You do not state what the basis material is.
I think what may be happening is one of two scenarios, possibly both.
1.) A thin nickel flash over the basis material (copper, kovar, bronze)
followed with a matte tin finish can be dissolved due to the fact that
you are pretinning the leads, which would cause at least some of the
nickel to be absorbed into the tin-lead solder if the component is held
in the tinning bath for too long of a dwell time. Later, when you
attempt to solder the part, the nickel could be completely absorbed, and
the only thing you would have left to solder to would be the basis
metal, which may or may not be solderable. This would lead to a
de-wetting defect.
Nickel's rate of dissolution into solder is much slower than that of
other metals. But if it is thinner than 20 uinches, it can easily be
completely removed from the basis metal in a two-step process of tinning
followed by soldering.
2.)If the nickel is allowed to oxidize prior to the matte tin finish
applied by the component vendor, then the matte tin is simply being
deposited (coated over), but not plated to the nickel due to the nickel
oxide barrier. When you attempt to tin the part in-house the matte tin
is dissolved into the plating bath exposing the nickel oxide barrier,
and then when you attempt to solder to it no intermetallic formation
takes place between the nickel and the tin/lead solder.
Standard fluxes such as RMA, organic water soluble, etc, do not remove
nickel oxides.
If the operators are able to eventually get acceptable solder joints, I
suspect that they are physically removing the nickel oxides with the
solder iron tips. Either that or the basis metal is solderable, but not
ideally so.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Chris Schaefer
Sent: Friday, December 28, 2007 11:00 AM
To: [log in to unmask]
Subject: [TN] Hand Soldering Matte Tin over Nickel Flash with SnPb Alloy
Greetings All,
I have a PTH TK type resistor component that is formed to fit an SMT
geometry land pattern. The component is Pb Free component plated using
Matte Tin over Nickel Flash. Once the component is soldered it exhibits
non- wetting/ poor wetting characteristics thus causing us to rework
until we have achieved acceptable results. Of course this is not what I
would like to see occur, but unfortunately at this time it is the
process. The installation process is as follows:
- Part is formed using a lead forming tool
- Is tinned using 25% RMA for 3 seconds and let to dry (somewhat); then
soldered (SnPb tin/ lead alloy) for 3-4 seconds in a 600F bath
- It is soldered (SnPb tin/ lead alloy) to the cca using a 6ooF solder
iron tip for
2-3 seconds
- END...
Apparently this has been an issue for years, but just recently was
brought up to our Engineering department. We tin the component to allow
for improved wetting characteristics, but this does not provide much
improvement. I have done some investigation on the internet looking for
reports and tests performed soldering Matte Tin over Nickel using
standard Tin/ Lead alloy, but have found very little information
regarding this. The component manufacture states our process as
described to them 'should work without issue, but this is not the case.
So my questions are what are the issues related to soldering matte tin
over nickel flash with tin/ lead alloy? What hand soldering parameters
are considered optimal/ ideal with a low-mass matte tin plated component
considering the information provided? Is it possible to get very similar
results using this combination of metallurgies versus using tin/ lead
plating and solder materials?
Any help would be appreciated.
Thank You,
Chris
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