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18 SMT Magazine • February 2016 role of interconnects in LeD lighting and where high reliability interconnects fit One important class of applications in the general lighting segment is high-power outdoor lighting such as street lighting. Customers (e.g., municipalities and building owners) expect very high reliability of such LED-based street lights, since major value propositions include energy savings as well as lumen maintenance over a long lifetime. For such applications, a B50, L70 lifetime of 35,000 hours is needed. For these high reliability and lifetime requirements, it is critical to have excellent assembly interconnect reliability (i.e., LED package to insulated metal substrate attach). The role of interconnects in LED assembly is fundamentally to: • Convey power and information efficiently and reliably over the rated life • Get the heat out faster and reliably over the rated life • Enable more light output, consistently, for longer time for the same package and system footprint New super-high and ultra-high-power LED package designs provide high lumen density that can enable significant system cost reduc- tions through fewer LEDs, smaller PCBs, and smaller heat sink size requirements. This, how- ever, puts significant performance demands on interconnects (solder joints in particular) due to the nature of the materials stack in the assem- bly, and the high operating temperature inher- ent in such systems. In the case of a super-high or ultra-high- power LED assembly on aluminum MCPCB, the ΔCTE between the LED and the MCPCB is 18– 20, which is quite high, as shown schematically in Figure 1. During thermal cycling experienced by the LED assembly in applications such as outdoor lighting or automotive, the high ΔCTE causes significant strain energy build-up in the solder joint between the LED sub-mount and the MCPCB, during the thermal cycling expe - rienced in use. This is shown schematically in Figure 2. This strain energy build-up causes mi- cro-cracking, and eventually, failure of the joint. Thus, for a given LED package structure and board material used, it is beneficial to use solder HigH-reliabilitY iNtercoNNects For HigH-PoWer led assemblY Figure 1: Schematic of chip-on-board stack with cTE mismatch.