Study of solder interconnect configurations and performance of vertical laser assisted assembled “3.5D” packages

The current study explains the performance of different solder interconnections (SAC305, SnPb, SnBi) associated with horizontal and vertical laser soldering in a 3.5D semiconductor package, using laser assisted bonding (LAB). In a 3.5D package, a device can be vertically bonded to the side of a 3D stack, thus enabling connection of different substrates to the stack. For a wide range of applicability, solder balls with high and low melting points, and varying solder compositions were selected, their corresponding laser reflow characteristics were recorded and analyzed. The metallurgical properties of the solder interfaces were analysed, in detailed pre- and post-thermal cycling tests using a focused ion beam (FIB), scanning electron microsope (SEM) and x-ray. The mechanical strength of the solder interconnections prior to assembly was measured using a shear test unit, and the corresponding fracture modes were inspected using an optical microscope. Moreover, real-time performance of 3D and 3.5D packages will be correlated with in silico electrical results based on the evaluated solder alloy SAC305. Finally, potential photonic applications of 3.5D optical packages, and future prospects concerning intended reliability and stability are highlighted.