Detection of Defects in Ceramic Substrate with Embedded Passive Components by Scanning Acoustic Microscopy

Z.Q.YU,G.Y.LIandY.C.CHAN Department of Electronic Engineering

City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong

zqyu@ee.cityu.edu.hk

Abstract Delaminations of the embedded passive components are

often associated with reliability problems. Understanding the delamination can help us quickly identify the failure cause at an early stage of the failure analysis. In this work, defects of buried capacitors have been detected by use of scanning acoustic microscopy (SAM). The results show that S A M can be a powerful analytical tool for the nondestructive evaluation of delaminations in buried passive components. T-scan is an effective method for determining the existence of the internal defects, and TAMI-scan is shown to he capable of detecting the location and size of the delamination and void.

I Introduction Ceramic substrates play an extremely important role in

electronic packages. Low temperature co-fired ceramic (LTCC) technology has the ability to integrate passive components such as capacitors into a monolithic package [l- 31, thereby freeing valuable hoard surface area for active components. Delamination is one of the major problems in the development of such components due to shrinkage mismatch of different materials. Understanding the delamination is very important for the development of such components.

The long term reliability of such buried components can be determined by two primary tests, the destructive physical analysis (cross sectioning) and the accelerated electrical and environmental life tests. However, one common problem with these tests is the destruction of the component under study. Although destructive physical analysis (DPA) or cross sectioning is a popular technique to verify the quality of the buried components, it only yields information about the location of the cross section and occasionally may be difficult to interpret correctly. To take full advantage of cross sectioning techniques the sectioning must be performed directly over the defective area. If the test is accelerated, as in the case of an electrical or environmental test, the objective is to stress the units to force infant mortality. As the test continues less fall out is expected, yielding only the highest quality devices. Actually, what remains are the parts that have not yet failed or dropped out as infant mortality. There is no definitive evidence that the component has maintained its mechanical integrity during the course of the testing.

Acoustic microscopy was introduced in the late 1980.. Today, it has proven to be a powerful analytical tool for the nondestructive evaluation of delaminations in plastic packages [4-71. This work reports the delamination detection of capacitors buried in a LTCC substrate by means of a scanning acoustic microscope (SAM).

Il Testing Methodology

techniques that used in industry. The Pulse-Echo scan and the Thru scan are two common

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Receive Transducer

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Figure 1. Block diagram of a S A M system

As shown in Figure 1, the Pulse Echo technique utilizes a pulsed generator and transducer to transmit focused bursts of ultrasound waves into a sample. The reflected wave or echo will then received by the same transducer. The transducer and sample are immersed in distilled as a medium for wave transmission. Besides it is safe environment for most packages. By plotting or manipulating the echo or reflected signals, a full image can be obtained. For a Thru scan, the transmitted ultrasound is collecting by another transducer at the other side of the package, imaging the entire thickness of the package. Manipulation of Pulse Echo data produces a Tomographic Acoustic Micro Imaging scan (TAMI-scan), which can show multiple sheets of horizontally cross- sectioned images. Many kinds of defects can be detected with this method. Generally, T-scan is used to detect the existence of defects, and TAMI-scan to give the depth and size of defects such as delaminations and chip cracks.

In T-seen Testing In this work a Sonix HS 1000 S A M was used to study the

internal structure of the capacitors embedded in a ceramic substrate. Details of the production and configuration of the substrates containing embedded capacitors are given in our

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previous work [SI. The equipment has a frequency range of 5 MHz to 180 MHz (ultrasound), allowing delaminatlons, cracks, and voids to be detected. To suit the ceramic materials used in this work, 15 MHz was chosen for T-scan.

Figure 2. T-scan image of a good LTCC substrate with embedded capacitors

Figure 2 shows a T-scan image of a good LTCC substrate with embedded capacitors. There are no evident defects. Firing profile is very important for the development of the capacitors embedded in a LTCC substrate. During heating, the majority of the solvent evaporates in the temperature range of about 50 to 200°C. Organic burnout takes place at temperatures between 2 0 0 T and 500°C. To remove organics completely and minimize the shrinkage mismatch, the parts must be soaked within this temperature range for a minimum of one hour. The densification rate must also be appropriate for both conductor and ceramic to minimize the shrinkage mismatch.

