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100 DESIGN007 MAGAZINE I AUGUST 2020 a standard, or an advanced and unachievable design because of their capabilities. And this illustrates the importance of knowing vendor capability in advance. Production capabilities change with tech- nological progress and the sophistication of production equipment. As a result, production design rules will be updated accordingly. Start- ing layout, the FPC designer should be familiar with IPC standards—such as IPC-2223, IPC- 2221, IPC-2222—and be aware of the product class and the type of FPC. According to IPC- 2223, there are five types of FPC constructions for adhesive substrates and the same five types for adhesiveless substrate. Figure 2 shows examples of Type 1 adhesive substrates and Figure 3 shows Type 2 adhesiveless substrates. The flex designer should be aware of the final assembly requirements. Connections between rigid and flex circuits can be achieved with B2B connectors, ZIF connectors, anisotro- pic conductive film (ACF) or paste, and hot bar soldering (Figure 4). Each of these techniques requires a different size and shape of the termi- nations in the layout. The choice of flex materials and the stackup depends on the budget, application, and mechanical restrictions. Since long ago, the most popular flex laminate material has been polyimide. Polyester films are popular for inex- pensive constructions. In addition to these widely used plastic materials, liquid crystal polymer (LCP) and polyethylene naphthalate (PEN) substrates are also utilized. FPC design procedure has these general rules regardless of application, type, and the vendor's level of sophistication: 1. All FPC corners must be rounded. 2. All traces' corners must be rounded. 3. All vias, plated through-holes, and SMT pad connections to traces must have teardrops/fillets. In addition to these three rules, it's good practice to make the SMT pads oval or round in shape. It is also possible to round corners of larger SMT pads (Figure 5). Since copper adds rigidity to the construction, the designer shall route traces in a staggered manner on the adjacent layers and avoid copper planes, a technique which eliminates the "I-beam effect" and increases the FPC's reliability. Decreasing the rigidity of a flex circuit's but- ton copper plating also reduces the amount of copper. Button plating is a production process that selectively deposits copper onto the pads and consequently barrels of vias and plated through-holes, and it requires additional work with Gerber files. The other method of flex copper plating is panel plating. During panel plating, copper is deposited on the entire copper pattern of the external layers and TH barrels. Flexibility can be increased by using crosshatched copper rather than solid copper planes. Figure 2: Type 1 single-sided adhesive substrate. Figure 3: Type 2 double-sided adhesiveless substrate. Figure 4: A hot bar connection.