Microstructure and adhesion properties of Sn-0.7Cu/Cu solder joints
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The microstructure and adhesion strength of the Sn–0.7Cu/Cu solder joints were examined. The acicular (Cu6Sn5)-phase was formed inside the solder and at the solder/Cu interface, mostly in the direction normal to the interface. Compared to the Sn–3.5Ag alloy, the Sn–0.7Cu solder showed comparable wettability, but lower microhardness and joint strength. After aging, the acicular -phase grew into a round and scallop-shaped morphology, thin ⑀(Cu3Sn)-phase evolved just above the Cu substrate, and the joint strength decreased linearly with aging time.
I. INTRODUCTION
Sn–Pb alloys are most widely used for the assembly and packaging of electronic components. However, due to environmental concerns arising from the toxicity of Pb, several Pb-free solders such as eutectic or neareutectic Sn–Ag, Sn–Bi, Sn–In, and Sn–Sb alloys, have been considered to replace Sn–Pb solders.1–5 Among them, the eutectic Sn–3.5Ag is one of the most promising and extensively studied candidates, for of its good wettability and ductility, and better mechanical properties and thermal resistance than Sn–Pb solders.3–5 The Sn–3.5Ag solder, however, costs 20 times more than Sn–37Pb, has a limited availability,6 and leads to excessive dissolution of Cu substrate.2 On the other hand, the eutectic Sn–0.7Cu has been evaluated to be equally attractive as Sn–Pb not only in performance, but in cost and availability.6 However, there is very limited published data on the basic physical properties of Sn–0.7Cu/Cu solder joints. The purpose of this work is to investigate the microstructure, wettability and adhesion strength of the Sn–0.7Cu alloy soldered to the Cu substrate.
II. EXPERIMENTAL
The Sn–0.7Cu alloy was prepared by encapsulating pure Sn and Cu metals in quartz tubes and then melting them at 800 °C for 1 h under Ar atmosphere to avoid oxidization. The solidified ingots were air-cooled and cold-rolled into 0.25- or 1.5-mm-thick sheets. The sheets were then annealed at 150 °C for 24 h for stress-relief and punched to disk specimens of 3- or 6-mm diameter. For comparison, the Sn–3.5Ag solder was also prepared J. Mater. Res., Vol. 17, No. 4, Apr 2002
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in a similar manner. The chemical composition of specimens was confirmed by inductively coupled plasma (ICP; Inductively Coupled Plasma Mass Spectrometer) (Yokogama Analytical Systems, Tokyo, Japan) analysis. The substrates used in this study were Cu lead frames (Hitachi, Tokyo, Japan) of which chemical compositions are shown in Table I. For soldering, the Sn–0.7Cu disks (1.5-mm thickness and 6-mm diameter) were ultrasonically cleaned in alcohol and placed on the Cu substrates coated with mildly activated rosin (RMA) type flux. Then, the entire specimens were put into a furnace, held at 290 °C for 10 min, and finally air-cooled. For the measurement of adhesion strength, lap shear specimens were prepared by soldering two pieces of lead frames with solder disks (0.25-mm thickness and 3-mm diameter) as shown in Fig. 1. The solder layer thickness was reduced
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