JP2004363347A - Multilayer printed circuit board - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a multilayer printed circuit board which can reduce EMI with a simplified structure. <P>SOLUTION: In the case of eight-layer structure, for example, ground layers G1, G2 formed of copper foil covering almost the entire part of the circuit board are allocated to the second layer and seventh layer. Moreover, the ground layers G1, G2 are almost entirely connected with a shield G3 by the copper plating to wrap the power supply layers P1, P2, signal layers S2, S3 and insulation layers I2 to I6 at the internal side of the circumference of the end of the circuit board. Accordingly, resonance between the power supply layers P1, P2 and the ground layers G1, G2 may be eliminated and radiation of EMI to the external side can be suppressed. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【０００６】
ここで、ｎは開放境界における外向き単位法線ベクトルである。
この場合、多層プリント基板の外形が、対象としている信号の波長に対して数分の１程度の寸法よりも大きければ、外形の影響は無視できなくなり、次の（２）式の周波数ｆｍｎで平行平板の共振が発生する。
【数２】

【０００７】
ここで、ｍ，ｎは０または正の整数（但し、ｍ＝ｎ＝０は除く）、ａ，ｂはプリント基板の縦及び横の外形寸法、Ｃ０ は真空中の電磁波の速度（光速度）、及びε_ｒは電源層とグランド層の間の絶縁層の誘電率である。
【０００８】
共振が発生した場合、内部では大きな電磁界が発生し、開放端部から電磁界が漏れて外部に放射される。電磁界上では開放端部は磁気壁のように振る舞い、端面に沿って次の（３）式で示す磁流Ｉｍが流れ、この磁流Ｉｍを放射源とした放射が発生する。
Ｉｍ＝Ｅｅｄｇｅ×ｔ ・・・（３）
【０００９】
ここで、Ｅｅｄｇｅはプリント基板端部の電界、ｔは電源層とグランド層の間の厚さである。
【００１０】
このように、信号配線のリターン電流の広がりや、層間接続用のビア配線からの結合等の影響で、電源−グランド層が励振されて空洞共振が発生し、開放部となったプリント基板端部が、スロットアンテナのように振る舞い、共振で発生された周波数の電磁波による大きなＥＭＩが発生する。このため、プリント基板の端部にバイパスキャパシタを接続したり、キャパシタと抵抗の組み合わせによるＡＣ終端を行ったり、このプリント基板をシールドケースに収納する等の大掛かりな対策が必要であった。
【００１１】
本発明は、簡単な構造でＥＭＩを低減できる多層プリント基板を提供するものである。
【００１２】
【課題を解決するための手段】
前記課題を解決するために、請求項１の発明は、信号層と、電源層と、第１及び第２のグランド層とが絶縁層を介して積層された多層プリント基板において、前記第１及び第２のグランド層を、それぞれ表面及び裏面から１層目或いは２層目に配置すると共に、これらの第１及び第２のグランド層で挟まれる前記信号層と電源層を包み込むように、該第１及び第２のグランド層の端部を導電性のシールド材で接続して構成している。
【００１３】
請求項２の発明は、請求項１の多層プリント基板における第１及び第２のグランド層を、基板のほぼ全面を覆う銅箔で形成し、シールド材を、銅めっきで形成するようにしている。
