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I am rather new to layout work. I am asking this question to sanity check that:

  1. The pastemask expansion is supposed to be negative (paste area smaller than the pad), right?
  2. My vendor said to use "0.1mm expansion" (I assume that means -0.1mm), but for minimum-size 0402 pads (0.6mm x 0.58mm), this causes the paste area to be only 44% of the copper area! Whereas people online say that the paste area should be 80% of the copper area. What's a better strategy -- a fixed -0.1mm expansion regardless of pad size, or doing it by area and manually setting it up a different expansion for different pad sizes?
  3. Is it a better idea to specify pastemask gerbers myself (and risk specifying the wrong amount of solderpaste), or leave it up to the assembly house (and risk them messing up special features like golden fingers)?

Thanks!

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The actual values depend on the needs of the individual footprint, so there is no right answer. You need to reach a compromise between what the component manufacturer suggests and what the board assembly house recommend, given their exact knowledge of their manufacturing tools and process and the thickness of the past mask used.

1: In Altium Designer a negative paste mask expansion means the hole in the paste mask is smaller than the pad dimensions. I usualy start with zero.

2a: To little paste causes weak joints.

and

2b: Too much paste escapes across the board causing solder balls which short things out. Or can cause toombstoning or floating where the surface tension forces are such that they lift one end or both ends the component up during reflow.

3: Listen to the assembly house, they know best.

I should add, that the assembly house often alter supplied paste mask data to match their process as a matter of course when the setup the job. Find out if they have done this and put the alterations back into your design, that way if you move houses or make a 2nd batch you can be sure of using the best data.

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