Coating LEDs to protect them is big business. The volume of manufactured LEDs globally is growing at an exponential rate and there is no end in sight.
However, the challenges to protect them are not small especially in an outdoor environment.
The LED circuits are placed in exposed areas and subjected to the full force of the elements. Then they are expected to survive for long periods of time.
Further, the protection placed on the circuits must not affect the light output of the LED (the lux) or cause heating problems due to thermal demands.
Finally, due to the low cost of the LED products in the first place then the protective method of coating and application has also to be extremely low in price.
This final point can be the most challenging. After all protecting the LEDs is relatively easy with certain coating materials. Coating the unit for the right price is the key challenge.
This price challenge is due in most cases to the insulative properties of the majority of coatings applied. Nearly all of the traditional coating methods require components like connectors not to be coated since they would ruin the electronic properties of that component.
This leads either to masking of components in process or selective processing that leads to increased costs in prodution.
To provide a high level of protection whilst being low cost is not a trivial task for a coating. Processes like Parylene, conformal coatings, encapsulates and potting compounds continuously find it difficult to meet all of these criteria and customers are continuously compromised.
So, is there an alternative protective coating for LEDs besides Parylene, conformal coatings and encapsulates?
Nexus has been examining a new, novel technique that may be able to meet all of the environmental demands for LEDs and actually be cost-effective.
This process is a Hybrid ALD (Atomic Layer Deposition)/CVD (Chemical Vapor Deposition) technique.
This method uses multiple layers of ultra-thin coatings with differing properties to build a completely protective coating.
The final coating built up is much thinner than the other traditional coatings including Parylene. However, its protective performance has been found to be superior to them all in most categories of testing so far.
Further, the really exciting part about this technology is the cost of processing.
Since the coating is extremely thin then it has been found that no masking is required.
This is because when components like connectors are joined together then the ultrathin coating does not prevent electrical connection. Even better, the physical protection is not compromised.
This means that the cost of process is purely the cost of application of the material and nothing else.
Since the process is relatively low cost then this does offer a very interesting alternative to the traditional coating materials.
Actually, although the technology and chemistry can be a little complex the process itself is fairly simple.
Once the process is set up in the machine the operator just loads, switches the machine on and unloads on completion.
This is a far cry from the sophisticated processes of robotic selective coating or the challenges of Parylene. Further, the process is actually very stable and in reality is tried and test in other industries.
So what does a hybrid ALD / CVD film look like?
The film is built up of alternating layers of ALD and CVD thin coating layers. The ALD is a ceramic-based material providing the insulating properties and the CVD film provides the barrier protection.
Once the required film thickness is achieved then a final hydrophobic layer is applied that combines with the ALD and CVD layers to provide a highly effective barrier.
So how well did the hybrid coating perform in protecting LEDs?
Nexus actually worked with live LED circuits from a customer.
The customer LED product was for outdoor application. For testing the customer used their own in-house test methods to prove the technology.
The LED circuit was exposed to customer tests for resistance against salt, moisture and temperature.
The test methods included:
- Initial test submerged in DI water dip for 12 hours
- Second test submerged in 25% concentration saltwater dip for 17 hours
- Third test 2 x 6 hour cycles in water ramped from room temperature to 70°C
After each test the boards were tested for failure or problems.
The LED circuit passed on all tests. All results achieved were completed with no masking of components and zero light loss in LED opacity.
The electrical connections were found to be excellent and the coating did not effect the integrity of the connectors.
So what about the cost of process?
Since the process is masking and de-masking free then the cost per unit is incredibly low and superior to nearly all the traditional methods of coating protection.
Further, the protective properties of the hybrid coating in nearly all cases is superior to the conventional methods.
So, you get a lower cost coating with a higher technical performance.
So, just how good is the hybrid coating as a protective material for electronics?
Generally, with protective coatings for electronics then Parylene is considered the gold standard in most cases.
So, we compared Parylene with the hybrid ALD / CVD material.
|Wear resistance/Handling Ease||Poor||Excellent|
|Water Vapor Transmission Rate||Good||Excellent|
|Temperature Resistance (extended time)||100°C||350°C|
|Adhesion to various materials||Poor||Excellent|
|Scalable to large production||Poor||Excellent|
|Process Time||8 – 12 hrs||8 – 12 hrs|
|Hydrophobicity||Good||Good – Excellent|
|Cost||High||Low – Med|
What we also identified for the material were some key properties for LEDs.
- The Water Vapor Transmission Rate (WVTR) is superior to Parylene so the coating is far more waterproof for the LEDs
- Coating adhesion is superior as it covalently bonds to the substrate. So, the lifetime of the material will be better on the circuit.
- The hybrid coating is UV stable whereas Parylene in general is not. This is an important criteria for coatings exposed outside on LEDs
- The coating stayed 100% transparent during testing (no loss of lux). That again is important for LEDs.
- The coating thickness of the hybrid material is x10 LESS than the Parylene. This aids light transmission and electric connectivity
So, in reality the hybrid ALD / CVD material could just be what the LED industry is looking for in protecting their circuits. Nexus will let you know how the material performs on other types of circuits shortly.
Need to find out more?
If further information on these topics and the key question you can go to our free eBook by clicking conformal coating design now.
If you are new to Nexus and our work on conformal coatings then a good place to go is our Start Here page.
Dr Lee Hitchens, Author of Nexus
Dr Lee Hitchens is the author of the Nexus conformal coating website and eBook.
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