JPH11145192A - Method for forming solder bump - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for forming a soldered bump, without conductive failures on the opposite face to the electrode forming face of a substrate in a simple process. SOLUTION: In forming a soldered bump 5' conducted with an electrode 12 formed on a substrate 11 via a through-hole 11a of the substrate 11 on the opposite face to the electrode 12 face, flux 14a with which conductive metallic powder with satisfactory solder wettability is applied to a soldered ball 5, then the soldered ball 5 is moved on the through-hole 11a on the opposite face to the forming face of the electrode 12, and the substrate 11 is heated so that the soldered ball 5 can be fused, and the electrode 12 can be energized through the through-hole 11a with the soldered ball 5. Even when a clearance is present between the soldered ball 5 and the electrode 12, the soldered ball 5 is fused, and then the fused solder is allowed to reach the electrode surface after being introduced by the metallic powder with satisfactory solder wettability, so that conductive failures can be prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for forming a solder bump on an electrode of an electronic component or a substrate.

[0002]

2. Description of the Related Art As a method for mounting electronic components, a method using solder bumps is known. In this method, a solder bump, which is a protruding electrode of solder, is formed in advance on an electrode of an electronic component, and the solder bump is soldered to an electrode such as a substrate. As a method of forming a solder bump, a method of transferring a solder ball onto an electrode of an electronic component or the like has been conventionally used.

[0003] By the way, CSP (chip size p)
In some cases, the surface on which the electrodes are formed and the surface on which the solder bumps are formed are not the same surface of the substrate.
That is, an electrode is formed on one surface of a tape-shaped substrate constituting a package, and a solder ball is transferred and melted on a surface opposite to the surface on which the electrode is formed, and a through hole of the substrate provided at the electrode position is provided. Then, the molten solder is conducted to the electrodes through the electrodes to form solder bumps. Hereinafter, a conventional solder bump forming method will be described with reference to the drawings. FIG. 4 is a partial sectional view of a conventional tape-shaped substrate.

In FIG. 4, electrodes 2 are formed on the lower surface of a tape-shaped substrate 1. A through hole 3 is provided at the position of the electrode 2 on the substrate 1, and a solder ball 5 coated with a flux 4 is mounted on the through hole 3. Since the diameter of the solder ball 5 is set to be larger than the diameter of the through hole 3, the solder ball 5 cannot enter the through hole 3. Therefore, the solder ball 5 contacts the electrode 2 because of the thickness of the substrate 1. Are separated by gaps.

[0005]

Thereafter, the substrate 1 is sent to the reflow and heated, but since the solder balls 5 are separated from the electrodes 2, the molten solder remains on the electrodes 2 even after the solder balls 5 are melted. There is a case where conduction failure occurs in which the electrode 2 does not reach the upper surface and solidifies without being electrically connected to the electrode 2. Therefore, as a countermeasure to prevent the occurrence of the conduction failure, the surface of the electrode 2 in the through hole 3 is plated with metal before the transfer of the solder ball 5, or the solder ball is filled with cream solder on the electrode 2. For example, a step is added prior to the transfer of the solder ball 5 such as transferring the solder ball 5.

In the case where the solder bumps are formed on the surface of the substrate opposite to the surface on which the electrodes are formed, additional steps such as metal plating and cream solder application are required to prevent conduction failure.
There was a problem that the process became complicated.

Accordingly, an object of the present invention is to provide a solder bump forming method capable of forming a solder bump having no conduction failure on a surface opposite to an electrode forming surface of a substrate by a simple process.

[0008]

According to a first aspect of the present invention, there is provided a method for forming a solder bump on a surface opposite to an electrode forming surface of a substrate, the solder bump being electrically connected to the electrode through a through hole in the substrate. A method of applying a flux mixed with a conductive metal powder having good solder wettability to a solder ball; and transferring the solder ball coated with the flux to a substrate surface opposite to the electrode forming surface. And mounting the substrate on which the solder balls have been transferred to melt the solder balls so that the electrodes are electrically connected to the solder balls through the through holes.

