CNES – Inspection of critical components for space missions using X-ray CT

19 May 2022
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The CNES Expertise Laboratory uses X-ray CT to carry out n on-destructive inspection of optocouplers for future space missions. A thorough understanding of failure mechanisms in critical components is one of the main ways to bring equipment reliability up to the level these missions demand. JUICE (JUpiter ICy moons Explorer) is the first large-class mission of ESA’s Cosmic Vision programme.

CNES supports this mission, whose goal is to make a single tour of the Jupiter system, including in-depth studies of three moons that may harbor subsurface oceans: Ganymede, Europa and Callisto.
RX Solutions nano-CT system at the CNES laboratory
RX Solutions nano-CT system at the CNES laboratory
Inspecting critical components at CNES: the high-voltage optocoupler.

An optocoupler is an electronic component that works as an isolator. It protects the electronic circuit and isolates critical components from high voltages.
It uses a light emitter, usually a light-emitting diode (LED), to send an internal optical signal to a photodetector and an amplifier. Optocouplers provide a very high degree of isolation between the electrical signal that drives the LED and the amplifier output, because there is no direct electrical connection between them, only the optical signal.

Failure analysis of optocouplers for space instruments with X-ray CT

On the JUICE mission, the PEP (Particle Environment Package) experiment is an instrument that measures particles and plasma with six sensors.
Among them, the JDC instrument (Jupiter Dynamics and Composition) is an ion mass spectrometer (1 eV–41 keV, M/ΔM = 30), designed and built by IRF (Kiruna, Sweden) with a technical contribution from the IRAP laboratory in Toulouse.
It aims to study populations of charged and neutral particles in the magnetic environments of Jupiter (a fast-rotating magnetosphere), Ganymede (a mini-magnetosphere) and around Europa and Callisto.
Some of the instrument’s electro-optical elements are charged by a static high voltage. For the instrument to work precisely, the high voltages must be handled with great accuracy. In addition, to enable certain measurements, the high voltage has to be swept very quickly at very precise intervals (rate > 1 kV/ms).
Custom optocouplers are therefore used in the high-voltage operating circuit.
The optocoupler designed by the IRAP laboratory has two independent LED chain branches, which provide circuit redundancy.
If functional defects show up on both branches, the faulty optocoupler can severely limit the scientific capability of the whole instrument.
In the analysis that follows, functional defects were observed on some optocouplers after a thermal cycling test.

Kateryna Kiryukhina, Technology Analysis Expert at CNES
Artist’s impression of JUICE © ESA ATG Medialab 2017

X-ray CT at CNES


To investigate the origin of this electrical behavior, a CT inspection was carried out. X-ray CT has become an essential technology for characterizing samples in 3D. It lets you inspect a sample non-destructively and view internal structures at high resolution.

A quick CT scan can generate a dataset for assessing a part’s structural, electrical and dimensional aspects. The origin of a defect, such as a solder anomaly, porosity, contamination or an interconnection problem, is quickly identified.

The X-ray CT inspection took place in the CNES laboratory, on an RX Solutions EasyTom 160 (a CT system from the Nano range). This system combines two X-ray sources, microfocus and nanofocus, so it can handle the most demanding applications, down to resolutions of 0.4 µm.
The optocoupler failure revealed by the electrical test could have several origins, linked to the components, to the wire bond interconnections and to thermomechanical stress. Hence the two different X-ray CT scans performed on the optocoupler: the first to view the whole sample, the second to zoom in on the LED and look in more detail at the die attach area.

Both CT scans give very sharp detail, with balanced contrast and brightness, which is what you need to see the optocoupler’s main internal components precisely. The four LEDs and the photodiode can be inspected accurately, along with the attach area of each LED.
Scan n°1 - the set-up
Scan n°1 - the set-up
Scan n°1 - results on the whole sample
Scan n°1 - results on the whole sample
Scan n°2 - the set-up
Scan n°2 - the set-up
Scan n°2 - zoomed results on the LED attach area
Scan n°2 - zoomed results on the LED attach area
Scan n°1 - the set-up
Resolution: 11 µm/voxel; scan time: 1 h 20; X-ray tube: 160 kV nanofocus; X-ray detector: flat panel
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Scan n°1 - results on the whole sample
The scan result is a complete 3D volume of the inside of the optocoupler, showing all its internal components. Performed at a resolution of 13 µm, the scan provides a dataset for inspecting the LED mounting areas in detail. It gives an overall rendering and lets you inspect the structure as a whole.
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Scan n°2 - the set-up
Resolution: 2 µm/voxel; scan time: 2 h 30; X-ray tube: 160 kV nanofocus; X-ray detector: CCD camera
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Scan n°2 - zoomed results on the LED attach area
A second scan, with the plastic casing around the optocoupler removed, focuses on the main electrical elements: the four LEDs and the photodiode. Identifying the most likely source of failure led us to run a third scan, at an increased resolution of 2 µm, targeting the LED and its connection area.
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X-ray CT is proven a key tool in the current manufacturing challenges of electronical components

After a first electrical test which has highlighted a defect on the optocoupler, tomography allows to find the root cause of this defect, in a non-destructive way. Within a few hours, the X-ray tomography inspection gave all the insights of the optocoupler and is the best technology to get the origin of the defect highlighted during the electrical tests.
 

This enables to choose the right design to ensure the reliability of the equipment to be flown in a few years in the Jovian environment and enable a wealth of new scientific discoveries!
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