JPH0715131A - Mounting method of electronic component - Google Patents

Abstract

PURPOSE:To provide a mounting method of electronic components with fewer soldering bridge defects between neighboring lead terminals. CONSTITUTION:Solder precoats 12a and 12b are applied, before printing cream solder 3a and 3b, on terminals of IC's with higher coplanarity, on a part or all of pads 2a and 2b that correspond with electronic component electrodes that require ample supply of solder, or on a part or all of the pads 2a and 2b of the other electronic components. An adhesive or sticky flux is formed on the cream solder 3a and 3b that correspond with the precoat 12a and 12b by painting.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface mounting method for electronic parts by reflow soldering on a printed wiring board.

[0002]

2. Description of the Related Art Generally, as surface mounting of electronic parts, solder cream is printed on a pad made of a copper material or the like on a printed wiring board so that terminals or electrodes of the electronic part correspond to the solder cream on the pad. After mounting the electronic components on the board, it is mounted by reflow heating. In such a mounting method, FIG. 4 is a plan view showing a conventional printed wiring board, and FIG. 5 is a plan view showing a mounting state of a conventional electronic component. In these figures, 1 is a printed wiring board, 2a and 2
b is a pad formed by etching a copper foil on a printed wiring board, 3a and 3b are substantially the same size on the pads 2a and 2b respectively, and cream solder formed by metal mask printing, and 4 is a pad 2a Narrow pitch IC of flat package type with narrow terminal pitch,
5 is a wide pitch IC of a flat package type with a wide terminal pitch to be mounted corresponding to the pad 2a, 6 is a terminal protruding from the narrow pitch IC 4 and the wide pitch IC 5 at a predetermined interval, and 7 is corresponding to the pad 2b. Small chip component to be mounted, 8 is a large chip component mounted so as to correspond to the pad 2b, 9 is electrodes arranged at both ends of the small chip component 7 and the large chip component 8, and 10 is a pad 2a by reflow heating. It is a solder bridge formed between the comrades. 6 (A) to 6 (C) are a series of cross-sectional explanatory views showing a mechanism of unsolder failure occurrence in a conventional IC mounting method, and FIGS. 7 (A) and 7 (B) show a mounting state after a conventional chip component reflow. FIG. In these figures, 11a and 11b are solders which are crystallized by reflow heating.

Next, the operation will be described. Generally, IC
Due to manufacturing errors and external forces during the transportation process, the heights of the lower tips of the terminals 6 become uneven, resulting in step differences.
The maximum value of all terminals in the above is commonly called coplanarity. In FIG. 6, before mounting the narrow pitch IC 4 and the wide pitch IC, both ICs have coplanarity a and b, respectively, as shown in FIG.
Since a wider pitch IC has a larger manufacturing error, the coplanarity is generally a <b.
When both ICs are placed on the corresponding pads 2a by the component mounting machine as shown in (B), the terminals 6 rush into the cream solder 3a due to the weight of each IC or the mounting pressure of the component mounting machine, The lower tip portion of the terminal 6 located below is stabilized in a state of being substantially in contact with the pad 2a. At this time, if the printed thickness c of the cream solder 3a is larger than the coplanarity a of the narrow pitch IC 4 and smaller than the coplanarity b of the wide pitch IC 5, as shown in FIG.
4, all terminals including the terminal 6 located at the uppermost position contact the cream solder 3a, but for the wide pitch IC 5, the terminals 6 located above the printing thickness c of the cream solder 3a do not contact the cream solder 3a. . When these are reflow heated, the cream solder 3a evaporates and scatters the blended flux component and solvent,
The volume is reduced to melt and form the solder. At this time, since all the terminals 6 of the narrow pitch IC 4 which were in contact with the cream solder 3a after mounting the IC (B) are kept in contact with the cream solder 3a during the reflow heating due to the adhesive force of the cream solder 3a, (C ), A normal fillet-shaped solder 11a is formed between the terminal 6 and the pad 2a.
Regarding the terminals 6 of the wide-pitch IC 5 that were not in contact with the cream solder 3a after mounting C (B), the corresponding cream solder 3a spreads only on the pad 2a due to the surface tension, and the solder 11a remains separated from the terminals 6a. Therefore, it is not soldered. If the printed thickness c of the cream solder 3a is made larger than the coplanarity a of the narrow pitch IC 4 and the coplanarity b of the wide pitch IC 5 in order to eliminate unsoldered solder, the solder supply amount increases and the narrow pitch IC
With respect to No. 4, since the terminal pitch is narrow, the solder bridge 10 as shown in FIG.

