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108 DESIGN007 MAGAZINE I JANUARY 2022 Figure 1 illustrates the difference in adhe- siveless flex cores vs. adhesive flex cores. Each of the above have specific uses depen- dent on the end use needs. However, if design- ers are uncertain as to what their options are and what material might be best, they can always contact the PCB fabricator's engineer- ing staff and ask for suggestions and recom- mendations. Incomplete or Insufficient Rigid-Flex Base Material Type Definition e base material chosen defines the perfor- mance limits of the rigid-flex circuit in process and in field operation in many applications. With most lead-free solders, the upper tem- perature excursions required for soldering can be as high as 260°C, which normally mandates the use of polyimide laminates. However, the material choice and its electrical proper- ties can affect other performance issues. One key area is in the management of characteris- tic impedance of the circuit, and assurance of signal integrity, with higher-frequency circuit designs becoming ever more common (these latter subjects will be given more attention later). Temperature range requirements for the rigid laminates used in rigid-flex construc- tions must also be considered and addressed. e rigid material should be high-temperature capable. Polyimide glass laminate is a common callout, but available improved epoxy resins are oen suitable. Copper Type and Thickness Callout While a number of different metal foils are available for making flexible circuits, copper is the most commonly used metal for making electronic interconnections. It is highly con- ductive, malleable (making it both flexible and foldable), relatively easily processed by etch- ing and plating, and relatively inexpensive. e type of copper used most oen for flexible circuits is rolled annealed copper (RA copper), which has the best properties for dynamic flex applications. When designing the copper, type and thickness choice should match the electri- cal and mechanical requirements for the appli- cation. icker copper is typically used for higher- power applications, and thinner copper for cir- cuits that require repeated bending (dynamic flexing). ere are many choices of copper thickness, but the most commonly used in the creation of flexible circuit laminates are pres- ently ½ ounce (17 µm or 0.7 mils) and 1 ounce (35 µm or 1 mil). Figure 1: Adhesiveless vs. adhesive flex.

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