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Introduction of Flexible Circuit Assembly

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Flexible circuits offer some unique challenges to the assembly process. The assembly materials and processes for populating and interconnecting components to a flexible circuit range are essentially identical to those used for standard rigid or rigid flex pcb, but there are some twists required, as will be shown. The assembly processes range from very simple methods, such as manual component insertion and hand soldering (which requires little or no fixturing), to fully automated methods, which normally require specially developed, design-specific and dedicated fixtures.

How, then, does one choose an assembly process and method for flexible circuits? To begin appropriately, it is necessary to consider a number of important factors: What is the flexible circuit base material? What types of components will be used? How many assemblies will be built? These and other important questions must be addressed before one can adequately make the proper decisions regarding assembly. These seemingly simple matters can greatly influence the pcb assembly choice. For example, it is commonly assumed that polyester circuits cannot be used in applications where soldering is required. The reason for this assumption is that polyester films have a low melt point and will be grossly distorted by the tin-lead soldering process. However, when properly fixtured, soldering can be used for joining components to polyester. A number of major OEMs have been doing just that for many years. They have developed methods that provide proper shielding of the body of the polyester circuit in process. The distortion of the material can be localized to areas adjacent to the point of connection.

With the advent of the well-meant but misguided legislation mandating the use of lead-free solders, there will be significant challenges ahead, as lead-free solders have a roughly 30-40ºC higher melting temperature than traditional tin-lead solders. Sadly, it appears that traditional tin-lead solders are actually more environmentally friendly, so the law does double damage. One alternative is to use lower-temperature solders, or conductive adhesives can also be employed. The important thing is that manufacturers not limit themselves in their thinking and that they be open to alternative solutions. Following is a brief review of the key elements of flex circuit assembly.
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