In general, large chip parts have a larger electrode surface area and larger chip part thickness than small chip parts. Therefore, the corresponding pad area on the substrate is increased and a normal solder fillet is formed. It is necessary to increase the amount of solder supplied for forming. In FIG.
When the cream solder thickness is set so that the solder supply amount becomes appropriate for the small chip component 6 as shown in (A),
For the small chip component 6, normal fillet-shaped solder 11b is formed between the electrode 9 and the pad 2b.
In the case of the large chip component 7, due to the insufficient supply of solder, the solder 11b can form only an insufficient solder fillet that contacts only a part of the vertical surface of the electrode 9, resulting in unsoldered and sufficient solder joining. It may not be possible to secure strength. On the contrary, when the cream solder thickness is set so that the solder supply amount is appropriate for the large chip component 6 as shown in (B), the solder supply amount becomes excessive for the small chip component 6, resulting in the electrode 9 and the pad. In order to form the solder 11b having an abnormal fillet shape that swells between 2b, there is a risk of solder peeling, and electrodes or terminals of both ends of the small chip component 6 itself or an adjacent electronic component (not shown) and a solder bridge are formed. May occur.

[0005]

Since the conventional mounting method is configured as described above, it is not suitable for mixed mounting of electronic components such as ICs having different coplanarity values of terminals and chip components having different external dimensions. However, since a fixed solder cream supply thickness cannot be applied, there are problems that solder bridges, unsolders, insufficient solder joint strength and the like frequently occur, and it takes a lot of time for repair and correction.

The present invention has been made in order to solve the above-mentioned problems, and soldering failure occurs in mixed mounting of electronic components such as ICs having different coplanarity values of terminals and chip components having different external dimensions. It is an object of the present invention to provide a mounting method of an electronic component which is free from the occurrence of defects and has good manufacturing quality.

[0007]

The electronic component mounting method according to the present invention is such that some or all of the pads are pre-coated with solder before the cream solder is printed on the pads on the printed wiring board.

Further, a solder precoat is applied on a pad corresponding to a part or all of the electronic components having a plurality of terminals protruding at a predetermined interval and requiring a solder forming thickness, and cream solder is printed on the pad. In addition, an adhesive or an adhesive flux is formed on the cream solder.

Further, before the cream solder is printed, solder is pre-coated on a pad corresponding to an electronic component which requires a large amount of solder supply.

[0010]

As described above, before the cream solder is printed on the pads on the printed wiring board, some or all of the pads requiring the solder forming thickness or the solder supply amount are pre-coated with solder. There are few defects.

[0011]

EXAMPLES Example 1. FIG. 1 is a plan view of a printed wiring board showing an embodiment of the present invention. In the figure, 12a and 12
b is a solder precoat formed on the pads 2a and 2b corresponding to the wide-pitch IC 4 and the large chip component 6 each having a large terminal coplanarity and having substantially the same size as the pads 2a and 2b, and the cream solders 3a and 3b are different from each other. Is formed by metal mask printing on the solder precoats 12a and 12b at the same time as the pads 2a and 2b corresponding to the narrow pitch IC 5 having a small coplanarity and the small chip component 7. 2A to 2C are explanatory views showing an IC reflow heating process in one embodiment of the present invention, and FIGS. 3A to 3C are explanations showing a chip component mounting process in one embodiment of the present invention. It is a figure.

