WO2010087053A1 - Bonding wire - Google Patents

Abstract

The purpose of the present invention is to so improve the bonding strength as to be suitable for the increased speed of working even at a low working stage temperature of about 150°C.  Bonding wire (P) for connecting an electrode (a) of an integrated circuit element and conductor wiring (c) of a circuit wiring board by means of a ball bonding method.  Temper heat treatment is performed to obtain bonding wire having a thickness 0.04 to 0.09μm of a coating layer (2) in such a manner that the coating layer (2) consisting of one or more kinds of gold, platinum, palladium, and silver is formed on the entire outer peripheral surface of a core (1) consisting of copper of not less than 99.99% by mass, that after the adhesion of the core (1) and the coating layer (2) is enhanced by applying diffusion heat treatment to the coated wire thereof, the wire is elongated up to wire diameter L: not less than 12μm and not more than 50.8μm, and further that tensile elongation becomes not less than 8%.  The bonding wire with the configuration is connected to the electrode and the conductor wiring with sufficient bonding strength, preventing work from being stopped even under the high-speed work at low temperatures.

Description

Bonding wire

The present invention relates to a bonding wire for connecting an electrode on an integrated circuit element such as an IC, LSI, transistor or the like and a conductor wiring of a circuit wiring board such as a lead frame, a ceramic substrate, or a printed circuit board by a ball bonding method, and its manufacture. It is about the method.

The connection method by this kind of ball bonding method is generally in the form shown in FIGS. 1 (a) to 1 (h). As shown in FIG. 1 (a), the wire P is inserted into the capillary 10a and the ball is attached to the tip thereof. From the state where (FAB: Free Air Ball) b is formed, the clamp 10b is opened, and the capillary 10a is lowered toward the electrode a on the integrated circuit element. At this time, the ball (FAB) b is captured in the capillary 10a and bonded to the center of the capillary 10a.

When the ball b comes into contact with the target electrode a (when the capillary 10a reaches the electrode a), the capillary 10a grips the ball b and applies heat, weight, and ultrasonic waves to the ball b, whereby the ball b and the electrode a Solid-phase bonding is performed to form a 1st bond and adhere to the electrode a (FIG. 1B).
When the 1st bond is formed, the capillary 10a moves up to a certain height (FIG. (C)) and then moves to a position directly above the conductor wiring c (FIGs. (D) to (e)). At this time, in order to form a stable loop, there is a case where a special movement is performed on the capillary 10a so as to attach a “garbage” to the wire P (see the solid line from the chain line in FIG. 4D).

Capillary 10a that reaches directly above the conductor wiring c descends toward the conductor wiring c and presses the wire P against the conductor wiring (2nd target) c ((e) to (f) in the figure). At the same time, heat, weight, and ultrasonic waves are applied to the pressed part, thereby deforming the wire P, and stitching for joining the wire P onto the conductor wiring c and a tail for securing the tail in the next step. A bond is formed (FIG. 1 (f)).

After both the bonds are formed, the capillary 10a moves up with the wire P remaining, and after securing a tail of a certain length at the tip of the capillary 10a, the clamp 10b is closed (by grabbing the wire P). The wire P is torn off from the bond portion (FIG. 1 (g)). At this time, since the tail bond temporarily fixes the wire P, the wire P forming the tail bond does not rise together with the capillary 10a.

The capillary 10a stops when it rises to the required height, and a high voltage is applied to the tip of the wire P secured at the tip of the capillary 10a with the discharge rod g to discharge (discharge) the spark. The wire P is melted, and the melted wire material becomes a spherical ball b by the surface tension and hardens (FIG. 1 (h)).

With the above operation, one cycle is completed, and thereafter, the electrode a and the conductor wiring c are connected by the ball bonding method by the same operation.

In the connection by this ball bonding method, a gold wire is mainly used as the bonding wire P. However, since gold is expensive, in recent years, an inexpensive copper wire having a copper purity of 99.99% by mass or more has been used. Has been done. At that time, since the surface is easily oxidized when the copper is bare, as shown in FIG. 2, a core material 1 made of a copper wire is coated with an oxidation resistant metal 2 is used.
As the coating metal (coating layer) 2, gold (Au), platinum (Pt), palladium (Pd), silver (Ag), nickel (Ni) or the like is employed (Patent Documents 1 to 3).

