JPH02194696A - Multilayered printed wiring board - Google Patents

Abstract

PURPOSE:To relieve the concentration of stresses so as to obtain an excellent resistance to thermal stress by providing inner-layer conductor circuits near the outermost layers. CONSTITUTION:A copper-plated laminated board on which a multilayered printed circuit board 1 is formed is respectively provided with, for example, two sheets of electrolytic copper foil for forming outer-layer conductor circuits 3 on both of the front and rear surfaces of an insulating board 2. In addition, electrolytic copper foil is used for forming inner-layer conductor circuits 6-9. This wiring board 1 is provided with the inner-layer power source circuit 7 having a large occupying area and an earth circuit 8 in areas which are respectively within d/4 from the front and rear surfaces of the board 2 having a thickness of (d) and, at the same time, the signal circuits 6 and 9 are respectively provided in the same areas. As a result, even when the board 2 swells or contracts by heat, the tensile or compressive force received by the plated section 5 of a through hole itself can be suppressed to that received by the double-sided printed wiring board and the resistance to thermal stress can be improved extensively without using any thermal land.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a multilayer printed wiring board, and particularly to a multilayer printed wiring board having through-hole plated portions.

[Conventional technology]

In general, multilayer printed wiring boards have through-holes for inserting electronic components, via holes for signal transmission, etc., by forming a conductive layer on the inner walls of the holes by plating. There is electrical connection between the outer layer conductor circuits or between these and the inner layer conductor circuits. Recently, there has been a noticeable increase in the number of layers in wiring boards and the reduction in the diameter of through holes due to high-density packaging.In particular, in multilayer printed wiring boards, interstitial via holes (IVH1 embedded) such as pallite holes or blind holes By using holes (generic term for blind holes), etc., we are dealing with a number of ways to increase density. However, as the number of layers increases, the board thickness of printed wiring boards increases, and - as density increases, conductor patterns and plating thickness tend to become thinner and thinner than conventional ones. Hall reliability is becoming increasingly important. This is also to prevent problems such as through-hole disconnection.

It is necessary to conduct sufficient consideration at the design stage to create a structure that can withstand reliability verification tests such as Ill standby solder reflow process and thermal cycle test.

Furthermore, even after the printed wiring board is completed, it is important to guarantee long-term reliability against mechanical stress (thermal stress) caused by expansion and contraction of the wiring board due to changes in ambient temperature.

Conventional multilayer printed wiring boards, for example, have the following characteristics: (1) High reliability, as described in the Printed Circuit Technology Handbook (edited by the Japan Printed Circuit Industry Association, published by Nikkan Kogyo Shimbun, February 1986), pages 86-87; (2)
Wiring capacity can be increased. (3) Power supply impedance can be set low and uniform, noise level is reduced, and operation is stabilized. (4) By providing a ground layer on the inner layer, it is possible to match the characteristic impedance, which is considered to take advantage of the characteristics, for example, as shown in Table 2.5.2 (p. 87) in the same handbook. It has a laminated structure like this.

Figure 4 is a cross-sectional view showing the through-hole portion of such a conventional multilayer printed wiring board. In Figure 1, (1) is a glass cloth-based epoxy resin copper-clad laminate (hereinafter referred to as copper-clad laminate). A multilayer printed wiring board (2) is a glass cloth-based epoxy resin laminate (hereinafter referred to as a glass-epoxy laminate) that constitutes the multilayer printed wiring board (1).
(3) is an outer layer conductor circuit formed on the outermost layer of the front and back sides of this insulating substrate; (4) is a through hole provided to penetrate the multilayer printed wiring board (1);
5) is a through-hole plating portion formed in the through-hole (4) to connect the front and back outer layer conductor circuits (3);
6) to (9) are inner layer conductor circuits provided inside the insulating substrate (2), where (6) is a signal circuit and (7) is an inner layer power supply circuit.

(8) is a ground circuit, and (9) is a signal circuit. The respective inner layer conductor circuits (6) to (9) are located approximately evenly with respect to the cross section, and have a structure that is approximately symmetrical with respect to the center line (lO) of the printed wiring board (1).

