Years ago the industry standard tin-lead finish was a 85Tin/15Lead ratio.
Now if these parts were pick-and-place on a PCB with immersion silver plating and if Sn96.3 Ag3.7 solder paste was used (*NOT* suckie SAC), how much of a decrease in the melting point would one see, or alternately what would the resulting MP be?
Years ago the industry standard tin-lead finish was a 85Tin/15Lead ratio. Now if surface mount parts like this were pick-and-place on a PCB with immersion silver plating and if Sn96.3 Ag3.7 solder paste (NOT sucky SAC) was used, how much of a decrease in the melting point would we see, or alternately what would the resulting MP be?
I don't know, but if you look at
If you must use these parts, one method is to use two lead-free solder baths, dip in one first then the second to finish. The second should be periodically moved to first and fresh solder used for second, if that makes sense.
Cheers
-- Syd
That might look good in theory, but from a real-life production viewpoint is economically a disaster even ignoring the time requires to do that. We are talking about thousands of devices - not onsies and twosies.
Yes, that method has been used, in extremis, for tens and hundreds. For thousands, use another part. ROHS has made downhole work easier.
Cheers
-- Syd
For what i am doing, there IS no other part.
Check your exemptions. You might not have to go RoHS.
Cheers
A WAG, but AFAIK Lead reacts with Acetic Acid to give Lead Acetate which is water soluble, (old name 'Sugar of Lead' - hang a sheet of Lead in wine which has gone sour to make it drinkable if poisonous) but Tin doesn't.
I've no idea if that could leach out the Lead, but there may be a chemist or two here who could advise.
Cheers
-- Syd
Well, if you want to dissolve the plating, a dip in dilute nitric acid (~10%) should do the trick. Rinse very thoroughly. Even with careful use, I'd be surprised if you can completely avoid getting any on the package, which will corrode the leads and decompose the encapsulant (stronger nitric can be used to burn away encapsulant, leaving behind a lot of filler and a shiny silicon die).
Tim
-- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
I have been able to obtain a lot of alloy melting point data, a lot of it with sufficient points to allow graphing. The data seems to imply that the melting point would be higher than the eutectic Sn62Pb36Ag2 melting point of 179C.
Presumption #2: 100 parts Sn96.3Ag3.7 and one part Sn85Pb15 (from device lead plating) gives Sn96.2Pb3.7Ag0.15 net alloy. Using tin/lead alloy MP graph gives 230C, substantially higher than the 221C of the solder paste.
Me neither (though I knew a man who was), but aren't you assuming complete mixing? That would imply a long time molten.
If these are SM parts, then you may be ok if you glue or clamp the part so there's no load on the pins and use lots of solder such that the pins are completely encased. Personally I wouldn't risk it. I wonder if companies who re-ball BGAs could help?
Good luck.
-- Syd
In my case, i am working with SOT-363 parts. I bet that most of those re-ball companies are either frauds or rip-off artists; i base that on the way they send UCE with lottza hype.
Very little mass then. Maybe a dab of RTV over the joints (or the whole part) is all you need.
Cheers
-- Syd
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