.

The current surface mount technology is developing in the direction of high precision, high speed and intelligence, which puts forward higher flatness requirements for PCB boards, which are the homes of various components.

In the IPC standard, it is specifically stated that the allowable deformation of PCB boards with surface mount devices is 0.75%, and the allowable deformation of PCB boards without surface mount devices is 1.5%.

In fact, in order to meet the needs of high-precision and high-speed placement, some electronic assembly manufacturers have stricter requirements for deformation. If required, the allowed deformation is 0.5%, and some even require 0.3%.

PCB boards are composed of copper foil, resin, glass cloth and other materials. The physical and chemical properties of each material are different. After being pressed together, thermal stress residues will inevitably occur, leading to deformation.

At the same time, during the processing of PCB, various processes such as high temperature, mechanical cutting, and wet treatment will also have an important impact on the deformation of the board. In short, the causes of PCB board deformation are complex and diverse. How to reduce or eliminate the deformation due to material characteristics? Deformation caused by differences or processing has become one of the complex problems faced by PCB manufacturers.

Analysis of causes of PCB board deformation

The deformation of PCB board needs to be studied from several aspects such as material, structure, pattern distribution, processing process, etc. This article will analyze and elaborate on the various causes of possible deformation and improvement methods.

The uneven copper surface area on the circuit board will worsen the board bending and warping.

Generally, circuit boards are designed with a large area of copper foil for grounding purposes. Sometimes the Vcc layer is also designed with a large area of copper foil. When these large area copper foils cannot be evenly distributed on the same circuit board When it is installed, it will cause the problem of uneven heat absorption and heat dissipation speed.

Of course, the circuit board will also expand due to thermal expansion and contraction. If the expansion and contraction cannot occur at the same time, it will cause different stresses and deformation. At this time, if the temperature of the board has reached the upper limit of the Tg value, the board will begin to soften, causing deformation.

The connection points (vias, vias) of each layer on the circuit board will limit the expansion and contraction of the board.

Most of today’s circuit boards are multi-layer boards, and there are connection points (vias) like rivets between layers. The connection points are divided into through holes, blind holes and buried holes. Where there are connection points, the board will be restricted. The effect of expansion and contraction will also indirectly cause the board to bend and warp.

Reasons for PCB board deformation:

(1) The weight of the circuit board itself will cause the board to dent and deform.

Generally, reflow furnaces use chains to drive the circuit board forward in the reflow furnace, that is, using both sides of the board as fulcrums to support the entire board.

If there are overweight parts on the board, or the size of the board is too large, there will be a dent in the middle due to its own weight, causing the board to bend.

(2) The depth of the V-Cut and the connecting strip will affect the deformation of the panel.

Basically, V-Cut is the culprit that destroys the structure of the board. Because V-Cut cuts grooves into the original large sheet, the V-Cut is prone to deformation.

The influence of laminating materials, structures, and graphics on plate deformation:

The PCB board is composed of a core board, a prepreg, and an outer layer of copper foil. The core board and copper foil are thermally deformed during lamination. The amount of deformation depends on the coefficient of thermal expansion (CTE) of the two materials.

The thermal expansion coefficient (CTE) of copper foil is about 17X10-6; while the Z-direction CTE of ordinary FR-4 substrate at the Tg point is (50~70)X10-6; above the TG point it is (250~350)X10-6 , X-direction CTE is generally similar to copper foil due to the presence of glass cloth.

Deformation caused during PCB board processing

The causes of PCB board deformation during processing are very complex and can be divided into two types of stress: thermal stress and mechanical stress.

Thermal stress is mainly generated during the lamination process, and mechanical stress is mainly generated during the stacking, handling, and baking processes of panels. The following is a brief discussion in process order.

1. Incoming copper clad laminate materials:

Copper-clad laminates are all double-sided with symmetrical structure and no graphics. The CTE of copper foil and glass cloth are almost the same, so there is almost no deformation caused by different CTE during the lamination process.

However, the size of the copper-clad laminate press is large, and there are temperature differences in different areas of the hot plate, which will lead to slight differences in the resin curing speed and degree in different areas during the lamination process. At the same time, the dynamic viscosity at different heating rates is also significantly different, so there will also be Local stresses due to differences in the curing process.

Generally, this stress will maintain balance after pressing, but will gradually release and cause deformation during subsequent processing.

2. Pressing:

The PCB lamination process is the main process that generates thermal stress. Similar to the lamination of copper-clad laminates, it will also produce local stress caused by differences in the curing process. The thermal stress of PCB boards is due to thicker thickness, diverse pattern distribution, and more prepregs. The stress will also be more difficult to eliminate than that of copper clad laminate.

