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Subject:	Re: Silk screen changing color
From:	Dennis Fritz <[log in to unmask]>
Reply To:	TechNet E-Mail Forum.
Date:	Fri, 3 Oct 2003 11:58:13 EDT
Content-Type:	text/plain
Parts/Attachments:	text/plain (38 lines)

Eric -

I assume you did not make the board, and are getting the yellowing effect
after your assembly operation.

Legend inks historically were kind of an "orphan" in specifications.  They
were only cosmetic - so the functional tests of documents like SM-840 for solder
mask did not apply.  Sounds like the legend is a two part epoxy, with the
curing agent an amine - noted for turning yellow upon degredation.  Ever see one
of the old two part epoxy solder mask cure ovens - yellow brown everywhere on
the inside from excess catalyst and solvent evaporation?

I suggest you try some of the testing in SM-840 on the legend - tape
adhesion, wipe with methylene chloride swab, etc for testing degree of cure.  Perhaps
the legend is cured - as defined by cross linking, but the fabricator added
too much catalyst and the excess is turning yellow.

Talk to the fabricator to see if he can tell why the legend is not yellow to
start if it is fully cured, and now changing color for you.

Incidentally, IPC is now forming a task group to write a specification on
legend inks.  Also, UL is watching the progress of that document, as legend is
becoming a larger % of the board surface.  For some reason, UL says they don't
want to find blazing legend on top of V-0 rated solder mask.

Denny Fritz
MacDermid, Inc

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