Oxygen-free copper (OFC)
or Oxygen-free high thermal conductivity (OFHC)
copper generally refer to a group of wrought high conductivity copper
alloys that have been electrolytically refined
to reduce the level of oxygen
to .001% or below .
Oxygen-free copper is typically specified according to the ASTM/UNS database . The UNS database includes many different compositions of high conductivity electrical copper. Of these three are widely used and two are considered oxygen-free.
- C10100 - also known as Oxygen-Free Electronic (OFE). This is a 99.99%pure copper with 0.0005% oxygen content. It achieves a minimum 101% IACS conductivity rating. This copper is finished to a final form in a carefully regulated, oxygen-free environment. Silver (Ag) is considered an impurity in the OFE chemical specification. This is also the most expensive of the three grades listed here.
- C10200 - also known as Oxygen-Free (OF). While OF is considered oxygen-free, its conductivity rating is no better than the more common ETP grade below. It has a 0.001% oxygen content, 99.95% purity and minimum 100% IACS conductivity. For the purposes of purity percentage, silver (Ag) content is counted as copper (Cu).
- C11000 - also known as Electrolytic-Tough-Pitch (ETP). This is the most common copper. It is universal for electrical applications. ETP has a minimum conductivity rating of 100% IACS and is required to be 99.9% pure. It has 0.02% to 0.04% oxygen content (typical). Most ETP sold today will meet or exceed the 101% IACS specification. As with OF copper, silver (Ag) content is counted as copper (Cu) for purity purposes.
IACS refers to an electrical conductivity standard developed in 1913. Advances in the refining process now yield OF and ETP copper that can meet or exceed the 101% standard. Note that OF and ETP coppers have identical conductivity requirements.
Oxygen plays a beneficial role for improving copper conductivity. During the copper smelting process, oxygen is deliberately injected into the melt to scavenge impurities that would otherwise degrade conductivity.
There are advanced refining processes claiming to reduce impurity levels to below the C10100 specification by reducing copper grain density. Copper of this form is sometimes referred to as crystalline copper. At this time, there are currently no UNS/ASTM classifications for these specialty coppers and the IACS conductivity of these coppers is not readily available.
For industrial applications, oxygen-free copper is valued more for its chemical purity than its electrical conductivity. OF/OFE grade copper is used in plasma deposition (sputtering
) processes, including the manufacture of semiconductors
and super conductor
components, as well as in high vacuum devices such as particle accelerators
. In any of these applications, the release of oxygen (and/or other impurities) can cause undesirable chemical reactions with other materials in the local environment.
Use in home audio
Many owners of high-end audio and video equipment value oxygen-free copper. Behind this demand is the belief that oxygen-free copper will have enhanced conductivity or other electrical properties that are significantly advantageous to low frequency (audio) signal transmission. However, conductivity specifications for C11000 ETP and C10200 OF coppers are identical. In practice, nominal conductivity differences between the three grades listed above are less than 1% at room temperature.
Note that the high conductivity electrical coppers discussed in this article are categorically distinct from coppers deoxidized by the addition of phosphorus in the smelting process. Phosphorous copper is typically used for structural and thermal applications where the copper material will be subject to temperatures high enough to cause hydrogen embrittlement. Examples including welding/brazing rods and heat exchanger tubing.