JP2689593B2 - Electronic component mounting method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a mounting method of an electronic component having a lead terminal. More specifically, the present invention relates to the shape of solder that is supplied in advance in reflow soldering for mounting an electronic component for surface mounting having lead terminals.

[Conventional technology]

In recent years, reflow soldering methods have been widely adopted for mounting electronic components on printed wiring boards. This reflow soldering method will be generally described with reference to FIGS. 4 and 5. FIG. 4 is a plan view showing the printed wiring board (1), and FIGS. 5 (A) and 5 (B) respectively show this board (1).
FIG. 3 is a plan view showing a flat-type electronic component mounted on. A pad portion (2) is provided on the substrate (1). The pad portion (2) is made of, for example, an exposed copper layer. The group of strip-shaped pad portions arranged in two rows at the lower left in the figure is for the bidirectional flat type electronic component shown in FIG. 5 (A). The group of pad portions at the lower right of the substrate (1) shown is for the four-direction flat type electronic component shown in FIG. 5 (B). As shown in FIGS. 5A and 5B, the flat type electronic component has a large number of lead terminals (5) around the electronic component. Now, in order to mount these flat type electronic components on a substrate, a solder paste is printed on the pad portion (2) of the substrate (1) using a metal mask or a screen mask, and lead terminals ( 5) is pressed and temporarily attached by the adhesive force of the solder paste, and then the solder paste is melted by a heating process using infrared rays or steam, and then solidified.

The main problem of the reflow soldering method having a lead terminal is the defective bridging (7) shown in the perspective view of the pad portion (2) and the lead terminal (5) after mounting in FIG. 6 (lead terminal (5 ) Occurrence of solder (3) between them and short-circuiting) and open failure (the lead terminal (5) is not connected to the pad part (2) and is floating)
Is the occurrence of. In addition, (6) is a connecting portion of the lead terminal (5). As the degree of integration of the electric circuit in the electronic component increases, the number of lead terminals drawn from the periphery of the electronic component increases and the interval of the arrangement of the lead terminals becomes narrower. Thus, when high-density mounting is performed, generally speaking, if the amount of solder is too large, brittle defects occur, and if the amount of solder is too small, open defects occur.

In the conventional technique, in order to prevent bridging defects, it is basically necessary to supply the solder paste so that it does not overflow from the pad portion or to reduce the amount of solder. For example, a feature of the reflow soldering method disclosed in Japanese Patent Publication No. 57-29072 is that a solder paste is screen-printed on the pad portion so as not to overflow from the pad portion. Until then, the amount of solder that had existed over the entire pad area due to solder plating and swelling around the pad area was reduced by screen-printing the solder paste as described above, and thus, for example, 0.635 mm. It is reported that the occurrence of bridging defects was eliminated even with a narrow lead terminal interval.

However, this method does not cause bridging defects when the amount of solder supplied is extremely small, but the strength of the joint decreases, causing a problem in terms of reliability, and securing strength that does not have a problem in reliability. If the amount of solder supplied is large, brittle defects occur. This is because it is difficult to supply solder paste accurately with the current process.
Even if the paste is printed so that it does not stick out from the pad part, the solder paste may stick out when the component is pressed after that, or the viscosity of the solder paste may drop during heating, causing it to stick out from the pad part and cause bridging failure. It will be.

In Japanese Patent Application Laid-Open No. 58-132940, when mounting an electronic component having lead terminals on a substrate, a solder paste layer having a width narrower than the length of the connection portion of the lead terminals is applied to the area surface where the pad portion is present on the substrate. It is characterized by being applied uniformly at one time. Until then, a solder paste layer having a width wider than the length of the connecting portion of the lead terminal was uniformly applied. In other words, this Japanese Patent Application Laid-Open No. 58-132940 also aims to reduce the amount of solder.

However, since the length of the connection of the lead terminals is very short, this method of printing solder paste with a width narrower than the length of the connection does not reduce the amount of solder even if the lead terminals are bonded to the pad. Due to the small number, the joint is unstable and cannot guarantee quality for many years.

