C110 vs C101 Copper: Which Grade for Busbars, Cold Plates and Machined Electrical Parts

01The Short Answer

02What the Designations Mean

The numbers are UNS designations with the trailing zeros dropped: C110 is UNS C11000, C101 is C10100, C102 is C10200 and C145 is C14500. The names describe how the copper was refined.[1]

C110, electrolytic tough pitch (ETP). Electrolytically refined copper that is cast in air, which leaves a small, deliberate amount of oxygen in the metal, typically a few hundredths of a percent, tied up as copper oxide particles. Minimum copper content is 99.90% (silver counted as copper). This is the commodity grade: if a supplier's stock list says only "copper bar," it is almost certainly C110.

C102, oxygen-free (OF). Melted and cast under a protective atmosphere so oxygen never gets in. Minimum 99.95% copper, maximum 0.0010% oxygen (10 ppm).

C101, oxygen-free electronic (OFE). The highest purity commercial copper: minimum 99.99% copper, maximum 0.0005% oxygen (5 ppm), with individual limits on more than a dozen trace impurities. It is the only grade whose specification guarantees a minimum of 101% IACS. It exists for electron devices, vacuum systems and anything where trace elements would outgas or contaminate.

C145, tellurium copper. Copper with 0.40 to 0.7% tellurium and a trace of phosphorus. The tellurium forms fine particles that break chips, which is the same job lead does in free-machining brass or sulfur does in 12L14 steel.

03Side by Side: C110, C102, C101 and C145

All values are from the Copper Development Association alloy data sheets. Conductivity is for annealed material at 68 °F; machinability is rated against C360 free-cutting brass at 100.[1]

Read down the first three columns and the point makes itself: for a part that is only machined and bolted together, the three pure coppers are the same material. Every difference that matters sits in the brazing, welding and embrittlement rows.

04Where the Oxygen Matters: Hydrogen Embrittlement

The oxygen in C110 is not dissolved; it sits in the metal as small particles of copper oxide. At room temperature those particles are harmless, and they even help conductivity a little by tying up impurities. The trouble starts when the part is heated in the presence of hydrogen: a furnace brazing atmosphere, a reducing torch flame, or some welding conditions. Hydrogen atoms are small enough to diffuse into hot copper. When they reach an oxide particle they react with it and form water vapor. Steam molecules are too large to diffuse back out, so pressure builds at the grain boundaries until the metal cracks from the inside.[1][2]

The oxygen-free grades have nothing for the hydrogen to react with, which is why the CDA data sheets list them as "will not hydrogen embrittle during brazing." C145 is deoxidized with phosphorus and is also immune.

The reverse is just as important for cost: a bolted busbar, a gasketed or o-ring sealed cold plate, a clamped heat spreader or a soldered joint never sees those conditions. Specifying oxygen-free copper for those parts buys nothing the part can use.

05Conductivity: A Smaller Difference Than the Price Suggests

IACS is the International Annealed Copper Standard, set in 1913, where 100% equals 58 MS/m. Refining has improved since then, so ordinary commercial copper now measures slightly above 100%. The CDA lists the same nominal value, 101% IACS, for C110, C102 and C101. What differs is the guarantee: C110 and C102 are certified to 100% minimum, and C101 to 101% minimum.[1]

One percent of conductivity is smaller than the effect of the things a designer actually controls. Cold working copper to a hard temper costs two or three percent. A 10 °C rise in conductor temperature raises resistance by about 4%. A poorly prepared bolted joint can add more resistance than a foot of bar. For busbar sizing, thermal performance of a cold plate, or heat spreading, treat the three pure coppers as identical and spend the effort on cross-section, joint design and plating.

Thermal conductivity follows the same pattern because the same free electrons carry both heat and current. All three pure grades list 391 W/m·K. For comparison, 6061-T6 aluminum is roughly 167 W/m·K, which is why copper is chosen for the highest heat flux cold plates in spite of being more than three times as heavy and harder to machine.

06Machining Copper, and Why C145 Exists

Pure copper rates 20 on a scale where free-cutting brass is 100, and the rating is the same for C110, C102 and C101. The metal is soft and ductile, so instead of shearing into chips it tends to smear ahead of the tool, weld itself to the cutting edge and push up a burr at every exit. None of that is a reason to avoid copper; it is a reason to plan for it:[1]

C145 is the answer when the part is mostly machining: a connector body with threads, cross holes and grooves, or a terminal stud made by the thousand on a lathe. At a machinability rating of 85 it cuts much like brass, holds threads cleanly and sheds chips instead of wrapping them around the tool. The price is conductivity, 93% IACS against 101%, and availability: C145 is a rod and bar product under ASTM B301, so it is not a candidate for a wide cold plate or a flat busbar cut from plate.[1][3]

The general rules for keeping machining cost down apply to copper with extra force; our DFM guide for CNC milling covers internal radii, pocket depth and tolerance choices.

07Which Grade for Which Part

On cost: C110 is the volume product and is stocked in the widest range of bar, plate and busbar sizes. Oxygen-free grades carry a premium and come in fewer stock sizes, which sometimes means buying a thicker plate and machining it down. Copper prices move daily, so get current numbers with the quote; the metal weight calculator will give you blank weight for any size.

08What to Put on the Drawing

A material note that says "copper" will be quoted as C110 by every shop that sees it, in whatever temper is on the shelf. If that is what you want, fine. If not, the note needs four things:[3]

Our RFQ guide covers the rest of the package.