My prior post discussed SMD (solder mask defined) vs NSMD (non solder mask defined) pads for BGAs. I received a comment on the post asking for a bit of clarification:
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 maneuverability factors outweigh this concern?
IPC-7905B does recommend NSMD pads for the most reliable solder joint. They do note however, that sometimes SMD pads are used to prevent pad lifting so it’s a bit of a trade-off. Their suggested compromise is to use mostly NSMD pads and SMD pads in the corners. I guess it you have solder joint issues, make sure that your PCB uses NSMD pads. If you have issues with pad lifting, try the SMD corners. If you have a lot of pad lifting, you might have a warping problem that needs a little more attention.
As far as the assembly process goes, we can build them either way without problems – unless there’s an underlying issue causing problems. The difference in pad area isn’t an issue for us – unless you also remove the soldermask from the trace going to the via. Doing that is bad.
Pop goes the weasel is okay
Pop goes the BGA is not
As pitch gets smaller, things get more difficult. Did that blinding flash of the obvious hurt your eyes? Mine too. It’s not always just more difficult though. Sometimes it’s different as well.
Like BGAs, for example. With 0.5 mm and larger pitch, we and pretty much everyone else recommend NSMD (non solder mask defined) pads. However, once the pitch drops to 0.4 mm, some manufacturers are recommending solder mask defined pads to prevent bridging between solder balls. Make sure your fab house can nail the mask registration.
With LGAs and QFNs, IPC recommends NSMD pads for 0.5 mm pitch and larger. Once the LGA or QFN (DFNs too) pitch drops below 0.5, does everyone suggest solder mask defined pads like with BGAs? No. Actually, IPC tells you to go ahead and remove the solder mask web between your land pads with 0.4 mm pitch parts. With spacing that close, the solder mask can’t reliably stick to the PCB. That would be messy. As to why a solder mask defined pad would not be a good thing, I can only speculate at this point. My assumption is that without the solder ball like the BGA has, the whole land area is needed to ensure a good solder joint.