Thermal Measurements

Thermal measurements are used in a variety of applications involving LEDs, laser diodes and other semiconductor devices. Thermal measurements, specifically junction temperature, are useful at all stages of semiconductor production from design and development to quality assurance. The basis for every semiconductor data sheet and end user design is junction temperature.  

The leading standard for optical measurement of high-power LEDs, the 40-page CIE TC 2-63, refers to LED junction temperature 204 times. According to its authors, members of the International Commission on Illumination, this parameter directly influences the LED’s photometric, radiometric and colorimetric performance. Junction temperature, or TJ, is also a key parameter in LED reliability performance as semiconductor, phosphor, and packaging failure mechanisms involve chemical reactions that have specific activation energy thresholds.

The IESNA LM-85  standard outlines the methodology to be used for the measurement and characterization of high-power LEDs. An LED or Laser Diode’s junction temperature (Tj) is a primary determinate for long-term reliability; it also is a key factor for photometry. For example, a typical white LED output declines 20% for a 50 °C rise in junction temperature. Because of this temperature sensitivity, LED measurement standards, like IESNA’s LM-85, require that junction temperature is determined when making photometric measurements.

What is Junction Temperature?

Junction Temperature (Tj) is the operating temperature of the LED, laser diode or other semiconductor device. 

What is In-situ Junction Temperature?

In-situ junction temperature is the junction temperature of the device eg. LED in its operating environment.  The operating environment might be when the device mounted in a burn-in fixture, integrated into an enclosed luminaire, or any other environment where the device is powered.  

When considering in-situ junction temperature, the following diagram provides a nice visual.  

Why is an Accurate Junction Temperature important?

Mass marketed products benefit from knowledge of the accurate junction temperature in many ways.  Primarily, to ensure proper design of an efficient fixture. With proper junction temperature measurement, metal can be reduced saving fixture and shipping cost. Secondly, to prevent losses due to support issues. Losses could be hundreds of thousands of dollars and higher if the products are returned due to failure under warranty.

For products used in the medical industry, knowledge of the accurate junction temperature is imperative for several reasons.  LEDs are improving rapidly obsoleting their predecessors.  To ensure that next generation devices perform consistently with obsoleted devices.   To ensure consistent color and operation, in-situ junction temperature testing should be performed on the devices and used to define other testing to be performed on the medical device.

For manufacturers of LEDs or other semiconductor devices then knowledge of the accurate junction temperature is important. Your datasheets and marketing materials are important. Using accurate data communicates quality to your customers.

If you are replacing an obsolete device with a new device then knowledge of the actual junction temperature of the device used in your existing product will simplify selection of a replacement device. To best compare the original LED with the replacement, the junction temperature (Tj) of each LED should be measured, when it is operating in-situ in the endoscopy product. This can be done with the Electrical Test Method defined in JESD51-51. The ETM does not require any thermocouples, instead it uses the LED diodes voltage vs temperature characteristic to infer temperature. 

Tj is a useful tool for qualifying and modifying a design because it exposes the effect of small changes in the product. For example if the new LED is slightly smaller than the old one and a larger encapsulation is being considered, the effect of various encapsulants on the heat flow out of the LED can be measured quantitatively in just a few minutes. In this way the final design can be adjusted for the ideal performance.

The Tj should also be compared with the manufacturer’s maximum junction temperature ratings to ensure they are below the maximum allowable temperature. Actually they should be 20-40% below this temperature. This is particularly important for pulsed overdrive operation. In addition the junction temperature should be used to determine the ideal stress temperature for the lumen maintenance testing that is done for the new part. For example if the in-situ junction temperature is found to be 100C at 2A, and the nominal thermal resistance for the devices when in the stress system is 6.5 degrees C/W, and the forward voltage at 2A is 3V, then the matching stress temperature would be 100C – 2 * 6.5 * 3 = 61C.

Likewise knowing the Tj in-situ allows the light quality characteristics such as chromaticity to be established. Chromaticity and lumen output shifts dramatically with temperature so it is important to know the actual operating Tj when assessing these parameters and when documenting them for the final product specifications.

If your product is a large LED / device then in-situ production line testing will provide insight into the production processes and shorten your time to market.

If you are considering burn-in of your devices, the junction temperature measurement may be determined or specified in order to ensure the burn-in conditions are consistent with the end product requirements.

Contact Vektrex for your In situ Junction Temperature solution.  

How To Measure Junction Temperature

What is JESD51-51?

The most common Tj measurement technique is the Joint Electron Device Engineering Council’s (JEDEC) JESD51-51 method. This technique, often called the Electrical Test Method (ETM), uses a two-level pulsed drive current. The high current level, called the heating current, heats the junction to its operating temperature. After an extended drive period, the current switches briefly to the measurement level. During this low-level interlude, forward voltage measurements are made. The device under test may be mounted on a temperature control platform to maintain a constant case temperature.

How to Use Forward Voltage to Measure Semiconductor Junction Temperature

Use the JEDEC Electrical Test Method to Determine Junction Temperature.

How to Use the JEDEC Electrical Test Method to Determine Junction Temperature.

JEDEC’s Electrical Test Method (ETM) defines a method to measure an LED’s Tj in-situ at full current under actual operating conditions.  

