In a message dated 4/19/99 3:35:01 PM Pacific Daylight Time,
[log in to unmask] writes:
> TN'ers:
>
> We're getting a handful of bridges at wave solder on a few designs. We've
> tried A, B, and C, but they continue to show up consistently on some styles
> of connector behind chip components, and on trailing leads of flatpacks.
>
> The machines is a fairly new Electrovert with convection preheat, Lambda
> wave, and chip wave. We're using Sn63 at 500F with RMA 25%.
>
> We're looking at a hot air knife retrofit, which would be turned off on
> everything but problem boards. We've had a demo, with mixed results.
Good:
> bridges were much improved, and no insufficients. Not-So: some frosty
> looking joints, which we expected, but also a lot of solder balls and
> splashes.
>
> We've heard that setup can be touchy, and we didn't expect to optimize on a
> two board sample, but we're not sure what we can conclude here.
> Unfortunately, the demo was not apples-to-apples - it was performed on an
> inerted machine with an A-type wave, and we're not sure if the splashing
and
> balls could be attributable to the increased surface tension.
>
> Anyone out there have good things to say about their hot air knife?
>
> Joe Kane
> Lockheed Martin Control Systems
>
Hi Joe,
As you've discovered, hot air knives can help in some instances, but are not
a cure-all for badly designed assemblies. When I say badly designed, I don't
mean it to sound as negative as it first seems. What I'm saying is DFM.
Usually, DFM is more or less an after-thought. The assembly is designed, and
thrown over the fence for us to build. The topic then comes up after the
board is laid-out and guys like you and me have all this heartburn trying to
build the damn thing without defects...am I hitting a nerve?
The first thing that hit me about your situation, is wave soldering
flatpacks. What masochist laid that out? Maybe there's no choice because of
what the assembly is required to do, but when some designer puts a flatpack
on the bottom of a board that is going to be wave soldered, he or she have
better done everything they can to avoid that to get my respect. Like I
mentioned in a post earlier, I still see individual resistors and capacitors
used where chip networks or arrays would have saved a ton of space, and just
maybe that flatpack didn't have to go on the bottom of the board.
That ain't easy stuff at all! Waving flatpacks that is. I don't think any
sort of air knife will help with that if it's not designed right! It can be
done, but things do have to be dead on as far as design, and the type of
flatpack you're trying to wave. I've seen designs where it also helps to
orient it diagonally 45-degrees in relation to the direction of travel as
it's entering the wave.
One question about that QFP first off, aren't you worried about popcorning? I
would be. But the popcorning issue aside, is there any thieving pads designed
in for the flatpack? If not, you're sunk as far as bridging goes, I don't
there's any type of air knife that will overcome that. Another thing, the
leads need to have very short shoulders coming out from the body to avoid
bridges (in my experience).
I've got a CDROM player that we buy that goes into a mechanical assembly that
we do, that has a waved 25-mil QFP on the bottom that has a good layout of
thieving pads that I'll take a picture of with my new digital camera (Yay! I
get to use it now!) and send it to ya' tomorrow, I'm at home now so that's
why I can't send anything now.
But to sum up, sometimes air knives work, and sometimes they don't, it just
depends...I was a lotta help huh?
-Steve Gregory-
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