JP 2003-133361 A JP 2004-64033 A JP 2007-12776 A

In the bonding wire P made of a metal-coated copper wire, the ball b needs to be made smaller with the miniaturization of integrated circuit elements due to the recent miniaturization of electronic components. For this purpose, the diameter L is preferably 50 μm or less (paragraph 0009, lines 12 to 14 of Patent Document 1).
In addition, when the ball b is in a downward bowl shape (reverse conical shape) in connection to the electrode a of the integrated circuit element, the electrode b is pressed by the sharp end of the ball b when the ball b is pressed against the electrode a. The ball b is preferably a true sphere as much as possible. In order to increase the sphericity of the ball b, the thickness t of the covering layer 2 is set to 0.001 or less of the core wire diameter (Patent Document 1, Claim 1), and the thickness t of the covering layer 2 is set to 0.001 to The thickness is set to 0.02 μm (Claim 1 of Patent Document 3), or the coating layer 2 is formed of an oxidation-resistant metal having a melting point higher than that of copper of the core material 1 (paragraph 0014 of Patent Document 2).
Further, in the case of BGA (Ball Grid Array) based on an organic substrate, if the heating temperature (stage temperature) is increased, warping occurs and bonding properties are significantly deteriorated. For this reason, various devices for ensuring sufficient bonding strength even when the heating temperature (stage temperature) at the time of bonding between the wire P and the electrode a or the conductor wiring c is low, for example, about 150 ° C., for example, In addition, the wire is drawn after the heat treatment (Patent Document 3, paragraphs 0020, 0054, etc.).

As described above, the bonding wire P coated with a copper wire with an oxidation-resistant metal has been well received by various devices from the past. There is a further demand for improved joint strength.

This invention makes it a subject to respond to the request.

In order to achieve the above object, the present invention adopts all the above-described devices, and the wire diameter L of the bonding wire P is 12 μm or more and 50.8 μm or less, and is made of copper having a purity of 99.99 mass% or more. The coating layer 2 having a thickness t of 0.02 to 0.09 μm made of one or more of gold, platinum, palladium and silver was formed on the entire outer periphery of the core material 1.

The reason why the wire diameter L of the bonding wire P is set to 50.8 μm or less is that the diameter L is set to 50 μm or less in the above-mentioned Patent Document 1, but if it is 50.8 μm or less, it does not change to 50 μm or less. This is because the ball b can be made smaller.
The lower limit of the wire diameter L is set to 12 μm or more because if it is less than 12 μm, it becomes difficult for the operator to pass the wire P through the capillary 10a before bonding, resulting in poor workability.
The reason why the copper purity of the core material 1 is 99.99% by mass or more is to ensure high conductivity of copper.

The thinner the thickness t of the coating layer 2, the lower the hardness of the ball b and the lower the possibility of damage to the Si chip (electrode a). However, if it is too thin, the copper of the core material 1 will be lost during stitch bond bonding. The degree of exposure becomes large, and only the stitch bond bonding property of a copper wire not having the coating layer 2 can be exhibited. For example, as can be understood from the experimental results of Examples and Comparative Examples described later, there is a possibility that two or more machine stops may occur. For this reason, the thickness t of the coating layer 2 shall be 0.02 micrometer or more from the experimental result of the Example and a comparative example.
When ball bonding is performed at a stage temperature as low as about 150 ° C., the thickness t is 0.04 μm or more, more preferably 0.04 μm or more, based on the results of continuous bonding test. When the stage temperature is lowered, the load required for stitch bond bonding increases, and when the thickness t of the coating layer 2 is in the range of 0.02 μm or more to less than 0.04 μm, the degree of exposure of the core copper increases and the continuous bonding property increases. This is because may be damaged.
On the other hand, if the coating layer 2 is thick, the hardness of the ball b increases and the possibility of damage to the Si chip (electrode a) increases. For this reason, the thickness t of the coating layer 2 shall be 0.09 micrometer or less from the experimental result of a postscript Example and a comparative example.

