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: 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 (USP4,801,489 = 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. Usually, in the electronic circuit board, a bypass capacitor to bypass high frequency noise is provided on the connector located at the end of substrate, power supply terminal and ground terminal. The required function of this bypass capacitor is to bypass the unnecessary high frequency component. Therefore bypass capacitor of relatively small capacitance is used. In the conventional printed circuit board, this bypass capacitor is mounted on substrate as a discrete part. Therefore soldering is required to mount the capacitor on the substrate, which impedes reduction of size of substrate. Moreover, the lead wire of bypass capacitor causes inductance at high frequency, as a result of which the frequency characteristics of impedance are degraded. Generally, in the printed circuit board, a proper number of bypass capacitors are provided between ground pattern and circuit pattern or between power supply pattern and ground pattern so as to bypass the unnecessary high frequency component. In the conventional printed circuit board, however, this bypass capacitor is mounted on the substrate by soldering as a discrete part. Therefore, soldering is required, and existence of bypass capacitor impedes the reduction of size of substrate. Moreover, the lead wire of bypass capacitor causes inductance at high frequency, as a result of which the frequency characteristics of impedance are degraded. In the conventional 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 11. The remaining claims set out optional features. An embodiment of the invention provides a printed circuit board on which a bypass capacitor is formed by using a conductive layer. An embodiment of the invention provides a printed circuit board on which a bypass capacitor is formed at a place by forming shield conductive layers on both sides of an insulation layer. 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. An embodiment of the invention provides a printed circuit board capable of providing a suppression effect for EMI prevention means on a layout affording easy connection to the power supply pattern even when the conductive layer and ground pattern connection area is restricted. In an embodiment a conductive layer is provided above the connector land, so that a capacitor is composed of this conductive layer and the connector land through the insulation layer. This capacitor serves as a bypass capacitor. Or in the case when the conductive layer exists above the power terminal land, a bypass capacitor is composed of this power terminal land and the conductive layer located thereabove. However, from the viewpoint of radiation noise suppression effect by reduction and unification of circuit impedance at unnecessary high frequency, the same effect is obtained irrespective of whether the conductive layer is connected to the ground pattern or power supply pattern. Accordingly, in the case when it is difficult to connect the ground pattern to the conductive layer owing to layout restrictions but it is rather easy to connect it to the power supply pattern, the conductive layer is connected to the power supply pattern, and at the connector the bypass capacitor is composed of this conductive layer and connector land. Or the bypass capacitor is composed of ground terminal land and conductive layer. Moreover, it is possible to compose a capacitor, using 1st and 2nd conductive layers by interposing a 2nd insulation layer between the 1st and 2nd conductive layers. Accordingly, if the 2nd conductive layer is connected to any part of the circuit pattern, the capacitor can be used as a bypass capacitor of the signal line. When the 1st conductive layer is connected to the ground line pattern, a bypass capacitor can be composed between power supply source and ground by connecting the 2nd conductive layer to the power line pattern. This bypass capacitor is also composed when the 1st conductive layer is connected to the power line pattern and the 2nd conductive layer is connected to the ground line pattern. 