JP2001077522A - Electronic component mounting method - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To suppress solder connection after preheating before mounting electronic components or before reflow, thereby reducing solder bridges and minimizing the amount of solder to minimize open defects. Provided is a method for mounting an electronic component that does not cause any problem. SOLUTION: A QFP1 and a printed circuit board 3 provided with a plurality of pad portions 4 in accordance with a mounting position of the QFP1.
And prepare. Then, using the mask member 8, the solder paste is printed on the pad portions 4 of the printed circuit board 3 to form the printed body 2. The shape of the printed body 2 is substantially band-like, and Q
A constriction 6 is formed at a portion of the lead terminal 5 of the FP 1 where the ground portion 5a contacts, so that the width thereof is reduced by about 10 to 20% as compared with other portions. Then, the ground portion 5a of the lead terminal 5 is electrically connected to the narrowed portion of the printed body 2 where the constriction 6 is formed by soldering.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for mounting an electronic component having lead terminals, and more particularly, to a method for mounting an electronic component having lead terminals on a printed circuit board by a reflow soldering method. It is related to the shape of.

[0002]

2. Description of the Related Art FIGS. 9A and 9B are schematic views showing a mounting structure by a conventional general reflow soldering method, wherein FIG. 9A is a cross-sectional view, and FIG. is there. In this mounting structure, a plurality of lead terminals J2 pulled out from the electronic component J1 at a predetermined interval are connected to a plurality of pad portions J4 provided at positions corresponding to the lead terminals J2 on the printed circuit board J3. It is electrically connected via a solder paste J5 printed in advance on the pad portion J4 (hereinafter, this mounting method is referred to as a mounting method of an electronic component for connecting lead terminals to the pad portion by a reflow soldering method).

Here, the number of lead terminals J2 extending from the periphery of the electronic component J1 increases due to the increase in the density of the electric circuit in the electronic component J1, and for example, a QFP (QuadFla)
tPackage) and SOP (SmallOutlin)
In ePackage), the pitch of the lead terminals J2 is becoming narrower. As a result, when the solder printed on the printed circuit board is melted, excess solder overflows from the pads and the lead terminals of the printed circuit board, and the solder is connected between the lead terminals to form a bridge, which causes a bridge defect. It has become.

[0004] As a measure against this bridge failure,
The amount of solder paste to be printed on the printed circuit board may be reduced, but in this case, an open defect occurs in which the lead terminal is not connected to the pad portion due to an insufficient amount of solder or flux. For example, in the reflow soldering method disclosed in Japanese Patent No. 2689593, a portion to which a lead terminal is connected is divided into both ends on a pad portion so as not to supply solder over the entire width of the pad portion. The solder is supplied in the state in which the solder is placed. In this case, although bridging due to excessive solder does not occur, the amount of solder is greatly reduced to cause an open defect, and the bonding strength and reliability are reduced.

[0005]

The results of a detailed study of the occurrence of a bridge by the inventors are shown below. FIG.
0 is a top view showing a printed state of the solder paste J5 in the conventional mounting method. As shown in FIG. 10, in the conventional general reflow soldering method, the solder paste J5 is printed on the entire surface of the pad portion J4. However, when the amount of the solder paste J5 is large, as shown in FIG. 9B, which is a view seen from above the printed circuit board J3 after mounting the electronic component J1, the mounting of the electronic component J1 is not performed. By load, that is, lead terminal J2
By pressing the solder paste J5, the solder paste J5 is crushed and protrudes in the horizontal direction, and is similarly connected to the solder paste J5 printed on the adjacent pad portion J4.

[0006] Alternatively, even when the solder is not connected at this time, usually, when melting the solder, the solder is preheated once at about 150 ° C and then melted at about 230 ° C.
During preheating in such a heating temperature profile, the solder paste is softened and connected. Then, it was found that when the solder melted in this state, a solder bridge was formed. In other words, even if the amount of solder that was conventionally considered to be excessive is given, if the solder paste between the adjacent pad portions is not connected by the preheating stage, the solder bulges when the solder is melted and the bridge is formed. Never be.

