But why should we accept this demarkation line? Let's not lower our
standards just for TIs convienience. Has anybody done any metalurgical
research into this?

Bev,
you said the parts are flux dependant, I agree totally. I have learned to
live in the realms of higher flux activity, and TI are probably the sole
reason for this!! Water soluble fluxes will solder almost anything. As BGA
technology takes over, No clean is the 'order of the day', and this too has
been choosen at the edges of IPC class 3 with healthy residues, to tackle
the lack of solderability of this finish.


   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.

Why didn't you take issue with TI for the problem, instead of training QA to
accept "no toe fillets, side fillets, poor wetting angle etc...."


>Author:  SteveZeva <[log in to unmask]> at 0UTG0ING
>Date:    3/27/98 12:21 PM
>

>
>     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.
>

Solder flowing to the lead and wetting angle are common indicators of what
is called solderability.


>     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.

Anybody know what the metalurgy of this boundary is? Why should we treat
this line different to when it is observed on tin/lead terminations? I don't
believe this line is safe!! I think it is a source of weakness, and can
develop with time into a crack.

Many inspectors see that as the solder appearing to not have
>wet very good.

Yes!!!

 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.

But why should we accept the difference in the first place????

>     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.

Maybe your assemblies are simple and uniform, but we have 12 layer, complex
assemblies where with a range of devices which put a wide enough range on
the delta T already. Again, why should we have to change our profile for TI?

>     Sure, the joints will look a little different, but that's okay.

How do you know it's ok? Have you performed metalurgical analysis on them?
The IPC 610 is based on visual characteristics for defects.

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.

What solder between bottom of the foot and the pad? 70% of the strength is
in the heel fillet! Yes.

 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...

If every other joint on the assembly has a toe fillet but TI devices don't
why should I accept this? OK so it conforms to IPC, but I would like to see
it there as an indicator that things are going well. The side fillets do add
strength to the joint.

>      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.

Yes, we too have been using this finish on devices for a long time.

 Once they switched to palladium
>they saw at least a 50% increase in yields...plus, they eliminated using lead
>completely.

SO WHAT!!!! This is not my problem!!! TI may see the advantages...I don't care!

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...)

Ah, now I see what you mean by higher profile temperatures! Lead free
technology with m.p. of 230 deg C. Nice.....very nice...(Electovert, your
ovens still only have a max. programmable temp. of 280 deg C!) Look,
California's electronics industry might try to shoot itself in the foot, and
tell the rest of the world how to behave, but first they'll have to develop
a working technology we can all use. Show me someone making complex
assemblies with lead free solder.


>     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.
>

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