JPH05179375A - Fine gold alloy wire for bonding - Google Patents

Abstract

PURPOSE:To provide a gold bonding wire excellent in heat resistance and having high reliability for connecting a semiconductor device to an external electrode because neck strength at the time of ball bonding is enhanced. CONSTITUTION:This fine gold alloy wire for bonding contains 3-50wt.ppm Al and 3-30wt.ppm Ca as elements of a first group in high purity gold of >=99.995% purity or further contains 3-30wt.ppm on or more among Y, La, Ce, Nd, Dy and Be as elements of a second group. The total amt. of the elements of the first and second groups is 9-100wt.ppm.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fine gold alloy wire having excellent heat resistance used for connecting electrodes on a semiconductor element and external leads, and more particularly, to vibration fatigue during semiconductor device assembly work after joining. The present invention relates to a gold alloy thin wire for bonding, in which the strength of the ball neck portion is improved in order to significantly reduce the disconnection due to.

[0002]

2. Description of the Related Art Conventionally, a gold alloy fine wire has been mainly used as a bonding wire for connecting an electrode on a semiconductor element and an external lead. A thermocompression bonding method is a typical method for bonding gold alloy fine wires. In the thermocompression bonding method, the tip portion of the gold alloy fine wire is heated and melted with an electric torch, and a ball is formed by surface tension.
In this method, the ball portion is pressure-bonded to the electrode of the semiconductor element heated in the range of 0 ° C., and then the connection with the lead side is further performed by ultrasonic pressure-bonding.

In recent years, there has been a strong demand for improving the characteristics of gold alloy thin wires as the bonding technology has improved. For example, as a result of the temperature just above the ball being elevated at the time of bonding, the crystal grains become coarse and the strength of the gold alloy wire deteriorates, resulting in a defect that the wire breaks due to vibration during assembly of the semiconductor device. As a measure for preventing this, a gold alloy thin wire having a higher pull strength than that of a conventional gold alloy thin wire is desired, using the pull strength after bonding as an index.

Further, with the recent generalization of thin semiconductor devices, gold alloy fine wires in which a large amount of elements are added to high-purity gold have been developed and used for the purpose of reducing the loop height after bonding. There is. For example, 5 to 1 calcium
Gold Bonding Wire Containing 00 ppm by Weight (JP-A-53-105968) 1 of lanthanum, cerium, praseodymium, neodymium and samarium
There is a gold bonding wire (JP-A-58-154242) containing 3 to 100 ppm by weight of one or more kinds and 1 to 60 ppm by weight of one or more of germanium, beryllium and calcium. ..
However, the pull strength is lowered due to the decrease of the loop height, and the hardness of the ball is increased due to the addition of a large amount of alloying elements, the joint strength after bonding is significantly deteriorated, and the reliability of the semiconductor device is lowered. Therefore, there is a demand for a gold alloy fine wire for bonding, which has a bonding strength equivalent to that of a conventional gold alloy fine wire having a relatively high loop height, a low loop height, and high reliability.

[0005]

DISCLOSURE OF THE INVENTION The inventors of the present invention have studied various conventionally proposed gold alloy thin wires for bonding, and as a result, these gold alloy thin wires for bonding have a high content of a specific additive element. Although it has higher tensile strength than pure gold wire, it may cause wire breakage during semiconductor device assembly as a result of coarsening of crystal grains directly above the ball, resulting in large variations in pull strength after bonding. It was confirmed that there was a problem that it was unreliable and its value was low.

In addition, the gold alloy fine wire for bonding, in which a large amount of an element for increasing the recrystallization temperature of gold is added in order to reduce the loop height after bonding, has a reduced pull strength due to the precipitation of crystal grain boundaries after bonding. What to do,
As the amount of additional elements increases, an oxide layer is formed on the ball surface due to oxidation of the additional elements during ball formation, and sufficient bonding cannot be achieved during thermocompression bonding with the electrodes. , The deformation rate decreases during pressure bonding, and a contraction hole is easily formed at the tip of the ball, which reduces the joint area with the electrode of the semiconductor element and reduces the shear strength. Wire breakage due to vibration due to vibration, etc., flow resistance of the sealing resin that the gold alloy thin wire receives during resin sealing, and difference in the coefficient of thermal expansion of the constituent materials of the semiconductor device due to various thermal cycles thereafter The gold alloy fine wire may be broken due to shearing force and the like, and as a result, there is a problem that the reliability of the semiconductor device is lowered.

[0007]

Means for Solving the Problems The inventors of the present invention have added a composite metal of aluminum and calcium, whereby the strength of the ball neck portion is high and the variation thereof is small, and the bonding gold with fine crystal grains in the ball neck portion is used. It was confirmed that a fine alloy wire can be obtained, and by controlling the amount of addition, a fine gold alloy wire for bonding with a low loop height can be easily manufactured industrially and the above-mentioned problems can be solved.

