Prevent Delamination in Rigid Flex PCBs
A rigid flex PCB is ideal for a wide range of applications, including wearables and medical devices. These flexible printed circuit boards can fold, flex and bend, saving space and ensuring that the device fits into its enclosure. However, the ability to fold and flex can increase the stresses that the board is subjected to, which may lead to delamination. To prevent this, it is crucial to design a rigid-flex PCB using the correct manufacturing techniques and follow best practice PCB designs.
Delamination in a rigid-flex board occurs when the layers of the flexible circuit separate from one another. This can be caused by a number of factors, including poor adhesion, moisture, or thermal stress. It can also be caused by excessive bending, which can cause the layer to warp or tear. If any of these issues occur, it can result in a failed product that is unreliable and difficult to use. To avoid delamination, it is important to design your flex circuit with the following best practices:
Using a cross-hatched ground plane on the flex area of the circuit. This will reduce the risk of cracking or peeling during the flexing process. The location of vias should be taken into consideration as well. Vias should be placed away from the flex areas of the board, and they should be positioned at least 30 mil away from the flex interface. Additionally, it is important to add teardrops to the pads to minimize stress concentration points and to ensure that the vias are able to withstand high currents.
How to Prevent Delamination in Rigid Flex PCBs
Solder reflow temperatures for rigid-flex are typically lower than the reflow temperature for a rigid PCB. This is because the flex material can absorb moisture that would otherwise convert to steam during the soldering process. This moisture can then rapidly heat and expand, which can cause delamination. To avoid this, the flex circuit should be pre-baked before assembly. The reflow oven should be heated to 225-250°F, which will allow the flex materials to absorb the moisture without causing delamination.
Stiffeners should be located close to the flex regions of the circuit, but they should not extend more than half of the bend radius. Otherwise, they can cause shorts that will damage the flex circuit and prevent it from folding and flexing properly. Additionally, stiffeners should not be located too close to traces or vias, as they can also lead to shorts.
A final tip is to make sure that the spacing between consecutive bends is greater than twice the thickness of the copper layer being bent. This will ensure that there is enough room to accommodate the desired bending radius without damaging the underlying layers.
Rigid-flex is becoming increasingly popular as a way to reduce cost and complexity while increasing the reliability of products that need to be able to flex, fold, or bend. However, implementing rigid-flex requires a thorough understanding of the manufacturing process and the ability to identify potential problems that could be caused by a lack of knowledge or experience. By avoiding the common DFM mistakes mentioned above, it is possible to create a reliable rigid-flex circuit that will withstand the test of time and repeated flexing.