According to a second aspect of the present invention, there is provided a method of forming a solder bump according to the first aspect of the present invention, wherein 80% or more of the metal powder is a solder particle having a particle size of 5 to 25 μm. I did it.

According to a third aspect of the present invention, there is provided the method of forming a solder bump according to the first aspect, wherein the metal powder has a particle diameter of 1 to 10 μm in an amount of 80% or more of the total amount. The metal powder had good solder wettability.

According to the invention described in each claim, a flux in which conductive metal powder having good solder wettability is mixed is applied to the solder ball, and the solder ball is mounted on the substrate surface opposite to the electrode forming surface. By this, even if there is a gap between the solder ball and the electrode surface, after the solder ball is melted, the molten solder is guided by the metal powder in the flux, reaches the electrode surface, and is securely joined to the electrode. The conduction failure can be prevented.

[0012]

Embodiments of the present invention will now be described with reference to the drawings. FIG. 1 is a partial cross-sectional view of a flux transfer portion of a solder ball transfer device according to an embodiment of the present invention, and FIGS. 2A and 2B are enlarged cross-sectional views of the flux transfer portion of the solder ball transfer device. FIGS. 3A, 3B, and 3C are partial cross-sectional views of the tape-shaped substrate.

First, a method of applying a flux to a solder ball will be described with reference to FIGS. The flux transfer unit 6 of the solder ball transfer device shown in FIG. 1 includes a container 13 and a squeegee unit 9. The container 13 has a horizontal flat bottom surface and stores the flux 14 on the bottom surface of the individual. The squeegee unit 9 includes two cylinders 8 for moving the two squeegees 7 up and down. 2
As shown in FIG. 1, the two squeegees 7 are inclined in different directions, and when the rod of the cylinder 8 is lowered, the gap between the lower end of the squeegee 7 and the bottom of the container 13 is maintained at a predetermined gap d. It is. When the squeegee unit 9 reciprocates horizontally in this state, the two squeegees 7
The flux 4 stored therein is applied on the bottom surface of the container 5 with a predetermined thickness d equal to the gap d described above.

Next, the flux 14 will be described. The flux 14 is obtained by mixing a conventionally used flux with a metal powder having good conductivity and good solder wettability. The ratio of the metal powder in the flux 14 is desirably 50 VOL% or more by volume ratio, and a metal powder such as gold, silver, or copper is used as the metal powder. Further, solder particles in which the solder is made into fine particles can be used as the metal powder.

In the present embodiment, since the method of applying the flux 14 to the solder balls uses a method by transfer, the particle size of the metal powder mixed in the flux 14 is set within a predetermined range so that good transfer can be performed. Need to fit. 80% of the total amount of gold, silver, copper, etc.
The above metal powder having a particle size in the range of 1 to 10 μm is used, and when solder particles are used, 80% of the total amount is used.
The above solder particles having a particle size in the range of 5 to 25 μm are used.

Next, application of the flux 14 to the solder balls will be described. As shown in FIG. 1, the suction tool 10 in which the solder balls 5 are vacuum-sucked into the suction holes 10a is positioned on the container 13 on which the flux 14 is applied. Next, as shown in FIG.
3 is lowered from the bottom surface to the position of height h. Height h
Is a height at which the lower end of the solder ball 5 does not come into contact with the bottom surface of the container 13 and is determined by actual trial as a height at which an appropriate amount of flux 14 is transferred to the lower surface of the solder ball 5 It is.

Next, the suction tool 10 is raised from this state. Then, the flux 14 applied to the bottom surface of the container 13 with a predetermined thickness d is transferred to the lower surface of the solder ball 5 and rises while adhering. In this way, as shown in FIG. 2B, the flux 14 is applied to the lower surface of the solder ball 5 by transfer.

