In a message dated 4/7/99 3:51:41 PM Pacific Daylight Time,
[log in to unmask] writes:
<< Hi folks
I have this "Adverse Effect" puzzle from last CE plant ispection ; stating we
don't have validated the adverse effect of repairs ; which unfortunately I
could not argue about (this inspectors are getting wiser every year !)
Now, before I'll scrap all the soldering irons and insist on thermal profiles
at all stages ; which is kind of radical ; and introduce hot air jets with
validated thermals ; which is kind of over the top ; or design it all in on
p&p ; which will take time : how do you folks address the fact we have
perfectly tuned reflows and than on the second stage we shock anything else
with 0-300'C instant ? Ok ; I can have the hotplates ; etc.; but did anybody
attached thermocouples to the manually handled component monitored throughout
the process ; and crosslinked those thermals with those specified by
component manufacturer (some caps do have very specific raise) ; flux
activation recommendations ; etc. ?
This is mainly SMD's issue ; the T/H parts with legs to dissipate the first
shock up to a degree are bit easier subject .
Thanks a lot for the help
Paul Klasek
http://www.resmed.com >>
Hi ya Paul!
The thing about the inspectors, don't ya' just hate that! Bet ya' wished the
job turnover was higher in that line of work, huh? Inspections would go a lot
smoother that way, and our lives would be easier...(GRIN) I can tap dance
with the best of them, but when they start learnin' my steps it can get
rough....hehehe.
Your question interested me, and I never have actually measured something
like what you're referring to. Never needed to, haven't built anything that
was that critical. So, digging into my "Rube Goldberg" bag o' tricks I did
just that. I know this ain't completely under "laboratory conditions". But
this is what I did:
1. Got me a scrap board that had some capacitors on it...0805's, 1206's, and
1812's. Don't know what the dielectric was that they were made with...but
they were solid ceramic ones. I figured to do this with caps since that's
what a lot of people think of when you start talking thermal shock....(least
I do).
2. Got my SuperMole Gold and had Trang solder the thermocouple tips into the
fillets as close as she could to the bodies each of the 3-sized caps and then
let everything cool back down again.
3. Then I had her remove each cap while I plotted what was going on heatwise
while she did that. (We have Metcals by the way and I have 700 degree tips..)
This is what I plotted:
0805's: Ramp from ambient to cap removal: 86.94 degrees C.
per second, and a peak of 216 degrees C.
1206's: Ramp from ambient to cap removal: 58.15 degrees C. per
second, and a peak of 196 degrees C.
1812: Ramp from ambient to cap removal: 43.52 degrees C.
per second, and a peak of 168 degrees C.
So what I see is that the more mass, the less shock. The peak I registered on
the 1812 cap is even below liquidous for a 63/37 so Trang was able to reflow
the fillet before the heat went to the cap. Again I know this isn't real
scientific, and there's probably a hundred reasons why this isn't real valid
data...but it's my best shot. I think that if you keep you tip temperatures
down, do things quickly, and kinda' ease into the fillet of a capacitor (you
know, solder from the outside in towards the terminations) it'll help
minimise the shock. Hand soldering is going to always be technique
dependant....that's why a good rework operator is worth their weight in gold.
But hey, what do I know?
Other than that, I think preheating is the way to go if you wanna be real
safe manually reworking something and not shock it. Pace makes some good
stuff along those lines...
By the way, check out
http://www.avxcorp.com/tech/techpdf/!smtinfo.pdf there's a
good paper there that talks about capacitor cracking.
Here's the Abstract:
"With the increasing use of multilayer ceramic capacitors in surface mount
technology, the understanding of the mechanical properties and thermal stress
resistance parameters of MLC's is essential for zero defect soldering and sub
ppm failure rates. In this paper, various aspects of SMT including zero
defect design, placement considerations, soldering techniques, thermal stress
resistance parameters, and post solder handling are reviewed. Special
emphasis is given to parameters responsible for thermal shock behavior of
MLC's with review of the effect of overall component thickness, temperature
gradients, and terminations of MLC's."
C-ya L8'er Mate!
-Steve Gregory-
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