Next, the operation will be described. Only the parts of the mounting process that differ from the conventional example will be described. I
Regarding the C mounting, after mounting the IC, as shown in FIG.
Due to the weight of C or the mounting pressure of the component mounter, the terminal 6 of the wide pitch IC 5 having a large terminal coplanarity rushes into the cream solder 3a, and the lower tip portion of the terminal 6 located at the lowest position is not the pad 2a but almost the solder. Precoat 12
Stabilizes in contact with a. At this time, if the printed thickness c of the cream solder 3a is larger than the coplanarity a of the narrow pitch IC4 and smaller than the coplanarity b of the wide pitch IC5 as in the conventional example, all terminals of the narrow pitch IC4 come into contact with the cream solder 3a. But wide pitch IC
With respect to No. 5, the terminals 6 located above the printed thickness c of the cream solder 3a do not contact the cream solder 3a. When these are reflow-heated, the flux component and the solvent blended with the cream solder 3a are evaporated and scattered, and the solder precoat 12a corresponding to the wide pitch IC5 is melted and softened, as shown in (B). The terminals 6 of the pitch IC 5 settle on the solder precoat 12a due to the weight of the wide pitch IC 5 itself. At this time, wide pitch IC5
Considering the coplanarity b of, solder precoat 1
If the thickness of 2a is made large enough, all terminals 6 of the wide pitch IC 5 can be formed by the adhesive force of the cream solder 3a.
Since it can keep contact with the cream solder 3a, as shown in (C), even after the reflow heating, the narrow pitch IC 4 and the wide pitch I
The normal fillet-shaped solder 11a can be formed between the pads 6 and all the terminals 6 of C5.

FIG. 3 shows chip component mounting.
3A shows each mounting process before mounting the chip component, FIG. 3B shows after mounting the chip component, and FIG. 3C shows each mounting process after reflow heating. In these figures, by additionally forming the solder precoat 12b on the pad 2b corresponding to the large chip component 8 which requires a large amount of solder supply, both the small chip component 7 and the large chip component 8 can be individually suitable. Since the solder supply amount can be set to a proper value, after the reflow heating, the normal fillet-shaped solder 11b as shown in (C) can be formed.

Example 2. If an adhesive or an adhesive flux (not shown) is formed on the cream solder 3a of the first embodiment by application or the like, the adhesive force causes the terminals 6, the cream solder 3a and the solder precoat 12 to be formed.
The contact with a can be maintained and reliable soldering can be performed.

[0015]

As described above, according to the present invention, the solder pre-coating is applied to some or all of the pads before printing the cream solder, so that ICs having different terminal coplanarities and electronic components having different external dimensions can be mounted together. With respect to mounting, a soldering defect is less likely to occur, and a mounting board with good manufacturing quality can be obtained.

[Brief description of drawings]

FIG. 1 is a plan view of a printed wiring board showing an embodiment of the present invention.

FIG. 2 is a cross-sectional explanatory view showing a heating process of an IC in one embodiment of the present invention.

FIG. 3 is a cross-sectional explanatory view showing a mounting process of a chip part according to an embodiment of the present invention.

FIG. 4 is a plan view showing a conventional printed wiring board.

FIG. 5 is a plan view showing a mounted state of a conventional electronic component.

FIG. 6 is a series of cross-sectional explanatory views showing a mechanism of unsoldered defect occurrence in a conventional IC mounting method.

FIG. 7 is a cross-sectional view showing a mounted state after the conventional chip component reflow.

[Explanation of symbols]

 1 Printed Wiring Board 2a, 2b Pads 3a, 3b Cream Solder 4 Narrow Pitch IC 5 Wide Pitch IC 6 Terminal 7 Small Chip Component 8 Large Chip Component 9 Electrode 10 Solder Bridge 11a, 11b Solder 12a, 12b Solder Precoat

Front page continuation (72) Inventor Hideki Suginaga No. 1 Baba Institute, Nagaokakyo City, Kyoto Prefecture Mitsubishi Electric Corporation Electronic Product Development Laboratory

Claims (4)

[Claims] 1. A cream solder is printed on a pad on a printed wiring board corresponding to a terminal or an electrode of an electronic component, the terminal or electrode is adhered to the pad by an adhesive force such as cream solder, and then heated and then cooled. As a result, in a method of mounting an electronic component by reflow soldering for mounting on a printed wiring board, solder pre-coating is applied to some or all of the pads before the cream solder is printed. 2. A solder precoat on a pad corresponding to a part or all of electronic components having a plurality of terminals protruding at a predetermined interval and requiring a solder forming thickness before printing the cream solder. Claim 1 characterized in that
How to mount the described electronic components. 3. The method for mounting an electronic component according to claim 1, wherein cream solder is printed on the solder precoat, and an adhesive or an adhesive flux is further formed on the cream solder. 4. The method of mounting an electronic component according to claim 1, wherein before the cream solder is printed, the solder corresponding to the electronic component requiring a large amount of solder is pre-coated with solder.