JP2010212439A - Circuit board - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a circuit board capable of controlling the characteristic impedance of signal wiring and exhibiting effects thereof for countermeasures against EMI. <P>SOLUTION: The circuit board 1 has a ground layer 5 and signal wiring 3a, 3b arranged on the ground layer via an insulator layer 2 wherein a shield layer 7 formed of a conductive raw material is formed on an insulating covering layer 4 for covering the signal wiring on a surface reverse to the ground layer. The insulating covering layer opposing the signal wiring requiring the control of characteristic impedance is set as an opening 7a of the shield layer where the shield layer is not layered. When the signal wiring is paired wiring for differential transmission, a distance U between both sides of the pair wiring at the position of an inter-linear distance S and an opening end 7b of the shield layer is set at a range 3S≤U≤20S, where S is the inter-linear distance of the paired wiring. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a ground layer and a circuit board in which signal wiring is disposed with respect to the ground layer through an insulator layer.

For example, a circuit board on which a device operating in a high frequency band is mounted has a characteristic impedance (hereinafter also referred to as Zo) of a signal transmission path (hereinafter also referred to as signal wiring) in order to suppress signal reflection and waveform distortion. It is necessary to match the input / output impedance of the device.
In order to match the characteristic impedance of the signal transmission path described above, a microstrip structure is adopted in which a ground layer is opposed to a signal transmission path (strip line) having an appropriate pattern width with an insulating layer having an appropriate thickness interposed therebetween. Yes.

The ground layer in the circuit board structure described above serves as an electrical reference plane that defines the characteristic impedance of the signal transmission path. In general, the characteristic impedance is often selected to be around 50Ω for single end and around 100Ω for differential transmission.
Note that Zo in the circuit board described above is a ratio of reactance L per unit length of the signal transmission path and capacitance C per unit area between the signal transmission path and the ground layer (reactance L / electrostatic capacity). It is a value approximated by the square root of the capacitance C).

By the way, in recent years, thin rigid boards and flexible circuit boards are frequently used as circuit boards for mounting the above-described devices. When such circuit boards are adopted, of course, signal transmission paths for the ground layer are naturally used. The interlayer is narrow (thin), and the value of the capacitance C between them increases almost in inverse proportion to the dimension between the layers.
Therefore, in the above-described thin multilayer circuit board, in order to obtain the desired Zo, the width of the signal transmission path is narrower (thinner) than that of a conventional circuit board with a thick interlayer, A means for suppressing the increase in the capacitance C must be employed.

  Thus, in order to obtain a desired Zo, when the signal wiring is to be formed thin, there is a case where the signal wiring has to be thinned so that the processing of the signal wiring is difficult. Even if signal wiring can be processed, the thinner the signal wiring, the higher the ratio of circuit processing accuracy and line width variation, and the Zo variation increases accordingly.

  For this reason, there arises a problem that signal reflection and waveform distortion occur in the signal wiring portion where the change of Zo is large. Furthermore, since the wiring resistance value of the signal wiring is increased, there is a problem that the higher the signal frequency supplied to the signal wiring, the worse the transmission characteristics.

  Therefore, in order to solve the above-described technical problem, the ground layer formed as a so-called solid electrode layer is removed in a mesh shape so that the opposing area between the ground layer and the signal wiring per unit area is substantially reduced. Proposals have been made to ensure the width of the signal wiring by making it smaller. This is disclosed in the following prior art documents.

JP 7-32463 A

  By the way, in the circuit board of the microstrip structure disclosed in the above-mentioned prior art document, although the above-mentioned ground layer side works effectively as an EMI (electromagnetic wave interference) countermeasure, on the opposite side of the signal wiring side, The effect cannot be expected. Therefore, it is conceivable that the outer surface layer (insulating coating layer) of the signal wiring that requires EMI countermeasures is covered with a conductive shield layer made of, for example, silver paste to substantially form a strip structure.

  However, when the shield layer is disposed on the surface layer side of the signal wiring, a capacitive coupling newly occurs between the signal wiring and the shield layer on the coverlay. That is, the capacitance between the signal wiring and the ground layer is added to the capacitance between the signal wiring and the shield layer, making it difficult to properly control Zo of the signal transmission path.

  The present invention has been made by paying attention to the technical problems described above, and can sufficiently control the characteristic impedance of the signal transmission path, and also exhibits the effect in the above-mentioned EMI countermeasures. It is an object of the present invention to provide a circuit board that can be used.

