Authored By:
Saurabh Gupta, Howlit Chng, James M Wade, Kyle Davidson,
Todd Smith, Kevin J Byrd, Jose I Hernandez, Juan Landeros
Intel Corporation
OR, USA
Summary
Tin-Bismuth (Sn-Bi) Low Temperature Solder (LTS) pastes are being widely adopted by the electronics industry due to the benefits of lowering the peak reflow temperatures in SMT assembly. Lower peak temperatures result in reduced package and board warpage which leads to fewer assembly defects and higher yields. However, the effects of the paste metallurgy, especially the Bi content, on board level reliability are being actively researched. Temperature cycling and shock reliability performance evaluations have frequently been published. However, there is a lack of data assessing the shear strength of the LTS joints for actual motherboard components. Shear strength of the solder joints plays an important role in preventing damage due to board handling and such data is also useful for new solder paste development and evaluations.
Shear strength data was collected using a Mark-10 Test Frame on several SMT components that are prone to handling damage such as headers, inductors, crystal components, clips, shields, and PEM nuts. The influence of several assembly process parameters, such as reflow profiles and paste volumes, as well as incoming board properties such as pad surface finish have been studied in this work. Both LTS and Tin-Silver-Copper (SAC) paste metallurgies have been evaluated. This study was conducted on a test board designed specifically for performing shear testing and on actual product boards.
This paper shows that paste metallurgy has the most significant effect on the shear strength. Other process parameters such as reflow profiles have no impact on the shear strength if they are within the spec window provided by the paste supplier. This data will be helpful in bolstering the confidence of ODMs to move to LTS paste metallurgies for SMT assembly.
Conclusions
Shear testing was conducted on several components such as headers, E-Caps, crystals on functional and test boards. The results show that the LTS has similar shear strength to the SAC joints. There are, however, differences in the failure mechanisms between LTS and SAC. Figure 11 illustrates the five failure mechanisms that are possible upon shearing a component. When the component itself breaks then that failure is represented as Mode #1. Mode #2 occurs when the joint between the component leads and the solder breaks. When the crack propagates through the solder joint, then it is Mode #3.
Interface cracking between the solder and the pad is represented as Mode #4. When the pad is pulled off completely from the PCB then it is shown as Mode #5. LTS failures are typically Mode #3 while SAC shows Mode #4 or Mode #5 depending on whether the pads are MD or SMD. Despite the differences between failure mechanisms, the shear strength values between the Sn-57Bi-X LTS and SAC305 solder pastes remain comparable. These results do not apply to all formulations of the Sn-Bi alloys and it is recommended to verify mechanical performance of the LTS paste before using it for any application.
Initially Published in the SMTA Proceedings
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