Hi, Dan,
Quite a story. We are ICT testing double sided boards and have settled on
0.037" test pads with no holes. The pad diameter was a much fought-over
compromise from the 0.050" diameter originally demanded by the jig vendor.
The smaller the pad diameter, the greater the chance of missing the target,
as you realised.
The problem with holes is penetration, or lack of it by the probes.
Basically, it is a hole - it has no electrical contact with anything, so
the probe is only in contact with the hole edges at the very lip, which
isn't too reliable as far as contact surface area goes.
Short answer: 0.040" diameter pads = OK. Holes in the test pads = not OK.
Pete Duncan
Dan Ratcliff
727-530-8762 To: [log in to unmask]
<[log in to unmask] cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
ADYNE.COM> Subject: [TN] ICT after double sided reflow
Sent by: TechNet
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05/18/01 03:53 AM
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727-530-8762
Hey all,
We have a multilayer .062 thk blind/buried/thru via OSP PCB designed as
a double sided reflow assembly. The assembly process went fine until
the boards reached ICT. The board has .030 dia. testpoints with and
without .010 dia. holes. Prior to this we used .040 dia test pads with
.010 holes which after researching the issue, we felt could be reduced
to .030. These testpoints function as ICT probe sites and as circuit
interconnect vias. When we tested the first PCB's we found that because
the testpoints were not soldered the OSP was not removed from the test
pads. As a result, the test probes (high force chisel probes) did not
reliably penetrate through the residual OSP to make contact with the
test land. For our second attempt, we applied a 1X paste print to the
testpoints prior to reflow. The result was that while we got a good
surface for the probe to contact, some solder and flux residue ended up
in the holes and the test probes did not reliably penetrate the flux
residue. For the third attempt, we overprinted the paste (.040) on the
testpoints in an attempt to create enough solder volume to fill the hole
and create a relatively flat surface to probe without the hole capturing
flux residue. Unfortunately due to the proximity of adjacent features
there were some testpoints which could not be overprinted. This
resulted in improved ICT yields but still left failures. The fourth
modification was to build a new ICT fixture which had tighter
tolerances, enhanced probe types and an alignment plate. This step
addressed the difficulty of hitting the .030 test pads reliably and
produced very good results. Finally we changed the paste chemistry by
using a differant solderpaste. This seemed to eliminate the remaining
solder issues because the paste has a higher metal content an the
residual flux did not prevent the probes from making contact with the
test pads. The production engineers drew the conclusion from all of
this that we can not reliably hit a .030 diameter test pad and that all
double sided reflow designs must have testpoints without holes in them.
This was OK when we had a microvia design. However, going forward we
are looking at very dense double sided reflow .093 thk OSP PCB's with
thru vias only. We lack the real estate to have a via and test land at
separate locations in densely populated analog areas. I would like to
hear how other folks are dealing with double sided reflow and ICT. Is
it really necessary to avoid holes in the test lands? Is it possible to
have reliable ICT test with .040 dia. probe sites and .010 dia. holes in
them?
Thx in advance for your help,
Dan Ratcliff
Sr. PCB Designer
Paradyne Corporation, Largo FL
727-530-8762
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