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Page 68


www.us-tech.com


March, 2020 Acoustically Examining A Ceramic Pressure Sensor Continued from previous page


Analyzing Images The acoustic image of the external surface of


the ceramic diaphragm is shown in Figure 1. There is an electronic timing gate used to select a “win- dow” of echo arrival times. The time is proportion- al to the feature depth within the material. In this figure, the gate is very narrow and


tures, which are exterior chips in the ceramic package, due to rough handling during manufac- turing.


The dark line near the center of Figure 1 is a


small vertical crack in the ceramic. It is dark because it is a narrow crack with steep walls. This crack could potentially break the seal surrounding the air held between the two disks. The crack may scatter ultrasound, but it has no flat surface to reflect ultrasound to the transducer, and thus appears black. The ceramic disk on this side of the sensor


flexes when pressure is applied. A crack here may distort data, and, if it reaches the interior, may permit the gas or fluid whose pressure is being measured to leak into the space between the two dicks and change the pressure reading. Figure 2 is a narrowly gated image of the inner


face of the flexible ceramic. The image was made using a transducer with a very high frequency of 230 MHz for maximum detail. The focal length was 0.25 in. (6.4 mm). Several items are visible:


l A is the backside of the ceramic diaphragm.


Figure 2: (A) Ceramic-to-metallization inter- face. (B) Ceramic-to-adhesive interface. (C) Delamination between ceramic and adhesive.


encompasses only the surface and immediate sub- surface. The thickness of the disk is about 0.02 in. (0.5 mm). An ultrasonic transducer with a frequen- cy of 100 MHz and a focal length of 0.15 in. (3.8 mm) was used to make this image. In this image, the surface of the diaphragm


shows a mottled appearance. This is due in part to the grain structure of the ceramic material and also in part to the slight surface roughness. Around the circumference are several dark fea-


The crack appears black, because most of the ver- tical extent of the crack lies above the narrow gate from which echoes are returning. The crack stops echoes from reaching the transducer and is therefore imaged as an acoustic shadow. The crack’s size and the fact that it reaches the sur- face suggest that it might leak when a fluid or gas applies pressure. Even if there is no leakage, the extent of the crack may already have altered the flexing properties of the ceramic. If so, the sensor may simply give an inaccurate reading, or the cracking may expand until the ceramic ele- ment breaks.


l To the left of this crack is a fainter, smaller


crack. l


(B and C) is where the diaphragm is bonded to the seal ring whose thickness creates the air space between the disks. The diaphragm appears to be bonded to the seal ring near B, but is delam-


The circular border in the image of the sensor


inated at C. l


Figure 3), the seal ring contains numerous air- filled voids, both circular and worm-shaped. Worm-shaped voids could have formed during assembly if too much pressure was applied to the sensor and an air pocket invaded the bond. The circular voids may have formed when air was trapped. Additional worm-shaped features and voids occur at the other end of the delamination.


At the bottom center of the image (enlarged in


l One of the chip-outs at the surface of the sensor is visible near the lower right of Figure 3 as a


Figure 3: Enlarged view of adhesive.


black indentation. In this image, the indentation itself is not what is being imaged, because it lies relatively far above the gated depth, and the echoes it has sent to the receiver arrived long before the echoes from the interfaces at this depth. Like the crack in Figure 2, it is imaged as an acoustic shadow and shows up as black. Contact: Nordson SONOSCAN, 2149 E. Pratt


Boulevard, Elk Grove Village, IL 60007 % 847-437-6400 E-mail: info@nordsonsonoscan.com Web: www.nordsonsonoscan.com r


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