JP2005103645A - Solder ball and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a solder ball having both the smooth surface and the clean surface, while having high sphericity and dimensional accuracy which are demanded for solder ball, and its manufacturing method. <P>SOLUTION: The solder ball consists mainly of Sn and contains 1.0-4.5mass% Ag and also 0.3-1.2mass% Cu, and, as other elements, 0.006-0.1mass% in total of one or more elements of Ge, Ni, P, Mn, Au, Pd, Pt, S, Bi, Sb and In. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a solder ball used for a semiconductor device or the like, and particularly to a solder ball having a smooth surface shape and excellent transportability and mountability, and a method for manufacturing the same.

BGA (Ball Grid Array), a semiconductor device packaging technology, is widely used. In order to form a solder pad by forming bumps on a carrier and finally bond to a substrate, this BGA is used to accurately connect hundreds, often thousands of solder balls per array on the carrier. Moreover, it can be mounted on the same plane. For this reason, solder balls are often required to have a precision close to a true sphere with a sphericity of 0.95 or more, a smooth surface, and a dimensional precision within ± 10 μm with a ball diameter of 100 to 1000 μm.
As a method for producing solder balls used in this way, a ball-in-oil method is generally used. This method is obtained by heating and dissolving finely cut solder in oil and then cooling. However, this method requires a process of manufacturing fine solder pieces and a cleaning process, which is uneconomical.

Recently, as a more economical method instead of this method, pressure and vibration are applied to the molten metal in the crucible to extrude the molten metal from the orifice provided in the lower part of the crucible, and the molten metal dropped from the orifice is rapidly cooled and solidified. A method of manufacturing solder balls is disclosed in US Pat. No. 5,266,098 and is called the uniform droplet spraying method. Based on this method, the present inventors have made improvements to produce solder balls and have proposed as Japanese Patent Application No. 2000-70535.
In the uniform droplet spraying device proposed in Japanese Patent Application No. 2000-70535, a vibration device using a piezo element or the like is installed at the top of the device, and there is a furnace for holding the molten metal below. Below the furnace is a hole called an orifice, and outside the orifice is a recovery chamber that controls the division and solidification atmosphere of the melt extruded from the orifice. A structure for collecting solder balls at the bottom of the chamber is proposed.
JP 2000-70535 A

Here, the above-described uniform droplet spraying apparatus will be described more specifically with reference to FIG. 1 in detail.
Vibration is generated by the piezo element (1), and the vibration is transmitted to the molten metal (4) through the vibration rod (3). Gas is supplied to the dissolution chamber (5) from the gas supply port (2) and is pressurized. Since the melt extruded from the orifice (6) by the pressure is vibrated, it is divided at regular intervals to form droplets (8).
In the electrode plate (7), a potential is applied between the electrode plate and the molten metal, thereby charging the droplets to the same charge and preventing the droplets from contacting each other. These droplets are spheroidized by their surface tension while falling in the chamber (9), and are cooled and solidified during flight in the gas atmosphere.
An inert gas is supplied into the recovery chamber (9) from the gas supply port (11) to prevent oxidation of droplets and the like. The pressure in the recovery chamber is lower than that in the dissolution chamber (5). Finally, it is collected at the bottom of the chamber as a spherical solidified body (10).

In the apparatus shown in FIG. 1, the furnace and the recovery chamber can control the atmosphere and reduce or increase the pressure. By making the furnace pressure higher than the recovery chamber, the molten metal is ejected from the orifice.
The vibration generated by the piezo element or the like is added to the molten metal in a furnace using a rod made of stainless steel or ceramic. The liquid column to which the vibration is applied has a flow rate at regular intervals. For this reason, the liquid column is cut at regular intervals to form droplets with a uniform volume.
In addition, uniform volume droplets become spherical due to their surface tension. Thereafter, the solder balls are solidified to obtain uniform spherical solder balls.
In addition, an electrode is installed around the liquid column, and a potential difference is provided between the liquid column and the electrode. As a result, the cut droplets are charged with the same sign, and the droplets repel each other, thereby preventing the shape and particle size from being poor due to contact during solidification. This method makes it possible to manufacture solder balls with good sphericity and dimensional accuracy with high productivity.