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. The distribution electrostatic capacitance between the signal pattern and the ground pattern on the one hand and the conductive layer connected to power supply pattern on the other hand is increased, and the circuit impedance reduced, by connecting the conductive layer to the power supply pattern in place of the ground pattern. As a result, any unnecessary high frequency component caused by ringing is grounded to the power supply pattern, which results in suppression of radiation noise. Since the signal pattern and the ground pattern are wholly covered by the conductive layer connected to the power supply pattern, the impedance of each circuit is unified, and therefore occurrence of impedance unmatching in high frequency transmission, and generation of unnecessary high frequency components resulting therefrom, are suppressed. Moreover, the ordinary shield effect given by the conductive layer itself is additionally present. Brief description of the drawings Fig. 1 and Fig. 2 show the first embodiment. Fig. 3 shows the impedance vs. frequency characteristics of a prior art construction and the embodiment. Fig. 4(A), Fig. 4(B), Fig. 5(A) and Fig. 5(B) show a cross section of printed circuit board which is the second embodiment. Fig. 6 (A) shows a cross section of printed circuit board which is the third embodiment of this invention. Fig. 6 (B), Fig. 6 (C) and Fig 6 (D) show the bent part of signal line pattern. Fig. 7 explains the effect of the printed circuit board which conforms to 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. This circuit pattern 2 includes a signal pattern 20, a ground pattern 21, and a power supply pattern 22. 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. In the embodiments shown in the figures, the conductive paste layer 5 and the overcoat layer 6 are formed only on the front surface of substrate 1. As a matter of course, it is desirable to form these layers also on the rear side of substrate 1. Fig. 1 shows the printed circuit board which is the first embodiment of this invention. Fig. 1 (A) shows the X-X' cross section. Fig. 1 (B) shows the plan. First, the circuit pattern shown in the figure is formed on the surface of the substrate 1. The circuit pattern includes signal pattern 20, ground pattern 21, and power supply pattern not shown in the figure. In this embodiment, a connector land 8 to which the connector is connected is formed, in addition to this circuit pattern, at the end of substrate 1 with a larger area than that of ordinary connector land 8a,at the same time when the circuit pattern is formed. These patterns are formed by the well known photolithography. After the required patterns are formed, the undercoat layer 4 is formed, leaving uncovered a partial area A of the ground pattern 21. It is desirable that the area A where the ground pattern 21 is exposed is formed at several places on the substrate. At least one place is required. This undercoat layer 4 is formed by screen printing so that the upper side of connector land 8 is also covered together with the above-mentioned circuit pattern. After the undercoat layer 4 is formed, the conductive layer 5 for shield is formed thereon. This conductive layer 5 is also formed by printing on the undercoat layer 4 so as to cover the upper side of connector land 8 together with the above-mentioned circuit pattern. In this example of embodiment this conductive layer 5 is made of paste. After the conductive layer 5 is applied, it is heated to cure, and then an overcoat layer 6 made of insulation resin material is formed on the whole upper surface of conductive layer 5. This overcoat layer 6 can be made of the same material as that of undercoat layer 4. It is easily formed by printing together with conductive layer 5 and undercoat layer 4. In the composition mentioned above the area C, where the connector land 8 and the conductive layer 5 located above it overlap on respective sides of undercoat layer 4, composes a capacitor. Since the thickness of the undercoat layer 4 is about 20 to 40 µm, the capacitance necessary for a bypass capacitor can be obtained if the area C has an appropriate area. Accordingly, the required bypass capacitor can be formed by composing the capacitor section as the area C shown in Fig. 1 (A) for all the connector lands connected to each signal pattern without solder-connecting the bypass capacitor as a discrete part. Thus, two cases are compared (in one case the connector land and the conductive layer compose the bypass capacitor, in other case the bypass capacitor is used as a discrete part according to the conventional method). There are following differences between them. First, when the bypass capacitor is used as a discrete part according to the conventional method, inductance L of the lead wire can be calculated by using the formula (1) assuming that length and diameter of two lead wires (metal wires) of bypass capacitor are 5 mm and 0.6 mm, respectively. L= µl 2 (ln 2l r -1) [H] where µ: magnetic permeability (=4π x 10 -7 [H/m]) l: length of lead wire (=5 x 10 -3 [m]) r: radius of lead wire (=0.3 x 10 -3 [m]) Accordingly, inductance of one lead wire is: Assuming that the electrostatic capacitance (C) of the bypass capacitor is 330 x 10 -12 (F), the series impedance Z consisting of the capacitance of the capacitor and the inductance of two lead wires can be calculated by using the formula (2). . Z = 1 2πfc 