Power Electronics

Thin-Film Chip Resistors Challenge Traditional Thick-Film Devices

Thin-film chip resistors are comparable in cost and have higher power ratings than their more traditional thick-film cousins.

It is easy to see that the trend toward downsizing components to create more powerful devices in smaller packages is influencing the resistor market. Nearly every design engineer is developing portable electronic devices that are smaller and lighter with more functionality and application features, and resistors comprise a large portion of the component count in such devices.

In the past, downsizing a design meant sacrificing power-handling capabilities, which had to be dealt with either by technology improvements to the active components being used, or by settling for less performance. Using higher-power devices enables the design engineer to downsize the current design without sacrificing power handling or performance, and makes it much easier to add features to the product. The plethora of new higher-power chip resistors and through-hole devices from many of the largest resistor manufacturers proves that downsizing and higher power handling is becoming a significant market trend.

LIVING WITH THICK-FILM TECHNOLOGY

Until recently, these resistors were based on thick-film technology. Designers are familiar and comfortable with the capabilities of a thick-film resistive element on a ceramic substrate. Improvements to the power-handling and heat-withstanding capabilities of thick-film inks allow resistor manufacturers to push the power-handling envelope further than ever before.

However, these higher-power, thick-film materials do not come without a price. Other substrate materials may be used to significantly change the thermal characteristics of a given chip size, but those substrate materials are in much shorter supply and dramatically increase the cost of manufacturing the resistor. Aside from certain niche applications that have no other options, the current market will not bear such price premiums for chip resistors.

THIN VERSUS THICK: WHAT ARE THE COSTS?

Historically, thin-film resistors were limited to precision applications requiring lower noise, lower parasitic inductance or capacitance, or tighter tolerances and temperature coefficient of resistance (TCR). Due to the nature of end-product applications — and the need for a much more controlled, stable, and clean manufacturing environment — thin-film resistors have been priced much higher than similarly sized thick-film devices.

There had been no development of high-power thin-film chip devices because the perceived price premium would be too great for mass implementation and volume usage. Recent developments in low cost thin-film materials and high-speed thin-film volume manufacturing have changed all of that.

In fact, the final barrier to the mass implementation of thin-film commodity chip resistors had been cost. To reiterate, thin-film materials used in the past were designed for high-precision applications. Their typical implementation would yield resistors with ±50 ppm/°C temperature coefficients and ±0.5% absolute resistance tolerance minimum, with scrap rates below 5%.

New thin-film materials, however, have been developed for low-cost, high-speed, high-volume manufacturing. Opening TCR requirements to ±100 ppm/°C and absolute resistance tolerance requirements to ±1% or 5%, the thin-film materials can be optimized for lower cost, while maintaining superior low-noise, high-stability, and low-parasitic properties. Loosening these parameters reverses the usual price relationship between thick-film and thin-film components.

Currently, high-power thick-film materials are significantly more expensive than thin-film materials. Until the costs for thick-film materials drop down to reasonable levels, using high-power thick-film devices will be limited to the few devices that also offer lower heat rise.

At the same time, these relaxed parameters have streamlined and expanded the thin-film production process to mass-production scale. This requires a significant commitment on the part of the manufacturer in terms of capital investment and floor space, but in the end these new materials — combined with the high-volume inline sputtering process — will lower manufacturing costs for this new commodity thin-film chip product to within 10% of the cost of standard commodity thick film chip resistors. With this last and most critical price barrier eliminated, thin-film resistors finally appear poised to become a significant portion of the general-purpose commodity resistor market.

TRACKING PERFORMANCE

Fig. 1 shows the company's RNCP series high-power anti-sulfur thin-film chip resistors, while the table lists the devices' electrical specifications. Just like their thick-film counterparts, high-power thin-film commodity resistors operate well at a power rating that is a step higher than traditionally associated with a given package size.

For instance, the 1206 size can operate at 0.5 W, the 0805 device at 0.25 W, and so on. They also require additional heat removal similar to most other thick-film devices; that requirement is a function of the 96% alumina ceramic used in both processes.

But that is where the two technologies diverge. Thin-film technology will outperform thick-film technology in nearly every electrical and stability parameter, and the difference is even more pronounced at higher power. Higher power implies a higher operating temperature, which favors the lower-TCR thin-film device.

Higher power over the life of the part also implies more mechanical stress, increased effects due to short-term overloads, and more stress due to thermal cycling — all which favor a thin-film technology resistive element. The power derating curve of these resistors is shown in Fig. 2, while Fig. 3 shows heat rise at full rated power.

The data in Fig. 2 and Fig. 3 was collected from a test setup that included 10 resistors, each mounted to a typical PCB and powered up to full rated power. This is a more practical test condition than the usual method, which uses a single device per board. Data was measured via a thermocouple connected to each device, and represents an average part temperature for each size.

Along with these benefits, there are other strengths that thin-film technology provides. Thin-film devices are sulfur-resistant due to the replacement of the printed silver palladium inner layer with a sputtered nichrome-based element. The silver in the inner termination of thick-film resistors combines with sulfur to form silver sulfide, which is non-conductive and eventually leads to an open circuit failure.

Many electronic products are expected to last more than five years. Even resistors with increased palladium and decreased silver cannot guarantee protection against the formation of silver sulfide. And, with the aforementioned advancements in thin-film manufacturing processes, there is no longer a significant cost difference to prohibit engineers from designing in parts that are 100% impervious to sulfur.

Thin-film resistor technology also provides a more environmentally friendly product because it doesn't require the Pb-containing glass layer that is standard for nearly every thick-film chip resistor in the market. Existing RoHS standards allow an exemption for Pb-containing glass in thick-film resistors, so it is still there. It is unknown whether this exemption will continue or if all thick-film resistor manufacturers will be forced eliminate this from their products.

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