Hi Leif,
 
I've run across the same issue in the past. One way to get around the bumps is to have your stencil vendor half etch all the vias into the bottom of the stencil. I've had to do that a few times...
 
Steve

--- On Mon, 9/28/09, Leif Erik Laerum <[log in to unmask]> wrote:


From: Leif Erik Laerum <[log in to unmask]>
Subject: [TN] Plug vias first or second.
To: [log in to unmask]
Date: Monday, September 28, 2009, 11:03 AM


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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