Random Voids Generation and Effect of Thermal Shock Load on Mechanical Reliability of Light-Emitting Diode Flip Chip Solder Joints

Abstract

To make the light-emitting diode (LED) more compact and effective, the flip chip solder joint is recommended in LED chip-scale packaging (CSP) with critical functions in mechanical support, heat dissipation, and electrical conductivity. However, the generation of voids always challenges the mechanical strength, thermal stability, and reliability of solder joints. This paper models the 3D random voids generation in the LED flip chip Sn96.5-Ag3.0-Cu0.5 (SAC305) solder joint, and investigates the effect of thermal shock load on its mechanical reliability with both simulations and experiments referring to the JEDEC thermal shock test standard (JESD22-A106B). The results reveal the following: (1) the void rate of the solder joint increases after thermal shock ageing, and its shear strength exponentially degrades; (2) the first principal stress of the solder joint is not obviously increased, however, if the through-hole voids emerged in the corner of solder joints, it will dramatically increase; (3) modelling of the fatigue failure of solder joint with randomly distributed voids utilizes the approximate model to estimate the lifetime, and the experimental results confirm that the absolute prediction error can be controlled around 2.84%.Materials 2020, 13, 94 2 of 16 power, heat generation, and packaging density are becoming higher and the operation condition is severer, the demand on the highly reliable LED CSP is dramatically increasing [9,10]. For the package to be effective, the solder joint plays critical roles in electric conduction, heat conduction, and mechanical connection. Actually, because the LED CSP always inevitably suffers the thermal cycling and even shocks from outside environments, the mismatches of thermal expansion among all packaging components under these conditions always generate periodic stress and strain in the package level [11,12]. The increase of loads on a LED CSP has put forward more requirements on the reliability of a solder joint, because the fatigue failure of the solder joint under the thermal cycling condition will lead to the failure of whole package. Therefore, the thermal shock load on the mechanical reliability of the LED flip chip solder joints has always been one of the critical bottlenecks in the development of LED CSP technology [13][14][15].Among all solder alloys, the lead-free Sn96.5-Ag3.0-Cu0.5 (SAC305) is considered as one of the most popular chip-attach candidates owing to its cost effectiveness, good solderability, and favorable mechanical strength [16,17]. It has widely been used in the flip chip soldering, but such solder connections are prone to failure over time under the thermal cycling [18] or mechanical bending test [19]. D. Kong et al. [20] used the Anand model to simulate the stress-strain response of the SnAgCu-based solder joint under the condition of thermal cycle loading and predicted its lifetime with the Manson-Coffin model. They found that the addition of a certain amount of Ce and Fe in the SnAgCu solder joint could significantly increase its fatigue ...

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