On the other hand, if the thickness of the solder paste is increased and increased, the melted solder remains stable while straddling the adjacent pad portions, and bridging defects cannot be removed.

However, the reflow soldering method is not enough to adjust the amount of solder to be supplied and to firmly and stably bond the pad part and the connection part of the lead terminal for many years, and to suppress the occurrence of bridging defects. Was required to be developed.

[Problems to be solved by the invention]

In other words, in the conventional reflow soldering method in mass production used when mounting electronic components on a printed wiring board, there is a bridging defect when the amount of solder is large, and there is an open defect when the amount of solder is small, and it has existed for many years. That is, there is a problem that the bonding stability cannot be obtained.

The present invention has been made in order to solve such a problem, and is a bridging defect in which an electronic component having a lead terminal is mounted on a printed wiring board with a sufficient solder amount and the adjacent lead terminals are shorted. It is an object of the present invention to provide a mounting method of an electronic component in which a highly reliable joint can be obtained by a reflow soldering method that eliminates the occurrence of

With the current solder paste printing technology, the thixotropy of the solder paste (the property that the viscosity decreases when shear stress is applied, that is, the viscosity decreases when printing) makes it possible to print with high accuracy. It is difficult to control the amount and printing accuracy, and dripping occurs during repeated printing, causing the solder paste to stick out of the pad. This dripping is inevitable in mass production. Furthermore, sagging occurs when electronic components are mounted and when heated. And in the current process, such dripping is inevitable.

Here, if the absolute amount of solder is small, dripping is small, and even a method of uniformly and uniformly applying solder does not result in bridging defects, but on the contrary, open defects are observed, and the bonding strength and reliability are deteriorated. The open failure is caused by a slight variation in the height of the connecting portion of the lead terminal. This open defect is difficult to find because the connection portion is pushed down when the probe pin is applied in the continuity test, and the probe is conducted to make an OK determination. In contrast, bridging defects are easy to detect. Therefore, avoiding open defects,
The predetermined amount is determined by increasing the amount of solder in order to secure the bonding strength and reliability. Although the solder paste is printed so that the print shape is as shown in FIG. 7 with respect to this predetermined amount, it is difficult to control the print amount and print accuracy with high precision, so the predetermined amount (target amount) It is supplied in a larger amount, or it hangs and sticks out from the pad part, causing the occurrence of bridging. FIG. 7 is a sectional configuration view showing the lead terminal (5) of the electronic component (4) and the pad portion (2) of the substrate (1), showing the state before joining and immediately before melting the solder paste (3). Shows.

This will be described in more detail with reference to FIG. 8th
Figures (A) and (B) are explanatory views showing the relationship between the volume of solder and the volume that can be filled by the conventional printed shape, and (A) is the solder before the joining of the lead terminal (5) and the pad portion (2). The state immediately before the melting of the paste is shown, and (B) shows the state after the melting of the solder paste after joining. If the total volume Va of the metal components of the solder paste (substantially equal to the volume immediately before melting the solder paste) is larger than the fillable volume Vb,
That is, in the case of Vb / Va <1, bridging occurs by bridging between pad parts. When 1 ≦ Vb / Va <A (A is a constant), depending on the amount of solder paste protruding from the pad, that is, the degree of sagging, it may or may not become a bridge, and Vb / Va ≧ A In that case, it does not become a bridge. Here, A is a constant larger than 1.

In other words, because the printing accuracy is poor, the solder supply amount increases,
Vb / Va <1, or dripping that is unavoidable in the current process mainly occurs during component mounting, and the solder paste sticks out between the pad parts by the time of heating reflow,
Vb / Va <A, which is accompanied by dripping, is occurring, and brittles are occurring.

Now, the problem is how to increase the value of Vb / Va (solder filling volume / solder supply amount).