I

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Figure 3. (a) T-scan image of a defective buried capacitor showing delaminations; (b) transmission signal at point 1: (c) transrmssion signal at point 2

Figure 3 (a) is the T-scan image of the buried capacitor fired in an inappropriate firing profile. Delaminations and voids may be seen as black areas. Figure 3 (b) and (c) show the signal received in areas 1 and 2. It is evident that no signal is received in the area of delamination. This defective sample had been soaked within temperature range of 200 to 50OoC for only about 40 rmnutes. The densidcauon raffi of the ceramic was somewhat fast. Thus delaminations and voids were produced due to the remnants of the organics and the sintering shrinkage mismatch.

Figure 4 (a) shows another T-scan image of the defective substrate with embedded capacitors. It is evident that voids, shown as black points, exist in the substrate. Figure 4 (b) and (c) show the signal at points 1 and 2 shown in 4(a). The results above demonstrate that T-scan is an effective method for the detection of the existence of defects in an LTCC substrate with embedded passive components and thus may be

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IV TAMI-scan Testing used for screening out internally flawed products from high reliability applications.

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Figure 4. (a) T-scan image of a defective buried capacitor showing voids; (b) transmission signal at point 1; (c) transmission signal at point 2.

The frequency of the sound waves was chosen to be 75 MHz for TAMI-scan. The physics of sound is such that if there is a delamination, the reflected wave will undergo phase inversion due to the presence of air at the interface. Thus phase information is utilized in coding the TAM1 image to

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Figure 5. TAMI-scan images of a good buried capacitor. (a) first electrodelceramic interface; (b) second electrodelceramic interface: (c) third electroddceramic interface: (d) fourth electrodelceramic interface. (e) Signature of different interfaces.

Figure 5 (a)-(d) shows TAMI-scan images of a good capacitor for foci at successively deeper electroddceramic interfaces. No evident delaminations are visible. Figure 5 (e) shows the signal reflected at different interfaces. The reflections due to each interface are recognisable.

Figure 6 shows TAMI-scan images of a defective buried capacitor. Tne delaminations and voids (white areas shown in Figure 6 (a)/) mainly exist at the first interface. The defects are not observed at the other electrodelceramic interface.

Figure 7 shows a TAMI-scan image of a cracked substrate with embedded capacitors. It is evident that the cracks exist in the internal substrate. The results above illustrate that TAMI- scan is an effective way for revealing, locating and sizing of internal defects such as delaminations, voids and cracks in buried passive components.

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Figure 6. TAMI-scan images of a defective buried capacitor showing delaminations and voids. (a) first electroddceramic interface; (b) second electroddceramic interface; (c) third electroddceramic interface; (d) fourth electrodelceramic interface.

V Conclusions Scanning acoustic microscopy offers a powerful tool for

visualising internal structures of embedded passive components. Both T-scan and TAMI-scan are used in this work to probe for defects in ceramic substrate with embedded capacitors. T-scan is shown to be an effective method for determining the existence of internal defects. It is fast and so is suitable for production line screening ont of internally flawed products from high reliability applications. TAMI-scan is shown to be capable of detecting the depth and size of the defects. SAM is potentially applicable for visualisation of internal structure of any essentially planar item, such as ceramic MCM substrates with buried passive components. As the passive elements of electronic assemblies become more and more complex in substrate, SAM will be an increasingly attractive resource to complement visual methodologies in the electronic packaging industry.

VI Acknowledgment This work was supported by a City University Strategic

Research Grant, project 9040106. The authors would like to acknowledge Dr. D. P. Webb for proof reading of this manuscript.

V n References I . Goldberg, Lee: ” Multilayer, Low-fire, ceramic substrates sport

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5. Nagdingam, I., S. D. Mohd-yusoff, P. Ramakrkhnan, R. Jaafar and C. Francis, “Scanning acoustic microscope (SAM - a measurement tool for plastic IC Packages,” Proceedings of the 1997 6th International Symposium on the Physical and Failure Analysis oflntegrated Circuits, pp. 244-9.

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8. Chan, Y. C. and G. Y. Li, “Fabrication and characterization of multilayer capacitors buried in low temperature CO-fired ceramic substrate”, Active and Passive Electronic Components, vol. 20, pp. 215-224, 1998.

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