The ABCs of LEDs – Substrates & Thermal/Temperature Sensitive

Returning to our series on everything LEDs, we will pick back up at letters “S” and “T” with substrates and thermal/temperature sensitive.

S – Substrates

The main substrates utilized in the production of LEDs are sapphire, gallium arsenide, or silicon carbide. There are also other alternative substrates such as gallium nitride (GaN) and silicon. Most advanced LED devices are built on sapphire (Al2O3) substrates. Onto the polished upper surface of the sapphire substrate an epitaxial (“epi”) layer of gallium nitride is grown using metal-organic chemical vapor deposition (MOCVD). In the world of substrates, gallium nitride on gallium nitride (GaN-on-GaN) and gallium nitride on silicon (GaN-on-Si) are the white-hot topics.

AlN is currently used in HB LED packaging but only in situations where there are no other feasible alternatives. The high cost pressure in the HB LED market, as well as the significant portion of total device cost that packaging entails, increases the need to minimize high-cost AlN usage.

The high cost of AlN substrates is attributable to a number of factors. The causes span a range from high cost of raw materials such as AlN powder to complex and expensive manufacturing steps.

T – Thermal/Temperature Sensitive

Poor quality solid-state lighting (SSL) with inadequate thermal designs or poor circuit designs can create electrical over-stress conditions. These conditions, along with a chemical compatibility issue in the manufacturing process, can all result in LED quality degradation or even complete luminaire failure.

The majority of LED failures are temperature related; an LED's performance and projected lifetime correlate tightly to thermal management and the resulting junction temperature of the LED semiconductor chip. Elevated junction temperatures cause a reduction in light output and accelerated LED lifetime degradation. Proper thermal management of an LED luminaire and mechanical construction are vital for performance. To assess the mechanical construction, an evaluation should include techniques such as X-ray photographs of LED soldering. Measuring actual thermal performance to validate thermal design assumptions is necessary to ensure quality and reliability of SSL products.

However, thermal measurements of SSL can be challenging. Positioning the thermal couple at the wrong point or having large amounts of photonic energy illuminate the thermal couple will result in an incorrect temperature measurement. Such errors could result in design issues that may compromise LED lifetime for the finished product.

Notice the difference in the thermal performance of the two identical printed circuit boards (PCBs) in the infrared photograph below. The PCB on the left is cooler in temperature, shown as a yellow color, while the PCB on the right is bright red and the LEDs are white hot, indicating a thermal interface issue with the PCB mounted on the right.

LEDs temperature comparison, LEDs, light emitting diode

The number and closeness of LEDs requires good thermally conductive die attach to keep the LEDs as cool as possible. High-brightness LED applications require maximum thermal transfer to achieve performance requirements.

Critical to the LED assembly process is a void-free eutectic solder interface between the diode and its substrate that provides the thermal and electrical connections needed to generate a stable transmission of light. Eutectic die attachments transfer the tremendous amount of heat generated by the diodes to maintain the temperature stability of the device.

To yield the optimal thermally conducting solder interface, the temperature profile of the attachment process must be repeatable and have the capability for a high temperature ramp rate. Once the interface is brought up to the proper eutectic temperature, the heating mechanism must maintain that programmed temperature with minimal overshoot. After the required amount of reflow time, the heating mechanism must controllably cool to minimize damage to the diode and to allow the eutectic material to reach metallurgical equilibrium. This equilibrium is reached through simultaneous application of active thermoelectric pulse heating and cooling gases.

LED matrix assembly is an extremely temperature-sensitive process that requires careful control during assembly. LEDs are sensitive to temperatures in excess of 300°C.

Sources:

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LEDs Magazine: “Cost-reduced AlN delivers thermals needed in HB LED packages”, http://ledsmagazine.com/features/10/9/7.

Solid State Technology Journal: “The gleam of well-polished sapphire”, http://electroiq.com/blog/2013/01/the-gleam-of-well-polished-sapphire/.

LEDs Magazine: “LED Japan/Strategies in Light Japan speakers include blue LED pioneer”, http://ledsmagazine.com/features/10/9/2.

LEDs Magazine: “LED system evaluation yields quality analysis”, http://ledsmagazine.com/features/10/9/8.

LEDs Magazine, “Improve LED manufacturing via in-line monitoring and SPC”, http://ledsmagazine.com/features/10/7/10.

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Janine Hueners

Marketing Specialist
Palomar Technologies, Inc.