solder mask onto pad or pad defined by solder mask?

We did this for BGA pads, but have not thought about other types. So, this may or may not apply for those.

Pad defined by:||Advantages||Disadvantages Non-solder mask||No point-of-stress concentration at edge mask and solder ball||Smaller pads have poorer adhesion strength, performance in rework and multiple reflows, and heat dissipation ||No CTE mismatch at edge mask and solder ball|| ||More space for routing traces between pads|| Solder mask||Larger pads have better adhesion strength, performance in rework and multiple reflows, and heat dissipation||Potential stress concentrations may start solder-joint cracking sooner ||||Less space to route traces between pads

Finally, we'd suggest using the same type of pad on the board as are on the BGA interposer.

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We had some bad experiences trying to work with soldermask defined BGA pads. If the opening in the soldermask is right to the edge of the copper/pad without a space it can leave reside on the pad and nothing will stick to it. See http://www.sipad.com/mask_defined_pads.htm

If you relieve the mask away from the edges of the copper it works much better but the pcb vendor has to make sure to develop the mask properly.

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