LED Assembly, Reliability & Testing Symposium Failure in LED Interconnects: Effect of Substrate Material and Solder Alloy LED Assembly, Reliability & Testing Symposium November 28-30, 2017 Maxim Serebreni1, Patrick McCluskey2, Gyan Dutt3, Ranjit Pandher3 1DfR Solutions, 9000 Virginia Manor Rd #290, Beltsville MD 20705 mserebreni@dfrsolutions.com 2CALCE/ Department of Mechanical Engineering, University of Maryland, College Park MD 20742 3Alpha Assembly, 300 Atrium Drive, Somerset NJ 08873 Gyan.Dutt@alphaassembly.com

Outline Introduction Fatigue failure in solder joints Solder Fatigue in LED on MCPCB Influence of Solder Alloy Thermal cycling & Die shear strength Fatigue Life Predictions Methodology Results Conclusion and Future Work

Introduction SMD Package-on-board (POB) & Chip-on-board (COB) LEDs. Coefficient of thermal expansion mismatch between metal board and ceramic submount under fluctuating thermal loads during operation. Dielectric material, Solder alloy and substrate material influence thermal fatigue life of solder interconnects. POB COB CTE = 3-8 ppm/C CTE = 17-24 ppm/C

Fatigue Failure in LED Solder Joints Solder interconnect fatigue failure is greatly influenced by the substrate material and solder alloy. Solder fatigue failure is influenced by the joint height as much as the effective CTE mismatch! LED on FR4 LED on Cu Heat Spreader Zhao, X. J., J. F. J. M. Caers, Sander Noijen, Ying Zhong, M. De Jong, Harry Gijsbers, Gorden Elger, and Harald Willwohl. "Potential interconnect technologies for high power LEDs assemblies." In Electronic System-Integration Technology Conference (ESTC), 2012 4th, pp. 1-8. IEEE, 2012. LED on MCPCB

Solder Fatigue in LED on MCPCB LED on MCPCB show dependence on metallic substrate (aluminum vs. copper), solder alloy and dielectric material Reference: Effect of Metal Clad Substrate Effect of Thermal Profile 1600 vs. 4500 cycles, 2.8X difference 1000 vs. 2800 cycles, 2.8X difference Effect of solder alloy Effect of Dielectric Material 1600 vs. 3400 cycles, 2.1X difference

Influence of Solder Alloy High Power ceramic submount LEDs assembled on Al MCPCBs with Pb-free Creep Resistant Alloy (CRA) and SAC305 Thermal cycling from -40°C to 125°C with 30 minute dwell 2X improvement in fatigue life under accelerate life testing! CRA β~1600 cycles Percent of LED population with the lights ON SAC305 β~850 cycles

Die Shear Strength The improved thermal fatigue properties of Creep Resistant Alloy can be seen from die shear strength after 1000 thermal cycles CRA alloy demonstrated highest shear strength in comparison to SAX305 and SACX0807 CRA SAC305 SACX0807

Analytical vs. FEA Prediction Methodology Each fatigue life prediction method has its strength and weaknesses. Analytical model calibrated for specific package type and substrate. FEA model provides high fidelity and can account for complex material interaction and geometry. Requires more material characterization and user inputs. Finite element Model Analytical Model 3D Stress-Strain Behavior 1D Stress-Strain Behavior Strain Energy density Strain Energy density Damage Law   Damage Law Cycles to Failure Cycles to Failure

Fatigue Life Prediction Analytical approach: First order approximation of stress and strain in solder joints. Combine empirical data with linear elastic prediction. Assumption include simplified geometry and temperature dependent material properties. Follows same damage law as FEA.

Fatigue Life Prediction Finite element approach: enables accurate representation of LED and solder joint geometry. Provides insight to the localized effects and global influence from system level. Highly sensitive to simulation methodology and material input. Die Epoxy Lens Metallization layer Copper pads Metal Dielectric layer Solder joints anode and cathode Thermal pad Solder

Thermal Cycle & LED Geometry Temperature cycles with 10 °C/min ramp rate and 30 minute dwell at each temperature extreme. Temperature change applied to entire LED and IMS package as experienced during accelerated thermal cycling conditions. Philips LUXEON Rebel white LED.

