JPS61182296A - Circuit board - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field of the Invention) The present invention relates to a circuit board, and particularly to a low dielectric constant insulating material used as a substrate material or an interlayer insulating film material.

(Technical background of the invention and its problems) Polyimide resin has excellent heat resistance and a relatively low dielectric constant (
(relative dielectric constant of about 3.2 to 3.5), it is used as a substrate material for circuit boards such as printed wiring boards and LSI mounting boards,
It is one of the insulating materials that has recently attracted particular attention as a material for interlayer insulating films in multilayer wiring boards.

By the way, as high-speed logic elements such as GaAS (gallium arsenide) ICs increase in speed, problems such as a reduction in signal propagation speed in the wiring on the substrate and crosstalk between wiring have become problems, so the substrate material Materials for interlayer insulating films are required to have as low a dielectric constant as possible. An example of an insulating material with a low dielectric constant is tetrafluoroethylene resin (
Although it is known to have a dielectric constant of 2.1 to 2.5 degrees, it has a problem of poor processability such as etching. Therefore, it is desirable to use polyimide resin, which has excellent processability, but is inferior to tetrafluoroethylene resin in terms of dielectric constant. As described above, polyimide resin and tetrafluoroethylene resin both have advantages and disadvantages when viewed as insulating materials for circuit boards.

(Object of the Invention) An object of the present invention is to provide a circuit board using an insulating material having a low dielectric constant and excellent workability.

[Summary of the invention]

The circuit board according to the present invention is characterized by using a low dielectric constant material made of polyimide resin containing bubbles.

〔Effect of the invention〕

The low dielectric constant insulating material for the circuit board according to the present invention is characterized by good workability because it is mainly composed of polyimide resin, and has a low dielectric constant because it contains air bubbles. Therefore, it is extremely effective as a substrate material for a circuit board for mounting high-speed logic elements, an interlayer insulating film material, etc.

[Embodiment of the Invention] FIG. 1 is a sectional view of a circuit board according to an embodiment of the invention. In the figure, a substrate 1 is constructed by forming an insulating layer 3 on a conductor layer 2, and a plurality of conductor wirings 4 are formed on the insulating layer 3.

The insulating layer 3 is made of a polyimide resin containing bubbles according to the present invention, and is formed to have a thickness of, for example, about 2 μm. In order to produce such a polyimide resin containing bubbles, for example, a varnish-like polyamide resin, which is a precursor of a polyimide resin, may be stirred using a suitable stirrer to mix bubbles into the polyamide resin. At this time, it is desirable to add an appropriate surfactant to prevent bubbles generated by stirring from agglomerating and escaping to the outside. However, it goes without saying that the material and amount of the surfactant must be selected so as not to impair the reduction in dielectric constant of the polyimide resin containing bubbles. After applying the varnish-like polyimide resin mixed with air bubbles onto the conductor layer 2, the polyimide resin containing air bubbles is heated, for example, to a temperature of about 300 to 350°C to cause a dehydration condensation reaction. An insulating layer 3 is formed.

The simulation results of the signal propagation speed on the conductor wiring 4 in the circuit board with this structure are shown below. As an embodiment of the present invention, an insulating layer 3 made of polyimide resin containing 40% of air bubbles by volume is formed to a thickness of 2 μm, and a strip-shaped wiring with a line width W=2 μm is formed as a conductor wiring 4. A circuit board was produced in which two circuit boards were formed with an interval of S=2 μm. On the other hand, as a comparative example, an insulating layer made of a normal polyimide resin (relative permittivity: 3.5) containing no air bubbles was prepared.

When determining the propagation speed on conductor wiring for two independent propagation modes, that is, even mode and odd mode, for these two types of circuit boards, it is found that for the comparative example circuit board on which an insulating layer made of ordinary polyimide resin is formed, each 1.84X10”m/S and 2.01 xl 0'
m/s is 5, whereas in the circuit board based on the present invention in which an insulating layer made of polyimide resin containing bubbles is formed, the speed is 2.3.
5x10111 m/s and 2. ．． 47X108m/
It becomes s. This is approximately 80% of the speed of light, and this improvement in propagation speed is an effect based on the reduction in the effective dielectric constant due to the inclusion of air bubbles in the polyimide resin constituting the insulating layer 3.

According to the circuit board of this example in which the insulating layer 3 made of polyimide resin containing bubbles is used as the substrate material, compared with the conventional circuit board in which the insulating layer is made of ordinary polyimide resin that does not contain bubbles. The signal propagation speed is 28% and 23% for even and odd modes, respectively.
The speed of propagation increases, and an ultra-high propagation speed of about 80% of the speed of light can be obtained. Therefore, this circuit board is suitable as a mounting board for ultra-high-speed devices such as GaAS IC.

