Issue link: https://iconnect007.uberflip.com/i/869081
56 SMT Magazine • September 2017 tant for WLPs, as it is difficult to handle and se- cure the parts. Experimental A 3.5 x 3.5 fan-in wafer-level chip scale package was used for rework and reball evalu- ation. The package uses standard 0.5 mm pitch SAC405 solder balls. Demount, desolder, and re- ball steps were all performed using a semi-au- tomated rework system. OM and CSAM inspec- tion were performed before and after to assess for mechanically- or thermally induced rework artifacts. Package Demount from Board: Figure 5 shows the set-up for package de- mount using a semi-automated rework system. The PCB assembly was clamped on a motorized XY stage and a vision system was used to man- ually center the WLP between top and bottom heaters. Top heater convection was localized us- ing a 10-mm nozzle attachment. The nozzle was lowered onto the board, and an automated rec- ipe was used to heat the part to reflow temper- atures. To prevent thermal artifacts and ensure complete solder liquidation, the WLP was heat- ed between 217 and 250°C for 60-90 s. At peak reflow of 235–250°C, the WLP was lifted from the PCB using vacuum suction through a met- al pick-up tube. Figure 6 shows a 3.5 mm vacu- um cup that was used to minimize mechanical stress to the WLP silicon backside. In order to accurately measure the WLP sol- der joint temperature, a board assembly was REWORK AND REBALL CHALLENGES FOR WAFER-LEVEL PACKAGES Figure 4: Schematic illustrating the WLP condition after demount, desolder, and reball. Figure 5: Schematic of set-up used to demount a WLP from a PCB.