JPH09326412A - How to attach the solder balls - Google Patents

Abstract

The present invention relates to a solder ball attaching method, and an object thereof is to provide a solder ball attaching method capable of surely fusing a solder ball to a circuit board. SOLUTION: The first step of forming a temporary fixing resist around the pad to which the solder ball of the circuit board is attached is made thicker than the thickness of the pad, and the solder ball forming the circuit board on which the resist is formed. The second step of pressing the transfer jig placed on the circuit board to transfer the solder ball to the pad portion of the circuit board through the flux, and the solder ball transferred to the circuit board are formed on the circuit board. And a third step of fusion bonding to the pad.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a solder ball mounting method, and more specifically, a circuit board on which a semiconductor element is mounted (hereinafter referred to as a circuit board) is arranged in a BGA (ball grid array) input / output. The present invention relates to a method of attaching a solder ball when attaching to a motherboard via a terminal.

Conventionally, when a component is mounted on a printed circuit board, it has been usual to place the circuit component on the surface thereof, insert the lead of the circuit component into the through hole, and solder it. However, in recent years, the packaging density of circuit components has been improved, the wiring patterns have been miniaturized accordingly, and the number of pins of the circuit components has increased significantly. With the increase in the density of such circuit boards, printed circuit boards having three or more layers have been used.

[0003]

2. Description of the Related Art As electronic devices become smaller and more sophisticated,
A printed circuit board (motherboard) needs to draw out many input / output terminals in a limited space. For this purpose, a BGA (Ball Grid Array) package (hereinafter referred to as BGA) has been used. The BGA has a matrix-shaped pattern configuration so that a solder ball in a matrix form can be attached to the bottom of a package on which an LSI or the like is mounted. It is to get the electrical connection between the package and the motherboard.

[0004]

The number of pins of the above-mentioned BGA having a maximum of 200 terminals has been put to practical use. In this case, when the solder balls are attached to the circuit board, it is difficult to accurately position the solder balls, and therefore, when the solder balls are melted, the adjacent solder balls are fused.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a solder ball mounting method capable of reliably bonding a solder ball to a circuit board.

[0006]

According to the present invention for solving the above-mentioned problems, the first step is to form a resist for temporary fixing around a pad to which a solder ball of a circuit board is attached, in a thickness thicker than the thickness of the pad. And a second step of pressing the circuit board on which the resist is formed from above the transfer jig on which the solder balls are arranged to transfer the solder balls to the pad portions of the circuit board via flux. The third step is to fuse the solder balls transferred onto the circuit board to the pads formed on the circuit board.

According to the structure of the present invention, since the resist for temporary fixing is formed around the pad to which the solder ball of the circuit board is attached to be thicker than the thickness of the pad, the solder ball can move because of the resist. The solder balls transferred to the circuit board can be accurately positioned, and the solder balls can be reliably fused to the circuit board.

[0008]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a flowchart showing an embodiment of the method of the present invention. The present invention provides a first step of forming a temporary fixing resist around a pad to which a solder ball of a circuit board is attached, which is thicker than the thickness of the pad, and a circuit board on which the resist is formed.
The second step of pressing the solder balls onto the pad portion of the circuit board via flux by pressing the solder balls from above the transfer jig on which the solder balls are arranged, and the solder balls transferred to the circuit board And a third step of fusion bonding to the pad formed on. By taking such a step, it becomes possible to print the resist for temporary fixing around the pad to which the solder ball of the circuit board is attached and to accurately position the solder ball transferred to the circuit board. The solder balls can be reliably fused to the circuit board.

FIG. 2 is a diagram showing an embodiment of a manufacturing process according to the method of the present invention. (A) A liquid resist of the first layer is printed around the pads to which the solder balls of the package as the circuit board are attached. Printing is performed using a screen printing method.

In FIG. 1A, 10 is a package as a circuit board made of LSI or the like. Package 10
On the back surface of 1), 1a is a liquid resist of the first layer, and 2 is a pad electrode (hereinafter simply referred to as a pad). here,
The pad is a metal conductor for electrically connecting the package and the printed circuit board (motherboard) via a solder ball. By printing the liquid resist 1a of the first layer, the surface of the pad 2 is also covered with the liquid resist.

(B) The first layer liquid resist 1a applied to the surface of the pad 2 is removed, and the first layer resist is cured. In FIG. 2B, 1 is a cured first layer resist. The resist has been removed from the surface of the pad 2.

(C) The second layer of liquid resist is further printed on the surface of the cured first layer resist 1. In FIG. 3C, 3a is a second layer liquid resist printed on the surface of the first layer cured resist 1. The surface of the pad 2
It is covered with this liquid resist 3a.

(D) The second layer liquid resist 3a applied to the surface of the pad 2 is removed, and the second layer resist is cured. In FIG. 3D, 3 is a cured second layer resist. The resist has been removed from the surface of the pad 2.