FIG. 2 is a sectional view of a circuit board according to another embodiment of the present invention. In this embodiment, the substrate 10 is formed on a base 12 made of a semi-insulating material such as GaAs or an insulating material such as alumina ceramic with a ground conductor layer 11 deposited on the back side. An insulating layer 13 made of polyimide resin containing bubbles is formed as a surface layer, and conductive wiring 14 is formed on the insulating layer 13.

Although not shown, if the base 12 is made of a semi-insulating material such as GaAs, a desired circuit can be formed on the base 12 by a semiconductor process and connected to the conductor/body wiring 14. An i-speed theory IC is constructed.

Furthermore, if the substrate 12 is made of an insulating material such as alumina ceramic, a chip-shaped digital IC or the like is mounted on it and connected to the conductor wiring 14 by wire bonding or other means, thereby forming a high-slid IC as a whole. shall be configured. The circuit formed or mounted on the base 12 may be formed or mounted on the insulating layer 13, or may be formed or mounted on the region where the insulating layer 13 is selectively removed.

According to the circuit board of this embodiment, the surface layer of the board 10, that is, the part directly under the conductor wiring 14, is made of a low dielectric constant insulating layer 13 made of polyimide resin containing bubbles, so that Not only can the signal propagation speed be improved compared to the case where conductor wiring is directly formed on the base 12 which has a high dielectric constant, but also the base 12 and the insulating layer 1
There is an advantage in that crosstalk noise can also be reduced by appropriately selecting the thickness relationship in 3.

To explain this second embodiment more specifically, for example, the base 12 is made of G with a dielectric constant of 12 and a thickness of 300 μm.
An aAs substrate is used, an insulating layer 13 is formed on it, and a line width W and an interline space S are each 2μ.
A conductor wiring 4 having a diameter of m was formed.

FIG. 3 shows the signal propagation speed in even and odd modes when the thickness of the insulating layer 13 is 2.5 μm and the dielectric constant is changed. Insulating layer 13 is 40% according to the invention
In the case of polyimide resin containing bubbles, the signal propagation speed of this model is 1 in both even and odd modes.
．． 45X10Bm/s and 2.52X10 days m/S. By the way, in the case of normal polyimide resin, the speed is 1.32X10' m/s and 2.18X10"m, respectively.
/S.

FIG. 4 shows the crosstalk level between the conductor wirings 14 when the film thicknesses of the insulating layer 13 are made of a polyimide resin containing 40% bubbles and a normal polyimide resin. The wiring length of the conductor wiring 14 is 3 mm, and the measurement conditions are as follows: A terminating resistor of 1Ω is loaded at one end of the conductor wiring 14.
A pulse signal of 0.1 ns between rising surfaces is applied here,
The signal propagation speed and crosstalk level were measured at the other end of the conductor wiring 14 to which a 10 MΩ terminating resistor was connected.

This measurement method is described in the literature (1) Yoshiwara et al.
Numerical analysis of high-speed pulse transmission characteristics in
This is the method described in 4). As is clear from this figure, when the insulating layer 13 is made of ordinary polyimide resin, 22
%, but when the polyimide resin containing bubbles according to the present invention is used for the insulating layer 13, the crosstalk level is reduced to about 17%. The effect of reducing this crosstalk noise is greater as the dielectric constant of the disconnected layer 13 is smaller.

It should be noted that the present invention can be modified in various ways without departing from the gist of the invention. For example, in the embodiment, an example was described in which the polyimide resin containing bubbles according to the present invention was used as the substrate material itself. Of course, it can also be used as an interlayer insulating film on a substrate or a protective film on various circuit boards.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view of one embodiment of the circuit board according to the present invention, FIG. 2 is a cross-sectional view of another embodiment of the circuit board according to the present invention, and FIG. 3 is a circuit board of the embodiment of FIG. 2. Figure 4 shows the relationship between the dielectric constant of the insulating layer and the signal propagation speed on the conductor wiring in the example shown in Figure 2. FIG. DESCRIPTION OF SYMBOLS 1... Substrate, 2... Conductor layer, 3... Insulating layer made of polyimide resin containing bubbles, 4... Conductor wiring, 1o.
. . . Substrate, 11 . . . Ground conductor layer, 12 . . . Base made of an insulating or semi-insulating material, 13 . Figure 1-5/LLI)'! Bell on T value

Claims (4)

[Claims] (1) A circuit board characterized by using a low dielectric constant insulating material made of polyimide resin containing bubbles. (2) The low dielectric constant insulating material is characterized in that it is used for the insulating layer in a circuit board formed by forming conductor wiring on a base formed by forming an insulating layer on a conductor. A circuit board according to claim 1. (3) The low dielectric constant insulating material is interposed between the base and the conductor wiring in a circuit board formed by forming the conductor wiring on an insulating or semi-insulating base. A circuit board according to claim 1. (4) The low dielectric constant insulating material is used for the interlayer insulating film in a multilayer wiring circuit board in which conductor wiring and interlayer insulating films are alternately formed. The circuit board according to item 1.