Fine Pitch Microdots Dispensing and Jetting Optimization in SiP Assembly using Welco Solder Paste

Abstract

With the massive advancement in heterogeneous SIP integration, today’s the industry required microscopic dispensations with greater accuracy and consistency requirements for MEMS cavity or 3D-mid soldering application which are integrated into the SIP assembly. Solder paste dispensing has been used for decades and it offers better flexibility by allowing dispensed solder into cavities and on top of the mounted components. Lines and irregular dispense patterns are also achievable using this method. The main drawback is the relatively slow speed at which solder paste can be dispensed when compared to screen printing. 3D screen printing has its own limitation in terms of stencil fabrication and paste deposition consistency at narrow pitch though it offers higher throughput. With the recent breakthrough in the microdots dispensing technology, fine pitch and high accuracy paste dispensing can be achievable without splashing and significantly reduces the deposit satellites in acceptable tolerances. Solder paste jetting can be considered a viable option other than dispensing. It is a non-contact dispensing fluid application where the transfer of momentum is through high accelerations and resulted in fixed volume amount by controlling the material fed into a chamber. It also enables multiple pass possibilities to customize solder paste deposits other than single dot variation. Customization can be done for volume, paste height, shape, position, and pad coverage. In terms of material perspective, the paste rheology is critical for both dispensing and jetting application. Stable and constant shear viscosity over time is crucial in ensuring consistent dispensability without defects using Type 6, 7 and even Type 8 powders. Adjusting the metal load can helps improving the paste fluidity flow rate but adverse effect will be less actual solder amount formation after soldering. This paper will study the effect of dispensing or jetting parameters on dispensability performance over time at fine apertures as well as needle gauge size configuration with response to the powder size used.