Study of 0.6mil silver alloy wire in challenging bonding processes
With competitive price and superior electrical/thermal conductivity and mechanical properties, more and more IC package industries have adopted copper (Cu) and palladium coated copper (PdCu) wires as the alternative to gold (Au) wire in the past decade. However, the high hardness and the excessive ultrasonic energy and bonding motions required during bonding of Cu wire limit its usage in areas such as memory packages and sensitive devices that are prone to damages on the pads and under-layer dielectrics. Silver (Ag) and Ag-alloy wires emerge as other alternatives since they have similar properties like wire hardness, elongation and breaking load at room temperature as Au wire while having a more competitive price. Process capability of ultra-fine (0.6mil) Ag-alloy wire, including free air balls (FAB) and bonding capability on aluminum (Al) die pads were first investigated. Factors affecting the FAB performance were studied and optimum settings were recommended. Comparison of bonding responses between Au, PdCu and Ag-alloy wires on challenging scenarios, such as overhang dies and die-to-die bonding, were also included in the study. Benchmarked Au process, Ag-alloy wire possesses great portability and wide first bond process window. Ag-alloy wire also demonstrates good bonding capability and loop shape control on challenging applications, such as overhang die and long die-to-die applications. However, further optimization of Ag-alloy process is still necessary to overcome its constrains, such as higher hardness and higher energy required during bonding of Ag-alloy wire.
A Study on the Effect of Bond Stress and Process Temperature on Palladium Coated Silver Wire Bonds on Aluminum Metallization
In the past ten years, the increasing price of gold has motivated the wire bonding industry to look for alternative bonding wire materials in the field of microelectronics packaging. A new candidate wire to replace gold is palladium coated silver wire. In this thesis, the effect of the two specific process parameters “bond stress” and “process temperature” on the ball bonds made with the new candidate wire are investigated. Using 20 μm diameter wire and various level-combinations of these process parameter, ball bonds are produced according to a special accelerated optimization method to result in a target diameter of 46 ± 0.5 μm and target height of 16 ± 0.5 μm. Three different levels are used for each of the specific process parameters. After pre-selecting a few process parameters, the accelerated method determines the levels for the process parameters “impact force” and “electric flame-off current” with a 2×2 design of experiments. Then, the ultrasound parameter is maximized up to a level where a pre-selected ultrasonic deformation occurs to the bonds, maintaining the target bond diameter and height. The bond quality is measured by measuring the shear strength of the bonds. The results show that • the bond geometry is not affected by the bond stress, • the optimized specific process parameters vary by less than ~0.5 % when bond stress values are varied from 60 to 100 MPa, • the variations in optimized parameters are larger than ~3.0 % when the BT is changed from 100 to 200 oC, • ball bonds achieve acceptable shear strength (> 120 MPa) when the values for both, bond stress and bond temperature, are high, • ultrasound level and shear stress interact, the higher shear stress the lower the ultrasound level required. An average shear strength of ~120 MPa is achieved with 11.4 % ultrasound, 100 MPa bond stress, and 200 oC bond process temperature. In summary, a robust methodology is presented in this thesis to efficiently optimize the ball bonding process as demonstrated with the new candidate wire has a bondability similar to that of gold wire with only minor adjustment in the bonding process needed.
Degradation and failure mechanisms in thermally exposed Au-Al ball bonds
During the manufacturing and the service life of Au-Al wire bonded electronic packages, the ball bonds experience elevated temperatures and hence accelerated interdiffusion reactions that promote the transformation of theAu-Al phases and the growth of creep cavities. In the current study, these service conditions were simulated by thermally exposing Au-Al ball bonds at 175 and 250 °C for up to 1000 h. The Au-Al phase transformations and the growth of cavities were characterized by scanning electron microscopy. The volume changes associated with the transformation of the intermetallic phases were theoretically calculated, and the effect of the phase transformations on the growth of cavities was studied. The as-bonded microstructure of a Au-Al ball bond typically consisted of an alloyed zone and a line of discontinuous voids (void line) between the Au bump and the bonded Al metallization. Thermal exposure resulted in the nucleation, growth, and the transformation of the Au-Al phases and the growth of cavities along the void line. Theoretical analysis showed that the phase transformations across and lateral to the ball bond result in significant volumetric shrinkage. The volumetric shrinkage results in tensile stresses and promotes the growth of creep cavities at the void line. Cavity growth is higher at the crack front due to stress concentration, which was initially at the edge of the void line. The crack propagation occurs laterally by the coalescence of sufficiently grown cavities at the void line resulting in the failure of the Au-Al ball bonds.