When the present inventors manufactured Sn-Ag-Cu-based solder balls, which are attracting attention as lead-free materials, by the above-described uniform droplet spraying method, there is a difference in the unevenness of the surface shape of the solder balls. occured.
In solder balls, the surface shape is very important. There are two main reasons for this.

The first reason is that, for example, in an apparatus for mounting solder balls on a BGA package, it is required to roll without stagnation in order to continuously supply solder balls. If the rolling is poor, the supply of solder balls stops in the middle of the apparatus, so adjustment of this supply part is necessary frequently, and the production efficiency is significantly reduced.
The second reason is that, for example, as a method of mounting solder balls on a BGA package with high accuracy, a nozzle portion arranged with high accuracy in accordance with the BGA package is evacuated, and the solder balls are adsorbed to this portion, and a predetermined position There is a way to arrange. At this time, if the unevenness of the surface of the solder ball is excessive, it cannot be adsorbed, a terminal on which the solder ball is not mounted is formed, and the BGA package itself becomes defective.

In addition, the solder ball is also required to have a clean surface.
If the surface of the solder ball is dirty, the oxide layer cannot be removed even if flux is used, and the bonding strength is reduced. Considering this, conventional cooling in oil is not preferable in terms of quality stability and productivity even if washing is performed.
Although it is technically possible to cool in a cryogenic environment such as in liquid nitrogen, a solder ball mainly composed of Sn is very brittle because it becomes αSn, which is a stable phase at a very low temperature. become. Since this brittle solder ball is chipped by classification or the like, a ball with high sphericity cannot be obtained.
An object of the present invention is to provide a solder ball having a smooth surface shape and a clean surface and a method for producing the same while maintaining high sphericity and dimensional accuracy required for the solder ball.

  As a result of intensive studies on the above-mentioned solder ball surface shape (unevenness), the present inventors have found that the smoothness of the solder ball surface shape in the Sn-Ag-Cu system, which is attracting attention as a lead-free material, is a trace element in the alloy. In order to find that there is a correlation in the content, and to obtain a solder ball having a smooth surface, the inventors have found that there is an appropriate amount of trace element added, and have reached the present invention.

  That is, the present invention mainly comprises Sn, contains 1.0 to 4.5 mass% Ag and 0.3 to 1.2 mass% Cu, and other elements such as Ge, Ni, P, Mn, Au, It is a solder ball in which one or more of Pd, Pt, S, Bi, Sb, and In is 0.006 to 0.1% by mass in total.

In the present invention, the above-mentioned solder ball is a solder ball obtained by cooling and solidifying a spheroidized droplet in a gas atmosphere.
The production method of the present invention mainly comprises Sn, contains 1.0 to 4.5 mass% Ag and 0.3 to 1.2 mass% Cu, and other elements include Ge, Ni, P, Mn , Au, Pd, Pt, S, Bi, Sb, In at least one kind of two or more kinds is held in the crucible with a total amount of solder of 0.006 to 0.1% by mass, and pressure and vibration are applied to the molten metal. It is a method for producing a solder ball that is applied and extrudes molten metal from an orifice provided at the bottom of the crucible, and cools and solidifies the spherical droplet dropped from the orifice in a gas atmosphere.

  A solder ball having a smooth surface shape becomes easy to handle when mounted on a BGA package, for example, and contributes to an improvement in productivity and an improvement in acceptance rate. Therefore, the solder ball and the manufacturing method thereof of the present invention are very important industrially.

