Wetting Behaviors of Sn-Based Solders on Cu and Pd Surfaces
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H.K. KIM, Y. WANG, A. MAHESHWARI* ,and K.N. TU Department of Materials Science and Engineering, University of California at Los Angeles, Los Angeles, CA 90095-1595 *Miniaturization Lab, Texas Inst. Inc., Temple, TX 76503-6102
ABSTRACT We have studied the surface morphology and wetting reaction of eutectic SnBi, eutectic SnPb, and pure Sn on Cu and Pd surfaces. In the case of Pd, the reactions were so fast that no quasiequilibrium wetting angle could be measured. The compound formation has changed not only the interfaces but also the surfaces. Due to the formation of a reaction band outside the solder cap, the SnPb solder is no longer wetting the Cu surface but rather the Cu-Sn compound surface. In the wetting interface between eutectic SnPb and Cu, the morphology of the scallop-like Cu-Sn compound grains shows that we may not apply the classical model of layered compound growth to analyze the rate of soldering reactions.
INTRODUCTION Rapid advances in packaging of electronic materials has dictated continued research on performance enhancement and reliability of solder joints. ' 2 The environmental concerns of Pb containing solders3 have prompted numerous studies on the usage of lead free solders. As a result, a number of lead free binary, ternary, and in some cases quaternary solder alloys are being looked at and their properties being refined.47 Lead based solders have performed well for almost two decades and have excellent properties as joining materials. Studies have clearly shown that Sn in Sn based solders preferential takes part in the reactive wetting reaction. " This evokes the question of the role of Pb in Pb containing solders that have performed so well until now. A clear understanding of the role of Pb in the wetting and structural capability of solders would provide us with useful insight to develop the right Pb free material for soldering applications. The primary goal of this paper is to understand the wettability of eutectic SnPb and several Pb free solders on substrates. Simple systems were chosen to study the problem. Binary eutectic solders of SnPb, SnBi and pure Sn were chosen as ideal materials. The substrates chosen for this study were Cu and Pd primarily to understand wettability and interfacial reactions in bulk and then correlate the acquired results to future experiments on thin film substrates. Not only it is important to understand the wetting issues related to the materials chosen but also the reaction mechanism at the interface and morphological growth of the intermetallics. This has prompted detailed experimentation for systematic comparison of solderability of different materials and then development of a simplified model to explain the experimental results.
183 Mat. Res. Soc. Symp. Proc. Vol. 390 01995 Materials Research Society
EXPERIMENTAL METHODS
For wetting experiment, three different solder materials (eutectic SnPb, eutectic SnBi, and pure Sn) were reflowed on Cu (99.95 % purity) and Pd (99.995 % purity) plates. The reflow of the solder samples was carried out in mildly
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