Thickness in Flexible PCB Boards
Keeping consistent thickness in a flexible PCB board is essential to its performance and longevity. Various factors can cause a variation in thickness, which could negatively impact signal impedance and flexing capabilities. These factors include:
Choosing the correct materials for a flexible pcb board is critical to ensuring consistent thickness. A flex PCB has unique design constraints that are based on its operating conditions and bending capability, so it is important to understand them before selecting the materials. It is recommended to use industry-standard ECAD/MCAD software to help EE and mechanical engineers select the best materials.
The thickness of a flex PCB depends on the layer count, material type, and other factors. The thickness is also determined by the design and layout of a flex circuit board. It is best to plan a layout that includes rigid sections and a flex section with plated holes and vias.
Ensure Consistent Thickness in Flexible PCB Boards
The layer count of a flex PCB can be up to 14 and is often determined by the amount of component connections and the complexity of the layout. The most common layers are reference planes, shielding, and solder mask/coverlay.
For a flex circuit board, it is important to consider the minimum bend radius and maximum number of flex cycles that it will experience. The bending of a flex circuit board can lead to stress and cracking, so it is important to plan for this during the design stage. A flex PCB with a minimum of a 10-mil bend radius is recommended, as it will increase the lifespan of the board.
Another consideration when planning a flex circuit board is to ensure that the traces are not close to the bend points of the circuit. This can prevent solder joints from delaminating during the bending process and damaging the copper. Using a jig or clamp to support the flex circuit board during bending is also a good idea.
When routing traces, it is important to avoid sharp angles and opt for curved paths instead. Straight traces can create stress and cracks during flexing, which can damage the copper and reduce the lifespan of the board.
Depending on the thickness needed to meet the bend requirements of the flex circuit, the stack-up can be adhesive-based or adhesiveless. The most common application of an adhesive-less flex core is stripline controlled impedance or radio frequency (RF)/electromagnetic interference (EMI) shielding.
A flex circuit board may require stiffeners for areas with many surface mount components and connectors. Stiffeners are made of a rigid material, such as aluminum or stainless steel, and are laminated to the flex circuit board. They can relieve strain, balance weight, and provide abrasion resistance.
When designing a flex circuit board, it is also important to keep in mind the type of coverlay, stiffeners, and adhesives that will be used. The choice of these materials can influence the cost and flexibility of a flex circuit board. For example, some flex circuits are coated with a photo-imageable or dry film coverlay to protect the copper from corrosion and to allow easy inspection of the design. A flex circuit can also be finished with a thermally cured acrylic or pressure-sensitive adhesive to hold the components in place. Adding metal house to selected areas of a flex circuit board can also increase its abrasion resistance and reduce the risk of electromagnetic interference.