Hi Kevin - yes, we have done both single, double and even post reflow
rework of the ImAg surface finishes. As you detailed, the ImAg suffers
solderability degradation with each thermal excursion and with leadfree
reflow temperatures 30-40C higher than tin/lead, the degradation happens
faster. The "thin" type ImAg surface finishes are disappearing due to the
thermal degradation issue. The use of inert atmospheres and increased flux
activity help but the simple fact is that we are losing some of our
process window bandwidth with the implementation of leadfree soldering
processes. I imagine we'll get some of that process window back as the
material suppliers work on making their products more robust.
Dave
"Glidden, Kevin" <[log in to unmask]>
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02/15/2010 12:19 PM
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Re: [TN] Lead-free soldering on ImAg
Dave, Did your investigations utilize double sided reflow as well as
single sided, plus secondary soldering? Do you see degradation to
solderability after multiple soldering processes?
We utilize ImAg finish in a traditional SnPb process, and we DO sometimes
see solderability issues. Even normal SnPb reflow temperatures can
accelerate the oxidation on the second side pads before they are printed
and placed. The two reflow passes required for double sided SMT can make
any post-SMT soldering (such as for hand soldering components or wire
attachment) even more difficult.
Your materials used (flux types & activity levels) can drastically effect
whether or not surface oxidation can be overcome to allow proper wetting.
Just as important (if not more) is controlling the exposure of the PCB to
sulfur compounds or moisture, both of which can begin and accelerate the
oxidation. Putting controls in place to ensure proper packaging and
moisture resistance can go a long way in preventing the PCBs hitting the
line right-off-the-bat with minor oxidation already started. This too is
included in the 4553....
You also mentioned silver thickness, and I have found more favorable
results utilizing the "thick" designation, not the "thin", both of which
are allowed in 4553....
-----Original Message-----
From: David D. Hillman [mailto:[log in to unmask]]
Sent: Monday, February 15, 2010 11:20 AM
To: [log in to unmask]
Subject: Re: [TN] Lead-free soldering on ImAg
Hi Ioan! I have conducted a number of leadfree soldering investigations
with SAC405 and SAC305 solder paste using Immersion silver pwbs in
accordance with the IPC-4553 specification and have had no issues. The
higher leadfree processing temperatures tend to degrade all printed wiring
board surface finishes a bit faster than in tin/lead processing. I
recommend you investigate to see if the Immersion silver meets the IPC
4553 requirements.
Dave Hillman
Rockwell Collins
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Ioan Tempea <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
02/15/2010 10:06 AM
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[TN] Lead-free soldering on ImAg
Dear Technos,
I know the ImAg has been discussed over and over and with the same
results, very good and reliable finish.
My little something now is all the proponents were mainly in high-rel
applications (so SnPb solder most likely) and I do not remember if RoHS on
Ag was discussed. I know it wouldn't make sense for this finish not to
work with SAC305 pastes, but I just want to make sure.
We have, guess what, wetting issues on two different boards, from the
same supplier. We have tried playing with the reflow recipe, no
improvement. The batch finished, but now we are attacking a few more
different models
from the same PCB fab.
So, did anybody have issues with ImAg because they were on lead-free? Any
particular parameters that are not a happy ingredient, like specific types
of paste, specific solder recipe milestones?
If it were the PCBs, what could be wrong except for the Ag thickness? I
mean XRF analysis would be cheaper than x-sectioning...
Thanks,
Ioan Tempea, ing.
Ingénieur Principal de Fabrication / Senior Manufacturing Engineer
T | 450.967.7100 ext.244
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