Flexible Printed Circuits
When designing flex or rigid-flex PCBs, balancing durability with component placement and routing can be challenging. Often, the quality of a flex circuit is determined by its physical construction, copper pattern and manufacturing process. However, even with careful consideration of these issues, reliability can be impacted by some gnarly material quirks that are difficult to avoid completely. These include copper work hardening, fatigue and outgassing. The good news is that a combination of design and manufacturing safeguards can mitigate these pitfalls and prevent them from causing problems down the road.
Outgassing is a process that occurs when moisture trapped within a flexible printed circuit is forced out by heat or high vacuum. This moisture can be caused by wave or hand solder reflow during production, use in ultra-high vacuum environments, or by the PCB’s manufacturing process itself. When outgassing occurs, it can lead to voids in the solder mask and other defects.
This is an important issue to address for PCBs that will be used in sensitive applications such as optical lenses or high-precision instrumentation. It is also a concern for PCBs that must be designed to withstand thermal cycles, as outgassing can cause condensation on critical components like chemical sensors and electrical relays.
Preventing Outgassing in Flexible Printed Circuits
One of the most effective ways to prevent outgassing is to ensure that the laminate and conductive layer materials have passed outgassing evaluation tests in accordance with IPC 1601. Another option is to bake the bare board, which removes any volatile molecules from the surface. This can be accomplished by placing the boards in an oven that is set to a temperature of 100 to 125 degrees Celsius for several hours.
A third option is to make sure that through-hole pads in flex PCBs have adequate plating and anchoring stubs to hold the solder. In some cases, a stub can be used for the entire length of a pad to help reduce the amount of solder needed to fill the hole. In addition, by using smaller coverlay access openings and ensuring that the traces are not too close to the anchor stubs, outgassing can be minimized.
The best way to prevent outgassing in flex circuits is to select the appropriate materials and fabrication techniques. This includes ensuring that the flex is made with a material with low CVCM, TML and WVR values and making sure that the plated hole diameter is at least 25um. It is also a good idea to bake the PCB before assembly.
Outgassing can be a problem for flex and rigid-flex circuits, but it isn’t impossible to prevent. The key is to take steps during the design phase to ensure that all of the aforementioned mitigations are in place. By following these tips, designers can be sure that their flex and rigid-flex circuits will be able to perform as intended in a variety of operating environments. To learn more about how Rush PCB can help you with your next project, contact us today!