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Old 09-05-2005, 02:31 PM
Grant Stockly Grant Stockly is offline
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<Last part of the previous post>

Perhaps it's time to comment on this thread.

Dichloromethane and dimethylformamide are relatively effective disrupters of most epoxies but their action is accompanied by great swelling because the polymer becomes engorged with the liquid before any significant solvation takes place. This will destroy wire bonds on an IC.

Fuming (essentially anhydrous) sulfuric acid acts by the completely different process of sulfonating reactive groups that remain on the polymer. The depolymerized and sulfonated byproducts are quite soluble not only in the acid but usually in water as well. The worst thing that you could do in this relatively straightforward process is to wash with water at intervals because this would initiate almost instantaneous corrosion. It would be advisable for a chemist, as someone trained in the handlingof reactive materials, to carry this out or at least to establish procedures and train others with less experience. The action of sulfuric acid in this regard is quite different than that of nitric. Nearly anhydrous nitric acid (completely anhydrous is extremely difficult to prepare) is a very powerful oxidizer and could lead to unstable, dangerous byproducts whereas the sulfonates resulting from the sulfuric acid reaction are relatively stable. Water must, of course, be prevented from splashing into any concentrated acid, especially sulfuric.

A very strong acid such as sulfuric behaves completely differently in the absence of water. Since most acids are highly hygroscopic and are sold as water solutions, most people do not observe this other side of their behavior. Without water to create an ionized electrolyte, corrosion of metals will not take place. I have de-encapsulated ICs for failure analysis in 200 degree sulfuric acid and been able to operate the IC without replacing the .001" aluminum wirebonds that it came with. I recall one instance where our company built prototype hybrid microelectronic circuits out of such de-encapsulated ICs when their supplier was late getting a new design on the market and the only ones available were already encapsulated.

The key is to realize that water must be excluded until the sulfonating acid has been completely rinsed away by a non-aqueous liquid. As Mr. Saxon said, there are simple and safe devices available for doing this operation. However, with proper care and protective gear it can be done in a beaker on a hot plate in a fume hood. A few ml.s of sulfuric acid are heated to drive off water until heavy vapors are observed over the liquid (which may darken during heating due to trace impurities). The IC is carefully lowered into the hot acid and a vigorous reaction ensues with the epoxy almost instantly washing into the solution. After a few seconds the IC is then quickly lifted out and held over a receiving vessel and flooded with a stream of ethanol. Only after this is a final rinse in deionized water carried out, followed by fresh electronic grade ethanol and forced drying in warm air.

The ready made devices which carry out the operation are typically a small bowl with a hinged lid from which air is withdrawn by a gentle vacuum. An inert metal feeder tube leads from a heated reservoir for the sulfuric acid and passes through the wall of the bowl to a position where the encapsulated device is secured. When the lid is closed and the slight vacuum applied, the hot acid is pulled into the bowl over the device. It is somewhat self-limiting in that, if the lid is opened, there is no driving force to bring more acid into the container. Naturally, the vacuum source needs to be protected by a trap and all waste products properly handled no matter how the procedure is carried out.

John Twilley
Conservation Scientist
(formerly, Manager of the Reliability Analysis Center, Teledyne Microelectronics)

From daemon Fri Jan 25 08:35:53 2002
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