Multilayer ceramic chip capacitors present the most ubiquitous of failures. Find out the problems and why they occur ….

It might be thought that most reliability problems in electronics would occur in highly complex semiconductor chips with hundreds of terminals. In fact, the most common electronic component to fail, as seen by the ERA's Reliability and Failure Analysis Service, is the multilayer ceramic chip capacitor (MLCC - see Figure 1).

Figure 1. A group of MLCCs

MLCC failures and their causes The effect of MLCC faults ranges from the minor through flattening battery powered equipment and causing fires inside cars and aircraft cabins. In some circumstances the faults can even appear to recover and then return. MLCC faults present certain characteristic features:
	faults often take weeks or months to show and are not picked up by normal screening techniques,
	faults almost always occur in high capacitance devices of low voltage rating,
	usually no fault is visible on the outside of the device,
	the capacitance of the device is not affected but the leakage current sometimes rises from nanoamps to milliamps or higher.

The structure of an MLCC
MLCCs comprise many (usually dozens) of thin layers of ceramic with electrodes between the layers connected alternately to either end of the device. A large total area is provided in a small footprint by stacking the layers.

The ceramic is a fine-grained mixture including barium titanate and similar materials with a high permittivity. It is formed into thin sheets in the unfired state and printed with electrodes of silver/palladium or nickel. The layers are stacked up, pressed and fired. The sheets are cut and electrodes formed on the ends for solder connections. By varying the area of the electrodes, the number of layers and the thicknesses of the ceramic, a wide range of values can be provided.

How MLCCs are damaged
Problems in MLCCs occur because the ceramic is brittle. During assembly, components pass through a series of processes, all of which can generate stresses- direct mechanical stresses and those derived from the difference in the coefficient of thermal expansion between the capacitor and the board.

The capacitors are first picked up and placed on the board by a vacuum nozzle or small jaws, either of which can impose high mechanical stresses if they are not set up correctly. They are either pressed into solder paste on the pads and reflow soldered, or glued onto the board and turned over for wave soldering. The latter process generates a rapid thermal shock as the whole capacitor is struck by the wave of molten solder. After soldering the boards are cut or broken into individual pieces from a large tile. MLCCs near the edges can be subjected to shock waves, if this separation is not carried out carefully.

Figure 2. the corner of an MLCC, in cross-section. The red arrows show the cracked line

The board may then be bent during mounting into a box. Finally the application of the electronic system may involve temperature changes for hundreds or thousands of cycles. Each change imposes stresses.

How damage causes electrical failure The mechanism of failure consists of the formation of the crack (shown in Figure 2 and in close up in Figure 3), followed by migration of metal from the internal electrodes along the crack. Once there is a continuous conducting path, a leakage current can flow. The track takes a while to grow, which is why failures normally occur in use and are missed by post production testing. The failure can appear to recover but it can also grow again by the same mechanism.

Figure 3. Close up of area denoted by the red box in figure 2. The crack bridges between the conductors

Figure 4 Monitoring of migration of silver down the crack is shown in figure 3

The extent of this growth can be assessed by using energy dispersive x-ray (EDX) analysis in the SEM. Figure 4 shows EDX spectra taken from points "Spectrum 2" (solid yellow) and Spectrum 3" (red line) in Figure 3. Increased levels of silver (Ag) can be seen in the crack at point 3.

The future - lead-free processes and MLCC failure
Many designs are now being made in lead-free, a trend which will increase as the 1st July 2006 ban deadline approaches. All the realistic alternative solders melt at temperatures about 40oC higher than the traditional tin/lead solders. It is evident, therefore, that lead-free assembly will increase the stress on vulnerable MLCCs further. Extra care will be required to prevent a surge in these types of failures.

How we can help
ERA's Reliability and Failure Analysis Service routinely identifies MLCC failures, diagnoses the underlying causes and proposes effective solutions. Call us on +44 (0)1372 367444 or email.