2πfL x 2 where f: frequency (Hz) The impedance at f = 100 MHz is calculated as follows: Z = 1 2π x 100 x 10 6 x 330 x 10 -12 2π x 2 x 100 x 10 6 x 2.51 x 10 -9 = 4.82 - 3.15 = 1.67 [Ω] In the case when a bypass capacitor is composed of the connector land and the conductive layer, it is sufficient to consider only the electrostatic capacity. Therefore, the impedance Z can be obtained from the formula (3). . where ε 0 :dielectric constant of vacuum (8.854 * 10 -12 [F/m]) ε r :relative dielectric constant of undercoat layer S: overlap area of connector land and conductive layer (m 2 ) d:thickness of undercoat layer (m) Assuming that ε r = 5, S =2 cm 2 , and d = 30 µm, we get: C = 8.854 x 10 -12 x 5 x 2 x 10 -4 30 x 10 -6 = 2.95 x 10 -10 [F] Z = 5.40 [Ω] The frequency characteristics of impedance from 10 MHz to 1000 MHz were calculated in a similar manner for both cases and the resulting data are plotted in the logarithmic graph (Fig. 3). In Fig. 3, the abscissa shows frequency f(MHz) whereas the ordinate shows the absolute value of impedance |Z|(Ω). |Z A | and |Z B | represent the frequency characteristics of impedance, respectively of the conventional example and of the embodiment. |Z AC | and |Z AL | represent the frequency characteristics of the absolute value of impedance, respectively caused by caused by the electrostatic capacitance of the bypass capacitor and caused by inductance of the lead wire of the conventional example. As is evident from Fig. 3, in the conventional example, the absolute value of the impedance caused by the inductance of lead wire increases as the frequency increases. Consequently, the absolute value of the impedance |Z A | also increases at frequencies above 100 MHz. In the embodiment of the present invention, the bypass capacitor does not have inductance. Accordingly, the impedance decreases as the frequency increases. Therefore this feature is most useful for the bypass capacitor. The example shown in Fig. 1 represents a bypass capacitor between the connector 1and 8 and the conductive layer 5. Since the power terminal land is formed at the end of substrate 1, a bypass capacitor can be formed between this power terminal land and the conductive layer 5. The effect can be also enhanced by additionally using a discrete bypass capacitor. In the embodiment shown in Fig. 1, the conductive layer 5 is connected to the ground pattern 21. It is allowed to connect the conductive layer 5 to the power supply pattern in place of ground pattern 21. Fig. 2 (A) and Fig. 2 (B) show the examples. That is, when the undercoat layer 4 is formed by printing, a part of the power supply pattern 22 is left uncovered and the conductive layer 5 is formed by printing thereon, so that the potential of the conductive layer 5 can be set to the power supply voltage. As a result, in the area C a bypass capacitor is formed between the connector land 8 and the power source. Nevertheless, since the power supply impedance can be ignored at high frequency, the same high frequency noise (radiation noise) suppression effect as that obtained in the example of embodiment shown in Fig. 1 can be obtained. In the embodiment shown in Fig. 2, the bypass capacitor can be also formed between the ground terminal land and the conductive layer. As shown in Fig. 1 and Fig. 2, the same effect can be obtained by connecting the conductive layer 5 either to the ground pattern 21 or to the power supply pattern 22. Therefore, in the case when the power supply pattern and ground pattern is restricted in the layout of substrate, it is possible to select for connection to the conductive layer 5 the more effective pattern from the viewpoint of high frequency noise suppression effect. In the embodiment mentioned above there is no need to mount an additional capacitor on the substrate by soldering. Therefore the size of the printed circuit board can be reduced. Moreover, since lead wires are not required, inductance does not occur, which enables excellent frequency characteristics of bypass capacitor to be obtained. The formation of the bypass capacitor requires merely an extension of the insulation layer and the conductive layer. Therefore there is no need to increase the printing process. Accordingly, a good effect is obtained, and production cost does not increase. Fig. 4 (A) and Fig. 4 (B) show a printed circuit board which is another embodiment of this invention. The circuit pattern 2 shown in the figure is first formed on the surface of the substrate 1. The circuit pattern includes signal patterns 20 and 20', a ground pattern 21 and a power supply pattern not shown in the figure. These patterns are formed by using well known photolithography. After the required patterns are formed, the first undercoat layer 4 is formed, leaving uncovered the partial area D of signal pattern 20' and