JPH0857680A - Cream solder composition and soldering method using the same - Google Patents

Abstract

PURPOSE: To provide a cream solder compsn. having excellent wettabrility and excellent mechanical strength in a joint part and a soldering method using this compsn. CONSTITUTION: This cream solder compsn. is produced by forming metallic powders prior to heating of a mixture or two-layered state 3 composed of metallic powder 1 having a 63Sn/37Pb compsn. and metallic powder 2 having an Sn/Pb/Bi compsn. as an essential component and is so formed as to include 0.1 to 8% Bi as the alloy compsn. after heating and melting. As a result, the ratio of the Bi content occupying the entire amt. after melting is kept at a lower ratio and, therefore, the degradation in the excellent mechanical strength which is the characteristics of 63Sn/37Pb is obviated and the impairment of the good wettability at the time of melting which is the characteristic of Sn/Pb/ Bi is averted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to cream solder, and more particularly to a technique for improving the wettability of cream solder mainly containing a metal powder of 63Sn / 37Pb composition.

[0002]

2. Description of the Related Art In the surface mounting of semiconductor elements such as printed wiring boards and ICs, the technique of joining individual constituent elements plays an extremely important role. In particular, in order to increase the degree of integration of the LSI substrate and enable high-density mounting, improvement in micro soldering (soldering of minute portions) technology is indispensable.

As a method for accurately mounting a minute semiconductor component at a fixed position on a circuit board, a method using cream solder is generally known. That is, by using the viscosity and surface tension of the cream solder that is attached in a desired pattern on the circuit board by the printing method, the semiconductor component is fixed and then heated by hot air, infrared rays, etc. to solder the fine component. I am trying to do. for that reason,
For such cream solder, low reflow temperature,
Demands for workability and reliability such as good wettability and bonding strength after cooling and solidification are becoming higher and higher.

In general, cream solder is prepared by mixing metal powder solder with highly viscous flux to form a cream. As the metal powder, for example, 6
Eutectic solder of 3Sn / 37Pb composition is widely used.

[0005]

However, these conventional cream solders have a problem that the two characteristics of wettability and reliability of bonding strength are not always compatible with each other. That is, for example, 63
Although the Sn / 37Pb eutectic solder is excellent in the mechanical strength of the joint, it has a problem that the wettability and its spreadability are not sufficient depending on the conditions and a bridge is often generated in some cases. To improve the wettability, some improvement can be expected by adding some activators, but basically it is not possible to obtain more than the characteristics of the alloy used. In addition, due to the problem of corrosion due to flux residue, the materials that can be used as activators are limited.

[0006] Therefore, in order to secure more excellent wettability, a composition of 46Sn / 46Pb / 8Bi containing Bi has been proposed (see Japanese Patent Application Laid-Open No. 1-24599). This 46Sn / 46Pb / 8Bi solder achieves good wettability and spreadability and a low melting point of 130 to 192 ° C. by setting the Bi content to 8 wt%, but it hardens and becomes brittle when Bi is added, Bonding strength is 63S
There is a problem that it is lower than the n / 37Pb eutectic solder.

The present invention has been made by paying attention to such a conventional technique, and a cream solder composition using a metal powder of 63Sn / 37Pb eutectic solder is excellent not only in bonding strength but also in wettability. It is an object of the present invention to provide a cream solder composition and a soldering method using the same.

[0008]

In order to solve the above problems, the present invention has a metal powder composition of 63 Sn / 37 Pb before heating.
A mixture of metal powder having a composition and metal powder having an Sn / Pb / Bi alloy composition, which is a cream solder composition containing 0.1 to 8% of Bi as an alloy composition after heating and melting, and this cream solder composition Is used for soldering.

Further, the present invention provides a soldering method which is performed by cream solder using a two-layer metal in which Sn / Pb / Bi metal powder is applied to the surface of the metal powder of 63Sn / 37Pb composition.

The cream solder composition of the present invention is
The reflow temperature of 63Sn / 37Pb solder is 210
The composition of the powder to be mixed by heating at ~ 230 ° C. is Sn / Pb metal powder, Sn / Pb / B.
The metal powder is not limited to i metal powder, and Ag, C
u, Sb, etc. may be contained as auxiliary components.

