SMT007 Magazine

SMT-Mar2017

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40 SMT Magazine • March 2017 REWORK AND REPAIR ON FLEX CIRCUITS tacting the flex circuit lead, thereby resulting in an electrical "open." For rework paste printing, co-planarity is a challenge when the stencil and surface to be printed are not coplanar. There- fore, paste print deposition using a syringe is of- ten used instead. Sometimes, conductive epox- ies are used in interconnecting devices to the flex material. While the curing temperature of these joining materials is much lower than the reflow temperature of more standard solder, it can make a mess. Even when the rework pro- cess is engineered properly, many times the lim- itation on rework is that the marginal cost of the assembly is far less than the burdened re- work cost, making the scrap pile a more attrac- tive economic alternative. There are some advantages to reworking flex circuits from a process standpoint. The lower thermal mass compared to a rigid PCB short- ens the duration time to reach liquidus when soldering to a flex board. This speeds up the rework process for replacement. In addition, many times this lowers the air temperature re- quired from the hot air system thereby result- ing in less potential component damage. The high temperature withstand properties of flex materials such as Kapton, Peek and high-tem- perature polyimide all give the flex rework pro- cess a larger process window. In terms of industry standards for PCB re- pair, the IPC 7711/21 Repair and Modification of Printed Boards and Electronic Assemblies covers the rework and repair processes for flex circuits. Each of the processes in the standard are listed in terms of their applicability to flex rework or re- pair with an "F" in the upper right-hand part of the process documents under the "Board Type" section heading. There is even a flex-specific standard of conductor repair. Conductor repairs on flex is covered in procedure 7.1.1. By way of an example of a conductor repair, Figure 2 illustrates a torn flex circuit as part of a rigid-flex board. The standard process found in IPC 7721 3.5.1 was used to repair the materi- al. The conductor runs had a copper foil jump- er installed to replace the damaged conductors and then they were soldered together for fur- ther rigidity. The outcome of this repair can be found in Figure 3. Rework and repair of flex circuit assemblies is evolving and remains a challenge as the in- dustry continues to adopt best practices from the rigid assembly world. SMT Bob Wettermann is the principal of BEST Inc., a contract rework and repair facility in Chicago. Figure 2: Ripped rigid-flex circuit. Figure 3: Repaired base Kapton material along with conductors.

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