Material Properties for Simulation Materials in the LED assumed to be linear elastic except for solder alloys SAC305 Linear elastic materials   Material Elastic Modulus (GPa) Poisson’s ratio CTE (ppm/ºc) PCB 24 0.138 17 Ceramic sub 300 0.28 8.2 Silicon 185 0.18 2.6 Silicone 130 150 Metallization 79 0.37 14.2 Copper 120 0.3 16.9 Aluminum 70 0.35 SAC305 Properties defined by creep equation 20 CRA CRA   Creep model Solder A1 α n H1 SAC305 277984 0.02447 6.41 54041 CRA 15E9 0.1538 4.0 123000

Dielectric Material Temperature dependent properties of the dielectric material. Hyperbolic tangent function fits storage modulus data. High Temperature (HT), MP (Multi-purpose), CML (Circuit Material Laminate) dielectrics. Coefficient of thermal expansion Storage modulus < Tg Before Tg: HT < CML < MP After Tg: HT < MP < CML MP < HT < CML HT MP CML a 0.5 0.425 b 1.2 1.4 1.5 S (°C) 55 25 35 Tg (°C) 140 85 CTE1 40 30 CTE2 95 110 120

Accumulated Strain Energy Density Accumulated creep strain energy density is lower for CRA compared to SAC305 in all simulated conditions. Lower energy >> Higher fatigue life Accumulated creep strain energy density magnitude higher for LED on MCPCB compared to FR4 due to larger in plane CTE mismatch. The higher temperature extreme of 125°C accumulates more creep energy compared to 80°C for SAC305 solder alloy per thermal cycle. LED FR4 LED MCPCB/HT Dielectric -40°C to 125°C -40°C to 125°C 20°C to 80°C 20°C to 80°C

FEA Results: LED on MCPCB Von-Mises stress and shear strain contours for LED on MCPCB with CML Dielectric. Stress distribution more uniform in anode and cathode pads. Peak strain occurs at the corner of solder joints. Location of fatigue crack initiation. SAC305: 80°C CRA: 80°C Stress Stress Shear Strain Shear Strain

FEA Results: LED on FR4 SAC305 CRA Accumulated creep strain energy contours for Luxeon Rebel LED on two substrates and solder alloys. -40°C to 125°C. Maximum solder creep strain occurs on ceramic side of the package. End of second thermal cycle Stress & strain at 125°C cathode SAC305 anode CRA

Fatigue Life Predictions LED on MCPCB from -40°C to 125°C for CRA and SAC305. Fatigue life improvement due to CRA solder alloy overcomes contribution of dielectric material type. FEA models does not account for dielectric influence as much as analytical model due to insufficient material properties at higher temperatures. Analytical model shows largest improvement in fatigue life by combining soft dielectric on MCPCB with CRA over stiffer dielectric and SAC305. FEA Analytical

Fatigue Life Predictions LED on MCPCB from 20°C to 80°C between CRA and SAC305. Analytical model predicts fatigue life improvements of 5X with CRA compared to SAC305. FEA predicts fatigue life improvement of 2.5-3X over SAC305. Analytical model accounts for empirical behavior correlated from test data. This is indicative to contribution of higher creep resistance at lower temperatures of CRA over SAC305. Influence of dielectric in fatigue life improvement diminished over lower temperature fluctuations. FEA Analytical

Fatigue Life Prediction LED on FR4 with SAC305 and CRA solder alloys for two thermal profiles. Lower CTE mismatch between submount (8.2ppm/°C) and FR4 (17ppm/°C) compared with aluminum (24ppm/°C). Under 20°C to 80°C profile CRA shows 4X fatigue life improvement over SAC305. Analytical model predictions more conservative than FEA. Real world fatigue data could fall between two prediction methods. 45,258 60,433 14,830 8728 767 4381 730 1567

Conclusion and Future Work Fatigue life of LED solder interconnects can be greatly increased by selecting Creep Resistant Alloy over SAC305. The benefit of solder alloy selection is further enhanced at lower operating temperatures in comparison to accelerated thermal cycling. Fatigue life of LED on MCPCB greatly depend on the dielectric material properties. Lower modulus dielectric greatly increase fatigue life for MCPCB applications. Fatigue life prediction models can account for the influence of solder alloy type, LED and substrate geometry. Empirical and FEA models enable fast integration of LED design and process parameters (i.e. solder alloy type) on fatigue life predictions. Thermal conductivity analysis will be included in future simulations to account for thermal gradient and heat sink influence on solder fatigue.

Thank you for your attention! Questions? Contact: Maxim Serebreni Gyan Dutt DfR Solutions Alpha Assembly Solutions mserebreni@dfrsolutions.com Gyan.Dutt@alphaassembly.com (301)-474-0607 (908)-279-3041