Have the the microvia filled with epoxy and plated over to eliminate the 
problem with solder going into the hole.  Conductive silver epoxy is often 
used.

Jeff Condit.

> Dear Members,
> 
> We have serveral very high density PCB with microvias on pads, the microvia
> is 4 to 5 mil, recently, our SMT house tell us two components contains
> microvia on pad are tombstoning after reflow sporadically, and maybe the
> reason is the pad with microvia on it need more solder when printing, since
> there is a hole on pad, so the stress is unblanced during reflow, thus
> tombstone occured.
> 
> Does any one have more experience on this? if you use microvia on pad in
> your design? I heard copper-Filled Microvias before, No holes observable
> after plating, but I do not know how to do this, where can I find more
> technical paper about this? Is it very expensive?
> 
> Your input is very appreciated! 
> 
> Best Regards,
> Luo.
> 
> 


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