TANFILM® VS. NICHROME
Moisture Performance Comparison
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By far, the two most popular materials used to build precision thin film resistors are tantalum nitride and nickel-chrome. Nickel-chrome (also known as nichrome) films dissolve when exposed to moisture in the presence of an electric potential. Tantalum nitride (also known as TaNFilm®) films do not dissolve when exposed to moisture in the presence of an electric potential.
Tantalum Nitride is one of a small number of metals referred to as "valve metals". A naturally occurring passivating layer of oxide that forms on the surface of the resistor exhibits a rectifying characteristic and opposes corrosion of the film in an electric field. The formation of this oxide layer occurs naturally in a tantalum nitride resistor and is referred to as self-passivation.
Nichrome resistors are not self-passivating. The packaging, sealing and mechanical protection of nichrome resistor films is critical to prevent failures in a nichrome chip when subjected to moisture. This fact may cause concern if the film is located in a package with relatively little protection such as a chip package. Chip resistor packages typically provide only a layer or two of epoxy on the top surface of the device as protection from environmental effects. If moisture permeates this barrier then the resistor film will be corroded.
Nichrome resistors were known to have moisture performance problems since their development by IRC in the 1950s. As late as 1991, General Electric ASCD, Binghamton, NY, discovered "open" failures during a powered moisture test of MIL-R-55342 chip resistors manufactured with nichrome films. General Electric published a GIDEP alert (EE-P-92-01) identifying the problem. General Electric identified the failure mechanism as follows.
"The failure mechanism is an electro-chemical interaction which dissolves the resistive film. In extreme conditions, open resistor element failures can occur in seconds. In less severe cases, a gradual increase in resistance value will occur. The resistance increase will continue until failure if the nichrome element is further exposed to moisture with voltage applied."
General Electric concluded that the moisture performance of nichrome resistors is dependent on the integrity of the package, commenting, "As a consequence, nichrome chip resistors will not survive even short-term exposure to moisture unless protected or placed in a hermetic enclosure."
The corrective action taken by GE was the creation of a source control drawing for tantalum nitride chip resistors.
To demonstrate the failure mechanism, a water drop test was performed by IRC on unsealed TaNFilm® and nichrome resistors. The test method was:
IRC TaNFilm® resistors are available in tolerances to ± 0.01% ratio and temperature coefficients to ± 2ppm/°C tracking in a wide variety of chip, network, leaded, unleaded, surface mount and through hole packages.
If we haven't convinced you to use TaNFilm® resistors in your next design, you can still buy nichrome resistors from IRC at low cost. Check out the PCF chip series.
For resisitive product solutions to your most challenging applications contact IRC today. If you have questions about this product or would like to order the videocassette version which shows this test in its entirety click here to contact us.