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Radiographic inspection Of Miniature Components Through Image Magnification

Using Computed Radiography

Sambamurthy E, R Gunasekaran, Cherian Thomas and C R Thomas

Rocket Propellant Plant, Vikram Sarabahai Space Centre, Trivandrum 695022, India

sambamurthy@vssc.gov.in , r_gunasekaran@vssc.gov.in, cherian_thomas@vssc.gov.in

Abstract

Radiography is one of the age old NDT techniques for testing wide range of components. It is

highly effective in revealing internal details of components. However general radiography

technique is inadequate for radiography of miniature components for viewing it's internal

details that are in the order of microns. Techniques like micro-focal radiography and nano-

focal radiography are evolving for such applications. Ironically computed radiography (CR)

is found to be highly promising technique for radiography of miniature components such as

printed circuit boards. The present work is focused on the image magnification applications

of computed radiography. Miniature components viz PCB, MEMS rate sensor, Li-ion cell that are used in launch vehicle/satellite components have been radiographed by CR. In some

of the components the details to be investigated are as low as 50 micron. CR is successfully used for examining these components and image magnification was achieved upto 20X

without deteriorating the image quality.

This paper covers the details of experiments carried out using computed radiography towards inspection of miniature components used in aerospace applications.

Key words: NDT, Radiography, Computed radiography, Image magnification, MEMS rate

sensor, PCB and Li-ion cell

1. INTRODUCTION:

Radiography is a widely used NDT technique for testing various components. Computed

radiography (CR) and digital radiography (DR) are upcoming digital technologies in the field

of radiography. Higher latitude, ability to process the image, easy archival, image quality on

par with the film, elimination of chemicals and darkroom etc are some of the advantages of

digital radiography. The prime component in computed radiography is specialised phosphor

screen called imaging plate (IP) that replaces x-ray film, for capturing images. When exposed

to x-ray Phosphor crystal lattice are excited and become trapped at high energy states. By

scanning laser beam over an exposed screen and detecting the emitted energy at each

location, image stored in the phosphor can be fully converted into digital.

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Radiography of miniature components is a challenging exercise as the details to be examined

are in the range of few mm to microns. Discerning such miniature details is very difficult in

conventional film radiography. Further, magnification of a digitised film results in reduction

in image quality. Special techniques like micro-focal and nano-focal radiography techniques

are used for this purpose, however CR images are found to be highly useful for discerning the

miniature details. In this current work trials have been carried out on printed circuit boards,

miniature electronic component, MEMS rate sensor and Li-ion cell. Details of the test results

are presented.

2. TEST METHODOLOGY:

In the present study all the components are tested with GE make x-ray machine of focal spot

size 0.4mm (mini focus) with an SFD of 1.5 meter. Exposure parameters are chosen to ensure

the sufficient grey values. CIT make super high resolution imaging plate is used for capturing

image. imaging plate is scanned by CIT make DR HD system with 16 bit scanner at 1200DPI

i.e a resolution of 21micron. 5Megapixel black and white screen is used for visualisation of

the digital image.

With the help of CIT make DR 1200 software images are magnified upto 30X and the

required details are seen. Image processing tools such as edge detection and localised contrast

enhancement also helped in visualising the required features.

Fig 1.1 CR imaging plate Fig 1.2 CR scanner

Fig 1.3 CR image acquisition and image display

process

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The reason for the magnification capabilities of the CR can be explained as follows. When

the image is scanned it is scanned with higher dots per inch(DPI).Scanning at such a higher

DPI gives a resolution of around 21 micron. This image is stored in tif format and the image

size is around 800 MB. Because of such a high resolution scanning, the information in

digitised image is large and therefore the image quality is maintained at higher

magnifications.

3. RESULTS AND DISCUSSION:

i. PCB inspection:

One of the PCBs used in satellite launch vehicle was found shorted during continuity checks.

