For both direct and indirect LED applications, the selection of adhesive candidates shrinks dramatically to include only low-VOC materials that are compatible with encapsulants. To simplify the selection process for the LED manufacturer, some adhesive companies have tested and validated products as meeting the stringent chemical protocol requirements for direct and indirect application. Such testing eliminates the possibility of unexpected field failures and greatly reduces research costs for the LED manufacturer.
Two types of LED compatibility screening tests are used: end-use device testing and laboratory testing. In end-use device testing, a finished luminaire is used. The finished device is energized and monitored over time for changes in color temperature and output. If color temperature or light output decreases over the test period, outgassing and associated degradation have taken place. This generalized device testing will not specifically identify any materials as the source of the incompatibility. Once the results of the tests are known, manufacturers can research the cause and specify different materials to eliminate degradation.
Figure 3. Laboratory testing of LEDs is directly focused on the effect of specified adhesives or sealants on the chip and silicone lens assembly.
Laboratory testing is directly focused on the effect of specified adhesives or sealants on the chip and silicone lens assembly. On an aluminum test panel populated with six test LEDs, the test adhesive is applied directly on the silicone lens of three test LEDs and indirectly beside the silicone lens on two test LEDs. The sixth LED is left untouched as a control. A cover lens is then bonded over the top of each test LED. None of the adhesive undergoes cure prior to testing. The same test is done a second time, but this time adhesive is cured prior to testing. The test panels are energized and monitoring of light color and intensity begins (see figure 3).
Variables being tested include compatibility of cured adhesive in contact with the LED, cured adhesive in proximity to the LED, uncured adhesive in contact with LED, and uncured adhesive in proximity to LED. This array of test conditions helps determine the compatibility of the adhesive within all expected and unexpected manufacturing conditions.
After extensive testing of 44 adhesives with three different LED die, some trends in the compatibility of adhesive chemistries can be seen. As a rule, non-corrosive silicones are fully compatible for both direct and indirect applications. Epoxies, modified silanes, methyl methacrylates, and polyurethanes are good choices for indirect contact, although there are exceptions. Single component cyanoacrylates tend to be incompatible due the blooming characteristics. However, a two-component, static-mix fast-cure instant adhesive passes as fully compatible for both direct and indirect contact.
Due to their thermoplastic nature, hot melt adhesives offer mixed results depending on proximity to the LED, chemical base of the hotmelt, and the LED being tested. Anaerobics as a family of adhesives do poorly in tests. But high-temperature anaerobic threadlockers are compatible for indirect contact as they resist outgassing at LED operating temperatures.
Compatibility should never be generalized by adhesive technology because of the variations and exceptions within chemistries. Therefore, it is critical that LED manufacturers work closely with their adhesive suppliers during device design. Actual end-use performance should always be verified on the final device before it goes into large-scale production.
Ed Fisher is a market application engineer for Henkel Corp.