Combined Printed Solder Paste and Printed Adhesive Process for Devices Going through Wave Solder

ABSTRACT The secondary side devices assembled on boards along with through-hole components are usually mechanically secured to the secondary side using an adhesive and then electrically connected using a wave solder. This paper documents an improved process for attaching secondary side devices that combines two screen print machines for paste and adhesive printing followed by placement machines and reflow. The first machine prints solder paste on the SMT pads for the secondary side passive and other non-BGA active devices. The second machine prints epoxy adhesive underneath the same devices. The design of the glue stencil is unique in that the underside of this stencil has etched relief areas to avoid disturbing the solder paste printed by the first print machine. Subsequent placement and reflow run as they would with any other SMT process. The adhesive cures in the oven during solder paste reflow. The addition of paste and adhesive allows for secondary side parts to be either shielded from exposure with a wave solder pallet or exposed to the wave solder. This process has some inherent advantages over placing glue dots with needle type dot dispense machine as follows: Solder joints are created before the wave solder. This assembly process allows secondary side heat sensitive BGAs and other nearby parts to be shielded from the wave solder while other less heat sensitive locations are still exposed to wave solder. The new process allows designs with higher part densities and unrestricted orientation on the secondary side (smaller pads and closer spacing than are allowed with a wave solder process). The new process can increase the beat rate of the secondary side by 30-50% versus adhesive machines that dispense dots individually. Defect rates with this process are typically much less than with a typical dot dispensing glue machine (57 dpm versus 175 dpm at the part level).