Hi Steve

We use nitric acid. The acid MUST be water- free ( purity for analytic reasons ) otherwise the aluminium will be eaten.

1) Examine the specimen with X-ray to find the chip
2) Grind a shallow deepening in the encapsulation above the chip
3) Heat the specimen to approx. 80 deg.C
4) Apply a droplet of acid into the deepening
5) Wait until the acid is getting murky or thickening
6) Rinse with Acetone
7) Repeat from point 4 until the chip is free

Precautions:

Nitric acid is corrosive thus
- Wear protective equipment such as gloves, protective clothing, glasses
- Use in a chemical compartment with forced air circulation

The acid- epoxy mixture mustn't become dry. The resulting crust is extremely difficult to remove
Do not use water or alcohol to rinse

Once you have the know how it's no big deal.

The equipment that Mike mentioned is called Jet-Etch. we did not make good experiences with it:
- For each opening size you want to etch you need another Teflon- mask ( very expensive )
- The equipment needs 2 litres of nitric acid that becomes contaminated in use
- The acid is heated up to 80 deg. C. In order to avoid the build up of pressure the fumes are leaving the apparatus by a tube. In our case the fumes condensed at the wall of the cabinet causing a flood of nitric acid that was not funny to clean up.

Best regards

Guenter

Guenter Grossmann

Swiss Federal Institute for Materials Testing and Research EMPA
Centre for Reliability
8600 Duebendorf
Switzerland

Phone: xx41 1 823 4279
Fax :      xx41 1823 4054
mail:     [log in to unmask]

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