JP2000223831A - Paste solder for surface mount by bga and method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To ensure a distance between an IC package and a circuit board and improve the strength of juncture by adding metal particles with a melting point higher than that of solder particles. SOLUTION: Coated metal particles 24 obtained by coating metal particles 20 with solder is turned into paste using a flux ingredient 26. When an IC package is surface-mounted using the paste solder thus constituted, first, the solder 22 coating the metal particles 20 is melted to fill the cavities among the metal particles 20 and provide continuity and juncture with electrodes. The metal particles 20 ensure continuity by themselves and are not melted during a reflow process. Therefore, the metal particles maintain the weight of the IC and prevent deformation. Further, the presence of the metal particles 20 prevents the formation of conical solder, and thus the reduction in cross-sectional area is prevented. In addition, the breakage of junctures is prevented even under stress owing to the strength of the metal particles 20.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a paste solder,
In particular, paste solder for surface mounting using a ball grid array, and IC chip parts or ICs using it
The present invention relates to a method of surface mounting a package on a circuit board.

[0002]

2. Description of the Related Art A lead frame such as a QFP is used for a surface mounting method of a mounting component such as an IC chip component or an IC package (hereinafter referred to as an IC package) on a printed circuit board or a wiring board (hereinafter referred to as a circuit board). IC used
There are a method of joining the package to the circuit board by soldering, and a method of joining the IC package to the circuit board using solder balls by a BGA or CSP method.

In recent years, the number of input / output terminals has increased along with the increase in size and speed of LSIs, and the terminal pitch of semiconductor packages has become narrower.
A, The joining method by the CSP method has been often used.

Here, the current mounting method of BGA (including CSP) is performed through a series of steps of supplying and assembling solder to a BGA type IC package and a wiring board, and then joining by reflow processing. Especially in the case of the BGA method, the distance between the terminals of the IC package and the circuit board is becoming smaller and smaller, while the number of terminals is also increasing. In order to realize the soldering, it is necessary to perform considerably high reliability soldering.

FIG. 1 is a schematic diagram showing a state of bonding between an IC package and a circuit board by the BGA method. In the following, for simplicity of description, the present invention will be described on the assumption that an electrode pad and an electrode pad are joined for convenience.

As shown in FIG. 1, according to the surface mounting method of the BGA method, solder balls 12 are arranged in a grid on electrode pads of an IC package 10, and the electrode pads (indicated by intersections of broken lines) are arranged in the same pattern. ) Printed circuit board
Place it on top of 14, as shown by the arrow in the figure. The solder ball is melted by a reflow process of heating the soldering assembly thus prepared to a temperature equal to or higher than the melting point of the solder, and the BGA type IC package is joined to the printed circuit board.

FIG. 2 is a side view of this situation.
As shown in (a), when the BGA type IC package 10 is placed on the printed circuit board 14, a solder assembly is formed with the solder balls 12 sandwiched between the upper and lower electrode pads 16. When this is subjected to a reflow treatment, the solder melts, and the cross-sectional shape of the solder joint portion 18 is changed according to the reflow temperature and the size of the solder ball at that time, as shown in FIG.
As shown in (c), the cross section becomes concave and convex.

For example, Japanese Patent Application Laid-Open No. 8-99189 discloses a BG
A method using a cream solder in which two types of solder alloy powders are used to form the bumps of A is disclosed. However, this method aims at preventing the displacement of the solder bumps, and the liquidus temperature of each solder alloy powder is lower than the reflow temperature.

Japanese Patent Application Laid-Open No. 9-198916 discloses fine particles in which spherical polymer particles such as polystyrene resin particles are coated with a conductive layer and further coated with a solder layer. The reason for using spherical polymer particles is that stress is generated due to a difference in coefficient of thermal expansion between the base materials when joining with solder balls, but stress can be absorbed, and non-conductive polymer particles are used. Is itself provided with heat resistance. In this case, since the spherical polymer particles are not deformed by the reflow treatment, they are soldered, for example, in the shape shown in FIG.