the partial area A of ground pattern 21. It is desirable that the area A where the ground pattern 21 is exposed is formed on several places of substrate. At least one place is required. After the first undercoat layer 4 is formed, the first conductive layer 5 for shield is formed by screen printing thereon. In this case the first conductive layer 5 is formed on the upper surface of the first undercoat layer 4 in a region on the left as shown in the figure, namely in the region not covering the upper part of signal pattern 20'. The reason why the first conductive layer 5 is formed so that the upper part of signal pattern 20' is not covered is that contact of the first conductive layer 5 to a second conductive layer 9 on the signal pattern 20' must be avoided, as stated later. The first conductive layer 5 mentioned above is made of copper paste. After applying the first conductive layer 5 it is heated to cure, and then the first overcoat layer 6 made of resin insulation material is formed so as to cover the first conductive layer 5. The first overcoat layer 6 can be made of the same material as that of undercoat layer 4. It can be easily formed by printing together with the first conductive layer 5 and undercoat layer 4. After the first overcoat layer 6 is formed, the second conductive layer 9 is formed in the area above the above-mentioned signal pattern 20'. This second conductive layer 9 is formed by screen printing, using the same material as that of the first conductive layer 5. Accordingly, it is connected to the signal pattern 20' in the area D. The region of formation of the second conductive layer 9 is chosen so that part of it (left side in the figure) is located overlapping a part of the first conductive layer 5. In the figure the second conductive layer 9 overlaps the first conductive layer 5 in the area C. In this area C the first overcoat layer 6 is sandwiched between the second conductive layer 9 and the first conductive layer 5, thereby forming a capacitor in this part. After the second conductive layer 9 is formed, the second overcoat layer 10 is formed on the whole surface of substrate. This second overcoat layer 10 is formed by using the same printing process and the same material as those applied for the first overcoat layer 6 and undercoat layer 4. In the composition mentioned above, the first conductive layer 5 and the second conductive layer 9 in the area C overlap each other through the first overcoat layer 6, and a capacitor is formed in this part. The first conductive layer 5 is connected to the ground pattern 21 in the area A, and the second conductive layer 9 is connected to the signal pattern 20' in the area D. Therefore the capacitor formed in this area C serves as a bypass capacitor connected to the signal pattern 20' Since the thickness of the first overcoat layer 6 is extremely small (20 to 40 µm), the bypass capacitor which is formed in the area C has a sufficient capacitance to act as the bypass capacitor. Since, moreover, the overlap area of first conductive layer 5 and second conductive layer 9 can be easily changed when designing the screen mesh layout, a bypass capacitor having a small capacitance or a bypass capacitor having a large capacitance can be easily formed. Above is shown an embodiment where the bypass capacitor is formed between signal pattern 20' and ground pattern 21. It is also allowed to connect the first conductive layer 5 to the power supply pattern in place of ground pattern 21. Even in this configuration the composition of area C does not change. A bypass capacitor having the required capacitance can be obtained. Fig. 5(A) and Fig. 5(B) show a bypass capacitor which is formed between the power supply pattern and the ground pattern. The difference in configuration between printed circuit boards shown in Fig. 4(A) and Fig. 4(B), and in Fig. 5(A) and Fig. 5(B), is that the second conductive layer 9 of printed circuit board is connected to the power supply pattern 22, not to the signal pattern 20'. This is the only difference. In all other respects they are the same. It is also allowed to connect the first conductive layer 5 to the power supply pattern 22 and the second conductive layer 9 to the ground pattern 21. In either case a bypass capacitor to absorb the high frequency noise between the power supply source and the ground pattern is formed in the area C. In the embodiment shown above, the same radiation noise suppression effect as that obtained on the conventional EMI-prevented printed circuit board can be obtained owing to existence of first conductive layer. Moreover, if the second insulation layer and the second conductive layer are laminated on the first conductive layer, a bypass capacitor can be formed in a part where the first conductive layer overlaps the second conductive layer. Therefore the radiation noise suppression effect is ensured and and at the same time the size of printed circuit board can be reduced. Since lead wires are not required, inductance does not occur, so that excellent frequency characteristics of the bypass capacitor can be obtained. Moreover, since all the insulation layers and conductive layers can be formed by using the known screen printing process, the production process can be simple. Fig. 6 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. 