Neubauer, Terri wrote:

> Hi TechNetters:
>
> I am interested in non-destructive testing of bare printed circuit boards.
> Has anyone used neutron radiography?  The information that I've managed to
> gather has been both scarce and contradictory as far as places that conduct
> testing and the usefulness of the results.  Can anyone shed some light on
> this topic?
>
> Thanks very much!
>
> Terri K. Neubauer
> Northrop Grumman - ESSD
> [log in to unmask]
>
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Hi Terri--

Over 10 years ago (in my previous work life) I had some experience using neutron-radiography as an
inspection tool for some on some heat exchangers that I was responsible for the design and manufacture
thereof.

What we did was to chemically mill a heat exchanger pattern of various sized ducts, manifolds, etc. about 1
mm deep into a sheet of 1.6 mm thick copper that was about 400 mm square.  The etched pattern was about 300
mm square, and consisted of a 30 X 30 matrix of individual heat exchanger locations on 15 mm centers. At
each of the 900 locations, there was to be mounted a "flip-chip" integrated circuit, mounted on a rigid
transposer, to convert from the die terminals to a pin-grid array.

 The etched patterns were then "wax" filled, surface ground flat, the wax was "sprinkled over" with silver,
and then the surface was electroplated with Cu for about 0.7 mm.  Holes were then drilled from the outer
surface to access the inner heat exchanger manifolds, the heat exchanger was then slowly heated from the
access holes to the other portions of the exchanges, a hot solvent was then pumped through the exchangers
channels to flush and clean the wax out of them.  There wasn't much contrast difference between the
residual organics (wax) and the copper using traditional x-ray radiography.  So we went to a local
radiographer to have neutron radiography evaluations performed.  These worked very well as the neutron
radiography have far superior resolution than x-ray, and to neutron radiography,  the copper is almost
transparent and organics are more dense.  We were able to locate all areas where the wax had not been
removed.   I can remember in the literature a neutron radiograph of a steam locomotive engine, the
detailing was fantastic, you could see just about every rivet in the pressure boiler.

Hope this helps

Ralph

Ralph Hersey & Associates
3885 Mills Way
Livermore, CA 94550-3319
PHN/FAX: 925.454.9805
e-mail: [log in to unmask]

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