Conductor Materials

Bare Copper

Bare copper slowly combines with oxygen at room temperatures to form copper oxide. Raising the temperature accelerates this reaction, and at about 180°C and higher, bright copper wire turns black in just a few minutes. Oxide film is a poor conductor of electricity and must be either removed or pre- vented from forming in order to assure reliability of connections. This is usually accomplished by coating the copper wire with another metal which ox- idizes more slowly at operating and processing temperatures. Thus, a coating is sometimes used to facilitate termination (soldering); sometimes as a processing aid (preventing oxidation of the copper at Teflon TFE extrusion temperatures); and sometimes to offer a lower-resistance connection ("Wire- Wrap" termination). Bare copper is satisfactory at temperatures up to about 100°C.

Tinned Copper

Tinned copper conductors are a soldering aid and are usually specified where this terminating method is to be used. Suitable for conductors continually exposed to temperatures not exceeding 150°C, tinned copper conductors are slightly more expensive than bare copper wires. However, the labor savings gained by using tinned copper more than offset the additional expense, especially when manual twisting and solder dipping of the stripped lead is re- quired.

Silver Coated Copper

Silver plated copper is made by electro-plating pure silver on 18 AWG wire which then is cold drawn to size and finally annealed. Minimum silver thick- ness is 40 micro-inches. Though higher in cost than tinned copper, silver-coated conductors are recommended for wires operating from above 150°C to about 200°C and in high frequency applications where, because of skin effect, higher conductivity of silver is desirable. They are readily wet by solder, permitting rapid soldering with hand irons. Care must be taken, however, to prevent solder wicking under the insulation, which may reduce conductor flex Life. Silver coated copper will oxidize after a few hundred hours at 250°C.

Nickel Coated Copper

Nickel plated conductor (50 micro-inches minimum nickel thickness) is recommended for Teflon TFE hook-up wire operating for prolonged periods at temperatures of from 200° to 260°C, and where silver coating is objectionable because of possible solder wicking. Ordinary soft solder does not wet nick- el as readily as it does tin or silver. It adheres well enough to make a good termination, but will not wick into the stranded conductor beyond the joint, thereby leaving flexibility unimpaired. Connections exposed to temperatures above the melting point of soft solder require special soldering techniques. The term "nickel clad" refers to a much thicker coating -10% to 30% of the radius of the strand.

Comparison of Conductor Coatings

Characteristic	Tin Plate	Silver Plate	Nickel Plate
Life Stability	Conductivity and solderability deteri- orate with heat aging at rated tem- perature due to migration of tin and copper and tin oxidation.	Excellent- no loss of conductivity with heat aging at rated tempera- ture. solderability shelf-Life remains good.	Conductivity remains stable with heat aging at rated temperature.
Crimp Terminability	good-but contact resistance in- creases with time and can be vari- able.	Excellent contact resistance re- mains low.	good- but contact resistance may vary with time. use plated steel ter- minal in some cases.
Solder	good originally. deteriorates with shelf life.	Excellent.	requires active flux.
Service Temperature	150C°	200C°	250C°

Conductor Properties Comparison Chart

Material	Maximum Con-tinuous operat-ing temperature	Corrosion resis-tance	Solderability	Relative Weight	conductivity relative to cop-per(%)	Tensile psi	Price
Bare Copper	150C°	Good(pooroxida-tionresistance)	Fair	1	100%	35	$
Tin Plated Cop-per	150C°	Good	Good	1	100%	35	$
Silver Plated Copper	200C°	Poor	Good	1	100-103%	35	$$$
Nickel Plated Copper	260C°	Good	Poor	1	96%	35	$$
Copper Cov-ered steel	200C°	Good	Fair	0.925	18-40%	100-200	$$
High strength alloy	200C°	Poor	Good	0.98	63-90%	60-95	$$$$
Stainless steel	870C°	Good	Poor	0.925	10-30%	70-300	$$$