Electronic – Solder Mask and Pad Area (SMD vs NSMD)

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I have been trying to figure out the "right" way to size pads for surface mount components, and I have found some conflicting wisdom floating around the internets.

Lets say I have this hypothetical part that requires two square pads, the centers of which are 15 units apart, and the edge that are 5 units on edge.

This SO post suggests that I should have my copper be the exact size of the pads, and the mask be slightly oversize to account for shrinkage and/or misalignment. This seems somewhat reasonable, as relative shift/shrink/misalignment can be tolerated. Under this scenario I might want the the masks to be 6×6 holes with the same center-to-center spacing as the underlying copper.

Conversley, in a different scenario if you were to oversize the copper to 6×6, you would gain the same tolerance to misalignment,and additionally some extra adhesion holding the pads down. This approach is suggested in some BGA/QFN packages I have been looking at. (Example 1) (Example 2) (Example 3)

I realize that there isn't really a "Correct" answer, but what motivates one arrangement verses the other?

Best Answer

Different sources give conflicting advices because they refer to different needs. You need three sizes to take care of: the copper area, the solder mask and the paste mask. Their sizes are usually determined by:

  1. The component itself: contact position and tolerances must be taken into account.
  2. Fabrication tolerances. Fabrication houses have different values for these, which you need to learn in advance, or at least estimate. With these you can calculate what will your pads look like if everything goes off-spec.
  3. The soldering method: SMD components that you might attempt to hand solder (e.g. in a prototype) should have pads that go a little over the component so you can place your soldering iron tip. If you are going fully automated, this is not an issue.
  4. The PCB finish (hot air leveling, gold flash, etc.) affects the "wettability" of the pad, which affects you if you're going into very small stuff.
  5. The available space in your board. If you need to fit many tracks and components in a tight space you will want your pads to be at their absolute minimum.
  6. The solder mask is important to avoid solder bridges, so if your solder mask tolerances are poor and your pads are too close to each other, you might need to make them smaller to make room for the solder mask.
  7. The thickness of your stencil determines the amount of solder paste that goes in your pad. You don't want your pad to overflow (e.g. big pads like center pads in SOIC chips), so your paste mask needs to be adjusted to avoid that if your your thickness is high.
  8. Mechanical properties. With large components like aluminium capacitors, the firmer the solder joint, the better.

This list is most probably incomplete, but it should give you an idea of what values you need to take care of in your particular project.

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