JP6790504B2 - Manufacturing method of printed wiring board and mask for screen printing - Google Patents

Description

The present invention relates to a method for manufacturing a printed wiring board and a mask for screen printing.

Surface mount technology (SMT) is a method of mounting electronic components on a substrate. In SMT, a solder paste is printed on a substrate, parts are placed on the substrate, and then heat is applied to perform soldering. As a package of surface mount components for reducing the mounting area and enabling high-density mounting, there are forms such as LGA (Land Grid Array) and BGA (Ball Grid Array). Since this package has no pins around the package and uses solder balls as electrodes, the amount of solder is large, and voids are likely to occur during soldering as compared with general chip parts. Patent Document 1 proposes to reduce voids by printing the solder paste offset from the electrode pad on the substrate and allowing the solder to flow to the electrode pad by self-alignment utilizing the wettability at the time of solder melting. ing.

Japanese Unexamined Patent Publication No. 2015-118988

If the solder paste is printed by shifting it from the regular position and the parts are also shifted from the regular position and placed on the solder paste, the voids can be reduced, but the parts may not return to the regular position and mounting defects may occur. There is.
An object of the present invention is to reduce voids and suppress mounting defects.

In the method for manufacturing a printed wiring board according to one aspect of the present invention, solder paste is applied to a substrate on which a plurality of lands are formed so as to straddle a part of the lands and the outside of the lands for each of the plurality of lands. The process includes a step of placing the electronic component on the solder paste so that the center of each of the plurality of lands and the center of the electrode of the electronic component corresponding to each of the plurality of lands overlap.

According to the present invention, when a part of the land and the solder paste applied so as to straddle the outside of the land are melted, they flow onto the land by self-alignment, and voids can be discharged by this flow. ..

It is sectional drawing which shows the manufacturing method of the 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 graph explaining the effect of void reduction. It is sectional drawing which shows the manufacturing method of the printed wiring board in the comparative example 1. FIG. It is a top view of the substrate and the mask in Comparative Example 1. It is a top view of the substrate and the mask in Comparative Example 1.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. It should be noted that each drawing is a schematic one and may differ from the actual one. In addition, the following embodiments exemplify devices and methods for embodying the technical idea of the present invention, and do not specify the configuration to the following. That is, the technical idea of the present invention can be modified in various ways within the technical scope described in the claims.

<< First Embodiment >>
"Constitution"
FIG. 1 is a cross-sectional view showing a method of manufacturing a printed wiring board.
2 and 3 are plan views of the substrate and the mask.
First, as shown in FIG. 1A, a plurality of lands 12 made of conductors and arranged at equal intervals are formed on the upper surface of the substrate 11. FIG. 2A is a substrate 11 showing only a portion of the entire substrate on which BGA and LGA components are mounted. The substrate 11 is square in plan view, and the land 12 is circular. The lands 12 are arranged at equal intervals in both the row direction (horizontal direction) and the column direction (vertical direction) at a distance of Dp.

Next, a step of applying the solder paste 15 to the substrate 11 will be described. First, as shown in FIG. 1B, a mask 13 for screen printing is placed on the upper surface of the land 12. As shown in FIG. 2B, the mask 13 is provided with a plurality of openings 14 at positions shifted outward from the center P of the mask 13 with respect to positions corresponding to each of the plurality of lands 12. The position of the land 12 is indicated by a dotted line. The opening 14 has the same shape as the land 12. Specifically, only the pair of openings separated by the line Lx that passes through the center P of the mask 13 and vertically divides the substrate 11 or the Ly that divides the substrate 11 horizontally are the intervals of the distance D1 (first interval). The other openings are provided at equal intervals (second interval) at a distance D2 that is smaller than the distance D1 and equal to the distance Dp. In addition, as an example accompanied by illustration, it is defined in each of Lx and Ly that the distance between the pair of openings separated is larger than the others, but the distance is not limited to this, and Lx or Ly. The same effect can be obtained even if either one of them has the above distance relationship.
Further, the virtual line passing through the center of each of the vertical rows of the plurality of openings 14 is parallel to the line Ly, and the virtual line passing through the center of each horizontal row is parallel to the line Lx.

FIG. 3A is a plan view of the state where the mask 13 is placed on the upper surface of the land 12 on the substrate 11. Since the opening 14 is displaced from the regular position corresponding to the land 12, the land 12 and the substrate 11 are visible in the opening 14.
The step of applying the solder paste 15 on the substrate 11 will be described. First, the solder paste 15 is applied through the opening 14 of the mask 13 using a squeegee of a printing machine (not shown) (screen printing), and then the mask 13 is placed on the substrate 11 as shown in FIG. 1 (c). Keep away from. 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. FIG. 3B is a plan view of the mask 13 separated from the substrate 11. Since the solder paste 15 is applied at a position shifted from the regular position corresponding to the land 12, a part of the land 12 is visible.

Next, the mounting step of the electronic component following the coating step of the solder paste 15 will be described. As shown in FIG. 1D, the center of each of the plurality of lands 12 and the center of each electrode 17 of the electronic component 16 corresponding to each of the plurality of lands 12 overlap on the solder paste 15. The electronic component 16 is placed.
Next, the solder paste 15 is melted by reflow. As shown in FIG. 1 (e), the molten solder 18 flows to a regular position by self-alignment, so that each land 12 and each electrode 17 of the electronic component 16 are joined.
The above is the manufacturing method of the printed wiring board.

