Steve,
I concur with what you've said. We also had accept/reject issues with
the palladium solder joints. The demarcation line along the side is
unsettling to some inspectors. We answered the great majority of
those questions by distributing to QA, a bulletin, with inspection
criteria and photographs of acceptable joints. Overall, we get good
results with these parts.
If I'm evaluating solderability problems on a board where the
palladium parts soldered decently, I take it as a general indicator
that the profile is probably OK.
Bruce Tostevin
Benchmark Electronics Inc.
Hudson, NH
______________________________ Reply Separator _________________________________
Subject: Re: [TN] Parts Solderability (2)
Author: SteveZeva <[log in to unmask]> at 0UTG0ING
Date: 3/27/98 12:21 PM
TGIF Ya'll!
The "peanut gallery" here is gonna throw his two-cents out in this
discussion, because it is something that really can be a pain sometimes,
especially if you're in a process engineering or quality postion. I've seen a
lot of time and energy burned over this issue in some instances...
The biggest difference I've seen between a tin/lead and a palladium
coated lead, is solder joint appearance. What I noticed mainly is that the
solder does not seem to flow as readily to a palladium coated lead as it does
with a tin/lead coated lead.
When you look at a palladium coated lead solder joint under a microscope,
it will tend to have a very strong demarcation line where the solder stops and
the lead begins. Many inspectors see that as the solder appearing to not have
wet very good. A tin/lead plated lead will have the solder wet and flow over
top of the foot making the overall joint look kind of rounded and
smooth...many times when you have that kind of variances in solder joint
appearance, the inspectors tend to excessively reject the palladium joints. In
my opinion, when they put a defect arrow on it, and have it reworked, all it
is accomplishing is making all the solder joints look the same, and nothing
more.
A little higher than normal peak temperature in your profile helps things
a little, but don't cook your boards! This helps the appearance some, but I
really don't think it's necessary. I feel the joints are good, the plating was
designed to be a drop-in replacement for tin/lead.
Sure, the joints will look a little different, but that's okay. From
everything that I've been taught and read, the most critical area of a solder
joint is the interface between the bottom of the foot of a lead and the the
top surface of the pad, next in importance would be the heel, and I've always
observed completely acceptable wetting in those two areas on palladium coated
leads. Side and toe fillets don't add a whole lot to the joint as far as
mechanical strength goes from every stress testing graph I've read. Don't get
me wrong, they do add a little to the equation, but not as much as you might
think...
I spoke with somebody from T.I. back when I first noticed it about
5-years ago... (that's right, they've been doing it for 5-years). The biggest
reason they switched I was told, was that the solder plating operation was the
messiest, low yielding process that they had. Once they switched to palladium
they saw at least a 50% increase in yields...plus, they eliminated using lead
completely. Something that most of us are probably going to have to face
sometime in the future. (even though some of us aren't really looking forward
to it...)
So unless I'm really missing something, from what I see, the only problem
with palladium coated leads is getting everybody to agree on what the
appearance of an acceptable solder joint should be.
Have a great weekend everybody!
-Steve Gregory-
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