To: George, Paul, Dave, John and others of interested in replacing Sn/Pb
plate with tin plate.

Glad that while I was out of the office the experts answered the problems
that many people are having with the tin sulfate plating bath To summarize:
Stannic oxide, SnO4. is a major contaminate of the tin stannous sulfate
plating bath and it forms a very small particle size virtually unfilterable
tetrahedral crystal which is insoluble in sulfuric acid solutions.  The
stannic crystal codeposits with the tin and causes grainyand rough surface
appearance in the solder joint and will also produce a possible dewet surface
when
subjected to solder testing with Sn/Pb solders.  The Ato-Tech flocculant
works well to allow filteration and remove particle from the bath.  And It is
best to hold the stannic oxide low.  Some of the new baths such as
methane-sulfonic acid old fluoroboric solutions resit formation of stannic
oxide crystals, but are considerable more   expensive to make up and to
operate.  The old bright acid tin that
was used by Dave's  company many years ago are more subject to whisker
growth.  Some of the other bright deposits have been reported to grow
whiskers when deposited over brass or may be copper.

 I have not experinced problems with stripping thin tin coating plated from a
bath with a high stannic content and they do act as a good etch resist but
are sbject to staining an graying out during the etch cycle; I think from the
absorption of etchant solution or mabe copper in the more porous plate
containi stannic compounds.  I also have not used as low as 0.0001 inch
thickness even for the etch-strip-SMOBC process.

When plating tin over nickel to be used as a final finish and plating tin
over copper in the same line it is desirable to decrease cross contamination
or possible imersion plate nickel with the copper in the acid pre-plate dips.
 Also when plating tin over nickel keep the 
time and rinse steps as short as possible between tin and nickel to prevent
passivation that may later show up as dewetting.  

The use of solder mask over the tin plate over copper is not a problem for
surface mount since the 232oC melting point of tin is not reached as it is
with wave solder.  However, I have seen orange peel on surface mount boards
even though the profile indicated no to point higher than 217oC.  A major
problem with soldermask over tin on copper is that the dri-film,slivers break
loosed and get trapped between conductors.  The LPI is better if the slivers
are not produced prior to coating..Best process is to apply LPI almost
immediatly after etching and baking.  Airknives are not the preferred 
drying method as they cause slivers to break loose.  When applying over
nickel the nickel should be thick enough to resist bending down and breaking
loose.     

Tin can been fused but it is more common to use a bright deposit; however,
the problem with slivers as mentioned still exists.

I note that at the SMI conference at San Jose in Sept. that Dr. Y Zhang et al
of Lucent Technologies is giving a paper on Tin as Alternate for Sn/Pb which
proposes to be a solution for some of the problems we are
discussing.......See you there.

Phil Hinton 
Hinton "PWB" Engineering
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