NONDESTRUCTIVE TESTING OF MULTILAYERS CERAMIC CAPACITORS

Abstract: A method for nondestructive testing of multilayer ceramic capacitors made from piezoelectric materials to determine the presence or absence of delaminations. The capacitor is inserted in an impedance bridge, a bias voltage of 1. to 2.5 times the rated voltage is applied and the impedance is measured as a function of frequency. A lower than expected impedance indicates a defective capacitor.

Description: This invention pertains to the field of nondestructive testing and in particular to the nondestructive testing of multilayer ceramic capacitors. Specifically, this method of nondestructive testing is directed to discovery of delaminations and nonvisible cracks in a ceramic capacitor body. In the prior art, the testing of multilayer ceramic capacitors for either delaminations or nonvisible cracks has been primarily destructive testing. As the name implies, this testing is restricted to samples of a lot and consequently is not effective with respect to individual units within the lot. Though nondestructive testing has been suggested, however, few economically successful techniques have been developed to date. Among the proposed or attempted means of nondestructive testing, there have been at least four alternatives. Ultrasonic scanning is a form of bulk testing which to date has been unable to isolate individual defective units within a lot and consequently has not been very successful. Neutron radiography is a very complex and expensive means of testing but has not yet provided an adequate resolution. The scanning laser acoustic microscope provides a very expensive means of testing but it does rely on human judgment and is relatively slow. Testing by acoustic emission is a very slow process. The delamination of the layers in a multilayer ceramic capacitor shortens the life of the device, significantly decreases its reliability and causes problems in soldering related to thermal expansion. The delamination of layers in a multilayer ceramic capacitor is not detected by the usual capacitance and resistence tests. The present invention provides a mean to determine the quality of multilayer ceramic capacitors, and in particular the absence of delaminations or other nonvisible cracks in the ceramic material. Certain classes of multilayer ceramic capacitors, when poled by a bias voltage, show piezoelectric effects. Standing acoustic waves are set up corresponding to the dimensions of the ceramic chip capacitor. Where delaminations are present, the resonance pattern is disturbed and the resulting changes can be used to distinguish delaminated capacitors from good or acceptable capacitors. The resonances occur generally between 0.25 MHz and 6 MHz. At these high frequencies testing can be very rapid with a potential for testing 6,000 capacitors per hour. The method of the present invention is characterized in that first the resonance frequency and the impedance at resonance for a good (acceptable) ceramic capacitor of a specific size, materials and capacitance is determined, said multilayer ceramic capacitor of that same size, capacitance and materials specified above is inserted into an impedance measurement bridge, a bias voltage is applied, this voltage being between 1 and 2.5 times the rated voltage of the capacitor, the impedance of said capacitor is then measured as a function of frequency, and finally in the comparison of what is a good or acceptable versus a bad or defective ceramic capacitor a high impedance at the resonance frequency indicating a good capacitor, a bad or defective ceramic capacitor showing a low impedance. Figures 1 and 2 are micrographs of good (acceptable) and bad (defective) Z5U ceramic capacitors taken in cross-section. Figures 3 and 4 are impedance (z) versus frequency (f) curves for the good and bad Z5U capacitors depicted in Figures 1 and 2. Figures 5a and 5b are micrographs of good X7R ceramic capacitors, the pictures being taken in cross-section. Figure 6 is a graph of impedance versus frequency for the capacitors of Figures 5a and 5b. Figures 7a and 7b are micrographs of bad X7R capacitors taken in cross-section. Figure 8 is a graph of the impedance versus frequency curves for the capacitors of Figures 7a and 7b. The present invention is directed to a method of nondestructive testing to determine the presence or absence of delaminations and concealed cracks within multilayer ceramic capacitors manufactured from materials having piezoelectric properties. In terms of the EIA standard designations and codes, the present method of nondestructive testing applies to what are known as class II and class III capacitors. These capacitors basically include materials which exhibit piezoelectric effects. Such capacitors would include capacitors of the types known in the trade as Z5U and X7R. It is possible with capacitors made from these materials to excite mechanical waves through electrical means. The frequency to match the mechanical resonance is determined by the length, width and height of the device made from these materials. When an electric field of one of two times rated voltage is applied, a mechanical wave feeds back a measure of the impedance. When this DC field is applied to the devices individually, standing acoustic waves are set up corresponding to the dimensions of the ceramic chip capacitor. If delaminations or hidden cracks are present, the resonance pattern is disturbed and the resulting changes can be used to distinguish the delaminated capacitors from good (acceptable) capacitors. Resonances occur generally between 0.25 MHz and 5 MHz. The Figures present some example which illustrates the method of the present invention. In Figures 1 and 2 there are presented illustrations of good and bad capacitors of the type Z5U with a size of 0.10 inches x 0.05 inches x 0.05 inches and having a capacitance of 100 nanofarads. It is evident from the micrograph of Figure 2 that there is a delamination in the lower third of the capacitor. This measured resonance frequency for the capacitor of Figure 1 yields a f L of approximately 1 MHz and a f W,H of approximately 1.7 MHz with the velocity v = 5 x 10³ meters/second. Turning now to the graphs of Figures 3 and 4, in Figure 3 one can determine that at 1.7 MHz, the impedance of the tested device of Figure 1 is relatively high and consequently the capacitor is a good capacitor. In Figure 4 the impedance of the tested device of Figure 2 at 1.7 MHz is substantially less than that shown for a good capacitor in Figure 3 and consequently there are delaminations or defects in that capacitor, as shown in Figure 2. Figures 5a and 5b are micrographs of good (acceptable) multilayer ceramic capacitors of the type X7R having the size of 0.125 inches x 0.095 inches x 0.055 inches and a capacitance of 67 nanofarads. The measured resonance frequency for these capacitors of Figures 5a and 5b is f L = 0.625 MHz, and f W = 0.875 MHz. Turning to the graph of Figure 6, one notices that at 0.625 MHz the impedance is very high, and at 0.875 MHz, the resonance frequency corresponding to this width, the impedance (z) is also high. This is true for both specimens. In Figures 7a and 7b, there are micrographs of two bad (defective) multilayer ceramic capacitors of the type X7R having the same measurements as the capacitor of Figures 5a and 5b. The delaminations in these capacitors are quite obvious from the photographs. Turning now to the graphs measuring the impedance versus the frequency for these bad capacitors, one notes that at the resonance frequencies of .625 MHz and .875 MHz, the measured impedance is significantly less than that for the good capacitors. Consequently, a good (acceptable) capacitor can be recognized by a large impedance value at the measured resonance frequencies. A bad (defective) capacitor can be recognized by the change in impedance which appears at resonance frequencies between a good and a bad capacitor. The impedance values at resonance are significantly lowered for a bad capacitor.

