>> Hello Technetters,
>> I need some advises. I have a 6- layers board. I try to wave solder it,
>> the solder at the ground point is not able to flow up to the top side of the
>> board, in fact, it doesn't flow above 75%.
>> We have tried using a solder-iron and set to temperature to 450 deg. cel. and
>> hopefully able to force the solder through the PTH. Too bad, it didn't work
>> anyway and it in fact has caused some lifted pads.
>> We use full blow Nitrogeneous wave-solder machine, and the flux we use is
>> only 1.2% solid content. And if the preheat temperature set to high, the flux
>> gets degraded before it even reaches the wave..
>> I would like to know if anyone could advise me how wave-solder such a board
>> with a great heavy thermal mass.
>> Thanks in Advance..
Greetings Kong,
WOW! Can't get top-solder even with a soldering iron cranked all the way up
to 450 degrees C.? Hoo-boy, sounds to me like there's some design issues that
need to be looked at on this board...but that don't help you now, right?
It definitely sounds like ya' need to do something special when ya' go to
touch-up those leads that aren't gettin' good top-solder. I think it's worth it
to go over some basics though.
First thing, make sure you're not reworking an acceptable solder joint. I've
had some experiences with the "no top-solder" problem where by once someone had
stopped for a minute and really took a close look at what was being touched-up,
found that at least half of it wasn't necessary. Ya' know, that's a pretty tough
defect to call visually, don't ya' think? I mean there's the blatant ones that
are easy to make a judgement on, but a lot of times when ya' have a board like
your working on now that may have a design problem that's causing less than
ideal top solder fillets, things can get blown out of proportion sometimes. An
ol' crusty career quality guy told me something one time that really has some
truth to it; "If ya' have to sit and stare at solder joint for more than just a
few seconds tryin' to decide if it's good or bad, it's most likely good."
'Nuff said about that...
What it sounds like to me has happened, and I've seen it before, is that
there's a solid copper internal plane (power, or ground) that's connected to
certain leads without thermally relieving the hole. Sometimes you can hold the
PCB up with light behind it and if the hole has been thermally relieved, it
should kinda' look something like this:
Internal plane
Area
,.-----.,
\ /<-----------------Thermal relief
Internal plane \.---./ Internal plane
Area .-. Area
|^~"| ( ) |"~^|
! ! '-' ! !
Thermal \ ', ,' /<-----Thermal relief
relief ------>^. / \ .^
'~ ~'
"One Thermally Relieved PTH"
Kinda' looks like a "mag-wheel" huh? This is supposed to be looking at the
plane that's connected to the hole from above, with the center circle being
where the lead would go into. Another thing that helps prevent the internal
planes from sucking all the heat when yer' tryin' to solder is to use a
"cross-hatched plane" in the internal layers instead of a solid copper plane.
Now, for the wave soldering. If you've already tried getting more heat into
the board to the point that yer' exhausting yer' flux before it hits the wave,
then heck, I don't know what else ya' can do...does yer' oven have topside
pre-heaters? I'm careful 'bout telling ya' to get things too hot, 'cause I don't
know if yer' waving the board in a fixture or not...ya' don't want the board
bowing on ya' and doing a "submarine-job" at the wave, DIVE!DIVE!DIVE! Man! I
hate it when THAT happens!
One thing that might help I used to do that I discovered quite by accident,
is to put the assemblies in a burn-in room or oven heat them thru and thru, and
take them out just prior to waving them. We had a pretty thick board that we
built and the fab was made in Florida...pretty humid there. We needed to bake
them before wave because of the blow-holes and voids we'd get because of the
moisture the fabs picked-up. Well, we had a few components on the board that had
some of the same problems that you're having, but not real terrible. What we
noticed was that the assemblies that we waved first after taking them out of the
burn-in room almost never had one teeny top-solder problem, while the assemblies
that were waved after they'd cooled down, would every once in a while. So that
might be something you want to try.
Kinda' the same thing applies when you got to touch-up the ones that you
HAVE to. With yer' soldering iron cranked-up to 450 degrees, yer' no doubt
gonna' lift some pads. Remember that ol' crusty quality guy I was talking about
earlier? He told me something else; "If ya' have to stay on a solder joint more
than just a couple of seconds with yer' iron to solder it, somethings wrong."
Typical FR-4 Tg (glass transistion temperature) is only around 145 degrees C.,
and with a 450 degree iron you can do some serious damage in 'nuthin flat! Ya'
might want to use some hot air to pre-heat the area that you touch-up, or
something I've seen some operators have to do, and it's awkward, is to use two
irons, one from each side...
__\/__
. / ^ _ \ .
|\| (o)(o) |/|
#------.OOOo----oo----oOOO.-----#
# Steve Gregory #
# SMT Process Engineer #
# The SMT Centre Incorporated #
# [log in to unmask] #
#________________Oooo.__________#
.oooO ( )
( ) ) /
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