[0010]

However, according to the method disclosed in Japanese Patent Application Laid-Open No. 8-99189, even if the displacement can be prevented, when the solder bumps are formed and assembled into a package, for example, as shown in FIG. 3) When the IC is energized in the state of (c), heat is generated in the IC package 10, and FIG. 3 (a), (b)
Stress is applied to the solder joint 18 due to thermal expansion in the direction of the arrow. When the thermal equilibrium is reached, when the thermal expansion of the printed circuit board 14 increases, a stress is generated in the opposite direction, and by repeating this, cracks due to fatigue are generated in the concave portions of the joints, further leading to breakage and poor conduction. . Fig. 3 (a)
, (B).

Furthermore, the recent high integration tends to increase the size of the IC chip itself and increase the weight of the IC package. Therefore, when mounting the IC package 10, the solder melted by the reflow process is crushed by the weight of the IC chip,
A change occurs in the direction indicated by the arrow in FIG. 4, and an accident in which a short circuit occurs between adjacent electrodes is expected.

Also, in the case of Japanese Patent Application Laid-Open No. 9-198916, it is extremely difficult to reliably mount one solder particle on each electrode pad of an IC package by BGA even if screen printing is used.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to solve the above-mentioned drawbacks observed in conventional solder bumps and to develop a technique for mounting a BGA type mounting component on a substrate by simple and inexpensive means. is there.

[0014]

Means for Solving the Problems In order to solve the above-mentioned problems, the present inventors have made various studies, and as a result, metal particles that do not melt at the soldering temperature together with the solder particles are obtained.
Alternatively, the inventors have conceived of forming a paste of composite particles obtained by coating such metal particles in advance with solder.

The paste solder thus obtained is patterned and applied on an IC package by screen printing, and the IC package provided with the paste solder on each electrode pad in this manner is mounted on a circuit board. When solder bonding is performed by reflow processing, the IC package and the circuit board are bonded together with a plurality of metal particles interposed, and at that time, each metal particle acts as an aggregate of a cement composition, Knowing that the separation distance between the IC package and the circuit board was ensured and the joining strength was greatly improved, the present invention was completed.

Here, the present invention is as follows. (1) A paste solder containing a solder particle and a flux component, the surface solder being a ball grid array further containing metal particles having a higher melting point than the solder particles.

(2) The paste solder according to the above (1), comprising 20 to 50% by weight of solder particles, 5 to 20% by weight of a flux component, and 30 to 70% by weight of metal particles. (3) The paste solder according to the above (1) or (2), wherein the size of the metal particles is 0.01 to 0.3 mm in diameter.

(4) A paste solder for surface mounting using a ball grid array, which is a paste solder containing metal particles coated with solder and a flux component, wherein the metal particles have a higher melting point than the solder.

(5) The paste solder according to the above (4), having a composition ratio of metal particles: 30 to 70% by weight, flux component: 5 to 20% by weight, solder: 30 to 50% by weight. (6) The paste solder according to (4) or (5), wherein the size of the metal particles is 0.01 to 0.3 mm in diameter.

(7) The paste solder according to any of the above (1) to (6) is screen-printed on one or both pads of a circuit board and a mounted component, and a chip component is assembled on the circuit board. Next, a reflow process is performed at a temperature equal to or higher than the melting point of the solder and lower than the melting point of the metal particles.

[0021]

Next, an embodiment of the present invention will be described more specifically with reference to FIGS. FIG.
FIG. 6 shows a case where each metal particle is coated with solder as an example. FIG. 6 shows a case where metal particles and solder particles are pasted together with a flux component.

FIG. 5 (a) is a schematic explanatory view of the paste solder formed according to the present invention.
20 has its periphery coated with solder 22 (shown in white), and such coated metal particles 24 are pasted by a flux component 26.

In using the paste obtained in this way, as shown in FIG.
Screen printing is performed on the 30 electrode pads 32. At this time, one to 100, generally two or more solder pastes containing a plurality of metal particles are supplied on each electrode pad by screen printing. Next, an IC package (not shown), which is a mounting component also provided with the electrode pad 32, is mounted thereon, thereby forming a soldering assembly. FIG.
See (c). After the reflow process, as shown in FIG.
The package and the printed circuit board 30 are connected via the metal particles. In FIG. 5 (d), the white area in the solder joint is the molten and solidified solder.