The stress existing in the PCB board is released during subsequent drilling, shaping or grilling processes, causing the board to deform.

3. Baking process of solder mask, characters, etc.:

Since the solder mask ink cannot be stacked on top of each other when curing, the PCB boards will be placed vertically in a rack for curing. The solder mask temperature is about 150°C, which is just above the Tg point of medium and low Tg materials. The resin above the Tg point is in a highly elastic state, and the board Parts are easily deformed under their own weight or strong wind from the oven.

4. Hot air solder leveling:

When leveling hot air solder for ordinary boards, the tin furnace temperature is 225℃~265℃ and the time is 3S-6S. The hot air temperature is 280℃~300℃.

The solder leveling plate enters the tin furnace at room temperature, and is post-processed and washed at room temperature within two minutes after coming out of the furnace. The entire hot air solder leveling process is a sudden heating and cooling process.

Due to different circuit board materials and uneven structures, thermal stress will inevitably occur during the cooling and heating process, resulting in microscopic strain and overall deformation and warping.

5. Storage:

PCB boards are generally stored vertically in racks during the semi-finished product stage. Improper adjustment of the rack’s tightness or stacking of boards during storage will cause mechanical deformation of the boards. Especially for thin plates below 2.0mm, the impact is more serious.

In addition to the above factors, there are many factors that affect PCB board deformation.

Prevention of PCB board warpage and deformation

Circuit board warpage has a great impact on the production of printed circuit boards. Warpage is also one of the important problems in the circuit board production process. The board with components on it will bend after welding, making it difficult for the component legs to be neat.

The board cannot be installed into the chassis or the socket inside the machine. Therefore, the warping of the circuit board will affect the normal operation of the entire subsequent process.

At this stage, printed circuit boards have entered the era of surface mounting and chip mounting, and the process requirements for circuit board warpage are getting higher and higher. So we need to find the reason why the circuit board is warping.

1. Engineering design:

Things to note when designing printed circuit boards:

A. The arrangement of prepreg sheets between layers should be symmetrical. For example, for a six-layer board, the thickness between layers 1 to 2 and 5 to 6 should be consistent with the number of prepreg sheets, otherwise it will easily warp after lamination.

B. Multilayer core boards and prepregs should use products from the same supplier.

C. The line pattern areas on the outer layer A and B should be as close as possible. If side A is a large copper surface and side B only has a few lines, this kind of printed board will easily warp after etching. If the line areas on the two sides are too different, you can add some independent grids on the sparse side for balance.

2. Drying plate before unloading:

The purpose of drying the copper-clad laminate (150 degrees Celsius, 8 ± 2 hours) before unloading is to remove the moisture in the board, and at the same time completely solidify the resin in the board, further eliminating the remaining stress in the board, which is helpful in preventing the board from warping. helpful.

At present, many double-sided and multi-layer boards still adhere to the step of drying the boards before or after cutting. However, there are exceptions for some board factories. Currently, the drying time regulations of various PCB factories are inconsistent, ranging from 4 to 10 hours. It is recommended to decide based on the grade of the printed boards produced and the customer’s requirements for warpage.

Both methods are feasible, whether to cut the pieces into panels and then bake them or to bake them as a whole piece and cut them out. It is recommended to cut the materials and then bake them. The inner layer boards should also be baked.

3. Longitude and latitude direction of prepreg:

The warp and weft shrinkage rates of prepregs after lamination are different, and the warp and weft directions must be distinguished when blanking and laminating. Otherwise, it is easy to cause the finished board to warp after lamination, which is difficult to correct even if pressure is applied to bake the board.

Many reasons for the warping of multi-layer boards are caused by not distinguishing the longitude and weft directions of the prepreg sheets during lamination, and they are stacked randomly.

How to distinguish longitude and latitude? The rolling direction of the rolled prepreg is the warp direction, and the width direction is the weft direction; for copper foil boards, the long side is the weft direction, and the short side is the warp direction. If you are not sure, you can check with the manufacturer or supplier.

4. Stress relief after lamination:

After completing the hot and cold pressing, the multi-layer board is taken out, the burrs are cut or milled, and then placed flat in an oven at 150 degrees Celsius for 4 hours to gradually release the stress in the board and completely solidify the resin. This step cannot be omitted.

5. Thin sheets need to be straightened during electroplating:

When using 0.4~0.6mm ultra-thin multi-layer boards for surface electroplating and pattern electroplating, special clamp rollers should be made. After clamping the thin plate on the fly bar of the automatic electroplating line, use a round stick to clamp the entire fly bar. The rollers are strung together to straighten all the boards on the rollers so that the plated boards will not deform.

