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ACID DECAPSULATION As a result, Digit Concept has been using the threewet chemical techniques known to be compatiblewith copper in automated acid equipment (iSA777): LowTemperature, recently lowered from10°C to 5°C [3] Acid Saturation with copper [4] Polarization of acid [5] LOW TEMPERATURE In 2011, it was demonstrated that temperature signifi- cantly changed the nondegradation of Cu bondings when this temperature was lower than ambient. When using a mix of 3:1HNO 3 H 2 SO 4 20%SO 3 , thedegradationof Cuwires was insignificant below 15°C. After a rinsing process with acetone, full contact of the circuit in 70°C water allowed surface oxidation to preserve the Cu wires. ACID SATURATION A recent presentation [5] confirmed that Cu satura- tion in nitric acid helped avoid damages to the Cu parts used in modern packages. Some examples are shown in Figs. 3-8. Other trials were performed with Al saturation, as shown in Figs. 9 and 10.

ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 20 NO. 4

Fig. 5 Flip chip package after opening. Fig. 6 Detail of Cu, not etched.

Fig. 7 Flip chip package after local opening. Fig. 8 No damage to Cu redistribution at top die level.

POLARIZATION In 1988, Dr. Grossenbacher was one of the first to use an acid polarization technique to detect contact between wires and acid. The polarization current greatly increases when this phenomenon occurs. In the 1990s, Rudolf Wolf noticed that the copper die pad was protected by a layer and used the polarization technique in the AS2000 decapsulation system. When this technologywas used on Cu/BCB, the three wet chemical techniques were func- tional, but required etch stops at the correct times to prevent etching of polyimide. During those tests, using the method of polarization at constant voltage, it was discovered that the current varied significantly over time. Further investigationswere conducted to understand and make use of this phenomenon. Fig. 9 Copper die pad and aluminum die surface without damage with HNO 3 -Cu + H 2 SO 4 -Al. Fig. 10 SEM photo of Al wire and Al die surface.

Fig. 2 SEMphotoofCuwirecrosssectionafterdecapsulation by nitric/sulfuric acidmix. In this case, the hot water rinse was not used and the resulting crystal growth was analyzed by EDX.

Fig. 3 HNO 3

with Cu after saturation turns to green color.

Fig. 4 Preservation of the entire frame in Cu.

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