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.
Ag alloy wire characteristic and benefits
Gold wire has been high volume production in IC packaging industry. With soaring price of gold in recent years and IC packaging search for cost reduction, copper wire offers 2nd alternative for wire bonding type assembly. But copper wire has drawbacks in control issues such as pad crack, aluminum splash, cratering and low throughput, Cu wire need a more complex multi-processing program to solve above problems, even if copper raw material cost is low, that cause the process costs are increased These limitations of copper wire are related to pad thickness and structure. Silver alloy wire can emerge as 3rd alternative as the cost can compete with copper and the properties of silver are nearly identical to gold while in the die to die package which benefits will be more apparent Many Study had performed for Silver alloy wire (88% & 95%), included the general workability, wire pull, ball shear, Al splash, pad-to-pad bonding. To get good workability result, Silver alloy wire properties have been studied and done the DOE to determine the best material properties windows. The key properties included the element composition and elongation, hardness, etc. IMC is another key point for the reliability performance, with the DOE of best wire bonding parameters, the IMC structure was analyzed with different condition of lead-frame based, substrate based. Combined with process flow and compound material types, IMC behavior has been observed. Electron Migration study had been performed as well to check if any side effect for Silver alloy wire bonding. Experiment results showed Silver Alloy wire bonding has better performance than copper. Silver alloy wire can be the mainstream to replace gold and copper wire, especially in pad-to-pad wire bonding type.
Effects of Pd addition on Au stud bumps/Al pads interfacial reactions and bond reliability
The main purposes for developing low-alloyed Au bonding wires were to increase wire stiffness and to control the wire loop profile and heat-affected zone length. For these reasons, many alloying elements have been used for the various Au bonding wires. Although there have been many studies reported on wire strengthening mechanisms by adding alloying elements, few studies were performed on their effects on Au bonding wires and Al pad interfacial reactions. Palladium has been used as one of the important alloying elements of Au bonding wires. In this study, Au-1wt.%Pd wire was used to make Au stud bumps on Al pads, and effects of Pd on Au/Al interfacial reactions, at 150°C, 175°C, and 200°C for 0 to 1200 h thermal aging, were investigated. Cross-sectional scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and electron probe microanalysis (EMPA) were performed to identify intermetallic compound (IMC) phases and Pd behavior at the Au/Al bonding interface. According to experimental results, the dominant IMC was Au5Al2, and a Pd-rich layer was at the Au wire and Au-Al IMC. Moreover, Au-Al interfacial reactions were significantly affected by the Pd-rich layer. Finally, bump shear tests were performed to investigate the effects of Pd-rich layers on Au wire bond reliability, and there were three different failure modes. Cracks, accompanied with IMC growth, formed above a Pd-rich layer. Furthermore, in longer aging times, fracture occurred along the crack, which propagated from the edges of a bonding interface to the center along a Pd-rich layer.