Is it a “drop-in” replacement for SAC 105 and SAC 305?

While SnCuNi (SCN) is a popular alloy for wave applications, its use in surface mount has not been well documented. Recent work has shown that, even though the melting temperature of SCN is higher than that of SAC alloys, a SAC-based assembly reflow profile can achieve satisfactory SCN solder joints.^1,2

The objective of this study was to further evaluate the reliability of SCN-based electrical interconnects. The potential use of SCN in portable product applications due to its lower cost was examined through the use of drop testing, and the performance was compared to commonly-used SAC solder joints.³ It has been suggested the increased ductility of lower-Ag alloys is desirable for high strain rate shock loading.⁴ The SCN alloy has no silver content, and therefore, one might expect it to have even greater ductility than SAC 105, thus making it more appropriate for shock loading.

The test board used for drop testing was of a modified Jedec design.⁵ It was a 1 mm thick, two-layer FR-4 construction, using a Cu OSP surface finish. The board dimensions were 132 x 77 mm. The pads were non-solder mask defined with a nominal diameter of 0.0145˝. Fifteen component locations were available, although only four symmetric locations were assembled for testing.

Amkor CABGA 208 components, using an ENIG surface finish, were supplied without solder balls. To these, 0.020˝ diameter solder balls were attached in-house using a tacky flux. Alloys included SAC 305, SAC 105 and SCN. A generic profile was used for ball attachment with a peak temperature of 250°C.

A total of 16 drop test boards were assembled. SAC 305 and SAC 105 components were assembled with SAC 305 paste, while SCN components were assembled with SCN paste. All drop test vehicles were assembled with a peak temperature of 246°C and 60s above 217°C and 32s above 227°C.
Drop testing was performed per Jedec JESD22-B111. The test boards were attached to the drop table at the four corners, with the components facing downward. The shock input was 1500-G with 0.5 ms duration, as measured on the shock machine.

Each component was connected to an event detector to monitor for electrical failure during the testing. Electrical failure followed the Jedec definition of “the first event of intermittent discontinuity followed by three additional such events during five subsequent drops.”⁵ Each board was dropped until all components had failed. Failure data are plotted as a two-parameter Weibull distribution in Figure 1. The characteristic lifetimes of SAC 305, SAC 105 and SCN are 45, 112 and 51 drops, respectively. The results indicate SCN provides equivalent reliability to SAC 305 in a drop/shock environment. However, both SAC 305 and SCN provide about half the lifetime of SAC 105 in this environment.

Failure analysis was conducted through the use of dye penetration and cross-sectional analysis. All tested components failed by pad cratering. Through failure analysis, there were no detected failures due to bulk solder fatigue or interfacial/intermetallic fracture. Therefore, direct comparisons between alloys are more readily made.

In this environment, a more ductile alloy under the given repeated shock load should increase the lifetime of brittle failure modes, such as pad craters. It was assumed that because SCN has no Ag content, it would perform very well in this environment. The solder strength indicates it is also weaker than both SAC 305 and SAC 105.3 The different microstructure due to different alloying elements is very likely to affect the mechanical behavior in the repeated high strain rate shock load.

SCN alloy appears to be well suited for use in surface mount applications with minimal adjustments to the assembly process. Reliability can be expected to be similar to that of SAC 305. Drop testing results showed that SCN assemblies were very similar to those with SAC 305; however, the reliability of each was less than SAC 105. SCN appears to be a drop-in replacement to SAC 305 systems, but may not perform well in high shock load environments where lower Ag-content solders are already optimized. 

References
1. U. Marquez de Tino, D. Barbini, L. Yang, B. Roggeman, M. Meilunas, “Developing a Reflow Process for Sn/Cu/Ni Solder Paste,” SMTA Pan Pacific Symposium, January 2009.
2. U. Marquez de Tino, “Developing a Reflow Process For Sn/Cu/Ni Solder Paste,” Circuits Assembly, April 2009.
3. B. Roggeman, U. Marquez de Tino and D. Barbini, “Reliability Investigation of Sn/Cu/Ni Solder Joints,” SMTA International, October 2009.
4. L. Garner, et. al, “Finding Solutions to the Challenges in Package Interconnect Reliability,” Intel Technology Journal, November 2005, vol. 9, no. 4, pp. 297-308.
5. JESD22-B111, “Board Level Drop Test Method of Components for Handheld Electronic Products,” 2003.

Ursula Marquez de Tino, Ph.D., is a process and research engineer at the Advanced Process Lab at Universal Instruments Corp. (uic.com); umarquez@vsww.com. Brian Roggeman is a process research engineer at the Universal Instruments Advanced Process Lab.