Printed circuit board

Abstract: The radiation noise suppression effect is enhanced by providing an insulation layer which is formed so that the circuit pattern is covered excepting at least a part of power source pattern or ground pattern on the substrate on which circuit pattern is formed, and a conductive layer which is formed so as to be connected to the uninsulated part of the power source pattern or the ground pattern on the insulation layer, by modifying pattern shape of the conductive layer and the insulation layer or by increasing or reducing the number of these layers.

Description: SPECIFICATION Background of the Invention (Field of the Invention ) This invention relates to a printed circuit board e.g. for use in electronic equipment, such as a printed circuit board for which electromagnetic interference (EMI) preventing means is taken. (Description of the Prior Art) The recent progress of electronic equipments is facilitating the trend toward increase of operation speed and density of circuit formed as printed circuit board, as a result which regulations are becoming more severe. One of the typical EMI preventing means which are conventionally applied is to reduce the radiation noise and incoming noise by covering printed circuit board with the shield case. However, this method causes a problem that the electromagnetic wave energy enclosed in the shield case is emitted outwards through cables. Moreover, since this method is not basically intended to reduce the electromagnetic wave energy which is generated due to high frequency characteristics of circuit. Therefore it is almost impossible to completely suppress the radiation noise. Hence, a new printed circuit board against EMI has been proposed (US-A-4,801,489 = EP-A-0 238 267). This printed circuit board features that an insulation layer (hereinafter this insulation layer is called the undercoat layer) is formed, excepting at least a part of ground pattern, on a substrate on which a signal line pattern (hereinafter referred to as signal pattern), a ground line pattern (herein after referred to as ground pattern) and a power line pattern (hereinafter referred to as power supply pattern) are formed, and a conductive layer such as conductive paste layer is formed thereon so as to be connected to an uninsulated part of the ground pattern. Usually, another insulation layer (hereinafter referred to as overcoat layer) is formed on this conductive layer. Radiation noise can be reduced in the printed circuit boards of such a configuration mainly for the following four reasons. Firstly, reduction of ground pattern impedance by conductive layer. Secondly, removal of high frequency component from signal pattern and power supply pattern by adjacent conductive layer. That is, since the undercoat layer which is formed with so-called solder resist is thin, about 20 to 40µm in thickness, the distributed electrostatic capacity of signal pattern and power supply pattern with respect to the conductive layer which is formed thereon and connected to the ground pattern is significant. Accordingly, unnecessary high frequency component which is generated by ringing is grounded to the ground pattern with high frequency, so that the radiation noise is suppressed. Thirdly, equalization of impedance of signal pattern and power supply pattern by conductive layer. That is, since the signal pattern and power supply pattern are covered by the conductive layer, the distance between these circuit patterns and the conductive layer connected to the ground pattern is equalized, and the impedance of each circuit pattern is equalized. As a result, impedance mismatch in high frequency transmission and generation of unnecessary high frequency component which is caused thereby are suppressed. Another reason is a shield effect by conductive layer itself. Owing to these four reasons, that is reduction of impedance of ground pattern by conductive layer, removal of high frequency component by adjacent conductive layer, equalization of impedance of circuit pattern and ordinary shield effect of conductive layer itself, the radiation noise can be effectively suppressed. However, in the conventional printed circuit board employing conventional EMI preventing means, the width of the ground pattern led to the IC ground pattern is small, so that the impedance at high frequency is high, resulting in occurrence of potential difference along its length and, thereby, in unstable ground level of IC. As a result of fluctuation of this ground level, the radiation noise is caused, and by the same reason the radiation noise is caused in the output signal of IC. Due to this, sufficient effect of EMI preventing means cannot be obtained. In the known printed circuit board featuring that the conductive layer is formed on the undercoat layer, the conductive layer is formed only around IC excepting the part just below IC. Therefore the effect of conductive layer could not be sufficiently utilized. As a result of persistent examination and tests of above-mentioned printed circuit boards it has been revealed that sufficient effect cannot be obtained as to reduction of radiation noise (high frequency noise) of electromagnetic wave for some shapes of signal pattern included in circuit pattern. There is a problem that if the interior angle of signal pattern of bent part is near 90*, the radiation noise reduction effect cannot be obtained sufficiently. The reason is unknown yet. It is assumed that the impedance mismatching occurs in the bent part, resulting in generation of reflected wave, which causes increase of equivalent high frequency impedance. Especially, for the fine