Electroless Ni Immersion Au Research Articles

Vertical Interconnections by Electroless Au Deposition on Electroless Ni Immersion Au Surface Finish

Electroless Au is used to vertically connect copper pillars with electroless Ni immersion Au surface finish on two stacked chips by a microfluidic electroless interconnection process. A very low bonding temperature of 50°C and a pressure-less bonding process can be achieved. The vertical interconnections are formed by a forced flow of electroless Au plating solution through a microchannel so that the reduced Au atoms self-assemble between the gaps of facing copper pillars to complete the interconnections. The deposited Au grains span the entire gap across the two copper pillars, suggesting that the rate-limiting step is more likely to be the nucleation rather than the growth of the Au grains. Four-point probe measurements show that the average resistance of the electroless-Au-bonded interconnections is low. Mechanical shear testing reveals that the bonding of the interconnections is strong. Furthermore, it is demonstrated that this process is capable of accommodating a high degree of pillar misalignment.

Effect of surface finish metallization on mechanical strength of Ag sintered joint

A micro-sized Ag paste was fabricated with the metal content of approximately 85% as a sinter-bonding material in this study. The sintering reactions and joint strengths of Ag sintered joints with three different surface finishes of Cu, Ag, and electroless Ni-immersion Au (ENIG) were evaluated at different sinter bonding times during the sintering process. Stable interfacial microstructures and relatively dense Ag sintered layers were formed in the three Ag sintered joints, and the interdiffusion behaviors between the Ag paste and substrate metals promoted in the formation of good metallurgical bonding during the Ag sintering process, regardless of surface finish. The bonding time of 10 min was sufficient for full sintering reactions between the chip and different substrate finishes. The shear strength did not increase with increased sintering time regardless of surface finish. The substrate metals strongly affected the die shear strengths of the Ag sintered joints, and the Ag-finished joint had a more stable interfacial microstructure and superior shear strength compared to the other metal-finished joints.

Electromigration Behavior of through-Si-via (TSV) Interconnect for 3-D Flip Chip Packaging

The electromigration of conventional Sn-37Pb (in mass%) solder bumps was investigated with a current density of 2.0 x 10 4 A/cm 2 at 393 K using three-dimensional (3-D) flip-chip specimens comprised of an upper Si chip and a lower FR-4 substrate. Electromigration failure of the Sn-37Pb solder bumps occurred with complete consumption of electroless Ni immersion Au (ENIG) under bump metallization (UBM) and void formation at the cathode side of the solder bump. Cross-sectional studies were conducted with scanning electron microscopy (SEM). Ni 3 Sn 4 intermetallic compound (IMC) layers were formed in both interfaces, while a typical eutectic structure of Pb- and Sn-rich phases was formed within the solder region. After 11 h current stressing, separation of the Pb- and Sn-rich phases occurred in the solder bump, while a void was formed in the cathode-side bump. After 18 h current stressing, the solder joints catastrophically failed due to the re-melting of the solder bump.

Reliability of Au bump flip chip packages with adhesive materials using four-point bending test

In this study, the bending fatigue reliability of Au bump flip chip packages with anisotropic conductive film (ACF) and non-conductive film (NCF) as adhesive materials was evaluated using a four-point bending test. Differential bending between the substrate and the Si chip was the dominant failure driver for the Au bump flip chip packages subjected to the bending test. The bending fatigue life of the NCF joint was longer than that of the ACF joint, regardless of the bending frequency. The main reason for the increased electrical resistance of the packages with ACF and NCF was the delamination between the adhesive material and the electroless Ni-immersion Au (ENIG) pad and between the Au bump and ENIG pad, respectively. For the ACF package, the bending stress was concentrated at the conductive particles between the Au bump and the ENIG pad during the bending fatigue test, resulting in a low bending fatigue strength. The delamination in the packages propagated with increasing bending fatigue cycles and frequency. The Au bump flip chip package with NCF was mechanically robust due to the higher initial strength and the larger bonding area between the Au bump and the ENIG pad.

