JPH098439A - Mounting technique - Google Patents

Abstract

PURPOSE: To prevent a bridge from being generated between leads and to make a junction part, which does not contain lead, form between the lead and a land by a method wherein a low-melting point metal layer is provided in the interface between the lead and the land, a heating and a pressing are simultaneously performed on the metal layer by a heating tool, the adherence of the lead to the land is improved and a diffusion of a low-melting point metal is generated in the respective mother materials of the lead and the land. CONSTITUTION: An insert metal layer 5, which is a low-melting point metal layer, is inserted in the interface between a lead 1a of an electronic component and a land 3 on a printed board as a junction medium and the metal layer 5 is heated and pressed by a heating tool 6. Then, a bonding flux is applied on the metal layer 5 for a mounting of the electronic component and the removal of an oxide film in the junction part between the lead 1a and the land 3 and a thermocompression bonding of the lead 1a to the land 3 is conducted by heating and pressing the metal layer 5 by the tool 6. An unnecessary bonding flux in the interface of the junction part is made to eliminate from the interface and interfacial diffusions of a low-melting point metal are mutually made to generate in the respective mother materials of the lead 1a and the land 3.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mounting method for a narrow lead pitch electronic component.

[0002]

2. Description of the Related Art In the electric and electronic industry in recent years, miniaturization of electronic parts and high-density mounting on a circuit board are rapidly advancing along with miniaturization of products. 1m for chip parts
mx 0.5mm size (1005 chip parts)
Is also used. In the quad flat package (hereinafter, referred to as QFP) shown in FIG. 4, the lead pitch has been narrowed from 0.5 mm to 0.4 mm and 0.3 mm. In addition, T represented by liquid crystal
Also in CP mounting, a narrower pattern pitch and a narrower lead pitch are rapidly advancing due to downsizing of displays and narrowing of frame.

The structure of the soldering portion in the conventional surface mounting of electronic components will be described below with reference to FIG. First, the cream solder 4 is supplied to the land 3 on the printed circuit board 2 by printing in a thickness of 80 μm to 150 μm, and then the electronic parts are mounted with the QFP on the printed circuit board by utilizing the adhesive force of the cream solder. Soldering was performed by melting the solder by raising the temperature in a heating furnace.

Further, although the toxicity of lead is becoming a serious social problem, about 40% by weight of lead is contained in the solder which is generally used at present.

[0005]

However, in the soldering method using the cream solder printing technique as described above, if the lead pitch becomes narrower, the printing will be limited. That is, there has been a problem that the solder bridge between the leads sharply increases as the lead pitch becomes narrower. At present, it is considered that the lead pitch of printing of cream solder is 0.3 mm.

In view of the above problems, the present invention directly joins the lead and the land by thermocompression bonding the respective base materials without using the cream solder, does not generate a bridge between the leads, and is lead-free. It is possible to form a joint portion not including.

[0007]

[Means for Solving the Problems] In order to solve the above problems, the mounting method of the present invention uses a low melting point metal layer (hereinafter referred to as insert metal) at the interface between the lead and the land without using cream solder. By providing and heating and pressing at the same time with a heating tool to improve the adhesion between the lead and the land and to cause diffusion, lead is not generated between the lead and the land base material without generating a bridge between the leads. It is possible to form a joint portion that does not include.

Further, the lead of the electronic component or the land on the printed circuit board is directly coated with the low melting point metal material in advance to form the bonding medium, and the coating thickness of the low melting point metal material as the bonding medium is It is characterized in that the total is 10 μm or less. Further, a laser is used as a heat source, and an optical crystal or glass having a high transmittance in a used laser wavelength region is used as a pressing tool to form a bonding portion at a laser irradiation portion.

[0009]

According to the present invention, by the mounting method described above, the insert metal at the interface between the lead and the land is melted by heating and pressing, the adhesion between the lead and the land is improved, and the lead and the land base material are diffusion-bonded. It is possible to solve the problem of toxicity due to the occurrence of bridge and the inclusion of lead in the solder, such as when using cream solder.

