A Joining Technique Using Multilayer Lead-Indium-Gold Composite Deposited In High Vacuum
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A JOINING TECHNIQUE USING MULTILAYER LEAD-INDIUM-GOLD COMPOSITE DEPOSITED IN HIGH VACUUM CHIN C. LEE, CHEN-YU WANG, YI-CHIA CHEN, AND GORAN MATIJASEVIC University of California, Department of Electrical and Computer Engineering, Irvine, CA 92717 ABSTRACT A joining technique for electronic devices has been developed. This technique uses a leadindium-gold multilayer composite solder deposited directly on GaAs wafers in high vacuum to prevent indium oxidation. The gold layer on the composite further protects the indium layer from oxidation in atmosphere. The GaAs dies are bonded to a gold-coated alumina substrate at a process temperature of 250'C. Nearly perfect joints are achieved as verified by a scanning acoustic microscope (SAM). SEM and EDX results indicate that the alloy joint consists of AuIn 2 grains embedded in an In-Pb solid solution phase, as predicted from the AuIn-Pb phase diagram. Compared to lead-tin solder, indium-lead solder has been shown by others to exhibit much better fatigue resistance and have much less of a scavenging effect. Thermal shock as well as shear tests confirm that a good die attach is obtained with the leadindium-gold composite. INTRODUCTION The semiconductor device package serves the multiple purposes of mechanical support, environmental protection, heat dissipation, and electrical connection. Reliable die bonding is a necessity for a good IC package. Commonly used bonding media include metal-filled organic adhesives and glasses, hard solders, and soft solders. Hard solders have excellent fatigue resistance and a high thermal conductivity. Due to elastic rather than plastic deformation, hard solders retain the stresses caused by the thermal expansion mismatch among the parts joined and may fracture in extreme cases. Soft solders have a high degree of plastic strain capability and can therefore deform to release the stresses developed. Soft solders, on the other hand, incur thermal fatigue and creep movement. The most commonly used soft solders are tin-lead alloys of various compositions. Regardless of the wide and popular use, tin-lead solders do have significant thermal fatigue. Furthermore, they exhibit a scavenging effect when the molten solder with a high tin content dissolves the gold film and quickly forms brittle gold-tin intermetallic compounds. Since electrodes are frequently gold plated, the rapid gold-tin interdiffusion in either the liquid or solid state can degrade bonding quality and thus requires careful evaluation. In spite of its seldom use in the industries, In-Pb solder has been shown to exhibit much better thermal fatigue lifetime than Pb-Sn solder [1-3]. Furthermore, molten In-Pb alloy dissolves gold much more slowly than Pb-Sn alloy, thus greatly reducing the scavenging effect [4]. Accordingly, the In-Pb alloy appears to be a worthwhile subject to study for electronic packaging. Fig. l(a) displays the In-Pb phase diagram [5]. The In-Pb binary system is quite simple in that indium and lead form continuous solid solutions except for the alloy with an indium compo
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