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. 6(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 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. 6 (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. 6 (C) and (D). Fig. 7 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. 6 (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. 6 (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. 6 (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. 6. 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 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 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); and a conductive layer (5) for connection to ground or a power supply potential, and formed on the insulation layer (4); characterised in that: (a) the insulation layer (4) is formed so as to cover part of the circuit pattern (2) excluding at least a part of the further pattern; and (b) the conductive layer (5) is formed so as to be connected to an uninsulated part of the further pattern, and to overlap a conductive portion (8,9) which is formed in addition to the circuit pattern (2) and which is connected to a part of the circuit pattern (2) other than the further pattern, to form a bypass capacitor. A printed circuit board according to claim 1, wherein said conductive portion is a conductive land (8) formed on said substrate (1). A printed circuit board according to claim 2, wherein said conductive land is a signal connector land to which a signal connector is connected. A printed circuit board according to claim 2, wherein said conductive land is a power supply terminal land to which a power supply connector is connected and said further pattern is the ground line pattern (21). A printed circuit board according to claim 2, wherein said conductive land is a ground terminal land to which a ground connector is connected and said further pattern is a power supply line pattern (22). A printed circuit board according to claim 1, wherein, a second insulation layer (6) is formed on said conductive layer (5), said conductive portion (9) is formed on the second insulation layer (6) and overlaps the conductive layer (5) so as to form said bypass capacitor, and the conductive portion (9) is connected to the signal line pattern (20'). A printed circuit board according to claim 1, wherein a second insulation layer (6) is formed on said conductive layer (5), said conductive portion (9) is formed on the second insulation layer (6) and overlaps the conductive layer (5) so as to form said bypass capacitor, and the conductive portion (9) is connected to the power supply line pattern (22) and said further pattern is the ground line pattern (21). A printed circuit board according to claim 1, wherein a second insulation layer (6) is formed on said conductive layer (5), said conductive portion (9) is formed on the second insulation layer (6) and overlaps the conductive layer (5) so as to form said bypass capacitor, and the conductive portion (9) is connected to the ground line pattern (21) and said further pattern is the power supply line pattern (22). A printed circuit board according to any one of the preceding claims, 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 any one of claims 1 to 9, wherein a bent part of said signal line pattern (20) in said circuit pattern (2) has the form of a circular arc. A method of manufacturing a printed circuit board comprising the steps of: (a) printing a circuit (2) 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); and (c) forming a conductive layer (5) on the insulating layer (4), characterised in that the insulating layer is not formed on at least a part of the said pattern (21 or 22) of the printed circuit (2) and the conductive layer (5) contacts the printed circuit (2) at the said pattern (21 or 22) and overlaps a conductive portion (8,9) which is formed in addition to the printed circuit (2) and which is connected to a pattern of the printed circuit (2) other than the pattern which the conductive layer contacts, to form a bypass capacitor. A method according to claim 11 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, HISHIYAHIGASHI-RYO, TATSUTA
Applicants: TATSUTA DENSEN KK, TATSUTA ELECTRIC WIRE & CABLE CO., LTD.
Application year: 1990
Main classification: H05K1
IPC codes: H05K1, H05K3, H05K9