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May 2003

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From:
Brian Ellis <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Sat, 24 May 2003 08:57:39 +0300
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Just a little addition to previous responses. You haven't said what kind
of instrument, tank size, solution volume and composition, etc.

Many years ago, my company (now defunct, to avoid charges of
commercialism) made the Microcontaminometer for applications such as
these. The smallest tank size was 26 x 26 x 5 mm, to take a single 1"
hybrid circuit and it had what was probably the most sensitive
conductivity cell in the world. I'm sure this would have given
meaningful results with a single flip chip. We had one client who used
it to measure the incoming ionic contamination individually on every one
of an ultra-critical high-impedance component with a glass casing about
2 mm diameter and 5 mm long (from memory), for example.  The MCM-1, of
which there were about 25 or 30 made, would be fairly useless now, as it
used an HP-85 computer to drive it and these went out of production in
1987. It was replaced by the PC-driven MCM-2, which was a generally
better instrument. If you could trace one of these on the 2nd hand
market, in working order, this would be your ideal solution, but there
weren't many made, possibly less than 10.

Whichever instrument you use, I suggest you use a 50% IPA solution. This
works better with W/S flux residues and gives you twice the sensitivity.
Use the smallest tank, available (make an ad hoc small one, if possible).

As has been said, the absolute value of ionic contamination is fairly
meaningless, but it can be used for process control by determining the
statistical consistency of the results. As the absolute value is not
meaningful, it doesn't really matter two hoots which area you use,
provided you always use the same.

Brian

Glenn Pelkey wrote:
> Hi Technetters,
>
>         My buddy and I have a disagreement on using Resistivity of
> Solvent Extract (ROSE) for measuring cleanliness on a Flip Chip board.
> So, we're throwing it to league of experts on this forum for feedback
>
>         Here's the story:  The board and assembly are very simple.  One
> flip chip die at about 0.002 square inches and one board about 0.490
> square inches.  Flux is applied only to the flip chip die, reflowed, and
> DI inline washed.
>
>         Here's the disagreement:  I say use 2X the surface area of the
> die for the calculation.  My good friend says use 2X the board surface.
> Of course, you can see one number is much higher than the other in ug/sq
> inch NaCl equiv.  My logic for using only the die area is to understand
> the process cleanliness at that location only, but I recognize the
> entire board surface could be cross contaminated in some manner.  We
> also loaded the machine with 32 of these units to get something
> measurable.  When we only put 4 in, nothing showed up, but the smallest
> entry for surface area is 0.1 square inches.
>
> Thanks for your input!
>
> Glenn
>
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