Knowing the advantages and disadvantages to using precision thin-film resistors will help you choose the correct one for your design

BY
KORY SCHROEDER,
Director
of Marketing & Product Engineering
Stackpole
Electronics
www.seielect.com

Advancements in thin-film materials and deposition technology have
provided the market with reasonably priced precision resistors and allowed
their use in a wide range of applications and end products. More specifically,
unique materials and design have yielded precision chip resistors with higher
power ratings, lower tolerance and TCR values, and resistance to
moisture/humidity — all without the use of precious metals that increase
costs. In addition, these cost-effective chip resistors are 100% lead-free and
RoHS-compliant without exemption. There are clear advantages and disadvantages
to using precision thin-film resistors. It is important to understand these
advantages and disadvantages in order to select the right precision resistor for
a given application or requirement.

Advantages
Precision thin-film resistors have progressed significantly in the
past 10 years. The 0.1%-tolerance and 25-ppm TCR resistors that, in the past,
were considered high-precision, expensive, and were difficult to find on the
shelf anywhere are now commonly found in stock. It should be noted that E96
resistance values are the most popular and common from all manufacturers. When
possible, engineers should focus their precision designs on the E96 series of
values. In doing so, they will have a better chance of finding parts for a
prototype or production build. While most thin-film resistor series are
available in E192 values, these values will rarely be in stock, may not be
marked, and may require a higher minimum-order quantity.

The overall electrical performance improvement for precision thin-film
resistors compared to standard thick-film chips is dramatic (see Table 1). Beyond the obvious improvements possible in terms
of tolerance and TCR, the measured performance characteristics to the industry
standard tests show the impressive capability of precision thin-film chips.

Table 1: Precision thin-film resistors
versus standard thick-film chips.


Test

Precision
Thin-Film Performance

Typical
Thick-Film Performance
Short-Time Overload ±0.05% ±2%
Load Life ±0.05% ±3%
Biased Humidity ±0.05% ±3%
Resistance to Solder Heat ±0.05% ±1%

 

Recent advancements in thin-film technology now allow production
of surface-mount chip resistors with tolerances down to 0.01% and TCR down to 1
ppm. While this performance still lags behind that of bulk metal foil
technology, the gap is narrowing. Furthermore, the cost differential between
the highest-precision thin-film chip resistor and a comparable bulk metal foil
resistor is more than an order of magnitude less. Now, design engineers with
extreme precision requirements have reasonably priced choices that didn’t exist
in the past. For example, Stackpole’s RNCF Series provides a wide range of
sizes and resistance values in tolerances as tight as 0.01% and TCRs as low as
1 ppm/°C.

1117_Special_Passives_Stackpole-Graph

Disadvantages
There
are two main disadvantages to thin-film technology. Pulse-handling for thin-film
resistive elements is limited due to the small amount of resistive material
present for this technology. For Nichrome thin-film resistive elements, which
are the most abundant thin-film resistors in the market today, there is a
possibility of moisture corrosion leading to resistance shifts and, eventually,
open failures.

Pulse-handling
for thin-film resistors is comparable to thick-film chip resistors for longer
pulse durations of 1 s or longer. This is due to the higher relative density of
thin-film elements compared to thick-film. However, for shorter pulse
durations, the amount of energy handling dramatically decreases. For pulse
durations of 0.1 ms, for example, the difference between thick-film chip pulse-handling
compared to thin-film is roughly an order of magnitude. However, precision thin-film
resistors are most often used in precision applications, measurement or
monitoring, and highly accurate controls. These applications will typically
have little or no pulse-handling required. If there are pulse conditions, then other pulse-limiting resistors can be
used in conjunction with the thin-film resistors to limit their exposure to the
pulse energy.

Moisture corrosion is a potential cause of failure for standard
Nichrome thin-film chips. Moisture causes the metals in the thin-film element
to oxidize, making them non-conductive and eventually leading to positive
resistance shifts and open-circuit failures. Fortunately, there are several
options to remedy this problem. For example, Stackpole’s RNCS series is a
proprietary passivated Nichrome thin-film technology that provides a moisture
proof barrier protecting the resistive element. In 3,000-hour biased humidity
testing, the passivated Nichrome thin-film technology shows very little
resistance shift. For applications that require the highest resistance to
moisture corrosion, Tantalum Nitride technology can be utilized. Tantalum
Nitride thin-film resistors can withstand thousands of hours under biased
humidity with almost no significant resistance shift at all.

Summary
Design engineers now have a
thin-film SMD alternative that offers a broad resistance and size range,
tolerances as tight as ±0.01%, and TCR down to 1 ppm/°C. The thin-film
resistors utilize new materials and innovative designs that provide an inherent
resistance to harsh environmental conditions like moisture and humidity while
lowering costs. Designed to meet the requirements of a wide range of
applications that need precise performance, thin-film SMD resistors deliver the
ruggedization and long-life performance without degradation.