SUMMARY OF THE INVENTION In view of the above problems, the present invention suppresses the connection of solder after preheating before mounting electronic components and before reflow, thereby reducing solder bridges and minimizing the amount of solder. It is an object of the present invention to provide a method of mounting an electronic component that does not cause open failure.

[0008]

According to the first aspect of the present invention, there is provided a method of mounting an electronic component for connecting a lead terminal to a pad portion by a reflow soldering method. Lead terminal (5)
The width of a portion in contact with the solder (2) is narrower than other portions of the solder (2). This can prevent the electronic component (1) from being connected to the adjacent solder (2) due to preheating before mounting or reflow, thereby reducing the number of solder bridges and minimizing the decrease in the amount of solder (2). Thus, it is possible to provide a method of mounting an electronic component that does not cause an open defect.

According to a second aspect of the present invention, in the first aspect of the present invention, the width of the portion of the solder (2) in contact with the lead terminal (5) is reduced to compensate for the reduced amount of solder. In addition, it is characterized in that the solder (2) is lengthened in its length direction. As a result, the total amount of solder (2) does not decrease, so that the open failure can be reliably prevented.

According to the third aspect of the present invention, it is possible to provide a mask member (8) suitable for use in the mounting method according to the first aspect. By using the mask member (8) of the present invention, the mounting method of claim 1 can be appropriately executed.

The reference numerals in parentheses of the above-mentioned means indicate the correspondence with the concrete means described in the embodiments described later.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS (First Embodiment) In this embodiment, a case will be described in which a QFP 1 is used as an electronic component and a solder 2 according to the present invention is supplied using a solder paste.
FIG. 1 is a cross-sectional view when the mounting method of the present embodiment is used. FIG. 2 is a schematic top view of the QFP 1. FIG. 3 is a top view of a printed body (solder according to the present invention) 2 in which a solder paste is printed on a plurality of pad portions 4 of a printed circuit board 3 according to the present embodiment, showing a state before the QFP 1 is mounted. Show. As shown in FIG. 1, the mounting structure of the present embodiment is configured such that the ground portions 5a of a plurality of lead terminals 5 which are drawn out from the QFP 1 at a predetermined interval as shown in FIG. 4 are electrically connected to each other by a printed body 2 formed thereon.

The lead pitch of the QFP 1 of this embodiment is 0.4 mm. The lead terminal 5 drawn out of the QFP 1 is, for example, a 42 alloy (iron-nickel alloy), which is plated with solder. FIG.
Is a partial top view of the printed circuit board 3 before the solder paste is printed.
It is provided on the printed circuit board 3 in accordance with the position of one of the plurality of lead terminals 5. The pad portion 4 is made of copper or the like. For example, as shown in FIG.
Is 1.8 mm, the width W1 is 0.23 mm, and the interval W2 between the pad portions 4 is 0.17 mm.

Further, the printed body 2 before the QFP 1 is mounted
As shown in FIG. 3, the shape is substantially band-shaped, and the width of the lead terminal 5 at the portion where the ground portion 5a contacts is reduced by about 10 to 20% as compared with the other portions. Has become. Specifically, a rectangular constriction 6 is formed so as to cut at right angles inward from two long sides of the pad portion 4. Hereinafter, the narrowed portion of the printed body 2 where the constriction 6 is formed is referred to as a cut portion 7. In this example, the length L2 of the cut portion 7 is 0.9 mm.

In FIG. 3, the printed body 2 is shown to be slightly smaller than the pad section 4, but this is for the sake of simplicity. It almost coincides with the pad portion 4.
Therefore, the dimensions L1 and W1 correspond to the length of the print body 2 and the width of the portion other than the cutout portion 7, respectively. The solder paste is formed by kneading solder powder and flux.

Next, a mounting method according to the present embodiment will be described. A QFP 1 and a printed circuit board 3 provided with a plurality of pad portions 4 in accordance with the mounting position of the QFP 1 are prepared. Then, using a mask member 8 such as a metal mask or a screen mask, the pad portion 4 of the printed circuit board 3 is formed.
The printed body 2 is formed by printing a solder paste thereon. Here, FIG. 5 is a view showing a part of the mask member 8. In the mask member 8, the printed body 2 is
The pattern 9 is formed so as to have a shape.