Next, the transfer of the solder balls 5 onto the substrate will be described. As shown in FIG. 3A, an electrode 12 is formed on the lower surface of the substrate 11, and a solder bump is formed on a surface opposite to the electrode forming surface, that is, on the upper surface of the substrate 11 through a through hole 11a. You. The suction tool 10 holding the solder balls 5 is moved onto the substrate 11, and then the suction tool 10 is lowered onto the through-hole 11a. Next, the vacuum suction of the solder ball 5 is released and the suction tool 10 is raised, so that the solder ball 5 is transferred to the position of the through hole 11 a of the substrate 11. At this time, as shown in FIG. 3B, the lower end of the solder ball 5 on the through hole 11a is
The flux 14 applied to the lower surface of the solder ball 5 is in contact with the surface of the electrode 12 although it does not contact the upper surface of the electrode 12 due to the thickness of 1.

Next, the substrate 11 on which the solder balls 5 have been transferred
Is sent to a reflow device and heated. As a result, the solder balls 5 are in a molten state. At this time, since the flux 14 contains a large amount of metal powder having good solder wettability, the molten solder is guided by these metal powders to cause a gap between the metal powders. Move downward while infiltrating. Then, after reaching the surface of the electrode 12, the molten solder is cooled and solidified to be joined to the surface of the electrode 12.

At this time, depending on the components in the flux 14,
The oxide film on the surface of the electrode 12 is removed, and good soldering is performed. As shown in FIG.
A solder bump 5 'is formed via 1a. Further, the metal powder mixed in the flux 14 is replaced with the solder bump 5 ′.
However, since these metal powders are conductive, they do not hinder conduction between the electrodes 12 and the solder bumps 5 '.

[0021]

According to the present invention, a solder ball is coated with a flux mixed with conductive metal powder having good solder wettability, and the solder ball is mounted on the substrate surface opposite to the electrode forming surface. Therefore, even when the solder ball is not in direct contact with the electrode, when the solder ball is melted, the molten solder infiltrates between the metal powders in the flux and is guided to the electrode surface, so that the molten solder remains on the electrode surface. It is possible to form a good solder bump which is reliably reached and joined and has no conduction failure.

[Brief description of the drawings]

FIG. 1 is a partial cross-sectional view of a flux transfer unit of a solder ball transfer device according to an embodiment of the present invention.

FIG. 2 (a) is an enlarged cross-sectional view of a flux transfer unit of the solder ball transfer device according to one embodiment of the present invention; and (b) a flux transfer unit of the solder ball transfer device according to one embodiment of the present invention. Enlarged sectional view of

3A is a partial cross-sectional view of a tape-shaped substrate according to an embodiment of the present invention. FIG. 3B is a partial cross-sectional view of a tape-shaped substrate according to an embodiment of the present invention. Sectional view of a tape-shaped substrate in the form of

FIG. 4 is a partial sectional view of a conventional tape-shaped substrate.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1, 11 Substrate 2, 12 Electrode 4, 14 Flux 5 Solder ball 6 Flux transfer part 9 Squeegee unit 10 Suction tool 10a Suction hole 11a Through hole

Claims (3)

[Claims] 1. A method of forming a solder bump on a surface opposite to an electrode forming surface of a substrate through a through hole of the substrate and forming a solder bump electrically connected to the electrode, the method comprising: Applying a flux mixed with good metal powder, transferring the solder ball coated with the flux to the substrate surface opposite to the electrode forming surface, and removing the substrate on which the solder ball is transferred. Heating the solder ball to melt the solder ball to make the electrode and the solder ball conductive through the through hole. 2. The method according to claim 1, wherein 80% or more of the total amount of the metal powder is 5%.
2. The method according to claim 1, wherein the solder bumps have a particle size of about 25 [mu] m. 3. The method according to claim 1, wherein 80% or more of the total amount of the metal powder is 1%.
2. The method for forming a solder bump according to claim 1, wherein the metal powder has a particle diameter of 10 to 10 [mu] m and has good solder wettability.