Quality and reliability investigation of micro-alloyed solders

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Supervisor:
Dr. Krammer Olivér
Department of Electronics Technology

In my thesis Bi and Sb doped micro-alloyed solders were compared to traditional tin-silver-copper lead-free and to tin-lead-silver solders. The solder joints were formed with vapour-phase soldering technology. The solder joints of surface mount resistances on a FR4 substrate have been characterized after several stages of TS (Thermal Shock) tests. The mechanical and structural comparative tests were performed after 200, 500, 1000 and 2000 cycles.

After the reflow soldering, tombstone effect was observed. The SAC305 solder was the worst, and the SACX0307 was the best, only the 3.6 % of the resistances were concerned, whilst in case of SAC305 it was 19.7 %.

After TS tests, in most cases the shear strength of the solder joints showed a steady decline. After 2000 TS cycles, the SnPbAg samples had the strongest joints with 15 N measured value. In terms of percentage decrease of the shear strength the SACX0807 and SACX0307 micro-alloyed samples with 48.8 % and 46.3 % were better than the SAC305 with 39 %.

By observing cross-sectional samples of the solder joints, it is revealed the path of the crack propagation is similar as in the literature. While increasing the number of thermal shock cycles, the thickness of Cu3Sn intermetallic layer (IML) became observable between the copper and the Cu6Sn5 IML. In case of lead-free solder joints the Cu3Sn IML grew continues, while after 2000 TS cycles in case of the SnPbAg joints the saturation of the thickness (1 mm) was observable. In terms of the growth of the total IML thickness, the SAC305 sample had the thickest (6 mm) IML after 2000 TS cycles. The SACX0807 solder type is more reliable than conventional SAC305 solder due to the better measured USS (Ultimate Shear Strength) and therefore using SACX0807 is recommended.

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