The reason why the coating metal is gold, platinum, palladium and silver is that these noble metals are generally used as coating materials for electronic materials and are relatively easy to obtain. Similar to the copper of the core material 1, the purity is preferably 99.99 mass% or more. The covering layer 2 may be formed of one of these noble metals, but may be a multilayer of two or more metals.

From the above, this bonding wire P is configured with a bonding wire P having a diameter L of 12 μm or more and 50.8 μm or less for connecting the electrode a of the integrated circuit element and the conductor wiring c of the circuit wiring board by the ball bonding method. The core material 1 is made of copper having a purity of 99.99% by mass or more, and the coating layer 2 having a thickness of 0.02 to 0.09 μm is formed on the entire outer periphery of the core material 1 with at least one of gold, platinum, palladium, and silver. The structure which formed is adopted.
In this configuration, in the bonding wire for ball bonding at a stage temperature as low as about 150 ° C., the thickness t of the coating layer is set to 0.04 to 0.09 μm.

Various methods for manufacturing the bonding wire P having these configurations can be adopted. For example, gold, platinum, palladium, and silver 1 are formed on the entire outer surface of the core 1 made of copper having a purity of 99.99% by mass or more. After forming the coating layer 2 of seeds or more, the coated wire is subjected to diffusion heat treatment to improve the adhesion between the core material and the coating layer, and then drawn to a wire diameter of 12 μm or more and 50.8 μm or less. It is possible to adopt a configuration in which the tempering heat treatment is performed so that the thickness becomes equal to or greater than%, and the thickness t of the coating layer 2 is 0.02 to 0.09 μm.

In these structures, it can be considered that palladium or platinum having a melting point higher than that of the copper is more preferable for the coating layer 2.
As described above, it has been confirmed that if the coating layer 2 is formed of an oxidation resistant metal having a melting point higher than that of the copper of the core material 1, the sphericity of the ball b is increased. When the copper melts into a true sphere by discharge, the melted portion creeps up the wire directly above due to the surface tension, but when the coating layer 2 is also melted, the creeping is worsened I thought it was difficult to become a true sphere. This is prominent when the coating layer 2 has a thickness of 0.02 to 0.09 μm.
Further, in the case of palladium and platinum, the margin of the condition for making the ball b a true sphere becomes wider with palladium. This is considered because the melting point of palladium is closer to the melting point of copper than the melting point of platinum. That is, when platinum is used for the coating layer 2, there is a time difference until the platinum melts after the copper is melted by discharge. If palladium is used for the coating layer 2, the difference in melting time is small, so that it is more likely to be a true sphere.

Incidentally, the melting point of the pure copper is 1083 ° C., the melting point of gold is 1064 ° C., the melting point of palladium is 1554 ° C., the melting point of silver is 962 ° C., and the melting point of platinum is 1772 ° C.
Further, when the thickness t of the coating layer 2 is 0.04 μm or more, machine troubles are reduced even at a stage temperature of 150 ° C. However, palladium is particularly adopted for the coating layer 2 and the thickness t of the coating layer 2 is 0.05 μm or more. It has also been confirmed that no machine trouble occurs even when the stage temperature is set at a lower temperature of 135 ° C. In the case of the palladium coating layer 2, it is considered that this is based on a synergistic effect with the fact that the difference in melting time between copper and palladium is small and the sphere is more likely to become a true sphere.

Furthermore, if the copper purity of the core material is 99.999% by mass or more, the possibility of damage to the Si chip (electrode a) decreases as can be understood from the experimental results of Examples and Comparative Examples described later. This is because the FAB hardness can be further reduced by setting the copper purity to 99.999 mass% or more.

