Meeting Advanced LED Thermal Management Requirements for Challenging Automotive Applications

Posted 12/4/2013 by Ravi M. Bhatkal, Ph.D.

LED technology is seeing widespread adoption in automotive applications, for both interior applications such as instrument clusters, displays and dome lights, and exterior applications such as tail lamps, turn signals, center mounted stop lights, daytime running lights and headlamps.  LEDs offer several advantages such as energy efficiency, long life, instant turn on and off, high luminance, high brightness, color control (e.g. pure red, orange, and white light), shock and vibration resistance, and styling and design freedom. 

Automotive LED lighting systems such as exterior lighting, can be described in terms of a hierarchy shown in Figure 1 below.


Hierarchy of an Automotive LED Lighting System

Figure 1:  Hierarchy of an Automotive LED Lighting System

For high power LED applications such as automotive headlamps, materials used in the packaging and assembly of LEDs need to be able to handle high thermal loads and high thermal cycle and vibration reliability requirements.  In this paper, we discuss the mitigation of thermal and reliability issues in high power LED lighting systems with two of Alpha®’s advanced materials platforms.

Key Issues in Automotive LED Lighting

Key issues in automotive LED lighting include:


Heat dissipation at all levels of the lighting system

-Heat buildup can lead to device failure

-Voiding under thermal pads leads to heat buildup, loss of efficiency and potential failures


Directionality of light output

-Applications such as headlights need good control of LED die/package tilt and optical axis


Reliability and Longevity

-Long term thermal cycling requirements and vibration resistance 

during operation are critical for reliable operationof the LEDs for the design life.


Efficiency and brightness improvement

-Efficiency is critical for reduction of energy consumption, while brightness improvement (increased light output) is needed to reduce cost per lumen.


Rework

-It is difficult to rework LEDs assembled, so first pass yield is critical


The Role of High Thermal and High Reliability Materials in Automotive LED Lighting

High thermal and high reliability die attach and assembly materials enable rapid heat extraction, thus enabling performance improvement, reliably, over the expected lifetime.

Key requirements for advanced die attach and assembly materials used in LED die attach and package-on-board assembly are:

Die Attach

-Very high thermal conductivity 

-Thin, uniform bond line for die tilt control and thermal performance

-No bleed-out or die top contamination

-High thermal cycle reliability with high CTE mismatch

-High creep resistance 


Package-on-Board Assembly

-Low/acceptable void level assembly

-Package tilt control

-High creep-fatigue resistant solder alloys leading to better thermal cycling performance 

-High vibration resistance -Excellent printability and first time right assembly


Two Key Alpha® Technology Platforms for High Thermal, High Reliability Requirements

Alpha®’s novel high thermal, high reliability materials for LED die and package attach can help improve the design window and performance.

High creep fatigue resistant solders

-Maxrel™ alloy based preforms and paste for die/package attach


Silver sintering die attach materials

-Argomax® Ag film for die attach

-Fortibond™ pressureless sintering


Maxrel™ Creep Resistant Alloy

Maxrel™ alloy is a creep resistant solder material that increases reliability of the solder joints subjected to thermal cycling in high CTE mismatch material stacks.  Figure 2 shows Maxrel alloy performance vs SAC305 alloy in thermal cycling of ceramic submount LEDs assembled on Metal Core PCBs.   High creep resistance of Maxrel™ alloy provides better thermal cycling performance.

Thermal Cycling Performance of Maxrel™ Alloy vs SAC305 Alloy

Figure 2:  Thermal Cycling Performance of Maxrel™ Alloy vs SAC305 Alloy

Maxrel™ alloy has unique reflow properties that minimize die / package tilt and die float.  This leads to more uniform bond lines and control of die tilt as shown in Figure 3.

Further, the higher creep resistance of Maxrel™ alloy provides increased reliability while reducing bond line, which in turn means that Maxrel™ alloy can provide lower thermal resistance in the joint.

Further improvements in reliability can be obtained by tailoring the ratio of modulus of the solder to the modulus of the dielectric.

Bond Line Uniformity of Maxrel™ Alloy Joint vs SnAg Alloy Joint


Figure 3:  Bond Line Uniformity of Maxrel™ Alloy Joint vs SnAg Alloy Joint

We will continue this discussion of Alpha's Advanced LED Thermal Management Technologies for Automotive Applications

in the next issue of the Alpha E-Newsletter in which we will feature ALPHA® Argomax® and Fortibond® products.


Acknowledgments: Bawa Singh, Oscar Khaselev, Ranjit Pandher, Siuli Sarkar, Anna Lifton


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