Heat treatable copper-iron alloys have a higher strength than copper whilst maintaining a reasonably high electrical and thermal conductivity and excellent welding, soldering and brazing properties.
This precipitation-hardenable alloy was developed by Olin Brass in the USA in 1964 (C19400). It offered a combination of high conductivity and strength and proved an excellent alternative to copper and brass for electronic and electrical applications such as lead frames and connectors.
In Europe the alloy is designated CW107C and appears in PD6641:1999 and as a leadframe alloy in EN1758:1998. There is no British standard for this alloy.
The precipitation heat treatment involves heating the copper-iron alloy to 800 to 900oC and water quenching. This is called solution treatment and the alloy is in its softest condition. Subsequent precipitation (age) hardening at lower temperatures of 425 to 500oC results in the formation of finely dispensed iron precipitates in the alpha matrix; these are responsible for the increased strength and hardness.
The copper-iron alloy may be stress relieved at 200-300oC.
The alloy has the following combination of properties:
- Tensile strength: 370-580 N/mm2
- Proof strength: 110-485 N/mm2
- % Elongation: 10-1
- Hardness (HV): 80-170
- Electrical conductivity: 60-65% IACS
- Thermal conductivity: 260-265 W/moC
Soldering, brazing and gas shielded arc welding are excellent. Laser welding and resistance welding are good. Coated metal arc, resistance, spot, seam and butt welding are not recommended.
This is rated at 20-25% which is satisfactory. Free machining brass is 100%.
Copper-iron has excellent cold formability and fair hot formability.
Resistance to Corrosion
Copper-iron has good resistance to corrosion in industrial and marine atmospheres. It is insensitive to stress corrosion cracking. However, it is susceptible to attack in the presence of ammonia, sulphur, hydrogen sulphide and mercury.
This is decrease in stress under constant strain, typically observed in steel bolts in turbine casings at high temperature which must be regularly tightened.
For copper alloys used in electrical contacts, it is important to maintain good contact force throughout the functional life of the product and have good stress relaxation resistance.
One indication of stress relaxation may be obtained by measuring the % of stress which is retained as a function of temperature and time. Results on copper iron strip test pieces tested after 1000 hours are:
% Retained Applied Stress
It is clear that the % retained stress decreases as the temperature increases; results such as this will be one factor used in the design of high performance components from copper-iron strip, such as clips, contacts, connectors, switches, pins and leadframes which operate in this temperature range of 100-200oC.
Strength at Temperature
Leadframes may be subject to temperatures as high as 350oC for a few minutes; tests have shown that copper-iron shows very little loss of strength after 100 minutes at 350oC.
Copper-iron has a combination of strength and weldability making it ideal for the following applications.
- Circuit breaker components
- Contact springs
- Electrical connectors
- Electrical springs
- Plug contacts
- Fuse clips
- Cable wrap
- Pin grid array (PGA) in semi-conductor industry
Copper-iron is available as strip, tube and wire.
Below are the specifications for Europe, US and Asia. Note that for USA and Asia, some compositions are not identical. For equivalent standards from other countries visit the Copper Key website.
Europe: CW107C (CuFe2P) (European Standard EN designation).
USA: C19400 (American Society for Testing and Materials ASTM designation).
Japan: C1940 (Japanese Industrial Standards, JIS designation).
Further information on copper-iron, and other conductivity materials, is available at the Copper Alloys Knowledge Base
Application Example 1: Leadframes
Leadframes connect the wiring from electrical terminals on the semi-conductor surface to the large scale circuitry on electrical devices and circuit boards.
Copper-iron, with its combination strength, electrical and thermal conductivity, formability, plateability, corrosion resistance and the ability to withstand temperatures up to 350o
C for a short time without softening, is an ideal choice for this application.
Application Example 2: Pin Grid Array
A pin grid array, often abbreviated PGA, is a type of integrated circuit packaging. In a PGA, the package is square or rectangular, and the pins are arranged in a regular array on the underside of the package. The pins are commonly spaced 2.54 mm apart, and may or may not cover the entire underside of the package.
There is a strong demand for materials of high thermal conductivity to dissipate the heat produced in the operation of the PGA, as well as high electrical conductivity. An ideal material for this application is CW107C in wire form, which is widely used as PGA lead pins for personal computers, and tin plated square wires have been adopted in connector terminals for automobiles.
The high electrical conductivity 65% IACS, high thermal conductivity 260 W/m o
C and high tensile strength of up to 580 N/mm2
are the reasons why CW107C wire is used for pin grid arrays in application such as a microprocessor.