Hi Tom!
Don't ya' just LOVE that? They bring the boards to you to build them
because there was something they liked about the way you do things or they
wouldn't have given you the P.O., but at the same time, they try to change
your processes. Well, that's part of the ol' contract assembly game...I've
been there, done that. I usually listen intently, then spew out gobs of thanks
for setting me straight, and say that on the next run of the boards I'll
implement the suggestions. I go back and do my own thing and that usually will
give me enough time to think of something else to say when I get asked about
my processes again...(GRIN). Don't get me wrong, by no means do I profess to
know it all, but one can usually tell in the first few minutes after talking
with someone if they're knowledgeable, and I'm always open for suggestions to
make my life easier. But when you work at a contract assembler, you gotta' be
able deal with those people who still look at us contract assemblers as
nothing but "crude stuffing houses" who need to be monitored and guided in
order to put out a quality product.
As for double-sided boards, either way is perfectly acceptable provided things
are done right...and one short out of 500 parts isn't a catastrophe, but I
will say that shorts on SMT capacitors normally are pretty rare, and can be an
indication of a more sinister problem, like flux entrapment in epoxy
voids...you have seen all the TechNet posts on this haven't you?
Now this is only MY preference, but I would rather do a double reflow and mask
the bottomside SMT (if I can) with the wave fixture, than to deal with epoxy.
There's a few reasons why;
1. Many mixed technology boards I've seen will need a wave fixture anyway
because they've got right angle connectors at the edges of the boards that
prevent them from being run by the fingers. So you can use the fixture to not
only mask the SMT on the bottom, but mask any plated tooling holes that
require it instead of (as in my case since I'm closed-loop with my water)
using a latex mask and waiting for it to cure, or kapton tape dots which both
have to be manually removed prior to washing the assembly after wave.
2. You don't have to worry about boards bowing while waving them.
3. Some assemblies are real sensitive to cracked capacitors, and you stand
more of a chance to crack them when hitting the wave, those issues are pretty
much eliminated with fixture masking. We built a board that went into an
ultrasound machine, and if there was even one capacitor cracked they could
tell by the quality of the display on the ultrasound system...there would be
little lines of static running through the screen.
4. Parts don't get knocked off the bottom of the board from the handling that
the board is exposed to stuffing the PTH. Unfortunately, when you talk
epoxy'ing parts on the bottom of a PCB, a lot of people are thinking about the
2-part, 2-ton epoxy that you glue your chair leg back on at home and don't use
the care they should when handling the boards. They don't realize that the
epoxy we use is ONLY strong enough to just hold the parts on through wave,
nothing more.
5. The last reason I always try and double-side reflow everything now, is
because I don't have a dispenser...(good reason huh?) I'm at a start-up now
and ya'll know how tight the money can be at a start-up...I guess I could
stencil the epoxy if I wanted to, but it is less headache to double-side
reflow.
But like I said earlier, either way is perfectly acceptable when things are
under control. I just have my preferences.
Try to suck it up, grit your teeth, and smile real big when you get some
"suggestions" to change your process from your customers...I know it takes
patience, but if you can be mellow it does make points. The guy will go home
feeling like he accomplished something that day, he'll get that "warm and
fuzzy" feeling that you're doing a good job (you probably have been doing that
already), and you won't have to waste any energy trying to change this guys
mind when he probably won't anyway...
-Steve Gregory-
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