JPS5814758B2 - multilayer wiring board - Google Patents

Description

[Detailed description of the invention] This invention relates to a multilayer wiring board.

In general, multilayer wiring boards are manufactured by first forming a circuit pattern on the inner copper-clad laminate using a well-known printing and etching method, then aligning the circuit pattern with the outer copper-clad laminate, and then sand-inching it through synthetic resin-impregnated glass cloth. After forming the structure, it is common to heat and press the laminated plates using a thermocompression laminating press to integrate the laminated plates of each layer, and then to establish conduction of internal circuits using the through-hole method.

However, when drilling holes in an integrated laminate using the through-hole method, resin, which is an insulator melted by the cutting heat of the hole drilling process, adheres to the inner layer copper foil in the hole, and the hole continues to be drilled. When forming copper plating on the inner wall of the hole, the electrical connection between the inner layer copper foil and the plating becomes incomplete due to the adhesion of the resin described above, resulting in a defective product as a multilayer wiring board.

Therefore, in order to prevent the resin from being fused onto the inner layer copper foil during hole-drilling work, conventional drills with good cutting performance were not used to their wear limits, and were used under conditions that seemed to be extremely wasteful. Efforts have been made to overcome this problem by providing a post-processing step that can remove even if the inner copper foil is fused onto the inner layer copper foil.

However, due to variations in the machinability of the drills used, variations in the drilling conditions, and variations in the post-processing process for removing fused resin, the inner layer copper in all the holes of multilayer wiring boards produced in large numbers is It is very difficult to completely eliminate the presence of fusion resin on the foil, and expensive multilayer wiring boards often suffer from poor conductivity due to this.

This invention was made in view of the above-mentioned circumstances, and aims to provide a multilayer wiring board with high reliability of electrical connection between inner layer copper foil and plating.

Next, the present invention will be explained in detail with reference to the drawings.

Figure 1 is a diagram showing a cross section of a conventional ordinary board after drilling a hole in a laminated and bonded multilayer wiring board. FIG. 2 is a diagram showing a cross section of a conventional conventional substrate after plating has been performed.

In FIGS. 1 and 2, 1 is a multilayer wiring board, 2 is a hole, 3 is an inner layer copper foil, 4 is an insulating base material, and 5 is an inner wall copper plating of the hole 2.

Figure 3 is an enlarged view of the part shown in the circle in Figure 2;
The relationship between the cross-sectional connection shapes between the inner layer copper foil 3 and the hole inner wall copper plating 5 is shown.

As shown in FIG. 3, the normal cross-sectional shape of the inner layer copper foil 3 is connected almost orthogonally to the hole inner wall copper plating 5 with the dimensions and thickness of the inner layer copper foil 3 as it is.

FIG. 4 shows a situation where the connection area between the inner layer copper foil 3 and the hole inner wall copper plating 5 is significantly reduced due to the presence of the fused resin 6.

In the example shown in FIG. 4, the connection area is reduced to about 1/2 compared to the normal connection area as shown in FIG. Naturally, it is expected that this will decline.

Therefore, even if the resin 6 is fused onto the inner layer copper foil 3, in order to ensure a necessary and sufficient inner layer connection area, only the inner layer copper foil portion facing the inner wall of the hole should be fused in the thickness direction of the inner layer copper foil. If it can be expanded, the reliability of the inner layer connections of the multilayer wiring board 1 should be increased.

This invention focuses on the above points, and the part of the inner layer copper foil 3 of the multilayer wiring board 1 facing the inner wall of the hole is shaped to expand in the thickness direction of the inner layer copper foil, thereby making it possible to improve the relationship between the inner layer copper foil and the plating. An object of the present invention is to provide a multilayer wiring board that has a significantly increased connection area and improved connection reliability.

FIG. 5 is a diagram showing a situation in which the portion of the inner layer copper foil facing the inner wall of the hole is expanded in the thickness direction of the inner layer by implementing the present invention, and 7 shows the expanded inner layer copper foil portion. show.

In the example shown in FIG. 5, due to the expanded inner layer copper foil 7, the connection area between the inner layer copper foil 3 and the mating portion 5 is approximately twice that of the normal one. Even if the same amount of resin 6 as shown in FIG. 4 is fused, the inner layer connection area will be approximately 1.5 mm larger than the normal connection area as shown in FIG.
Since the number of connections increases by five times, the reliability of the connection is sufficient.

Next, details of tools used in carrying out the invention and data for carrying out the invention will be explained in detail.

FIG. 8 is a side view showing the shape of a drill for drilling, which is used in carrying out the present invention.

In FIG. 8, S represents the shank portion, F represents the flute portion, D0 represents the drill tip diameter of the flute portion, and D1 represents the drill diameter of the flute portion in front of the shank portion.

Generally, in order to expand the hole and simultaneously expand the inner copper foil as cutting progresses, it is considered best to use a flute whose diameter increases from the tip to the shank. Ta.

