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April 2001

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Subject:
From:
"Wenger, George M (George)" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Wed, 4 Apr 2001 18:48:14 -0400
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Ioan,
If the pads are solderable after wave soldering then you probably don't have
a "solderability" problem.  If you were seeing partial wetting or de-wetting
on the pads you might have a "solderability" problem.  If you are
experiencing non-wetting" on random pads then you probably have a
"soldering" problem.  If you see random or isolated pads that don't take
solder during wave soldering but do take solder after wave soldering then
I'd suggest that the pad isn't touched by molten solder during the wave
soldering process.  If it was it should have wet.  Examine the pads that
don't wet and pay particular attention to their geometry and dimensions.
Also check the solder mask thickness and size of the solder mask well
surrounding the non-wet features.  Our experience is that random
non-soldered pads are usually caused by what we refer to as the "solder mask
well effect".  We have a thick in-house solder mask for high voltage circuit
packs and found that when the solder mask was 3 to 4 mils thick we always
got many random non-soldered pads.  When we control the solder mask
thickness to 1.5 to 2.5 mils thick we eliminate not all but the majority of
the non-soldered pads.  When we use a typical LPI solder mask with a
thickness of 0.8 to 1.2 mils we see random non-soldered pads very
infrequently, that is if our wave soldering processes are in-control.  We
never see non-soldered pads on the bottom side if the features are vias.  If
they are test pads or component sites that don't contain a via we find that
the flux we apply ( and we do use X-33-04 in one factory) collects in the
well between the solder mask and the pad.  When it is heated and volatilizes
a bubble covers the pad and blocks solder from touching the pad.  When this
happens solder doesn't wet the pad.  Vibrating the board or the wave during
wave soldering helps "push around" these bubbles allowing solder to touch
the pad.

Try using a chip wave along with a laminar wave or if you already have a
chip wave try increasing its pressure or oscillation.


Regards,

George
George M. Wenger DMTS
Bell Laboratories Princeton, Supply Cain Network
Engineering Research Center FMA / AQA / RCA Lab
(609) 639-2769 (Office); 3210 (Lab); 2346 (Fax)
[log in to unmask]


-----Original Message-----
From: Tempea, Ioan [mailto:[log in to unmask]]
Sent: Wednesday, April 04, 2001 1:43 PM
To: [log in to unmask]
Subject: [TN] Random absence of soder joints


Hi,

some time ago, Mike Bailey brought up a situation of random non-wetting on
the secondary side. Well, I run into a somewhat similar problem right now.

After wave soldering, I see SMT passives that are on the solder side
exhibiting either non-wetting at all, or one side soldered while the other
one looks just like before the wave, no sign of wetting. Even on the pads
without components, the situation is the same, some pads have absolutely no
trace of solder, while the adjacent ones collect solder without any problem.
Not related to shading, since bad spots are even far from any component. Not
related to glue contamination either, since I have trouble even with pads
without components.

To make it even more complicated, there is no problem to hand solder the
non-wetted pads, after the wave.

I refreshed the flux, ran the assemblies on two totally different machines
(different preheating, fluxing), with the same poor result

The process is:
glue & insertion + reflow at 120C for the bottom side, only passives
populated
paste & insertion, reflow for the top
wave with no-clean X33 flux from Multicore and regular eutectic solder
board is HASL

I know this is a mild flux, but it does a great job for us on much more
demanding assemblies. I will try a stronger one, but cleanliness is a big
issue.

Now, I would like to know if Mike found something and if he could fix
anything.
Besides that, any hint that Technet could give would be largely appreciated.

Thank you,
Ioan

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