fig2 a illustrates a preferred embodiment of a method of forming a bonding wire of the present invention , which is similar to fig1 a except that the polarity of the voltage applied by a d . c . voltage source 14 between a metal wire 1 , made of copper , aluminum , palladium or an alloy of combination thereof , and a consumable electrode 2 is reversed . that is , the wire 1 , supported by a capillary chip 5 acting as a bonding tool is connected to the positive terminal of a d . c . high voltage source 14 , and the consumable electrode 2 is connected to the negative terminal thereof . the discharge region is held in an inert gas atmosphere of a gas such as argon to prevent oxidation of molten portions of the wire 1 . with this arrangement , only the tip portion 6 of the wire 1 is heated when thermal electron emission occurs on the surface of the electrode 2 , and thus only the tip portion is melted . that is , the molten portion is defined strictly at the tip portion of the wire 1 , resulting in a completed ball 18 as shown in fig2 b . the stability of the discharge between the wire tip at the positive potential and the consumable electrode at the negative potential may be degraded to some extent compared with the case where the opposite potentials are employed . however , it has been found by the present inventors from analysis of the phenomena of melting of a wire having a diameter of 25 microns and tip ball formation that a non - melted portion is unavoidably left in a portion of wire when the polarities of the wire and the electrode are negative and positive , respectively . according to the present invention , the stability of the discharge is improved by increasing the voltage and using an inert shielding gas such as argon , which can more readily be converted to a plasma than air . in order to stably form a ball 18 having a diameter two to three times that of the metal wire 1 for a wire of diameter in a range of 15 to 35 microns , the discharge current and discharge time may be selected suitably from ranges of 30 to 200 ma and 1 to 10 msec , respectively . fig3 is a graph showing experimental data of discharge current and discharge time obtained for a copper wire having diameter of 25 microns . in fig3 the hatched area shows preferred range of these parameters . in the above - described embodiment , only the tip portion of the wire is heated and it is heated concentrically to thereby melt that portion completely , resulting in a ball having no non - melted portion . although this method is very effective , there may occur a problem in some situations in that , due to such complete melting , the mechanical strength of the neck portion ( the boundary portion between the ball portion and the wire portion ) is lowered during a bonding operation , resulting in looping of the wire portion or breakage of the neck portion . this may be ameliorated by leaving the neck portion partially non - melted . according to another embodiment of the present invention , the above problem is effectively eliminated by making the discharge voltage applied between the wire and the consumable electrode alternating with the electric energy when the polarity of the wire is positive being larger than that when the polarity is negative . fig4 a illustrates an arc when an alternating voltage is applied by an a . c . source 24 between the wire 1 and the discharge electrode 2 . in fig4 a , when the polarity of voltage applied to the wire is positive , an arc 3 is formed between the tip of the wire and the discharge electrode 2 , as shown by solid lines , whereupon the tip portion is concentrically heated as in the preceding embodiment . when the voltage polarity of the wire is switched to negative , the arc 3 tends to form in a wider area of the wire , as shown by chain lines , as in the conventional ball formation process for gold wire . by repeating the alternating polarity cycles to the applied voltage , a small non - melted portion 28a is left in the neck portion of a ball 28 , as shown in fig4 b . the discharge region may be held in an inert gas atmosphere to prevent oxidation during melting as in the preceding embodiment . in order to form such a non - melted portion 28a efficiently , the voltage applied to the wire 1 should be selected so that the electric energy supplied to the wire 1 when the polarity is positive is larger than that when the polarity in negative . for example , the voltage v 1 applied to the wire 1 when its polarity is positive is made larger than the voltage v 2 applied to the wire when its polarity is negative , as shown in fig5 a . alternatively , it is possible to make the time t 1 for which the voltage of a positive polarity is applied to the wire 1 longer than the time t 2 for which the voltage of negative polarity is applied , as shown in fig5 b . empirically it has been found preferable that the ratio of the energy supplied to the wire when its polarity is positive to the energy supplied thereto when its polarity is negative is about 3 to 4 . the above embodiments have been described as using an electrode wire made of any of copper , palladium , aluminum , or alloys or combination thereof . it should be noted that , in order to obtain good bonding , the hardness of the ball formed at the tip of the wire must be compatible with the hardness of the electrode on the semiconductor chip to which the wire is to be bonded , which electrode is usually made of aluminum . the vickers hardness hv of an aluminum electrode is usually in a range of 35 to 40 , while that of the ball formed at the tip of the copper wire is 60 or more . therefore , the hardness of a copper wire is too high to obtain good bonding to an aluminum electrode . in order to make the hardness of the wire and the electrode compatible with each other , it is necessary to either lower the hardness of the copper wire or increase the hardness of the aluminum electrode . as to the copper wire , the copper should be as pure as possible , preferably 99 . 99 % or more . by using such copper , the hardness of the wire can be lowered to about 50 to 60 hv . in addition , in order to prevent oxygen from mixing into the ball and to thereby restrict any hardness increase caused thereby to prevent degradation of the roundness of the resulting ball due to oxide film formation , the oxygen concentration of the inert gas or reducing gas should be 5000 ppm or less . it has been found by the inventors that the hardness of the aluminum electrode can be increased by adding thereto a minute amount of an impurity . the impurity may be selected from materials such as mn , mg , cu and / or si , or gases such as o 2 , n 2 , etc ., and the amount thereof may be 10 to 5000 ppm . the hardness of aluminum containing such an impurity is increased to 45 to 60 hv , which is compatible with hardness of the copper wire . furthermore , other mechanical properties of the aluminum electrode are also improved by the addition of the impurity . for example , the shear strength of an ordinary aluminum electrode is only about 5 to 7 kg / mm 2 , which is not sufficient for the wire to withstand the subsequent molding process for encapsulating the semiconductor chip in resin . that is , the semiconductor chip is usually molded with a resin which may be of the thermoplastic type , in which case molding is carried out at a high temperature . when an ordinary aluminum pad is sujected to such conditions , it tends to break or deform internally , causing the resultant device to be unstable in operation . fig6 is a graph obtained by plotting critical values p of pressure applied in a direction shown by a double arrow in fig7 which illustrates a copper wire 1 ball - bonded to an aluminum electrode 20 formed on a silicon chip and measured when the aluminum pad is broken , with the shear strength of the aluminum pad being taken as a parameter . it is clear from fig6 that the pressure value p increases with shear strength in a range below about 8 kg / mm 2 . the damage to the aluminum electrode subjected to the critical pressure in this range is usually inner breakage of the pad . on the other hand , for an aluminum electrode having a shear strength of about 8 to 12 kg / mm 2 , the damage which occurs is breakage of only interface regions thereof , while for an electrode having a shear strength larger than 12 kg / mm 2 , the semiconductor chip is damaged due to the inner stress of the electrode . therefore , the shear strength of the aluminum electrode should be in a range of about 8 to 12 kg / mm 2 . fig8 is a graph showing the relation of the shear strength of an aluminum film to the amount of impurity contained therein when copper is selected as the impurity . from fig8 it is clear that the shear strength of the aluminum film increases with an increase in the amount of the impurity , and the desired shear strength is obtained when the amount of copper is in a range of about 100 to 500 ppm . as described hereinbefore , according to the present invention , ball - type non - gold bonding wires are greatly improved by improving the ball formation at the tip of the wire , lowering the hardness of the ball , and increasing the mechanical strength of the aluminum electrode .