[Problem to be solved by the invention]

Conventional multilayer printed wiring boards have the structure shown above.
As mentioned above, the main focus of the design was to reduce crosstalk caused by electrostatic induction, electromagnetic induction, etc., and to pursue electrical characteristics such as improving characteristic impedance. Therefore, if a temperature change is applied to the printed wiring board,
Due to the difference in linear expansion coefficient between the insulating substrate (2) and the through-hole plated part (5), tensile stress is generated on the through-hole plated part (5) when heated, and compressive stress is generated when cooled, resulting in excessive thermal stress. There was a risk that so-called corner cracks or barrel cracks would occur in the through-hole section due to fatigue phenomena when heat stress was applied or repeated thermal stress was applied over a long period of time, resulting in wire breakage.

As a conventional measure against such thermal expansion and contraction, an inner layer conductor circuit with a thermal land is used, for example, as described in pages 232 to 233 of the above-mentioned handbook. However, since there are lands in the inner layer even with this method, it is not very effective against stress concentration at the connection between the inner layer conductor circuits (6) to (9) and the through-hole plating section (5), as described later. In addition, this method does not allow
This method had drawbacks such as a low degree of freedom in design, required precision in workmanship, was not suitable for high-density packaging, and the inner layer conductor circuit was expensive, so it could not necessarily be said to be an effective method. Therefore, conventionally, in an environment where thermal stress occurs in a printed wiring board, if the thickness of the through-hole plating portion (5) is thin, the thermal stress that occurs cannot be coped with, and there is a risk of wire breakage.

For example, Figure 5 shows an example in which thermal stress in a four-layer wiring board with thermal lands was analyzed using a structural analysis simulation program. 5)
Stress concentration was observed at the connection portion of the inner layer conductor circuit (12), indicating that disconnection is likely to occur in this portion. The validity of the analysis has been confirmed in actual thermal shock tests and thermal cycle tests. , 6+ua, when the temperature change is 100°C, it is less than 25μ-. In order to deal with this, it was necessary to ensure reliability by applying a thick through-hole plating part (5) and increasing the mechanical strength of the through-hole plating part (5) itself (for example, in the above case, 50μ - or more plating thickness is required).

However, this method requires a lot of time for the plating process.

It has the disadvantage of high manufacturing costs due to poor finishing accuracy (roughened surface), which reduces yield, and when trying to reduce manufacturing costs, it approaches the limit of its ability to handle thermal stress. Thin plating had to be applied, which not only made the process difficult, but also left concerns about reliability.This risk is particularly noticeable in multilayer printed wiring boards, as shown in Figure 6. As shown in Figure 2, this is not so much the case with double-sided wiring boards that do not have inner layer conductor circuits.

The present invention has been made in order to eliminate the structural defects of such conventional multilayer printed wiring boards.

By locating the inner layer conductor circuit of the printed wiring board at the optimal position, stress concentration is alleviated, and multilayers with plated through holes with excellent heat stress resistance can be created without making any changes to the conventional manufacturing process. The purpose is to provide printed wiring boards.

[Means to solve the problem]

The multilayer printed wiring board of the present invention is a multilayer printed wiring board having through holes, in which inner layer conductor circuits are provided at positions close to the outermost layers on the front and back sides of an insulating substrate.

In the multilayer printed wiring board of the present invention, all the inner layer conductor circuits can be provided so as to be located in the outermost area of the areas obtained by dividing the wiring board into four equal parts in the thickness direction.

In addition, the inner layer power supply circuit and the ground circuit are respectively placed on the next layer of the outermost layer conductor circuit on the front and back sides, and the wiring board is placed 4 times in the thickness direction.
It can also be provided so as to be located in the outermost area of the equally divided areas.

[For production]

A structural analysis was performed while changing the position of the inner layer conductor circuit, with a focus on bringing the stress concentration of the multilayer printed wiring board closer to the value of the double-sided circuit board. It can be seen that the stress generated decreases as the distance from the center line of It was found that this is the case where the area is located in the outermost d/4 area (area A in FIG. 3). These inner layer conductor circuits, which are easily affected by position, are the inner layer power supply circuits and ground circuits that occupy a particularly large area among the inner layer conductor circuits, and are extremely difficult to use for circuits that do not take up much area, such as signal circuits. There is no effect.

