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Reliability Study of Low Silver Alloy Solder Pastes
Reliability Study of Low Silver Alloy Solder Pastes
Paper presents the reliability study of lead-free solder joints reflowed using various lead-free alloy solder pastes after thermal cycling tests.
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Authored By:
Jennifer Nguyen, David Geiger and Murad Kurwa
Flextronics International
Milpitas, CA, USA

Sn3.0Ag0.5Cu (SAC305) is currently the most popular near eutectic lead-free alloy used in the manufacturing processes. Over the last several years, the price of silver has dramatically increased driving a desire for lower silver alloy alternatives.

As a result, there is a significant increase in the number of alternative low/no silver lead-free solder alloys available in the industry recently. Our previous study showed that many alternative low silver solder paste materials had good printing and wetting performance as compared to SAC305 solder pastes. However, there is lack of information on the reliability of alternative alloy solder joints assembled using alternative low silver alloy solder pastes. In this paper, we will present the reliability study of lead-free solder joints reflowed using various lead-free alloy solder pastes after thermal cycling test (3000 cycles, 0 degrees C to 100 degrees C). Six different lead-free pastes were investigated. SAC305 solder joints were used as the control. Low and no silver solder pastes and a low temperature SnBiAg solder pastes were also included.

There is no significant difference in the intermetallic layer thickness of the solder joints assembled with SAC305 and other alternative high temperature low-Ag, Pb-free alloy solder pastes (SAC0307, SnCuNi, Material C,Material D). In general, the IMC thickness of these materials slightly increased after thermal cycle test, but the change was negligible. The IMC thickness of the SnBiAg solder joints after reflow process was typically thinner than those of the high temperature lead-free alloys. The IMC layer of SnBiAg solder joints grew during the thermal cycle testing, to a similar IMC thickness to the other lead-free alloys. The thickness and composition of the intermetallic layers was not determined to affect the reliability of the solder joint during the thermal cycle testing.

The thermal reliability of alternative lead-free solder joint varied depending on the package types and the component size. In our study, this factor affected the solder joint's thermal reliability more than the impact of alloy composition of the solder paste. The 2512 resistors was failed first as compared to other tested components. Complete failure and sever cracking were seen for most 2512 resistors after 3000 thermal cycles (0 degrees C to 100 degrees C). No complete failure was observed for small chip component such as 0603, 0402, 0201 components after the testing.

Severe cracking and some failure were also observed for BGA196, BGA228, BGA97 and QFN88 after the thermal cycle testing. Minor cracking and no failure was seen for BGA1156, BGA64, QFN32, and QFP208 and QFP100 components. In general, solder joint assembled with SAC305 solder pastes still performed better than low Ag alloy solder pastes. Unexpectedly, low temperature SnBiAg solder joint performed well after thermal cycle test when it was the single alloy in the solder joint. When SAC 305 BGA reflowed with SnBiAg solder pastes, more defects and failures were seen. Further reliability study should be done for alternative lead-free alloy solder paste materials.

Initially Published in the IPC Proceedings

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