Stress–Strain Characteristics of Tin-Based Solder Alloys for Drop-Impact Modeling

Abstract

The stress-strain properties of eutectic Sn-Pb and lead-free solders at strain rates between 0.1 s -1 and 300 s -1 are required to support finite-element modeling of the solder joints during board-level mechanical shock and productlevel drop-impact testing. However, there is very limited data in this range because this is beyond the limit of conventional mechanical testing and below the limit of the split Hopkinson pressure bar test method. In this paper, a specialized drop-weight test was developed and, together with a conventional mechanical tester, the true stress-strain properties of four solder alloys (63Sn-37Pb, Sn-1.0Ag-0.1Cu, Sn-3.5Ag, and Sn-3.0Ag-0.5Cu) were generated for strain rates in the range from 0.005 s -1 to 300 s -1 . The sensitivity of the solders was found to be independent of strain level but to increase with increased strain rate. The Sn-3.5Ag and the Sn-3.0Ag-0.5Cu solders exhibited not only higher flow stress at relatively low strain rate but, compared to Sn-37Pb, both also exhibited higher rate sensitivity that contributes to the weakness of these two lead-free solder joints when subjected to drop impact loading.

Effect of processing parameters on fracture toughness of lead-free solder joints

“…Many studies have shown that Young's modulus can be assumed to be independent of strain rate from quasi-static to very high strain rates (up to 300 s À1 ) [9,[36][37][38][39][40]. The present FEM showed that this was particularly true for the calculation of J ci , which decreased by only 2% when the solder Young's modulus was increased from 51 GPa to 110 GPa.…”

“…[23,[30][31][32][33]. Table 2 lists the values that were used, taken from the SAC305 properties provided in [34,35] and the stress-strain curves of [36]. Linear interpolation was used to estimate the yield stress at strain rates where the yield stress at the corresponding strain rate was not reported directly.…”

Comparison of solder joint fracture behavior in Arcan and DCB specimens

“…Since the solder yield stress is a function of strain rate [20,[34][35][36][37], the SAC305 properties were defined using the data of [38,39] and the stress-strain curves of [40] as shown in Table 1. The Young's modulus and Poisson's ratio of SAC305 were assumed to be, respectively, 51 GPa and 0.4, while those for the copper were taken to be 124 GPa and 0.35 [38,39].…”

High-Speed Cyclic Bend Tests and Board-Level Drop Tests for Evaluating the Robustness of Solder Joints in Printed Circuit Board Assemblies

“…The higher susceptibility to brittle fracture is because the flow stress of the bulk solders increases with strain rate, 14 while the dynamic fracture toughness of the brittle intermetallic compound (IMC) tends to decrease with increasing strain rate. 15 It is well known that the fracture toughness of a structure is a function of the size and distribution of the defects within the structure.…”

“…12, where the flow stress was characterized at 0.1 strain at strain rate of 50 s -1 (Table V). 14 The following observations were made: (i) BLDST-life decreased with increased flow stress of the solder alloys, (ii) the OSP pad finish outperformed the ENIG pad finish in general (more so for solder alloys of lower flow stress), but both finishes displayed similar sensitivity towards the flow stress of the solder alloy-a 100% increase in the flow stress of the solder alloy resulted in approximately 50% reduction in BLDST-life, (iii) High-Speed Cyclic Bend Tests and Board-Level Drop Tests for Evaluating the Robustness of Solder Joints in Printed Circuit Board Assemblies doping the solder alloys with minute amount of Ni increased the BLDST-life in general, but the effects varied-doping was much more effective for OSP than for ENIG pad finish; it was much more effective for SAC101 than for SAC305 solder; and doping has no effect at all for the SAC305_ENIG solder joint.…”