EoW September 2010

technical article

Usually a 0.4mm thick strip is provided with a 3μm coating of tin on both sides. When the scrap is directly recycled, the resulting CuFe2P alloy contains an impurity of tin at around 1.5%. This has a major effect on work hardening behaviour and on the electrical conduc- tivity of the alloy, which falls drastically when the tin content exceeds 0.3% (see Figure 2). There is, therefore, a need for a new alloy with comparable properties to CuFe2P but which can be recycled without difficulty, even when coated with tin. Pure copper/tin alloys such as CuSn 0.15 have the potential to be used as alternatives. When coated with tin, the scrap can be fed directly into the material cycle (see Table 1). Moreover, the mechanical and techno- logical properties correspond relatively well to those of CuFe2P. There are, however, distinct weaknesses in terms of softening behaviour and relaxation resistance (see Tables 2 and 3). A look at the newly developed alloy BB05xi shows a different situation. The targeted harmonisation of the alloy elements (tin, nickel and phosphorus) gives the material mechanical and technological properties comparable to CuFe2P, together with the softening and relaxation (stress creep of the component at high temperature) properties profile required for further processing (see Figure 3) and for the intended application.

Relative electric conductivity in %

Tin content in %

Figure 2 ▲ ▲ : Influence of the tin content on the conductivity of CuFe2P

BB01 C14410/15

SB02 C19400

BB05xi Balance 0.2 – 0.8

Copper

Balance

Balance

Tin

0.12

-

Zinc Iron

<0.10 <0.02 <0.02

0.13

<0.05 <0.02

2.4

Nickel

–

0.1 – 0.6

Phosphorus

<0.015

0.03

0.008 – 0.05

Table 1 ▲ ▲ : Comparison of the chemical composition of various bronzes

It is fed back into the material/production cycles as a cathode. This procedure is energy intensive and is, therefore, expensive relative to direct melting.

(Restriction of Hazardous Substances) directive, which came into force on 1 st July 2006, bans the typical lead-tin compounds that were formerly used. The integration of the functional pure tin coating in the material cycle is described in detail below. The choice of material for connectors is based primarily on physical criteria such as electrical conductivity, modulus of elasticity, thermal relaxation and pro- cessing characteristics, ie ductility and bendability, and welding behaviour. Issues relating to partial or total surface protection are of secondary importance, as are the basic availability of the materials and material costs. An examination of production and punching waste reveals that, in many cases, it is not given the attention that, on ecological and economic grounds, it deserves. This is illustrated by the following example. During production from hot-dip tinned CuFe2P (C19400) of large lead frames for ABS and ESP systems about 50% to 70% scrap is produced. None of this can be directly recycled (fed back into the melting process). It has to go through time-consuming smelting and be electrochemically separated.

Table 2 ▼ ▼ : Comparison of the technological properties of various bronzes

BB01

SB02

BB05xi

Electric conductivity Soft [% IACS] Thermal conductivity [W/mK] Coefficient of thermal expansion [Rt-100ºC]

>83

63

>62

360

260

250

17.7 x 10 -6

17.7 x 10 -6

17.7 x 10 -6

Elastic module [GPa]

128

123

126

Table 3 ▼ ▼ : Comparison of the technological properties of various bronzes

Strip thickness 0.3mm

BB01

SB02

BB05xi

Rm [MPa]

450 410

450 420

425 380

[MPa]

Rp

0.2

A50 [%]

4

9

6

HV

130

145

125

Softening temperature [ºC (1 h)]

300

350

350

Bendability [180º GW R/S] Bendability [180º BW R/S]

1

0

0.5

1

1

0.5

68

EuroWire – September 2010