Under Bump Metallization Structure Research Articles

Effect of Electromigration-Induced Joule Heating on the Reliability of Sn-Ag Microbump with Different UBM Structures

The effect of current stress-induced Joule heating on two different under-bump metallization (UBM) structures in Sn-Ag microbumps was investigated with current stressing at 150°C and a current density of 5 &!thinsp;× 104 A/cm2. Both Ni UBM and Cu UBM configuration microbump structures underwent extensive electromigration (EM) testing, with results revealing a longer lifetime with the Cu UBM configuration than the Ni UBM structure. The observed EM failure mechanism in the Ni UBM configuration was identified as a void formation within the bump interconnected Al trace and not due to damage accumulation inside the microbump structure. The intermetallic compound developed inside the microbump was formed and maintained its stability throughout the current stressing period. To identify the main driving force of damage accumulation in the Al trace, the current density and temperature distributions in the Sn-Ag microbumps were analyzed numerically via the finite element method. The simulation results showed higher Joule heating with the Ni UBM than the Cu UBM microbump configuration, along with the bump geometrical contribution of add-on higher Joule heating in the Ni UBM microbump structure.

Finite element modelling for the investigation of edge effect in acoustic micro imaging of microelectronic packages

In acoustic micro imaging of microelectronic packages, edge effect is often presented as artifacts of C-scan images, which may potentially obscure the detection of defects such as cracks and voids in the solder joints. The cause of edge effect is debatable. In this paper, a 2D finite element model is developed on the basis of acoustic micro imaging of a flip-chip package using a 230 MHz focused transducer to investigate acoustic propagation inside the package in attempt to elucidate the fundamental mechanism that causes the edge effect. A virtual transducer is designed in the finite element model to reduce the coupling fluid domain, and its performance is characterised against the physical transducer specification. The numerical results showed that the under bump metallization (UBM) structure inside the package has a significant impact on the edge effect. Simulated wavefields also showed that the edge effect is mainly attributed to the horizontal scatter, which is observed in the interface of silicon die-to-the outer radius of solder bump. The horizontal scatter occurs even for a flip-chip package without the UBM structure.

The effect of pre-aging on the electromigration of flip-chip SnAg solder joints

The effect of pre-aging on electromigration is investigated in this study using flip-chip SnAg solder joints. The solder joints were pre-aged at 170°C for 1 h, 3 h, 5 h, 10 h, 25 h, and 50 h, and then they were subjected to electromigration tests of 0.9 A at 150°C. It was found that the average failure time increased about three times when the joints were pre-aged for 3 h to 25 h. But it decreased when the joints were overaged. It is proposed that the major contributor to the prolonged failure time may be the densification of the nickel and copper under-bump metallization (UBM) and the solder due to the aging treatment. The pre-aging treatment at 170°C may stabilize the microstructure of the solder. The vacancies in the solder were annihilated during the heat treatment, causing a slower diffusion rate. In addition, the UBM structure became denser after the pre-aging process. Thus, the denser UBM structure may lead to slower consumption rates of the nickel and copper layers, resulting in the enhancement of electromigration resistance.

Preparation and Temperature Cycling Reliability of Electroless Ni(P) Under Bump Metallization

The reliability of electroless Ni(P) under-bump metallization (UBM) was evaluated via temperature cycling and solder bump shear strength tests. Commercial diodes and dummy dies fabricated in-house were used as substrates for the electroless Ni(P) UBM deposition. Solder bumps were formed after reflowing eutectic 63Sn37Pb solder foils over the Ni(P) UBM. The solder bump shear strength was measured before and after different temperature cycling. The results from this study showed that the UBM thickness and dimension had important effects on the solder bump shear strength and reliability. Both the larger UBM dimension and larger UBM thickness tended to induce higher stress in the UBM, which resulted in the lower solder bump shear strength and lower temperature cycling reliability. A better UBM structure solution for high current electronic packaging application is indicated in this paper

Damages and Microstructural Variation of High-lead and Eutectic SnPb Composite Flip Chip Solder Bumps Induced by Electromigration

The electromigration behavior of the high-lead and eutectic SnPb composite solder bumps was investigated at 150 °C with 5 × 103 A/cm2 current stressing for up to 1711 h. The diameter of the bumps was about 125 μm. The underbump metallization (UBM) on the chip side was sputtered Al/Ni(V)/Cu thin films, and the Cu pad on the board side was plated with electroless Ni/Au. It was observed that damages occurred in the joints in a downward electron flow (from chip side to the substrate side), while those joints having the opposite current polarity showed only minor changes. In the case of downward electron flow, electromigration damages were observed in the UBM and solder bumps. The vanadium in Ni(V) layer was broken under current stressing of 1711 h while it was still intact after current stressing of 1000 h. The electron probe microanalyzer (EPMA) elemental mapping clearly shows that the Al atoms in the trace migrated through the UBM into the solder bump during current stressing. Voids were found in the solder bump near the UBM/solder interface. The Sn-rich phases of the solder bumps showed gradual streaking and reorientation upon current stressing. This resulted in the formation of uniaxial Sn-rich phases in the middle of the solder bump, while the columnar and fibrous Sn-rich phases were formed in the surrounding regions. The formation mechanism of electromigration-induced damage to the UBM structure and solder bump were discussed.