SMT007 Magazine

SMT-Jan2018

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92 SMT007 MAGAZINE I JANUARY 2018 attach the replacement pad with the connect- ing circuit by lap soldering the two together. Mix the two-part epoxy per the manufactur- er's suggestions, direct the mixture and apply pressure to the replacement pad and laminate and cure per the manufacturer's instructions. (This technique also works for still attached but lifted only pads.) Make sure to inspect for anomalies and perform an electrical continuity test as a post repair check. In a second repair method (IPC 7721 Proce- dure 4.7.2), a dry film-backed plated replace- ment pad is adhered to the PCB laminate by a heat and pressure-activated dry film. All the process steps in the above epoxy method apply until you get to the sequence where a repair material is chosen. Once the replacement pad in the circuit frame is identified, it is cut out from the frame using a sharp blade. Only in this case the technician has to make sure that they cut through both the replacement pad as well as the adhesive backing. Now the pad is soldered to the trace if required. A special tip and handheld heating tool similar to a solder- ing iron is placed into a fixture which controls the downward pressure. Through this heated tip temperature and tip pressure the adhesive cures. The other steps are similar in nature to the epoxy repair method. One of the most challenging types of pad repair are those with embedded vias. These vias, connected to other board layers, will still need to maintain their connectivity to the connected layer post repair. There are numer- ous approaches to maintaining these very diffi- cult repairs including but not limited to small jumper wires of small copper trace intercon- nections. There are two methods for repairing damaged pads. Both the two-part epoxy/resin and the dry film techniques will do the job. In either event, a highly-skilled and trained technician can make most of these types of repairs when financially viable and where the risks of reli- ability have been mitigated based on the end use operating environment. SMT007 Reference 1. IPC 7711/21 Rework of Electronic Assem- blies/Repair and Modification of Printed Boards and Electronic Assemblies. Bob Wettermann is the principal of BEST Inc., a contract rework and repair facility in Chicago. Figure 2: Replacement pads of commercial circuit frame. Figure 3: Replacement pad held down with Kapton tape for epoxy/resin curing process.

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