In other words, as in the present invention, if the inner layer conductor circuit of the multilayer printed wiring board is placed as close as possible to the outermost layer of the front and back surfaces of the insulating substrate, a multilayer printed wiring board having through-hole plating parts that can withstand repeated thermal stress can be obtained. .

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a sectional view showing a through-hole portion of a multilayer printed wiring board according to an embodiment. In FIG. 0, (1) to (11) indicate the same or equivalent parts as in FIG. 4.

The copper-clad laminate on which the multilayer printed wiring board (1) is formed is
For example, electrolytic copper foil with a thickness of 35 μm is used to form an outer layer conductor circuit (conductor pattern) (3) on both the front and back surfaces of an insulating substrate (2) made of a glass-epoxy laminate with a thickness of 1.6 μm. have. In addition, inner layer conductor circuits (6) to (
9) uses, for example, an electrolytic copper foil with a thickness of 35 μm. The above multilayer printed wiring board (1) is an insulating substrate (2
), an inner layer power supply circuit (7) and a grounding circuit (8) occupying a large area are placed in the area A within d/4 of the front and back sides for the thickness d of the
is located in this part, and signal circuits (6) and (9) are also located in this part.

Therefore, even if the insulating substrate (2) expands or contracts due to heat, the tensile or compressive stress applied to the through-hole plated portion (5) itself can be suppressed to the waves of the double-sided printed wiring board, without using a thermal land. A highly reliable multilayer printed wiring board with significantly improved heat stress resistance can be obtained. - As an example, the thickness of the inner layer conductor circuits (6) to (9) is 35 μm,
Through hole plating part (5) thickness 20μm, substrate (2
) A 6-layer printed wiring board (1) with a finished thickness of 1.6s+a was subjected to a repeated thermal cycle test with a temperature change of ±100°C. Although through-hole disconnection occurred, the printed wiring board of the present invention was found to withstand over 1000 cycles.

In the above embodiment, all inner layer conductor circuits (6) to (9)
) is placed within d/4 on the front and back sides, but
The positions of the signal circuits (6) and (9) do not greatly affect the generated stress, so as shown in Figure 2, only the inner layer power supply circuit (7) and ground circuit (8), which occupy a large area, are placed on the front and back sides d/4. signal circuit (6), (
9) can produce the same effect even if it is placed closer to the center line than d/4. Furthermore, although the above embodiments have been described using a six-layer printed wiring board as an example, a multilayer printed wiring board having a layer structure of less than or more than that can produce the same effect. Furthermore, the present invention utilizes the subtractive method (
There is no difference depending on the printed wiring board manufacturing method such as the etched foil method or the full additive method, and the effect is exactly the same regardless of the printed wiring board manufacturing method.

〔Effect of the invention〕

Since the multilayer printed wiring board of the present invention has the inner layer conductor circuit located close to the outermost layer, it is more resistant to thermal stress than conventional ones, and reliability verification such as solder reflow process and thermal cycle test during manufacturing is possible. You can obtain something that can withstand repeated thermal stress over a long period of time, including testing.

[Brief explanation of the drawing]

1 and 2 are cross-sectional views showing the through-hole portion of a multilayer printed wiring board according to another embodiment, and FIG. 3 is a cross-sectional view showing the through-hole portion of the printed wiring board when the position of the inner layer conductor circuit is changed. A graph showing the results of analyzing the generated stress using a structural analysis simulation program. Figure 4 is a cross-sectional view showing the through-hole section of a conventional multilayer printed wiring board.
Figure 5 is a graph showing the results of an analysis using a structural analysis simulation program of the stress generated in the through-hole portion of a conventional four-layer printed wiring board, and Figure 6 is a graph showing the stress generated in the through-hole portion of a conventional double-sided printed wiring board. It is a graph showing the results of analyzing stress using a structural analysis simulation program. In each figure, the same reference numerals indicate the same or corresponding parts, (1)
is a multilayer printed wiring board, (2) is an insulating substrate, (3) is an outer layer conductor circuit, (4) is a through hole, (5) is a through hole plating part, and (6) to (9) are an inner layer conductor circuit. .

Claims (1)

[Claims] (1) A multilayer printed wiring board having through holes, characterized in that an inner layer conductor circuit is provided at a position close to the outermost layer on the front and back sides of an insulating substrate.