select adhesives for a rigid flex board
Rigid flex PCBs offer significant advantages over traditional printed circuit boards, including higher reliability and signal integrity and cost reduction. They are used in a variety of applications where space-efficiency and weight reduction is required, such as in portable medical devices. However, there are a number of unique challenges with rigid-flex design that must be addressed to ensure proper functionality and long life.
The copper layer on a rigid flex board can be adhered to the board in a variety of ways, from an epoxy-based system to a thermally-stable acrylic adhesive. It’s important to choose the right adhesives for your design based on the physical threats your board is likely to face, such as vibration or mechanical stress. For example, for a board that will be subjected to high temperature environments, the adhesive should have a low ion content and be resistant to decomposition.
When routing traces in the rigid-flex section of your circuit, it’s important to avoid placing them near the edge of the flex section. This will help to prevent the flex from cracking or peeling. You should also consider adding tabs or anchors to the vias in the flex region to help prevent them from peeling as well. Additionally, it’s important to keep special copper features and vias and plated through holes 50-60 mil away from the transition zone of a rigid-flex circuit.
How do you select adhesives for a rigid flex board?
Another common mistake in rigid-flex circuits is leaving copper exposed to the environment. This can lead to corrosion, oxidation, and shortened life-span. Using a protective mask for the copper or applying an epoxy coating to the copper surface can help prevent these issues. Additionally, using a rigid-flex design with a conductive layer that is electrodeposited instead of rolled annealed can help reduce the risk of environmental problems.
A flex core can be added to the rigid-flex design to increase its flexibility. The flex core can be bonded to the rigid section of the circuit board with an adhesive or inserted into a hole in the drilled stiffener. When designing a flex circuit, you should use a hatched pattern for the reference planes to help maintain flexibility. Ideally, the pair pitch and diagonal pitch of the hatched pattern should be equal to minimize impedance.
Lastly, it’s important to select the correct materials for your rigid-flex design. For instance, you should use FR-4 or FR-5 materials for the rigid-flex sections of your circuit board to allow it to resist the physical threat from vibrations and other stressors. It’s also essential to use a good soldering paste for the assembly process, which is especially important in high-frequency circuits.
Developing a rigid flex board requires specialized skills and the proper tools. A good option is to use a PCB design software package that integrates all aspects of flex and rigid-flex PCBs in one suite of industry-standard ECAD/MCAD tools. This will enable you to easily incorporate flex and rigid-flex features into your design, verify clearances, and ensure that your product meets your mechanical requirements.
