Issue link: https://iconnect007.uberflip.com/i/408601
16 SMT Magazine • November 2014 rework cHaLLenges For smartPHones anD tabLets continues feaTure An example of an epoxy fillet removal tech- nique for package-on-package (PoP) or ball grid arrays (BGA) is the use of a handheld convec- tion tool and dental picks, or the use of a solder- ing iron with very small solder tips. The den- tal pick or solder tip is heated and then used to carve the epoxy away from the component. This process requires a skilled operator as no au- tomation process exists. To facilitate the ease of carving the epoxy, the dental pick tip must be polished to a mirror finish using extremely fine grit sandpaper. The polished tip, when heated, will achieve the temperature needed to change the physical properties of the epoxy. Even a skilled operator cannot guarantee success in re- moving the epoxy fillet. Care must be taken to ensure that the tool does not damage the sur- face of the PCB, component to be removed, or adjacent components. The next step is to remove the component from the PCB by using infrared or convection to heat the board and a retractable vacuum noz- zle or other mechanical means, like a tweezers nozzle, to physically remove the component. In densely packed mobile devices, BGAs are often mirrored on the PCB. Removal of the top BGA requires precision control of the temperatures. If the solder on the bottom BGA becomes liq- uidous at 217°C during the top BGA's removal profile, the solder underneath the bottom BGA expands in volume, forcing the solder to ooze at high pressure through the softened epoxy which has exceeded its glass transition tempera- ture. This can be seen as small solder balls that have appeared out of the BGA (Figure 4). The result of this phenomenon is the ap- pearance of a poorly executed surface mount reflow process, and the possibility of functional failures as shorts. When an X-ray inspection is conducted and compared to original solder joints, a difference in size can be seen between the original printed circuit board and the circuit board after rework. The bottom BGA now has opens as solder has been lost. The solution is to keep the bottom chip below 217°C. 208°C, in general, is the ideal rework temperature for most parts. This often requires the use of a spot heater in the preheater to locally heat and cool components in a vertical position during re- work. With the correct temperature applied to the bottom BGA, the top part can be reworked at lead free temperatures. At this point, the choice must be made to scrap the part or recover the component for later reuse. Some production-level manufacturers re- claim components, or reball and reuse some parts because of cost (e.g., memory, processors, and Wi-Fi chips). Energy-efficient and faster DDR memory costs up to $35 each, while pro- cessors can also cost up to $35 apiece and NAND flash memory and Wi-Fi chips often costing as much as $41 each. PoPs with both processor and memory are up to $50 combined. The cost of these components can represent a signifi- cant portion of the cost of the PCB. If a PCB is damaged due to poor rework operations, some money can be reclaimed from refurbishments of the high cost components. Reusing compo- nents after removal is a hidden operation, often kept from most OEM manufacturers. However, contract manufacturers recycle parts as a cost-savings measure. Removal and reuse of components is common in the devel- opment lab environment. Component cost for a prototype may be high and the time needed to acquire a new part may exceed the time avail- able which increases development time and costs. This is true in and out of warranty rework markets. If they are to be reused, the compo- Figure 4: Solder expelled through epoxy during removal.