JPH088526A - Mounting method for electronic part, printed wiring board and electronic part - Google Patents

Abstract

PURPOSE:To prevent generation of faulty soldering due to the deviation of part supply position by a method wherein at least a set of pad and lead, which are corresponding with each other, are formed widely in the direction of arrangement, and an electronic part is shifted to the appropriate position by the surface tension of fused solder. CONSTITUTION:Only a part of a lead 12 is different on an IC 10A. On the other hand, the pad 16, to be provided on a printed wiring board 14A, is formed in such a manner that it conforms to the leads 12 and 12A of the IC 10A. Solder and flux are fed to pads 16 and 16A, and the IC 10A is mounted by a mounter. As the lead 12A and the pad 16A, located at both ends of the IC 10A, have wide width, a part of them is overlapped, and the lead 12A and the pad 16A are prevented from coming off. When solder, which is melted by heating, is allowed to flow under the above-mentioned state, the IC 10A moves on the pads 16 and 16A by the surface tension of the molten solder 18A, and they are automatically corrected to the right position.

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 electronic parts suitable for mounting ICs having leads with a narrow pitch, a printed wiring board used for carrying out this method, and an electronic part.

[0002]

2. Description of the Related Art In order to reduce the size and weight of electronic equipment, electronic parts are becoming smaller and more densely mounted. Therefore I
It is required that the electronic components such as C be of the surface mounting type and that the lead interval be further narrowed. For example, a lead pitch narrowed to 0.5 mm or less is also used.

As an example of an electronic component having such a narrow pitch lead, TSOP (THIN SMALL OUTLINE PACKAG)
A method of mounting an E) type IC will be described with reference to FIGS.
FIG. 5 is a perspective view of an example of a TSOP type IC (FPT-20P-M04), and FIG. 6 is a view showing a position shift state at the time of mounting.
VI arrow view in FIG. 7, and FIG. 7 are explanatory views of the mounting method.

In FIG. 5, reference numeral 10 is an IC having a rectangular and plate-shaped package, and two ICs are provided on a pair of short sides.
Zero gull wing type leads 12 are provided. The leads 12 are divided into groups of five, and two pairs of leads 12 are provided at wide intervals on both short sides of the package 10. That is, as shown in FIG. 6, the lead 12 of one set has a pitch of about 0.5 mm and a lead interval of 0.3 m.
m, the lead width is 0.2 mm, and the interval between adjacent groups is about 1.3 mm.

Pads 16 are formed at intervals corresponding to the leads 12 on the printed wiring board 14 on which the IC 10 is mounted. That is, when a circuit pattern is formed on a copper-clad laminate having a copper foil adhered on its surface by etching, these pads 16 are also formed by the same process. Then, these pads 16 are plated with copper.

Solder 18 is supplied to these pads 16 as shown in FIG. There are various methods for supplying the solder 18. For example, cream solder on each pad 1
6 includes a method of printing using a squeegee, and a method of covering a portion other than the pad 16 with a plating resist and performing thick film plating of solder.

A flux 20 having adhesiveness is supplied onto the solder 18 of the printed wiring board 14 as shown in FIG. 7B. For example, the flux 20 is continuously applied along the arrangement direction of the pads 16 using a computer-controlled dispenser. This flux 20 is IC1
It serves as an adhesive that temporarily fixes the IC 10 when mounting 0, and also improves the solderability by cleaning the soldering surface.

Next, the IC 10 is carried from an automatic mounting machine (mounter) to a predetermined mounting position and temporarily fixed by a flux 20 as shown in FIG. 7 (C). That is, IC10
Each lead 12 is aligned and temporarily fixed so as to come on the corresponding pad 16. When the entire printed wiring board 14 is heated by the infrared heater 22, the solder 1
8 melts (reflows), and each lead 12 is soldered to the corresponding pad 16 as shown in FIG. 7 (D). Figure 7
In (D), 18A indicates molten solder.

[0009]

2. Description of the Related Art Here, when the pitch of the leads 12 becomes narrower as the IC 10 becomes smaller and the density thereof becomes higher, I
A slight positional deviation when supplying C10 onto the pad 16 by the mounter becomes a problem. If the displacement of the supply position of the IC 10 by the mounter is sufficiently small as compared with the lead pitch, the lead 12 moves to the vicinity of the center of the corresponding pad 16 due to the surface tension of the molten solder 18 when the solder 18 reflows. That is, self-correction is possible by reflow.

