Copper End Terminations for BME Capacitors

June 1, 2001
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The transition of the multilayer ceramic capacitor (MLCC) market away from palladium-containing electrodes to nickel-containing, base metal electrodes (BMEs) is clearly the result of the dramatic increase in the price of palladium to more than $1,000 per troy ounce. Some MLCC manufacturers have pursued the technique of using so-called ultra-low fired dielectrics with up to 100% silver electrodes, but the inferior properties of the ultra-low fired dielectrics have limited the acceptance of this approach.

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.
From a processing point of view, the major difference between copper end terminations for BME capacitors and conventional terminations for precious metal electrode capacitors is the firing atmosphere. Copper end terminations are fired in a nitrogen atmosphere, with a controlled level of oxygen.

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 Development

Resin/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.

Figure 1. Rheology behavior of two copper end termination pastes with different copper powders.
Figure 1 shows the rheology behavior of two pastes with the same vehicle but different copper powders. Paste 2 has a typical thixotropic behavior, which is desirable for good cosmetic coverage.

Figure 2. Viscosity stability of copper end terminations with different copper powders (Brookfield Model: RV, Spindle: SC4-14, at 10 rpm, 25°C).
The effect of the copper powder on the stability of paste viscosity is shown in Figure 2, in which the correct selection of a copper powder for surface coating contributes to a longer shelf life for the paste.

Figure 3. Top (a) and bottom (b) show very different fired densities due to different copper particle size distributions.
The fired density of the end termination is also strongly related to the selection of copper powders and, of course, the glass, an ingredient that will be discussed later. Figure 3 illustrates terminations that contain the same glass ingredient in the same amount, but contain a different copper powder. Figure 3 (a) shows a porous termination structure due to the relatively large particle size distribution of the copper powder.

Figure 4. Top (a) and bottom (b) show dramatic changes in end termination coverage due only to a difference in the copper powder.
The copper powder selection also affects the apex-to-side ratio of the end termination, as shown in Figure 4. It is clear that by changing only the copper powder, termination coverage can be altered dramatically.

Figure 5. Blisters due to the selection of an inappropriate copper powder.
The selection of an inappropriate copper powder can also lead to the so-called blister problem, in which blisters occur between the dielectric body and the end termination. This is a different problem from the blistering caused by unclean vehicle burnout, when blisters occur primarily in the end termination. Figure 5 shows blisters caused by the selection of an inappropriate copper powder. Obviously, both types of blisters must be avoided.

Figure 6. Top (a) and bottom (b) show different surface glass behavior due to different copper powders.
The problem of surface glass bleeding can also be traced back to the selection of the copper powder, as shown in Figure 6. Both terminations have the same formulation, except for the copper powder.

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.

Figure 7. Copper green film adhesion to BaTio3 (a) and (b), and to Al2O3 (c) and (d) were greatly improved by additives.
Figure 7 compares the green film adhesion of copper end termination pastes using the ASTM method 3359 cross cut adhesion test. This test uses a cutting tool of known geometry, size and hardness to rate the green adhesion qualitatively on a scale of 0-5. It was shown that, with minor additions of some additives, the green strength can be greatly enhanced. Another interesting phenomenon is that the polarity of the materials in the base ceramic influences the results of this test.

Figure 8. The effect of additives on copper paste rheology behavior.
Rheology is also greatly affected by the addition of some minor additives, as shown in Figure 8.

Successful Applications

The development of copper end terminations for BME capacitor applications is complicated, due to the interaction among multiple elements within the formulation as the termination is processed under a nitrogen atmosphere. With a thorough understanding of the role of each ingredient, and the careful selection of the most appropriate elements within the formulation, a product platform of copper end terminations has been developed. These end terminations can be customized for various applications and scaled up for full production volumes.

Acknowledgements

The authors would like to thank Kerry Sams for making and testing pastes. Thanks also to our customers, for their kind feedback and comments.

For More Information

For more information about copper end terminations for BME capacitors, contact Weiming Zhang (wzhang@4cmd.com), J. Thomas Hochheimer (thochheimer@4cmd.com) or David Malanga (dmalanga@4cmd.com) at Heraeus Inc., Circuit Materials Division, 24 Union Hill Road, West Conshohocken PA 19428; (610) 825-6050; fax (610) 825-7061; or visit http://www.4cmd.com.

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