The PCB is visually inspected and critical areas like pins of the IC's are examined under a

microscope for locating the shorting however no anomaly is noticed.

The PCB is radiographed with CR and the image is magnified to 15X, all the pins were

inspected. It was found that one of the pin was found to be shorted with another. Fig 2.1

shows the digital radiograph of the PCB and fig 2.2 shows the magnified image(15X) of the

shorted IC.

ii. MEMS rate sensor:

MEMS rate sensor is one of the important launch vehicle component. It consists of ball grid

arrays(BGA). Lead ceramic balls of 0.5mm are used for soldering the IC.

BGA joints are radiographed to inspect

i) dry joints/ insufficient solder

ii) bridging/shorting due to surplus solder

Fig 2.1 CR image of PCB (X 1)Fig 2.2 Magnified image of IC

(X15)

Shorting

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iii) voids due to the gas bubble within solder.

iv) misplacement/misalignment due to inaccurate placement of the components.

Among the above mentioned defects bridging between solder joints is more critical defect

because it can directly result in functional disorder of the product.

MEMS rate sensor shown in fig 3.1 is radiographed with CR and the digital image is shown

in fig 3.3.The image is magnified to 24X and at this magnification it is possible to resolve the

required features. Magnified image is shown in fig3.4 All the Ball Grid Array( BGA)s are

found normal and the CR is used as mandatory for inspection of this product.

iii. MEMS based initiators for micro thrusters:

Micro thrusters are important satellite components used for specialised applications like orbit

correction operations. MEMS based initiators are essential for initiation of the micro

thrusters. Initiator hardware is shown in fig 4.1.It is manually filled with 1-2 mg of charge

slurry. Fig 4.2 shows the initiator with charge. For proper initiation the charge should be in

contact with the resistor. In order to ensure no gap between the charge and the resistor

5mm

Fig 3.4 :Region of interest( X 24)

Fig 3.3 :MEMS

rate sensor( X 1)

Fig 3.2 :IC to be

testedFig 3.1 :MEMS rate

sensor

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radiography is felt essential. Initiator is radiographed with CR and for resolving the required

features the image has been magnified to 30X.

Magnified image is shown in fig 4.3 and it shows the improper filling of the charge. Based on

NDT feedback, process changes were introduced and new initiator is processed. The

magnified (X30) image of modified initiator is shown in fig 4.4 where proper filling of the

charge is noticed.

iv. Li-ion cells:

Li-ion cell is another important satellite launch vehicle component. It is radiographed to

inspect the alignment of the stack and to verify the shorting between case and stock. Digital

radiograph of Li-ion cell is shown in fig5.1.It could not reveal the required features. Here the

gap between the case and stock is in the order of few microns. It is magnified to 15X and the

corresponding image is shown in fig 5.2 . Proper stack alignment is noticed and no short is

noticed between the case and stock.

Resistor

Fig 4.1 initiator without charge Fig 4.2 initiator with charge

Fig 4.4 proper charge

filling ( X 30)

Fig 4.3 improper charge

filling ( X 30)

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4.CONCLUSIONS:

From the above mentioned studies, CR is highly usefull in addressing the specific

radiography requirements of miniature components. In all the above mentioned cases

conventional radiography and other digital radiography techniques could not able to reveal

the required features. It is one of the easiest solution for image magnification requirements

however it requires huge computer memory for image archival.

5. REFERENCES:

1.HalmshawR. Industrial radiology, theory & practice. 2nded. London: Chapman & Hall;

1996.

2. L Koen et al, “Computed radiography Exposure Indices in mammography”, SA Journal of

Radiology, 2008

3.Bavendiak et al, “New Digital Radiography Procedure Exceeds Film Sensitivity

Considerably in Aerospace Applications”, ECNDT, 2006

Fig 5.1: image of

Li-ion cell ( X1)

Fig 5.2: image of Li-ion cell (X 15)Gap between

case and stock

Proper stocking