Laser soldering behavior fo Sn-Ag solders
Eutectic or near-eutectic Sn-Pb solders have dominated electrical and electronic assembly since the industry was established. Concern about toxicity of Pb has stimulated research into Pb-free alternatives. An attractive candidate is eutectic SnAg. Higher melting temperature (221{degrees}C) promises higher strength and serviceability, but also poses processing challenges. Laser soldering offers potential, but effects on microstructure and properties must be determined. Eutectic, 96.5% Sn-3.5% Ag solder paste containing RMA-SMQ51AC flux was used to make joints on 132-pin quad flat-pack components on FR-4 fiberglass PWB. A computer-controlled 80W Nd:YAG laser system operating in the continuous mode scanned the beam across preprinted solder paste on pads to produce joints. A eutectic structure f Ag{sub 3}Sn rods in a nearly-pure Sn matrix dominated the microstructure of all joints, along with various Cu-Sn intermetallics, but with significant differences attributable to processing parameters. The finer structure of laser soldered joints resulted in 50% higher microhardness, suggesting higher static, creep and thermomechanical fatigue strength. Eutectic morphology, Cu-Sn intermetallic dendrites in the bulk, and Cu-Sn intermetallic at interfaces have been related to system physical metallurgy and processing parameters.
- OSTI ID:
- 115510
- Report Number(s):
- CONF-9404233--
- Country of Publication:
- United States
- Language:
- English
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