【００１４】
本発明によれば、以上のように多層プリント基板を構成したので、次のような作用が行われる。
【００１５】
例えば、信号層に流れる電流によって電源層の電位が変動し、この電位変動によって電源層とグランド層の間で空洞共振が発生したとする。電源層は、第１及び第２のグランド層とこれらのグランド層の端部を接続するシールド材とで、包み込まれているので、この多層プリント基板から外部に放射される電磁波は低減される。
【００１６】
【発明の実施の形態】
図１は、本発明の実施形態の多層プリント基板を概念的に示す断面図である。
この多層プリント基板では、表面から順に、信号層Ｓ１、グランド層Ｇ１、電源層Ｐ１、信号層Ｓ２、信号層Ｓ３、電源層Ｐ２、グランド層Ｇ２、及び信号層Ｓ４を形成する８層の配線パターンが、絶縁層Ｉ１，Ｉ２，…，Ｉ７を介して順次積層されている。即ち、表面及び裏面の信号層Ｓ１，Ｓ４とその内側のグランド層Ｇ１，Ｇ２との間は、それぞれ絶縁層Ｉ１，Ｉ７によって絶縁されている。また、グランド層Ｇ１，Ｇ２とその内側の電源層Ｐ１，Ｐ２との間は、それぞれ絶縁層Ｉ２，Ｉ６によって絶縁されている。更に、電源層Ｐ１，Ｐ２の間には、絶縁層Ｉ３，Ｉ５を介して、信号層Ｓ２、絶縁層Ｉ４及び信号層Ｓ３からなる内層パターンが形成されている。
【００１７】
なお、表面の信号層Ｓ１には、ＬＳＩ等の電子回路部品を搭載するための電極等を備えた配線パターンが形成されている。また、図示していないが、各層の配線パターンの間は、絶縁層を貫通して設けられたビアポスト等によって接続されている。
【００１８】
一方、この多層プリント基板における端部の周囲は、２層目のグランド層Ｇ１と７層目のグランド層Ｇ２との間が、銅めっきによるシールドＧ３でほぼ全面的に接続され、その内側の電源層Ｐ１，Ｐ２、信号層Ｓ２，Ｓ３及び絶縁層Ｉ２〜Ｉ６が包み込まれている。
【００１９】
なお、このような多層プリント基板は、例えば次のようにして製造することができる。
【００２０】
先ず、グランド層Ｇ１，Ｇ２のサイズが、他の電源層Ｐ１，Ｐ２及び信号層Ｓ１〜Ｓ４よりも、一回り大きく形成された８層の多層プリント基板を、従来と同様の製造工程を用いて製造する。次に、この多層プリント基板の周辺部で、シールドＧ３を形成すべき箇所に、細長いスルーホール設ける。この時、信号層Ｓ１〜Ｓ４を含む内部領域と、その外周部との間が切り離されないように、例えば四隅に接続領域を残しておく。
【００２１】
更に、多層プリント基板の周辺部に設けられたスルーホールを銅メッキで充填した後、その外側の周辺部を切断して除去する。これにより、グランド層Ｇ１，Ｇ２間が、銅めっきによるシールドＧ３でほぼ全面的に接続され、その内側の電源層Ｐ１，Ｐ２、信号層Ｓ２，Ｓ３及び絶縁層Ｉ２〜Ｉ６が包み込まれた多層プリント基板が完成する。
【００２２】
次に、この多層プリント基板における電磁放射の動作を説明する。
この多層プリント基板では、電源−グランド層の平行平板構造を、グランド層Ｇ１、電源層Ｐ１、信号層Ｓ２，Ｓ３、電源層Ｐ２、及びグランド層Ｇ２の構成にし、２つのグランド層Ｇ１，Ｇ２の端部を接続している。これにより、端部から電磁界が漏れない構造となり、外部への放射は抑制される。また、グランド層Ｇ１，Ｇ２の端部が閉じていることから、グランド層の面積が広がったことになる。従って信号層Ｓ１〜Ｓ４を流れる電流の帰路も確保され、信号伝送上の問題も生じない。
【００２３】
図３（ａ）〜（ｃ）は、本実施形態のＥＭＩ低減効果の試験結果の一例を示す図である。
【００２４】
図３（ａ）は、図２と同様にＥＭＩ対策を施していない従来の多層プリント基板を用いた試験回路、同図（ｂ）は図１と同様のＥＭＩ対策を施した本発明の実施形態の多層プリント基板による試験回路である。
【００２５】
いずれも、基板の外形サイズは２３０×１５０ｍｍで、８層構造となっている。また、全長２１０ｍｍの信号配線は、信号層Ｓ１，Ｓ４に分割して配置され、これらの信号層Ｓ１，Ｓ４の間の接続は、ビアポストを介して行われている。なお、この信号配線の特性インピーダンスは５０Ωであり、一端とグランド層Ｇ１との間に信号入力用ＳＭＡコネクタを介して試験信号が与えられ、他端は終端用の２つの５ｐＦのキャパシタを介して、それぞれグランド層Ｇ１と電源層Ｐ１に接続されている。
【００２６】
