Using a polymer termination for multilayer capacitors forestalls damage during PCB assembly
For safety critical applications such as aerospace and military, failure caused by a short circuited capacitor could be disastrous. One of the most reliable components for surface mount communications is the multilayer ceramic capacitor, in which a block of ceramic dielectric is embedded with layers of metal. The electrode layers are connected into a parallel plate structure via “caps” of metallization and terminations that are applied to opposite ends of the block (Figure 1). Still, even multilayer ceramic capacitors can fail if a fracture occurs because of PCB bending. These devices can also be affected by flexing or stress caused by temperature cycling, a circumstance that can make them vulnerable to cracking. Such cracks may not manifest themselves immediately, and even electrical testing can fail to detect the cracks, thus causing the potential for disaster. Figure 1. Multilayer chip capacitor structure.Fortunately, greater degrees of board bending without incurring capacitor damage are now possible by adding polymer termination to the capacitors. Ordinarily, when a circuit board is deflected, it will take on the form of an arc. The outer surface of the board stretches and elongates the distance between the solder lands on which the chip is mounted. This situation stresses the chip as shown in Figure 2, the solder joint will be deformed, and the chip may well crack. Figure 2. Board bending may bread chip capacitors.Figure 3 shows a characteristic crack resulting from PCB bending. The crack is contained within the terminated area of the chip, running from the lower edge of the termination toward the main face. An electrical short circuit results when the crack enters the electrode overlap. The fault may not be detected until some time after the fracture occurs (Figure 4). A decline in the chip’s insulation resistance (IR) may be delayed until the structure is penetrated by a conductive medium such as atmospheric moisture. Figure 3. Typical mechanical crack. Figure 4. Electrical short circuit as a result of mechanical fracture.Tracking Down the Problem
On individual boards, the electrical problem may be transient. As the result of thermal treatment, often applied inadvertently, the board may function for a time, only to fail again later. Although the problem may seem to be isolated to a single capacitor, more sophisticated analysis may reveal that other capacitors have also cracked but have yet to affect board operation. Measures can be taken to reduce damage. For example, most fractures occur during de-panelization so this separation of daughterboards and motherboards should be attempted only with purpose-built jigs and never by hand. The possibility of damage from transient incidents—i.e., uncontrolled problems—should be recognized as well. Simply changing the source of the capacitors will not pinpoint and rectify the true cause. What had been considered a one-off occurrence will inevitably happen again. There are generally three dielectric categories of capacitors available: COG (NPO), X7R, and Y5V. Sizes usually range from 0201 (0.5 mm long x 0.25 mm wide) to 2225 (5.6 mm long x 6.4 mm wide). Analysis of field failures shows that no one size is more vulnerable than another. Small capacitors proved no stronger than large, and thin capacitors were no weaker than thick ones. Interestingly, COG capacitors did not often figure in cracking incidents. Strength Testing
A bend test can be used to evaluate chip strength. Chip capacitors are soldered to a test board, which is then inverted over a pair of horizontal support rods (Figure 5). The board is deflected at a given speed to a fixed extent, and the effect on the capacitor is assessed. Recently, researchers conducted a structured “Bend Test” program to determine the parameters that affect a chip’s ability to withstand bending forces. The most common electrical parameter is change of capacitance value. However, capacitance change is seldom implicated in “real incidents.” Instead, micro-sectioning was adopted as the key evaluation parameter. In over 15,000 chip capacitors that were bent and micro-sectioned, an immediate change of IR was observed in less than one percent of parts that were subsequently determined to have cracked. Correlating the failures determined by micro-section with those suggested by capacitance measurement demonstrates that capacitance change was a feature of only a portion of broken parts. Figure 5. Bend test.The only significant difference in strength across a broad matrix of capacitor design and build parameters can be found between barium titanate-based (the key material used in the X7R and Y5V dielectric categories) and neodymium oxide based components (the base material of the COG [NPO] category). COG capacitors fail at bend deflections approximately double those at which similar X7R and Y5V parts fail. Circuit board solder pattern is also relevant. Land widths narrower than the chip width were found to increase board strength significantly. See Figure 6. Figure 6. Chip geometry influences mechanical fracture.Similarly, the edge of the termination band relative to the edge of the solder pad is a factor. If the capacitor termination edges are positioned “inboard” of the solder lands, the assembly with withstand greater deflection without damage. The use of soft solder 50Sn and 50Pb, more than doubled the average deflection at failure, as compared to more commonly used 62Sn/36Pb /2Ag solders. The influence of the type of solder used poses a cautionary note for those anxious to switch to alternative solders to comply with the RoHS directive. Some equipment, for example safety-critical products for the military, space, and aerospace sectors, is currently exempt from the RoHS Directive. Experts in these industries are concerned over the risk of tin “whiskers” forming on electronic components and breaking off to cause a short circuit. Using a tin-lead finish and careful choice of terminations can help allay this concern.Strengthening the Chain
In the entire chain of materials that make up a chip assembly—board, land, solder joint, chip termination, and chip ceramic—the weakest link is often the ceramic. Even if changing properties of ceramic is not an option, other materials can be improved to strengthen the overall chain. For example, glass-based conductor materials usually used for terminations can be replaced with a conductive plastic. A silver-loaded epoxy polymer termination is flexible, thus reducing the stress between the PCB and the ceramic capacitor. It can be applied using conventional termination techniques; but instead of being sintered at approximately 800 degrees C, it is cured at 180 degrees C. The polymer termination has a fibrous structure, and its mechanical and electrical properties remain largely unaffected by extremes of heat and chemical treatments (Figure 7). After the polymer termination process stage, the capacitors are plated with nickel and tin using the same methods employed for industry-standard sintered silver-terminated capacitors, so soldering characteristics remain unchanged. Figure 7. Polymer termination microstructure.The electrical parameters of a polymer-terminated chip are indistinguishable from a conventional part. Reliability testing confirms that the polymer has no negative effect on electrical or environmental performances. Actually, a polymer-terminated X7R or Y5V capacitor will afford a bend test deflection at failure, which is almost double that of the same capacitor with a conventional termination. This performance places them in the same category with COG capacitors amongst which “real life” failures are almost unknown. Conclusion
Millions of polymer-terminated X7R capacitors were evaluated in applications known to be subject to problems caused by capacitor cracking. During the trials no parts were identified to have failed as a result of chip fracture. In fact, polymer termination can be used with other products such as surface-mount pi-filters, EMI chips, X2Y filters, and class X and Y surge and safety capacitors. All are RoHS compliant and compatible with lead-free soldering processes.Matthew Ellis is an applications engineer at Syfer Technology Ltd. in Arminghall, Norwich UK. He received his BSEE degree with honors from the University of Leeds in Yorkshire. His current work focuses on multilayer ceramic capacitors and high voltage applications. He can be reached at [email protected].
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