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solder mask onto pad or pad defined by solder mask?

shen_zte

#31614

solder mask onto pad or pad defined by solder mask? | 8 December, 2004

can anyone tell me the benifit of the solder mask onto pad(part) or pad defined by solder mask?and how to excute the SMT process ? thanks.

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#31621

solder mask onto pad or pad defined by solder mask? | 9 December, 2004

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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#31655

solder mask onto pad or pad defined by solder mask? | 10 December, 2004

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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