SMT007 Magazine

SMT007-Jan2020

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90 SMT007 MAGAZINE I JANUARY 2020 There are a couple of approaches that hold promise in preventing this solder "squirt out" during the BGA rework process. One of the methods is through the use of a mechani- cal mill to completely ablate the component from the PCB. No direct heat is applied, but the ensuing debris, mechanical stresses on the joint, and precision required does not make this rework method an option in every case. Another rework method being resurrected is the use of a highly focused laser as the heat source [1] . This method relies on bringing the temperature above liquidus on the solder joint very quickly, thereby not giving the opportu- nity to reflow neighboring devices while also not allowing the underfill to squirt out. The challenges of reworking these underfills can be mitigated to a great extent through: 1. More thorough training of the operators 2. Having the right tool for the right job 3. Having the right process and site preparation process 4. Controlling collateral heat damage 5. Proper reflow profile development 1. Operator Training BGA rework technicians must be fully trained and must have their skills practiced and developed. These technicians need to be conversant in material selection, includ- ing underfill softening agents, fluxes, and sol- ders. They also need to understand the inter- relationship of underfill softening and solder reflow temperatures. In addition, knowledge of how the softening agents may/may not interact with the materials on the PCB would prove helpful. Since X-ray analysis is many times the process debugging tool, the BGA rework technician needs to be schooled in reading X-ray images. 2. The Right Tool for the Right Job In order for the process to have a chance of success, the right tools—including the type of rework heat source, a profiling tool, X-ray equipment, and solder excavating tools—need to be used. This will help to have a controlled rework process with repeatable and predict- able results. 3. Proper Site and Process Preparation There are several processes, as well as site preparation steps, to consider once the com- ponent on a PCB with underfill has been removed. Proper moisture sensitivity con- trol (per IPC/JEDEC J-STD-033) methods will help prevent both the board and the com- ponent from moisture-induced damage dur- ing the rework process. Proper bake-out heat cycles will drive out the moisture from both the component and the board to prevent component "popcorning" and board delami- nation. An accurate assessment of solder ball size, solder mask condition, and a search for missing or contaminated pads at the com- ponent rework as well as neighboring sites will help determine the effectiveness of the rework process. 4. Collateral Heat Damage There are numerous problems that can arise from the reflow of neighboring underfilled component solder connections; for example, oxidation, de-wetting, pad and lead damage, solder ball formation, wicking, starved sol- der joints, and component damage are the big- gest anomalies post-rework. Depending on the rework heating source, there can be a variety of heat shielding methods employed [2] . 5. Proper Profile Development The rework profile for underfilled PCBs mostly duplicates the original reflow profile. To protect neighboring underfilled components due to the below solder liquidus softening tem- This phenomenon may be most pronounced for pads that are no-connects underneath the device.

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