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I was surprised when saw a magnet picking some random 0.125 W resistors leads. I was always thinking that leads are made of copper alloy. Are there some subtle reasons to make them using metals other than tin plated copper or copper alloys ?

Thank you

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Main (and probably only) reason: copper is expensive. I've commented before that cost can outweigh other factors in design and production, and that goes also for components. Every milli-cent counts. The alloy used (no, I don't know what it is) may have a higher resistance than copper, but over the whole the difference will be negligible. I guess that for very low value resistors (< 0.01\$\Omega\$) copper may be used. Copper is also used for certain power devices because it conducts heat better. These 500\$\mu \Omega\$ examples from Isabellenhütte illustrate both:

enter image description here

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Leadwires are attached to provide both electrical and mechanical support, and must survive the component fabrication process before they are assembled.

Glass/metal sealing for diodes and some kinds of resistors makes Kovar and similar alloys (iron/nickel based) a natural choice of lead material, because it adheres to glass and doesn't cause stresses on cooling. Even when (for power diodes like 1N4001) the leads are heavy copper for cooling, the button that seals against the glass is a magnetic material welded to the copper. Nickel, also ferromagnetic, is frequently employed on surface mount components, because a thin layer of nickel will hold solder, where a thin layer of copper might dissolve into the solder instead.

Copper isn't suitable for thin items that must be fired at high temperatures (it oxidizes), and has chemical incompatibility with some materials (only a few high-tech ICs have copper near the silicon parts).

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With resistors, there are two technical reasons I can think of:

  • Heat: Copper is not only a very good electrical conductor, it is also a very good conductor for heat. Sometimes, you want to run a large and leaded resistor's body at 155 °C (or even 175 °C, if its specification allows), but for safety reasons and regulations (UL, mostly), you may be limited to 130 °C at the solder joint (for standard FR4 and solder). With some sort of steel alloy, the heat remains around the resistor, and can be radiated or convected into the air around it, while not so much heat is transferred into the board.

  • Mechanical Strength: Sometimes, resistors are not mounted flat onto the board. Instead, the leads are bent to allow for upright mounting or to keep the resistor at a certain distance from the board, or both. (Sometimes, it is because of constraints of pick-and-place machinery, sometimes it is done to save space, sometimes, you want the hot resistor at a distance from the board - see "Heat"). When the board is exposed to mechanical shocks or vibrations, the resistor's leads last longer when they are made from some stronger material like steel. The process of forming the leads automatically may also be a bit rough on the leads, and steel may be the better choice.

Both reasons are especially true for NTC inrush limiting resistors. You want them to run as hot as you can (e.g. 175 °C) because then, the resistance is low and the losses are low. At the same time, you need power to make and keep the resistor hot, and in order to save power and keep the solder joints from becoming hotter that (e.g.) 130 °C, you don't want the heat to go away from the resistor's body into the board . And typically, an NTC's leads are bent to keep the body 5...10 mm above the board, requiring some mechanical strength of the leads. All of these reasons favor steel.

Then, of course, cost may also be an issue that favors steel alloy.

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  • \$\begingroup\$ Your first point ("heat") is actually a case for copper, and against the steel alloy. You want to get rid of the heat in any possible way, so not just convection, but also the conduction. \$\endgroup\$ – stevenvh Jul 9 '11 at 7:47
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    \$\begingroup\$ @stevenvh, this may really be the other way round for leaded components. I've mentioned NTC resistors, where it's definitely true that you don't wand any possible way, and it is also true for some ways of mounting power resistors some 5...10 mm above the board's surface. Try convincing UL that 175 °C are a good idea right on the board's copper traces and they will tell you ;-) Using the board as a means of heat dissipation was not as popular as today when leaded components were still the norm instead of SMDs - especially for cheap (brown) board material commonly used in consumer electronics. \$\endgroup\$ – zebonaut Jul 9 '11 at 7:53
  • \$\begingroup\$ OK, I see your point. I have yet to see a power application where the heat flow to the PCB is seen as a bad thing. Of course 175°C is too hot, but that's where good design comes in: large copper areas instead of narrow traces, and such. In one product we used 105\$\mu\$ copper for this instead of the standard 35\$\mu\$. \$\endgroup\$ – stevenvh Jul 9 '11 at 8:03
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    \$\begingroup\$ An example of a power design where heat transfer into and from the board is bad could be a switching supply where you have to mount the electrolytic caps somewhat close the the (hot) transformer and semiconductors: It may or may not be good that the heat from the transformer goes into the board, but it's always bad if heat goes from a hot copper trace into the caps. Of course, the caps produce heat themselves, but are usually cooler than anything in their neighborhood, so passive heating is an issue. That said, I think their lead material is chosen primarily for mechanical strength and cost. \$\endgroup\$ – zebonaut Jul 9 '11 at 8:14
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The only reason I ever learned is this:

In the old days, some components were easily damaged by heat. The leads were made of steel so a person would be less likely to destroy the component while manually soldering, since steel is about 20x less thermally conductive than copper. (Manual soldering was the only assembly technique for years.) Typically it was semiconductors and some capacitors that had this kind of lead - and still do.

The other answers have a valid point about strength, though: in leaded resistors sometimes the little caps inside the epoxy that connect the copper leads to the thin-film resistive element are made of steel - I assume for strength. I have never seen a resistor lead made of steel though.

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