Bev,
How about "Water Soluble vs. No-Clean Pastes: The Factors that Affect
Performance", Surface Mount Technology, Jan. 93 issue, pp 35-40.
Rob Schetty
LeaRonal Inc
Freeport, NY USA
-----Original Message-----
From: Bev Christian <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: Monday, March 30, 1998 3:24 PM
Subject: Re: [TN] Re[2]: [TN] Parts Solderability (2)
>Eddie,
>I too would like not to see the demarkation but line, but 1) for some
>components we were sole-sourced and 2) TI gave us a deal we could not
refuse
>so...
>Secondly, we are using no-clean only in all our plants, so I was referring
>to different no-cleans and their efect on d. There is a paper on this
>somewhere. Anyone out help us out on a the ref?
>
>regards,
>Bev Christian
>Nortel
>
>
>> ----------
>> From: Eddie Brunker[SMTP:[log in to unmask]]
>> Sent: Monday, March 30, 1998 5:22 AM
>> To: [log in to unmask]
>> Subject: Re: [TN] Re[2]: [TN] Parts Solderability (2)
>>
>> 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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