Yuan-Wei Chang

Electromigration Mechanism of Failure in Flip-Chip Solder Joints Based on Discrete Void Formation

In this investigation, SnAgCu and SN100C solders were electromigration (EM) tested, and the 3D laminography imaging technique was employed for in-situ observation of the microstructure evolution during testing. We found that discrete voids nucleate, grow and coalesce along the intermetallic compound/solder interface during EM testing. A systematic analysis yields quantitative information on the number, volume, and growth rate of voids, and the EM parameter of DZ*. We observe that fast intrinsic diffusion in SnAgCu solder causes void growth and coalescence, while in the SN100C solder this coalescence was not significant. To deduce the current density distribution, finite-element models were constructed on the basis of the laminography images. The discrete voids do not change the global current density distribution, but they induce the local current crowding around the voids: this local current crowding enhances the lateral void growth and coalescence. The correlation between the current density and the probability of void formation indicates that a threshold current density exists for the activation of void formation. There is a significant increase in the probability of void formation when the current density exceeds half of the maximum value.

A new failure mechanism of electromigration by surface diffusion of Sn on Ni and Cu metallization in microbumps

Microbumps in three-dimensional integrated circuit now becomes essential technology to reach higher packaging density. However, the small volume of microbumps dramatically changes the characteristics from the flip-chip (FC) solder joints. For a 20 µm diameter microbump, the cross-section area and the volume are only 1/25 and 1/125 of a 100 µm diameter FC joint. The small area significantly enlarges the current density although the current crowding effect was reduced at the same time. The small volume of solder can be fully transformed into the intermetallic compounds (IMCs) very easily, and the IMCs are usually stronger under electromigration (EM). These result in the thoroughly change of the EM failure mechanism in microbumps. In this study, microbumps with two different diameter and flip-chip joints were EM tested. A new failure mechanism was found obviously in microbumps, which is the surface diffusion of Sn. Under EM testing, Sn atoms tend to migrate along the surface to the circumference of Ni and Cu metallization to form Ni3Sn4 and Cu3Sn IMCs respectively. When the Sn diffuses away, necking or serious void formation occurs in the solder, which weakens the electrical and mechanical properties of the microbumps. Theoretic calculation indicates that this failure mode will become even significantly for the microbumps with smaller dimensions than the 18 µm microbumps.

Study of discrete voids formation in flip-chip solder joints due to electromigration using in-situ 3D laminography and finite-element modeling

Nowadays, the microelectronics industry broadly uses the flipchip technology to enhance the packaging density. However, the small size and the unique geometry of the flip-chip solder joints induce the electromigration (EM) reliability issue. In this study, a Pb-free solder joints (SAC1205) was EM tested by a current of 7.5×103 A/cm2. During the tests, a three-dimensional (3D) X-ray laminography method was applied to in-situ observe the microstructure evolution. The laminography method allows for the non-destructive observation and provides the quantitative analysis among three dimensions. After EM testing for 650 hr, a new EM failure mechanism was found rather than the well-known models, the pancake void propagation and the under-bump-metallization dissolution. According to the laminography images at different testing stages, many voids simultaneously formed and grew during the entire procedure of testing. Most of them distributed in the current crowding region, but a few also located in the low-current-density region. As the testing time increased, voids grew bigger, coalesced with each other, and finally became large voids which occupied the interface and caused EM failure. The finite-element (FE) method was also applied to analyze the interplay between the microstructure evolution and current density redistribution. A series of 3D FE models were built based on the laminography images at different testing stages. The current density distribution from the FE analysis indicates that the multiple voids formation does not affect the global current density distribution until the voids merged together and became very large voids in the late stage of EM testing. The relieving of the global current crowding in the pancake void model was not found in this new EM failure mechanism. It was the local current crowding found in the new model that responsible for the EM retardation.

Microstructure evolution in microbumps for 3D-IC packaging

In this study, we would analyzed the solid state reaction between Sn2.5Ag solder bump and Cu/Ni under-bump-metallization (UBM). After 150 °C thermal aging, we observed that the intermetallic compounds (IMCs) at chip side and interposer side both were Ni<inf>3</inf>Sn<inf>4</inf> IMCs. It indicated that the solder did not react with Cu and the Cu layer was completed. As thermal aging time increased, the thickness of Ni<inf>3</inf>Sn<inf>4</inf> IMCs and the Ag<inf>3</inf>Sn grain size increased. In addition, the dispersed Ag<inf>3</inf>Sn compound would aggregate to form plated Ag<inf>3</inf>Sn compound as the thermal aging time increased. The growth kinetics of Ni<inf>3</inf>Sn<inf>4</inf> was volume diffusion (n=0.5) domination the diffusion process of Sn and Ni atoms. We calculated the Ni<inf>3</inf>Sn<inf>4</inf> IMCs growth rate constant was 0.067 μm/hr^½ at 150 °C aging. As the aging time increased, the concentration of Ag in the remaining solder bumps also increased. When the concentration of Ag was over 3.5 wt.%, the probability of plate-like Ag<inf>3</inf>Sn compound appeared in the solder bumps would increased. We used the Ni<inf>3</inf>Sn<inf>4</inf> IMCs growth rate constant to define the critical volume of Sn2.5Ag solder was 1621.0 μm³. If the volume of the Sn2.5Ag microbumps were below the critical volume, the concentration of Ag in the remaining solder would over 3.5 wt.%, and the plate-like Ag<inf>3</inf>Sn would appear in the remaining solder after 1000-hr thermal aging at 150°C.

Agglomeration of Ag 3 Sn compounds in microbumps during reflow for 3D-IC packaging

In this study, we study the metallurgical reactions at liquid state for different thickness Sn2.5Ag solder microbumps with Cu/Ni UBM. After 260 °C reflow, we observed that the intermetallic compounds (IMCs) at chip side and interposer side both were Ni<inf>3</inf>Sn<inf>4</inf> IMCs. As reflow time increased, the thickness of Ni<inf>3</inf>Sn<inf>4</inf> IMCs and the concentration of Ag in the solder increased. We calculated the Ni<inf>3</inf>Sn<inf>4</inf> IMCs growth rate constant was 0.45 μm/min^1/2. In addition, the dispersed Ag<inf>3</inf>Sn compound agglomerate to form plate-like Ag<inf>3</inf>Sn compound as reflow time increased. We used the Ni<inf>3</inf>Sn<inf>4</inf> IMCs growth rate constant to define the critical volume of Sn2.5Ag solder was 1088 μm. If the volume of the Sn2.5Ag microbumps was below the critical volume, the concentration of Ag in the remaining solder would over 3.5 wt.%, and the plate-like Ag<inf>3</inf>Sn would appear in the remaining solder after 10 min of reflowing at 260 °C.