EoW September 2010

technical article

During further processing at high tempera- tures the thickness of the alloy layer that forms between the base material and the tin coating of tin-coated BB05xi is comparable to that of CuFe2P. Production lines therefore do not have to be converted to accommodate this new composite material (Figure 4). Moreover, this new alloy is significant as the tin-plated scrap from the individual stages of the value chain is directly recyclable. A comparison of the metal values BB05xi and CuFe2P also does not justify the difference between the costs of indirect and direct recycling of production and punching scrap, which in this sector are usually 20% to 25% of the metal value – a factor of considerable importance in times of high and increasing raw material prices. With a scrap percentage of, for example, 70% the smelting costs can rapidly match the production costs, casting doubts on the economic feasibility of the whole process. The use of a tin-coated phosphor bronze is therefore a worthwhile alternative to tin-coated copper-iron alloys from both an ecological and an economic point of view (the additional use of electricity and acid for the electrolytic treatment of the scrap is eliminated). 2.2 Development 2 Copper-tin alloys are used for connectors and components for electronic and electrical engineering applications as they have good to very good spring characteristics, good resistance to electrical and thermal stress, low stress relaxation and excellent bendability and solderability. Usually a small amount of phosphorus is added to alloys of this type for the purpose of deoxidation, which is why they are also referred to as phosphor bronzes. The properties of this group of alloys depend mainly on their tin and phosphorus content, and to a lesser extent on the other added alloy elements. By means of suitable processing they can be adjusted for use in a wide range of applications. The many industrial applications for these alloys range from high quality connectors and sockets for electronic modules to electrically conductive contact springs. In the past “downgrading” was carried out as an efficient method of selecting a phosphor bronze. In other words, the technological properties of a low-alloy

Relaxation in %

Temperature in ºC

Figure 3 ▲ ▲ : Comparison of the relaxation behaviour of CuFe2P and BB05xi

Alloy layer in µm

Exposure time in h

Figure 4 ▲ ▲ : Formation of the alloy layer at 180°C after hot-dip tinning

phosphor bronze were adjusted so that its spring characteristics and processing properties corresponded to those of the original high-alloy phosphor bronze. How- ever, certain constraints had to be taken into consideration. The tin and phosphorus content influence the work hardening behaviour and ductility of phosphor bronzes to a considerable extent, and a clear relationship has been found between the achievable bendability and the tin content. Figure 5 shows how increasing the tin content has a positive influence on the bendability at constant strength. Against this background it was logical to develop a higher-alloy phosphor bronze.

Another reason to do so was the demand for miniaturisation of connectors, as a reduction in cross-section reduces the contact force at constant deflection of the spring element. For a defined constant force, therefore, the spring element has to be redesigned – the design stress increases accordingly. One solution for this is the newly developed alloy BB95, a 10% phosphor bronze. At a yield stress level Rp0.2 > 720 MPa, the bendability of BB95 in BW90° R/S is superior to that of an 8% tin bronze by a factor of 2. Depending on the intended application, BB95 can be hardened to a yield stress level Rp0.2 of 800 MPa, and the high strength variety to >950 MPa.

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EuroWire – September 2010