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I'm trying to reflow a custom shaped strip onto a PCB, but I'm unsure about what size pad I need on the PCB.

  • The strip is 13mm x 6mm x 0.3mm.
  • The PCB surface finish is ENIG.
  • The pad is likely to be mask defined (because of thermal transfer, and helping to prevent the pad from getting pulled off the PCB). I don't think this last point is relevant to my question, but maybe it is.

Can the PCB pad exactly match the 13mm x 6mm x 0.3mm strip, or does it need to be slightly larger to allow solder to form that concave fillet around the edges? What would happen if the pad exactly matches the pad size?

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  • \$\begingroup\$ custom shaped strip of what? \$\endgroup\$
    – BeB00
    Aug 23, 2022 at 2:54
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    \$\begingroup\$ without knowing too much detail, definitely make it a bit larger so that you can see those fillets. \$\endgroup\$
    – Bryan
    Aug 23, 2022 at 2:55
  • \$\begingroup\$ What's the purpose of the strip? \$\endgroup\$ Aug 23, 2022 at 3:02
  • \$\begingroup\$ It's a custom strip of pure nickel. It's for cold welding a battery cell since I can't cold weld directly to the PCB pads (or at least, not easily). \$\endgroup\$
    – Dan
    Aug 23, 2022 at 3:03
  • \$\begingroup\$ Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. \$\endgroup\$
    – Community Bot
    Aug 23, 2022 at 4:41

1 Answer 1

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You can get a sound solder joint between component and pad, whether the pad is exact size, or slightly larger.

The main issue is, how do you know whether you have a sound solder joint?

With an exact size pad, you may need to have exquisite control over your solder process, cleanliness, temperature profiles etc to know that it works every time. If not, you might have to accept that you find failures late in the day at your final test, or worse still with customer returns, or try to use something like x-rays to image the joint (difficult to image a thin layer of solder under a thick layer of nickel).

With an oversized pad, you can see immediately whether your joint is good, but it costs you more board real estate.

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  • \$\begingroup\$ Thanks Neil_UK. That was a great answer! I am struggling with board real estate a bit. Do you think a 13.5mm x 6mm pad would be good enough for a 13mm x 6mm nickel strip? The 6mm width is difficult to increase, but I can probably afford up to 14mm for the length if I absolutely have to. \$\endgroup\$
    – Dan
    Aug 23, 2022 at 7:45
  • \$\begingroup\$ @Dan It's entirely up to you, you and your colleagues' confidence in your processes and attitude to risk. Run some test assemblies and see what happens. \$\endgroup\$
    – Neil_UK
    Aug 23, 2022 at 8:04
  • \$\begingroup\$ Awesome! Thanks Neil :) \$\endgroup\$
    – Dan
    Aug 24, 2022 at 3:39

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