FLEX007

Flex-July2018

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64 FLEX007 MAGAZINE I JULY 2018 typically incorporate a wide range of mate- rial options. There are hundreds of different flexible circuit constructions that all fit under the general category of a "flexible circuit." Each flexible circuit is customized not only in shape and electrical interconnect design; it is also customized with the materials that meet the supplemental specific application requirements of the OEM, many of which were mentioned earlier. In addition to these material selection options, there is also a long list of design-related features that are incorporated into the conduc- tive pattern. These look to address potential reliability issues that may surface due to the environmental and mechanical forces the flex is subjected to. For instance, a very specific material may be needed for the flexible circuit to withstand exposure to a unique chemical within an appli- cation. However, that material may have ther- mal expansion characteristics that can impact plated through-hole integrity. Another exam- ple may be a flex circuit design that requires dynamic flexing, but the layout has sharp cor- ners or plated through-holes in the flex zone that can introduce reliability problems. Unfortunately for the OEM, this wide combi- nation of material options and reliable design knowledge for the application-specific solution is something that only an experienced flex cir- cuit manufacturer with high material science expertise can address. Furthermore, individual designers and engineers at OEMs are not in a position to be knowledgeable about the ever-changing material solutions and the unique flex circuit design considerations compared to the flex manu- facturer. The OEM designer may be involved in a flex cir- cuit design once every two years, likely designing signifi- cantly more rigid PCBs for his company's product develop- ment work. Such infrequency does not enable the designer to become an expert in flex circuits and all of their options to solve different problems that arise. He has no choice but to rely on the flex- ible circuit manufacturer for this expertise and guidance. With rigid PCBs, the OEMs don't have to worry about the supplemental mechanical ele- ments woven into a flexible circuit application. The rigid PCB industry is nearly identical from one manufacturer to another; once the PCB is completed, it can be shopped around to the cheapest source. In addition, the board itself remains identical in construction no matter who builds it so the risk of introducing a prob- lem by selecting an alternative supplier to the PCB is low. But with flex, the unique characteristics of the flexible circuit created by the material choices and design factors can change even with minor or unpredictable differences from one flex manufacturer to another. For instance, one flexible circuit manufacturer may stan- dardize on one type of bond ply that is 'equiv- alent' to another according to the manufac- turer of the bond ply. Yet, unknowingly, the two have different behaviors when subjected to the OEM's specific application and one fails. This is a risk an OEM can't afford to take by changing from one flex supplier to another. The original flexible circuit has proven to meet the challenges of the application through the OEM's testing protocol and making a switch to another vendor could introduce something Table 1: Flex circuits are similar to rigid PCBs, with these important exceptions.

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