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CNES - critical components inspection with X-ray CT for space exploration

by Verona on 19 May 2022 at 11h59

CNES laboratory, using X-ray Computed Tomography, carries out non-destructive inspection of optocouplers for space missions. An in-depth understanding of critical components’ failure mechanisms is one of the main aspects to raise the reliability of the equipment to the level required by the criticality of these missions. JUICE - Jupiter Icy moons Explorer is the first large-class mission in ESA’s Cosmic Vision 2015-2021 program. CNES is supporting the mission aimed to accomplish a unique tour of the Jupiter system that will include extensive studies of three potentially ocean-bearing satellites: Ganymede, Europa and Callisto.

Critical components' investigation at CNES - the case of a high-voltage Optocoupler 

An optocoupler is an electronic component that operates as an isolator. It protects the electronic circuit and isolates critical components from high voltages. Optocoupler uses a light emitter, typically a Light-Emitting Diode (LED), to provide an internal optical signal to a photodetector and amplifier. The optocouplers provide a very high degree of isolation between the electrical signal that drives the LED and the output of the amplifier because there is no direct electronical connection between them, only the optical signal.
 
 
 
On JUICE mission, the PEP (Particle Environment Package) experiment is an instrument to measure particles and plasma through 6 sensors. Among them, the instrument JDC (Jupiter Dynamics and Composition) is an ionic mass spectrometer (1 eV-41 keV, M/DM = 30) designed and manufactured by IRF (Kiruna, Sweden) with a technical contribution of the IRAP laboratory in Toulouse. It aims to study the populations of charged and neutral particles in magnetic environments of Jupiter (quickly rotating magnetosphere), Ganimede (mini-magnetosphere), and in the vicinity of the moons Europe and Calisto.
Some electro-optical elements of the instrument are polarized by static high voltage. For the precise operation of the instrument, the high voltages must be operated with high precision. In addition, to enable some of the measurements, the high voltage must be swept very quickly at very precise intervals (rate > 1kV/ms). Custom optocouplers are therefore used in the operating circuit of the high voltage.
The optocoupler designed by the IRAP laboratory has two independent branches of LED chains, ensuring redundancy in the circuit. If functional defects are found on the two branches, the faulty optocoupler can severely limit the scientific capacity of the whole instrument. In the following analysis, functional defects were observed on some optocouplers after a thermal cycling test.
Kateryna KIRYUKHINA
Technology analysis specialist

X-ray Computed Tomography at CNES lab

To investigate the origin of this electrical behavior, a tomography inspection was performed. X-ray Computed Tomography, also known as CT scanning, has become, over the years, an essential technology for non-destructive 3D characterization. It offers the possibility to inspect a sample in a non-destructive way and to visualize at high resolution the inner structures.

A quick CT scan can generate a CT dataset to assess the structural, electrical, and dimensional aspects of a workpiece. The origin of a defect will as soldering defect, presence of voids, contamination, interconnection issue... will be promptly identified. The X-ray CT inspection was realized in the CNES laboratory, on an RX Solutions EasyTom 160 CT system. This setup, combining micro and nano X-ray sources allows addressing the most challenging applications, at very high resolution (down to 0,4 µm). 
The failure of the optocoupler, highlighted by the electrical test, might have several origins, related to components and bonding wires interconnections and thermo-mechanical stress.
Hence the two different X-ray scans were performed on the optocoupler: the first scan for the visualization of the whole sample and the second one to zoom in on the LED to observe in more detail the die-attach area.
 
Both scans enhance sharp details likewise a balanced contrast and brightness that are needed to accurately see the main inside components of the optocoupler. The 4 LEDs and the photodiode can be precisely inspected as well as the die attach areas of each LED.

Scan n°1 - set-up of the whole sample

Resolution: 11µm / voxel, Scanning time : 1h20, X-ray tube: Nano-focus 160 kV, X-ray detector: flat panel

Scan n°1 - results on the whole sample

The scan output consists in a 3D volume of the inside of the optocoupler, including any internal components. Performed at 13 µm resolution, the scan offers a set of data to inspect the LEDs mounting areas in full detail. It offers a global rendering and allows to inspect its structure as a whole.

Scan n°2 - set-up of the die attach area

Resolution: 2µm / voxel, Scanning time : 2h30, X-ray tube: Nano-focus 160 kV, X-ray detector: CCD Camera

Scan n°2 - results on zoom of the die attach area

A second scan, removing the plastic protection around the optocoupler, focuses on the main electrical elements, the 4 LEDs and the photodiode. The identification of the most likely source of failure guides a third scan with an increased resolution to 2 µm targetting the LED and its connecting area.

Scan n°1 - set-up of the whole sample

Resolution: 11µm / voxel, Scanning time : 1h20, X-ray tube: Nano-focus 160 kV, X-ray detector: flat panel

Scan n°1 - results on the whole sample

The scan output consists in a 3D volume of the inside of the optocoupler, including any internal components. Performed at 13 µm resolution, the scan offers a set of data to inspect the LEDs mounting areas in full detail. It offers a global rendering and allows to inspect its structure as a whole.

Scan n°2 - set-up of the die attach area

Resolution: 2µm / voxel, Scanning time : 2h30, X-ray tube: Nano-focus 160 kV, X-ray detector: CCD Camera

Scan n°2 - results on zoom of the die attach area

A second scan, removing the plastic protection around the optocoupler, focuses on the main electrical elements, the 4 LEDs and the photodiode. The identification of the most likely source of failure guides a third scan with an increased resolution to 2 µm targetting the LED and its connecting area.

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!
 
 
Click here to download the full application note here

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The technical progress expected in the automotive industry in the coming years is important and very challenging, particularly with the development of electric vehicles. Renault Group has made innovation one of the keys to its success with its Guyancourt Technocentre, one of the largest automotive research and development center in Europe, using X-ray CT. Read more here

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