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Technical Article

PCB Footprints and Soldermasks

Time:2017-07-31 10:25:59 Click:

Electronic Design Automation (EDA) typically contains all PCB footprints for commonly used components and packages but sometimes the footprints are modified a little or in most cases their dimensions are a bit off. In such cases when footprints are different on the parts or PCB footprints are different, soldering issues arise. Solder mostly pulls a component to its best possible orientation but the part is shifted by one of the pads for a pin on the IC being soldered which causes a lot of distress in prototyping or production. The unit conversion between English and metric units sometimes causes an issue when an IC is large enough to create a pad-to-pin mismatch because the difference between dimensions of pads and pins gets added up. Package of each footprint has to be verified and significant effort is required in reviewing to avoid such issues. In case of addition of a new EDA package, manual footprint design or use or supplied footprint, verification and review is mandatory. 

 Although silkscreen is not a critical part of PCB layout process, small or unclear text or symbols cause serious problems at the time of manual assembly or troubleshooting. Silkscreens should be well documented as they are primarily a way for the user to understand and interact with the PCB design. Polarities of various components such as capacitors (tantalum or electrolytic) or diodes is to shown clearly. Marking orientation of various IC packages is important (such as mentioning the first pin) helps when a PCB is being stuffed. Font size should be visible and correct abbreviations for components should be used. 

 Soldermask is one of the important feature of a component’s footprint. In order to reduce the soldering issues, vias when used in pads should be covered with soldermask. Excess of solder on a pad also creates a soldering issue. If a stencil is used for spreading solder paste on a PCB, excess solder can be caused by any solder mask with relatively bigger openings. Because of surface mounting, SMT components don’t face this issue, only components which have a big solder pad under them (for heat sinking purposes on QFN and power ICs) are pushed up from the board because of this solder which hinders pin connections. A changed soldermask that has 50% coverage can be used on such locations. Soldermask in the areas that are to soldered has to manually removed, which a tedious and painstaking process which we should keep away from. 

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