  The circuit board according to the present invention, which has been made to solve the above-described problems, includes a ground layer and a signal wiring disposed through an insulator layer with respect to the ground layer, and capacitive coupling between the ground layer and the signal wiring. By controlling the characteristic impedance of the signal wiring, an insulating coating layer is formed on the signal wiring disposed on the insulator layer, Further, a shield layer made of a conductive material is formed, and an opening of the shield layer where the shield layer is not laid is formed on the insulating coating layer facing the signal wiring.

In this case, when the signal wiring in which the characteristic impedance is controlled is differentially transmitted, and the distance between the pair wirings is S, both outer sides of the pair wiring and the shield layer It is desirable that the distance U to the open end of the lens is set in a range of 3S ≦ U ≦ 20S.
This effect can be remarkably exhibited in a thin circuit board in which the insulating coating layer between the signal wiring and the shield layer has a thickness of 100 μm or less.

  In addition, a linear guard pattern having the same potential as that of the ground layer is disposed on the insulator layer along both outer sides of the pair wiring that is differentially transmitted. A circuit board configured such that the central portion in the width direction is positioned at the opening end of the shield layer can also be suitably employed.

  In this case, the ground layer and the guard pattern are preferably connected via a via hole, and the guard pattern and the shield layer are connected via an opening formed in the insulating coating layer.

  According to the circuit board described above, the insulating coating layer facing the signal wiring that needs to be controlled by Zo is formed in the opening portion of the shield layer where the shield layer is not laid. The degree of capacitance coupling between the two can be greatly reduced. Thereby, appropriate control of the characteristic impedance of the signal wiring can be achieved.

  On the other hand, the signal wiring is provided with a circuit board capable of exhibiting a sufficient effect for the above-mentioned EMI countermeasures although the shield effect is slightly lowered by forming the opening of the shield layer. can do.

BRIEF DESCRIPTION OF THE DRAWINGS It is the laminated structure figure which showed 1st Embodiment of the circuit board concerning this invention by sectional drawing. It is the top view which showed the signal wiring part on the insulator layer shown to FIG. 1A. It is the top view which showed a preferable example of the ground layer. It is a diagram which shows the relationship between the distance U of a signal wiring and a shield layer opening, and the distance S between the lines of a signal wiring. It is the laminated structure figure which showed 2nd Embodiment of the circuit board concerning this invention by sectional drawing. It is the top view which showed the signal wiring part on the insulator layer shown to FIG. 4A. It is the laminated structure figure which showed 3rd Embodiment of the circuit board concerning this invention by sectional drawing. It is the top view which showed the signal wiring part on the insulator layer shown to FIG. 5A.

Hereinafter, a circuit board according to the present invention will be described based on the embodiments shown in the drawings. FIG. 1A is a cross-sectional view showing the laminated structure of the first embodiment, and FIG. 1B is a plan view showing signal wiring portions on the insulator layer shown in FIG. 1A at substantially the same scale. .
The configuration shown in FIGS. 1A and 1B shows an embodiment in which the signal wiring that requires control of the characteristic impedance is a pair wiring that is differentially transmitted.

  In the circuit board 1 shown in FIGS. 1A and 1B, a signal wiring is arranged via a ground layer and an insulator layer with respect to the ground layer, and the signal wiring (in FIG. A microstrip structure is formed for the part (pair wiring indicated by symbol A), and a shield layer is added to the part of the signal wiring (pair wiring indicated by symbol B in FIG. 1B) that does not require control of characteristic impedance. is doing.

In this embodiment, a film-like base substrate is employed as the insulator layer 2, and signal wiring that is differentially transmitted is provided on one surface (the upper side in FIG. 1A) of the base substrate 2. 3a and 3b are formed, and an insulating coating layer 4 is laminated on the upper surface (the upper side in FIG. 1) of the signal wirings 3a and 3b via an adhesive layer (not shown).
On the other hand, a ground layer 5 is formed on the other surface (lower side in FIG. 1) of the base substrate 2, and a second insulating coating layer is formed on the lower surface of the ground layer 6 via an adhesive layer (not shown). 6 is formed.

  A shield layer 7 made of a conductive material is formed on the insulating coating layer 4 that covers the signal wiring on the opposite side of the ground layer 5 through the base substrate 2, and the characteristic impedance needs to be controlled. On the insulating coating layer 7 facing the signal wirings 3a and 3b (a pair wiring indicated by a symbol A in FIG. 1B), an opening 7a of the shield layer where the shield layer is not laid is formed.

  The base substrate 2 that functions as a central insulator layer in which the signal wiring is disposed has a function as a core of the circuit board 1. The material of the base substrate 2 includes a resin film, a fiber substrate, and the like. Materials etc. can be mentioned.

