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28 FLEX007 MAGAZINE I OCTOBER 2018 sections are not inadvertently bonded together. Additionally, when utilizing a flex core as an outer layer in a multilayer construction it is important to note that all copper features are typically plated on the outer layers. If it is your intention to flex this device to any extreme or more than once it would be advisable to have only the holes and pads plated on the flex cir- cuit side (Figure 4). Electrodeposited copper is brittle and plat- ing the traces on the flex side may crack when flexed. These structures are relatively straight- forward in terms of processing but it is once again worthwhile to check with the fabrica- tor's engineers for review and comment before submitting the design for quote. 8. Flexible circuit structure with multiple discrete breakout sections: One of the most attractive features of flexible circuits is their ability to serve as miniature wiring harnesses. Flex circuits can interconnect—in three-dimen- sional space—the various elements (e.g., mod- ules, switches, and power sources) of an elec- tronic product. In many cases, such as with one and two metal layer circuits, this is a primary objective that can be easily accomplished by simply designing the circuit with discrete routes to desired terminations in panel form and then routing, punching or cutting the circuit from the panel. In the case of multilayer and rigid- flex circuits, this activity is not so simple and much more attention to design and process detail is required. Because of the infinite variety of design possibilities, it is not possible to give a generic recommendation beyond consulting with the fabricator's manufacturing engineer to pre-determine a best approach (Figure 5). 9. Flex layers with multiple layer separa- tions and discrete routes: As with the pre- vious discussion, it is possible to prefabricate internal layers with the same multiple discrete routes and, moreover, to route them from sepa- P L A TED TR A C ES A ND S MD (F UL L P L A TE) P L A TED HOL ES A ND PA DS R I GI D S EC TI ON ` ` MA S K PLATED COPPER LAYER 1 LAYER 2 LAYER 3 RIGID LAMINATE CORE 10 MILS NO-FLOW PREPREG 6 MILS COVERLAY 1 MIL POLYIMIDE ADHESIVE 1 MIL ACRYLIC 1/2 OZ COPPER ADHESIVELESS POLYIMIDE FLEX 1 MIL COVERLAY 1 MIL POLYIMIDE ADHESIVE 1 MIL ACRYLIC BUTTON PLATE ONLY. PLATED PAD SMALLER THAN THE CIRCUIT PAD AND NO PLATING ON TRACES. FLEX SECTION MA S K 1/2 OZ COPPER 1/2 OZ BASE COPPER 1/2 OZ COPPER 1/2 OZ COPPER Figure 4: For extreme flexing, consider plating only the holes and pads on the flex circuit side. Figure 5: Flex with multiple discrete breakout section.