Brad,
We are using .062 thick board (our standard). Currently we have .016 drill,
.012 PTH, .004 traces, .0055 distance. We have used Polyimide for many
years, and have on small occasions used HiTemp FR-4, but I am uncertain
whether maybe Polyimide should still be the way to go. I will approach our
PCB supplier and ask if they felt they could get better results using the HT
FR-4, but this will be something that all of our management will have to
agree on, as we will be straying from the narrow path we have traveled on
for so long. :o)
Thanks for your input.
Regards,
Rob
-----Original Message-----
From: [log in to unmask] [mailto:[log in to unmask]]
Sent: Wednesday, September 18, 2002 11:37 AM
To: [log in to unmask]
Subject: Re: [TN] Close pitch PCB failure
Hello Rob,
The nominal distance of .007 is tight. In the case of .8mm BGA the PCB
planes will have an .0275 clearance and I have drilled .012 diameter holes
in the .100 thick board. That renders .00775 PTH wall (drill wall) to edge
of copper. It doesn't get much tighter than that. The same configuration;
signal layers use a .019 diameter pad with the distance from PTH wall edge
(drill) to edge of pad of .0035, with .004 trace and .004 spacing this
renders .0075 PTH wall to trace edge. These are tight numbers and are BASIC;
all that "slop" is the fabrication allowance. Increase those numbers by
decreasing board thickness allowing a smaller drill diameter (aspect ratio
<9/1).
FR4 is the best material to achieve success. Polyimide usually brings on
need for etch back and there simply is no real estate to do that. Thermount
is wonderful material, but is best used for its propensity to have
consistent lower Er and its mainstay is low XY CTE. I do not recommend its
usage for these tight configurations. Folks used to recommend it because
its very nice registration qualities, but that Kevlar may have micro
ratcheting -in my opinion- and may induce the CAF condition.
If your stuck in a tuff design I would recommend:
1. lower thickness (mils count)
2. decrease trace widths (half mils count)
3. FR4 170 deg C Tg stuff
4. new drills, one up
5. have a plan "B" ie redesign
6. group crossing of fingers
Good luck,
Boston Brad yer PCB pal
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