Improving the performance of thin film resistors for high temperature environments and electrical loads

As lead-free technology matures, high-temperature solder alloys are now available that can be used at solder point temperatures as high as 140-150°C without losing reliability. An example is the Innolot project, which includes material suppliers, device suppliers and automotive system developers, which successfully developed a solder paste containing SAC alloys and halogen-free flux for heavy-duty electronic equipment assembly.

As solder materials optimized for high operating temperatures become available on the market, engineers have more freedom to use lead-free electronics in higher ambient conditions, including under-the-hood (UTH) in automobiles, transmission or brake systems, and other applications such as drilling and mining equipment or industrial drives.

However, there are still some factors that affect the reliability of solder joints during thermal cycling. These factors include not only the characteristics of the solder alloy, but also the design of the device terminations and the quality of the plating. In fact, in lead-free soldering, these device-related factors are more important than in SnPb assembly.

Tin whisker growth and cracking of solder joints are the leading causes of failure in lead-free assemblies. The industry has developed a process called "safety termination" that tightly controls the plating thickness and coating, and uses a nickel liner to mitigate these effects. For example, the nickel liner blocks metal leaching from the solder joint, thereby maintaining an optimal metallization structure. The use of low-diffusion nickel alloys in safety termination improves the integrity of this barrier. In addition, close control of the tin plating process, including current density and the composition and purity of the electrolyte, allows for optimal tin plating thickness, thereby reducing tin whisker growth.

Additional improvements in the interface design can further improve the reliability of the solder joints. The optimized method of the end cap attachment uses a specified air buffer to relieve the stress of the solder joint. This method can effectively solve the stress caused by CTE mismatch, which can cause the solder joint to crack. Further improvements in the end cap design can minimize the stress on the solder joint. The interface between the end cap and the PCB pad is designed to leave a certain gap to allow the solder to creep during thermal cycling.

Optimizing the termination in this way can effectively improve the reliability of lead-free solder joints, provided that the plating characteristics can be adequately controlled. This is just one factor that allows designers to use the latest high-temperature solder alloys to create systems for more demanding environments. However, the thermal performance of the device itself must also be considered, especially when it comes to high-volume market devices such as thin film resistors. Using such devices can save space and cost compared to relatively expensive specialty devices such as wirewound resistors or large-sized thick film resistors with lower power density.

One key challenge that must be overcome is related to the maximum center temperature that such devices must withstand. For most common resistors, manufacturers generally specify 125°C as the maximum temperature, or at most 155°C. Assuming a small size such as 0102 or 0805, a commonly used industry standard case size, the power dissipated in the resistor under load can be enough to overheat the device, which is already operating at a maximum solder point temperature of nearly 150°C.

Therefore, further improvements are needed to thin film resistors to enable them to withstand higher film temperatures. The core issue of the research is to improve the properties of thin film materials and the electrical insulation system used in the manufacture of thin film chip resistors.

Improved resistance film

The main component of general-purpose thin-film resistors is a nickel-chromium alloy device using nickel-chromium technology. Recent research conducted by Vishay Draloric/Beyschlag has found a way to improve this basic compound, making the device more reliable over the same temperature and humidity range. This method adds a third component to the nickel-chromium matrix, optimizing the matrix and making the resistance parameters uniform.

This new generation of film enables manufacturers to produce thin-film resistors that can withstand surface temperatures of 175°C and are stable at or above the maximum allowable operating temperature of enhanced lead-free solder alloys, 155°C. The new mixture is also engineered to have a higher activation energy for improved stability (by a factor of 10) and reliability.

High temperature spray painting system

Vishay has also developed a high-temperature paint system that can be used continuously at temperatures up to 175°C and provide sealing and moisture protection over the life of the device. This specially developed system, filled with epoxy acrylate, has been released and passed a series of tests at 175°C upper temperature limit for 1000 hours of storage and humidity levels specified in HAST 121 (Highly Accelerated Temperature and Humidity Stress Test).

By using optimized terminations and new thin film technologies and packaging materials, the new generation of thin film resistors achieves stability, reliability and high loads never seen before in high-volume market resistors of the same form factor. Comparing the performance of the latest high-temperature (HT) MMU0102 thin film resistors (equivalent to the form factor of 0805) with the traditional MICRO_MELF thin film equivalents and corresponding commercial resistors in 0805 and 0603 packages, the HT enhanced resistors have significantly higher load capabilities than thick film technology and are superior to thin film technology in terms of basic power density.

in conclusion

Recent advances in lead-free solder alloys give designers more confidence when assembling automotive or industrial systems, where the target environment encounters sustained high temperatures, wide temperature cycles, and requires high reliability. Using these solders, operating temperatures can be maintained at high temperatures of 155°C without sacrificing solder joint reliability. However, for traditional thin-film resistors, this temperature is close to the maximum recommended temperature. Even with relatively small load currents, ohmic heating can cause the device temperature to exceed the maximum recommended temperature, compromising stability and reliability.

The new thin film resistor technology uses optimized device termination and high temperature materials. Under constant or higher load levels than can be achieved by current lead-free surface mount devices, the new resistor can guarantee performance in high temperature applications, while achieving higher stability and smaller size. Similar high temperature resistor devices with MINI-MELF size (0.5W, equivalent to 1206 size) and rectangular chip size (automotive series, rated for 175 ℃) have also appeared.