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38 SMT Magazine • March 2017 Flex circuits are used in a variety of appli- cations for the interconnection of conductors which need to be bent or exercised continuous- ly. This interconnection technology has his- torically been used as a wire interconnection. There are a variety of versions of this type of circuit. One of them is the dual access flex cir- cuit which is a single-sided flex circuit that is manufactured so that the conductive material can be accessed from both sides of the flex. A double-sided flex circuit is a circuit having two conductive layers, one on each side of the base layer within the circuit. Trace patterns, specif- ic to your needs, can be created on both sides of the substrate film. They can be interconnect- ed where desired with copper plated through- holes. A multilayer flex circuit combines sev- eral single-sided or double-sided circuits with complex interconnections, shielding and/or surface mounted technologies in a multilayer design. Rigid flex circuits combine the best of both rigid printed circuit boards and flexible circuits integrated. Circuits are typically inter- connected between the rigid and flex circuits through plated through-holes. There are a variety of benefits to the flex circuits. One of the major benefits of a flex as- sembly is the nearly error-free implementation of wiring in lieu of labor intensive hand wir- ing. They are also able to configured, unlike their rigid counterparts, as complex 3-dimen- sional configurations as they can be contorted in to a variety of shapes. As the name implies, the materials used in flex circuits can be bent back and forth numerous times meaning they can be used in in highly repetitive applications such as on print heads. When weight is an is- sue, flex circuits are a good alternative to rigid boards and wires as both the dielectric material and conductor runs are very thin. Over the last several years the flex industry has seen a growth in demand. It is now a 10 billion WWD industry with growth rates in the 7–10% year range. With this surge in usage of flex, the stan- dards for rework (replacing devices while still meeting the initial specification and function- ality) and repair (repairing the physical damage on a flex circuit) of these type of electronic in- terconnection circuits has not kept pace. There are some rework challenges that come from the very nature of flex circuits. First of all, it is difficult to hold the flex circuit flat. The flex- ible nature of the Kapton or other base flex ma- terial, which makes it so attractive in the appli- cation, makes it challenging from a rework per- spective. In order to retain the flatness of the as- sembly, it needs to be taped to be held down. In some cases, a vacuum fixture, a relatively pric- ey endeavor, is fabricated for flex circuit rework. When placing fine pitch components, the vac- uum structure of such fixtures has a significant influence. If the vacuum is directly under some of the leads of a fine pitch component, there is a likelihood any vacuum will "pull" the flex into the hole, preventing the component from con- by Bob Wettermann BEST INC. Rework and Repair on Flex Circuits FEATURE COLUMN: KNOCKING DOWN THE BONE PILE Figure 1: Rigid-flex circuit board assembly.

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