Copper End Terminations for BME Capacitors
Instead, it is expected that more than 75% of MLCCs will incorporate BME technology within the next few years. To address this growing demand, a new product platform of copper end terminations, compatible with BME capacitors and customized to various applications, has been developed.
Like silver end terminations, which serve precious-metal electrode MLCCs, the ideal copper end terminations for BME MLCCs should incorporate:
- sufficient green strength to survive all handling processes;
- good interaction with the inner electrode to ensure electrical connection;
- good bonding to the dielectric to ensure high, stress-free mechanical adhesion;
- a good cosmetic appearance, with no dimples; and a smooth, dense and glass-free surface; and
- uniform chip coverage, with an apex-to-side ratio of <3 and corner coverage of >20 micrometers.
It is this difference that has created many challenges in the course of developing copper end terminations. These challenges include the selection of compatible vehicles, additives, copper powders and glasses. It is also critical to understand the interaction between each element within the formulation to the processing and the final properties of the material.
Copper End Termination DevelopmentResin/Vehicles—An appropriate resin should burn out easily and cleanly under nitrogen. If the decomposition rate of the resin is too slow for a clean burnout before the glass begins to melt, the decomposition gas will be trapped in the glass and form bubbles.
The result is a phenomenon known as blistering, in which parts of the end termination exhibit swelling. Blistering reduces the bonding strength between the end termination and the chip, leading to poor capacitor performance.
Another desired resin property is green strength sufficient to allow the end termination to survive all processing steps.
Copper Powders/Flakes—Copper powders and flakes are the most critical ingredients in the copper end termination. The particle size, particle size distribution, particle morphology and particle surface chemistry all have a strong effect on the performance of the copper end termination. Factors such as paste rheology and stability, end termination cosmetics and coverage, as well as fired density and electrical connection, are all affected by the choice of copper powders and flakes.
The problem of pinholes on the end termination, which may allow the nickel plating solution to penetrate into the surface between the end termination and the dielectric body, may be the result of either of two factors: a lack of wettability between the glass and the copper powder; or the hard agglomeration of copper powders, which cannot be broken up by a three-roll mill. Thus, highly agglomerated copper powders should be avoided.
Glass—The first requirement for glass is the stability of the composition when it is fired under a nitrogen atmosphere. Thus, the composition should not have any oxides that are easily reduced, such as bismuth oxide (Bi2O3).
The second requirement of glass is acid resistance. At the least, the glass should resist the nickel plating bath solution to avoid penetration. Thus, glass with a high silica content is a good option.
Because the most important role of glass in formulation is to assist in sintering, the third requirement of glass is good wettability to the selected copper powders.
In addition to these requirements, the glass should have a suitable softening point, proper viscosity at firing temperature and a strong attachment to the dielectric bodies. If the softening point is too low, it will trap the unburned resin. However, if the softening point is too high, the end termination will not densify sufficiently.
Proper attachment of the glass to the dielectric bodies produces good adhesion of the end termination to the chip. However, too severe a reaction will induce micro-cracks on the chips, which is not desirable.
Additives—The role of additives in an end termination formulation is somewhat dramatic. Using only a very small percentage of proprietary additives can significantly improve the adhesion of the green film to the dielectric and dramatically enhance the thixotropic behavior of the paste.