LED test standards, packaging material challenges
Emerging LED test standards and HB-LED packaging materials pros/cons were two of the topics covered at MEPTEC's The Heat is On event in late March.
Figure 1. Junction temperature and thermal resistance calculations. (Source: Mentor Graphics)
András Poppe, marketing manager in Mentor Graphics' MicReD division, took on a number of important LED testing challenges in his presentation. He noted the importance of defining thermal resistance for LEDs: some LED vendors neglect the effect of radiant flux (Popt, i.e., emitted optical power) in their calculations, which results in better data than is actually achieved in the field. Still another example where calculations need to be carefully weighed is determining the amount of cooling needed for reliable LED operation (Fig. 1). In this case, hot lumens need to be taken into account, essentially, calculating the light output under real operating junction temperatures (i.e., the actual operating conditions) rather than laboratory test conditions. (He further explains these concepts in a podcast interview, at electroiq.com/podcasts.html).
Figure 2. Comprehensive LED testing. (Source: Mentor Graphics)
Poppe also presented a comprehensive LED testing solution (Fig. 2), noting that the JEDEC JC15 committee is working on this issue along with the CIE (Commission Internationale de l'Eclairage). Additional comments can be heard in his interview, along with a discussion of challenges associated with testing AC mains-driven LEDs—a major trend because they can be substituted for conventional light bulbs.
Meanwhile, presenter Daniel Duffy, research scientist in Henkel's Advanced Technology Group, summarized the encapsulant materials used for high-brightness LEDs (HB-LEDs), specifically the pros and cons of epoxy and silicone. The material challenges for epoxies are temperature stability and color (aging); and for silicone, contamination and adhesion, as well as barrier properties. In the future, epoxies will have to be stable with respect to blue light (T>150°C); and silicone material will have to fulfill the condition T<Tg CTE <60ppm/K. "Silicone encapsulants are very stable," said Duffy. "But it is not enough—future power demands require higher levels of photo-thermal stability."
Die-attach material challenges include transparency, CTE, interfacial TC, and adhesion properties. Such materials will need to have stable thermal resistance, high TC, matched TCE, and good adhesion properties. Duffy specifically mentioned adhesion as a critical property for LED packaging. "Delamination leads to increased interfacial thermal resistance," he said. "Localized temperature increases can shorten device life." Furthermore, cracking can lead to weakening of wire bonds and cracks; also, delamination weakens barrier protection.
Duffy also discussed the outlook for new and/or enhanced materials for HB-LED applications. Quantum dots in particular, he said, are "very interesting," and likely "will play a continuous role in the future" once the industry figures out how to tune the interactions between them and the rest of the materials involved in LED packaging. "They offer a wide variety of colors, tunability of color, and lots of options for tuning their performance with temperature, with time and, maybe even other optical effects we're not even considering now...they're here to stay." The challenge, he noted, will be getting them into materials for higher-power applications. — Debra Vogler, senior technical editor