JPH01102946A - How to mount electronic components using reflow soldering - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field 1] The present invention relates to a method for mounting electronic components on a printed wiring board. More specifically, the present invention relates to a reflow soldering method for mounting flat type electronic components using cream solder. Note that cream solder refers to a paste made by kneading solder particle powder and flux.

[Prior Art] In recent years, reflow soldering has been increasingly used to mount electronic components on printed wiring boards. The reflow soldering method will be generally described with reference to FIGS. 5 to 8. FIG. 5 shows a printed wiring board (1). This board is provided with a land portion (2). Land part (
2) consists of an exposed copper layer, for example. 2" at the bottom left of the illustration
The group of rectangular land portions arranged in rows corresponds to the bidirectional flat type electronic component shown in FIG. 6(^). The land portion on the right side of the illustrated board (1) is for a four-way flat type electronic component shown in FIG. 6 (8). As shown in Figures 6 and 7, flat type electronic components have many lead terminals (5) around them. Now) To mount these flat type electronic components on a board, it is necessary to Print cream solder on the land part (2) using a metal mask or screen mask, press the lead terminal of the electronic component onto this, temporarily attach it using the adhesive force of the cream solder, and then use infrared rays, steam, etc. The solder is melted through a heating process and then solidified.

The problems with reflow soldering are the occurrence of bridge defects (solder crosses between the lead terminals and causes a short circuit) and open defects (the lead terminals are not connected to the land and are floating). be. As the degree of integration of electrical circuits within an electronic component increases, the number of lead terminals drawn out from the periphery of the electronic component increases, and the spacing between the lead terminal arrays becomes narrower. In this way, when high-density packaging is performed, if the amount of solder is too large, a bridge failure will occur, and if the amount of solder is too small, an oven failure will occur.

Conventional technology was based on reducing the amount of solder. For example, the reflow soldering method disclosed in Japanese Patent Publication No. 57-29072 is characterized in that cream solder is screen printed on the land portions so as not to overflow from the land portions.

The amount of solder that had previously been present over the entire land area and swelled around the land area due to solder plating was reduced by screen printing the cream solder as described above, and in this way, the amount of solder was reduced to 0.635M, for example.
It has been reported that bridge defects no longer occur even with narrow lead terminal spacing. However, this method requires no forming (
(refer to Figure 7). In the case where the lead terminal (5) is formed as shown in the lower part of Fig. 8, a bridging failure occurs even if the cream solder (3) is applied in an area less than the area of the land part (2). .

It has been formed. Regarding things, JP-A-58
There is a -1329404 publication. When mounting a flat type electronic component with formed or lead terminals on a board, a cream solder layer with a width narrower than the length of the connection part of the formed lead terminal is applied to the surface of the board in the area where the land part is located. It is characterized by being uniformly deposited on the surface. Until then, a cream solder layer with a width wider than the length of the connection part of the lead terminal was applied in seven different ways. In other words, this Japanese Patent Application Publication No. 58-1329
Publication No. 40 also attempts to reduce the amount of solder. By the way, in the method that is the basis of this technology, in which the cream solder layer is evenly and uniformly applied to the surface of the area where the land portions are located on the board, if the distance between adjacent land portions becomes narrow, melting may occur. The solder remains stable across adjacent lands, making it impossible to eliminate bridging defects.

Further, the application of the cream solder of JP-A-58-132940 is usually carried out using a dispenser. This is suitable for small-volume production, but is not suitable for mass production. This is because, if such adhesion, which is also called flat coating, is performed by printing using a metal mask, for example, in order to perform mass production, the necessary amount of solder cannot be secured. To explain in more detail, if the thickness of the metal mask is increased, cream solder cannot be handled through the mask, and the thickness must necessarily be reduced. Thus, when this method of JP-A-Sho is applied to mass production, open defects occur. Even if there is no open defect and the lead terminal is bonded to the land, the bond is unstable due to the small amount of solder, and long-term quality cannot be guaranteed.

In this way, the amount of solder is sufficient to firmly and stably maintain the bond between the land part and the connection part of the lead terminal for many years, and the reflow soldering method is suitable for mass production and suppresses the occurrence of bridge defects. There has been a need for the development of

[Problems to be solved by the invention] In other words, in conventional reflow soldering in mass production, when the amount of solder increases, bridge defects occur, and when the amount of solder decreases, open defects occur and long-term bonding occurs. The problem is that stability cannot be achieved.

The present invention has been made to solve such problems, and it is possible to firmly mount a flat type electronic component having formed lead terminals on a printed wiring board with a sufficient amount of solder, and to connect the adjacent lead terminals. Reflow soldering aims to provide a method to eliminate the occurrence of bridge short circuits.

[Consideration of the problem] With the current technology for printing cream solder, due to the thixotropic property of cream solder (the property that the viscosity decreases when shear stress is applied, that is, the property that the viscosity decreases during printing). , sagging occurs during repeated printing. This sag is unavoidable in mass production. Furthermore, sagging occurs when electronic components are mounted and when heated. Such sag is unavoidable in the current process.