図３（ｃ）は、上記試験回路から放射された電磁界強度を電波暗室３ｍで測定した測定結果であり、横軸にＥＭＩ周波数［ＭＨｚ］、縦軸に放射電界強度［ｄＢμＶ／ｍ］を示している。この試験では、従来の多層プリント基板と本実施形態のＥＭＩ対策を施した多層プリント基板のそれぞれに、試験信号としてレベルが０ｄＢｍの正弦波を３０〜１０００ＭＨｚの間でスイープして与え、各周波数における放射電磁界強度を測定している。図３（ｃ）に示すように、本実施形態のＥＭＩ対策を施した多層プリント基板では、最大放射電界強度で１３．４ｄＢ低減され、また周波数特性ではピークがほぼ消滅するまでに低減されていることが確認された。
【００２７】
このように、本実施形態の多層プリント基板は、２つのグランド層Ｇ１，Ｇ２の端部をほぼ全面的に接続し、電源層Ｐ１，Ｐ２の端部が外部に露出されないように構成している。これにより、電源−グランド層間で発生した電磁界を閉じ込め、外部への放射を抑制することができる。また、グランド層は、開放端部を持たないので、接地としての働きが強化され、端部以外の放射も抑制される。これにより、バイパスキャパシタやノイズ対策部品を使用したり、シールドケースに収納する等の大掛かりな対策を必要とせずに、簡単な構造でＥＭＩを低減できるという利点がある。
【００２８】
なお、本発明は、上記実施形態に限定されず、種々の変形が可能である。この変形例としては、例えば、次のようなものがある。
【００２９】
（ａ） ８層の多層プリント基板を例に説明したが、層数は任意である。
【００３０】
（ｂ） 表面及び裏面から２層目にグランド層Ｇ１，Ｇ２を設けているが、１層目にグランド層を設けても良い。また、一方の面の１層目と他方の面の２層目に、それぞれグランド層を設けるようにしても良い。
【００３１】
（ｃ） 製造方法は、例示した方法に限定されない。例えばグランド層Ｇ１，Ｇ２で挟まれる６層基板を形成した段階で、これらのグランド層Ｇ１，Ｇ２間を銅メッキで接続する。更にグランド層Ｇ１の表面にプリプレグによる絶縁層Ｉ１を介して信号層Ｓ１となる銅箔を貼付け、この銅箔をエッチングして信号層Ｓ１を形成する。同様に、グランド層Ｇ２の表面に絶縁層Ｉ７を介して銅箔を貼付け、この銅箔をエッチングして信号層Ｓ４を形成する、という方法も可能である。
【００３２】
【発明の効果】
以上詳細に説明したように、請求項１の発明によれば、第１及び第２のグランド層を、それぞれ表面及び裏面から１層目或いは２層目に配置し、更に、これらの第１及び第２のグランド層の端部を導電性のシールド材で接続している。これにより、内部の信号層と電源層が、第１及び第２のグランド層で包み込まれ、簡単な構造で外部へのＥＭＩの放射を低減することができる。
【００３３】
請求項２の発明によれば、第１及び第２のグランド層を、基板のほぼ全面を覆う銅箔で形成し、更にシールド材を銅めっきで形成している。これにより、ほぼ確実に、ＥＭＩの放射を低減することができる。
【図面の簡単な説明】
【図１】本発明の実施形態の多層プリント基板を概念的に示す断面図である。
【図２】従来の多層プリント基板の一例を概念的に示す断面図である。
【図３】本実施形態のＥＭＩ低減効果の試験結果の一例を示す図である。
【符号の説明】
Ｇ１，Ｇ２ グランド層
Ｇ３ シールド
Ｉ１〜Ｉ７ 絶縁層
Ｐ１，Ｐ２ 電源層
Ｓ１〜Ｓ４ 信号層[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a multilayer printed circuit board for mounting electronic circuit components at a high density, and more particularly to a technique for reducing EMI (unwanted electromagnetic radiation).
[0002]
[Prior art]
In order to mount electronic circuit components such as an LSI (large-scale integrated circuit) at a high density, a multilayer printed circuit board in which a plurality (generally, four or more layers) of wiring patterns for passing power and signals are used.
