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Design007-Oct2020

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94 DESIGN007 MAGAZINE I OCTOBER 2020 The heavy-copper flex can have plated vias, as well as multiple traces and circuitry. Some heavy-copper flex circuits can have components designed onto the surface, although they must be placed clear of any bend area. Heavy-cop- per flex-based circuits are used in applications where a flat, stiff, circuit board will not fit. Typi- cally, heavy-copper flex circuits are single- or double-sided circuits with simple, open designs. Crowded, highly populated circuits do not bend well because the components do not bend. When designing a bendable, heavy-copper flex circuit, consideration must be given to the placement of the components. All components must be kept clear of the bend lines. If only a small bend radius is required, some com- ponents, such as capacitors, will allow a very small radius. Multilayers are possible but with limits on layer counts and significantly decreased ability to bend. As a multilayer is a composite con- struction consisting of many layers, it imparts a larger stiffness in the inability of adjoining layers to move in relation to each other, creat- ing a rather stiff but bendable package. A typical flex solder mask or cover coat can be used. The solder mask must be sprayed or applied in such a way as to cover the sides of the tall, heavy-copper traces. When a cover coat is used, extra layers of glue sheets are used to fill the gaps in between the higher heavy- copper traces. An additional use of bendable, heavy cir- cuit boards is the creation of vertical surface buss bars made by combining the attributes of a bendable flex board with heavy copper and designing the fingers to be soldered into the main PCB. These heavy-copper fingers that protrude off the board are made by removing the flex material portion with a sculpting laser, leaving the copper behind (Figure 1). A bendable buss bar can snake around the surface vertically, supplying power and car- rying heavy currents. The base printed circuit could be less expensive—typically a 1- or 2-oz. FR-4 PCB with the vertical, bendable power board carrying the current. Thermal dissipa- tion would be increased because the buss bar would be vertical, increasing the thermal shear and airflow. Single-layer, double-layer, and Figure 1: Heavy-copper leads off the flex PCB.

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