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CuSn intermetallic compounds (IMC's), formed at the interface between the solder and the copper substrate are found to play a dominant role in determining the thermal fatigue life of surface mount solder joints fabricated from a conventional infrared reflow process. In order to predict the growth of this IMC layer during the operating life of the solder joint and its effect on the thermal fatigue life, the formation characteristics of the IMC's in 0805 and 1206 LCCC solder joints are systematically studied in this investigation. Only the stable CucSns'X rpphase intermetallic compound was observed in all as-solidified solder joints as confirmed by scanning electron microscopy (SEM) and energy dispersive x-ray (EDX). The mean layer thickness was found to increase almost linearly with reflow time up to about 200 s. The thickness of the interfacial IMC layer increased with increasing reflow temperature for 0805-type solder joints up to around 250 "C and reached a saturated thickness of 2.5 pm beyond this temperature. Additional intermetallic formation due to higher reflow temperature or longer reflow time would appear as C u S n whiskers in the bulk solder of the joint. The copper land pad size and quality of component lead metallization were also found to greatly affect the formation of C u S n IMC in surface mount solder joints, and hence its reliability in terms of thermal fatigue life and mechanical properties.

The effect of Cu-Sn intermetallic compounds (IMC) on the fatigue failure of solder joints has been studied by means of shear cycling. The samples consist of leadless ceramic chip carriers (LCCC) soldered onto FR-4 printed circuit boards (PCB), and are prepared by conventional reflow soldering using a 63Sn-37Pb solder paste and then aged at 150 C for 1, 4, 9, 16, 25, 36, and 49 days. The specimens are subjected to low cycle fatigue shear tests controlled by the displacement. The results indicate that the fatigue lifetime of the solder joints depends on the thickness of the IMC layer between the Cu-pad and bulk solder, and the quantitative relationship between the lifetime and thickness can be described as a monotonically decreasing curve. The greatest decrease is over the thickness range up to 2.8 m, when the IMC/bulk solder interface becomes flat, corresponding to a lifetime decrease to 62% of the as assembled value. For further increase in IMC thickness the lifetime decreases more slowly. Evidently, the effect of the Cu-Sn intermetallic compounds on the joint fatigue lifetime is not only concerned with the IMC thickness but also the interface morphology. A thick and flat IMC layer has a deleterious effect. The results of X-ray diffraction and metallographic analysis show that cracks initiate underneath the component metallization, and propagate along the IMC/solder interface, then toward the fillet. The Cu 3 Sn ("-phase) is formed between the Cu-pad and -phase, and grows more quickly than thephase during storage and long term operation or aging tests. However, the Cu 3 Sn makes only a small direct contribution toward fatigue failure.

Our previous investigation [1], revealed the formation kinetics and characteristics of copper-tin (Cu-Sn) intermetallic compounds (IMC) in leadless ceramic chip carrier (LCCC) surface mount solder joints during infrared (IR)-reflow soldering. The present study focuses on the solid state growth of the interfacial Cu-Sn IMC in LCCC surface mount solder joints under prolonged annealing at elevated temperature. A thick Cu-Sn IMC layer at the Sn-Pb solder/Cu interface of a surface mount solder joint (which can be achieved by prolonged aging at high temperature or after long term operation of surface mount technology (SMT) electronic assemblies) makes the interface more sensitive to stress and may eventually lead to fatigue failure of all SMT solder joint. The microstructural morphology of the Cu-Sn IMC layer at the solder/Cu pad interface in all annealed LCCC surface mount solder joints is duplex and consists ofphase Cu 6 Sn 5 and "-phase Cu 3 Sn IMC. Both Cu-Sn IMC phases thicken as the aging time increases. On the other hand, at the interface close to the component metallization, the growth of both theand "-phase were shown to be suppressed, with more a pronounced effect on "-phase, by Ni originating from the metallization. The mean total layer thickness was found to increase linearly with the square root of aging time and the growth was faster for higher annealing temperature. The activation energy for the growth of interfacial Cu-Sn IMC layers and the pre-exponential factor, D o , for diffusion were found to be 1.09 eV and 1:68 2 10 04 m 2 /s, respectively, for the 0805 LCCC surface mount solder joint using eutectic Sn-Pb solder. The pad size and quantity of Sn-Pb solder employed in LCCC surface mount solder joints were shown to have little effect on the solid state growth rate of interfacial Cu-Sn IMC layers.

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