Technetters,

We have a problem with certain PCBs where plugged vias have a solder  
mask "mound" that creates a coplanarity issues and also causes the 
screen to be elevated. The elevated screen results in excessive solder 
deposit and massive amount of shorts for the finest pitch parts. We 
believe the root cause is the practice of plugging the vias with solder 
mask after they have been covered. This seems to result in these mounds 
instead of a planar surface. From what we understand, this is the 
preferred order of the two steps by PCB vendors.

Why is this preferred?
Is there a IPC spec that covers this issue?
Is it reasonable to specify that the plugging must result in no mounds 
and 0 elevation of the vias?
Is there a particular process weakness from the PCB vendors that causes 
this defect?

Thank you.

-- 
Leif Erik Laerum

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