Furthermore, the coating layer 2 is formed by known means such as electrolytic plating, electroless plating, and vapor deposition. Generally, the wire P is formed by sequentially passing a copper rod having a large wire diameter through a tool called a die. Since it is finished to a predetermined wire diameter, the coating layer 2 is formed by the above means with an appropriate wire diameter in the middle of this process. At this time, the wire diameter of the core material 1 at the time of coating is determined by workability and cost. The coated wire coated with a metal such as palladium on the entire outer surface is subjected to diffusion heat treatment at 200 to 500 ° C. (temperature of the coated wire) to improve the adhesion between the core material 1 and the coating layer 2, and then the wire diameter is 12 μm or more. The thickness t of the coating layer 2 can be set to 0.02 to 0.09 μm by performing a tempering heat treatment so that the wire is drawn to 50.8 μm or less and further has a tensile elongation of 8% or more.
The reason why the tensile elongation is 8% or more is to improve the stitch bondability and obtain more stable bonding properties.
At this time, if the coating layer is made of palladium, the copper purity of the core material can be 99.999% by mass or more, and the thickness t of the coating layer can be 0.05 to 0.09 μm.

Since the present invention is configured as described above, a bonding wire obtained by coating a noble metal on pure copper having stable bonding strength can be obtained.

It is an explanatory diagram of the ball bonding connection method, (a) ~ (h) is the middle Sectional drawing of the bonding wire which concerns on this invention Relationship diagram of Auger peak intensity and coating layer (plating layer) depth

Examples 1 to 45 and Comparative Examples 1 to 13 shown in Table 1 were manufactured, and the degree of damage of the Si chip (electrode a) at the first bonding portion of the bonding wire P and the continuous bonding property test were performed.
That is, first, a copper wire having a diameter of 0.2 to 0.8 mm made of pure copper having a copper purity of 99.99% by mass (in Table 1: 4N) and pure copper having a copper purity of 99.999% by mass (in Table 1: 5N) is used. Prepare and coat the copper wires with noble metals such as Au, Pd, Pt and Ag by electrolytic plating, rewind the coated wires, pass through an annealing furnace, and then rewind them with a take-up reel. Diffusion heat treatment was performed. As the annealing furnace, an electric furnace having a furnace core tube having a furnace length of 1 m was used, and nitrogen gas was allowed to flow through the furnace core tube. The furnace temperature was 500 ° C. or more and 800 ° C. or less, the temperature of the coated wire was 200 to 500 ° C., and the traveling speed of the coated wire was 5 to 60 m / min. After performing the above diffusion heat treatment to improve the adhesion between the copper wire (core material) 1 and the coating layer 2, the wire is drawn to a wire diameter of 15 to 50.8 μm, and the tensile elongation is 8% or more. Conditioning heat treatment was performed to obtain bonding wires P (Examples 1 to 45 and Comparative Examples 1 to 13) having a thickness t of the coating layer 2 of 0.001 to 0.112 μm. In Table 1, an example showing two kinds of noble metals is a double coating of the two kinds of metals with the same thickness.

Next, Table 1 shows the results of the following evaluation on each bonding wire P.
“Thickness of coating layer (surface coating layer) 2”:
Measured by Auger electron spectroscopy. Sputtering was performed from the surface of the coating layer with Ar, and the spectrum was measured for each unit time sputtering. The unit in the depth direction was the sputtering time, and the distance in the depth direction of the target material was calculated in terms of SiO ₂ from the sputtering rate of the standard sample SiO ₂ measured in advance. For the coating layer, as shown in FIG. 3, in the graph in which the Auger peak intensity of the target coating layer element is on the vertical axis and the depth is on the horizontal axis, the distance from the position where the Auger peak intensity is 84% to the position of 16% is It was set as the thickness of the coating layer.
"Purity of core material (copper)":
Cu purity was determined by measuring the concentration of all elements by GDMS (glow discharge mass) analysis.
“Continuous bonding” (1):
Bonding machine performs 10,000 continuous bonding, "A" if no machine stop occurs, "B" if one machine stop occurs, "D" if more than one machine stop occurs It was. At this time, if the stage temperature is lowered, the continuous bonding becomes difficult. Therefore, it was performed at two levels of 200 ° C. (± 5 ° C.) and 150 ° C. (± 5 ° C.).
“Si chip damage at 1st junction” (2):
After bonding, in order to evaluate the Si chip damage directly under the 1st ball joint, the ball joint a and the electrode film were dissolved with aqua regia, and the cracks of the Si chip were observed with a light microscope and SEM. At this time, when observing 100 joints, one or no minute pits of 5 μm or less are seen as “A”, and when two or more minute pits of 5 μm or less are observed, it is practically harmful. Therefore, “B” was given as “D” when cracks of 5 μm or more were observed.
"Comprehensive evaluation":
The evaluation of (1) is A for both 200 ° C. and 150 ° C., and the evaluation of (2) is “A”, the evaluation of (1) is A for both 200 ° C. and 150 ° C., and (2 ) Was evaluated as “B” when B was evaluated and “1” was evaluated as A when the evaluation was 200 ° C., and “C” when B was evaluated at 150 ° C. In addition, even one having D is “D” because it is a practical problem.