However, when the lead pitch is narrowed, the displacement of the IC 10 due to the mounter becomes relatively large, and the leads 12 may temporarily be fixed to the positions separated from the pads 16 as shown in FIG. 6, for example. In this case, even if the solder 18 is reflowed, the molten solder 18A is still attached to the lead 1
2 does not come into contact and the self-correction due to the surface tension is not performed. Therefore, soldering failure will occur.

[0011]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and when reflow soldering a surface mount component such as an IC, it is possible to prevent the occurrence of a soldering defect due to a displacement of a component supply position. It is an object of the present invention to provide a possible electronic component mounting method, a printed wiring board and an electronic component which are directly used for implementing this method.

[0012]

According to the present invention, in this mounting method, solder is supplied in advance to a large number of pads arranged on a printed wiring board so as to be aligned in a straight line, and a large number of leads of an electronic component are positioned on the corresponding pads. In the mounting method of electronic parts which are combined and reflow soldered, at least 1 corresponding to each other.
A method for mounting an electronic component, characterized in that the pad and the lead of the set are formed widely in the arrangement direction of the pad and the lead, and the electronic component is moved to an appropriate position by the surface tension of the molten solder during reflow soldering. Achieved by.

The printed wiring board is characterized in that a large number of pads corresponding to the leads of electronic components to be mounted are arranged in a straight line, and pads at both ends of these pads are widened in the arrangement direction. Achieved by.

The electronic component is achieved by a surface-mounting electronic component, characterized in that, of a large number of leads arranged in a straight line, the leads at both ends are widened in the arrangement direction.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a diagram showing a positional deviation state of the present invention, FIG.
Is a layout view of pads on a printed wiring board, FIG. 3 is an explanatory view of a self-correction action, and FIG. 4 is an enlarged view of relevant parts.

The IC 10A used in this embodiment is the same as the IC 10 shown in FIG. 5, except for a part of the lead 12. That is, only the leads 12A, 12A at both ends of each set of five leads 12 have a width (0.4 mm) twice the width (0.2 mm) of the three leads 12 between them. Further, while the distance between the leads 12 and 12 is 0.3 mm, the distance between the lead 12A at the end and the lead 12 adjacent thereto is set to 0.325 mm.

On the other hand, the pad 16 provided on the printed wiring board 14A is formed so as to fit the leads 12 and 12A of the IC 10A. That is, only the pads 16A, 16A at both ends of each set of five pads 16 have a width (0.5 mm) twice the width (0.25 mm) of the three pads 16 therebetween. Also all pads 16, 16A
Are all 0.25 mm.

Solder 18 and flux 20 are supplied to the pads 16 and 16A by the same procedure as described with reference to FIG. 7, and the IC 10A is mounted by the mounter. At this time, the IC 10A is usually slightly displaced from the pads 16 and 16A. As shown in FIG. 1, the IC 10 may be displaced from the correct position, and the lead 12 may come off the pad 16.

Here, since both the lead 12A at the end and the pad 16A at the end are wide, it is assumed that the lead 12A and the pad 16A partially overlap each other without coming off. When the solder 22 is reflowed by the heater 22 (see FIG. 7) in this state, the IC 10A moves on the pads 16A and 16 due to the surface tension of the molten solder 18A (FIG. 1) and is automatically corrected to the correct position (self-correction). Correction effect).

This self-correction action will be described with reference to FIGS. The lead 12A is shown in FIG. 3 for one pad 16A.
If the solder 18 melts in the state of being displaced in the width direction as shown in FIG. 5, the surface tension of the molten solder 18A acts on the two surfaces a and b of the lead 12A and the pad 18A to reduce the positional deviation, and the self-correcting force is reduced. F is generated.

Surface tension F acting on the surface a of the lead 12A
₃ (FIG. 4) is F ₃ = [surface tension per unit length] × [contact portion length] = f ₃ × sin θ × L. F here
₃ is a surface tension acting between the molten solder 18A and the lead 12A, which is 0.50 gf / cm. θ is a contact angle on the surface a of the molten solder 18A, which is about 45 °. Since the contact length L is 0.050 cm, F
₃ = 0.50 x 0.71 x 0.050 = 0.0177g
f.

Since the same surface tension F ₃ acts on the surface b of the pad 16A, the self-correcting force F acting on one lead 12A is F = 2F ₃ = 0.035 gf.
If this force F is larger than the drag force R that prevents the movement between the pad 16A and the lead 12A, the IC 10A can move and a self-correction action occurs. Therefore, the magnitude of this drag force R is next determined.

