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Impact of Substrate on Materials Reliability of High Power LED Assemblies
Impact of Substrate on Materials Reliability of High Power LED Assemblies
What role does the substrate type play in the LED package-to-board assembly reliability? This paper presents a study to help answer this question.
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Authored By:
Ranjit Pandher, Ph.D. and Ravi Bhatkal, Ph.D.
Alpha Assembly Solutions, a MacDermid Performance Solutions Business
South Plainfield, NJ, USA

Kurt-Jurgen Lang
Osram Opto Semiconductors, Regensburg, Germany

Summary
New high power LED package designs provide high lumen density that can enable significant system cost reductions through fewer LEDs and smaller PCBs. The materials stack determines the CTE mismatch between the high power LED ceramic sub-mount and the substrates (including Metal Core PCB or FR4 substrate types). Choice of the substrate can impact reliability of the solder joints in thermal shock/cycling. One of the key questions is: What role does the substrate type play in the LED package-to-board assembly reliability?

This paper presents a study to help answer this question. Assembly of high power ceramic sub-mount LEDs was conducted with both aluminum MCPCB and FR4 substrates with multiple solder alloys. Thermal cycling was conducted under the conditions of -40C to 125C for 1000 cycles. Solder joint strength was measured at multiple intervals during thermal cycling by conducting package shear. The impact of substrate type is quantified for multiple solder alloys and recommendations are presented. Solder alloy characteristics and failure mechanisms that impact reliability for a given substrate are discussed.

Conclusions
Performance of LEDs assembled on three types of the substrates has been compared. Key observations are:

1. LEDs on FR4 boards show minimal drop in shear strength irrespective of the solder alloy.
2. Due to high mismatch in CTE, the MCPCB-MP substrates show a steep degradation of shear strength upon temperature cycling.
3. HR Pb-Free alloy showed highest strength and little drop in shear strength after thermal cycling.
4. In thermal cycling test, HR Pb-Free alloy showed stable performance on all types of substrates.

In conclusion, for a given alloy (SAC305 or HR Pb-Free), FR4 shows the least drop in shear strength of the joints. In the case of MCPCB-MP substrates, HR Pb-Free shows the highest reliability.

Initially Published in the SMTA Proceedings

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