Further, by using a laser as a heating source and thermocompression bonding using a pressure tool having excellent transmittance in the laser wavelength range, damage of electronic parts due to heat can be reduced and bonding without lead bridges is possible. Become.

[0011]

【Example】

(Embodiment 1) Hereinafter, a mounting method according to an embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows a cross section of a joint portion in a mounting method according to a first embodiment of the present invention. In FIG. 1, 1a is a lead of the QFP 1 shown in FIG. 4, 2 is a printed circuit board, 3 is a land, 5 is insert metal which is a low melting point metal layer, and 6 is a heating tool.

The mounting method constructed as described above will be described below with reference to FIGS. 1 and 2. FIG. 1 is a cross-sectional view of a first embodiment in which an insert metal 5 which is a low melting point metal layer is inserted as a joining medium at an interface between a lead 1a of an electronic component and a land 3 on a printed board, and a heating tool 6 heats and pressurizes the same. is there.

In this embodiment, after inserting the insert metal 5 at the interface between the lead 1a of the electronic component and the land 3 on the printed circuit board, bond flux is applied for mounting the electronic component and removing the oxide film at the joint. , Thermocompression bonding is performed by heating and pressurizing with the heating tool 6, unnecessary bond flux is discharged from the interface to the joint interface, and interfacial diffusion is caused in the respective base materials of the lead 1 a and the land 3. A joint portion is formed directly on the interface between the lead 1a of the electronic component and the land 3 on the printed board without using solder.

The thickness of the insert metal can be appropriately selected, but is preferably 10 μm or less. When the thickness of the insert metal exceeds 10 μm, the diffusion of the leads and the base material of the lands is hindered by the insert metal during thermocompression bonding, resulting in insufficient joint strength. Thickness of insert metal is 1
When it is 0 μm or less, it becomes possible to form a good bonding interface between the lead and the base material of the land.

Next, FIGS. 2A and 2B show the coated portion of the insert metal 5, in which the surface of the lead 1a and the surface of the land 3 are directly coated with the insert metal 5. Before mounting the lead 1a of the QFP1 coated with the insert metal 5 on the land 3 coated with the insert metal 5, a bond flux is applied to make the land have an adhesive force to facilitate the fixing of the QFP1 and to lead 1a and The oxide film formed on the surface of the land 3 is removed,
A clean metal surface can be exposed at the bonding interface. Next, as in the embodiment of FIG. 1, by heating and pressurizing with the heating tool 6, unnecessary bond flux is discharged from the interface to cause a diffusion at the interface between the lead and the land. It was possible to form a good joint between the base materials.

As described above, according to this embodiment, when the electronic component is mounted, the solder and the land of the electronic component are thermocompression bonded to each other through the insert metal without using the cream solder. A good joint could be formed between the respective base materials of the lead and the land without causing a bridging defect. (Embodiment 2) Hereinafter, a second embodiment of the present invention will be described with reference to the drawings.

FIG. 3 is a sectional view of a joint portion of a mounting method showing a second embodiment of the present invention. In the figure, 1
a is a lead of the QFP 1 shown in FIG. 4, 2 is a printed circuit board, 3 is a land, and 5 is an insert metal which is a low melting point metal layer, and has the same structure as that of FIG. On the other hand, in FIG. 1, heating and pressing are performed by the heating tool 6, whereas in FIG. 3, the heating tool is divided into a pressing tool 7 and a laser heating source 8.

With respect to the mounting method constructed as above, the joining process will be described below with reference to FIG. Before mounting the lead 1a of the QFP1 coated with the insert metal 5 on the land 3 coated with the insert metal 5, a bond flux is applied to make the land have an adhesive force to facilitate the fixing of the QFP1 and to lead 1a.
Also, the oxide film formed on the surface of the land 3 is removed to expose a clean metal surface at the bonding interface. The above steps are the same as in the first embodiment. On the other hand, the difference from the first embodiment is that
By using a laser as the heating source 8 and rapidly raising the temperature of the laser irradiation part, the joining time is shortened and the damage to the electronic parts due to heat is reduced. In this case, by using the pressure tool 7 having excellent transmittance in the laser wavelength region, the energy loss of the laser was minimized, and efficient heat energy supply was enabled. By simultaneously performing heating and pressurizing in this way, unnecessary bond flux at the joint interface is discharged from the joint interface, diffusion is caused at the respective base material interfaces of the lead 1a and the land 3, and excellent adhesion is achieved. It enabled the formation of a good joint. Further, by using a laser as the heating source 8 and using glass or an optical crystal, which is a material having excellent transmittance in the laser wavelength range, as the pressing tool 7, bonding can be performed in a short time, and the insert into the pressing tool 7 can be performed. Adhesion of metal was also reduced.

