Special Issue PaperThe growth mechanism of intermetallics between solders and metallized substrates, after thermal aging, are investigated. The solders used in this study are unleaded Sn-Cu-Ni solder and eutectic Pb-Sn solder. The Pt-Ag/Al 2 O 3 , Cu block and the electroless Cu/Pt-Ag/Al 2 O 3 are employed as the metallized substrates. Microstructure evolution of the interfacial morphology, elemental, and phase distribution are probed with the aid of electron-probe microanalyzer (EPMA) and x-ray diffractometry. Two kinds of intermetallics, Cu 3 Sn and Cu 6 Sn 5 , are formed at the solder/Cu interface. However, for the solder/Pt-Ag system, only Ag 3 Sn is observed at the interface. The thickness of Cu 3 Sn, Cu 6 Sn 5 , and Ag 3 Sn compound layers for all solder/metallized substrate systems shows a t 0.5 dependence at 100, 125, 150 and 170∞C. According to the calculated activation energy and diffusion constant, the growth rate of Cu 3 Sn and Cu 6 Sn 5 intermetallics in the electroless Cu metallized substrate is relatively higher than that for Cu block one at the range of 100∞C to 170∞C. However, the growth rate of Cu 6 Sn 5 and Ag 3 Sn is reduced in the Sn-Cu-Ni solder with respect to the eutectic Pb-Sn solder. On the other hand, the Sn-Cu-Ni solder system exhibits a thicker Cu 3 Sn intermetallic layer than the eutectic Pb-Sn solder after various aging times at 100∞C. The thickness of Cu3Sn in the eutectic Pb-Sn solder is, however, thicker than that for Sn-Cu-Ni solder at 170∞C.

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The electroless Ni (EN) plating method was employed to metallize the AIN ceramic substrates. The EN-plated AlN substrate was bonded with the Cu foil to form a sandwichlike AIN-EN/Cu/EN-AlN assembly by hot pressing in vacuum with a pressure of 6.5 MPa for 30 min. For the bonding temperature below the Ni-P eutectic temperature of EN at 88O"C, the samples were bonded through solid state diffusion. On the other hand, the samples were bonded via a liquid phase media through both wetting and diffusion if the bonding temperature was above 880°C. An optimum adhesion strength around 10 MPa occurred within bonding temperature range 600-700°C. The fracture took place in the ENKu interface for samples bonded below 6OO"C, while fracture took place in the AIN/EN interface above 700°C. The increasing temperature enhanced interdiffusion of Cu and EN to form a strong bond, yet resulted in a large residual thermal stress in the AIN/EN interface. The bonded samples with as-received AlN exhibited higher adhesion strength than those with polished AlN because there existed a residual compression perpendicular to the .41N/EN interface at surface irregularities in the as-received AlN, which resulted in an additional shear strength offset in the adhesion test. The adhesion strength of samples with etched AIN was the highest as compared to those of as-received and polished AlN, although the surface roughness of the etched AlN was the same as that of the as-received one. It is argued that the etched surface of AlN with micro-etched holes provides the anchor sites for interlocking with the EN film, which results in a good mechanical bonding in the joint. However, a mechanical trimming on the edges of bonded samples would damage the joint, and a low adhesion strength is instead observed.

Microelectronic packaging has experienced exciting growth in the past years, and solder joints plays important roles in the reliability of package system. Lead-tin (Pb-Sn) alloys are the most prominent solders for the interconnection and packaging of modern electionic components and devices. However, there are environmental concern on the toxicity of Pb. These concerns have inspired a great deal of research to study the feasibility of lead-free replacement alloys. It is known that the presence of intermetallic is often an indication of good wetting in solder joints. Nevertheless, excessive intermetallic growth may be detrimental to joint reliablity. The purpose of this study is to investigate the growth evolution between unleaded Cu-Sn-Ni alloys and metallized layer. These results will help to characterize the effect of aging on the microstructural and mechanical properties of solder / metallized layer / substrate systems.

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