(E) In this state, a flux for adhering solder balls to the surface of the pad 2 is applied, and the solder balls arranged on the transfer jig are transferred to the pad 2. Actually, the solder balls are transferred to the package 10 as shown in FIG. In FIG. 3, reference numeral 11 is a transfer jig in which depressions are provided at predetermined intervals. The solder ball 4a is placed in advance in the depression of the transfer jig 11, and the flux 5 is applied to the top of the solder ball 4a. Then, the package is pressed from above in the direction indicated by the arrow to transfer the solder ball 4a to the package 10 side. The solder balls 4a are temporarily fixed to the pads 2 of the package 10 with the flux 5. The diameter of the solder ball 4a is about 0.7 mm. FIG. 2E shows the package in which the solder balls 4a are transferred in this manner, which is turned upside down from the state shown in FIG. Solder ball 4a
Is regulated by the hardened resists 1 and 3 formed around it, does not roll (move), and can be positioned accurately.

According to this embodiment, the resist for temporary fixing is formed around the pad to which the solder ball of the circuit board is attached to be thicker than the thickness of the pad. Therefore, the solder balls transferred to the circuit board can be accurately positioned, and the solder balls can be reliably fused to the circuit board.

(F) The solder ball 4a is fused to the pad 2. In the state shown in (e), the solder ball 4a has a temperature of 200 ° C to
The solder ball is fused to the pad 2 by applying heat of about 240 ° C. Reference numeral 4 is a solder ball terminal formed by fusion bonding. FIG. 4 is a view showing a state of the solder ball terminal thus formed. Since the position of the solder balls is regulated by the resist, the solder balls can be accurately positioned and the adjacent solder balls do not melt.

When the solder ball terminal 4 thus formed is fused to the mother board, it is performed as shown in FIG. In the figure, reference numeral 20 denotes a printed circuit board (motherboard) including a package and circuit components such as resistors and capacitors.
Pads are formed on the mother board 20 at the same intervals as the package 10, and the pads and the solder ball terminals 4 are brought into contact with each other and fused by heat treatment (reflow). The solder balls 4 are fused with the pads of the mother board 20, and the package 10 is electrically connected to the mother board 20.

The thickness of the resist formed around the pad needs to be thicker than the thickness of the pad, which is 30 μm. Therefore, the thickness of 40 μm to 50 μm is required. Since it is difficult to form a resist having such a thickness by printing once, in practice, as described above, the
By printing a resist having a thickness of about μm to 25 μm twice, a thickness of about 40 μm to 50 μm is obtained.

FIG. 6 is an illustration of a resist forming method. First, the first layer resist 1 is printed on the surface of the package 10, and the second layer resist 3 is printed thereon. In this case, the second layer resist 3 is printed with a slight offset from the first layer resist 1. This is to facilitate the formation of the resist and to have a shape convenient for the position regulation of the solder balls 4a.

In the above-described embodiments, the case where the package is used as the circuit board is taken as an example, but the present invention is not limited to this, and it goes without saying that the present invention can be applied to other types of circuit boards. Yes.

Further, in the above-described embodiments, the case where the screen printing method is used when applying the liquid resist on the circuit board is taken as an example. However, the present invention is not limited to this, and other methods such as a spray coating method and a curtain coating method can also be used. Also,
At that time, a plurality of times of resist formation may be used to obtain a desired resist thickness, and the second and subsequent resist formation methods may not be the same as the first resist formation method.

Besides the liquid resist, a film-shaped resist may be used. When the film resist is used, the film resist and the circuit board are first bonded together. Next, exposure, development and main curing are performed using a mask. When using a film resist, unlike the case where a liquid resist is used, provisional drying is not necessary. Incidentally, as the resist forming method, the former screen printing method using a liquid resist is advantageous in terms of productivity and uniformity of coating amount.

In the case of the above embodiment, solder flux is applied to the solder balls by, for example, a transfer method,
Next, the solder balls to which flux was previously applied were transferred onto the pads, but other than this, when temporarily fixing the solder balls to the pads via the solder flux, an appropriate amount of solder flux is screen-printed or dispensed. Etc. may be used to apply to the pad, and then a solder ball may be adhered to the pad.

Further, in the case of the above embodiment, a typical Pb-Sn eutectic composition (Pb 63%) was used as the composition of the solder ball, but the present invention is limited to this composition. However, Pb—Sn solder balls having a composition other than the eutectic composition may be used. Incidentally, the semiconductor device according to the present invention is connected to an electronic circuit board (printed circuit board) for mounting electronic components by a heat treatment (reflow) via a solder ball terminal, and like the above-described embodiment, A typical solder ball composition is Pb-Sn.
When the eutectic composition of the system (Pb 63%) is used, it becomes possible to connect terminals with other electronic parts by batch heat treatment,
It is advantageous in terms of productivity.

