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Display Your Know How: Footprints

PCBTech

Which footprint option is best, A or B?

 

 

Let’s say your PI specialist advises that this part requires maximum power transfer according to the component rating… Which library footprint do you choose?

Simply answer by letter or include any reason to support your answer. Alternatives and opinions are welcome!

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  • There is no simple answer to this question. B is going to be a better option for most requirements but the split shown will only give about 30% paste coverage. Using fewer larger tiles would increase the percentage.

    Using too much paste risks the device "floating" on the pad which may cause solder balls and/or the remaining pins to be lifted away from their pads resulting in opens. Using too little paste may cause the joint starvation and reduce power and thermal transfer.

    Don't trust data sheets for paste. In my experience they rarely match real-world requirements. Ask your manufacturer for guidelines and work with them for the best compromise.

  • in reply to eDave

    Both of these footprints will work however none of them are optimal. For SMD components your better off using rounded or concave rectangles for the pads. The main reason pertains to the stencil. Laser's cant create square corners. With standard rectangular footprints the paste cant fill the corners due to the way the paste stencil is made.

    Best Regards.

  • in reply to excellon1

    Yes also good point excellon1. Does your stencil supplier have a recommended minimum arc radius for stencil openings? Our experience is that this radius would typically be very small? (circa <= 50um)

    Perhaps there are also other reasons for rounding stencil corners..?

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