[Means for solving the problem]

The mounting method of the electronic component of the present invention, an electronic component having a plurality of lead terminals protruding at a predetermined interval, in a position corresponding to the lead terminal on the printed wiring board, to the pad portion to which the solder is supplied in advance. What is joined by reflow soldering, the solder is divided into the pad portion at the tip side and the root side of the lead terminal, and the solder supplied to the root side and the lead terminal are supplied so as not to contact, The solder is melted so that the solder supplied to the base side is used for joining between the lead terminal and the pad portion after the solder supplied to the tip side.

[Action]

In the present invention, in the solder melting process, the solder supplied to the root side of the lead terminal is used for joining between the lead terminal and the pad portion after the solder supplied to the tip side, and the Since the value of (volume for filling with solder / solder supply amount) is increased, bridging can be suppressed even if a large amount of solder is supplied or some sagging occurs.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a cross-sectional view of a main part showing a method of mounting an electronic component according to an embodiment of the present invention, showing a state in which an electronic component immediately before melting of solder, in this case, solder paste, is placed on a substrate. In the figure, (1) is a printed wiring board, (2) is a pad portion, (3) is a solder paste, (4) is a flat type electronic component, and (5) is a lead terminal output from the electronic component (4). , (6) are solder connection portions which are the tip portions of the lead terminals (5). The connection part (6) has an inclination angle of 3 to 4
Degrees. The pad section (2) is copper. Solder paste (3) is 10-150μ of tin-lead eutectic solder (melting point 183 ℃)
It is a basted product obtained by kneading m particle powder (about 90 weight percent), a flax component (about 10 weight percent) and special additives. The flux consists of 50 weight percent solids and 50 weight percent solvent. Solids include rosin, activators, thixotropic agents, and stabilizers. The solvent component is glycol or the like. The special additive takes printability and viscosity at room temperature into consideration. The solder paste used in this example has a poise of 109,900,000 cPoise at room temperature and 2-5 cPoise at 183 ° C. The lead terminal (5) is 42 alloy (iron-nickel alloy), to which 90Sn-10Pb solder plating (melting point 215 ° C) is applied. Pad width is 0.40 mm, pad spacing is 0.25
mm. So the adjacent pad and lead pitches are 0.65 mm.

Now, the difference from the conventional one shown in FIG. 7 is that the solder paste (3) is divided into the pad portion (2) on the tip side and the root side of the lead terminal (5), and the divided solder on the root side. The paste (3) is printed and supplied so as not to come into contact with the lead terminals (5).

After mounting the electronic component (4) on the board (1) on which the solder paste is printed in this way, the solder paste (3) is heated and melted in an infrared furnace to perform soldering. FIGS. 2A to 2D are explanatory views sequentially showing the behavior of the solder paste (3) during soldering in this embodiment, and FIG.
(B) shows the start of melting of the solder paste, (C) shows the middle of the progress of the wetting of the lead wire, and (D) shows the final state where the divided solder pastes are combined. As shown in FIG. 2, the solder paste (3) starts to melt by being heated by infrared rays (FIG. 2B), and the divided lead tips are in contact with the lead terminals (5). As shown in Figure (C), the lead wire (5) is filled with the progress of wetting, and is subsequently merged with the other solder on the base side, and the final formation is the conventional non-divided printing. FIG. 2D shows the same soldering shape as the final shape resulting from the shape's solder paste (FIG. 2D).

Normally, the area where the lead terminals (5) and the solder paste (3) are in contact with each other is the place where the most dripping occurs, but as shown in FIG. Value (solder filling volume Vb ₁ / solder supply amount Va ₁ ) can be temporarily increased when wetting progresses, so the solder that squeezes out between the pad parts (2) is also absorbed on the pad parts (2). The occurrence of bridging can be suppressed. FIGS. 3A and 3B are explanatory views showing the relationship between the volume of the solder on the tip side of the lead terminal and the fillable volume thereof in this embodiment, and FIG. 3A is a state before joining and immediately before melting the solder paste. (B) shows that after melting the solder paste,
The state of FIG. 2 (C) is shown.