pitch high integration circuit board, CAD is used for designing the circuit pattern. Most of circuit boards having the circuit pattern which has been designed with the aid of this support device feature that the bent part of signal pattern has angle of 90*. As the rule of wire width and pitch of signal line of such a board is made more strict, the radiation noise becomes more noticeable. The layout of printed circuit board is varied depending on uses. Hence, in some cases the sufficient ground pattern area to connect a conductive layer cannot be provided. In these case the area to connect the ground pattern and the conductive layer is small, and high frequency impedance is increased, resulting in reduction of effect. To prevent potential difference in the ground circuit, it is desirable to connect the ground pattern and the conductive layer in several places. If such a connection is impossible owing to restriction of layout, the requirement to EMI could not be satisfied sufficiently. Summary of the invention According to the present invention there is provided a printed circuit board according to claim 1 and a method of manufacturing a printed circuit board according to claim 6. The remaining claims set out optional features. An embodiment of the invention provides a printed circuit board which addresses the above mentioned problem by forming a conductive layer also between IC pin insertion holes. An embodiment of the invention provides a printed circuit board on which an electromagnetic wave radiation noise reduction effect given by a conductive layer is not degraded even when the signal pattern pitch is made finer. The unnecessary high frequency component can be reduced by arranging a part of the conductive layer so that the IC pin land is not contacted between IC pin insertion holes, as a result of which radiation noise can be suppressed. If the interior angle of a bent part of signal line pattern is set to approx. 135° or more, it seems that signal reflection at this bent part is reduced. As result of this, the radiation noise which occurs on the whole printed circuit board is reduced. Similarly, the radiation noise caused on the whole printed circuit board is reduced if the bent part of signal line pattern is made in the form of circular arc. These facts have been proved experimentally by the inventor as is discussed later. Brief description of the drawings Fig. 1 (A) and Fig. 1 (B) show a cross section and a plan of a printed circuit board which is an embodiment of the invention. Fig. 2 explains the effect of the printed circuit board of the embodiment. Fig. 3 (A) shows a cross section of printed circuit board of another embodiment of this invention. Fig. 3 (B), Fig. 3 (C) and Fig 3 (D) show the bent part of signal line pattern. Fig. 4 explains the effect of the printed circuit board of this embodiment of the invention. Description of the preferred embodiment In the embodiments, a circuit pattern 2 made of copper foil is formed on the upper surface of a substrate 1 which is composed of an insulation material such as epoxy resin, phenol resin, glass fiber or ceramics. These patterns are formed by known photolithography. After formation of required pattern, an undercoat layer 4 is formed. This undercoat layer 4 is solder resist layer made of resin insulation material. The undercoat layer 4 can be easily formed by screen printing. After the undercoat layer is formed, a conductive paste layer 5 is formed thereon by screen printing. The conductive layer is made of copper paste and is formed on the undercoat layer 4. The material, for example, of the following composition, can be used for the copper paste. Other compositions are also applicable. Basically, the copper paste is made by mixing the fine copper particles which serve as filler, binder designated to bond reliably these particles and various additives to maintain the conductivity stably for a long time. Concretely, it is prepared by mixing 100 wt. portion of metal copper powder, 10 to 25 wt. portion of resin mixture (consisting of 35 to 50 wt. % of melamine resin, 20 to 35 wt. % of polyester resin, and 15 to 30 wt. % of resole type phenol resin), 0.5 to 2 wt. portion of fatty acid or its metallic salt and 0.5 to 4 wt. portion of chelate forming agent. Metallic copper powder of flake, resin, spheric or amorphous type can be used. It is desirable that particle diameter be less than 100 µm, especially 1 to 30 µm. After the conductive paste 5 is applied, it is heated to cure. Then the overcoat layer 6 made of insulation resin material is formed on the whole surface of conductive paste layer 5. This overcoat layer 6 can be made of the same material as that of undercoat layer 4. The conductive paste layer 5 and the undercoat layer 4 can be easily formed by printing. Fig. 1 (A) and Fig. 1 (B) show the cross section and the plan view of the vicinity of an IC mount part of a circuit board which is an embodiment of this invention. Fig. 1 (A) and Fig. 1 (B) show Y-Y' view cross section and X-X' view cross section, respectively. The circuit pattern 2 shown in the figure is formed on the surface of the substrate 1. First the circuit pattern 2 shown in the figures is formed. This circuit pattern 2 includes the signal pattern, ground pattern, power supply pattern and through-hole copper foil parts. After the necessary patterns are formed, the undercoat layer 4 is formed, leaving uncovered a partial area of the ground pattern which is not shown in the figure and the land (IC pin land part) 2a of the through-hole 11 which is used to insert the IC pin. This undercoat layer 4 is solder resist layer made of resin insulation