Steve is a serious guy, he may be angry and jump from the wall..we'd
better give a reasonable answer..
As all know, you get a ternary reaction (maybe more depending on
contaminations and flux type and nickel P % etc), which are named Beta
Sn, Ag3Sn and Cu5Sn6. And as you know well, IMCs are created when
meeting the ENIG. According to various laws, these IMCs will in fact
consume more and more tin, the IMC becomes thicker, and the joint will
be stronger with time. That's positive. Then it becomes complicated.
Depending on the thickness of EN, the nickel will give place to copper
to diffuse in the Sn direction. If the EN is thick, this won't happen,
but, as we know, some make sometimes too thin and porous EN, giving way
to Cu atoms to move. The Sn will diffuse as well towards its contrahent
copper. If there is a atomic flux imbalance, or what it's called, there
will be vacancies, or voids in the IMC interface..yeah right..Kirkendall
voids. That's why it's so important with a good EN or even a alloying
additional element like Ag. If everything is allright according to the
spec, nothing will happen of this kind. If not, the interface may be
brittle and be sensitive to sudden mechanical shocks, and your SOIC will
jump off the board. There may be other phenomenon, like local silver
segregation, tin segregation, though less likely to happen so fast. I
have just sniffed a little upon this topic. If you begin to dig further,
you will drown. There are thousands of pages written about all minor
reactions, phases, grain growth, diffusion speeds etc. Vernier
Engelmaier can probably speak for days on this, with little help from
strong coffee and schnapps.
All this is Arrhenius fault..
My guessing is that nothing will happen if you have a perfect ENIG,
performs a perfect soldering profile and use SOICs with perfect lead
finish. You will get a better and stronger joint with time. If not
having those good conditions...you may get weakened and bad solder
joints.
And...you sly fox..you already knew all this. Did I get into kind of a
trap? He-he....I didn't say I was telling the truth...eenemeeenimeeeny
Inge
Snore..zzzzz
Inge
-----Ursprungligt meddelande-----
Från: TechNet [mailto:[log in to unmask]] För Steve Gregory
Skickat: den 18 april 2007 20:38
Till: [log in to unmask]
Ämne: [TN] Off the wall question...
Hi all,
Just curious, what would you think that a SAC 305 solder joint on a SOIC
that was made on a ENIG finished polyimide board would do after spending
140 - 168 hours at 350 F. ?
Just wondering is all...
-Steve Gregory-
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