[0011]

The effect of adding Bi is to improve the wettability of the solder, but the addition of Bi also leads to a decrease in the mechanical strength of the joint after cooling and solidification. It is preferable to keep the amount as small as possible without impairing the desired strength. In general, the amount of Bi added is 3% or less, which secures a certain wettability and suppresses the strength reduction to the extent that it does not hinder practical use.
In the present invention, Bi is 0.1 to 0.1 as an alloy composition after heating and melting.
8%, preferably 0.1-5%, more preferably 0.
It is supposed to contain 8 to 3%. Bi content of 0.1
The reason why the content is set to ˜8% is that if it is less than 0.1%, the wettability, especially its spreadability is reduced, and if it is more than 8%, the mechanical strength is reduced.

In the cream solder composition according to the present invention, the metal powder composition before heating is, for example, the conventionally used metal powder of 63Sn / 37Pb composition and 46Sn / 46P.
Since it is mixed with a metal powder having a b / 8Bi composition and is heated and melted during soldering to form a ternary alloy, the proportion of the Bi amount in the total amount can be suppressed to a low level. The mechanical bond strength, which is a characteristic of the / 37Pb composition, is not reduced. At the same time, when it melts, it exhibits excellent wettability, which is a characteristic of adding 8Bi. On the other hand, this wettability means that the surface tension is small and the wetting angle is small (0 ° to 90 °), and it is possible to exhibit the characteristics that the occurrence of bridges and the like and the solder breakage are good. Is.

The present invention is not intended to lower the melting point by adding Bi alloy, but to mix Bi alloy as a kind of wetting promoter utilizing the specific wettability of Bi alloy. Therefore, the alloy to be mixed is 63Sn /
Not limited to 37Pb composition, other alloys with poor wettability,
For example, when combined with 96.5Sn / 3.5Ag,
A cream solder having a high melting point and good wettability can be obtained.

Further, by using a two-layer metal powder in which a metal having a composition of Sn / Pb / Bi is applied to the surface of a metal powder having a composition of 63Sn / 37Pb, the Sn / Pb / Bi metal is melted first, and then 63Sn / The characteristics of Sn / Pb / Bi metal can be brought out by the two-step melting behavior that the 37Pb metal powder melts. Further, when forming the two-layer metal powder, Sn was added to the metal powder of 63Sn / 37Pb composition.
For coating with / Pb / Bi metal powder,
It can be easily carried out by a mechanical method as described later, and the thickness can be easily controlled.

[0015]

The present invention will be described in more detail with reference to the following examples. FIG. 1 shows a metal powder of 63Sn / 37Pb composition and 4
It is a graph showing the melting behavior of each powder when the metal powder having a composition of 6Sn / 46Pb / 8Bi is mixed in a powder state and heated. A shows the state of the metal powder of 63Sn / 37Pb composition, B shows the state of the metal powder of 46Sn / 46Pb / 8Bi composition. C shows the temperature profile of the heating furnace. A region E shows that the metal powder having a composition of 63Sn / 37Pb is in a molten state, and a region F shows 46Sn / 46Pb /.
It shows that the metal powder having a composition of 8Bi is in a molten state. As you can see from this graph, 4 minutes after heating,
6Sn / 46Pb / 8Bi metal powder, and about 2.5
63 minutes later, 63Sn / 37Pb metal powder begins to melt. Thereafter, diffusion and wetting and the liquid phase of both powders proceed, and most of the particles fuse to each other at around 165 ° C. And 3 minutes later 18
Melting of Sn-Pb eutectic crystal and α solid solution started at 3 ° C,
Then, upon cooling and solidification, the ternary alloy according to the present invention is crystallized.

A cream solder was obtained by mixing the above-mentioned mixed powder and flux, and soldering was performed at a reflow temperature of 220 ° C. using this cream solder, and the wettability and the joint strength were tested. The mixing ratios of the respective metal powders at that time, the respective component ratios, and the results are shown in Table 1 below.
Shown in As a wettability test method, the sample
One character is printed perpendicularly to the pattern in a width of 300 to 420 μm with a 0 μm metal mask, 0.5 mm pitch QFP
The package was mounted and the number of bridges after reflow was evaluated. As a test method of the bonding strength, after the mold resin and the lead pin of the QFP package were cut by a cutter, a wire was hooked on each lead pin and pulled at a speed of 50 mm / min, and the stress at the time of breaking was measured.

[0017]

[Table 1]

As a result, good results were obtained in that the solder wettability had the alloy characteristics of simply incorporating Bi, while the strength had the characteristics of the final alloy.

