Component Analysis: Thin Film Resistors Provide a Solution Impermeable to Sulfur

The current trend towards miniaturization is pushing resistor technology to its limits. For example, the packaging alone for a 0201 chip resistor accounts for approximately 60% of the total component cost. For some designs that use the same resistor value in the same adjacent position on the board, chip component arrays can help alleviate layout and packaging issues. However, this is not applicable to all manufacturers.

Recent developments in thick film and thin film technology have enabled higher power ratings to be achieved for a given chip size. It is well known that thin film resistor components offer numerous performance advantages over thick film resistor components, where the only significant advantage is cost.

With the latest material and process advances, this apparent cost difference can be significantly reduced, which is likely to have a significant impact on the chip resistor market. At present, it is reasonable to expect that commodity thin film chip resistors with a tolerance of 1% and a temperature coefficient of resistance (TCR) of ±100×10-6/°C will be priced at about the same level as thick film resistors of equivalent precision.

In high sulfur environments, such as automotive equipment, industrial equipment, and heavy agricultural and construction equipment, common thick film chip resistors can experience resistance shift problems due to the formation of silver sulfide. The sulfur seeps through the plating and shielding layers and comes into contact with the silver to form silver sulfide (see Figure 1).

Figure 1 Sulfur penetrates through the electroplating layer and the shielding layer and contacts with silver to form silver sulfide

Silver sulfide is non-conductive, and continued exposure to sulfur will mean more silver sulfide is formed until all the silver is completely converted to silver sulfide. The conductive layer is thus interrupted and the component becomes an open circuit. This is a particularly frustrating phenomenon for any automotive or industrial equipment manufacturer, as it is a potential failure that is completely undetectable at the time of manufacturing. Some automotive and industrial equipment manufacturers have successfully prevented the formation of silver sulfide by sealing the electronics, but this approach is not feasible in all cases and is not a reliable method to ensure protection against sulfur contamination.

Thick and thin films

The internal terminations of thick film resistors are usually plated to varying degrees using a silver/palladium process. These relatively inexpensive termination materials have a higher silver content, but it is often the silver in the internal terminations that is susceptible to sulfur contamination.

Although it may be possible to find thick film materials with lower silver content, until now these alternatives have been more expensive and therefore seem unlikely for volume production. On the other hand, thin film chip resistors use sputtered, nickel-chrome-based internal terminations that contain no silver and usually no other precious metals. This means that nickel-chrome thin film materials are more stable in price than thick film materials with higher gold, palladium or platinum content.

Only chip resistors whose internal terminations do not contain silver or copper materials, or whose internal terminations are protected by an intermediate layer that is impermeable to sulfur, are completely immune to sulfur contamination. Competitive thick-film-based solutions exist on the market that offer some protection against sulfur, but they are still not completely immune to sulfur contamination—over time, they will eventually fail and become an open circuit.

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Likewise, we know that the effects of sulfur contamination can be exacerbated by the misalignment of the immersion-plated protective passivation layer. In this case, slowing down the immersion process can minimize the effect, but doing so will also increase manufacturing costs and reduce manufacturing throughput. Because thin-film internal terminations are not affected by sulfur contamination, the accuracy of this process is not critical.

It is clear that thin film resistor technology is more resistant to sulfur contamination in terms of its internal termination. In addition to this, resistors using thin film technology also have overall stability, lower noise, and lower parasitic capacitance and inductance (depending on the resistance value). Figure 2 shows a common thin film resistor, which is a significant improvement over thick film chip resistors, especially its larger resistance values.

Figure 2 Typical current noise levels depending on nominal resistor value

In the past, this improvement in lower electrical noise was only important in high-end audio applications. However, thick film resistors have difficulty meeting the noise requirements of today's latest high-speed communication equipment, such as routers, bridges, and DSL modems. Many factors contribute to thick film resistors being noisier, and one of the most significant differences between thin and thick film technologies is in the laser trimming characteristics. Once fired, thick film materials behave virtually like glass. Therefore, as the material cools, laser trimming creates numerous tiny microcracks around the trimmed area. These microcracks are a source of parasitic capacitance and errant current paths, all of which inherently lead to reduced performance when handling high-speed communication signals.

To reduce the effects of laser trimming on thick film components, manufacturers often add a layer of insulating glass to stabilize the laser trimming. This layer contains trace amounts of lead and is known to be important for maintaining the long-term reliability of thick film resistors; due to its importance, this insulating glass layer is currently exempted from the RoHS standard.

However, it is unclear whether this exemption will continue to exist or the industry will require a lead-free laser trimming stabilization alternative. Thin-film technology can provide a "greener" or more environmentally friendly resistor because it does not require the leaded glass used in almost all thick-film chips.

develop

A recent development in thin film technology, as well as thick film technology, has resulted in higher power ratings for a given EIA standard chip resistor size (see Table 1). This important advance allows engineers to miniaturize designs without sacrificing power handling capabilities. This breakthrough is critical for increasing current designs, reducing the size of future designs, producing smaller end products, or providing more functionality in the same size product.

The most important factor affecting the mass adoption of thin film technology for chip resistor manufacturing is its cost. The typical price point for thin film chip resistors with the worst tolerances and TCRs is 10 to 100 times higher than their closest thick film counterparts. To reduce this difference, suppliers need to completely redesign thin film materials to meet the needs of high-speed, low-cost production. It also requires the development of a high-speed, inline thin film manufacturing process. The final step in reducing costs is to relax the requirements for thin film materials from high-precision levels (0.1% tolerance and 25×10-6/°C TCR) to commercial, general-purpose and commodity levels (1% tolerance and 100×10-6/°C TCR). These three advances can be applied simultaneously to chip resistors that are only 10% to 20% of the price of common commodity thick film chip resistors. That may be the most eye-catching development of all.