The best PCB design method: six things to consider when selecting PCB components based on component package. All of the examples in this article were developed in Multisim design environment, but the same concepts still apply even though different EDA tools are used.

1. Consider the selection of component package

In the whole schematic drawing stage, we should consider the component packaging and pad pattern decisions that need to be made in the layout stage. Here are some suggestions to consider when selecting components based on component package.(1) Remember that the package includes the electrical pad connection and mechanical dimensions (x, y, and z) of the component, i.e., the shape of the component body and the pins connecting the PCB. When selecting components, you need to consider any installation or packaging restrictions that may exist on the top and bottom layers of the final PCB. Some components (such as polar capacitors) may have high clearance limits, which need to be considered in the component selection process. At the beginning of the design, you can draw a basic board outline shape, and then place some large or position critical components (such as connectors) that you plan to use. In this way, we can see the Virtual Perspective of the circuit board (without wiring) directly and quickly, and give relatively accurate relative positioning and component height of the circuit board and components. This will help to ensure that the components of the PCB can be properly placed in the outer packaging (plastic products, chassis, frame, etc.) after assembly. You can browse the whole circuit board by calling the 3D preview mode from the tool menu.(2) The pad pattern shows the actual pad or via shape of the soldered device on the PCB. These copper patterns on PCB also contain some basic shape information. The size of the pad pattern needs to be correct to ensure proper welding and to ensure the correct mechanical and thermal integrity of the connected components. When designing PCB layout, it is necessary to consider how the circuit board will be manufactured, or how the pad will be welded if it is manually welded. Reflow soldering (flux melting in a controlled high temperature furnace) can handle a wide range of surface mounted devices (SMDS). Wave soldering is usually used to weld the reverse side of a circuit board to fix through-hole devices, but it can also handle some surface mount components placed on the back of PCB. Usually, when using this technology, the bottom surface mount devices must be arranged in a specific direction, and in order to adapt to this welding method, the pad may need to be modified.(3) The selection of components can be changed throughout the design process. Early in the design process to determine which devices should use electroplated through holes (PTH) and which should use surface mount technology (SMT) will help the overall planning of PCB. The factors that need to be considered are device cost, availability, device area density and power consumption. From a manufacturing point of view, surface mount devices are generally cheaper than through-hole devices and generally have higher availability. For small and medium-sized prototype projects, it is better to choose larger surface mount devices or through-hole devices, which is not only convenient for manual welding, but also conducive to better connection of pad and signal in the process of error checking and debugging.(4) If there is no ready-made encapsulation in the database, the custom encapsulation is usually created in the tool.

2. Use good grounding method

Ensure that the design has sufficient bypass capacitance and ground level. When using integrated circuits, make sure to use the appropriate decoupling capacitor near the power supply end to ground (preferably the ground plane). The appropriate capacity of the capacitor depends on the specific application, capacitor technology and operating frequency. When the bypass capacitor is placed between the power and ground pins and close to the correct IC pin, the EMC and susceptibility of the circuit can be optimized.

3. Assign virtual component packaging

Print a bill of material (BOM) for checking virtual components. Virtual components have no associated encapsulation and will not be transferred to the layout phase. Create a bill of materials and then view all virtual components in the design. The only entries should be power and ground signals, because they are considered virtual components and are processed specifically in the schematic environment and will not be transmitted to the layout design. Unless used for simulation purposes, the components displayed in the virtual part should be replaced by components with packages.

4. Make sure you have complete BOM data

Check that there is sufficient and complete data in the BOM report. After the bill of materials report is created, it is necessary to carefully check and complete the incomplete information of devices, suppliers or manufacturers in all component entries.

5. Sort according to component label

To facilitate the sorting and viewing of the bill of materials, ensure that the component labels are numbered consecutively.

6. Check the redundant gate circuit

In general, all redundant gate inputs should have signal connections to avoid hanging the input. Make sure you check all redundant or missing gate circuits and that all disconnected inputs are fully connected. In some cases, if the input is suspended, the whole system will not work properly. Take the Dual OP AMP, which is often used in design. If only one of the op amps is used in the Dual OP AMP IC, it is suggested that either the other op amp be used, or the input end of the op amp that is not used should be grounded, and an appropriate unit gain (or other gain) feedback network should be set up to ensure the normal operation of the whole amplifier.In some cases, the IC pin may not be in the normal working range. In general, only when the IC device or other gates in the same device are not working in the saturated state — the input or output is close to or in the component power rail, the IC can meet the requirements of the index. Simulation usually fails to capture this situation because simulation models do not generally connect multiple parts of the IC together to model the levitation connection effect.