This drag force R is considered to be a frictional force acting between the lead 12A or the pad 16A and the molten solder 18A. If this friction coefficient is μ and the pressure is P, R =
It is calculated by μ × P. Here, the pressure P is defined as follows: M is a pressure due to its own weight, H is a pressure due to surface tension, and F ₀ is buoyancy.
It is obtained by P = M + H−F ₀ .

M is eight leads 12 when the solder is melted.
Since it is considered that the IC 10A is supported by A and the eight pads 16A, assuming that the weight of the IC 10A is 0.3 gf, M = 0.3 / 8 gf.

H is the length of the contact edge of the molten solder 18A (2d + 2L) (d is the width of the contact portion, L is the length of the contact portion).
H = (surface tension) × (2d + 2L) × cos θ = 0.50 × 2 (0.050 + 0.0175) × cos 45 ° = 0.048 gf.

Since the buoyancy F ₀ is very small, if omitted, P =
It becomes 0.085 gf. If the frictional resistance coefficient μ is 0.3, the resistance force R = μP = 0.025 gf. This resistance force R is F−R = 0.
035-0.025 = 0.01 gf, and this difference (F
-R) leads 12A of IC 10A to pad 16A
Attracted towards the center of. And the narrow lead 1
When 2 touches the corresponding pad 16, the self-correction force F increases, and the IC 10A can move to the correct position with a larger force.

In this embodiment, the leads 12 are divided into four groups of five, and the leads 12A at both ends of each group and the corresponding pads 16A are wide, but the present invention is not limited to this. Widening at least one of the leads in a row and the corresponding pad provides the self-correcting action of the present invention, and the present invention includes such.

However, in order to allow the IC to be held on a wide pad even if a normal lead having a wide width is disengaged from the pad, at least three leads which are not aligned and a pad corresponding to these leads are arranged. It is necessary to widen and. Therefore, it is desirable to widen the leads at the four corners of the IC and the corresponding pads. That is, it is preferable to widen the leads at both ends among the leads arranged in a straight line.

The present invention is suitable for a narrow-pitch lead IC, but can also be applied to surface-mounting electronic parts other than ICs. In addition, SOP (Sm with gull wing type lead
Not limited to allOutline Package) type ICs, other types of leads, such as SOJ (Small Outline J-bend Package) type and PL, which have leads bent inward in a J shape
CC (Plastic Leaded Chip Carrier) type IC and QFP (Quad Fla) with gull wing type leads on four sides
t Package) type IC etc.

[0030]

As described above, according to the invention of claim 1, a part of the leads of the surface mount type electronic component and the corresponding pads on the printed wiring board side are widened in the arrangement direction. Even if a slight misalignment occurs when the electronic parts are supplied to the printed wiring board by the mounter, the electronic parts move to the correct position due to the surface tension of the molten solder as long as wide leads and pads overlap each other (self-correction). Action). Therefore, it is possible to prevent defective soldering. Further, it becomes unnecessary to remarkably enhance the positioning accuracy of parts of the mounter.

According to the inventions of claims 2 and 3,
The printed wiring board and the electronic component used directly for carrying out the invention of claim 1 can be obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of leads and pads according to the present invention.

[Fig.2] Pad layout of printed wiring board

FIG. 3 is an explanatory diagram of a self-correction action.

FIG. 4 is a partially enlarged view of the same.

FIG. 5 is a perspective view showing an example of a conventional electronic component.

FIG. 6 is a view on arrow VI showing the leads and pads.

FIG. 7 is an explanatory diagram of a component mounting process.

[Explanation of symbols]

 10 IC as an electronic component 12 Lead 12A Wide lead 14, 14A Printed wiring board 16 Pad 16A Wide pad 18 Solder 18A Molten solder 20 Flux 22 Heater

Claims (3)

[Claims] 1. An electronic component in which solder is supplied in advance to a large number of pads arranged on a printed wiring board so as to be aligned in a straight line, and a large number of leads of the electronic component are aligned with the corresponding pads for reflow soldering. In a mounting method, at least one set of the pad and the lead corresponding to each other is
Widely formed in the array direction of pads and leads,
A mounting method of an electronic component, wherein the electronic component is moved to an appropriate position by surface tension of molten solder during reflow soldering. 2. A printed wiring board characterized in that a large number of pads corresponding to leads of electronic components to be mounted are arranged in a straight line, and pads at both ends of these pads are widened in the arrangement direction. 3. An electronic component for surface mounting, characterized in that, among a large number of leads arranged in a straight line, the leads at both ends are widened in the arrangement direction.