Hi Ioan,
There a several things you can do to prevent the solder wicking down the
hole with the current design, and that's where I would focus.
1) Via plug - From opposite of component side, 50~75% of via, nonconductive
epoxy.
2) Via fill - From opposite of component side, +2 mils, -10 mils of
conductor surface, nonconductive epoxy.
3) Via fill and plate over - Conductive or nonconductive epoxy. No hole in
via for SMT.
Cost goes up as you go down the list, but you don't need to respin the
board, reduced to no solder starvation, and little to no rework.
Glenn
Disclaimer: Any ideas or opinions expressed here do not necessarily reflect
the ideas or opinions of my employer.
-----Original Message-----
From: Tempea, Ioan [mailto:[log in to unmask]]
Sent: Thursday, March 11, 2004 5:30 AM
To: [log in to unmask]
Subject: [TN] Reliability of via in pad
Hi Technos,
I've got an annoying one. I am looking at an assembly for automotive, which
has 100+ SMT resistor and capacitor pads with vias in them, on both sides of
the 9 layer PCB. Now, I know that I will expect solder starvation, since the
solder will flow into the via. I could eventually patch this by plugging the
vias on the opposite sides of the concerned pads and make slightly bigger
apertures in the stencil. Or even charge more and touch-up all the opens.
But what does via in pad mean in terms of reliability?
1. There will be voids, which are going to become even more important on
lead-free (the customer is after the european market). Any reliability
concerns related to that? Once again, I'm talking automotive.
2. PTH reliability. 100+ vias will have totally random quantities of solder
in them. I remember one of Werner's contributions (Entschuldigung Herr
Engelmeier, aber... I lost the e-mail) saying that less than 50% fill
degrades the reliability. Am I right?
A good fix would be, in case they refuse to re-spin the board and move the
vias, to fill the holes. What kind of specification should I issue for
filling, in terms of material and fill percentage?
Any other concerns?
Thanks,
Ioan
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