In the above embodiment, when the solder paste is printed on the pad portion, it may be printed with the solder paste slightly protruding, and even if the thickness of the solder paste is increased under the condition that the printing accuracy is not so good. no problem.

Further, instead of the solder paste, the solder may be supplied in a divided shape by soldering or the like.

Further, in the above embodiment, infrared rays were used as the heating heat source, but it is also possible to use a VPS device that heats by utilizing the latent heat of condensation of steam, or to apply a hot plate. In addition, tin-based materials for solder paste
Although the lead eutectic solder is used, a solder having a different compounding ratio from this may be used, or an In-based low melting point solder having different components may be used. The pad portion (2) is not limited to copper, and eutectic solder may be coated, for example. 42 lead terminals (5)
Even if the alloy is not 90Sn-10Pb soldered,
For example, a copper alloy or the like may be used. Furthermore, the inclination of the connecting portion (6) is 3
It is not limited to 4 degrees, but about 10 degrees is good.

Furthermore, in the above embodiment, the electronic component having a pitch lead of 0.65 mm has been described, but it goes without saying that the present invention can be applied to an electronic component having a narrower pitch lead such as a pitch of 0.50, 0.40 mm.

〔The invention's effect〕

As described above, according to the present invention, an electronic component having a plurality of lead terminals protruding at a predetermined interval is provided on a printed wiring board at a position corresponding to the lead terminals, and a pad portion to which solder has been supplied in advance. In the method of mounting the electronic component on the printed wiring board by heating and melting the solder after placing the lead terminal in contact with the lead terminal, the lead terminal is provided with the solder on the pad portion. Is divided into the tip side and the root side, and is supplied so that the solder supplied to the root side and the lead terminals do not contact, the solder is melted, and the solder supplied to the root side is supplied to the tip side. Since it is designed to be used for joining between the lead terminal and the pad portion after the solder, the bridging, which has been the most difficult point in the past, even in the case where the lead terminal has a narrow pitch. Good and to prevent the open failure,
For example, as an electronic component mounting method for a next-generation computer or the like, there is an effect that a highly reliable joint having excellent cost performance can be obtained.

[Brief description of the drawings]

FIG. 1 is a cross-sectional configuration diagram of a main part showing an embodiment of the present invention, and FIGS. 2 (A) to (D) are explanatory views showing the behavior of a solder paste in the heating process of this embodiment in order. FIGS. 4A and 4B are explanatory views showing the relationship between the volume of the solder on the tip side of the lead terminal and the fillable volume thereof, respectively. FIG. 4 is a plan view showing the printed wiring board, and FIG. A)
(B) is a plan view showing a flat type electronic component,
FIG. 6 is a perspective view showing the shape of a brittle defect, FIG. 7 is a cross-sectional view of the principal part showing the printed shape of a conventional solder paste, and FIGS. 8 (A) and 8 (B) are solders each having a conventional printed shape. It is explanatory drawing which shows the relationship of the volume and the volume which can be filled. In the figure, (1) is a printed wiring board, (2) is a pad portion, (3) is solder paste which is solder, (4) is a flat type electronic component, and (5) is an electronic component (4). The lead terminal (6) is a tip portion of the lead terminal (5), which is a solder connection portion. In the drawings, the same reference numerals indicate the same or corresponding parts.

Claims (1)

(57) [Claims] 1. An electronic component having a plurality of lead terminals projecting at a predetermined interval is placed on a printed wiring board at a position corresponding to the lead terminals by applying the lead terminals to a pad portion to which solder has been supplied in advance. After being placed in contact with each other, heating is performed to melt the solder, and then cooling is performed to mount the electronic component on the printed wiring board. In the method, the solder is attached to the pad portion at the tip side and the root side of the lead terminal. Divided into, and supplied so that the lead terminal and the solder supplied to the root side do not contact, melt the solder, the solder supplied to the root side from the solder supplied to the tip side after the above A mounting method for an electronic component, characterized in that the mounting method is used for joining between a lead terminal and a pad portion.