That is, the gist of the present invention is as follows. (1) High purity gold (purity 99.995% or more)
As a group element, a gold alloy thin wire for bonding, containing 3 to 50 ppm by weight of aluminum and 3 to 30 ppm by weight of calcium. (2) In addition to the elements of the first group, 3 or more of yttrium, lanthanum, cerium, neodymium, dysprosium and beryllium are added as the second group of additive elements.
The gold alloy fine wire for bonding according to the preceding item 1, which contains -30 to 30 ppm by weight and the total amount of the elements of the first group and the additive elements of the second group is 9 to 100 ppm by weight.

The structure of the present invention will be further described below. The high-purity gold used in the present invention is gold having a purity of at least 99.995% by weight or more, and the balance being inevitable impurities. If the purity is less than 99.995% by weight, it will be affected by the impurities contained therein. In particular, with a gold alloy thin wire for high loops in which the amount of alloying elements added is relatively small, the effect of the amount of alloying elements added according to the present invention cannot be sufficiently exhibited.

Aluminum has a large solid solution limit in gold, and it has been conventionally known that aluminum has a high recrystallization temperature, but addition of aluminum alone does not have a sufficient effect. By adding calcium together with aluminum to high-purity gold, the strength of the ball neck portion is increased, the pull strength is improved, and the disconnection due to various vibrations before the resin sealing step can be reduced. This effect is not sufficient if the amount of aluminum added is less than 3 ppm by weight while calcium is added, because the crystal grains in the ball neck portion are not stably refined. When the amount of aluminum added exceeds 50 ppm by weight,
The ball does not become a true sphere at the time of ball formation, and a strong oxide of aluminum and calcium is generated on the ball surface, and it becomes difficult for both new metal surfaces to come out when the ball and the electrode are pressure bonded at the time of bonding with the electrode. Since the bonding strength decreases, the range of the amount of aluminum added is set to 3 to 50 ppm by weight.

Calcium is known to be an element that improves heat resistance, but if it is less than 3 ppm by weight, the effect of refining the crystal in the ball neck portion cannot be obtained. Also,
Addition of more than 30 ppm by weight makes the crystal in the ball neck finer and improves the heat resistance, but it causes shrinkage holes at the ball tip during ball formation, and oxidation during ball formation causes strong oxidation on the entire surface of the ball. When a product is generated and the bonding interface area between the ball and the electrode is decreased, the bonding strength (shear strength) is decreased. Further, the hardness of the ball becomes large due to an increase in the amount of calcium added, and if a load necessary to secure sufficient bonding with the electrode is applied, the semiconductor element below the electrode may crack. Therefore, the amount of calcium added was set to the range of 3 to 30 ppm by weight. The above-mentioned problems do not occur even if the contents of aluminum and calcium are the respective upper limits. It is considered that the reason why such an effect is obtained by the combined addition is that aluminum increases the solid solution amount of calcium in gold and increases the strength of the grain boundary.

The purpose of adding the elements of the second group is to further facilitate wire drawing by work hardening of the gold alloy thin wire during wire drawing. When the elements of the second group are added alone, unless added in a large amount, there is no effect in improving the room temperature strength due to work hardening during wire drawing. However, by adding together with the elements of the first group, work hardening during wire drawing becomes large, and wire breakage due to insufficient tensile strength of the gold wire during wire drawing can be prevented. Further, the addition of calcium has the effect of further improving the heat resistance by the combined addition and the effect of increasing the high temperature strength, and as a result, the effect of the present invention described above in which aluminum and calcium are added in combination can be further improved.

When the addition amount of these elements of the second group is less than 3 ppm by weight, the effect of promoting work hardening is unstable and the combined effect of improving heat resistance is not sufficient. On the other hand, if the amount of addition exceeds 30 ppm by weight, shrinkage holes at the tip of the ball, which were not generated by the addition amount of only the elements of the first group, are easily generated, resulting in reduction of shear strength.
Therefore, the addition amount of the elements of the second group is 3 to 30 weight p.
The range was pm.

When the total amount of the elements of the first group and the second group is less than 9 ppm by weight, the crystal grains in the ball neck portion are not stably refined and the effect is not sufficient. On the other hand, 100 ppm by weight.
Beyond the value, the ball does not become a true sphere during ball formation by an electric torch, a sufficient bonding area cannot be obtained when bonding with an electrode on a semiconductor element, the bonding strength is significantly reduced, and contact with other electrodes occurs. Therefore, the total amount of the elements of the first group and the second group is set within the range of 9 to 100 ppm by weight.

[0015]

EXAMPLES Examples will be described below. Gold purity is 9
Using 9.995% by weight or more of electrolytic gold, the mother alloys containing the above-mentioned additive elements were individually melted in a high-frequency vacuum melting furnace and cast. It should be noted that melting and casting a mother alloy to which calcium and aluminum are added together has the effect of improving the calcium addition yield.

