Thermal Stability and Electromigration Durability of Annealing Twinned Ag-Pd alloy Wires
Date Issued
2015
Date
2015
Author(s)
Lin, Hsin-Jung
Abstract
In response to the shortcomings of electronic package wire bonding with gold wire and copper wire and the requirement of low resistivity in high-frequency IC products, Ag alloy wires, which have physical properties similar to those of Au wire, are being developed. Unlike Cu wire, which has a high hardness, Ag alloy wire will not damage an IC chip or result in poor bonding strength. Improvements to the alloy design and the drawing and annealing processes have allowed the development of binary Ag-Pd alloy wire and ternary Ag-Au-Pd alloy wire containing a large number of annealing twins. In contrast to the apparent grain growth in a conventional Ag-4Pd wire during aging at 600ºC, the grains of this annealing-twinned Ag alloy wire remain almost unchanged. The high thermal stability of the grain structure leads to a smaller heat-affected zone near the free air ball of this twinned wire. The annealing twins in this material also confer the merits of increased tensile strength and elongation with aging time, which increase the reliability of wire-bonded packages. The bonding interface of the Ag-alloy wires and Al pad containing various Pd elements was also studied. Thermal aging temperatures of 100ºC to 200ºC and aging times of 0hr to 1,000hr were used for bond strength testing, and interface reactions were observed. The bonding interface of the Ag alloy wire and Al pad was quite complete. The amounts of the added elements also affected the bonding strength and interfacial reaction. The thickness of the intermetallic compounds (IMC) caused embrittlement and produced large amounts of contact holes due to the decrease in the contact strength. After 1,000 hours of thermal aging at 200ºC, bonding strength was still very high. The growth of the IMC layer was observed to occur in two stages. The first stage of the reaction takes place with the aluminum pad for the silver alloy wire, quickly generating an IMC layer. In the second stage, a Pd-rich layer forms between the silver wire and the aluminum pad. Since the diffusion coefficient of palladium is slow, the IMC layer grows slowly in this stage. In this study, the conductivity and lifetime during current stressing were increased by reducing the addition of Au or Pd. The annealing-twinned Ag-Pd wire has a much higher lifetime than annealing-twinned Ag-8Au-3Pd because silver has high electrical and thermal conductivity. As a result of these characteristics, the Joule heating and temperature during current stressing are lowered, thereby retarding the electromigration. The electrical resistivity of this Ag-4Pd bonding wire, manufactured with a conventional method, is 3.7 μΩ．cm, which is close to the values of traditional 3N Au wire (3.5μΩ．cm), Pd coated Cu wire (1.8μΩ．cm), and Ag-8Au-3Pd ternary alloy wire (5μΩ．cm). The electrical resistivity of the annealing twinned Ag-4Pd wire is 3.5μΩ．cm. An annealing twinned Ag-4Pu wire has been produced as an alternate material for a previously developed Ag-8Au-3Pd ternary alloy wire to meet requirements for high reliability, low electrical resistivity and low cost. Under electrical stressing with a current density of 1.23x105 A/cm2 for various times, the grains in this annealing twinned wire grow much more slowly than the grains in the conventional Ag-4Pd wire. The breaking load and elongation of this annealing twinned Ag-4Pd wire are also higher than those of conventional wire. Furthermore, the annealing twins increase the durability to electromigration of this Ag-4Pd wire under electrical stressing with various current densities.
Subjects
electronic package
Ag-Pd alloy wire
annealing twins
aging
electromigration
intermetallic compound (IMC)
Type
thesis