The most important point of the present invention is to contain 0.006% by mass or more of a specific element other than Sn, Ag, and Cu when Sn—Ag—Cu is a basic element.
In the present invention, specific elements other than Sn, Ag, and Cu constituting the Sn—Ag—Cu-based eutectic are basically Ge, Ni, P, Mn, Au, and Pd in terms of improving the surface shape. , Pt, S, Bi, Sb, In are selected.
Among these elements, Ge has a beneficial effect such as improving oxidation resistance. Ni, P, Mn, Au, Pd, Pt, S, In, and the like are also preferable because they have the effect of lowering the melting point and the effect of increasing the bonding strength, and can achieve solder characteristics suitable for the application. Sb is an element that also has the effect of improving the strength.

  FIG. 2 shows an example of a form of a ball having a smooth surface shape, while FIG. 3 shows a form of a ball having a very uneven surface shape.

The inventor presumes the reason why the surface shape becomes uneven as follows. When observing the metal structure of the solder ball having an uneven surface shape, it is considered that it is composed of an Ag-Sn or Cu-Sn based intermetallic compound phase dispersed without a clear crystal grain boundary and with a β-Sn single crystal. It is a simple metal structure.
Moreover, even if it is a composition system which is not a eutectic composition, when the metal structure has no dendrite, the surface shape becomes uneven.
On the other hand, when the metal structure of the solder balls having a smooth surface is observed, dendrite is recognized in a composition system that is considered to be β-Sn polycrystal and is not a eutectic composition.
In the present invention, it is considered that the solid-liquid coexistence region is widened because it contains a large amount of a specific element, and at the same time the melting point is lowered, so that it can be gradually solidified. It is presumed that this makes it possible to generate a plurality of crystal grains and to grow dendrite.

  When the solder is rapidly solidified into a ball shape, the shape change due to volume shrinkage at the time of solidification has no time to correct it to a spherical shape by surface tension, so that solidification is completed, resulting in an uneven shape. On the other hand, if solidification progresses relatively slowly, even if some solidify and volume shrinkage occurs, and there are irregularities on the surface of the droplet, we think that there is time to correct the shape to a curved surface again by surface tension Yes. In addition, it is assumed that the surface acts as a nucleus for heterogeneous nucleation, and since this nucleation part takes a smooth shape, it is considered that the proportion of the smooth surface inevitably increases in the polycrystalline body. .

In the case of the present invention in which the basic element is Sn-Ag-Cu, the lower limit of the specific element is 0.006% by mass. In this system, since both Ag and Cu are contained in Sn, an effect close to that when added as a trace element is obtained, and the effect of smoothing the surface even with a small amount is recognized.
In the present invention in which the basic element is Sn-Ag-Cu, the upper limit of the additive element is 0.1% by mass. This is because, in this system, when the specific element is contained in an amount of 0.1% by mass, there is no difference in surface shape improvement from the case where the specific element is contained beyond this.

In the present invention, the basic element is mainly composed of Sn and is 1.0 to 4.5 Ag mass% Ag and 0.3 to 1.2 mass% Cu. The reason for this is the “Research and Development Project for Lead-Free Solder Standardization” results report presentation, June 22, 2000, Shibaura Institute of Technology Sponsor: Japan Welding Association, Planning: Lead-free solder standardization This is because, as reported by the Research and Development Project Committee, this basic composition system is particularly excellent in properties as a solder.
For example, by adding an appropriate amount of Ag or Cu to Sn, it is possible to optimize the melting point and improve the wettability.

The solder ball of the present invention applies pressure and vibration to the molten metal in the crucible using, for example, the uniform droplet spraying apparatus shown in FIG. 1, and extrudes the molten metal from the orifice provided at the bottom of the crucible, and drops from the orifice. The molten metal may be spheroidized and cooled and solidified in a gas atmosphere. A gas atmosphere is an inert gas atmosphere of nitrogen, argon or carbon dioxide, or a clean surface by cooling in a reducing gas atmosphere in which these gases are partially mixed with H ₂ or CO gas. The solder ball which has can be obtained.