material. It can be formed easily by screen printing. After the undercoat layer 4 is formed, the conductive paste layer 5 is formed by screen printing on the place 12 just under IC on the undercoat layer 4 and on the peripheral part 9 so that the peripheral part 9 and the place 12 are continuous with one another. In this case the masking part to be used is IC pin land 2a and a clearance is formed between adjacent pin lands 2a. The conductive paste layer 5 is heated to cure. Then the overcoat layer 6 made of insulation resin material is formed on the whole surface of substrate 1, except that it leaves uncovered the IC pin land part 2a. The overcoat layer 6 can be made of the same material as that of undercoat layer 4. The conductive paste layer 5 and the undercoat layer 4 can be easily formed by printing. In the arrangement described above, the conductive paste layer 5 is formed as shown in Fig. 1 (B) so that it does not contact the IC pin land 2a in the area between IC pin insertion holes. Therefore the unnecessary high frequency component which is generated in this part can be reduced, so that the radiated noise can be suppressed. In this embodiment, the undercoat layer 4, the conductive paste layer 5 and the overcoat 6 can be easily formed by conventional screen printing. Accordingly, the EMI- suppressing printed circuit board can be easily made without using complicated process. Moreover, since the above mentioned conductive paste layer is formed between the IC pin insertion holes, namely between through-holes, the unnecessary high frequency component which is generated in this part can be reduced, so that the radiation noise can be suppressed. Especially, for the IC pin to which a clock or pulse signal is always supplied, the arrangement described in this embodiment is useful from the viewpoint of reduction of high frequency noise. Fig. 2 shows a graph where the abscissa shows percentage of area treated between the IC pins of printed circuit board, namely percentages of conductive paste layer which is formed in all the areas between IC pins, and the ordinate shows the radiation noise reduction effect. As is evident from this figure the radiation noise reduction effect is enhanced if the treated area between IC pins is increased. Accordingly, it is better to increase the treated area between the IC pins. Even when the conductive paste layer is formed on 1/4 of the total area between the IC pins, radiation suppression effect of about 7 dB is obtained. Therefore it is evident that the partial formation of a conductive layer between the IC pins is also included in this invention. It is better to form in turn the undercoat layer 4, the conductive paste layer 5 and the overcoat layer 6 also on the rear surface of the substrate although it is not shown in Fig. 1. In this embodiment the connection point of conductive paste layer 5 is assumed to be the ground pattern. The same effect can be obtained when the connection point of the conductive paste layer 5 is provided on the power supply pattern. In this embodiment, a part of conductive layer is formed also between IC pin insertion holes. Therefore the unnecessary high frequency component which is generated in this part, namely radiation noise, can be suppressed. Especially, on a printed circuit board the radiation noise is generated frequently on the IC pins since clock and pulse signal are always supplied to this part. If the arrangement of this embodiment is applied, radiation noise is suppressed effectively. Also, an ordinary screen printing process can be applied for production. Only the conductive layer arrangement pattern must be changed. Therefore the production cost is not raised. Fig. 3 shows a printed circuit board which is another embodiment of this invention. First, the circuit pattern as shown in the figure is formed on the surface of substrate 1 made of insulation material such as epoxy resin, phenol resin, glass fiber or ceramics. The circuit pattern includes the signal pattern 20, and the ground pattern 21. These patterns are formed by well known photolithography. After the required patterns are formed, the undercoat layer 4 is formed, leaving uncovered partial area A of the ground pattern 21. This undercoat layer 4 is solder resist layer made of resin insulation material. It is desirable that the exposed area A of the ground pattern 21 is formed on several places on the substrate. It must be formed at least on one place. After the undercoat layer 4 is formed, the conductive paste layer 5 for shield is formed by screen print. In this example of embodiment this conductive paste layer 5 is made of copper paste and covers almost the whole area of the undercoat layer 4. After the conductive paste layer 5 is applied, it is heated to cure. And then the overcoat layer 6 made of insulation resin material is formed on the whole upper surface of the substrate 1. This overcoat layer 6 can be made of the same material as that of the undercoat layer 4. It can be easily formed by the printing process together with the conductive paste layer 5 and the undercoat layer 4. As shown in Fig. 3(B) the signal pattern 20 is wholly arranged so that the interior angle of bent part B is set to approx. 