Marie
Given that the capability for conductive via fill materials seems to go up to 6:1 (or maybe 8:1- though voiding starts to become a factor). Since the aspect ratio on this board (for via fill) is almost 13:1, I can see the concern with trying to fill these vias.
But I don't understand the need for the via fill to be conductive. The typical filled via process is to copper plate the filled vias, fill them, then drill and plate the rest of the vias. This puts copper over the fill material. Once you do that, the conductivity of the fill material is irrelevant. And the good news is that non-conductive fill materials can go up to 13 or 14:1 aspect ratio. While difficult, this looks feasible.
Are these BGAs dogboned or are the drills in the BGA pads (via-in-pad)? That's the most logical step for this board. If you're going to fill the vias, then you might as well as go with via and pad and free up all that real estate on the board. That's my recommendation here.
Mike McMaster
RF Product Engineer
Merix Corporation
503-992-4263
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From: Marie-Elisabeth MAGNIN[SMTP:[log in to unmask]]
Reply To: [log in to unmask]
Sent: Friday, March 14, 2003 2:01 AM
To: [log in to unmask]
Subject: Re: [TN] 2.3mm thick PCB and BGA
Hi Peter
Thanks for your quick answer. Here are some further explanations :
application is avionics ! I also have concerns about copper plating in the
hole. This is the first point I told my customer. We just met a similar
problem last week on a 1.6mm thick PCB and 0.2mm via holes, the copper layer
broke when changing wrong BGA's.
to save place, the idea is to use via pads as test pads : via holes should
not be epoxy filled, but filled with a conductive material to create a
"large" test pad to contact on the test fixture (large despite a round pad
of 0.6mm diameter). Unfortunatly, it cannot be filled during wave soldering
(the process will only be reflow).
ME Magnin
mailto:[log in to unmask]
http://www.firstec.ch
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