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Required parameters for resistors in aeronautics

15 Jan 2013  | Dominique Vignolo

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Nowadays, the two main concerns of aircraft manufacturers are increasing fuel efficiency and conforming to anti-pollution regulations. To increase fuel efficiency, electronics are being moved close to their function in order to decrease the weight of the aircraft by reducing cabling. To comply with anti-pollution regulations, electric engines are being used to move the aircraft on the ground.

However, these changes have also created new performance requirements for electronic components, including resistors. In this article, we will discuss different resistor types for aeronautics applications – from strap resistors to high-precision chip resistors and resistor networks – and their required parameters, including high-temperature capabilities for stringent operating conditions, long-term stability, low TCR, and tight tolerances.

Historical applications
Over the past eight years, aircraft manufacturers have used high-temperature parts in a number of applications. One of these was landing and braking monitoring systems, where brake temperatures were measured and Wheatstone bridges were used to monitor hydraulic and tire pressure. In this type of application, the electronics were located in the wheel, and high temperatures reached them within an hour.

In terms of performance, these systems required components with operating temperature ranges from –55°C to +175°C, but this quickly needed to be expanded to +200°C. Drifts in performance were acceptable, as long as the parts would come back to their original values at +155°C.

Components with good long-term stability were also required, as the measurements had to remain stable for the life of the aircraft. The expected drift after several thousand hours of life could not exceed a given per cent. Finally, the components had to exhibit good behaviour during acceleration, vibration, and harsh environments. SMD products were shown to be the best under such conditions.

Several electronics were utilised in landing and braking monitoring systems. In addition to the Wheatstone bridge mentioned above, SMD wraparound chip resistors were included because they were easy to handle and offered a standard soldering process. In addition, high-temperature strap resistors were needed to activate or de-activate functions, while a chip resistor with TCR from 10 ppm to 25 ppm adjusted the gain of an operational amplifier.

Another application was a sensor that measured the temperature of helicopter turbines.

Like the aircraft braking monitoring systems, this sensor required components with an operating temperature range from -55°C to +200°C, very good long-term stability, and excellent behaviour during acceleration, vibration, and harsh environments. The application utilised SMD wraparound chip resistors. A set of resistors with values of 40 Ω, 80 Ω, 160 Ω, 320 Ω, 640 Ω, 1.28 kΩ, 2.56 kΩ, or 5.12 kΩ was able to withstand the HMP (high melting point) process and working temperatures up to +200°C.

Recent applications
With new regulations aimed at reducing pollution and saving fuel, more and more high-temperature applications are showing up. For example, engine temperatures are monitored so they can be regulated by a computer. This means electronics can be found inside the engine, where temperature can be very high.

Taking into account that the average life of an aircraft is 25 to 30 years, the load-life stability of the components used at high temperatures is a key parameter for aeronautics applications. The goal is to find the best compromise between handling the power and enhancing long-term stability.

Thermal model
On miniaturized SMD components, the heat generated within the resistor is removed to the surrounding environment first by conduction from the resistive layer – or junction – through the body of the chip and to the solder pads. The heat then spreads by conduction within the PCB, and by convection from the PCB to the ambient.

The components are so small compared to the PCB that heat removal from direct convection and/or radiation from the resistor body is ignored in the very simple, but well recognised, model:

1. Tj = Ta +Rthja x Pd = Ta +(Rthjsp +Rthspa) x Pd = Ta +Rthjsp x Pd +Rthspa x Pd

2. Tsp = Ta +Rthspa x Pd

where

Tj is the temperature of the resistive layer, or junction

Ta is the ambient temperature around the PCB

Tsp is the temperature of the solder pad, underneath the solder joint (it is almost equal to the solder joint temperature)

Pd is the power dissipation of the resistor

Rthja is the thermal resistance between the resistive layer and the ambient

Rthjsp is the thermal resistance between the resistive layer and the solder joint

Rthspa is the thermal resistance between the solder joint and the ambient, and takes into account the conduction within the PCB and the convection from the PCB to the ambient

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