Thermal imaging and dimensional measurement of microelectronics

09 December 2014

Chris Jones, managing director at Micro-Epsilon UK, discusses how the latest thermal imaging cameras and confocal displacement sensors are helping with the inspection of a variety of electronic devices, circuit boards and MEMS.

Manufacturers of electronic components and printed circuit boards (PCBs) are increasingly turning to non-contact temperature measurement systems to capture, analyse and optimise the thermal behaviour of electronic components and fully assembled PCBs. By using thermal imaging cameras – in R&D, new product development and high volume production – hot spots and defects can be detected quickly and reliably, without influencing the target object.

Detailed, real-time analysis of the thermal behaviour of integrated circuits can be carried out using infrared thermal imaging cameras which are able to capture and store thermal video and images with high optical resolution at high frame rates. Camera detectors can provide good thermal sensitivities  – typically 80mK-40mK –  enabling extremely small temperature differences to be detected. 

A new thermal imaging camera from Micro Epsilon –  the thermoIMAGER TIM 640 – is said to be able to record radiometric video images at an optical resolution of 640x480 pixels and is believed to be the smallest VGA-resolution infrared camera available today. With dimensions of 45 x 56 x 90mm and weighing 320g it is able to record radiometric video images at 32Hz and a VGA resolution of 640x480 pixels. At ambient temperatures of between 0 and 50°C, it  is possible to measure object temperatures ranging from -20°C to +900°C. For process integration, the camera is supplied with the  company’s TIM Connect software as standard which allows users to monitor and document measurements and to edit infrared video images. 

A USB 2.0 interface allows video recording at 32Hz – a useful feature for short-term thermal measurement activities, which need to be analysed in slow motion playback. Individual snapshots can be taken from these recordings. Measurement points are freely selectable and can be analysed via user-defined areas of interest. Alarms can also be set up and displayed, as well as maximum, minimum and average temperatures.

Dimensional measurement of MEMS
Micro Electro Mechanical or Micro Electronic Systems (MEMS) is the technology of very small devices which typically comprise components of between 1 to 100 micrometres in size. Normally machined from silicon, they will normally consist of a central unit that processes data and several components that interact with the surroundings such as micro-gears and micro-sensors.

The latest confocal chromatic sensors offer extremely high sensitivity and sub-micrometre resolution, which can provide significant advantages when inspecting the shape and size of MEMS structures during or post-production. These sensors can be integrated to linear X-Y stages, machine tools or special purpose inspection systems.

IFS 2405 sensors, from Micro-Epsilon, for example, are designed for measurement tasks that require maximum precision. As well as distance measurements on reflective and transparent materials, the sensors can also be used for one-sided thickness measurement of clear film, printed circuit boards or layers. The maximum resolution of these sensors is 0.01µm and maximum linearity is 0.3µm.

Confocal measuring principle
The confocal chromatic measuring principle works by focusing polychromatic white light onto the target surface using a multi-lens optical system. The lenses are arranged in a way that white light is dispersed into a monochromatic light by controlled chromatic deviation (aberration). A certain deviation (specific distance) is assigned to each wavelength by a factory calibration. Only the wavelength that is exactly focussed on the target surface or material is used for the measurement. This light reflected from the target surface is passed through a confocal aperture onto a spectrometer, which detects and processes the spectral changes.

Confocal measurement offers nanometre resolutions and operates almost independently of the target material. A very small, constant spot size, typically <10 µm, through the measurement range of the sensor is achieved. Miniature radial and axial confocal versions are available for measuring the internal surfaces of drilled or bored holes, as well as the measurement of narrow apertures, small gaps and cavities.

Powerful controllers
Traditionally, confocal controllers perform poorly when trying to compensate for difficult and changing surface conditions, particularly in high speed surface scanning tasks. The confocalDT 2451 and 2471 controllers from Micro-Epsilon, however, use newly developed software algorithms which enable the controllers to compensate in real-time, giving users the ability to scan surfaces very rapidly and at high resolution. The controllers also provide high-speed triggering interfaces that allows synchronisation with encoders and other motion control devices. The result is a controller that provides stable, high accuracy measurements, down to nanometre resolution.

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