《Effect》
Next, the action and effect of the embodiment will be described.
First, the mechanism by which the void 19 is generated will be described.
Prior to reflow, the flux and the like contained in the solder paste 15 are vaporized during preheating to preheat the substrate, solder paste, electronic components, etc., and reflow, and gas is generated. Since the volume of the vaporized gas expands, some of the vaporized gas is discharged from the molten solder 18, but some of the vaporized gas cannot be discharged depending on the melting time and the shape of the solder, and the gas that cannot be completely discharged is called void 19. Become.

Conditions in which voids 19 are likely to occur include a short melting time, a large soldering area, a shape in which vaporized gas is difficult to be discharged, and difficulty in self-alignment. The shape that makes it difficult for vaporized gas to be discharged is that 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 substrates. The shape is covered with.

Next, a mechanism capable of discharging the void 19 will be described.
The molten solder 18 is attracted to the land 12 and the electrode 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 air, so that the void 19 is suppressed from staying. Therefore, as the area of the surface of the molten solder 18 that comes into contact with air increases, gas discharge is promoted and voids 19 decrease.

In the embodiment of the present invention, as described above, the solder paste 15 is applied at a position shifted from the land 12 and the electrode 17. After that, the solder 18 melted by heating is first attracted to either the land 12 or the electrode 17. When the state of the molten solder 18 at this time is observed from the lateral direction, what was rectangular before heating is stretched into a parallel quadrilateral shape by pulling. As a result, the area of the surface of the solder 18 that comes into contact with air is increased, and the discharge of vaporized gas is promoted.

Next, the force acting on the electronic component 16 will be described.
The solder paste 15 is displaced from the center of each of the plurality of lands 12 on a virtual line passing from the center P of the substrate 11 to the center of the lands 12 in a direction away from the center P. Subsequently, the solder 18 melted by heating is first attracted to the electrode 17 of the electronic component 16 and stretched. As a reaction of the force that attracts the solder 18, a force that attracts the solder 18 is generated in the electronic component 16. However, the force acting on the electronic component 16 cancels out the similar force generated on the electrode 17 at a 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 stays at a position corresponding to the plurality of lands 12, mounting defects can be suppressed.

FIG. 4 is a graph illustrating the effect of void reduction.
Here, a 118-pin LGA is used, and the land 12 has a 1.5 mm pitch. When the shift amount (shift amount) was set to 0 mm, the content of void 19 was about 20%. On the other hand, when the amount of shift was set to 0.2 mm in both the vertical direction and the horizontal direction, the content of void 19 was reduced to about 5%. Further, when the amount of shift was set to 0.3 mm in both the row direction and the column direction, the content rate of the void 19 was reduced to about 2.5%. Further, when the amount of shift was set to 0.4 mm in both the row direction and the column direction, the content rate of the void 19 remained at about 2.5%. From the viewpoint of joint strength and reliability, the void content should be 35% or less at the maximum, the recommendation is 10% or less, and the target is 5% or less.

Next, a comparative example will be described.
[Comparative Example 1]
FIG. 5 is a cross-sectional view showing a method of manufacturing the printed wiring board in Comparative Example 1.
6 and 7 are plan views of the substrate and the mask in Comparative Example 1.
Here, only the points different from the above-described embodiment will be described, and the common parts will be omitted. In this Comparative Example 1, the case where the solder paste 15 is applied to the regular positions corresponding to the plurality of lands 12 is shown. First, as shown in FIG. 6A, the lands 12 are arranged at equal intervals at intervals of distance Dp, and as shown in FIG. 6B, the openings 14 are also arranged at equal intervals at intervals of distance Dp. They are arranged, and all are arranged at the positions corresponding to the lands 12.

Therefore, when the mask 13 is placed on the upper surface of the land 12, only the land 12 can be seen from the opening 14 as shown in FIG. 7A. In this state, when the solder paste 15 is applied and then the mask 13 is peeled off, the land 12 is covered with the solder paste 15 as shown in FIG. 7B. In this way, when the solder paste 15 is applied without shifting from the regular positions corresponding to the plurality of lands 12, the area of the side surface of the solder 18 melted during the reflow is smaller than that of the present invention, and the vaporized gas is discharged. Is not sufficiently performed, and as shown in FIG. 5 (e), the content of void 19 increases.

Although the above description has been made with reference to a limited number of embodiments, the scope of rights is not limited thereto, 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 components 17 Electrodes 18 Molten solder 19 Voids

Claims (2)

When applying the solder paste to a substrate on which a plurality of lands arranged at predetermined intervals are formed , only a pair of solder pastes separated by the center of the arrangement are provided at the first interval. , The solder paste is separated from the center of the substrate for each of the plurality of lands so that the solder pastes other than the pair of solder pastes are provided at equal intervals at a second interval smaller than the first interval. A process of shifting in the direction and applying so as to straddle a part of the land and the outside of the land.
It is characterized by including a step of placing the electronic component on the solder paste so that the center of each of the plurality of lands and the center of the electrode of the electronic component corresponding to each of the plurality of lands overlap. Manufacturing method of printed wiring board. A screen printing mask used to apply a solder paste to each of a plurality of lands formed on a substrate.
With multiple openings arranged at predetermined intervals,
In the plurality of openings, only a pair of openings separated by a center of the arrangement are provided at the first interval, and openings other than the pair of openings are smaller than the first interval. A screen printing mask characterized in that the plurality of openings are provided so as to be offset from the center of the substrate with respect to each of the lands by being provided at equal intervals at a second interval. ..

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