As described above, when the BGA type IC package is surface-mounted using the paste solder according to the present invention, first, the solder coated on the metal fine particles is melted, and the empty space of the metal fine particles is filled. Realizes bonding with electrodes. On the other hand, the metal fine particles themselves have the effect of securing conduction and preventing the deformation (crushing) of the IC because they do not melt even during the reflow treatment. In addition, the presence of metal particles prevents the formation of conical (concave) portions of the solder, thereby preventing the cross-sectional area from decreasing.
At the same time, due to the strength of the metal particles, the joint can be prevented from being broken even when stress is applied.

FIGS. 6 (a) to 6 (c) show a case where a solder paste in which metal particles 40 and solder particles 42 are pasted using a flux component 44 is used. Fig. 6 (a) or
(c) corresponds to FIGS. 5 (a) to 5 (c), respectively, except that the configuration of the solder paste is different, and the mounting operation is the same, and finally obtained by a reflow process. The solder joint is the same as in FIG.

The metal particles used in the present invention are not limited as long as they have a higher melting point than the solder used. Generally, the metal particles are made of silver, copper, nickel or an alloy thereof. From the viewpoint, copper is preferred. The particle size is not particularly limited as long as it does not impair the screen printability, but the separation distance between the IC package and the circuit board is usually required to be 0.1 to 1 mm.
The diameter is set to 0.3 to 0.01 mm because a plurality of, usually 10 to 100 pieces are connected between them.

Here, the metal particles coated with solder that can be used in the present invention can be produced by an electroplating method, a hot-dip solder plating method, a vapor deposition method, or the like.

For example, in the case of the electroplating method, a plating bath is constituted by dissolving a solder component in an appropriate compound form in an aqueous solution or a molten salt, and immersing metal particles in a container in an appropriate container and immersing the bath as a cathode. Should be performed.

Further, according to the hot-dip solder plating method, when the solder is melted, separately prepared metal particles are thrown into the solder, and then pulled up, a substantially uniform coating is formed on the surface of each metal particle. In one embodiment of the present invention, the composition ratio of the metal particles, the solder coating and the flux component is as follows.

Metal particles: 30 to 70, preferably 50 to 70% by weight Solder coating: 30 to 50, preferably 20 to 30% by weight Flux component: 5 to 20, preferably 10 to 15% by weight In the embodiment, the mixing ratios of the metal particles, the solder particles, and the flux components are as follows.

Metal particles: 30 to 70, preferably 60 to 70% by weight Solder particles: 20 to 50, preferably 20 to 30% by weight Flux component: 5 to 20, preferably 10 to 15% by weight Conventionally, those used for surface mounting of IC packages may be used, for example, 60Pb-4
0Sn solder is exemplified. It is only necessary that a predetermined strength can be secured together with the metal particles after the melting by the reflow treatment.

Conventional flux components may be used, and there is no particular limitation in the present invention. Typically, there is a rosin-based flux. Next, the IC according to the present invention
The surface mounting method of a mounting component such as a package will be described below.

First, the above-described components are mixed in a predetermined amount to form a paste, thereby obtaining a paste solder. The paste solder prepared as described above is printed on each electrode pad of the IC package by, for example, a conventional screen printing method. Preferably, approximately 10 to 100 metal particles are supplied on each electrode pad.

After the screen printing, the IC package is placed on a circuit board, and is put into a heating furnace as a soldered assembly and then subjected to a reflow process. The reflow temperature at this time is higher than the melting temperature of the solder and lower than the melting temperature of the metal particles. The reflow process itself is already known, and the present invention may be performed according to such a known method except for the temperature conditions described above.
Thus, according to the present invention, a reliable and high-strength solder joint can be obtained with a simple operation.

[0035]

EXAMPLE In this example, an IC package was surface-mounted on a circuit board in the manner shown in FIG. A paste solder having the following composition was prepared, and a solder layer was provided by screen printing on the electrode pads of a large number of IC packages provided in a grid pattern, and this was mounted on a circuit board. In this way, a solder assembly was formed, which was then subjected to reflow treatment to form solder bumps and joined.

