Nondestructive Detection of Defects in Miniaturized Multilayer Ceramic Capacitors using Digital Speckle Correlation Techniques

The novel application of a digital speckle correlation method (DSCM) was demonstrated for the in-situ and nondestructive detection of cracks in small objects such as multilayer ceramic capacitors (MLCs) in surface mount printed circuit assemblies. A combined DSCM and double lens optical arrangements was employed for the measurement of minute surface deformations in ~ C S . An improved cross algorithm instead of the original fullfield search method was developed based on the unimodal character of the DSCM and reduced the operation time by an order of magnitude without sacrificing the measuring accuracy. The internal cracks in MLCs that contributed to the thermal displacements on the MLC surface afler the electrical loading could be uniquely identified using this improved DSCM This technique was found to be extremely sensitive to the presence of internal cracks in MLCs of different sizes ("1206", "0805", "0603" and "0402") created by thermal shock and has been shown to be more reliable and user-friendly than other conventional nondestructive techniques. A resolution of better than 20 nanometers in surface deformation measurements is achieved within 0.0 I pixel resolution. The location and the size of defects, as obtained from the DSCM, correlate well with destructive physical analyses and surface temperature variation analyses performed on the respective samples microscopy [3] and acoustic emission [4]. M[ore recently, acoustic microscopy has been shown to be one of the most sensitive nondestructive: inspection techniques for the detection of cracks and delaminations in MLCs [3,5]. However, these inspection methods are usually applicable to isolated c:ornporients and their applications to PCBAs are rather liniited. In our primary work [6], it has been demonstrated for the first time that the electronic speckle pattern interferometry (ESI'I) is a new tool for the in-situ and non-destructive detection 'of flaws in MLCs of surface mount PCBAs. When intemal cracks are present in a MLC, short-circuiting between the correspoinding electrodes after adding the electrical loading gives rise to thermal displacements (as a result of Joule heating of the M1.C) at th'e defect point on the MLC surface. The resolution of the ESP1 was Ci.5h iabout 0.3 p m). This sensitivity would limit its application to leakage currents of MLCs greater than about 10-5 A, but can be increased by improving the resolution of'the optics system Digital speckle correlation techniques [S j were first reported as early as in 1982, but have not been widely applied in optical metrology nor nondestructive testing such as the inspection of defects in miniaturized electronic components In t!is work, a combined digital speckle corrdatioti melhod (DSCM) and double lens optical arrutigemenl was employed to enable the high resolution and precision measurement for the in-situ and nondestructive detection of minute surface deformations in MLCs INTRODUCTION DIGITAL SPECKLE CORRELATION METHOD In recent years, miniaturized electronic components are used extensively in portable electronic products Usually, surface mount technology is employed to assemble them on a printed circuit board (PCB) to form a printed circuit board assembly (PCBA) During surface mount processes, the PCB is subjected to a number of thermal and mechanical stresses. In particular, the environmental stress screening frequently used in a stringent quality control procedure can also induce severe thermal and mechanical stresses on the PCBA. Multilayer ceramic capacitors (MLCs) are thought to be more susceptible to these problems due to its multilayer metaVdielectric structure. For example, the thermal mismatch between the metal electrodes, the ceramic dielectric and the termination material of a MLC can give rise to cracks or delaminations as a result of applied thermal stresses. Clearly there is a demand for detecting such defects at assembly level in a nondestructive way A variety of nondestructive techniques for examining the defects of MLCs are primarily based on X-ray radiography [ I ] , neutron radiography [2], acoustic The digital speckle correlation method (DSCM) was developed and applied to deformation measurements [8,9]. Thc underlying principle of this method is to compare the two images #>fan object before and after loading and to obtain the point pairs of maximum correlation coefficient C. The point pairs of maximi.m C is the same point on the object before and after loading According to the positions of these point pairs in the two images, the displacements of the object to be measured (;an be obtained. The equation ol'the correlation coefficient C in our DSCM 1s 21 8 0569-5503/94/0000-0218 $3.00 ' 1 994 IEEE Figure 1 h4LC sample mounted on a test fixture where f(xi,yj). g(xi*,y,*) are the intensity distributions of the two speckle images, the overline f and g are the average values Equation (1) is different from the corresponding equations in the references [8,9] and gives a sharper ridge of correlation coeficient disrribution. Thus it greatly improves the measuring accuracy of our DSCM when compared with other DSCM equations. Another improvements of this DSCM are that the operation time is greatly reduced and the requirement lbr equipment is less stringent A new cross algorithm [lo] instead of the original full-field search method was developed based on the unimodal character and approximate symmetry of the correlation coefficient distribution The principle of this new algorithm is first to search for the peak point along the perpendicular direction (V direction) and then in the horizontal direction (U direction) until crossing through the first peak point. Usually the second peak point may become the point pixel of the maximum C (if it is not large enough, the cross algorithm will be repeated again) Some other calculation techniques such as the three point judgment method and 0 618 optimum seeking method are also used in the algorithm After searching for the pixel position of maximum C, a hrther subpixel processing must be completed using a linear or spline interpolation algorithm to get the measuring precision of 0.01 pixel The new algorithm will reduce most of the computing work (almost 10 times) in searching for maximum C. thus relaxing the computing requirements In our experiments, the computing time of one point (pixel) i s around 3 seconds when using a 486 PC and can be improved hrther by better software and hardware design. EXPERIMENTAL PROCEDURE The test samples studied in this work were X7R-type MLCs chosen from different vendors and their characteristics are summarized in Table I Before testing the electrical parameters (insulation resistance, capacitance and dissipation factors) of all samples were measured to ensure that they were within specifications Physical cracks in a MLC were created by thermal shock (samples were pre-heated to a pre-set temperature, subsequently quenched into ice-water, dipped in alcohol and dried at room temperature) Each test sample was mounted onto a custom-made printed circuit board (Figure I ) using conventional surface mount technology Figure 2 Experimental apparatus of the digital speckle correlation method (DSCM) Table 1 Characteristics of X7R-type MLC samples I Grouo 1 Size Code( Dimension 1 (volt) In this work, a combined DSCM and double lens optical arrangement [ 1 1 3 was employed for the measurements of minute surface deformation in MLCs. As shown in Figure 2, a 5 mw HeNe laser and a expending lens were used to illuminate the sample. A double lens optical apparatus, which consisted of a e105 mm camera lens and a f = 130 zoom camera lens (or a 8x micro objeclive lens), was designed to obtain a high magnification specklegram of the MLC sample. This specklegrani wa!; recorded by a CCD camera A PC and VC-32 image grabbcr 'were used to display the specklegrams of the tesi samp'es at difTerent conditions on the TV monitor and the image data were savt:d iir a hard disk concurrently. Electrical loading, typically at rated working voltage of a MLC, was applied to the sample whose I-V properties were measured automatically, every ten seconds, by means of a Keithley 61 7 Programmable Electrometer in conjunction hit11 a PC-based data capturing system. Subsequently, a custom software. based on the above DSCM. was used IO calculate thr: displacement distributions of the MLC surface and the calculated re!iults could be displayed or printed out in two dimensional or three dimensional mesh graphs. In the. 3-D graph. the I . axis is the resultant deformation in the x and y direi,tions of the M I X sample and is defined as In addition to DSChf testing, a surface temperature measurement was also carried out to observe the thermal variation after electrical loading was applied. The micro-sections of representative samples were examined by destructive physical analysis (DPA) techniques in an attempt to correlate with the DSCM results