High-Reliability Interconnects for High-Power LED Assembly

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The LED revolution is on. Now more than 50 years in the making, LED technology enables the digitization of light. This analog-to-digital transformation of light sources, from traditional light sources such as incandescent, fluorescent and HID, to LED, enables a whole new range of applications and markets, from displays, to automotive and general lighting, and emerging markets such as indoor vertical farming and horticulture, Internet of Things (IoT) and visible light communications. Our ability to rapidly engineer applications-specific new product designs, coupled with appropriate LED light sources and driver and control electronics, is resulting in rapid adoption of this platform.

The LED general lighting segment, in particular, is growing at the fastest rate, with unit volumes of LED retrofit lamps, luminaires and tubes forecast to grow by 50% year-on-year 2016. This is driven by rapid technological advances, simultaneous cost reductions and scale of manufacturing in LED package technologies as well as systems integration technologies.

The key issues that need to be addressed by the LED lighting supply chain include:

• Efficiency, brightness and light quality improvement (lumens per watt)

– Higher efficiency of electrical power to light and lumen output per package for reduced energy consumption

– High heat dissipation

– Good light extraction and directionality

• Reliability and lifetime improvement (lumen maintenance over life)

– Reliability of the package and the system determines their lifetime and lowers maintenance costs

– Vibration and high-temperature thermal cycling performance

• Cost/lumen over the lifetime ($/lumen and lumens/$)

– Increased efficiency and brightness, and increased reliability and lifetime reduce the cost per lumen over life

• New form factors

– Greater integration and design freedom

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 reductions through fewer LEDs, smaller PCBs, and smaller heat sink size requirements. This, however, puts significant performance demands on interconnects (solder joints in particular) due to the nature of the materials stack in the assembly, and the high operating temperature inherent in such systems.

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Editor's Note: This article originally appeared in the February 2016 issue of SMT Magazine.