Stencil Design for Rigid Flex PCBs
Rigid-flex PCBs are a type of printed circuit board that combine flexible circuit boards with rigid components. This unique design allows for a larger area of the board to be used while still providing rigidity and stability that are needed in certain applications. While this type of PCB can offer a number of benefits, there are some issues that can be encountered when designing a rigid-flex circuit board.
The most important issue that can arise in a rigid flex circuit is thermal z-axis expansion, which can cause soldering defects such as bridging and solder balls. This can also lead to intermittent responses after the board has been assembled and soldered. To minimize these issues, it is important to consider the size of the aperture openings and the overall pad size in the stencil design process.
Aperture openings must be sized correctly to ensure proper expulsion of the solder paste from the stencil. This will help to prevent the spread of excessive solder deposit which can contribute to bridging and other common issues. In addition, the amount of solder paste that is applied to the pads is also an important factor in preventing these issues. The optimal amount of paste to apply is roughly two-thirds of the wall thickness of the aperture.
Optimizing Stencil Design for Rigid Flex PCBs
Stencil material is another crucial consideration when designing for a flex circuit. Stainless steel and polyester are common choices for stencil material, and both provide high levels of durability. Stencil thickness is also important, as it can influence the amount of solder paste that is expelled through the aperture. This can impact the quality of the solder joints on the finished product, so selecting the right material for your needs is critical.
Other important factors to consider when designing a flex circuit include the bending radius and how traces will be routed. Bending radius is the distance between two planes in a part or assembly that are separated by a given radius. The smaller the bending radius, the more flexible the part will be. Traces that are straight or run in a perpendicular direction can create stress points when they bend, so it is important to curve traces as much as possible. Adding in dummy traces can also help to decrease the areas of high stress on a flex circuit.
Lastly, it is important to decide how much flexing your board will need up front. If it will only need to be bent in one static position – like in a handheld ultrasound device – then you can be more liberal with the number of layers, copper type, and so on that you can use. However, if your board will need to be continually moved, flexed, or folded during operation, you should limit the number of layers and choose a less dense copper material. This will reduce the weight of your board and help it to withstand a greater amount of strain. Stiffeners can also be added to the extreme ends of a flex circuit to add additional mechanical stability if needed.