Extended Capabilities with Precision Resistors from Vishay Foil Resistors
Standard Vishay Foil Resistors products are used in the most demanding Space, Military, Metrological, ATE, Medical and commercial applications from deep in the Earth to the farthest reaches of Space. However, some applications are so extraordinarily demanding that they require extraordinary expansion of standard technology to develop specific new products for their unique applications. Such new developments may require not only special stabilization and screening, but they could require new configurations and technology refinements as well. Listed here is a sampling of a few of the unique characteristics of the Foil resistors upon which customers have depended in their extension to ever-greater levels of precision and reliability.
- Power Current Sense:
The flat planar configuration of the resistance element attached to high-thermal conductivity substrates makes it possible to remove heat from the resistor very quickly. The resistor’s very low Temperature Coefficient of Resistance registers only a very small change in resistance to whatever residual heat remains. Together, these characteristics allow use of the Foil technology in high-speed high-power pulse applications with little distortion. For example, a high-speed production of RF semiconductors using direct-etching ion beams uses Foil as the sense resistor in the deflection amplifier circuit. Regardless of the rapid switching of high power to this sense resistor, the resistor does not drift and the equipment does not have to pause for stabilization as it indexes to each new semiconductor. This process produces RF semiconductors at a spectacular rate, thereby saving the producer millions of dollars in equipment utilization.
- Fast response time:
The Foil resistor has an inherently fast response time due to reduction of inductance and capacitance in its internal design. The ability to switch the most precise resistor available and still do it at very high speed makes the Foil resistor most suitable for some unique applications. For example, a seismic oil-exploration system must quickly switch through a series of gain-changing networks so that it could register the initial reverberations from the upper layers of earth and follow down to the deepest, weakest signals is a matter of milliseconds. Any delay in switching the gain-changing network (a series of feedback resistors around operational amplifiers) would miss distinct and potentially oil-bearing layers in the earth’s strata. Foil resistors in this application not only provide the speed and precision necessary, they also have the very low TCR that enable the equipment to be used from high-temperature high-humidity forests of Indonesia to the frigid low-humidity icescapes of Alaska and the Antarctica when used in a hermetically sealed enclosure such as VHP100.
- Thermal stabilization:
Fast thermal stabilization along with low TCR are essential in applications such as in fighter jets that could be operating at 120C on the desert floor and be in the minus 65C upper atmosphere just seconds later. Vishay Foil resistors are used extensively in both military and commercial avionics.
- High Temperature:
Foil resistors can and are used for “down hole” applications (logging data while drilling deep oil wells) where the temperature can exceed 275C degrees centigrade. This is achievable due to the absence of solder joints in the construction of through-hole resistors (spot welding) or special construction of the SMD products, as well as the resistors’ element which is able to take exposure to high temperatures with no immediate damage.
The devices can withstand radiation and have been used in applications subject to atomic radiation. Even when packaged in radiation-degradable molded epoxy, the nickel-chromium resistance alloy remains intact in outer orbits. Foil resistors and networks, in both molded and hermetic packages, are used extensively in the outer space.
- Shock and vibration:
Industry standards are based on the collective sustainability of all products and the threshold is what the most susceptible product can withstand. Above and beyond the industry standard, individual part specifications may include higher levels of shock and vibration sustainability. The Vishay Foil resistor with leads bonded to the back of the substrate and wrap around terminations can sustain extended shock and vibration. This applies, for example, to jet aircraft, truck-, tank-, and ship-mounted military equipment, air-drop emergency equipment, missiles, etc.
- Long term stability:
The maintenance of initial resistance through temperature, load, and time is a very real requirement in fixed resistors. With 30 years of data to support the statement, Vishay Foil resistors are the most stable. In non-repairable mission-critical applications, Deep Space probes must withstand the shock and vibration of launch, traverse the thermal extremes of space, and be ready to respond to commands after years of travel. These applications also exemplify the ability of the Foil resistor to maintain exceptional long-term stability even after the harshest exposure.
- Matched resistors:
With the ability to adjust the resistive elements to a few parts per million and with identical temperature response regardless of value, the Vishay Foil precision resistor networks (PRND) are the solution to matched performance even if the resistance range is vastly different. And special ¼ -inch square wirebondable chips containing as many as 15 binarily-related resistors maintaining ratios of 0.01% and tracking within 0.1 ppm/degree C. Such Foil resistors can be supplied in a wide variety of molded packages, hermetic packages, surface mounted chips, and wirebondable chips.
- PMO – Post Manufacturing Operations Enhance the Already Superior Stability of Foil Resistors:
These Post Manufacturing Operations (PMOs) are uniquely applicable to resistors made of resistive foil and they take the already superior stability of foil devices one step further. They constitute an exercising of the resin that bonds the foil to the substrate. The operations that are employed are:
- Temperature Cycling
- Short Time Overload
- Accelerated Load Life
Temperature Cycling is done initially in the chip stage of all production and will eliminate any fallout. The cycling exercises the bonding resin and relaxes the Foil without reducing the bonding strength. A small reduction in resistance is tolerable during this PMO.
Short Time Overload
Short Time Overload (STO) occurs when a circuit is subjected at one point in time to a temporary, unexpected high pulse (or overload) that can result in device failure. This STO is performed on all resistors during manufacturing, with a function to eliminate any hot spots if they exist.
Accelerated Load Life
The standard load-life curve of a Foil resistor exhibits a significant portion of its change in the first 500-1000 hours, after which the curve begins to stabilize. The Accelerated Load Life exercise applies a load for a specified amount of time to eliminate this knee in the load-life curve. Upon delivery, the resistor will be on the flat part of the curve for your convenience. The amount of acceleration is a function of the application and should be worked out between VishayPG Applications Engineering department and your design team.
Q: Can we use PMOs on other resistor technologies?
A: Applying these same operations to resistors of thick film, thin film, and wirewound have vastly different consequences and can drive these devices out of tolerance or open circuit. These devices experience too many failures to discuss here. On the other hand, these operations are an enhancement to Foil performance and should be considered when the level of stability required is beyond the published limits for standard products.