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September 2017 • SMT Magazine 57 sacrificed for temperature profiling. A WLP was demounted using a test recipe, and holes were drilled through the WLP board-side pads at the center, top right, and bottom left corners. Ther- mocouples were inserted through the backside of the PCB, until flush with the board-side pads. The thermocouples were then bonded and cured using a thermally-conductive epoxy. Af- ter the thermocouples were attached and tested, the PCB assembly was clamped to the X-Y stage of the rework system. A thin coating of flux was applied to the WLP board-side pads, and a vi- sion system was used to pick, align, and place a fresh WLP on to the PCB. The package was re- flowed to the board, and heater times and tem- peratures were iteratively adjusted until the crit- ical reflow parameters were satisfied. Package Desoldering After package demount, residual solder de- bris must be removed to provide a smooth and even surface for reball. Conventional desolder- ing is performed using the solder wicking tech- nique with a hot air pencil and braided wire. Al- ternatively, package desoldering was performed using a no-contact vacuum scavenging method that eliminates mechanical stresses and ensures repeatable temperature control. Figure 7 shows a schematic of the vacuum scavenge set-up, in- cluding the vacuum nozzle and custom fiber- glass-resin fixture. The WLP was placed front- side up in a spring-loaded fixture and clamped to the rework stage. The top and bottom heat- ers were used to heat the component above sol- der liquidus temperatures while a vacuum noz- zle descended 0.1-0.25 mm above the compo- nent. The molten solder debris and flux were then vacuum-suctioned in a pre-programmed XY raster pattern. Because the WLP is so small, only two passes were required to remove the re- sidual solder from the package-side. The scav- enging height and velocity, scavenge pattern, and top and bottom heater temperature were all controlled using a semi-automated recipe. To reduce artifacts and ensure liquidus temper- atures, a test WLP was profiled by attaching a thermocouple to the component backside. Figure 7: Schematic demonstrating the tool set-up for vacuum scavenging a WLP. Figure 6: Photograph showing the 10 mm nozzle, metal pick-up tube, and 3.5 mm cup used during vacuum pick-up of the WLP. REWORK AND REBALL CHALLENGES FOR WAFER-LEVEL PACKAGES