Good correlation has been established between high speed shearing of solder joint at component level and board level drop tests, endorsing high speed shearing as a viable quality assurance test for manufacturing and incoming inspection. The high speed shear characteristics of solder joints under different test conditions (shear speed, shear angle, and temperature) and aging conditions (multiple reflow, temperature humidity, and salt spray) have been evaluated. Preliminary S-N characteristic for SnPb_OSP and SnAg_OSP solder joints have been generated using high speed cyclic bends test. These could be devolved into a life prediction model for board level solder joints in product drop impact.
IntroductionCompared to SnPb solder, ternary Pb-free solder alloys have been found to be particular susceptible to brittle fracture in the intermetallic compounds (IMC) [1]. This has hightened the need for more stringent quality control for manufacturing process, mainly the ball attachment process, and raw materials, such as pad finishing on the substrate and PCB.It is not economically viable to use board level drop tests (BLDT) such as described in JEDEC STD JESD22-B111 [2] for quality control. Rather, a low cost component level test is preferred. The basic requirement for a valid component level test is to be able to reproduce the failure mode observed in BLDT. However, the industry-practice component level shearing of solder joints at up to 1mm/s could only induce bulk failure in all solder joints, including those with Pb-free solders [3,4], especially in the case of unaged samples [5]. On the other hand, brittle IMC fracture has been successfully reproduced in the impact shearing of Pb-free solder joints at component level [3,4]. IMC failure has also been observed in pulling of solder joints at lower speeds [3, 5]; but clampinginduced damage, especially in the case of fine solder joints, remains a serious concern.High speed shearing of solder joints have been reported using the split Hopkinson bar technique (3 m/s) [1, 6], miniature Charpy tester (1 m/s) [7], motorised shear tester [3], and Micro-Impactor (0.6 m/s) [4].The impact shear strength and impact toughness of a number of solder alloys and pad finishes have been investigated [4] and it has been found that while the impact shear strength of some Pb-free solder joints could be higher than that of eutectic SnPb solder joints, the impact toughness of all Pb-free solder joints investigated are inferior to that of eutectic SnPb solder joints. However, it was unclear what characteristics of impact shear, if any, could be correlated with the BLDT. This is to be investigated in this paper.
