JP2019062000A - Screen printing mask and printed wiring board - Google Patents

Abstract

To suppress generation of voids.SOLUTION: Solder paste 15 is applied to a plurality of locations separated from one another on one land 12 formed on a substrate 11.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a screen printing mask and a printed wiring board manufactured using the mask.

  There is a surface mounting technology (SMT: Surface Mount Technology) as a method of mounting an electronic component on a substrate. In SMT, solder paste is printed on a substrate, parts are placed thereon, and then heat is applied for soldering. There are forms such as LGA (Land Grid Array) and BGA (Ball Grid Array) as a package of surface mounting components for reducing the mounting area and enabling high density mounting. Since this package is soldered at the bottom of the package, voids are more likely to be generated as compared with general chip components. Patent Document 1 proposes that solder paste be printed offset from an electrode pad on a substrate, and that void be reduced by causing solder to flow to the electrode pad by self alignment utilizing wettability at the time of melting the solder. ing.

JP, 2015-118988, A

Since the upper surface of the solder is covered by the component to be mounted, the voiding gas can only be discharged from the side of the solder. Therefore, only flowing the solder by self-alignment can not sufficiently discharge the gas, and voids may be generated.
An object of the present invention is to suppress the occurrence of voids.

  A mask for screen printing according to an aspect of the present invention is characterized in that a plurality of openings are formed for one land formed on a substrate.

  According to the present invention, since the solder paste is applied to a plurality of locations separated from each other on one land, the total area of the side surfaces of the solder, which is the surface from which the gas is discharged to the outside, is increased. This can suppress the occurrence of voids.

It is sectional drawing which shows the manufacturing method of a printed wiring board. It is a top view of a substrate and a mask. It is a top view of a substrate and a mask. It is a figure which shows an Example. It is a figure showing comparative example 1. It is a figure which shows the comparative example 2. FIG. It is a figure which shows a modification. It is a figure which shows a printed wiring board.

  Hereinafter, embodiments of the present invention will be described based on the drawings. Each drawing is schematic and may be different from the actual one. In addition, the following embodiments illustrate apparatuses and methods for embodying the technical idea of the present invention, and the configuration is not specified to the following. That is, the technical idea of the present invention can be variously modified within the technical scope described in the claims.

<< Embodiment >>
"Constitution"
FIG. 1 is a cross-sectional view showing a method of manufacturing a printed wiring board.
First, as shown in FIG. 1A, a plurality of lands 12 made of a conductor are formed on the top surface of the substrate 11.
Next, as shown in (b) of FIG. 1, a mask 13 for screen printing is placed on the top surface of the land 12. In the mask 13, a plurality of openings 14 are formed for one land 12, and at least one opening 14 straddles both a part of the land 12 and the outer substrate 11 in the land 12. It is formed in position. Then, using a squeegee of a printing machine (not shown), the solder paste 15 is applied through the opening 14 of the mask 13 (screen printing).

  Next, as shown in FIG. 1C, when the mask 13 is separated from the substrate 11, solder pastes 15 are applied to a plurality of places separated from each other on one land 12. The solder paste 15 is applied so as to protrude from the land 12 onto the substrate 11 at at least one location. Thereby, the solder paste 15 is applied so as to straddle a part of each land 12 and the outside of each land 12. That is, a part of the solder paste supplied from one opening 14 is applied to the upper surface of the land 12 and the rest is applied to the upper surface of the substrate 11.

Next, as shown in (d) of FIG. 1, the solder paste 15 is arranged such that the centers of the plurality of lands 12 and the centers of the electrodes 17 of the electronic component 16 corresponding to the plurality of lands 12 overlap. The electronic component 16 is placed on top of the
Next, the solder paste 15 is melted by reflow. Each of the lands 12 and each electrode 17 of the electronic component 16 is joined by flowing the melted solder 18 to a proper position by self alignment as shown in FIG.
The above is the method of manufacturing a printed wiring board.

Next, specific configurations of the substrate 11 and the mask 13 will be described.
FIG. 2 is a plan view of a substrate and a mask.
(A) of FIG. 2 is the board | substrate 11 which showed only the part in which BGA and LGA components are mounted, and it is a square by planar view. A plurality of lands 12 a to 12 c are formed on the substrate 11. The lands 12 a are disposed at the center of the substrate 11 and are square in plan view. The lands 12b are arranged around the lands 12a, and are arranged four by four along a direction parallel to each side of the substrate 11. Each of the lands 12b is long in a direction orthogonal to each side of the substrate 11 in plan view. It is rectangular. The lands 12 c are disposed around the lands 12 a and on the sides corresponding to the four corners of the substrate 11 and are square in plan view.

