Mac Dermids processing guide speaks specifically about packaging
requirements.

Our company just issued instructions to our boardhouses as to packaging
for Alternate finish boards. Boards must be packaged in lots of 25
generally, although 10 to a pack is acceptable if they are larger boards
and more than 25 if they are smaller than 2"X 2". This is standard practice
at our houses and we assume at most others. We dont want to open every bag
of boards for inspection so after QC checks a few boards at incoming we
will reseal them. This includes boardhouse over runs left on their shelves
for planned release shipments.

the board house must put a paper date label on the outside of all packages.

The top and bottom board in the package must be covered by sulphur free
paper. (Since Immersion Silver finish is below the board surface we are not
worried about surface scratches) Plastic will tarnish boards surface.

I have 30 Silver boards in my desk wrapped in paper. They have been there
over 1 year. The top and bottom boards are very tarnsihed but all the rest
are fine.

Packages must be vacumm sealed. ( if house has those capabilities otherwise
we will seal them at receipt)

Another caveat is if you have to deal with a boardhouse which sends the
boards out for processing. Legend print is recommended before Immersion
Silver is applied since chemicals or heat used in screening can cause
tarnish, staining and unsolders unless they are using a UV ink.

No dessicant is to be inserted inside vacuum sealed packs but is allowable
inside packing boxes.

The secret to success is to treat all boards as if they are Silver
Immersion, this avoids tricky document changes and special handilng
procedures on the assembly floor. Operators should not be touching any
boards surface with bare hands, washing should be kept to a minimum on All
boards.

We are having a very positive experience with Immersion Silver transition
from HASL. 15 assemblies which were problems before are now switched over.
All BGA boards and anything with 20 mil fine pitch is automatically Silver
Immersion. We use both Alpha Level and Mac Dermid processed boards.

Aperture selections of stencils for fine pitch is where we are
concentrating now, since you are starting with 50% less solder you need to
increase the stencil aperture size but on fine pitch with a 1:1 ratio you
have shorts on everything less than 20 mil spacing. Better to get shorts
than opens but we want to reach the perfect balance. If anybody is already
there would appreciate the feedback, if not we will post it when we get
there.
Dave Chapman
Circuit Service

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