[0003]
FIG. 2 is a sectional view conceptually showing an example of a conventional multilayer printed circuit board. This multilayer printed board has, for example, eight layers of wiring forming a signal layer S1, a ground layer G1, a power layer P1, a signal layer S2, a signal layer S3, a ground layer G2, a power layer P2, and a signal layer S4 in this order from the surface. Each of the patterns has a structure in which the patterns are stacked via an insulating layer. Also, unlike this, an eight-layer structure of a signal layer S1, a ground layer G1, a signal layer S2, a power layer P1, a ground layer G2, a signal layer S3, a power layer P2, and a signal layer S4 is arranged in this order from the surface. Multilayer printed circuit boards are also widely used.
[0004]
As described above, in the multilayer printed circuit board, the power supply layer and the ground layer are provided separately from the signal layer in order to supply a stable power supply to the LSI and the like and increase the shielding effect between the wiring layers. These power supply layers and ground layers are so-called solid wirings so as to cover almost the entire surface of the substrate. This enables stable power supply to the mounted electronic circuit components and reliable operation by these electronic circuit components.
[0005]
[Problems to be solved by the invention]
However, the conventional multilayer printed circuit board has the following problems.
That is, the power supply system is composed of a pair of a power supply layer and a ground layer, the power supply layer and the ground layer have a parallel plate structure, and the ends thereof are open. Therefore, the internal electromagnetic field is as shown in the following equation (1).
(Equation 1)

[0006]
Here, n is an outward unit normal vector at the open boundary.
In this case, if the outer shape of the multi-layer printed circuit board is larger than a dimension about several times smaller than the wavelength of the target signal, the influence of the outer shape cannot be ignored, and the multi-layer printed circuit board is not parallel at the frequency fmn of the following equation (2). Plate resonance occurs.
(Equation 2)

[0007]
Here, m and n are 0 or positive integers (however, m = n = 0 is excluded), a and b are the vertical and horizontal outer dimensions of the printed circuit board, and C0 is the speed (light speed) of electromagnetic waves in vacuum. , And ε _r are the dielectric constant of the insulating layer between the power supply layer and the ground layer.
[0008]
When resonance occurs, a large electromagnetic field is generated inside, and the electromagnetic field leaks from the open end and is radiated to the outside. On the electromagnetic field, the open end behaves like a magnetic wall, and a magnetic current Im represented by the following expression (3) flows along the end face, and radiation using the magnetic current Im as a radiation source is generated.
Im = Edge × t (3)
[0009]
Here, Eedge is the electric field at the end of the printed circuit board, and t is the thickness between the power supply layer and the ground layer.
[0010]
As described above, the power supply-ground layer is excited by the influence of the spread of the return current of the signal wiring, the coupling from the via wiring for interlayer connection, etc., and cavity resonance occurs, and the end portion of the printed circuit board which has become an open part. However, it behaves like a slot antenna, and large EMI is generated by electromagnetic waves of a frequency generated by resonance. For this reason, large-scale measures such as connecting a bypass capacitor to the end of the printed circuit board, performing AC termination by a combination of a capacitor and a resistor, and storing the printed circuit board in a shield case have been required.
[0011]
The present invention provides a multilayer printed circuit board that can reduce EMI with a simple structure.
[0012]
[Means for Solving the Problems]
In order to solve the above problem, the invention according to claim 1 is directed to a multilayer printed circuit board in which a signal layer, a power supply layer, and first and second ground layers are laminated via an insulating layer. A second ground layer is disposed on the first or second layer from the front surface and the rear surface, respectively, and the second ground layer is formed so as to surround the signal layer and the power supply layer sandwiched between the first and second ground layers. The end portions of the first and second ground layers are connected by a conductive shield material.
[0013]
According to a second aspect of the present invention, the first and second ground layers in the multilayer printed board of the first aspect are formed of copper foil covering substantially the entire surface of the substrate, and the shield material is formed of copper plating. .
[0014]
According to the present invention, since the multilayer printed circuit board is configured as described above, the following operation is performed.
[0015]
For example, it is assumed that the potential of the power supply layer fluctuates due to the current flowing in the signal layer, and that the potential fluctuation causes cavity resonance between the power supply layer and the ground layer. Since the power supply layer is surrounded by the first and second ground layers and a shielding material connecting the ends of these ground layers, electromagnetic waves radiated from the multilayer printed board to the outside are reduced.