From this test result, when the coating layer thickness t is less than 0.02 μm, both continuous bonding properties at 200 ° C. and 150 ° C. are reduced (Comparative Examples 1, 2, 4, 5, 9, 10, 12). It can be understood that when the thickness is close to 0.02 μm, the former continuous bonding property is satisfactory (Comparative Example 2), and when it is 0.02 μm or more, both continuous bonding properties are satisfactory (Examples 1 to 45). Comparative Examples 3, 6-8, 11, 13).
On the other hand, when the coating layer thickness t exceeds 0.09 μm, the ball b becomes hard and damage to the Si chip (electrode a) is recognized (Comparative Examples 3, 6 to 8, 11, 13).
In addition, when pure copper 5N was used and the coating layer thickness t was 0.04 μm or more (Examples 5, 12, 13, 16, 20, 27, 32, 34, 37, 44, 45), the overall evaluation was It can be seen that even when the stage temperature is as low as about 150 ° C., the continuous bonding property is good and effective for ensuring the continuous bonding property. In particular, it is remarkable when the coating layer thickness t is 0.05 μm or more (Examples 12, 13, and 16).

P Bonding wire 1 Core material 2 Coating layer a Integrated circuit element electrode b Bonding ball c Circuit wiring board conductor wiring

Claims (7)

A bonding wire (P) having a wire diameter (L) of 12 μm or more and 50.8 μm or less for connecting an electrode (a) of an integrated circuit element and a conductor wiring (c) of a circuit wiring board by a ball bonding method, (1) is made of copper having a purity of 99.99% by mass or more, and a thickness (t) of 0.02 to 0.09 μm on the entire outer periphery of the core material (1) by one or more of gold, platinum, palladium and silver. A bonding wire characterized in that a coating layer (2) is formed. The bonding wire according to claim 1, wherein the thickness (t) of the coating layer (2) is 0.04 to 0.09 μm. The bonding wire according to claim 1 or 2, wherein the coating layer (2) is made of palladium or platinum whose melting point is higher than that of the copper. The bonding wire according to claim 1, wherein the coating layer (2) is palladium whose melting point is higher than that of the copper, and the thickness (t) is 0.05 to 0.09 μm. The bonding wire according to any one of claims 1 to 4, wherein the core material (1) has a copper purity of 99.999% by mass or more. 5. The method of manufacturing a bonding wire (P) according to claim 1, wherein the electrode (a) of the integrated circuit element and the conductor wiring (c) of the circuit wiring board are connected by a ball bonding method. Then, a coating layer (2) made of one or more of gold, platinum, palladium and silver is formed on the entire outer periphery of the core material (1) made of copper having a purity of 99.99% by mass or more, and the coated wire is subjected to diffusion heat treatment. After improving the adhesion between the core material (1) and the coating layer (2), the wire diameter (L) is drawn to 12 μm or more and 50.8 μm or less, and the tensile elongation is 8% or more. A method for producing a bonding wire, characterized by performing tempering heat treatment. The copper purity of the core material (1) is 99.999% by mass or more, and the coating layer (2) is made of palladium and has a thickness (t) of 0.05 to 0.09 μm. Item 7. A method for producing a bonding wire according to Item 6.

Images