Flex Circuit PCBs Bend Without Damage
There is a limit to the amount of strain flex circuit PCBs can bend without damage. Bending them puts tensile and compressive forces on the layers, which can eventually cause delamination. Understanding this limit and how to prevent it helps ensure the continued functionality and performance of a product.
A major reason for the popularity of flex circuits is their ability to replace wire harnesses in applications that would be too difficult or impractical with rigid-flex circuit boards. Examples of such products include automobiles, aerospace equipment, and medical devices. Because of this, these circuits must withstand harsh environments like high temperatures and exposure to chemicals, shock & vibration, and bending. Therefore, it is crucial that the flex materials and manufacturing processes chosen meet these requirements.
To achieve a higher level of flexibility, a flex circuit pcb needs to be made with a thinner material than a rigid-flex board. The most common choice is polyimide, which is highly flexible and tolerant to the rigours of production. It can also withstand multiple solder reflow cycles and maintain its shape under temperature fluctuations. Another benefit of polyimide is that it is impervious to flammability and moisture.
Can Flex Circuit PCBs Bend Without Damage?
The layer stack for a flex circuit is also important. Ideally, the copper should be positioned on the neutral bend axis, which experiences the least stress during flexing. This minimizes stress concentration at the corners of the flex, and extends the usable life of the board. Plating through-holes in the area surrounding the neutral axis should be avoided, as they can experience stress and fatigue due to alternating compression and tension in these areas.
A key factor in determining the flex circuit’s tensile strength is its thickness, which can be affected by the number of layers and copper weight. A flex circuit with a thicker overall stackup will require a larger bending radius, as this will increase the pressure on the layers at the edges of the bend. A smaller bending radius can also create a gap between the outer flex and the conductive coverlay, increasing the chance of stress cracking or failure of the traces on the inside of the bend.
Several other factors contribute to the overall flex circuit’s flexibility, such as trace and component placement. Traces should be spaced apart as much as possible, and components should be placed in a way that they are not touching each other. This will allow the flex circuit to maintain its elasticity while keeping connections secure. In addition, plated through-holes should be avoided or used sparingly, as they can cause stress concentration and cracking. Instead, use teardrop (pad fillets) shaped vias with large diameters to reduce the likelihood of stress concentration spots.
To maximize the flex circuit’s flexibility, the drill to copper distance should be as close to zero as possible. This will minimize stress and movement of the drill holes, as well as reduce the risk of failures and delamination. It is also advisable to keep the area around a bend free of discontinuities like cutouts and slits, which can generate stress concentrations and deform and weaken the layers in the flex zone.