Chip capacitors benefit from low-fire approach

Product designers in the communications infrastructure, medical, military, automotive and industrial control sectors are demanding robust, high quality devices, right down to the smallest passive component.

Differences between high fire (more than 1150 deg C) and low fire (up to around 1100 deg C) multilayer ceramic chip capacitors (MLCCs) should be appreciated.

Both processes have evolved significantly in recent years to meet growing demand for higher capacitances in smaller packages and at reduced cost.  For volume consumer markets, and for applications requiring very high capacitance devices, high fire MLCCs are the obvious choice. But, low fire devices have a reputation for high reliability and capacitance values are increasing.

Precious palladium

Some 15 years ago, high fire MLCCs were constructed with electrodes containing at least 80 per cent palladium. The price of this precious metal skyrocketed, and manufacturers swiftly investigated switching to alternative base metal electrode (BME) materials.

Nickel, costing 50 times less in electrode powder form, was the obvious choice.  The manufacturers successfully evolved their high fire processes to suit, but at a cost.  They had to make huge investments in new equipment, such as precision-controlled inert gas kilns.

Early reliability problems caused some anxiety, but have now been largely resolved, though as recently as 2004, a humidity related failure mechanism was reported.

Further, BME materials do not feature a particularly high dielectric constant.  Techniques had to be developed to produce ultra-thin dielectric tapes which could then be applied in multiple layers in order to increase the electrode count.  This was important for the Y5V dielectric types used widely in Far Eastern consumer markets, as these are more prone to capacitance loss.

An advantage was the ability, to increase dramatically the capacitance values, by building up the number of layers, while at the same time, reducing overall package size.  It is not unusual for BME parts to feature more than 200 electrode layers, with a dielectric thickness of just a few microns.  Capacitance ranges soared, initially breaking the 1µF barrier, and now exceeding 200µF.

Structurally stable

Conversely, low fire MLCCs require a much lower palladium content in the electrode material (typically 30 per cent). European manufacturers also sought to evolve their processes to reduce palladium content, now largely down to 10 per cent, although it cannot be replaced entirely.

Low fire dielectric materials feature additional glass or ceramic frit type ingredients, which enable them to be sintered at a lower firing temperature. Syfer’s low fire MLCC manufacturing differs from standard high fire primarily in its use of a wet, screen print process.  An advantage is that these low fire dielectric materials, combining powder, solvent and low levels of binder, create a dense, close-knit dielectric lattice.  Once sintered, the material forms a highly reliable, structurally and electrically stable component, regarded as considerably more rugged than high fire parts using an ultra-thin tape process.

In general the low fire process maintains thicker dielectric layers than for an equivalent BME part, due to the higher dielectric constant of the low fire material. This gives a lower volt per micron rating, than for the equivalent BME device.  As a result, the low fire MLCC will generally produce less capacitance loss when a voltage is applied, which is also regarded as enhancing the usability of the product.

The drawback of the thicker dielectric layer, is that there is a limit to capacitance values. But even so, Syfer has ranges of C0G and X7R devices offering higher capacitance values. A typical device in the X7R range, for example, features a 680nF capacitance in a 0805 footprint package, which is a cost-competitive, high reliability alternative to BME parts.

Meanwhile there are growing environmental concerns over the use of nickel electrodes in BME MLCCs. Manufacturers are even considering alternatives to nickel for plating terminations, due to its high toxicity.  It will be interesting to see how the continuing debate on hazardous substances evolves.