On Monday 22 August 2005 12:04 am, Joel N. Weber II wrote:
> One concern I've seen expressed with Pb free parts is the possibility
> of tin whiskers forming. Tin whiskers simply don't form if there's
> enough Pb mixed in with everything, but these days often the pins on
> many modern parts don't contain any Pb even if you're using lead in
> your solder. As far as I can tell, the industry is generally moving
> towards a matte tin finish for most parts.
Lead is not the only metal that prohibits the growth of tin whiskers.
In fact, silver, palladium and copper all prohibit the growth of tin whisker
as good or better (specifically, Sliver and palladium) than lead. When
plating copper with tin, you want an alloy of tin that is 93% tin and the
rest silver or palladium or some other metal. This is true for Immersion,
electroless and electroplating. When plating copper with nickel, you use
phosphorous in the same ratio (93% nickel, 7% phosphorous). As an example,
the following immersion plating solution, if applied correctly, will not grow
tin whiskers:
1.) Stannous chloride 3.8 g
2.) Silver Chloride 0.25 grams
3.) Thiourea 49.5 g (Complexing agent to keep copper from interfering with
the tin plating process).
4.) 66 degree Baume Sulphuric acid 12 ml.
5.) Distilled water 1 liter (1000 ml)
These days, poor PCB hygiene and/or contaminated rinse, pretreatment or
plating solution are the main cause of tin whisker growth. This is the
process for immersion tin:
1.) Clean (Mechanical and/or chemical)
2.) Rinse with distilled water
3.) Pretreatment with solution similar in composition to plating solution.
For tin, we use 12ml of 66 degree baume sulfuric acid in 1000ml of
distilled water,
4.) Plate
5.) Rinse with distilled water
6.) Dry
If you mess up any of the 6 steps you'll get uneven plating and tin whiskers
or worse.
Regards
Marvin
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