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April 1997

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Subject:
From:
[log in to unmask] (Jerry Cupples)
Date:
Fri, 4 Apr 1997 10:18:35 -0600
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Dave Hillman asked:

>     Hi Jerry -
>
>     With the decline of ceramic components and the increase of plastic
>     components with this be an issue again or because of past lessons
>     learned be a non-issue?

For conventional plastic packages, it is likely a non-issue as it can be
solved in various easy ways (coatings, metallization, polysilicon
passivation...).

For chip scale packages, it could be a concern where lead bearing solders
are in intimate contact with MOS dice. I can imagine various types of
solder bumping approaches that might increase the exposure. It is also a
concern (and I use that term advisedly) for _any_ material containing alpha
emitters which would be in intimate contact with a memory cell on a die.

The die coat materials used in RAM (at least those in use about 10 years
ago) were pure polymers known to be free of these emitters. The environment
has lots of these nasties like radon gas and other decay products. In the
day of the 16K RAM, there were nearly no precautions taken to deal with
alpha emitters, and it was the type of problem which took sensitive and
thorough lab/theoretical analysis to measure. There were experiments
conducted deep in salt mines, as I recall.

My comments are based on old information. That said; my opinion is that for
those of us who make boards, it is no concern. For those involved in chip
scale packaging, review the literature and talk to a physical chemist or
two.


regards,


Jerry Cupples
Interphase Corporation
Dallas, TX USA
http://www.iphase.com


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