Here, if the absolute amount of solder is small, there will be less dripping and bridge defects will not occur, but on the contrary, open defects will be observed and the joint strength and reliability will decrease. Open defects occur due to slight variations in the height of the connection portion of the formed lead terminal. This open defect is difficult to detect because when a probe pin is applied during a continuity test, the connection part is pushed down and conduction occurs, resulting in an OK determination. On the other hand, bridging defects are easy to detect. Therefore, a predetermined amount of solder is determined to avoid open defects and to ensure bonding strength and reliability.

Now, it has been found that when a flat type electronic component having formed lead terminals is mounted on a board using the above-mentioned predetermined amount of cream solder, the locations where bridges occur have specific properties. This will be explained with reference to FIG.

Figure 9 (A) shows the connection part (6) of the formed lead terminal (5) temporarily attached to the cream solder (3) applied to the land part (2) of the board by its adhesive force. Indicates the condition. FIG. 9(B) shows the state immediately after the cream solder has melted, and shows that the thickness of the cream solder (3) has decreased.

Thereafter, the melted solder forms a meniscus between the land portion and the connection portion of the lead terminal, and flows stably. This flow has a special effect, especially at the connection part (6) of the lead terminal (5), where it rises sharply from the vicinity of the root, creating a hook that promotes the occurrence of bridging in this area. . Thus, as shown in FIG. 9(C), the attachment of the connecting portion (6) creates a bridge near the root. However, when viewed from the side, it looks like FIG. 9(D). In other words, as shown in FIG. 8, bridges (7) particularly occur along the vicinity of line (10).

Further investigation into this phenomenon revealed the following. This will be explained with reference to FIGS. 10 to 12. In addition, the 11th
The figure is a perspective view of FIG. 9(B), and FIG. 12 shows a cross section taken along the xn-xn line of FIG. 11 and shows changes therein. Now, if the thickness (1) at the center of the molten solder spanning between the lands is less than a certain critical value, it will be divided into two parts and the bridge will be completely eliminated. This is because molten solder does not wet the substrate. However, if it exceeds the critical value, it becomes thicker and becomes metastable, solidifying and causing bridging failure. In other words, as shown by the dotted line (b), if the thickness (1) is small with the critical value tCb as the boundary, it will shift to the left and the bridge will disappear, and if the thickness (1) is larger than the critical value tcb, it will shift to the right and become quasi-stable. The bridge becomes defective. In other words, the surface energy of the molten solder reaches its maximum at the critical value tab when the cream solder (the dripping) that has protruded from the land melts and connects to each other and straddles the land. There are low plateaus on either side of it.

Note that the state on the right where the bridge is defective has a higher energy unit than the stable state on the left because the board straddles the board.

Now, the reason why bridge defects are concentrated on the line (10) in Figure 8 is that the molten solder, which has a property of getting wet with the metal of the lead terminal, is not attached to the connecting part (6) of the lead terminal (5). As the meniscus is completed in the rising area (see S in Figure 9 (C)), the cream solder protrudes (there may be more than one) that spans and connects the land area. The inventor determined that this is because the particles also move toward the line (1G), accumulate there, increase the thickness (1), and exceed the critical value tab. Furthermore,
The above-mentioned critical value changes depending on whether there is a flow of molten solder that replenishes the thickness (1) (see S in Figure 9 (C)), when there is no flow, or when there is a flow in the opposite direction. I also found out. Therefore, it was found that if this flow was controlled, it would be possible to eliminate the occurrence of bridging defects.

For this reason, by controlling the flow that occurs when the paste state of cream solder changes to the molten state of solder, the critical value can be changed from the conventional tCb to tCa as shown by the solid line (a) in Figure 10. It can be seen graphically from FIG. 10 that it can be made larger and therefore the rate of bridging can be lowered.

This is because the thickness (1) falls between the conventional low critical value tCb and the high critical value tea according to the present invention.
While the method of the present invention moves toward eliminating bridges,
Since the conventional method tends toward bridge generation (P), the rate of bridge generation decreases by the difference.

[Means for solving the problem] Now, the problem is how to control the flow of molten solder. The method according to the present invention is characterized in that at least the front end of the connection part, and at most the lower surface up to the base of the connection part, has a surface condition that is relatively more easily wetted by solder than that of the remaining part of the lead terminal. do.

[Function] In this way, the flow of cream solder during melting is drawn toward the tip of the connection rather than from the tip of the land to the root of the connection, or collected on the lower surface of the connection, suppressing the conventional flow (S) and forming a bridge. (P) is suppressed.

[Example] An embodiment of the present invention will be described below with reference to FIG.