135°. If the pattern of each signal line is set to 135° as shown in the figure, signal reflection at the bent part B is reduced, so that increase of high frequency impedance is prevented. Accordingly, as compared to conventional arrangement where the bent part B is set at a right angle, generation of radiation noise (high frequency noise) is less. Although suppression of high frequency noise generation at one bent part B is insignificant, significant effect is obtained if the effects of all the bent parts are combined since one substrate usually has many signal line bent parts. The result of experiment shows that if the interior angle of this bent part is less than 135°, the effect reduces, but at wider angle than 135° almost the constant effect is obtained. If the interior angle of bent part is larger than 135° as shown in Fig. 3 (B), the specified effect can be obtained. It has been proved that the effect can be obtained by giving a radius (circular arc) to the bent part B as shown in Fig. 3 (C) and (D). Fig. 4 shows a graph where abscissa shows frequency and ordinate shows electric field strength. It compares the conventional printed circuit board "a" where the bent part B is at a right angle, the printed circuit board "b" which is the embodiment shown in Fig. 3 (B) where the interior angle of the bent part B is set to approx. 135°, and the printed circuit board "c" which is the embodiment shown in Fig. 3 (C) where a radius is given to the bent part B and the interior angle is set to approx. 135°. As is evident from the figure, excellent magnetic wave suppression effect can be obtained if the printed circuit boards "b" and "c" of the embodiment of this invention are used. Although the graph of printed circuit board "d" which is the embodiment shown in Fig. 3 (D) is not shown, it occupies an intermediate position between the printed circuit boards "a" and "b", not better than "b" and "c" but evidently better than "a". It is desirable to form the conductive paste layer 5 and the overcoat layer 6 also on the rear side of the substrate, although they are not shown in Fig. 3. In the embodiment of the invention mentioned above the conductive paste layer 5 is connected to the ground pattern 21. It is also allowed to connect this conductive paste layer 5 to the power supply pattern not shown in the figure in place of the ground pattern 21. In the embodiment mentioned above, increase of high frequency impedance on the bent part can be prevented by setting the interior angle of the bent part to at least approx. 135° or giving a circular arc to this bent part. For this reason, and owing to the effect of the conductive layer, the effect of suppression of radiation noise emitted from the whole circuit board is greatly enhanced.

Claims: A printed circuit board, comprising: a substrate (1); a circuit pattern (2) formed on the substrate (1) and including a signal line pattern (20) and a further pattern, which is one of (i) a power supply line pattern (22) and (ii) a ground line pattern (21); an insulation layer (4) which is formed on the substrate (1) over the circuit pattern (2) so as to cover the circuit pattern (2) except at least a part of the further pattern; and a conductive layer (5) for connection to ground or a power supply potential, formed on the insulation layer (4) and connected to an uninsulated part of the further pattern, characterised in that : (a) the substrate has a plurality of Integrated Circuit pin insertion holes (11) through it, with respective Integrated Circuit pin connection lands (2a), for mounting an Integrated Circuit; and (b) the conductive layer (5) is formed so as to extend over at least a part of the area between the plurality of Integrated Circuit pin insertion holes (11) without contacting the respective Integrated Circuit pin connection lands (2a). A printed circuit board according to claim 1, wherein an internal angle of a bent part of said signal line pattern (20) in said circuit pattern (2) is set to approximately 135 degrees or more. A printed circuit board according to claim 1, wherein a bent part of said signal line pattern (20) in said circuit pattern (2) has the form of a circular arc. A printed circuit board according to any one of the preceding claims in which the plurality of Integrated Circuit pin insertion holes (11) includes one for a pin carrying a clock or pulse signal. A printed circuit board according to any one of the preceding claims in which the conductive layer (5) is effective to suppress radiation from the signal line pattern (20) in the range of 30 MHz to 300 MHz. A method of manufacturing a printed circuit board comprising the steps of: (a) printing a circuit including a power supply line pattern (22) or a ground line pattern (21) on a substrate (1); (b) forming an insulating layer (4) on said printed circuit (2) but not on at least a part of the said pattern (21 or 22) of the printed circuit (2); and (c) forming a conductive layer (5) on the insulating layer (4) so as to contact the printed circuit (2) at the said pattern (21 or 22), characterised in that : (a) the substrate has a plurality of Integrated Circuit pin insertion holes (11) through it, with respective Integrated Circuit pin connection lands (2a), for mounting an Integrated Circuit; and (b) the conductive layer (5) is formed so as to extend over at least a part of the area between the plurality of Integrated Circuit pin insertion holes (11) without contacting the respective Integrated Circuit pin connection lands (2a). A method as according to claim 6 in which the insulating layer (4) and the conductive layer (5) are formed by a screen printing process.

Inventors: MORIMOTO SHOHEI, MURAKAMI HISATOSHI, NAKATANI FUMIO, TERADA TSUNEHIKO, WAKITA SHINICHI, MORIMOTO, SHOHEI, MURAKAMI, HISATOSHI, NAKATANI, FUMIO, TERADA, TSUNEHIKO, WAKITA, SHINICHI, MORIMOTO, SHOHEI, HISHIYASHIGASHI-RYO, TATSUTA EL., TERADA, TSUNEHIKO, MIDORIGAOKA DANCHI 32-204,
Applicants: TATSUTA DENSEN KK, TATSUTA ELECTRIC WIRE & CABLE CO., LTD.
Application year: 1990
Main classification: H05K1
IPC codes: H05K1, H05K9, H05K3