[0016]
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1 is a sectional view conceptually showing a multilayer printed circuit board according to an embodiment of the present invention.
In this multilayer printed circuit board, in order from the surface, an eight-layer wiring pattern forming a signal layer S1, a ground layer G1, a power layer P1, a signal layer S2, a signal layer S3, a power layer P2, a ground layer G2, and a signal layer S4 Are sequentially laminated via insulating layers I1, I2,..., I7. That is, the signal layers S1 and S4 on the front and rear surfaces and the ground layers G1 and G2 inside the signal layers S1 and S4 are insulated by the insulating layers I1 and I7, respectively. The ground layers G1 and G2 and the power supply layers P1 and P2 inside the ground layers G1 and G2 are insulated by insulating layers I2 and I6, respectively. Further, an inner layer pattern including the signal layer S2, the insulating layer I4, and the signal layer S3 is formed between the power supply layers P1 and P2 via the insulating layers I3 and I5.
[0017]
Note that a wiring pattern including electrodes and the like for mounting an electronic circuit component such as an LSI is formed on the signal layer S1 on the surface. Although not shown, the wiring patterns of each layer are connected by via posts or the like provided through the insulating layer.
[0018]
On the other hand, around the edge of the multilayer printed circuit board, the ground layer G1 of the second layer and the ground layer G2 of the seventh layer are almost completely connected by a shield G3 made of copper plating. The layers P1 and P2, the signal layers S2 and S3, and the insulating layers I2 to I6 are wrapped.
[0019]
In addition, such a multilayer printed board can be manufactured, for example, as follows.
[0020]
First, an eight-layer multilayer printed circuit board in which the sizes of the ground layers G1 and G2 are slightly larger than those of the other power supply layers P1 and P2 and the signal layers S1 to S4 is manufactured by using the same manufacturing process as before. To manufacture. Next, an elongated through hole is provided at a position where the shield G3 is to be formed in the peripheral portion of the multilayer printed board. At this time, connection regions are left at, for example, four corners so that the inner region including the signal layers S1 to S4 and the outer peripheral portion are not separated.
[0021]
Further, after filling through holes provided in the peripheral portion of the multilayer printed board with copper plating, the peripheral portion outside the through hole is cut and removed. Thus, the ground layers G1 and G2 are almost completely connected by the shield G3 made of copper plating, and the power supply layers P1 and P2, the signal layers S2 and S3, and the insulating layers I2 to I6 are wrapped inside the multilayer print. The substrate is completed.
[0022]
Next, the operation of electromagnetic radiation in the multilayer printed circuit board will be described.
In this multilayer printed circuit board, the parallel plate structure of the power supply and the ground layer is made up of the ground layer G1, the power layer P1, the signal layers S2 and S3, the power layer P2, and the ground layer G2, and the two ground layers G1 and G2 are formed. The ends are connected. As a result, a structure in which the electromagnetic field does not leak from the end is suppressed, and radiation to the outside is suppressed. Further, since the ends of the ground layers G1 and G2 are closed, the area of the ground layers is increased. Therefore, the return path of the current flowing through the signal layers S1 to S4 is secured, and no problem occurs in signal transmission.
[0023]
3A to 3C are diagrams illustrating an example of test results of the EMI reduction effect of the present embodiment.
[0024]
FIG. 3A is a test circuit using a conventional multilayer printed circuit board that is not subjected to EMI measures as in FIG. 2, and FIG. 3B is an embodiment of the present invention in which EMI measures are applied as in FIG. This is a test circuit using a multilayer printed circuit board.
[0025]
In each case, the outer dimensions of the substrate are 230 × 150 mm and have an eight-layer structure. The signal wiring having a total length of 210 mm is divided into signal layers S1 and S4, and the connection between these signal layers S1 and S4 is made via via posts. Note that the characteristic impedance of this signal wiring is 50Ω, a test signal is applied between one end and the ground layer G1 via a signal input SMA connector, and the other end is connected via two terminating 5 pF capacitors. Are connected to the ground layer G1 and the power supply layer P1, respectively.
