The need to protect LEDs from the long –term exposure of a harsh environment is becoming more and more required. This is doubly so for LEDs used outdoors.

The alternative coatings used to protect LEDs are many. They include Parylene, conformal coatings, ultra-thin fluoropolymer materials and encapsulates.

However, none of the material processes offer the perfect protection without adding significant cost to the production process, and ultimately the circuit board and product.

Further, to reduce costs in production some of the coating materials used compromised performance. So, there is a typical balance of costs versus protection.

Now, this may all change with the introduction of a very new coating process that takes many of the performance benefits of the best coatings but does not suffer the associated problems of increased production costs.

This process is a hybrid ALD (Atomic Layer Deposition)/CVD (Chemical Vapor Deposition) technique.

So what is a Hybrid ALD / CVD technique?

Most people in conformal coating have heard of Parylene. Parylene is a CVD (chemical vapor deposition) process where the Parylene material is applied in a vacuum chamber and the coating builds up on the surface of the circuit.

The new hybrid process uses CVD as one of its film processes. However, how it differs is that the method also uses another technique, ALD (atomic layer deposition).

Further, these two processes, ALD and CVD are applied sequentially. They are deposited as ultra-thin coatings (nanometer scale thickness) one on top of the other.

This build up of multiple layers of ultra-thin coatings (alternating ALD and CVD films) with differing coating properties produces a completely different hybrid coating that outperforms the individual coatings produced by ALD and CVD alone.

Further, the final layer applied is a hydrophobic barrier that further enhances the performance of the coating.

So how does the hybrid film differ to traditional materials like Parylene and liquid conformal coatings for protecting LEDs?

First, the hybrid coatings protective performance has been found to be superior to them all in most categories of testing so far. It provides both improved electrical and physical properties that protect the circuits.

A major issue for LEDs is light loss when the film is applied over the LEDs. For the hybrid coating the coating shows zero loss of light and this is a great advantage.

Also, the material is both temperature and UV stable. The coating will survive up to 350C and does not degrade in UV light. Again, these are great advantages for the performance of the hybrid coating.

It is also hydrophobic. This property rejects water from the surface and improves the film properties enormously.

However, what makes this coating exceptional is its final property. That is the coating does not require the circuit board to be masked when the coating is being applied!

So, why does the hybrid film not need masking like other traditional conformal coatings and Parylene?

The difference is film thickness.

The hybrid film is much thinner than the other traditional coatings including Parylene. Typical film thicknesses can be as low as 0.1um.

Since the coating is extremely thin (<< less than traditional coatings) 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. The component mating parts connect together and no loss of connection can be measured.

Even better, the physical protection of the film is not compromised. In fact, all of the components including connectors are protected.

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 in LED processing.

Doesn’t the hybrid ALD / CVD process sound complex to operate?

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, waits for the coating to be applied and unloads on completion.  

This is a far cry from the sophisticated processes of robotic selective coating or the challenges of Parylene. Further, the application process is actually very stable and in reality is tried and test in other industries for a long time.

So, how well did the hybrid coating perform in protecting the LEDs?

Nexus, an online conformal coating database, actually worked with live LED circuits from a customer and tested the hybrid ALD / CVD material.

The customer LED product was an outdoor application. The LED customer used their own in-house test methods to prove the technology.

As part of the testing the LED circuit was exposed to 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 affect the integrity of the connectors.

So what about the cost of process?

Since the hybrid film process is masking and de-masking free then the cost per unit is incredibly low. This makes the material 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 ALD / CVD coating as a protective material for electronics?

Generally, with protective coatings for electronics then Parylene is considered the gold standard in most cases.

So, Nexus compared Parylene with the hybrid ALD / CVD material.

Table courtesy of Nexus

What Nexus 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
• The film is hydrophobic so repels water and aids the performance of the coating.

So, in reality the hybrid ALD / CVD material could just be what the LED industry is looking for in protecting their circuits.

Nexus will be performing further tests on the material to see how it performs on other types of circuits shortly.

Need to find out more?

For further information on the hybrid ALD / CVD materials then contact us directly. We can help you trial the coating.

Or, check out the Nexus article, Coating LEDs with a hybrid ALD / CVD Process.

Call us on +44 (0) 1226 249019, email your requirements on sales@schservices.com