"Course: BGA Rework-A Primer-BGA Reflow"
After part placement, the applied solder must be reflowed to attach the BGA to the PCB pads. Preheating the entire PCB bottomside to between 70°C and 125°C is critical to minimizing PCB warpage during the heat cycle. The thermal profile already developed should have the following basic characteristics:
In addition, care must be taken to ensure that adjacent components are not damaged during the reflow process. This care must be extended to components with low melting points such as connectors or standoffs or adjacent moisture-sensitive devices. Care should also be taken to not overheat any internal thermal grease if the BGA is capped.
After successfully attaching the BGA to the PCB, and allowing the board to be cooled down, the perimeter row of solder joints should be visually examined. The solder joint between the ball and the PCB pad should be similar in size and shape to the ball/part joint. If a water-soluble paste is used, the reworked area needs to be cleaned. Cleaning parameters should those be used in the original assembly process.
PCB warping and bowing should be minimized by the BGA rework process. This can be a serious problem in the case of very thin PCBs or boards with an uneven thermal mass distribution. The bowing can be exacerbated by the proximity of other components such as pin-in-hole connectors that may anchor the PCB, thereby enhancing the localized bowing under the BGA. If the PCB is bowed or warped, they might not be able to properly fit into the final assembly or opens may be formed.
BGA rework reflow sources commonly used include both hot air as well as infrared heat sources. IR reflow is preferred for micro miniature devices as the lack of air flow prevent the component from being disturbed during reflow. Hot air reflow is used for faster profiles, especially desirous when there is a great volume of BGA rework to be accomplished.