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Required parameters for resistors in aeronautics

15 Jan 2013  | Dominique Vignolo

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Resistors manufacturers need only take care of Rthjsp, but must carefully consider their choice of material, the resistor pattern, terminations, etc. Manufacturers who also improve thermal stability can offer resistors that can withstand higher and higher temperatures without undergoing significant drifts. This removes limitations on Tj.

The control of all the others parameters – namely Ta, Pd, and Rthspa – are addressed by the customer's assembly designers. Designers must take the PCB material, the thickness and layout of the copper tracks, the cooling system, and the interaction between surrounding components into consideration. Their designs are becoming more and more computer aided, which is the only way to face the increasingly stringent requirements of new electronic equipment in terms of miniaturisation, power dissipation density, temperature exposure, reliability, etc.

A poor thermal management might induce melting or reduced reliability of the solder joints; reduce PCB performance (even burn-out); and lower chip resistor performance due to high reversible or irreversible drifts. This is why thermal management is so important.

Data
The information included with standard chip resistor datasheets – nominal power Pn and derating curves – does not allow customers to get the best performance from their chip resistors, and might even mislead them.

In order to allow customers to use the above thermal model, manufacturers need to provide them with Rthjsp for standard and enlarged-termination parts, in addition to experimental data relevant to chip resistors of standard sizes mounted on various PCBs. These PCB should be chosen to represent the standard and best cases in terms of thermal resistance. This is mainly to help designers who cannot calculate thermal resistance by themselves so they can complete their CAD approach.

The following are abbreviation meanings for the data below:

PCB sCu – A PCB with a thickness of 1.6 mm, double sided, 35µm thick copper (minimum), at least 50% copper coverage both sides

PCB MCu – A PCB with a thickness of 1.6 mm, double sided, 70µm thick copper (minimum), at least 80% copper coverage both sides

Enlarged wraparound (W/A) resistors are equipped with bottom metallisation covering their backside, with the exception of a 0.5 mm wide insulation path.

Experimental Data

Table below shows maximum drifts, which can be guaranteed for various Tj. These maximum drifts are higher than the experimental data to provide safety margins for even the worst design cases.

All the above figures concern absolute drifts. These temperature-induced drifts can be drastically reduced using resistor networks or arrays. With such devices, the matching that is achieved through construction can lead to ratio drifts somewhere between two to five times lower than the absolute drifts.

Derating curve
Usual Datasheet Derating Curve

Pn = Nominal power and it is defined by benchmarking. This is almost the same for a given size across all suppliers

Tc = The highest limit of the temperature range. For example, +155°C is the Tc for an extended military range

Pd gives the maximum allowed power dissipation for a Ta lower than Tc

The truncation at +70°C is historical and totally arbitrary

The Pd values in a usual datasheet derating curve are not sustained by any thermal design considerations. They are very conservative and prevent design engineers from receiving the full capabilities of the resistors.

Meaningful Derating Curve


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