(Sn-Ag) eut +Cu soldering materials, part II: Electrical and mechanical studies
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Section I: Basic and Applied Research
(Sn-Ag)eut + Cu Soldering Materials, Part II: Electrical and Mechanical Studies R. Kisiel, W. Gasior, Z. Moser, J. Pstrus´, K. Bukat, and J. Sitek
(Submitted 20 January 2004) Electrical (solder resistivity and solder joint resistance) and mechanical (tensile strength and shear strength of solder joints) parameters of the binary eutectic Sn-Ag and two alloys close to the ternary eutectic Sn-Ag-Cu composition were investigated. The four-probe technique was used for the measurement of electrical parameters. Special equipment was constructed for the tensile strength measurements and also for determination of the shear strengths of solder joints between a typical circuit component and a Cu contact on a printed circuit board (PCB). It was found that electrical and mechanical properties of the three alloys studied are comparable to data in the literature for traditional Pb-Sn solders. A joint resistance below 0.3 m⍀ (⍀ =ohm) and shear strength of above 20 MPa were found for an individual solder joint between a circuit component (in the current study a “jumper” resistor) and a copper surface on a PCB.
1. Introduction
2. Electrical and Mechanical Measurements of Solders in Wire Form
The majority of electronic assemblies in use today use Pb-Sn solders for interconnection. Emerging environmental regulations worldwide have targeted the elimination of Pb usage in electronic assemblies due to the inherent toxicity of Pb. The most promising alternatives for replacement of PbSn solders are a family of near eutectic Sn-Ag-Cu solders.[1] For practical application of soldering materials, electrical and mechanical properties play an important role above and beyond the wetting characteristics reported in Part I of this work.[2] Measurements of these electrical and mechanical properties of the same alloys as in Part I, but in 1 mm diameter wire form, are reported here for the binary Ag-Sn eutectic alloy and two Ag-Cu-Sn alloys with compositions (Cu/at.%⳱0.46 and 0.74) near the ternary eutectic. The electrical resistivity of any such replacement solder must be sufficiently low to permit current flow without heating a solder joint. On the other hand, the mechanical strength of the solder has to be sufficiently high to attach a component to a printed circuit board (PCB). As an example, data[3,4] for resistivities and shear strengths of common solders used for microelectronic applications are listed together with resistivities of pure metals in Table 1. The resistivities of all three Pb-free solders in the current study were lower than the resistivity of eutectic Pb-Sn. The differences are neither too high nor too low to meaningfully affect the overall functionality of any electronic circuit in current use.
For resistivity measurements, the four-probe method was used. A Keithley 2001 (WUT, Warsaw, Poland) multimeter was used as the detecting instrument. Appropriate equipment for applying tensile forces with accuracies of 0.1 N was constructed. Then the electrical and mechanical measurements were m
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