Electrical – Designing footprint for LFCSP CP-8-13 what should be the pad size

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I am scratching my head to create a footprint for ADA4817 which comes in LFCSP CP-8-13 package. I reckon this package is only used by Analog Devices.

The main problem is I do not know what should be the pad size for the pins. The pins are stated to be 0.25mm in width and 0.40mm in height with a semi-circle ending:

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So these are my concerns:

  1. What should be the pad size be (copper) for each pin
  2. What should be the glue size be (for stencil)
  3. The copper shape should be a rectangle, or I should care for that round ending on the pads?
  4. My other concern is the distance between pins and the exposed pad. Can I get away by making the exposed pad footprint smaller than the actual exposed pad size? e.g. going with a 1x1mm square instead of 1.4×1.4mm?

Best Answer

So, there can be no general answer. The shape and size of pads depend on very many things, among these:

  • Purpose, usually a mixture of the following, dominated by one purpose:
    • Thermal transfer
    • mechanical stability
    • high current capability
    • impedance matched high frequency signalling
  • PCB fabrication tolerances
  • Component placement tolerances
  • thermal and mechanical aspects of the soldering process

So, if you're doing a board with a high-quality PCB manufacturer to be assembled by high-end pick and place machines and soldered with a well-calibrated reflow oven, then the pads will usually be minimally, if at all, wider than the pins, unless certain pads should be wider to allow for an all-around "solder taper" for more current or heat transfer capabilities.

If on the other you're sending this off to some unknown least-cost manufacturer and plan to assemble the board by hand, by all means, make the pads as wide as possible without risking solder bridges, and add a lot of exposed are outside the pad outline, where you can actively transfer heat into the solder paste.

Now, considering the free space between pins is but 0.25mm, I'd say: go for pads that are exactly as wide as your pins, and pray you get your manufacturing tolerances low enough!

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