The solid-state lighting industry continues to develop LEDs with higher lumens per watt, in alignment with Haitz Law, which states that the cost per lumen falls by a factor of 10 every decade and the amount of light generated per LED increases by a factor of 20. These advancements are driving the need for packaging and thermal substrate technologies that deliver better thermal management, are more reliable, and are cost-efficient.
There are a variety of thermal substrates used in LED packaging. One widely used type of thermal substrate is Metal Core Printed Circuit Boards (MCPCBs). The selection of the thermal substrate is key for designers to optimize LED performance.
LED designs also require a variety of optics and reflective materials to get the light out. This adds additional cost and complexity while affecting overall system reliability. The materials utilized in LED packaging, particularly the high thermal conductive substrate, will be key for designers in improving LED performance, simplifying the design process and reducing overall costs.
In this article we will cover the importance of substrates in relation to thermal management, reliability and color issues in LED products. In particular, the benefits of replacing FR-4 boards or thermally conductive epoxies with metal-core laminate boards (MCBs) will be addressed, as well as how polyimide materials and their unique properties, including reliability, thinness, robustness, and bendability are being leveraged for broader designs and applications in LED lighting.
Despite the lighting industry’s ability to provide more lumens per watt than ever before, an LED’s reliability and performance depend greatly on how it is packaged and mounted. The main drivers of technology development in this area are directly related to increasing lumens per watt while decreasing overall costs.
Approximately 30 percent of the energy passing through an LED chip is converted to light, while 70 percent is converted to heat, which directly affects the reliability and the color temperature of the LED. LED lighting manufacturers, in order to deliver on the value proposition of a longer lifetime than incandescent bulbs, have to dissipate heat.
LED chip manufacturers try to keep the junction temperature within the semiconductor at 75-85°C, which allows more light output from the device along with better light color and longer life. With more thermally conductive substrates, there is the potential capability to use fewer LED die in a design to get the desired light output.
Importance of Thermal Substrates
The first consideration in managing the heat in all LEDs, and particularly in higher-power applications, is determining what materials are used to make the circuit boards. Traditionally, aluminum-based FR-4 glass-reinforced and epoxy-based materials have been used. By replacing thick FR-4 boards with metal-core laminate boards (MCBs) such as DuPont™ CooLam™ thermal substrate, heat dissipates from the LED faster due to the thin dielectric material.
DuPont has developed acrylic and polyimide technologies to meet the needs of industries as varied as aerospace, automotive and consumer electronics. Forty years ago the company introduced its Kapton® polyimide film technology, and subsequently Pyralux® flexible circuit materials. Polyimides possess a unique combination of properties that make them ideal for a variety of applications in many different industries. The ability of polyimide to maintain its excellent physical, electrical, and mechanical properties over a wide temperature range has opened new design and application areas. It has excellent chemical resistance; there are no known organic solvents for the film. Polyimide has the highest UL-94 flammability rating – V-0 – and does not melt or burn. Its outstanding properties permit it to be used at both high and low temperature extremes where most other organic polymeric materials would not be functional.
While polyimide laminates were not made with LEDs in mind, the properties they demonstrate make it ideal for adoption by the solid-state lighting industry. The developments in the LED industry, as well as the high power/heat dissipation and electrical requirements, create the perfect intersection of market needs and materials science.
Benefits to the Lighting Industry
CooLam™ thermal substrates were developed for use in submount, chip-on-board, and metal core PCB LED packaging applications. These thermal-clad laminates are a composite of metal foil and proprietary thermally conductive polyimide dielectric bonded to a metal base, and they provide an ideal insulated metal substrate for high-brightness LED lighting.
The thin polyimide layer (Figure 1) exhibits very low thermal resistance. This thin dielectric layer allows for heat to pass readily while accommodating smaller packages. The dielectric is placed between the copper layer that fabricators convert to pads to mount the LED chip and the aluminum heat sink.
Additionally, in testing for breakdown voltage and sustained voltage, the polyimide demonstrates extreme dielectric strength that reaches the levels required by the lighting industry to receive UL recognition. These properties are important in the lighting world to ensure UL recognition of a luminaire or replacement bulb, and UL looks at both the materials used as well as how the components and device are fabricated.
The reliability and robustness of polyimide-based thermal substrates enhances the lifetime benchmarks set for LEDs, as the LED chip’s performance is only as reliable as the materials in which it is packaged (Figure 2). Thermal cycle testing is one measure of an overall system’s reliability with a variety of failure modes. One such failure mode would be lead-free solder cracking. This can occur because of the transfer of mechanical force due to the coefficient of thermal expansion mismatch that exists between metals. The aluminum (base) will grow at 20 parts per million, dielectric at 35 parts per million, copper at 12 parts per million, Sn-Ag-Cu (SAC) solder at 17 parts per million, and ceramic at 8 parts per million. Something has to give and usually it’s the solder. One must also consider the modulus, or relative stiffness, of the materials as well.
Polyimide material is 85 megapascals and the competitive material is 80,000 megapascals. The polyimide material acts like a stress buffer and won’t transfer that mechanical stress through the stack, resulting in a longer life for the LED package.
Another recent advancement in technology is a unique way to bond copper to aluminum with a polyimide dielectric that results in a novel bendability that has the potential to revolutionize luminaire and fixture design in the years to come. When the solid-state lighting industry first began, manufacturers used either FR4, which is rigid, or metal core printed circuit board, which is an aluminum plate, so everything the lighting industry designed for LEDs was flat. But the bendable polyimide substrate shows promise to allow lighting companies to direct and control the light output of the LEDs by bending the board and directing light where it is needed.
The next generation of this technology allows a user to do a one-time bend of aluminum, dielectric and copper together (Figure 3). With the current technology, lighting designers have to use optics and diffusers or multiple boards that they have to solder together. But a bendable substrate will allow simplified designs that will cost less to manufacture and less to install, and will last longer over time.
The key to LEDs delivering on their promise of longer life and more efficient electrical performance is in controlling the heat inside the LED chip package. The most effective way to do that is with substrate materials that dissipate heat conductively through the package while providing additional benefits, such as MCPCBs that are bendable, that will transform solid-state lighting designs and the lighting industry itself.
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