Next, the grounding portions 5a of the plurality of lead terminals 5 pulled out from the QFP 1 are pressed against the cut portions 7 of the printed body 2 and temporarily mounted. After that, it is placed in a heating furnace such as an infrared furnace and once preheated at about 150 ° C.
Before and after, soldering is performed by melting the solder paste.
As described above, the mounting of the QFP 1 on the printed circuit board 3 is completed.

According to the present embodiment, since the amount of the solder paste in the portion (cut portion 7) to which the lead terminal 5 is connected is smaller than in the other portion of the printed body 2, the amount of the solder paste in the portion where the QFP 1 is mounted is reduced. The collapse of the solder paste is suppressed, and the connection with the adjacent solder paste during mounting can be reduced. Further, it is possible to suppress the solder paste from being softened and connected during the preheating. Therefore, formation of a solder bridge can be significantly reduced.

In addition, since the width of the printed body 2 is not reduced except at the cut portion 7, the amount of the entire solder paste is not reduced so much that an open defect does not occur. Further, even in a portion where there is no solder due to the cut of the printed body 2, the solder spreads after soldering, and the bonding state between the lead terminal 5 and the pad portion 4 is no different from the conventional method.

Here, a study example that led to the formation of the printed body 2 as in this embodiment will be described below. QFP1
As described above, the one having a lead pitch of 0.4 mm was used, and the printed circuit board 3 having the pad portion 4 having the above dimension example corresponding to the QFP 1 was used. Then, after printing a solder paste on the pad portion 4 to form a substantially belt-shaped printed body, the QFP 1 was mounted on the printed circuit board 3 and soldered as described above. Here, the width of the substantially band-shaped printed body is uniform over the length direction,
The occurrence of bridges was examined by changing the width.

FIG. 6 is a graph showing the rate of occurrence of bridges when the width of the printed material is variously changed. In this figure, "after mounting" means after pressing the ground portion 5a of the lead terminal 5 against the printed body, and "after preheating" means "after preheating in the heating temperature profile at the time of soldering" as described above. Show. According to this, it can be seen that when the width of the print is reduced to 0.21 mm or less, the bridge generation rate is significantly reduced, and when the width is reduced to 0.2 mm or less, no bridge occurs even after preheating. As described above, the fact that no bridge occurs after preheating means that no bridge occurs due to melting of the solder paste.

However, when the width of the printed body is uniformly reduced as described above, the amount of the solder paste is reduced, and an open failure occurs. Also, considering that the cause of the bridge is that the solder paste is connected at the portion of the printed body where the grounding portion 5a of the lead terminal 5 abuts, the width of the printed body at the portion where the grounding portion 5a abuts is reduced. You can make it narrow.

Therefore, in order to suppress both the open defect and the bridge defect, as in the present embodiment, the shape of the printed body 2 is reduced in the width of only the portion of the lead terminal 5 that contacts the ground portion 5a. . For example, based on the example of FIG. 6 described above, the print body 2 has a width (corresponding to W1 in FIG. 3) other than the notch 7 of 0.23 mm and a width of the notch 7 of 0.2.
The shape is 1 mm or less, preferably about 0.2 mm. Note that, in order to suppress the occurrence of an open defect, the thickness is not reduced more than necessary.

It should be noted that in the present embodiment, the constriction 6
Is a rectangle, but the constriction 6 may be trapezoidal, triangular, or a shape including a curve as shown in FIG. 7, which is a top view showing another shape of the printed body 2. However, the amount of the solder paste supplied to the printing body 2 is set to be substantially the same as that of the rectangular shape. In order to obtain such a shape, it is only necessary to change the shape of the pattern 9 on the mask member 8. In addition, for example, infrared rays are used as a heating heat source at the time of soldering, but a VPS device that heats by utilizing latent heat of condensation of steam may be used, or a hot plate may be applied.

