November_EDFA_Digital

ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 20 NO. 4 34

GEOLOCATION OF CUWIRES DURING SENSITIVE IC ACID DECAPSULATION (continued from page 31)

The result is: • Oxidation occurs at the anode: Cu = Cu 2+ + 2e-/ Reduction occurs at the cathode: 2H + + 2e - → H 2 • Sulfate ions are attracted by the anode: Cu 2+ + SO 4 2- = CuSO 4 • CuSO 4 is deposited on the anode, which insulates the electrode. Intensity diminishes. Under the following conditions, testing confirmed the above result: • Anode: Copper electrode • Cathode: Stainless steel electrode • Electrolyte: 5:1 nitric/sulfuric • Voltage: 15 V When the sample is dampened prior to decapsulation (Fig. 12), a noticeable decrease in intensity occurs—and etching takes approximately 30 times longer than with a dry sample (Fig. 13). The intensity was observed to be roughly twice as high in a dry sample than a dampened sample. This confirms that a samplemust be dried before being etched. Intensity represents the crossingof electrons inside the circuit and consequently the creation of Cu 2 + at the elec- trode surface, hence the copper dissolution. The intensity decrease that is observed, more or less rapidly depending on the conditions, corresponds to the protectionof copper by the creation of a passive layer. To see if it was possible to distinguish the various areas reached during the acid etch, Digit Concept has been investigating the current signature during polarization. In

Fig. 11 One of the first acid decapsulation tools with the use of polarization, 1992.

The article “Failure Mechanisms in Encapsulated Copper Wire-bonded Devices” [6] proves the efficiency of electrolysis on components with copper bondings. As described in that article, electrolysis consists of polarizing the component pins so that the anions (sulfate anions in this case) go to the anode (bondingwires), thus generating a protective layer on the wire surface. The goal of the Digit Concept research on bonding detectionwas to pinpoint themoment when the bonding wires became uncovered. Because acid is conductive and electrolysis has nonegative impact on copper wires, it was possible tomeasure the intensity during contact between the acid and the bonding wire. When current measure- ment was combinedwith electrolysis on the copper wires, progression of the etch process was able to be detected. A change in intensity all along the etching process was observed. Indeed, the current passes through the circuit due to ions moving inside the solution as well as electrons moving in the electrodes and external wires. Some exchanges of the charge carriers occur at the inter- face’s electrode solution, which corresponds to electron exchanges between ions and the electrodes, namely localized electrochemical reactions at the surface of the electrodes. As the component remains functional after an opening with electrolysis, it was confirmed that the observed intensity variation corresponds to the progres- sion of the etching process. The copper is protected at first, but as it is uncovered by acid, the intensity tends to diminish. This is in accordance with a 2016 publication, [6] which confirms that a layer of sulfate ions is deposited on the copper. Because solid CuSO 4 is electrically insulating, this ties in with the results.

Fig. 12 Dampened Cu inside N:S 5:1 with 15 V polarization. Fig. 13 Dry Cu inside N:S 5:1 with 15 V polarization.

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