Hi Ioan,
We have designed boards with via in pad (VIP) for well over a decade and assuming you have a good solder joint, reliability in Telecom product has not been a problem. We did not evaluate for automotive applications. Having said that, let me also say that VIP is great for the designer but a royal pain for the assembler. It was only intended for designs where absolutely necessary, but it quickly got out of hand when designers realized that by checking off "yes" to VIP, the computer time required to route a board was significantly reduced. It essentially became a defacto standard on many of our designs. Some lessons learned were that you need to carefully control the finished holes size. If you specify a 10 mil nominal finished hole there is a world of difference between a 13 mil received and one that is truly 10 mils. Also, as discrete sizes get to 0603 you need to require a smaller finished hole. We used enlarged stencil openings as well as step stencils in order to get the n!
eeded solder paste applied. Another issue is to make sure that the via is not shared on both sides of the board with discretes or other surface mount pads. I would be glad to discuss further offline if you have additional questions or comments. We also did some work with plugging the vias, but found it not to be necessary if the surface mount process was optimized, so the cost wasn't justified. It is a slippery slope, as once you get proficient at one set of pad sizes, the designers are going to want to expand the use of VIP to other sizes and feature types.
Thanks,
Robert Furrow
Printed Circuit Board Engineer
Supply Chain Networks
Lucent Technologies
978-682-2260 [log in to unmask]
-----Original Message-----
From: Tempea, Ioan [mailto:[log in to unmask]]
Sent: Thursday, March 11, 2004 8:30 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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