Design tricks for tight bends.

Since you are designing flex layers into a circuit, it’s a safe bet that circuit is intended to bend. The key question is, will it survive being bent, twisted or otherwise contorted? When considering flexing, focus on a few key variables: the flex thickness, bend radius, bend angle and number of bend cycles over a lifetime. Some flexes operate in dynamic applications that experience continued flexing. Examples include the hinge in a laptop, a printer head, a drawer-type movement or a rotating mechanism. Other applications involve a limited number of bend cycles, where the circuit is bent into position and may move again during product servicing. Finally, some circuits bend into place and never move, experiencing only minor vibration. Let’s start with the static applications. Generally, if the part is bending less than 60°, it is almost impossible to cause damage. The IPC rule of thumb for static bending is simple. For one or two copper layer designs, the bend radius should be 10X the thickness of the flex. For three or more copper layers bonded together, the rule is 20X the thickness of the flex.

Heat dissipation varies with installation orientation and ambient conditions.

What are the upper temperature limits for a flexible heater?

Depending on the material selection, flexible heaters can conservatively run at temperatures exceeding 200°C. Many variables must be considered when pushing the upper limits, however. The main variables are power (watt) density, ambient temp, heatsink type and heatsink bonding method.