While most flexible circuits are used in a relatively dry environment, there are applications where water contact or submersion is required. Knowing in advance what types of water contact variables can affect the performance of the flex will help you to create a successful design.
You need to first evaluate the application and answer the following questions.
- Will the immersion in water be permanent or temporary? If temporary, for how long will it be immersed?
- What is the highest voltage potential between adjacent conductors, and will there be any high-speed signals requiring controlled impedance?
- Will this be straight freshwater or will there be salts, acids, alkalis, or other dissolved chemicals in the water?
- Will the solution (and circuit) be static or dynamic?
- What is the water temperature?
Plain water will not attack polyimide film or thermoset adhesive, which along with copper are the usual components of a flexible circuit. However, these materials are hygroscopic, so they will quickly absorb water whether it is from humid air or actual submersion. As the materials absorb water, their electrical properties (namely dielectric strength and dielectric constant) change.
This is the reason for the first two questions on the duration of the submersion and the voltage. If the circuit will only be submersed for a few seconds and will then return to a dry environment you have nothing to worry about. However, the circuit materials absorb moisture very quickly so any submersion over twenty or thirty minutes could affect the way the circuit performs. Now it will depend on the voltage that the conductors will be carrying and/or the need for controlled impedance. For example, as the moisture content of polyimide film rises to saturation (approximately 2.8% water), the dielectric strength drops from 8600 volts/mil to 6800 volts/mil. This is still very respectable, so unless your application has extremely high voltage you will probably not have any issues.
The dielectric constant of the insulating materials is another matter. As the moisture content of polyimide rises the dielectric constant also rises. At zero percent relative humidity, the dielectric constant is 3.0 and at one hundred percent relative humidity the dielectric constant is 3.8. This can have a significant impact on the impedance of the circuit and should be accounted for during the design process.
As mentioned earlier, plain water will not attack most flex circuit materials. However, any elements dissolved in the water that would cause a solution to become extremely alkaline can attack the circuit. Also, many final finishes like solder or tin at the termination area are very prone to corrosion when exposed to acids, bases, salts, and a host of other compounds. If you are unsure of the water purity, it would probably be a good idea to conformal coat the entire assembly prior to installation.
Final questions on your application would cover mechanical issues.
If the water is still, or if the flexible circuit is securely anchored, you should not have any issues on the mechanical front. However, if the water is dynamic and the circuit is allowed to move with the water, it too will be dynamic. If this is the case, you will need to examine what type of motion the circuit will be subjected to and ensure that your design does not have any stress concentrating features that could result in a failure due to this motion.
The last question has to do with the water temperature. If the water temperature is between freezing and normal air temperature you should not have any problems. But elevated temperatures will make the circuit very fragile. As the circuit becomes fragile, the effects of any mechanical forces such as water motion or handling can result in damage to the flex.
What is the Benefit?
You can successfully design a flex circuit that can temporarily or permanently be immersed in water. It is just important to weigh all the factors that can impact the performance of the flex and then design accordingly. Flexible Circuit Technologies has experience in these types of applications and can guide you on your way to a robust design!
Flexible Circuit Technologies provides the industry’s most experienced Application Engineering support and is well positioned to guide you to success in getting to a cost-effective design that will perform flawlessly in your given application.
Contact us and work with our Applications Engineering team to get started.
