Principles for selecting electronic components, how to choose PCB components?

Components are the basic elements that constitute a circuit and are the final result of circuit principle analysis and calculation. In circuit principle analysis, it is necessary to know the structure, characteristics, parameters of each component, its role in the circuit, and its impact on the entire circuit; in circuit parameter calculation, each component parameter is also a factor in circuit calculation. The final result facilitates the reasonable selection of component specifications and models. Correct selection of components is the key to realizing circuit functions, and selection methods and techniques are very important. How to quickly select components in PCB design?

1. Consider the choice of component packaging

Throughout the schematic drawing stage, component packaging and land pattern decisions that need to be made during the layout stage should be considered. Here are some suggestions to consider when selecting components based on their package.

Remember, the package includes the component’s electrical pad connections and mechanical dimensions (X, Y, and Z), which is the shape of the component body and the pins that connect to the PCB. When selecting components, consider any mounting or packaging constraints that may exist on the top and bottom layers of the final PCB. Some components (such as polarized capacitors) may have height headroom restrictions that need to be considered during the component selection process. When you first start designing, you can start by drawing a basic circuit board outline shape, and then place some large or location-critical components (such as connectors) that you plan to use. In this way, a virtual perspective view of the circuit board (without wiring) can be seen intuitively and quickly, and the relative positioning and component height of the circuit board and components are given relatively accurately. This will help ensure that the components fit properly into the outer packaging (plastic, case, frame, etc.) after the PCB is assembled. Call the 3D preview mode from the tools menu to browse the entire board.

The land pattern shows the actual pad or via shape of the soldered device on the PCB. These copper patterns on the PCB also contain some basic shape information. The size of the land pattern needs to be correct to ensure proper soldering and to ensure the correct mechanical and thermal integrity of the connected components. When designing the PCB layout, you need to consider how the board will be manufactured or, if soldered by hand, how the pads will be soldered. Reflow soldering (where flux is melted in a controlled, high-temperature furnace) can handle a wide variety of surface-mount devices (SMDs). Wave soldering is generally used to solder the reverse side of the circuit board to secure through-hole components, but it can also handle some surface-mount components placed on the back of the PCB. Typically when using this technique, the underlying surface mount components must be aligned in a specific orientation, and the pads may need to be modified to accommodate this soldering method.

Component selection can be changed throughout the design process. Determining early in the design process which devices should use plated through holes (PTH) and which should use surface mount technology (SMT) will help with overall PCB planning. Factors to consider include device cost, availability, device area density, power consumption, etc. From a manufacturing perspective, surface-mount devices are generally cheaper than through-hole devices and generally have higher availability. For small and medium-sized prototype projects, it is best to use larger surface-mount devices or through-hole devices, which not only facilitates manual soldering, but also facilitates better connection of pads and signals during error checking and debugging.

If there is no ready-made package in the database, a custom package is usually created in the tool.

2. Use good grounding methods

Make sure the design has adequate bypass capacitors and ground planes. When working with integrated circuits, make sure to use appropriate decoupling capacitors close to the power supply terminals to ground (preferably the ground plane). The appropriate sizing of the capacitor depends on the specific application, capacitor technology, and operating frequency. When bypass capacitors are placed between the power and ground pins and placed close to the correct IC pins, the electromagnetic compatibility and susceptibility of the circuit can be optimized.

3. Assign virtual component packages

Print a bill of materials (BOM) for inspection of virtual components. Virtual components do not have associated packages and will not be passed to the layout stage. Create a bill of materials and view all virtual components in your design. The only entries should be for power and ground signals, as they are considered virtual components and are only handled exclusively in the schematic environment and are not transferred to the layout. Unless used for simulation purposes, components shown in the virtual section should be replaced with components with encapsulation.

4. Make sure you have complete bill of materials data

Check that the bill of materials report contains sufficiently complete data. After the bill of materials report is created, it should be carefully checked to complete any incomplete device, supplier or manufacturer information in all component entries.

5. Sort according to component number

To aid in sorting and viewing the bill of materials, make sure component designations are numbered consecutively.

6. Check for redundant gate circuits

Generally speaking, all redundant gate inputs should have signal connections to avoid floating inputs. Make sure you check any redundant or missing gates and that any unwired inputs are fully connected. In some cases, if the input is left floating, the entire system may not work correctly. Take the dual op amps often used in design. If only one op amp is used in a dual op amp IC component, it is recommended to either use the other op amp, or ground the input end of the unused op amp, and place a suitable unity gain (or other gain ) feedback network to ensure that the entire component can work properly.

In some cases, ICs with floating pins may not function properly within specifications. Typically an IC can operate to meet specifications only if the IC device or other gates in the same device are not operating in saturation and the inputs or outputs are close to or at the component supply rails. Simulation often cannot capture this situation because simulation models generally do not connect multiple parts of the IC together to model the floating connection effect.