Screaming Circuits: SMD vs. NSMD


SMD vs. NSMD

In general, we and just about everyone else on the planet recommend NSMD (non solder mask defined) pads for your BGA land. Using copper to define the land gives more precision than using solder mask and, more importantly, gives a more reliable solder joint.

BGA on HASL close with vias between pads There are a few cases where SMD (solder mask defined) pads might be more appropriate. The Beagleboard folks, with their 0.4mm pitch Ti OMAP processor found that at that small a pitch, NSMD pads increased the chances of bridging.

Interestingly, IPC writes that the main advantage of SMD pads is to prevent pad lifting. They further note that since the corner balls are most likely to have lift issues, due to the greater concentration of stresses. If you have concerns about corner pad lifting, you might want to use SMD pads just in the corner for a little extra holding power. (ref IPC-7095B, 6.2.2)

Duane Benson
Danger! 50,000 Ohms

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Comments

My understanding was that SMD pads were preferred from a solderable area standpoint. To use an example from your picture, the pad in the upper-left corner has a larger surface area than any of the others by virtue of the larger exposed trace leading to it. With an SMD pad, this trace would be masked and the solderable area would be identical to the others. Have you found that other manufacturability factors outweigh this concern?

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