  The mask 13 is provided with several opening part 14a-14c corresponding to each of several land 12a-12c, as shown in FIG.2 (b). The positions of the lands 12a to 12c are indicated by dotted lines. Four openings 14a formed corresponding to the lands 12a are provided, respectively, at positions which straddle both the lands 12a and the outer substrate 11 of the lands 12a. The four openings 14a are formed on the land 12a so as to be shifted in the four directions along the diagonal line.

The openings 14 b formed corresponding to the lands 12 b are provided two by two along the longitudinal direction of each land 12 b. The openings 14b located inside the substrate 11 are all formed at positions overlapping with part of the land 12b, and the openings 14b located outside the substrate 11 are part of the land 12b and the substrate outside the land 12b. It is formed in the position which straddles both 11 and. The openings 14 b located outside the substrate 11 are formed offset to the outside of the substrate 11 along the longitudinal direction of the lands 12 b.
The openings 14 c corresponding to the lands 12 c are provided one by one for one land 12 c. The opening 14 c is formed at a position straddling both the land 12 c and the outer substrate 11 in the land 12 c. The opening 14 c is formed offset to the outer periphery of the substrate 11 along the diagonal of the substrate 11 with respect to the land 12 c.

  FIG. 3A is a plan view of the state in which the mask 13 is placed on the top surface of the land 12 on the substrate 11. The openings 14a, the openings 14b located outside the substrate 11, and the openings 14c are offset from the regular positions (positions immediately above the lands) corresponding to the lands 12a to 12c, respectively. 12 c and the substrate 11 are visible. On the other hand, since the openings 14b located inside the substrate 11 are all at positions overlapping with part of the lands 12b, only the corresponding lands 12b are visible and the substrate 11 can not be seen.

FIG. 3B is a plan view of a state in which the mask 13 is separated from the substrate 11 after the solder paste 15 is applied. Solder pastes 15 are applied to four places separated from each other on one land 12 a, and each applied so as to protrude from the land 12 a onto the substrate 11. Similarly, solder paste 15 is applied to two places separated from each other on one land 12 b, and the side located on the outer side of the substrate 11 is applied so as to protrude from the land 12 b onto the substrate 11. The solder paste 15 is applied to one land 12 c only at one site, and applied so as to protrude from the land 12 c onto the substrate 11.
(B) of FIG. 3 shows a state in which the application of the solder paste 15 is completed on the substrate 11. Thereafter, through the component mounting process and the reflow process, the printed wiring board 20 as shown in FIG. Complete.

<< Operation effect >>
Next, the operation and effect of the embodiment will be described.
First, the mechanism of generation of voids will be described.
Prior to reflow, in preheating for preheating the substrate, solder paste, electronic parts and the like, and during reflow, the flux and the like contained in the solder paste 15 is vaporized to generate gas. The vaporized gas may be expelled from the molten solder 18 because of its volume expansion, but it may not be expelled depending on the melting time and the shape of the solder, and the expelled gas becomes void. .

  The conditions under which voids are easily generated include a short melting time, a large area for soldering, a shape in which vaporized gas is difficult to be discharged, and difficulty in self alignment. With the shape where vaporized gas is hard to be discharged, the electrodes of electronic parts are arranged at high density like LGA (Land Grid Array) and BGA (Ball Grid Array), and the top and bottom of each solder are parts and board It is a shape covered with

Next, the mechanism that can discharge the gas that is the source of the void will be described.
The melted solder 18 is attracted to the lands 12 and the electrodes 17 and flows. During this flow, the molten solder 18 is agitated, and the vaporized gas is discharged from the surface of the molten solder 18 in contact with the air, so that the retention of the gas is suppressed. When the upper surface of the molten solder 18 is covered by the component, the side surface of the molten solder 18 provides a relief for the gas that is the source of the void. Therefore, as the area of the side surface of the molten solder 18 in contact with air increases, the discharge of gas is promoted and the void is reduced.