By using a predetermined amount of the mother alloy containing each additive element thus obtained and electrolytic gold having a gold purity of 99.995% by weight or more, a gold alloy having the chemical composition shown in Table 1 was prepared in a high frequency vacuum melting furnace. Melt casting, rolling the ingot, and then drawing at room temperature to obtain a gold alloy fine wire with a final wire diameter of 25 μmφ, and continuously annealing it in the air atmosphere to give an elongation value of about 4%. Adjust so that

Table 1 also shows the results of examining the tensile strength at room temperature, the loop height, the vibration rupture rate, the ball shape and the bonding strength of the obtained gold alloy thin wire. The loop height of the junction is 80 after the electrode on the semiconductor element and the external lead are joined using a high-speed automatic bonder, and then the height of the loop formed and the electrode surface of the semiconductor element are 80 with an optical microscope. The loop height was determined by measuring the difference between the two distances.

The vibration rupture rate is 5 pieces of iron-42% nickel lead frame (bonding span; 2 mm, having 6 IC packages in which 64 inner lead pins are arranged in four directions) for mounting a semiconductor element in a cassette. Then, the above-mentioned 25 μmφ gold alloy fine wire is bonded to the electrode on the chip and the inner lead by the gold alloy fine wire by automatic high-speed bonding, and then housed again in the cassette. ,
After vibrating the gravitational acceleration at each level of 1G, 2G, and 3G for 1 hour, the state of disconnection of the joint was inspected with an optical microscope, and the ratio of the number of disconnection was evaluated as a percentage.

The ball shape is determined by observing a gold alloy ball obtained by arc discharge by an electric torch with a scanning electron microscope using a high-speed automatic bonder. Nothing that could not be joined to the electrode in a good shape was marked with X, and good one was ○
It was evaluated by the mark. The bonding strength is the pull strength obtained by fixing the lead frame and the semiconductor element to be measured after high-speed automatic bonding with a jig and then pulling the central part of the gold alloy thin wire after bonding, and measuring the tensile strength at the time of breaking the thin wire 100 pieces. The variation was evaluated. In addition, the jig was moved parallel to the semiconductor element at a position 5 microns above the fixed semiconductor element electrode to shear and rupture the bonded gold balls, and the maximum load during peeling was measured 100 shear strength. It was judged based on the result of obtaining the variation.

Tables 1 and 2 (continued from Table 1) show the evaluation results of the gold alloy thin wires produced within the composition of the present invention.
Table 4 (continued from Table 3) shows the evaluation results of the gold alloy fine wire including the addition amount deviating from the component composition of the present invention. From the conventional experience, in the case of a gold alloy fine wire, the pull strength tends to decrease as the loop height decreases. The results of Tables 1 to 4 also show the same tendency. In comparison of the pull strengths of the gold alloy thin wires having almost the same loop height, the pull strengths of Table 2 are all Table 4
The result is smaller than the value of and the variation is also small. Further, when an excessive amount of alloying element is added, the shear strength is lowered and its variation is increased.

It is empirically known that, even in the case of vibrational disconnection, the disconnection occurrence rate tends to increase as the loop height decreases.
When breaking with 1G vibration strength, the reliability of the manufactured semiconductor device is low, and when breaking with 2G vibration strength,
If the occurrence rate of disconnection is 20% or less, the reliability of the semiconductor device is sufficiently satisfied. Regarding the occurrence rates of wire breakage in the gold alloy fine wires having almost the same loop height in Tables 2 and 4, the present invention results in a lower wire breakage occurrence rate, and the reliability of the semiconductor device is also sufficient.

Regarding the shear strength, it is generally considered that there is no problem in the case of a gold alloy fine wire of 25 μm if the shear strength is 50 g or more. In the case of Table 2, all satisfy the value of 50 g or more, but in the case of Table 4, it may be less than 50 g, which is insufficient as a gold alloy thin wire for bonding. In the evaluation of the ball shape, all the components within the scope of the present invention shown in Table 1 formed normal balls (see Table 2), but the components shown in Table 3 showed abnormal balls such as contraction holes at the ball tip. (Table 4
There is something that becomes.

As described above, when the composition of the present invention is deviated, the joining strength of pull strength and shear strength is insufficient,
It is clear that the rate of occurrence of vibration disconnection is high, which reduces the reliability of the manufactured semiconductor device.

[0024]

[Table 1]

[0025]

[Table 2]

[0026]

[Table 3]

[0027]

[Table 4]

[0028]

The gold alloy fine wire of the present invention has a small loop height variation, a high bonding strength, a small variation, a low occurrence rate of vibration disconnection, and a normal and stable ball shape. Bonding is possible, and the same effect is obtained even when the wire diameter is 18 to 30 μm, so that it has industrially useful characteristics.

Claims (2)

[Claims] 1. High-purity gold (purity of 99.995% or more)
In addition, as a first group element, a gold alloy thin wire for bonding containing 3 to 50 ppm by weight of aluminum and 3 to 30 ppm by weight of calcium. 2. In addition to the elements of the first group, 3 to 30 ppm by weight of one or more elements of yttrium, lanthanum, cerium, neodymium, dysprosium and beryllium are contained as a second group addition element, and the first element is added. The gold alloy fine wire for bonding according to claim 1, wherein the total amount of the group element and the second group additive element is 9 to 100 ppm by weight.