The composition of the paste solder used in this example was as follows. Metal particles: 65% by weight of copper particles having a diameter of 0.05mm Solder coating: 60Sn-40Pb 25% by weight of solder Flux component: 10% by weight of rosin-based flux The internal structure of the solder bump thus obtained is shown in FIG.
(d) is as schematically shown. As shown in the figure, the solder bump obtained by the present invention is composed of a solder layer containing metal particles like an aggregate, and a circuit board and an electrode pad portion of an IC package are provided at a predetermined interval and joined as much as possible. I knew it was there.

When a shear test was performed on the BGA package-mounted circuit board thus obtained, the shear strength was 800 g per bump.

[0038]

As described above, according to the present invention, a solder bump is not pushed out due to the weight of an IC package at the time of melting as in the prior art, and the formation of a recess at a joint is prevented. The practical benefits are great, as certain spacing is ensured, bonding strength is greatly improved, and these can only be achieved by conventional screen printing.

[Brief description of the drawings]

FIG. 1 is a schematic explanatory view of bonding between an IC package and a circuit board by a BGA method.

FIGS. 2A and 2B are views for explaining a state of bonding between a printed circuit board and an IC package according to a conventional method. FIGS. 2A and 2B show bonding before and after reflow processing, respectively. It is sectional drawing of a part.

FIGS. 3 (a) and 3 (b) are schematic explanatory views showing a state where stress is applied to a joint in a conventional method.

FIG. 4 is a schematic explanatory view of a state where a short circuit occurs in a solder joint in a conventional method.

FIG. 5 (a) is a schematic illustration of a paste solder according to the present invention, and FIG. 5 (b) is a view of the printed circuit board when the paste solder is screen printed.
FIG. 5D is a schematic illustration of a solder joint when an IC package is mounted thereon, and FIG.

FIG. 6 (a) is a schematic diagram of another embodiment of the paste solder according to the present invention, and FIG. 5 (b) is a diagram showing the paste solder of the printed circuit board when the paste solder is screen-printed. c) is a schematic illustration of a solder joint when an IC package is mounted thereon. [Explanation of Signs] 10: IC package, 12: Solder ball, 14:
Printed circuit board, 16: Electrode pad, 18: Solder joint, 20: Metal particles, 22: Solder, 24:
Coated metal particles, 26: flux component, 30: printed circuit board, 32: electrode pad, 40: metal particles,
42: Solder particles, 44: Flux component

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yoshinori Kusunoki 50 Nodacho, Noda-cho, Kariya-shi, Aichi F-term in Yuken Industry Co., Ltd. (reference) 5E319 AA03 AB05 AC01 BB05 CC33

Claims (7)

[Claims] 1. A paste solder for surface mounting using a ball grid array, further comprising a paste solder containing solder particles and a flux component, the metal solder having a higher melting point than the solder particles. 2. The method of claim 1, wherein the solder particles comprise 20 to 50% by weight of the solder particles, 5 to 20% by weight of the flux component, and 30 to 70% by weight of the metal particles.
The paste solder according to claim 1. 3. The method according to claim 1, wherein the metal particles have a diameter of 0.01 to 0.3.
3. The paste solder according to claim 1, wherein the thickness of the paste solder is mm. 4. A paste solder for surface mounting using a ball grid array, which is a paste solder containing metal particles coated with solder and a flux component, wherein the metal particles have a higher melting point than the solder. 5. The paste solder according to claim 4, having a composition ratio of metal particles: 80 to 95% by weight, flux component: 5 to 20% by weight, solder: 30 to 50% by weight. 6. The metal particles having a diameter of 0.01 to 0.3.
The paste solder according to claim 4 or 5, wherein the thickness is in mm. 7. The paste solder according to claim 1, which is screen-printed on one or both pads of a circuit board and a mounted component to assemble a chip component on the circuit board, and then at least the melting point of the solder. And performing a reflow treatment at a temperature lower than the melting point of the metal particles.