Next, a soldering method using a two-layer metal powder in which Sn / Pb / Bi metal powder is applied to the surface of the metal powder of 63Sn / 37Pb composition will be described. As shown in FIG. 2, 63Sn / 37Pb having a diameter of 20 to 45 μm
On the surface of the metal powder 1, Sn / Pb having a particle size of 0.1 to 5 μm
The / Bi metal powder 2 is sprinkled and adhered to obtain the two-layer metal powder 3. In this example,
A "hybridization system" (manufactured by Nara Machinery Co., Ltd.) is used as an apparatus for adhering the Sn / Pb / Bi metal powder 2. That is, this device collides the Sn / Pb / Bi metal powder 2 with the surface of the 63Sn / 37Pb metal powder 1 in a rotating rotating machine to mechanically adhere the Sn / Pb / Bi metal powder 2. Then, by continuing this rotation for a predetermined time, the Sn / Pb / Bi metal powder 2 is flattened and smoothed to obtain the two-layer metal powder 3 having the surface layer 4 having a substantially uniform thickness. [See FIG. 3]. The thickness of the surface layer 6 is controlled by Sn / P
This can be easily performed by appropriately adjusting or selecting the collision speed and the particle size of the b / Bi metal powder 2, and thus the Bi content in the final alloy can be adjusted to about 0.1 as intended by the present invention. It becomes possible to set it in the range of up to 8%.

The surface layer 4 is 46Sn / 46Pb.
The thickness t of the surface layer 4 when the / 8Bi metal is used can be derived by the following equations (1) to (3).

[0021]

[Equation 1]

From these equations, the composition of the surface layer 4 is 46S.
The main examples of the relationship between the thickness t of the surface layer 4 and the Bi% in the final alloy when n / 46Pb / 8Bi and R ₁ = 22.5 μm are as follows.

[0023]

[Equation 2]

That is, the surface layer 4 is 46Sn / 46.
When Pb / 8Bi is used, Bi in the final alloy after melting
% To 0.1 to 5%, the thickness t of the surface layer 4 is set to 0.
It is understood that the thickness may be 1 to 8.5 μm.

Then, a cream solder obtained by mixing the two-layer metal powder 3 and a flux was obtained, and the same tests as above were conducted for wettability and joint strength when soldering was performed using this cream solder. . The results are shown in Table 2 below. For the evaluation of the bonding strength, see 19
A strength of 0 [gf] or higher was defined as a practical strength.

[0026]

[Table 2]

Further, the same result as in [Table 2] was obtained when the flux was applied and then the two-layer metal powder 3 was further adhered onto the flux for soldering.

[0028]

As described above, the cream solder composition and the soldering method using the same according to the present invention are provided with a metal powder of 63Sn / 37Pb composition and Sn / Pb /
Since the metal powder having the Bi composition is placed in the mixed state or the two-layer state and then melted, the ratio of the Bi amount in the total amount can be suppressed to be low, and therefore 63S
It has the effect of not lowering the excellent mechanical strength which is the characteristic of n / 37Pb, and also the good wettability at the time of melting which is the characteristic of Sn / Pb / Bi, in particular, not impairing its spreadability. is there. And again Sn / Pb / B
The characteristic of i has an effect that bridging or the like is less likely to occur and solder breakage is improved.

[Brief description of drawings]

FIG. 1 is a graph showing melting behavior when two kinds of metal powders are mixed and heated.

FIG. 2 is a schematic cross-sectional view showing an example of the two-layer metal powder of the present invention.

[Explanation of symbols]

 1 63Sn / 37Pb metal powder 2 Sn / Pb / Bi metal powder 3 Two-layer alloy 4 Surface layer

Claims (3)

[Claims] 1. 63Sn / 37Pb or Sn / Pb /
In a cream solder composition containing a metal powder of Bi composition and being heated and melted at the reflow temperature of 63Sn / 37Pb solder, the metal powder before heating is Sn / Pb / Bi whose main component is metal powder of 63Sn / 37Pb composition. It is a mixture with a metal powder having a composition, and Bi is 0.
A cream solder composition comprising 1 to 8%. 2. A soldering method for applying the cream solder composition according to claim 1 to the surface of a base material. 3. The soldering method according to claim 2, wherein a two-layer metal powder in which Sn / Pb / Bi metal is applied to the surface of the metal powder having a composition of 63Sn / 37Pb is used.