PCB Design Best Practices: Six things to consider when making PCB component choices based on part footprints. All the examples in this article are developed using the NI Multisim design environment, however the same concepts apply when using different EDA tools.
1. Think about component footprint decisions
Throughout the schematic drawing phase, consider footprint and landpattern decisions that will need to be made in the layout phase. See the following suggestions for things to consider when making component choices based on part footprints.
-Remember that footprints include both the electrical pad connections and the mechanical (X, Y and Z) dimensions of the part. This includes the body outline of the part as well as the pins that attach to the PCB. as one of pcb assembler in China, ALLPCB suggetsed When selecting components, consider any housing or packing restrictions on both top and bottom sides of the final PCB. Some components (such as polarized capacitors) may have height clearance restrictions that need to be considered as part of the component selection process. When initially starting a design, consider drawing a basic board outline shape and placing some of the larger or critically-placed components (such as connectors) that are planned to be used. In this way, a quick virtual rendering of the board (without routing) can be visualized to give a relatively accurate representation of the relative positioning and component heights of the board and components. This will assist in ensuring that the parts will fit inside the packaging (plastic, chassis, mechanical frame, etc…) after the PCB assembly Invoke the 3-D Preview mode from the tool menu to review your board.
-Component choices can change throughout the design process. Choosing which parts should be Plated Through Hole (PTH) or Surface Mount Technology (SMT) early in the design process can assist the entire planning of the PCB. Consider parts costs, availability, part area density and power dissipation among other things. From a manufacturing perspective SMD components are typically less expensive than through-hole parts and are typically more readily available. For small to medium prototyping projects, larger SMDs or through-hole parts may be preferred to facilitate hand soldering and to facilitate better pad and signal access for troubleshooting and debugging steps.
-If a footprint is not available in the database, it is common to create a custom footprint from within the tool.
2. Use good grounding practices
Make sure the design has sufficient bypass capacitors and ground planes. When using ICs, make sure appropriate decoupling capacitors are used near the supply to a ground location (ground plane preferably). Appropriately sized capacitors depend on the application, the capacitor technology and the frequencies involved. When bypass capacitors are placed across the power and ground pins and located close to the appropriate IC pins, a circuit’s electromagnetic compliance and susceptibility performance will be optimized.
3. Assign virtual parts footprints
Run a bill of materials (BOM) to check for virtual parts. Virtual parts do not have footprints associated with them and will not be transferred to layout. Generate a BOM and review all the virtual components on the design. The only entries should be power and ground signals as these are considered virtual parts and are specially handled in the schematic environment and not the layout. Unless used solely for simulation purposes, parts that are shown in the virtual section should be replaced with parts having footprints.
4. Ensure you have complete BOM Data
Check for adequate data in the BOM report. After running the BOM report, review it and work on populating any incomplete part, vendor or manufacturer information for all of these parts.
5. Sort reference designators
To assist in the sorting and reviewing of the BOM, ensure reference designators are contiguously numbered.
6. Check spare gates
Typically, all of the spare gates should have the inputs connected to a signal to prevent the inputs from floating. Make sure you review any spare or forgotten gates so that unwired inputs can be adequately connected, if required. In some cases, if the inputs are left to float, the whole system may not function properly. For example, consider a dual opamp used in a design. If only a single opamp is used in a dual-section IC part, it is suggested that either a single-section IC be designed-in or for the unused portions, a ground be placed at the input and an appropriate unity gain (or otherwise) feedback network be placed with the amplifier to ensure the correct functionality of the part.