Pb-Free Alloy Alternatives
This paper compares solder compositions and micro alloyed SAC solders in terms of assembly and mechanical/thermal fatigue properties.
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Authored By:
H.J. Albrecht, P. Fruhauf, K. Wilke
Siemens AG, Berlin, Germany
Summary
The paper compares different solder compositions and micro alloyed SAC solders in terms of assembly and mechanical / thermal fatigue properties. Solder materials under test were: SAC405, SAC305, SAC2704, SnAg(1.2-2.7)Cu(0.2-0.5), SAC105, SAC105Ni, SCNGe, SAC1302Ni0.05, SAC1305Ni0.05x
Assembly and reliability data of high Ag content alloys and low Ag content alloys will be presented and discussed in relation to the acceptance criteria for different applications. The acceptability of any alloy may vary from products and is dependent on the basic alloy data especially the long time behavior under test and / or field conditions. The influence of the assembly parameter will be discussed after visual inspection, X-ray and microsectioning at the initial state.
A ranking list for reliable alloys will be presented for critical discussion and further requirements. Fatigue properties analyzed were shear and pull data, recrystallization, crack initiation and growth, electrical continuity. Based on the fatigue behavior the damage mechanism for different solders was analyzed and compared to characterize the possibility of the definition of the acceleration factor between alloys and / or TCT conditions. Acceptance criteria for lead-free solders will be discussed in relation to the modification of alloys and test conditions. A critical re-view is formulated for possibilities to transform test to field conditions and further demands to generate lifetime results.
Conclusions
The goal of this study was to determine the influence of different TCT conditions (T and ramp / hold times) related to the board level reliability of SAC solder with different constitutions in terms of Ag content and micro alloying elements, compared to SnPb solder. Thermal fatigue reliability appears to be dependent on processing, microstructure, and microalloy content, and those de-pendencies have yet to be characterized completely and understood.
Modifying the ramp and hold times during TCT offers non-uniform damage results. Fast ramping generates more defects in solder joints. The acceptability of different alloys can be taken from the degradation of electrical, mechanical and interface data (all in strict relation to the microstructure) like shear and pull forces as well as crack lengths based on commonly applied acceptance criteria as listed above.
The selection of the reliable lead-free solder will be influ-enced by the finishes on component and laminate side. Formed intermetallics are more-element IMC with differ-ent mechanical properties. Studies are required to gener-ate more data for the complete evaluation of solder joint reliability results.
Initially Published in the SMTA Proceedings
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