The method utilizes two currents; the operating current and a measurement current that is approximately 2-3% of the operating current. The method specifies to first profile the LED forward voltage versus temperature characteristic.  Second, to drive the LED at the intended operating current. After stabilization, rapidly pulse down to a measurement current for a brief interval and forward voltage measurement samples are taken. A sample waveform is shown below.

Vektrex TJ Measurement Toolkit based upon a Spikesafe Precision Pulsed Source Measure unit with bias module meets the requirements to execute this waveform. Requirements include:

  1. Fast pulsing is required to handle the sharp transition from heating to measurement current levels. JEDEC ETM requires the transition to be within us’s.  With slower pulsing, the electrical transient will slow measurement and interfere with the accuracy … need better words here  Using two instruments; one for each current, is slow and cumbersome.  Syncronization will be difficult.  With SpikeSafe SMU, the low current source (aka measurement source) is hardware integrated with us transition times.
  2. Continuous Power Conversion to support the high duty cycle of the heating current waveform. Spiksafe SMU includes continuous power conversion with no duty cycle limitations.
  3. High-bandwidth digitizer with high voltage to support the ensemble Tj measurement of multiple LEDs. Spikesafe SMU includes an integrated digitizer with a 400V range.
  4. Semi-automated software that captures the Vf trend and extrapolation. Spikesafe SMU bias option includes software and the calculations to determine junction temperature and thermal resistance.
  5. Instruction and Junction Temperature calculation spreadsheet to simplify and automate calculations.

What is the JEDEC Electrical Test Ensemble Junction Temperature Measurement?

The JEDEC ensemble junction temperature measurement is used to calculate junction temperature for multiple devices in series or one entire LED or laser array. Due to the increased heat for multiple devices, the ensemble TJ is generally higher than the Tj for a single device.

How to Simply Profile LED Vf Versus Temperature.

Very simply, it may be possible to use the heating of the device to profile forward voltage versus the temperature characteristic. A precision thermal platform may be used to control temperature and profile voltage versus temperature. As an alternative, the device may be placed in a thermal environment to better control temperature. The most difficult solution is to use a chamber.

How to Measure Junction Temperature for Mini and Micro LEDs

The small size of an LED or laser and surrounding optics makes Tj impossible to sense directly. Instead, it is measured indirectly using the LED or laser diode inherent voltage/temperature characteristic. Please refer to the ‘How to use the JEDEC Electrical Test Method to Determine Junction Temperature” section. The junction temperature for Individual devices may be determined. In addition, consider determining the ensemble Tj associated with multiple devices in series.

How to Use LM-80 Case Temperature Data To Determine LED Junction Temperature

During LM-80 reliability tests, the case temperature of an LED is measured at a point specified by the LED manufacturer.  LED samples are powered for long periods at elevated temperatures. The case temperature is monitored during this stress test. The reported case temperature may be used as the basis for selecting junction temperature however this data does not accurately represent the in-situ junction temperature of a device integrated into a complete product. The Tj for a complete product would include heatsinks, optics and housing.    

In practice it is also difficult to measure the Tj once the LEDs are packaged into a complete luminaire since the case temperature point is usually inaccessible. For this reason, the JEDEC ETM is recommended.

How to Use Thermocouples to determine LED Junction Temperature

Thermocouples may be used to determine junction temperature. Thermocouple measurements, however, are not junction temperature measurements. Thermocouple measurements lead to heat being conducted away from or toward the LED introducing error. Thermocouples themselves may also be affected by the LED light.  Often the thermocouple results are useless.

How to Use Manufacturer Specified Thermal Resistance Data to determine Junction Temperature

With a reasonable case temperature measurement, Tj may be calculated using the manufacturer published thermal resistance value.  However, the published values are typical.  In many cases, the value depends upon a thermal path through a solder connection.  Individual solder connections vary.  It is possible to have voids and other defects in the solder connecting the LED to the MCPCB or heat sink that increase the thermal resistance.  When this happens the actual Tj value is inaccurate and can be well above the calculated value.

How to Use an InfraRed Camera to Determine Junction Temperature

Infrared cameras are often used to measure junction temperature.   Often, the image is too coarse to distinguish the LED junction from surrounding elements.  In addition, many luminaires include optics such as diffuser lenses that block access to the individual LED chips, making it impossible to accurately measure the luminaire in its true condition. Finally, IR camera positioning must be exact for each measurement or results will be inconsistent and unusable.

How to Use Thermal Impedance Analyzers to Determine Junction Temperature

Structure functions are calculations that transform thermal transient measurement results into a thermal resistance versus thermal capacitance graph. The thermal resistance versus thermal capacitance graph acts as a visualization tool and provides insight into each layer the heat passes through. The calculations needed to produce structure functions are complex and special 3rd party are generally used to perform them.

Sophisticated thermal-impedance analyzers such as the T3STER  are good at determining detailed characteristics of single LEDs but thermal impedance analyzers are complex instruments that are difficult to understand and use resulting in errors.   In addition, they often lack the power to drive a complete luminaire.  Finally, in-situ Junction Temperature measurement would not be possible.

Products For Junction Temperature Measurement

Products for Junction Temperature Measurement Include:

TJ MEASUREMENT TOOLKIT – The Tj Measurement Toolkit automates and simplifies In-Situ Junction Temperature Measurement. 

AUROTEK JD-2020 – The Aurotek JD-2020 LED Thermal Resistance Analyzer is a high-speed instrument designed to conduct transient/in-line thermal resistance (RΘ) measurements.