SMT007 Magazine

SMT-Feb2016

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20 SMT Magazine • February 2016 amount of strain energy build-up in the sol- der joints, and can thus reduce the resistance increase and forward voltage increase that could result from such a strain energy build- up. This can provide a direct benefit to limit power draw to maintain current input to the LED, and maintain lumen output over a lon- ger period of time. For LED circuits connected in series, this becomes an especially important feature. Parting Thoughts LEDs are enabling the "digitization of light" and therefore, an "analog-to-digital transfor- mation" in any application that uses light. Our ability to engineer the light wavelength spec- trum gives us unprecedented control over light generation and allows us to tailor the light being produced, to the specific application at hand. Our ability to rapidly engineer applications-spe- cific new product designs, coupled with appro- priate LED light sources and driver and control electronics, is resulting in rapid adoption of this platform. This adoption is only accelerating, due to dramatic performance improvements, coupled with economies of scale and systems joints w ith improved mechanical and thermal fatigue/creep and vibration resistance. A new class of creep-resistant and vibration resistant alloys has been developed, that can provide this capability, via a micro-structural control approach. These advanced alloys have been developed with special additives for improved thermal stability for high temperature opera - tion and higher thermal fatigue and vibration resistance. These alloys have been commer- cially implemented for high power LED as- sembly. Figure 3 shows and example of compo- nent shear data for a high power ceramic based LED assembled on a metal core PCB using such a creep resistant solder alloy-based solder (Maxrel), compared with SAC305 and a low sil- ver alloy (SACX0807), over 1000 thermal cycles from -40°C to 125°C. It is clear that the creep re- sistant alloy maintains excellent shear strength even after 1000 thermal cycles under high CTE mismatch conditions. Such creep resistant alloys can provide sig- nificant benefits, in addition to maintaining solder joint integrity over a long time. In par- ticular, improved creep resistance limits the Figure 2: Schematic of stress-strain hysteresis showing strain energy build-up in solder joints, caused by thermal cycling and cTE mismatch. (Source: NiST) HigH-reliabilitY iNtercoNNects For HigH-PoWer led assemblY

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