Knowledge

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.
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