In addition, as a result of the study by the present inventors, in the apparatus of uniform droplet spraying method that employs cooling in oil or the method in which solder pieces are heated and melted and cooled in oil, heated oil is used to correct the shape. It can be made spherical by surface tension and can be gradually solidified by creating a temperature gradient in the oil. That is, the solidification rate can be slowed.
On the other hand, in the apparatus of the uniform droplet method that employs cooling in a gas atmosphere, since it is solidified spherically in the air, there is no need to correct the shape, but solidification proceeds very rapidly. Therefore, the problem of surface irregularities, which is a problem in the present invention, becomes significant. That is, when the present inventor performs cooling in a gas atmosphere in order to obtain a clean surface, it is very important to contain an appropriate amount of a specific element in order to smooth the surface shape.
Thus, since it is estimated that the unevenness | corrugation of a solder ball becomes remarkable when a solidification rate is fast, application of this invention is especially effective with respect to the air-cooled solidification solder ball with a quick cooling rate.

Examples of the present invention will be described. The manufacturing apparatus using the uniform droplet spraying method shown in FIG. 1 holds the molten solder in a crucible, applies pressure and vibration to the molten metal, and extrudes the molten metal from an orifice provided at the bottom of the crucible. Solder balls with a diameter of 600 μm were manufactured by cooling and solidifying spherical droplets dropped from the orifice in a nitrogen gas atmosphere.
Table 1 lists the tested compositions and summarizes the results of experiments conducted on the correlation between each composition and the surface shape. The solder ball evaluation method will be described. The surface shape as shown in FIG. A surface having an uneven surface as shown in FIG.

  No. 1 to 10 are the results for an alloy system having Sn—Ag—Cu as a basic element. No. containing 0.006% by mass or more of the specific element of the present invention. The surface shapes of 1 to 5 were smooth. No. 1, 2, 3, 4 and 5 are respectively added with Au, Pd, Ge, In, Ge and In. On the other hand, the content of the specific element is less than 0.006. The surface shape of 6-10 became uneven. From this result, it can be seen that in order to smooth the surface shape in a system containing Sn—Ag—Cu as a basic element, it is necessary to contain 0.006% by mass or more of a specific element.

  300 solder balls of the present invention shown in Table 1 were prepared, and surface cleanness was confirmed visually and with an electron microscope to confirm that the surface cleanliness was maintained. Next, the sphericity and dimensional accuracy were measured. The sphericity was 0.95 to 0.97, and the dimensional accuracy was in the range of 600 μm ± 7 μm.

It is a schematic diagram which shows an example of the apparatus which manufactures the solder ball of this invention. It is a microscope picture which shows the surface form of the solder ball of this invention. It is a microscope picture which shows the surface form of the solder ball of a comparative material.

Explanation of symbols

1. 1. Piezo element; 2. Gas supply port Vibrating bar, 4. Molten metal, 5. Lysis chamber, 6. 6. orifice, Electrode plate, 8. Droplet, 9. Chamber, 10. 10. spherical coagulum, Gas supply port

Claims (3)

Mainly composed of Sn, containing 1.0 to 4.5 mass% Ag and 0.3 to 1.2 mass% Cu, and other elements such as Ge, Ni, P, Mn, Au, Pd, Pt, S Solder balls, wherein one or more of Bi, Sb, and In is 0.006 to 0.1% by mass in total. The solder ball according to claim 1, wherein the spheroidized droplets are cooled and solidified in a gas atmosphere. Mainly composed of Sn, containing 1.0 to 4.5 mass% Ag and 0.3 to 1.2 mass% Cu, and other elements such as Ge, Ni, P, Mn, Au, Pd, Pt, S , Bi, Sb, In, or a total of 0.006 to 0.1% by mass of a molten solder is held in a crucible, and pressure and vibration are applied to the molten metal, and the bottom of the crucible A method for producing a solder ball, comprising: extruding a molten metal from an orifice provided in the nozzle, and cooling and solidifying a spherical droplet dropped from the orifice in a gas atmosphere.