Mechanical Characterization of the Heat Affected Zone of Gold Wirebonds Using Nanoindentation

Abstract

Abstract-With increasing miniaturization in microelectronics the wirebonds used in IC packages are witnessing a thrust towards fine pitch wirebonding.To have a precise control over loop height of the wirebond for fine pitch wirebonding, it is imperative to do mechanical characterization of the wirebond. The present work studies the mechanical properties of gold wire and wirebond using nanoindentation. The wirebond specimen surface was planarized using mechanical polishing.The loop height of the gold wirebond is directly proportional to the length of the heat affected zone (HAZ) above the ball of gold wirebond. Metallographic preparation of gold wirebond cross section reveals the presence of undesirable coarse grain structure in HAZ due to recrystallization and grain growth in the gold wire adjacent to the ball. The recrystallization temperature of our gold wire was found using D.S.C. to be 340.66 0 C. The doping elements present in the gold wire used, were identified using TOF-SIMS. Nanoindentation of the gold wire was done at different maximum loads to observe the hardness variation with load. The nanoindentation of gold wirebond has confirmed a v-shaped hardness profile in the HAZ. The hardness minima for the particular gold wire used with a ball size ratio of 2.4 was observed at distance of 160-170 µm from the neck of the ball. The elastic modulus was found to vary randomly and to be independent of the microstructure in the wirebond. A yield stress profile based on empirical hardness-yield strength correlation has been predicted for the gold wirebond.

The Fracture Resistance of Bonded Au Wires for Interconnection

“…Based on our criteria and nano-indentation results, HAZ length ( ) is found in the five samples and the grain size from the ball neck along the wires is measured according to the grain intercept count method (ASTM E112-96). The average grain size across the wire diameter , and measured , and the calculated yield stress ( ) from [5] are summarized in Table I.…”

“…Particularly, the large tensile stress along the length direction is induced on curing the epoxy of significantly larger coefficient of thermal expansion (CTE) than the wire [1], [2]. Under this stress, the region close to the ball neck in HAZ has been observed as the most vulnerable region to fracture, which has been considered due to the strength degradation in HAZ with increasing grain size [5], [6]. Hence, efforts have been made in retarding recrystallization and preventing grain growth in the Au alloys by controlling the type and amount of dopants [7].…”

“…For the wires to be deformed to a desired wire shape, sufficient ductility is also required [4]. Currently, there is no deep understanding of the factors determining the fracture resistance of the wire [5]. In addition, there are a large number of variables involved in the wire-looping process, and the variables affect the fracture simultaneously [2].…”

Curvature-Corrected Hardness Measurement Technique for Nanoindentation of Bonding Wire

“…In particular, mechanical properties of bonding wires should be improved to obtain higher performance characteristics such as higher reliability, strength, elongation and cost reduction with smaller wire diameters. (1)(2)(3)(4) We know from our experience that hardness is approximately three times as large as strength. Hence, the hardness test is widely adopted and used as a simplified practical mechanical testing method for determining the strength properties of various materials.…”

Some thoughts on bondability and strength of gold wire bonding

“…Jalar et al [21] carried out a nanoindentation test to measure the hardness of the Au base metal and the intermetallic compound of the Au ball bond and found that the intermetallic compound has higher hardness than the Au base metal. Hardness is proportional to yield strength, based on the Tabor relationship [22]. Hence, the formation of the intermetallic compound, with its higher yield strength, increases the strength at the bonding interface.…”