[0026]
FIG. 3C shows the measurement results obtained by measuring the electromagnetic field intensity radiated from the test circuit in an anechoic chamber 3 m. The horizontal axis represents the EMI frequency [MHz], and the vertical axis represents the radiated electric field intensity [dBμV / m]. Is shown. In this test, a sine wave having a level of 0 dBm is swept between 30 and 1000 MHz as a test signal and applied to each of the conventional multilayer printed circuit board and the multilayer printed circuit board subjected to the EMI countermeasure of the present embodiment. The radiation field strength is measured. As shown in FIG. 3C, in the multilayer printed circuit board in which the EMI countermeasures of the present embodiment are applied, the maximum radiated electric field intensity is reduced by 13.4 dB, and the frequency characteristic is reduced until the peak almost disappears. It was confirmed that.
[0027]
As described above, the multilayer printed circuit board according to the present embodiment is configured such that the ends of the two ground layers G1 and G2 are connected almost entirely, and the ends of the power supply layers P1 and P2 are not exposed to the outside. . Thus, the electromagnetic field generated between the power supply and the ground layer can be confined, and the radiation to the outside can be suppressed. Further, since the ground layer does not have an open end, the function as a ground is strengthened, and radiation other than the end is suppressed. As a result, there is an advantage that EMI can be reduced with a simple structure without using a large-scale measure such as using a bypass capacitor or a noise countermeasure component or storing the component in a shield case.
[0028]
Note that the present invention is not limited to the above embodiment, and various modifications are possible. For example, there are the following modifications.
[0029]
(A) Although the description has been given by taking the multilayer printed circuit board having eight layers as an example, the number of layers is arbitrary.
[0030]
(B) Although the ground layers G1 and G2 are provided in the second layer from the front surface and the rear surface, the ground layers may be provided in the first layer. Further, ground layers may be provided on the first layer on one surface and the second layer on the other surface, respectively.
[0031]
(C) The production method is not limited to the exemplified method. For example, when a six-layer substrate sandwiched between the ground layers G1 and G2 is formed, these ground layers G1 and G2 are connected by copper plating. Further, a copper foil to be the signal layer S1 is attached to the surface of the ground layer G1 via the insulating layer I1 made of prepreg, and the copper foil is etched to form the signal layer S1. Similarly, a method of attaching a copper foil to the surface of the ground layer G2 via the insulating layer I7 and etching the copper foil to form the signal layer S4 is also possible.
[0032]
【The invention's effect】
As described in detail above, according to the first aspect of the present invention, the first and second ground layers are arranged on the first or second layer from the front surface and the back surface, respectively. The end of the second ground layer is connected with a conductive shield material. Thus, the internal signal layer and power supply layer are wrapped by the first and second ground layers, and the EMI radiation to the outside can be reduced with a simple structure.
[0033]
According to the second aspect of the present invention, the first and second ground layers are formed of copper foil covering substantially the entire surface of the substrate, and the shield material is formed of copper plating. As a result, EMI radiation can be almost certainly reduced.
[Brief description of the drawings]
FIG. 1 is a sectional view conceptually showing a multilayer printed circuit board according to an embodiment of the present invention.
FIG. 2 is a sectional view conceptually showing an example of a conventional multilayer printed circuit board.
FIG. 3 is a diagram illustrating an example of a test result of an EMI reduction effect of the present embodiment.
[Explanation of symbols]
G1, G2 Ground layer G3 Shields I1 to I7 Insulating layers P1, P2 Power supply layers S1 to S4 Signal layer

Claims (2)

In a multilayer printed circuit board in which a signal layer, a power supply layer, and a first and second ground layer are laminated via an insulating layer, the first and second ground layers are first layers from the front surface and the rear surface, respectively. Alternatively, the first and second ground layers may be disposed on the second layer, and the ends of the first and second ground layers may be electrically shielded so as to surround the signal layer and the power supply layer sandwiched between the first and second ground layers. A multilayer printed circuit board characterized by being connected by a material. 2. The multilayer printed circuit board according to claim 1, wherein the first and second ground layers are formed of copper foil covering substantially the entire surface of the substrate, and the shield material is formed of copper plating.

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