In the above-described embodiment, the case where the resist is formed around the pad by the screen printing method is taken as an example, but the present invention is not limited to this, and other forming methods can be used. .

[0026]

EXAMPLE When the solder ball terminal defect rate was obtained by variously changing the resist thickness, the pad diameter and the solder ball diameter, the results shown in FIG. 7 were obtained. Numbers 1 to 3 are data for comparison. Nos. 4 to 15 show the solder ball terminal defect rates according to the present invention. The pad thickness is 30 μm. As is clear from the figure, in the case of the comparative example in which the resist thickness is smaller than the height (thickness) of the pad, the defect occurrence rate is high when the reflow is performed to form the solder ball terminal. It is considered that this is because the solder ball transferred onto the pad is likely to be displaced due to falling from the relatively high position of the pad onto the lower position of the resist.

Further, in the embodiment, when the resist thickness is higher than the height of the pad, the solder ball transferred onto the pad does not have a terminal defect such as displacement. It is considered that this is because the positional deviation due to the fall of the solder ball from the pad during the reflow is unlikely to occur. Therefore, when a solder ball terminal is formed by performing reflow by forming a resist higher than the pad, a misalignment of the solder ball terminal from the pad or a failure due to bonding of adjacent solder balls may be prevented. Can be prevented. As is clear from FIG. 7, according to the present invention, it is possible to reduce the pad diameter while keeping the defect occurrence rate at the time of forming the solder ball terminal very low.

When the pad diameter is increased, the solder ball terminal defects are inevitably reduced. However, on the contrary, the height of the solder ball after the solder ball connection is also reduced (due to the spread to the pad), and it becomes difficult to mount the package on the motherboard. A solder ball diameter / pad diameter ratio of 1 to 2 is recommended. It is preferably in the range of 1.0 to 1.6. This ratio may be determined by the number of pins of the package and the wiring design.

The greatest feature of the present invention is that the resist thickness is larger than the pad thickness. The thickness of the pad depends on the circuit pattern design specifications including the pad and the purpose of use. For example, it is determined by the initial copper foil thickness of the glass epoxy copper clad laminate and the circuit board manufacturing process such as polishing and plating thereafter. The thickness of the pad is generally 10 to 50 μm. In this case as well, the resist thickness may be equal to or larger than the thickness of the formed pad, but the maximum value of the resist thickness is within a range that does not hinder package mounting on the motherboard. There is no problem if you select it. For example, about 40% or less of the height of the solder ball fixed to the pad is desirable. Preferably, the thickness is 10 to 10 from the thickness of the formed pad.
A thickness of 50 μm is preferable. Also in the resist formation within the above thickness range, the above range may be taken into consideration for the ratio of solder ball diameter / pad diameter. For example, even if the ratio is 2.0 or more, good solder ball connection can be realized within the resist thickness range, but the pad strength (solder ball connection strength) decreases, and the package is mounted on the motherboard. May adversely affect the reliability of.

[0030]

As described above in detail, according to the present invention, the first fixing resist is formed around the pad to which the solder ball of the circuit board is attached to be thicker than the thickness of the pad. And a second step of pressing the circuit board on which the resist is formed from above a transfer jig on which solder balls are arranged, and transferring the solder balls to the pad portion of the circuit board via flux. The third step of fusing the solder balls transferred to the circuit board to the pads formed on the circuit board enables temporary fixing around the pads to which the solder balls of the circuit board are attached. By forming the resist thicker than the thickness of the pad, the movement of the solder ball is restricted by the resist. It becomes possible to position to,
The solder balls can be reliably fused to the circuit board.

[Brief description of drawings]

FIG. 1 is a flowchart showing an embodiment of the method of the present invention.

FIG. 2 is a diagram showing an example of an embodiment of a manufacturing process according to the method of the present invention.

FIG. 3 is a diagram showing how solder balls are transferred to a package.

FIG. 4 is a diagram showing a state of a solder ball terminal formed on a package.

FIG. 5 is a diagram showing how a package is attached to a motherboard.

FIG. 6 is an explanatory diagram of a resist forming method.

FIG. 7 shows experimental data according to the present invention.

[Explanation of symbols]

 1a Liquid resist 1 Cured resist 2 Pad 3a Liquid resist 3 Cured resist 4a Solder ball 4 Solder ball terminal 5 Flux 10 Package

Claims (1)

[Claims] 1. A first step of forming a temporary fixing resist on a periphery of a pad to which a solder ball of a circuit board is attached, in a thickness thicker than the thickness of the pad, and a circuit board on which the resist is formed, The second step of pressing the solder balls from above the transfer jig on which the solder balls are placed to transfer the solder balls to the pad portions of the circuit board via flux, and forming the solder balls transferred to the circuit board on the circuit board. And a third step of fusion-bonding to the formed pad.