Hi Richard - in response:
1. If a leadframe is made from Alloy 42, is there ALWAYS a concern
that the part will stress-crack after thermal cycling, especially on
stiffer leaded devices?
* If you read through some of the recent publications on alloy 42
leads you will find that no, an alloy 42 lead frame does not
automatically result in solder joint cracks. The lead material is just
one variable in how a solder joint survives each use environment. But
because alloy 42 leads are less compliant (than say copper leads) it
does mean that you are trading off some reliability. Take a look at
Engelmeir's, or Lau's or Iannuzzelli's publications.
2. Does the exact sequence of plating have any effect on the
survivability of the device after cycling? For example, do Alloy
parts that are overplated with nickel or silver, then tin-lead survive
better than Alloy 42 frames with only tin-lead?
* I believe that the plating sequence doesn't matter provided its good
to begin with. But, I don't have any data to support that and I think
most of the published papers are dealing with solder or solder/nickel
over alloy 42.
3. Is there general agreement that this problem reared it's head
because of the attempt to use commercial parts for military or harsh
environments? This would imply that engineering is choosing the wrong
parts for the application, and that the original part manufacturer is
making 'good' commercial parts. (And further, that an 'acceptable' mil
part no longer exists!)
* A general agreement by industry? That would be neat! Seriously, this
problem is the result of misapplication of a material in a design
where it is not optimum. Both the designer and assembler need to do
their homework (hopefully as a team) to avoid having problems like
this.
4. If the root of the cracking problem results from the manufacturer
allowing the Alloy 42 to tarnish before tin plating, is there anything
that can be done except for the horrific task of trying to
solderability test an SMT part?
* Solderability testing is the only way I know of for catching the
problem. Working closely with your component vendors would be one
method of avoiding a plating problem too.
Dave Hillman
Rockwell Collins
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______________________________ Reply Separator _________________________________
Subject: ASSY: Alloy 42 Plating Questions
Author: [log in to unmask] at ccmgw1
Date: 10/14/96 12:09 PM
There has been a recent flurry of discussion on Alloy 42.
We've also seen some post-thermal failures in which Alloy
42 MIGHT be implicated. I have some specific questions I
hope someone can clarify. I've just rejointed TechNet and I
haven't been able to find the last months' discussion in
the archives. :-))
1. If a leadframe is made from Alloy 42, is there ALWAYS a
concern that the part will stress-crack after thermal
cycling, especially on stiffer leaded devices?
2. Does the exact sequence of plating have any effect on
the survivability of the device after cycling? For
example, do Alloy parts that are overplated with nickel or
silver, then tin-lead survive better than Alloy 42 frames
with only tin-lead?
3. Is there general agreement that this problem reared
it's head because of the attempt to use commercial parts
for military or harsh environments? This would imply that
engineering is choosing the wrong parts for the
application, and that the original part manufacturer is
making 'good' commercial parts. (And further, that an
'acceptible' mil part no longer exists!)
4. If the root of the cracking problem results from the
manufacturer allowing the Alloy 42 to tarnish before tin
plating, is there anything that can be done except for the
horrific task of trying to solderability test an SMT part?
Thanks,
----------------------
Richard Huziak
Manufacturing Engineering
SED Systems
Saskatoon, SK, Canada
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