Printed-circuit-board (PCB) substrate material, most commonly FR-4 glass epoxy, is a poor conductor of heat. Conversely, copper is an excellent conductor of heat. So, more copper area on a PCB is ideal from a thermal-management perspective. Thick copper, such as 2-oz. foil (68 microns thick), conducts heat better than thinner copper. Unfortunately, using thick copper is expensive, and makes it difficult to achieve fine geometries. Therefore, the use of 1-oz. (34 microns) copper has become commonplace. For external layers, this is often ½-oz. copper plated up to 1-oz. thickness.
Solid-copper planes used on inner layers of multi-layer boards work well to spread heat. However, since these planes are normally placed within the board stack-up, the heat can get trapped inside the board. Adding copper areas on the outer layers of the PCB and placing many vias to connect, or “stitch,” these areas to the inner planes helps transfer heat out of the planes.
On two-layer PCBs, spreading heat may prove more difficult due to the presence of traces and components. Providing as much solid copper as possible with good thermal connections to the motor-driver IC is a necessity. Putting copper pours on both outer layers and stitching them together with many vias helps spread the heat across areas cut by traces and components.
Trace width: wider is better
Since the current in and out of a motor-driver IC is large (exceeding 10A in some cases), carefully consider the width of PCB traces in and out of the device. The wider the trace, the lower the resistance. Traces must be sized so that excessive power isn’t dissipated in the trace resistance, which causes the trace to heat up. Too small a trace can act as a fuse and burn open.
Designers often use the IPC-2221 standard to determine appropriate trace widths. This specification has charts that show the copper cross-sectional area for various current levels and allowable