Preventing Warpage of Printed Circuit Boards During Rigid Flex Circuits

Printed Circuit Boards During Rigid Flex Circuits

Rigid flex circuits combine rigid sections with flexible ones to create a single, integrated PCB for complex, three-dimensional applications. They offer several advantages over traditional rigid boards including improved flexibility, signal integrity and lower manufacturing costs. However, the fabrication process of these advanced PCBs can be more challenging and requires specialized equipment and expertise. One common issue during the production of rigid flex circuits is warping. This occurs when the board becomes warped during the soldering process, and can affect the quality of the finished product. The good news is that there are ways to prevent this from happening.

Choosing the right materials is key to avoiding warping. Achieving the desired physical and mechanical properties for a rigid-flex circuit board starts with understanding the material’s thermal, chemical and electrical properties. This knowledge helps designers choose the right copper, etch and dielectric layers for their design. It also helps ensure that the board is able to accommodate the required thickness for each flex and rigid section, as well as the appropriate bend radius.

To reduce the chance of warping, designers can use a layer stack-up that minimizes thermal mismatch and stresses on the board. Using a mix of high and low flow prepregs, and pairing signal and reference layers with corresponding dielectric materials will allow for optimal performance. Similarly, choosing an even copper distribution across the board will minimize stress and improve overall mechanical stability.

Preventing Warpage of Printed Circuit Boards During Rigid Flex Circuits

Additionally, ensuring that the flexible sections are properly bonded to the rigid sections will help prevent damage during bending and flexing. This is especially important for flex sections that need to withstand high levels of flexing and folding, such as those found in wearables or foldable devices. Lastly, minimizing the number of reflow and wave cycles can also help prevent warping.

When a board undergoes multiple soldering and reflow cycles, it can become warped as the copper wicks away from the pads. To minimize this, the designer can specify a limit to the number of reflow and wave cycles for the board, as well as specify a lower maximum temperature for the process. By rigorously implementing these steps, manufacturers can ensure that rigid flex circuits meet the highest quality standards, providing reliable and long-lasting performance in their intended applications.

The transition zone between the flex and rigid sections of a circuit is susceptible to a variety of defects including adhesive squeeze out, protruding dielectric materials, crazing, and haloing. These defects can be caused by uneven surface areas between the flex and rigid sections, or by differences in the thermal expansion rates of the different layers. To avoid these defects, the CM can add a bead of epoxy to the flex-to-rigid area.

Keeping these tips in mind will help designers to create a rigid-flex circuit that will perform at its best. These techniques can reduce the time it takes to get a new product to market by eliminating the need for additional connectors and cables, as well as improving the aesthetics of the end-product. To learn more about designing and manufacturing rigid-flex PCBs, join the experts at Altium for a free webinar.

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