1. Consider the selection of component packaging
During the entire schematic drawing phase, you need to consider component packaging and pad design decisions in the layout phase. When selecting components, the following suggestions are worth considering.
Please remember that packaging includes electrical pad connections and mechanical dimensions (X, Y, and Z) of the component, i.e. the shape of the component body and its pins connected to the PCB. When selecting components, it is necessary to consider any installation or packaging limitations that may exist in the upper and lower layers of the final PCB. Some components, such as polar capacitors, may have high height limitations, which must be considered in the component selection process.
1. If there is no ready-made package in the database, a custom package is typically created within the tool.
2. Use an effective grounding method.
Ensure that the design incorporates sufficient bypass capacitors and ground planes. When using an integrated circuit, make sure to place a suitable decoupling capacitor near the power terminal and ground (preferably a ground plane). The appropriate capacitance of the capacitor depends on the specific application, capacitor technology, and operating frequency. By placing the bypass capacitor between the power and ground pins and positioning it close to the relevant IC pin, you can optimize the circuit’s electromagnetic compatibility and susceptibility.
3. Allocate virtual component packages.
Print a bill of materials (BOM) for reviewing virtual components. Virtual components lack associated packaging and will not be transferred to the layout stage. Create a BOM, then examine all virtual components in the design. The only items should be power and ground signals, as these are considered virtual components processed only in the schematic environment and will not transition to the layout design. Unless used for simulation purposes, the components shown as virtual should be replaced with packaged components.
4. Ensure complete bill of materials data.
Verify that sufficient data is included in the BOM report. After generating the report, carefully review and complete any missing device, supplier, or manufacturer information for all component entries.
5. Sort according to component labels.
To facilitate the sorting and viewing of the BOM, ensure that the component numbers are sequentially numbered.
6. Check for redundant gate circuits.
Generally, the inputs of all redundant gates should be connected to avoid floating input terminals. Ensure that you have reviewed all redundant or missing gate circuits and that all unwired input terminals are fully connected. In some instances, if the input terminal is left floating, the entire system may fail to operate correctly. Take the dual op amp often used in designs as an example: if only one of the op amps in a dual op amp IC is utilized, it is advisable to either use the other op amp or ground the input of the unused op amp, deploying an appropriate unity gain (or other gain) feedback network to ensure normal operation of the entire component.
In some cases, ICs with floating pins may not function properly within the specified range. Typically, only when the IC device or other gates in the same device are not operating in a saturated state can the input or output be close to or within the component power rail, allowing the IC to meet the specification requirements. Simulations often fail to capture this scenario, as the simulation models usually do not connect multiple parts of the IC together to reflect the effects of floating connections.
The above outlines six essential techniques for component selection in PCB circuit board design.