Therefore, as an example, the flute length F = 9.0 mm, the tip drill diameter Do = 0.9 mm, the drill diameter D1 of the flute portion in front of the shank portion = 1.0 mm, and the diameter of the flute portion F is from the drill tip to the shank. Using a drill with a structure that gradually increases in size in the direction of As a result of observation using a metal distribution microscope, it was found that the inner layer copper foil had a shape that was considerably expanded in the thickness direction, as expected.

FIG. 6 shows the results of measuring the inner layer copper foil thickness t (unit: microns) and the maximum copper foil thickness tMAX (unit: microns) of the portion expanded in the thickness direction using a metallurgical microscope.

In FIG. 6, the horizontal axis represents the inner layer copper foil thickness t, and the vertical axis represents the maximum copper foil thickness tMAX in the direction of the inner wall of the hole.

As can be seen from Figure 6, tMAX is always about 2 with respect to t.
If the inner layer copper foil thickness t is 5 microns larger, for example, 20 microns, the maximum copper foil thickness tMAX on the inner wall of the hole is 45 microns, and the inner layer copper foil on the inner wall of the hole is pushed out in the thickness direction and the inner layer is connected. It can be seen that the area has increased approximately 2.3 times.

The above four types of multilayer wiring boards that showed such results are:
After copper plating was performed on the inner wall of the hole, a well-known printing and etching process was performed to form an outer layer pattern, which was then completed and used as a test sample to evaluate the reliability of the inner layer connection.

At the same time, a similar multilayer wiring board manufactured using a conventional manufacturing method was provided as a comparison sample.

There are many test methods for evaluating the reliability of the connection between the inner layer copper foil of a multilayer wiring board and the copper plating on the inner wall of the hole, as reported in various literature.
A so-called solder heat resistance test was employed in which a test sample was floated on molten solder kept at a constant temperature of 0° C. for 10 seconds, then taken up, and left in a desiccator for 4 hours to cool to room temperature.

In addition, the inner layer connection can be evaluated by measuring the DC resistance between the two holes connected to the inner layer using a 4-terminal method using a milliohmmeter, and capturing the phenomenon of incomplete connection as an increase in the DC resistance value. The quality of the test is determined by connecting the measurement point where the ratio of the measured value R0 (unit: milliohm) after the test to the measured value R0 (unit: milliohm) before the test is R/R0 > 1.2. It was considered as a point.

FIG. 7 shows the cumulative incidence of connection abnormalities with respect to the number of repetitions of the solder heat resistance test for the test sample manufactured by implementing the present invention and the comparative sample manufactured by the conventional manufacturing method.

This test result is the result of a test sample corresponding to an inner layer copper foil thickness of 20 microns, and in the figure, the ○ mark is the result of the conventional example, and the X mark is the result of the sample implementing the present invention.

As can be seen from FIG. 7, the reliability of the inner layer connections of the multilayer wiring board manufactured by implementing the present invention, in which the inner layer copper foil on the inner wall of the hole is expanded in the thickness direction, is lower than that of the conventional one. In comparison, it can be said that it is quite high.

Although this invention has been explained in detail above, the following two points can be mentioned as effects of this invention.

First, by forming the inner layer copper foil on the inner wall surface of the hole in a shape that is expanded in the thickness direction, the inner layer connection area is greatly increased, so that a multilayer wiring board having high inner layer connection reliability can be obtained.

Second, the inner layer connection area can be significantly increased to the extent that the decrease in the inner layer connection area due to resin fusion can be wiped out, so the extra step of removing the resin fused onto the inner layer copper foil can be omitted. This makes it possible to reduce product costs.

[Brief explanation of the drawings] Fig. 1 shows a cross section of a conventional ordinary board after drilling holes in a laminated and bonded multilayer wiring board, and Fig. 2 shows a cross section of a conventional board after drilling holes in a laminated and bonded multilayer wiring board. A diagram showing a cross section of a conventional ordinary board after copper plating has been applied to the inner wall of the hole, Figure 3 is an enlarged view of the part shown in the circle in Figure 2, and Figure 4 is a connection between the inner layer copper foil and the copper plating. FIG. 5 is a diagram showing a situation in which the area has been reduced due to the presence of the fused resin; FIG. FIG. 6 is a diagram showing an example of the relationship between the inner layer copper foil thickness t and the maximum copper foil thickness tMAX of the inner wall surface of the hole when the present invention is implemented, and FIG. 7 is a diagram showing a test sample produced by implementing the present invention. FIG. 8 is a diagram showing the cumulative incidence of connection abnormality with respect to the number of repetitions of the solder heat resistance test of comparative samples manufactured by the conventional method. FIG. In the figure, 1 is a multilayer wiring board, 2 is a hole, 3 is an inner layer copper foil, 4 is an insulating base material, 5 is a hole inner wall copper plating, 6 is a fused resin, 7 is an inner layer copper foil that has been pushed out, S is the shank part,
F indicates the flute portion, Do indicates the diameter of the tip of the drill, and D1 indicates the diameter of the drill near the shank.

Claims (1)

[Claims] 1. A multilayer, characterized in that the inner layer metal foil facing the inner wall of the hole has a shape that is expanded in the thickness direction of the inner layer, thereby increasing the connection area between the plating on the inner wall surface of the hole and the inner layer metal foil. wiring board.