Claims: 1. A method for non-destructive testing of multilayer ceramic capacitors made from piezoelectric materials to determine the presence of delamination of the layers and non-visible cracks in said capacitors, characterized in that first the resonance frequency and the impedance at resonance for a good ceramic capacitor specific size, materials and capacitance is determined, a number of capacitors of said specific type and size is inserted sequentially in an impedance bridge, a bias voltage to each of said capacitors is applied sequentially, said voltage being between 1 and 2.5 times the rated voltage of said capacitors, the impedance of each of said capacitors is measured sequentially as a function of frequency and finally said impedances are compared to an expected impedance, such that capacitors having shifts to lower than expected impedance values at resonance frequencies are rejected as defective. 2. A method for non-destructive testing of a multilayer ceramic capacitor made from piezoelectric materials to determine the presence of delamination of the layers and non-visible cracks in said capacitor, characterized in that first the expected resonance frequency and the expected impedance at resonance for said specific of capacitor is determined, said capacitor is then inserted in an impedance bridge, a bias voltage is applied to said capacitor, said voltage being between 1 and 2.5 times the rated voltage of said capacitor, the impedance of said capacitor is measured as a function of frequency and finally said impedance is compared to said expected impedance, such that a lower than expected impedance value at resonance frequencies indicates that said capacitor is defective.

Inventors: BOSER, OTMAR HERMANN
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1985
Main classification: G01R27
IPC codes: G01R31, G01R27, G01N27