Hao Wen Hsueh

An investigation into the crystallization and electric flame-off characteristics of 20 μm copper wires

Copper wires are used in electronic packaging, however the workability and reliability still need to be improved. This work investigates the microstructural characteristics and mechanical properties of annealed wires and un-annealed wires. In addition, the interface bonding characteristics of Al pads are also studied. Experimental results indicate that at the two annealing conditions of 610 °C/0.02 s and 510 °C/0.4 s, 20 μm copper wires possessed a fully annealed structure. Compared with the un-annealed wire, the annealed tensile strength and the annealed hardness decreased, and the annealed elongation increased. Through thermal crystallization, the matrix structure transformed from long, thin grains to equiaxed grains and a few annealed twins. The microstructure of the free air ball (FAB) after an EFO process consisted of column-like grains, and grew from the heat-affected zone (HAZ) to the Cu ball. As for bonding testing, the pull strength of the bonded samples increased with increasing the Al film thickness (from 76 nm to 800 nm).

Intermetallic Phase on the Interface of Ag-Au-Pd/Al Structure

Three wires, Au, Cu, and Ag-Au-Pd, were bonded on an Al pad, inducing IMC growth by a 155 hr high temperature storage (HTS) so that the electrical resistance was increased and critical fusing current density (CFCD) decreased. Observations of the Ag-Au-Pd wire after HTS (0–1000 hr) indicated that IMC between the Ag-Au-Pd wire and Al Pad was divided into three layers: Ag2Al layers above and below the bonding interface and a polycrystal thin layer above the total IMC. A high percentage of Pd and Au existed in this 200 nm thin layer, and could suppress Al diffusion into the Ag matrix to inhibit IMC growth. After PCT-1000 hr, a noncontinuous structure still remained between the IMC layer and interface, and the main phase of IMC was (Ag, Au, Pd)2Al with a hexagonal structure.

A study on electromigration-inducing intergranular fracture of fine silver alloy wires

In this study, Pd-coated Cu, Ag (purity = 4 N), and Ag alloy (Ag-8Au-3Pd) wires were employed to measure the tensile properties during current stressing using the so-called dynamic current tensile (DCT) test. Both the tensile strength and elongation of the wires decreased dramatically in the DCT test, particularly of the Ag-based wires, and the fracture morphology of the Cu-based and Ag-based wires was ductile fracture and intergranular fracture, respectively. Compared to the Cu-based wires, electromigration occurred more easily in the Ag-based wires, and it always generated voids and cracks at the grain boundaries; therefore, the fracture morphology of the Ag-based wires was intergranular fracture owing to the weakened grain boundary. Further, the results indicated that the Ag-based wires could not carry a higher current density than the Cu-based wires, primarily because their extremely low strength and elongation in current stressing might cause serious reliability problems.

Recrystallization of Ag and Ag-La Alloy Wire in Wire Bonding Process

Sliver wire was the novel material to replaced gold wire in wire bonding process, and rare earth element was often added to improve the properties of silver wires. The annealing effect (at 225°C~275°C for 30min) on the tensile mechanical properties of silver wires with φ=20μm was investigated. In addition, the microstructural characteristics and the mechanical properties before and after an electric flame-off (EFO) process were also studied. Free-air ball (FAB) of 85μm diameter from 20μm diameter pure silver wire was too huge for bonding process, otherwise the silver wire was added 0.05 wt.% lanthanum to form Ag-La alloy wire to reduce the diameter of FAB. FAB of Ag-La alloy wire with a 55μm diameter, and can avoid short-circuited. In addition, microstructures, tensile properties and the micro-hardness of Ag-La alloy wires indicated that the best annealing temperature was 425 °C.