Electromigration Behavior in Sn-37Pb and Sn-3.0Ag-0.5Cu Flip-Chip Solder Joints under High Current Density

The electromigration of conventional Sn-37Pb and Pb-free Sn-3.0Ag-0.5Cu (in wt.%) solder bumps was investigated with a high current density of 2.5 × 104 A/cm2 at 423 K using flip-chip specimens comprised of an upper Si chip and a lower bismaleimide triazine (BT) substrate. Electromigration failure of the Sn-37Pb and Sn-3.0Ag-0.5Cu solder bumps occurred with complete consumption of electroless Ni immersion Au (ENIG) underbump metallization (UBM) and void formation at the cathode side of the solder bump. Finite element analysis and computational simulations indicated high current crowding of electrons in the patterned Cu on the Si chip side, whereas the solder bumps and Cu line of the BT substrate had a relatively low density of flowing electrons. These findings were confirmed by the experimental results. The electromigration reliability of the Sn-3.0Ag-0.5Cu solder joint was superior to that of Sn-37Pb.

Effect of Stress State on Growth of Interfacial Intermetallic Compounds Between Sn-Ag-Cu Solder and Cu Substrates Coated with Electroless Ni Immersion Au

Excessive intermetallic compound (IMC) growth in solder joints will significantly decrease the reliability of the joints. IMC growth is known to be influenced by numerous factors during the component fabrication process and in service. It is reported that, other than temperature and holding time, stress can also influence the IMC growth behavior. However, no existing method can be used to study the effect of stress state on IMC growth in a controlled manner. This paper presents a novel method to study the effect of stress on interfacial IMC growth between Sn-Ag-Cu solder and a Cu substrate coated with electroless Ni immersion Au (ENIG). A C-ring was used and in-plane bending induced tensile and compressive stresses were applied by tightening the C-ring. Results revealed that in-plane compressive stress led to faster IMC growth as compared with in-plane tensile stress.

Effects of electromigration on microstructural evolution of eutectic SnPb flip chip solder bumps

The flip chip solder bump was investigated in situ with current stressing in an ambient temperature of 423 K. For the in situ investigation of the solder joints, a single flip chip package was examined during whole investigation time. Cross-sectional studies were conducted with the scanning electron microscope (SEM). Cu mini bumps were formed on a Si chip for the solder bumping and electroless Ni-immersion Au (ENIG) surface treatment was conducted on the Cu pads in substrate side. Cu 6Sn 5 and Ni 3Sn 4 intermetallic compound (IMC) layers were formed in the both interface, while a typical eutectic structure of Pb-rich and Sn-rich phases were formed within the solder region. After current stressing of 6 h, separation of the Pb-rich and Sn-rich phases were occurred in the left solder bump, while a void was formed under the Cu 6Sn 5 IMC layer in the right side solder bump. After current stressing of 7.5 h, the solder joints were catastrophically failed due to the remelting of the solder bump.

Solid-state interfacial reactions between Sn–3.5Ag–0.7Cu solder and electroless Ni-immersion Au substrate during high temperature storage test

The interfacial reactions between Sn–3.5Ag–0.7Cu solder and an electroless nickel-immersion gold (ENIG)-plated Cu substrate were investigated during aging at 200 °C for up to 1000 h. During aging, the Ni(P) layer was transformed partially into an Ni 3P layer, and then the Ni 3P layer was also transformed into an Ni–Sn–P ternary layer. After aging for 1000 h, Cu 6Sn 5 and Cu 3Sn IMCs were formed below the Ni–Sn–P layer. During aging at 200 °C, the interfacial IMC was transformed sequentially into (Cu,Ni) 6Sn 5, (Ni,Cu) 3Sn 4 and finally (Cu,Ni) 6Sn 5.