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November 2009

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Subject:
From:
Steven Creswick <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Steven Creswick <[log in to unmask]>
Date:
Wed, 25 Nov 2009 11:37:28 -0500
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text/plain (88 lines)
Inge,

My observations when sectioning LED's, and Si devices was that I could make
problems if I rushed the process too quickly.  

I always go very slowly.  I don't rush the cut.  Don't rush the polishing
operation.  Go slowly!  If I rush the cutting or polishing, I frequently end
up with cracks, chip-outs, etc.

If you were working with unpackaged [Flip Chips], I would suggest that you
may also wish to visualize the stress that the potting material puts on your
chips.

It may also be possible for your potting material to induce enough stress
inside a packaged device too, but I might not put that very high on my
likelihood list.  It seems that the cohesive strength of the Si is greater
than the interfacial strength between the Si and the mold compound.

I have not seen it inside packaged devices, but can acknowledge its
possibility - Can you afford to look at a part using reflected acoustics?
If you could 'safely' get a chip off the board without causing more damage,
one could look at it in a through-transmission mode.  If the separation were
as gross as you indicate, it should show up very easily with a low freq
transmission mode transducer.

Another option may be to do some fine focus x-ray examination of the board
and suspect vias to satisfy yourself of their integrity, then sacrifice the
IC by chemical de-encapsulation processes.. 

I've only seen device cracks like you describe when large CTE's/and or
thermal excursions were involved - [eg - power MOSFET soldered on Cu and
power cycled to death, or large chips using unfilled (optical) molding
compounds].

You have all the fun parts!

Steve C



-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Inge
Sent: Wednesday, November 25, 2009 11:03 AM
To: [log in to unmask]
Subject: [TN] Preparing for microvia examination

Schrekxscht!
That was the way the aliens from planet Mars saluted us in the funny film
that ended in their heads exploding when a guy played Elvis Presley on his
car radio. What does that have to do with microvias?  Anyone with experience
from cutting a board into small pieces and mold them in epoxy for cross
section analysis? I use a thin blade, high rpm diamond blade for cutting.
The board was equipped with SuperBGAs down to smallest QFNs. We suspected
via problems and looked for such after polishing. In fact we found extremly
good barrels and no sign of via issues. Instead we saw that some
cross-sectioned semi chips had fine microcracks. In some cases, the
microcrack ran all the way across the chip, i.e. the whole active area was
more or less separated from the rest of the chip.My q is this:  has anyone
of you got microcracks in the semi chips caused during the cutting
operation? I,ve cross sectioned hundreds of boards during my time here, but
never seen cracks like these be caused by the diamond cutting. Can there be
a Schrekxscht when you use the diamond saw?
/Inge

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