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JANUARY 2020 I DESIGN007 MAGAZINE 85 Tear Relief Figure 4 shows the most common and ef- fective method of eliminating tears in a flex- ible circuit. Copper tear stops are not advised; they have been shown to have limited value in keeping a tear from starting or propagating. Vias Through-holes can connect all layers at a via point. Blind vias connect outer layers to adja- cent layers but do not extend through the cir- cuit. Buried vias connect internal layers; how- ever, they do not extend to the outer layers. Blind and buried vias will increase the cost of the circuit but can increase usable PCB real es- tate on non-drilled layers. SMT Access Openings The two most common cover materials are polyimide film and flexible solder mask. The methods for creating access openings in the two materials are very different and carry dif- ferent design requirements. Access openings in polyimide film are created by drilling, rout- ing, or punching, which limits the size and shape of the openings to what can be done with a round bit or a tool. For this reason, SMT access openings in polyimide film are ei- ther round or oval. Also, gang access of multi- ple SMT pads is a common design practice on flex circuits. Flexible solder mask, like regular PCB solder mask, is photo-defined, so any shape opening is possible. Solder mask openings should be made slightly larger than the SMT pads to ensure that the mask does not get on the pads if there is any misregistration in the printing process. Controlled Impedance and Signal Integrity The speed at which electronic devices are op- erating is continually increasing. The result is that the characteristic impedance of all parts of the electronic assembly, including any flex or rigid PCBs in the system, need to have matching impedance. Impedance mismatches will cause signal reflections and degradation at each mis- match point, which results in erroneous signals and, ultimately, device malfunction. The characteristic impedance of flex can be determined before manufacturing using an im- pedance calculator. Your flex fabricator can as- sist you with these calculations, or you can buy or download an impedance calculator. A number of factors will affect the characteristic impedance of a flex PCB. The main contribu- tors are: • The dielectric constant of the insulation materials used to construct the circuit Figure 4: A proper tear-relief design option. Illustration of a standard two-layer circuit construction.