Graham,
When testing to the Bellcore spec, and when using the Bellcore pattern,
there is a design flaw in the pattern. If you examine the contact pads
(edge fingers) for patterns 2 and 4, they are right next to patterns 1 and
5, respectively. Go guard or ground traces. That is why we re-designed
one of the Bellcore patterns on the IPC-B-25A test board, to eliminate the
design flaw.
The patterns are 25 mil lines and 50 mil spaces. The contact fingers are
also 50 mils. So, if you have a clean pattern, and poor flux residues for
wire attach spanning the space between the contact pads, you get poor SIR.
If you have good flux residues between the contact pads, then the chance of
meeting the high required SIR levels of Bellcore becomes greater. The
Bellcore requirement to use an R or RMA flux and no cleaning was historical
in nature. Rosins are a natural encapsulant, with high SIR values. Using
those fluxes meant (usually if you knew what you were doing) that any poor
SIR readings were attributable to the pattern and not to the flux of wire
attach. Research at Bellcore in the 70s and 80 showed that if you tried to
clean off the flux of wire attach, you removed the protective nature of the
rosin, but left the activator, plus you had the potential of cleaning the
intended residues from the test pattern.
I had addressed the problem in two ways: For the patterns that we used
when I was at CSL, I had slots milled between the contact fingers for 1 and
2 and between 4 and 5, creating a physical gap that flux could not bridge.
The second way was to have customers solder paste or solder the contact
fingers as part of their test processing, so that all I had to do was
fluxless wire attach. It worked pretty well.
Doug
(and me put down my Mountain Dew? When you pry it from my cold dead
lifeless hand.......)
Graham Naisbitt <[log in to unmask]>@ipc.org> on 09/10/2002
07:52:54 AM
Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond
to Graham Naisbitt <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
To: [log in to unmask]
cc:
Subject: [TN] SIR Testing issue
Hello Techies
I do hope you can help with this one:
I have a board fabricator who is testing to GR-78-CORE Issue 1 September
1997.
In section 14.4.3 - Terminating Test Patterns that are hand soldered using
R
or RMA flux.
It states: "do not remove flux."
His questions: Why? Also, how does he do this as this (flux residues) will
influence his test results?
So, anyone care to answer....Doug, put that Mountain Dew down for just a
moment... MoonMan, is this covered in your book....Steve, can't use your
new
toy for this soldering job...
Regards, Graham Naisbitt
--
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