In the figure, (1) is a printed wiring board, (2) is a land portion, (3) is cream solder, (4) is a flat type electronic component, and (5) is a lead terminal from the electronic component (4). , (6) is a connection portion formed by forming the lead terminal (5). The angle of inclination of the connection part is 3 to 4 degrees. The land portion (2) is made of copper. Cream solder (3) is made by kneading tin-lead eutectic solder (melting point 183°C) particle powder of 10 to 150 μm (approximately 90 weight percent), flux component (approximately 10 weight percent), special additives, etc. It is a paste-like substance that is mixed together. The flux consists of 50 weight percent solids and 50 weight percent solvent. The solid content includes rosin, activator, thixotropic agent,
and stabilizers. The solvent component is glycol or the like. Special additives are used in consideration of printability and viscosity at room temperature. The cream solder used in this example was
00,000 to 900,000 C poise, and 2 to 5 C poise at 183°C. The lead terminal (5) is made of 4270I (iron-nickel alloy).

The width of the land portion is 0.40711I11. The land interval is 0.257111. Therefore, the pitch between adjacent lands is 0.65 am.

Now, the difference from the conventional one shown in FIG. 9 (^) is that in the conventional case, the entire lead terminal is coated with 90Sn-10pb solder plating (melting point 215°C> (12)), whereas in the present invention, the connecting part is plated. Solder plating (12) on the bottom surface of the tip of (6)
, which makes it more susceptible to molten solder than other parts. Therefore, in the present invention, it is difficult for the molten solder to flow when completing the meniscus in the portion of the connecting portion (6) that rises suddenly from the root. Therefore, from the tip of the land portion (2) to the connecting portion (
6) The flow toward the root (S) is suppressed, reducing bridge formation (P).

FIG. 2 shows an embodiment of the pond, in which the entire lower surface of the connecting portion (6) is plated with solder (12).

FIG. 3 shows still another embodiment, in which the lower surface of the connecting portion (6) is plated with solder (12), and the remaining portion is coated with solder resist (13).

FIG. 4 shows yet another embodiment, in which solder plating (12) is applied to the entire periphery of the tip of the connecting portion (6).
4. In the above embodiment, VPS (i-vapor condensation soldering method) was used as a heating heat source, but it may be heated by placing it in an infrared oven or by applying a hot plate. Furthermore, although tin-lead eutectic solder is used as the cream solder material, solder with a different blending ratio may be used, or In-based low melting point solder with different components may be used. The substrate may also be an FPC (flexible printed circuit board) or the like in addition to a glass/epoxy copper-clad laminate.

Further, the inclination of the connecting portion (6) is not limited to 3 to 4 degrees, but may be up to about 10 degrees.

[Effects of the Invention] As explained above, the reflow soldering method of the present invention can be used even in products having narrow pitch lead terminals.
This method has the effect of preventing bridge defects and open defects, which were the most difficult problems in the past, and providing extremely cost-effective and highly reliable bonding as a mounting method for electronic components such as next-generation computers.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing one embodiment of this invention, FIG. 2 is a diagram showing another embodiment of this invention, FIG. 3 is a diagram showing still another embodiment of this invention, and FIG. 4 is a diagram showing another embodiment of this invention. 1 and 5 showing still other embodiments of the invention are diagrams showing a printed wiring board, FIG. 6 is a diagram showing a flat type electronic component, and FIG. 7 is a side view of a flat type electronic component that is not formed. , Figure 8 is a diagram showing the mounting plane using the conventional reflow soldering method, Figure 9 is a series of diagrams showing the mechanism of bridging failure, and Figure 10 is a diagram showing the critical value for whether or not a bridging failure occurs. The conceptual diagram shown in Fig. 11 is a perspective view of Fig. 9 (B), and Fig. 12 is a perspective view of Fig. 9 (B).
The figure is a sectional view taken along the line xu-xu in FIG. 11. In the figure, (1) is a printed wiring board, (2) is a land portion, (3) is cream solder, (4) is a flat type electronic component, and (5) is a lead terminal from the electronic component (4). , (6) is a connection part, and (12) is solder plating. Note that the same reference numerals in each figure indicate the same or corresponding parts.

Claims (3)

[Claims] (1) An electronic component that has at least two lead terminals protruding from the electronic component at a constant distance from each other, and the lead terminals are formed to have a connection part,
By printing viscous cream solder at room temperature on the land portions of a printed wiring board having land portions corresponding to the lead terminals, adhering the connection portions to the land portions with adhesive force, heating, and then cooling, In a method of mounting electronic components on a printed wiring board by reflow soldering, at least the front end of the connection part, and at most the bottom surface up to the base of the connection part, is relatively more wetted with solder than the rest of the lead terminal. A method for mounting electronic components by reflow soldering, which is characterized by providing an easy surface condition. (2) A method for mounting electronic components by reflow soldering according to claim 1, in which the surface is made easily wettable by solder by applying solder plating. (3) A method for mounting an electronic component by reflow soldering according to claim 1, wherein a solder resist is applied to the remaining portion of the lead terminal so that the lower surface of the connecting portion is relatively more easily wetted by the solder.