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January 1999

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From:
"Stephen R. Gregory" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Tue, 12 Jan 1999 12:59:13 EST
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Good Day Ya'll!

     We just got a new customer here last week and built two 10-board proto
kits for them. Those boards are a type of power supply, and we're going to be
doing another board that they call a 4-port RS-232 + 48 digital I/O PC/104
Module. All 3 assemblies are roughly 3.5" square and are mixed technology.

     There's another thing common between all three assemblies besides their
size, what they call dual-mount headers. Each board has two of these, one is a
64-pin (2X32 pins), and the other is a 40-pin (2X20 pins). These are basically
real long gold plated headers with plastic female type receptacles at one end.
They're installed with the header pins going THROUGH the board and soldered.
The header pins stick out the backside of the board 10mm, and you have your
female sockets on the topside, so now what you have is a stackable type of
board connection system.

Cool connection system, BUT, there's a big problem with soldering these dad-
blamed things! You can't wave solder them because: 1.) They're gold plated, no
solder on the contact area (solder fillet must stay below .100" as measured up
the pin from the surface of the PCB). 2.) Even if the gold plating wasn't an
issue, they stick out 10mm on the bottom...you'd need a wave that shoots up
and looks like "Old Faithful" to reach the bottom of the board!

"So hand solder em' ya big weenie!" some of you may be muttering under your
breath now. Well there's a problem there too. The way these two connectors are
laid-out on the boards couldn't be any worse, they're parallel right next to
each other, spaced .100" apart. We've got 4-rows of 10mm long gold plated
pins, .100" apart, that we've got to solder and not get any solder on the gold
plating...just peachy huh?

My senior rework lady Trang, got us through the two proto kits of 20-boards.
She somehow was able to thread her soldering iron down thru the pins and
solder the connectors in without so much as a pimple of solder on the pins
(She simply amazes me sometimes! Goes to show what clean livin' and no
caffeine can do for ya!) But we simply can't do that in production, it'll take
forever, and I don't wanna burn Trang out, she's the only one steady enough to
do it.

So I called around to get quotes on some preforms. I've seen daisy-chained
donut preforms that you can use for headers and what have you that has the
same kind of footprint. I've used preforms before, but the kind for edge clips
for little SIP modules, not for PTH. So I called Indium, AlphaMetals, Bow
Solders, and Kester...they all make preforms. Everybody has quoted me, but
Kester told me that they think I may still may have a problem. They said that
from their experience, I'm going to have difficulty obtaining consistent
solder joints using preforms. They said that unless I can ensure very
consistent heating between all the pins, that I'll probably wind-up with some
joints being insufficient because the solder will tend to wick to the hottest
pin. I was planning on using a heat gun to provide the heat, and maybe even
going so far as to make a nozzle that will enclose all the pins. But I can't
put it in a reflow oven unless I want to hand solder eight 10-pin SIP RPACKS,
and a 2-pin metal canned crystal afterwards...I still need to look and see if
the plastic on the headers can take the heat anyway.

So after reading this journal, my question is (boy, I sure take the scenic
route sometimes don't I?); have any of you had any experience with the kind of
preforms I'm thinking of using? Is what Kester saying true?

'Preciate any info at all!!

-Steve Gregory-

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