LED encapsulation or potting of the LED fixtures prevents the ingress of moisture, the increase product reliability and insulation. Encapsulated LED fixtures have applications in environments where there is excess moisture content in the atmosphere, since the exposure of electronics in a water rich environment can trigger the corrosion of the metals in the electronics. Potted light fixtures can also be designed as an explosive proof fixture.
When choosing a potting material, there are several factors that need to be considered. The potting compound and the LED board have different a coefficient of thermal expansion, so the hardness of the material has to be chosen so that the material does not disturb the position of the LED on the board. The UV stability material needed to be considered, the UV degradation causes the yellowing of the material. Based on the tests conducted, QSil 216 was preferred due to its UV stability and softness. The fixtures with QSil 216 encapsulated were put in field test and some of them sent back due to the board failures. The failure reason of the fixtures has not yet been concluded, but it could be due to the chemical degradation of the LED phosphor by reacting with the QSil Primer. Another possibility is that there is a good chance that during the assembly process the employee didn’t spread the primer uniformly over the PCB. To overcome these issues we approach many companies for an alternative solution. A solution could be a polymer that can adhere to the PCB without any primer.
Companies contacted on LED Encapsulation methods
- Quantum Silicones
QSil 213 was suggested when asked about hardness – which has a similar hardness to the QSil 216. (Both are not flame resistant rated). Most of the silicone products from QSil need primers for adhesion and to uniformly spread the primer over the board, the technical support suggested to coat the PCB with two layers of primer. He said that if the primer is completely cured before putting the silicone on top, there is less chance of it reacting with the phosphor. He also mentioned that the hardness of the silicone can be controlled to some extent by how fast the mixture is cured.
A silicone with durometer Shore A 80 was suggested, which could be a solution for the dust resistance.
- Dow Corning
They gave their presentation on the LED encapsulation methods.
Tests shows that clear polyurethane could be a strong candidate for encapsulation, as it can adhere without a primer. Resinlab provides optically clear polyurethane (has no UL V0 94 rating). The sample was tested in CRS Electronics.
- Epic Resin
- Nagase America
Still under discussion
- Clearpur – Dutch company
The company provides clear polyurethane with UL-V0 94 rating but they only supply in Europe.
- Intertronics – UK based company
The company supply optically clear polyurethane (has no UL V0 94 rating)
- Innovative polymer
The company also supply optically clear polyurethane (has no UL V0 94 rating)
Four quick tests were conducted on LED boards with no extrusions
- One issue with the softer Silicone is that dust builds on the surface and it is difficult to clean the surface. To overcome this, a layered encapsulation was tested with hard Epoxy on top and QSil 216 on the bottom. The epoxy is found to be UV unstable, so we have to conduct the same test with another UV stable compound.
- QSil 216 was used on the LED board without using any primer. The result was poor adhesion to the board.
- UR6060 polyurethane from Resinlab was used without any primer. Much better adhesion than the QSil 216 was observed. UR6060 (Shore A 60) is harder than QSil 216 (Shore A 40).
- The extrusion can be designed in such a way that there are fins surrounding the LED board. The fins will be able to increase the area of adhesion and protect edges.
New ideas that should be tested
- Optically clear die adhesives can be an alternative for primer. The die adhesives are already used in LED chip manufacturing.
- The methods that are widely used on the solar encapsulation processes can be used in the LED encapsulation. For instance, the ionomer sheet lamination process for Silicon wafers.
- The idea of finned structures surrounding the LED board can be elaborated.
- The edges of the LED boards can be left without the dielectric coating, so it can be tested as to how well the polymer adheres to aluminium.