Multilayer ceramic capacitors consist of a plurality of interleaved and staggered layers of an electrically conductive film of metal (termed "electrode"), formed by the deposition (usually by screen printing or variations thereof) of a thick film paste and electrically insulating layers of a ceramic oxide (termed "dielectric"), formed by laying a cast dielectric tape or by casting a dielectric slurry over the dried electrode. The component is then fired to sinter both the dielectric and electrode into a monolithic and mechanically strong part with desired electrical properties. MLC capacitors are well known in the art. U.S. Pat. No. 2,389,420, for example, describes the structure, manufacture and properties of monolithic multilayer ceramic capacitors (MLCs).
In order for the capacitor to be joined to other electrical components and circuitry, an electrical connection to the electrodes must be made. This is done by way of what is called the "termination". Terminations are usually formed by applying a thick film paste to the end of the capacitor where the electrodes are exposed, usually by a dipping process or variations thereof, and firing to remove the organic components of the termination paste and to sinter the metal phase of the termination. MLC termination inks are usually dispersions of precious metals in an organic vehicle, with the metals ranging from 1:1 ratios of Pd and Ag, to pure Ag. Ternary terminations comprising Pt/Pd/Ag are also used, as are compositions containing non-precious metals and other combinations of precious metals. The termination also contains finely divided glass particles (termed "frits") which act to promote adhesion of the termination to the dielectric body. Dispersions of crystalline oxides can also be used. Firing of the termination is done to sinter the precious metal powder into a highly conductive solid form and to flow the glass frit to promote adhesion of the termination to the MLC dielectric body. Typical metal concentrations in termination inks range from 60 to 80 weight percent, with the frit comprising 12 to 0% and the organic vehicle comprising the remainder.
Three main kinds of MLC terminations exist. The so called "lead attach" terminations are used for MLCs where wire leads are attached to the termination using soldering and the capacitor is then encapsulated. Lead attach terminations contain mainly silver as the metal phase, with added frits to promote adhesion of the termination to the dielectric body. The so called "hybrid terminations" are usually Pd/Ag in composition, with the Pd:Ag ratio ranging from 1:1 to 1:4. These terminations are typically used to attach MLCs to hybrid circuits. The third type of termination, the so called "plateable base" terminations, typically contain only Ag as the metal phase, and are electrochemically plated with a predominantly Ni layer after termination firing. The Ni layer is present to control leaching of the termination metal phase during soldering to prevent solder de-wetting and soldering failure, especially when aggressive soldering processes are used.
Soldering of the termination to outside components and circuitry is done a number of ways. In the past, connection was typically done through wire leads attached to the termination and "lead attach"60 termination compositions were used extensively. The MLC body and part of the leads were encapsulated in a material which protected the part from the environment and helped adhere the leads to the MLC body. Recently, a new means of joining MLC parts to other circuitry termed "surface mounting" has been developed. Using this technique, unencapsulated MLC chips are directly soldered onto printed circuit boards using either reflow or wave soldering techniques. In the reflow technique, an amount of a solder paste is applied to the MLC termination and the printed circuit board. Then the solder is melted by infrared heating, boiling vapor condensation, or laser heating. The other principal technique of soldering is wave soldering. In this method, the MLC is glued to the printed circuit board usually using a heat curable epoxy composition then the printed circuit board is passed through an agitated bath of molten solder at a specified rate.
Wave soldering is the most aggressive soldering technique used in surface mounting. During wave soldering the MLC part is subjected to a rapid increase in temperature due to immersion of the part in the solder wave. In addition, it is also subjected to a rapid cool after leaving the solder wave but the time rate of temperature change during the cooling phase is less than the heating phase. The rapid temperature changes of wave soldering are ameliorated somewhat by the use of pre-heaters which heat the printed circuit board and the components on the board before immersion in the solder wave. But even with preheating, the thermal shock experienced by the parts is severe. The use of wave soldering is particularly stressful to MLC parts due to the rapid and large increase in temperature the part experience.
The seventy of the wave soldering process with MLC parts with terminations plated with Ni is worse yet since solder seems to rapidly wet the Ni plating, causing more rapid heat transfer from the solder to the MLC, and a rapid temperature rise in the interior of the MLC part as the heat of the solder is conducted from the termination through the electrodes.
The combination of wave soldering and Ni-plated Ag-based terminations has given rise to a particularly insidious, widespread and well known problem in the MLC industry termed "thermal shock cracking". The rapid heat transfer from the solder bath to the MLC part during wave soldering of Ni-plated parts causes cracking of the dielectric body which is usually manifested as cracks on the surface of the MLC part which usually intersect the termination. These cracks can cause failure of the MLC parts soon after wave soldering or can cause failures during the service lifetimes of the parts.
This invention deals with a means of reducing or eliminating the incidence of thermal shock cracking of MLC parts through the composition of the termination ink.