Golden Rules For Preventing Parts Headaches At PCB Layout
Preventing parts headaches at printed circuit board (PCB) layout involves a number of key design considerations and precautions. They include:
- Carefully evaluating PCB CAD design tools because each commercial version has trade-offs. Some allow parts or components to be easily designed on the PCB, while others make it more difficult. One tool in particular creates a component in a one-step process, but provides limited access to component pins. Another tool takes three to four steps to create a component. However, it incurs more time, yet is more flexible. Radial components at the CAD level pose potential issues due to the possibilities of coefficient of thermal expansion mismatch. To avoid problems, it’s best to select the proper CAD tool for a given PCB layout. But regardless of the tool used, PCB layout requires that IPC design guidelines 7351 must be stringently followed when designing through-hole components on a PCB.
- Keeping in mind fabrication tolerances. Sufficient margin on the pads of a through-hole component must be maintained. Otherwise, “breakout” can occur if requirements aren’t maintained for the annular ring. Also, correct coefficient of thermal expansion must be carefully maintained for plastic- and ceramic-based leadless chip carrier components, especially Pb-free, to avoid component thermal damage during high temperature cycles.
- Making sure component mechanical specifications are carefully read and correctly implemented. It’s vital that mechanical footprints on the PCB’s physical surface are viewed from top side. A few component manufacturers do inadvertently specify their components with a bottom side view without noting them as such. By placing a component specified this way on a PCB, the pin-out is changed 180° and PCB functionality is adversely affected. Checking out component height requirements is also important. Heights of components must comply with the PCB enclosure’s mechanical dimensions.
- Placing special attention to the PCB application. For example, the end product may be subjected to harsh environments with considerable vibration. Special component placement and robust PCB design must be applied to guard against these conditions.
- Not overlooking component life cycles. A big headache can occur when specifying an obsolete, soon-to-be-obsolete, or non-existent device.
- Taking special pre-cautions when replacing an obsolete component. Replacements or substitutions must be carefully evaluated to assure specs are correct and pin-to-pin compatibility.
- Following proper guidelines for component placement. Components must be placed at least 1.5 millimeters or more from the PCB’s edge. A pick and place line requires components to be as much as three to five millimeters from the PCB’s edge. A PCB undergoes different manufacturing cycles, and each machine has its own limitations. Hence, it is important to follow set guidelines to avoid damaging expensive components like a $3,000 ball grid array (BGA).
- Creating press fit components with very tight tolerances, ensuring there’s no wiggle room.
- Assuring design-for-assembly (DFA) engineers are highly trained and experienced.
- Checking that high voltage or current traces are substantially distant from one another to avoid thermal problems.
- Providing thermal relief so that components can be properly soldered on a PCB.
- Applying heat sinks to dissipate heat from thermal intensive components.