I've spent from 6:30 until now (8:50) with Jim Lohr, who have reviewed for me the materials he has, and other stuff he knows about, for the wire bond rupturing concern.  (I know there are many other problems and concerns with this assembly, but we only talked about the wire bonds and the flex leads.)


I think that these data support a particular root cause for the wire bond ruptures.


The design, glob-top and all, is OK, on the evidence of more than two dozen full-up boards made with either zero wire bond breaks, or a very few.


Then, on a particular production day late last spring, boards for the first time began to show hundreds of open wire bonds. After than, there were some days on which boards with zero defective wire bonds were made, and other days on which the boards had many dozens of broken wire bonds.  This strongly suggests a production-lot related problem.


A number of observations (Diane Kolos's images, observations by several different people at different times and locations) show that the glob-top resin is debonded and separated from the pc board material at (and near) the wire bonding site.  This places huge stresses onto the bond heel, which Diane's images show has (in two distinct cases) ruptured unter tensile stress --- just as we would expect to happen when this separation happens.


Inspection of the assembly process has shown that a Kapton tape was applied to precisely the region that was later used for wire bonding, and on the ends that are rupturing.  Vapor degreasing and plasma etching were used to prepare this region for later wire bonding.


Kapton tape is supplied by MMM with two alternative adhesives: acrylic-based or silicone-based.  We know that the manufacturer had silicone-based adhesive in its assembly areas: Jim cound not say whether we know whether this silicone-based Kapton tape was applied to the MCMs, or not.  Now, all we can say is that this is possible.


It would certainly explain the lack of adhesion of the glob-top to the polyimide pc board: silicone contaminants cannot be removed by normal vapor degreasing or by normal plasma etching or combinations of these. And silicone contamination is effective at limiting adhesion of glob-top resins (including resins) in monolayer thicknesses.


Diane removed the glob-top resin from a specimen she was working on, and noted that the adhesion was sharply limited or not present. Examination of the exposed surface using EDS did not show any contaminating materials --- but it could not have seen anything thinner than about 1 um, and silicone is effective as an adhesion-limiting layer at a thickness of less then 1 nm --- a thousand times thinner than EDS can "see".


Jin and I sketched some other tests that can be carried out to gather evidence bearing on this possible root cause.  We hope these may help in deciding what risk there is in using the several hundred already made assemblies.



Henning Leidecker