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I'm designing a breakout (debug) card for the FPGA Evaluation Board VC707. For this I'm using two FMC connectors (MC-HPC-10), for footprint and design of the connector I got help from the website (FMCHUB).

It's an open-source HW project. It's a fairly accurate design and it helped a lot.

I'm soldering these connectors at home with a reflow oven. The connector has no balls on it. The footprint has an issue, that the solder paste openings are much bigger than the solder mask openings:

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Is this common for a BGA grid? Should I rescale the solder paste (stencil opening) to the same size of the solder mask opening, or is this going to be fine?

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There are two types of pads.

  1. Solder mask defined pad (SMD pad). In SMD pad the mask openings is smaller than the copper pad. And paste should be equal or smaller than the mask opening. SMD pads suit fine pitch components, often used with BGAs
  2. Non-Soldermask defined pad (NSMD pad) also called Copper defined pads. In NSMD pad mask opening is bigger than copper pad. And paste should be equal or smaller than copper pad.

In your case pads are SMD so there should be paste <= mask opening.

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The openings for the STENCIL to which solder paste is applied (also called the paste mask) is slightly smaller than the pad, whereas the openings for the SOLDER MASK itself, which becomes integral to the PCB, is slightly larger than the pad.

This is because tolerances aren't perfect and you really don't want the solder mask to cover the pad. It's main job is to stop bridges so as long as it exists between pads it should do it's job.

So did you actually mean solder mask? Or did you really mean the paste mask?

You say "much bigger though" but the solder mask is usually bigger than the pad by only 0.1mm/3-4 mils/3-4 thousandths larger in diameter.

Similarly, the paste max is usually something like 0.1mm/3-4 mils/3-4 smaller than the pad diameter.

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