As described above, in the present embodiment, the laser is used as the heating source and the bonding is performed by using the pressure tool having the excellent transmittance in the laser wavelength range to reduce the heat damage to the electronic parts and reduce the inter-lead gap. Since bridgeless bonding is possible and the adhesion of insert metal to the pressure tool can be reduced, it is possible to reduce the number of times the pressure tool is cleaned, and electronic components can be mounted efficiently. Further, since the mounting method does not use solder, the problem of toxicity of lead contained in solder can be solved. In the first and second embodiments, narrow lead pitch electronic components (particularly QF
Although it is described as the joining process of P), the same effect can be obtained for joining in the tape carrier package (TCP).

[0020]

As described above, according to the present invention, the low melting point metal layer is inserted as a joining medium at the interface between the lead frame of the electronic component and the land on the printed circuit board, and further, the mounting of the electronic component and the removal of the oxide film at the joining portion. Therefore, by applying bond flux, and simultaneously heating and pressurizing with a heat tool, the bond flux is discharged from the interface between the lead of the electronic component and the land on the printed circuit board, and the joint portion is formed by diffusion of the lead and the land base material. Can be formed to improve the adhesiveness, and bridge defects between the leads of a narrow lead pitch electronic component can be eliminated. Also, since no solder is used, the problem of lead toxicity is eliminated.

Further, by using a laser as a heating source and thermocompression bonding with a pressure tool having an excellent transmittance in the laser wavelength range, it is possible to reduce the damage of electronic parts due to heat and to achieve a bonding without a bridge between leads. Further, since the adhesion of the insert metal to the pressure tool can be reduced, the frequency of cleaning the pressure tool can be reduced and the electronic components can be mounted efficiently.

[Brief description of drawings]

FIG. 1 is a sectional view showing a first embodiment of the present invention.

FIG. 2 (a) is a cross-sectional view of the electronic component lead coated with insert metal. (B) It is sectional drawing at the time of covering the land with insert metal.

FIG. 3 is a sectional view showing a second embodiment of the present invention.

FIG. 4A is a perspective view of a quat flat package (QFP) as an example of an electronic component. (B) It is sectional drawing of a quad flat package (QFP).

FIG. 5 is a sectional view of a soldering portion of a conventional electronic component.

[Explanation of symbols]

 1 Electronic Component 1a Electronic Component Lead 2 Printed Circuit Board 3 Land 4 Cream Solder 5 Insert Metal 6 Heating Tool 7 Pressurizing Tool 8 Laser Heat Source

Claims (4)

[Claims] 1. When mounting an electronic component, a low melting point metal material is inserted as a bonding medium at the interface between the lead of the electronic component and the land on the printed circuit board to bond the electronic component and to remove an oxide film from the bonding portion. Is applied, and thermocompression bonding is performed by heating and pressurizing with a heating tool, causing interfacial diffusion between the respective lead and land base materials, and solder is interposed between the electronic component lead and the land interface on the printed circuit board. A mounting method characterized by directly forming a joint portion. 2. The mounting method according to claim 1, wherein the lead of the electronic component or the land on the printed circuit board is directly coated with the low melting point metal material in advance to form the bonding medium. 3. The mounting method according to claim 1, wherein the coating thickness of the low melting point metal material as the bonding medium is 10 μm or less in total. 4. A laser is used as the heating tool, and an optical crystal or glass having excellent transmittance in the laser wavelength range of the heating tool is used as the pressing tool, and the bonding portion is formed at the laser irradiation portion. Mounting method described in 1.