Without this measure, the sheet will bend after plating a copper layer of 20 to 30 microns, and it will be difficult to repair.

6. Cooling of the board after hot air leveling:

During hot air leveling, the printed board is subjected to the high temperature impact of the soldering bath (about 250 degrees Celsius). After being taken out, it should be placed on a flat marble or steel plate to cool naturally, and then sent to the post-processing machine for cleaning. This is very good for preventing the board from warping.

In order to enhance the brightness of the lead-tin surface, some factories put the board into cold water immediately after being leveled with hot air, and then take it out after a few seconds for post-processing. This kind of hot and cold impact may cause warping of some types of boards. Curved, layered or blistered.

In addition, an air flotation bed can be installed on the equipment for cooling.

7. Treatment of warped boards:

In a well-managed factory, 100% flatness inspection of printed boards will be carried out during final inspection. All unqualified boards will be picked out, placed in an oven, baked at 150 degrees Celsius under heavy pressure for 3 to 6 hours, and allowed to cool naturally under heavy pressure.

Then remove the pressure and take out the board, and then check the flatness. This can save part of the board. Some boards need to be baked and pressed two to three times before they can be flattened. If the above-mentioned anti-warping process measures are not implemented, baking and pressing some boards will be useless and they will have to be scrapped.
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Customized thick copper circuit board, the best choice for stability and reliability!

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Circuit boards play a vital role in electronic equipment, and the customization of their thick copper plates is crucial to the stability and reliability of the equipment. This article will discuss in detail the customization of thick copper plates for circuit boards from four aspects: the advantages, customization process, application scenarios and future development trends of thick copper plates.
1. Advantages
As a special form of circuit board, thick copper plate has excellent thermal conductivity, good electrical performance and high strength, which plays a significant role in improving the stability and reliability of circuit boards. The thick copper plate can not only reduce the temperature rise of the circuit board and improve the heat dissipation performance, but also reduce the line loss of the circuit board and improve the accuracy of signal transmission. In special application areas such as high frequency and high power, thick copper plates are an indispensable choice. The advantage of thick copper plates is that they can provide better current carrying capacity, reduce the parasitic resistance of the circuit board, reduce the temperature of the circuit board, and extend the service life of the equipment. Therefore, customizing thick copper circuit boards can improve the reliability and stability of electronic equipment and is favored by more and more customers.
2. Customization process
The customization of thick copper plates for circuit boards usually requires the following processes: design confirmation, process discussion, material selection, production and processing, quality inspection and other links. The design confirmation stage is to determine customer needs and specifications. The process discussion stage includes determining the production process and requirements. The material selection stage is to select suitable copper and aluminum-based materials. The production and processing stage is to make circuit boards according to the design requirements. The quality inspection stage is to inspect the finished product. Conduct strict testing to ensure reliable quality. Due to the high production requirements for thick copper sheets for circuit boards, customers need to work closely with suppliers to ensure that every step of the process meets the requirements, so as to obtain customized products with reliable quality.
3. Application scenarios
Circuit board thick copper plates are widely used in communication equipment, military equipment, medical equipment, aerospace and other fields. In communication equipment, thick copper plates can increase signal transmission rate, reduce signal distortion rate, and ensure stable operation of equipment; in military equipment, thick copper plates have strong anti-interference and earthquake resistance, and can adapt to various harsh environments; in medical equipment Medium and thick copper plates can improve the safety and stability of equipment and ensure the accuracy of medical operations; in the aerospace field, thick copper plates can meet the requirements for lightweight and high performance of equipment and ensure long-term stable operation of equipment.
4. Future development trends
As the requirements for stability and reliability of electronic equipment continue to increase, the market demand for thick copper sheets for circuit boards will continue to grow. In the future, customized thick copper plates will pay more attention to the selection of environmentally friendly materials and develop more energy-saving and environmentally friendly thick copper plates for circuit boards; at the same time, with the development of technologies such as intelligence and the Internet of Things, thick copper plates for circuit boards will play an important role in the development of new smart devices. It has an important impact on applications and becomes an indispensable key component in future electronic devices.
The customization of thick copper plates for circuit boards is the best choice to improve the stability and reliability of electronic equipment, which is of great significance and broad prospects. By continuously optimizing the process flow and expanding the application fields, circuit board thick copper plates will be more widely used in various types of electronic equipment in the future, injecting more vitality into the development of the electronics industry.