Olin said:

> It's hard to imagine a legitimate reason for doing this.  Perhaps you
could
> provide one so that people don't feel they are helping you do something
they
> would dissapprove of?

Actually, we (Celestica) do a lot of decapsulation of parts for failure
analysis and reliability checking.  I haven't been involved in any specific
cases for a few years, but I can come up with five reasons right off the top
of my head why you would remove the potting compound (plastic around the
chip):

1.  Illustrate defects in the Al wire assembly process.  This is mostly
bends caused by mishandling.  Acid is used for this.
2.  Do (surface) analysis of the chip to Al wire joints.  Usually done by
grinding.
3.  Do surface analysis of the lead frame and Al wire joints.  This can be
particularly important in dipped (tinned) SMT devices to see if flux has
traveled along the lead frame to cause dendritic growth.  This is normally
accomplished by mechanical (cutting, breaking, grinding) methods.
4.  Look for/illustrate voids in the encapsulant.  Acid.
5.  Look for mechanically damaged/cracked chips in the encapsulant.  Acid.

The reason why a card stuffer like us will do this type of failure analysis
is because chip manufacturers are a) slow to respond and b) incented to find
problem causes that are not of their own doing.  Claiming EOL/EOS can often
be done after a quick probe with a Huntron Tracker (most of the problems
listed above will give a similar signature to EOL/EOS) without opening up
the package.  Doing this is an easy way out for a chip manufacturer and
generally results in an audit of the PCB assembler and no actions back on
the chip manufacturer.

We developed the capability of looking inside chips because we were tired of
getting reports back saying EOL/EOS and then explaining to our customers why
we were zapping chips.  I haven't heard of Celestica being accused of ESD
damaging a chip in six years, so developing this capability was worth it and
I've seen a lot of amazing things that should never have gotten out of the
chip plant.

The point of all this is to say is that there are a lot of legitimate
reasons for opening up a chip, especially if you are concerned with the
quality of your product.

Sorry for the long dissertation on the subject, but watching the failure
analysis process going down into the chip and seeing the chemists find that
a failure was a result of a chip Di-Water wash being contaminated with
bacteria is just fascinating.  (This was an actual situation that I was
involved with by the way.)


Going back to the original question, the reason why some acids are chosen
over others is based on what you want to leave behind to look at.  For
example, if you were interested in looking for evidence of dendrites between
pins in the epoxy, you would pick an acid that doesn't eat away at copper.
If you're interested in doing analysis of the chip's Al interconnections and
wire bonds, you would choose another acid that does not attack aluminum.
Sorry, I don't know which different acids are used for different
investigations - I do know that there are standard references explaining
what to use to look for different defects.

myke

--
http://www.piclist.com hint: The list server can filter out subtopics
(like ads or off topics) for you. See http://www.piclist.com/#topics