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September 2017 • SMT Magazine 71 TOP 5 BGA REWORK CHALLENGES TO OVERCOME leveling of the pads is found in 4.2.1. The improper meth- od for pad leveling involves 'scrubbing the deck.' This oc- curs when the wick is pushed back and forth along the un- derside area of the BGA pad site. This can result in the mask being scratched and pads being bent up or lift- ed off. The proper technique (Figure 8) calls for the solder braid to be moved in an up and down fashion onto the pads thereby not scratching the surface or prying the pads up. One way to overcome the problems associated with these contact types of pad prepping processes which rely on operator skill is to use a non-contact method for site preparation. Several higher end rework systems are now equipped with programmable non- contact scavenging systems. In these systems, a heated nozzle comes in to proximity of the PCB to reflow the solder, while at the same time fol- lowing the contour of the PCB surface and vac- uuming up the molten solder from the board. This way results in no damage to the pads or board surface. In addition, the skill level of the rework technician is removed as a variable. The downside of this approach is lessened through- put as both the programming time of the scav- enging system and the extended cycle time of the scavenging operation makes this somewhat cumbersome for most rework. The proper BGA removal profile needs to be developed to make sure the removal of the BGA will not unnecessarily cause pad lifting for re- moval of the BGA. The process for developing such a profile is like that of profiling a PCB for assembly, only done in reverse and on a small- er scale. First, a solder sample board is analyzed to determine the proper location of the vari- ous thermocouples. Typically, one is embed- ded in the die of the BGA, one each in two cor- ner balls, one or more on the neighboring parts and one or more scattered throughout the BGA ball pattern depending in the idiosyncrasies of the PCB's thermal characteristics. Attachment of the thermocouples includes drilling and glu- ing them into the board, the solder balls, and the actual die itself. After this the technician di- als in a reflow profile based on experience. Sol- der profiles are developed and the temperatures at these locations are measured. From there, ad- justments to the profile, removal of neighbor- ing parts, or proper heat shielding will fully op- timize the profile for minimized reflow time and damage to the neighboring components, while gaining the most consistent collapse of the sol- der balls. Bottom-side heat is especially neces- sary with thermally massive boards or for those processed using lead-free solder. This will pre- vent thermal damage to the PCB if the rework process is being done in higher liquidus tem- perature solders such as lead free. Multi-zone bottom-side heaters are sometimes necessary for higher layer count boards and to maintain a smaller temperature differential over the entire component area. Finally, the proper board sup- port will help prevent warping of the board. If the pads have been turned up by more than one pad thickness or have been partially or completely ripped off the board there are two Figure 8: Proper site preparation technique eliminates mask and pad damage.