The pad portion 4 is not limited to copper, and may be, for example, a printed board 3 coated with eutectic solder. The lead terminals 5 may also be made of, for example, a copper alloy. Further, as the electronic component 1, a flat type such as SOP can be used. Furthermore, the dimensions of the pad portion 4 and the printed body 2 are merely examples, and are not limited to these values, and can be changed as appropriate.

(Second Embodiment) This embodiment is a modification of the first embodiment, in which the shape of the printed body 2 is modified.
FIG. 8 is a top view of the print 2 of the present embodiment. As shown in FIG. 8, similarly to the first embodiment, a rectangular constriction 6 is formed in a portion of the printed body 2 where the ground portion 5 a of the lead terminal 5 is connected. Further, a portion facing the tip of the lead terminal 5 when the QFP 1 is mounted (FIG. 8)
The length of the printed body 2 in the section A) is extended to compensate for the reduced amount of solder paste due to the formation of the constriction 6 in the printed body 2. That is, the print amount of the solder paste is set to be the same amount as when the constriction 6 is not formed. In addition, the dimensions of the pad portion 4 and the degree of the constriction 6 are all the same as in the first embodiment, and the mounting method is also the same as in the first embodiment.

According to the present embodiment, the same effects as those of the first embodiment can be obtained. Further, in a portion which does not affect the formation of the bridge, since the amount of the solder paste which is reduced by forming the constriction 6 is supplied so as not to reduce the amount of the solder paste, the open failure can be more reliably prevented. . In this example, the printed body 2
Extends in the direction of the tip of the lead terminal 5,
If extended in the opposite direction, a bridge may be formed. Therefore, it is preferable to extend in the direction of the tip of the lead terminal 5 as in the present embodiment. The shape of the constriction 6 is the first
Various shapes can be applied as in the embodiment.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view when the mounting method of the first embodiment is used.

FIG. 2 is a schematic top view of a QFP.

FIG. 3 is a top view of the printed body according to the first embodiment.

FIG. 4 is a partial top view of a printed circuit board before printing a solder paste.

FIG. 5 is a view showing a part of a mask member.

FIG. 6 is a graph showing a relationship between a width of a printed body and a bridge occurrence rate.

FIG. 7 is a top view showing another shape of the print in the first embodiment.

FIG. 8 is a top view of a printed body according to the second embodiment.

FIG. 9 is a schematic diagram showing a mounting structure by a conventional general reflow soldering method.

FIG. 10 is a top view showing a printed state of a solder paste in a conventional mounting method.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Electronic component, 2 ... Solder, 3 ... Printed circuit board, 4 ... Pad part, 5 ... Lead terminal, 8 ... Mask member, 9 ... Pattern.

Claims (3)

[Claims] A plurality of lead terminals (5) drawn out from an electronic component (1) at predetermined intervals are provided at positions corresponding to the lead terminals (5) on a printed circuit board (3). For a plurality of band-shaped pad portions (4),
In a method of mounting an electronic component which is electrically connected to the pad portion (4) via a substantially band-shaped solder (2) supplied in advance, a portion of the solder (2) in contact with the lead terminal (5). Wherein the width of the electronic component is smaller than that of the other part of the solder (2). 2. The solder (2) is lengthened in its length direction so as to compensate for the reduced amount of solder by reducing the width of a portion of the solder (2) that contacts the lead terminal (5). The method for mounting an electronic component according to claim 1, wherein: 3. A plurality of lead terminals (5) extending from the electronic component (1) at predetermined intervals are provided at positions corresponding to the lead terminals (5) on the printed circuit board (3). For a plurality of band-shaped pad portions (4),
A mask used to supply the solder (2) to the pad portion (4) when the pad portion (4) is electrically connected to the pad portion (4) via a substantially band-shaped solder (2) supplied in advance. A member having a substantially band-shaped pattern (9) corresponding to a supply pattern of the solder (2), wherein the pattern (9) is the lead terminal (5) of the supplied solder (2); A width of a portion corresponding to a position where the pattern abuts is narrower than other portions of the pattern (9).