  In the embodiment of the present invention, as shown in FIG. 1C, the solder paste 15 is applied to a plurality of locations separated from each other on one land 12, so the side surfaces of the solder 15, ie, the origin of the voids, are applied. The way of getting rid of the Therefore, by separating the solder pastes 15 from each other, the gas can be discharged also from the newly generated side surfaces, and the generation of voids can be further suppressed. The solder paste 15 is applied so as to protrude from the land 12 onto the substrate 11 at at least one location. In this case, the solder 18 melted by heating is first attracted to either the land 12 or the electrode 17. When the state at this time is observed from the lateral direction, those which were rectangular before heating, It is stretched to a parallelogram shape by pulling. As a result, the area of the surface of the solder 18 in contact with air is increased, and the discharge of the vaporized gas is promoted.

Next, the force acting on the electronic component 16 will be described.
The solder paste 15 is shifted substantially in the radial direction from the center P of the substrate 11 with respect to each of the lands 12a to 12c, as shown in (b) of FIG. Therefore, the solder 18 melted by heating is first drawn to the electrode 17 of the electronic component 16 and stretched. As a reaction to the pulling force of the solder 18, a force is generated on the electronic component 16 toward the solder 18. However, the force acting on the electronic component 16 is to be offset with the similar force generated at the other symmetrical position on the other side of the center P of the substrate 11 and the electronic component 16 moves. Can be suppressed. Therefore, since the electronic component 16 remains at the position corresponding to the plurality of lands 12, mounting defects can be suppressed.

Here, an example will be described.
FIG. 4 is a diagram showing an example.
(A) in a figure is planar view, (b) is sectional drawing, (c) is a photograph which shows the generation | occurrence | production condition of a void. Here, the solder paste 15 is applied to two places separated from each other on one land 12, and the solder paste 15 is applied so as to protrude from the land 12 onto the substrate 11 at one place. . By applying the solder paste 15 in two divided portions, new side surfaces 15a are generated, and the gas can be discharged also from these side surfaces 15a. As a result of implementing such a method of application in the area shown by the dotted line in (c) of FIG. 4, the occurrence of the void 19 could be suppressed to 1% or less. (Void 19 can not be checked visually because it is 1% or less.)
The void generation rate is calculated based on Japanese Industrial Standard JIS C 61191-6.

Next, a comparative example will be described.
Comparative Example 1
FIG. 5 is a diagram showing Comparative Example 1.
(A) in a figure is planar view, (b) is sectional drawing, (c) is a photograph which shows the generation | occurrence | production condition of a void. Here, the case where the solder paste 15 is applied to the regular position corresponding to the land 12 is shown. As shown in (c) of FIG. 5, in the region indicated by the dotted line, the voids 19 were generated by about 30% at the maximum.

Comparative Example 2
FIG. 6 is a view showing comparative example 2;
(A) in a figure is planar view, (b) is sectional drawing, (c) is a photograph which shows the generation | occurrence | production condition of a void. Here, the case where it shifts (shifts) and applies the solder paste 15 from the regular position corresponding to the land 12 is shown. As a result of carrying out such a method of application in the area shown by the dotted line in (c) of FIG. 6, the generation of the void 19 could be suppressed to 5% or less. That is, although generation | occurrence | production of the void was able to be suppressed rather than the comparative example 1, generation | occurrence | production of the void was not able to be suppressed like the Example.

<< Modification >>
In the above embodiment, the solder paste 15 applied to the inside of the substrate 11 with respect to the land 12 b is not protruded from the land 12 b, but the present invention is not limited to this. Similar to the solder paste 15 applied to the outside of the substrate 11, the solder paste 15 applied to the inside of the substrate 11 may also protrude from the land 12b.
FIG. 7 is a diagram showing a modification.
As described above, if the inner side of the substrate 11 is also applied so as to protrude from the land 12 b, the area of the side surface in contact with air increases when flowing by self alignment, thereby further promoting the discharge of gas. Can.

  Although the foregoing has been described with reference to a limited number of embodiments, the scope of rights is not limited to them, and modifications of the embodiments based on the above disclosure are obvious to those skilled in the art.

11 substrate 12 land 13 mask 14 opening 15 solder paste 16 electronic component 17 electrode 18 melted solder 19 void 20 printed wiring board

Claims (3)

A screen printing mask used to apply a solder paste to lands formed on a substrate, the mask comprising:
A mask for screen printing characterized in that a plurality of openings are formed for one of the lands.   The said opening part is formed in the position which straddles both a part of said land, and the said outer side board | substrate in the land, The mask for screen printing of Claim 1 characterized by the above-mentioned.   The printed wiring board manufactured using the mask for screen printing of Claim 1 or 2.