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110 DESIGN007 MAGAZINE I JANUARY 2022 Oen additional copper may be plated onto the circuit, and this should be accounted for in the specification as well. If the designer is uncertain, he should contact engineering sup- port for guidance. Coverlayer or Solder Mask Over Flex Circuits Coverlayers, or cover coats, are polymer materials used to cover and protect the copper traces of the flex circuit product. As implied, there are a number of different options avail- able for protecting the circuits, and they serve different design requirements in terms of cost, performance, and flexural endurance optimization. When specifying the choice, it is important to call out not just the type of coverlayer material but also the thickness requirement. is can be very important in certain types of constructions, especially when a flex circuit will experience dynamic flexing during use. In terms of cost, a flexible solder mask is gen- erally the least expensive. Some one- or two- layer flex circuits, that will not be subject to multiple flex cycles or extreme radius bends, can be coated with an epoxy-based solder mask that is designed to flex without cracking. is, however, is not recommended when the design requires any dynamic or extreme flex- ing. e other option is the laminated cover- layer. ese are typically materials that have a makeup that is identical to the flex core mate- rial's and are best suited for dynamic flexible circuit applications. e coverlayer material is a polyimide sheet with acrylic adhesive on one side. It is typi- cally pre-machined to create openings in the sheet where the final finish is required. e coverlayer sheets are usually applied in a lami- nation press using special pads to assure con- formity around the copper features on the flex layer. For rigid-flex circuits, the coverlayer is typically cut to only protrude into the rigid portion by no more than 50 mils. e purpose of this is to allow all the plated holes in the rigid-flex to be void of any acrylic adhesive, as it can affect the hole wall plating integrity. Figure 2 shows an example of flexible sol- der mask and coverlayer being used in flex circuits. It is worthwhile to note here that bond ply used to laminate flex layers together is like a coverlayer, but it has adhesive on two sides. It is further worth noting that prepregs (glass cloth which has been pre-impregnated with a thermosetting resin), used for making rigid circuits, are used in the construction of rigid- flex circuits where they serve in the role of bond ply. It is important to note that coverlayer mate- rial can come in typical thickness inter vals from .5 mil – 5 mils (12-125 µm) of poly- Figure 2: Flexible solder mask and coverlayer used in flex circuits.

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