Flexible Circuit Substrates

The thickness of a flexible circuit substrate is a crucial parameter that determines the performance and manufacturability of a flex PCB. Thicker circuits require a larger assembly area, higher manufacturing costs, and tighter tolerances than thinner ones. However, thicker circuits also have the advantage of greater thermal stability and are less likely to experience stress due to temperature fluctuations. Determining the right thickness for a given project requires balancing electrical, mechanical, and manufacturing considerations. Prototyping and testing different flex PCB designs early in the design process and working closely with a specialized flex manufacturer skilled in thin flex fabrication will help to identify and optimize the best flex circuit thickness for your application.

The dielectric material that makes up the majority of a flexible circuit PCB is usually a polyimide film such as Kapton, which provides the perfect balance between flexibility, thermal stability, and cost. It’s also highly tolerant of solder reflow cycles and high temperatures, making it ideal for the demanding requirements of a flex circuit. Polyimide can be fabricated at ultra-thin thicknesses down to 6 microns, allowing for the creation of extremely thin multilayer flex circuits.

Stiffener layers (known as insulator plates in the PCB world) are an integral component of most flex PCBs. These are typically FR-4 or Kapton, with the latter being used for rigid sections of a flex circuit that need to maintain a certain degree of rigidity. Stiffener thicknesses can range from 0.002 to 0.059 mm, and they are incorporated into the flex circuit design to protect the conductors and components that have been attached to the stiffener.

Thicknesses of Flexible Circuit Substrates

In order to ensure that a flex circuit will bend properly, it is important to follow the recommended design guidelines that dictate where and how traces should be routed within the board. A common rule is to keep traces away from areas that will need to bend, as this can lead to breakage of the copper circuits and reduce overall lifespan of the flex circuit. When a flex circuit does need to be routed through a bend area, it is generally preferable to use S curves rather than angles to prevent the bending from causing stress on the traces and resulting in damage.

Another important factor to consider when choosing a flex circuit thickness is the thermal expansion and contraction rate of the material being used. Choosing a material that is low in CTE, and ideally near the glass transition temperature (Tg), will help ensure that it can withstand a reasonable amount of flexing without causing stress cracks in the copper.

In order to avoid stress fractures in a flex circuit, it is often necessary to use a wider bend radius than is typically used for rigid boards. This is because the larger bend radius helps to reduce the forces on the flex circuit that could cause damage to it in the event of a failure, while still providing sufficient flexibility. To further reduce the chances of a flex circuit becoming damaged, it is also advisable to make sure that the conductors are curved rather than straight and staggered to prevent l-beaming (the overlapping of traces). This can cause stress that could eventually crack the copper, and this can be avoided by using S curves.