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34 SMT Magazine • March 2017 tent height. In the case of solder paste printing applications, it's crucial that the assembly is at a specific height for even distribution of paste. If the assembly doesn't enter the machine proper- ly, the paste won't print correctly and gasketing issues can arise, which will cause manufactur- ing defects later in the assembly process. In the case of SMT pick-and-place ma- chines, the pallet for the flex circuit ensures that the assembly is also presented to those machines at the ideal height. High-speed SMT placement machines don't leave much room for error, so the flex assembly must be ade- quately prepared and supported for those fast placements. Beyond tooling support, manufacturing engineers and technicians need to pay atten- tion to the soldering heat applied to flex cir- cuits. Since flex circuits are typically very thin, there's nowhere for the heat to go besides into the barrel, so thru-hole parts must be soldered at lower temperatures. Hand soldering a flex circuit can be challenging, and requires an experienced technician to utilize the perfect technique. "When hand soldering a normal, rigid as- sembly you can touch the PCB for three to five seconds with a temperature range of 600– 800°F," commented Cathy Cox, process engi- neer at Lectronics. "With certain flex circuits, you can only touch that board for one to two seconds at most, and the heat is much lower at about 580°F." In the case of testing flex circuits, the same support and tooling rules apply. All flex circuits require appropriate backing to support the as- sembly during the testing process. In the case of flying probe testing, a special custom fixture should be developed to provide that support to the PCBA when it's being probed. For in-circuit testing (ICT), a rigid back- ing is usually designed into the clamshell fix- ture. While this can add mechanical challeng- es for ICT development, it's needed to ensure safe testing of the flex circuit. Additionally, low force probes should be used to avoid any unnec- essary damage to the flex's fragile surface. "We also make special provisions to guar- antee the orientation of flex circuits during the testing process," commented Tom Newman, test engineering manager at Lectronics. "Flex assemblies don't always have tooling holes to use as guides, so it's important to use a fixture or cut-out that the flex assembly can sit into per- fectly." Traditionally it can also be difficult to ac- cess test points on flex assemblies. Without ex- posed thru-hole vias, test technicians have lim- ited access to probe the assembly. Additional- ly, designing in appropriate test access points is sometimes overlooked with flex products. Electrical engineers should design in appropri- ate test points during the product development stage. De-panelizing flex assemblies can also be tricky. In many cases the only option for de- panelization is by hand, which can lead to poor quality control and repeatability issues in man- ufacturing. Laser de-panelizing is gaining popu- larity, and while much more precise than hand cutting, it's also a far more expensive solution. However, as laser cost continues to go down, these machines should become more common- place. Lectronics' engineering team works close- ly with bare board fabrication suppliers to op- timize the array design and perimeter connec- tions within the flex panels. They have specif- ically developed proprietary techniques to im- prove and optimize the de-panelization process where special tooling is no longer required. As flex circuits continue to solve design con- straints for space, heat, weight, and bend re- quirements in a variety of product applications, electronic manufacturing suppliers will devel- op new manufacturing parameters to assemble these flexible products. While flex circuits can be temperamental and play by their own rules at times, all these thin, fragile, bendy, heat-sen- sitive circuits really need is the right support system. SMT Davina McDonnell is the director of marketing at Saline Lectronics. To read past columns, or to contact McDonnell, click here. SUPPORTIVE TOOLING: THE MAGIC INGREDIENT FOR FLEX CIRCUIT ASSEMBLY