Examples of the material constituting the resin film include polyimide resins such as polyimide resins, polyamide resins, and polyamideimide resins, thermosetting resins such as epoxy resins, and thermoplastic resins such as liquid crystal polymers.
Among these, a polyimide resin or a liquid crystal polymer is preferable. For example, in the case of polyimide resin, it is excellent in heat resistance and mechanical properties and is easy to obtain. In the case of a liquid crystal polymer, it is suitable for high-speed signal transmission due to its low relative dielectric constant, and it has excellent dimensional stability due to its low hygroscopicity.

  Examples of the fiber base material used for the insulating layer include glass fiber base materials such as glass fiber cloth and glass non-fiber cloth, or inorganic fiber base materials such as fiber cloth and non-fiber cloth containing inorganic compounds other than glass, aromatic And organic fiber base materials composed of organic fibers such as aromatic polyamide resins, polyamide resins, aromatic polyester resins, polyester resins, polyimide resins, and fluororesins. Among these base materials, glass fiber base materials represented by glass fiber fabric are preferable in terms of strength and water absorption.

  When a fiber base material is used for the insulator layer, the fiber base material is preferably used in a state of being impregnated with a resin. As the resin impregnated in the fiber base material, a thermosetting resin such as an epoxy resin or an acrylic resin is preferably used, and among these, an epoxy resin is preferable from the viewpoint of heat resistance.

The thickness t1 of the base substrate 2 is preferably in the range of 1 to 100 μm, more preferably in the range of 5 to 50 μm, and more preferably in the range of 10 to 30 μm.
By setting the thickness of the base substrate 2 to be equal to or greater than the lower limit value, it becomes easy to make the signal line width equal to or greater than the processing limit. On the other hand, by setting the thickness to be equal to or less than the upper limit value, rigidity is high. It is possible to suppress the occurrence of excessive thickness and retain the characteristics of a thin substrate such as a flexible circuit board that is flexible.

  The signal wirings 3a and 3b arranged on one surface of the base substrate 2 may be directly provided on the base substrate 2, or may be provided via an adhesive. Then, at the end portion of each signal wiring or an appropriate intermediate portion, it is bonded to a mounting pad such as a semiconductor device (not shown) and functions as the circuit board 1.

  It is preferable that the insulating coating layer 4 covering the signal wirings 3a and 3b through an adhesive layer (not shown) is made of a resin material. Examples of the resin material include polyester resins, polyimides, and liquid crystal polymers. Among these, polyimide is preferable. Thereby, heat resistance and flexibility can be improved.

The thickness t2 of the insulating coating layer 4 is preferably 1 to 100 μm, more preferably 5 to 50 μm, and more preferably 10 to 30 μm.
By setting the thickness t2 of the insulating coating layer 4 to be equal to or greater than the lower limit value, it becomes easy to maintain the strength of the resin layer within a practical range, and by setting the thickness t2 or less to the upper limit value, the slidability and the flexibility are maximized. It becomes easy to make it exhibit.

  In addition, as a material constituting an adhesive layer (not shown) interposed between the base substrate 2 and the insulating coating layer 4, for example, an acrylic resin, an epoxy resin, a polyimide resin, or the like may be used. it can. Among these, an epoxy resin is preferable, and thereby heat resistance and flexibility can be improved.

  The conductive portion as the ground layer 5 disposed on the other surface (back surface) of the base substrate 2 is made of a conductor made of a copper material. In this conductor, a large number of holes formed in a parallelogram opening are formed in the portion facing the signal wiring (pair wiring indicated by symbol A in FIG. 1B) that requires impedance control. A specific configuration of the ground layer 5 will be described later in detail with reference to FIG.

The second insulating coating layer 6 laminated on the lower side surface (lower side in FIG. 1) of the ground layer 6 with an adhesive layer (not shown) is preferably made of a resin material. Examples of the resin material include polyester resin, polyimide, liquid crystal polymer, and the like.
Among these, polyimide is preferable. Thereby, heat resistance and flexibility can be improved.

  The thickness of the second insulating coating layer 6 is not particularly limited, but is preferably 5 to 50 μm, and particularly preferably 10 to 30 μm. This makes it easy to maintain the strength of the resin layer within a practical range by setting the thickness of the insulating coating layer 6 to be equal to or greater than the lower limit, and maximizing slidability and flexibility by reducing the thickness to the upper limit or less. It is easy to make it to the limit.

  The material constituting the adhesive layer (not shown) interposed between the ground layer 5 and the second insulating coating layer 6 is interposed between the base substrate 2 and the insulating coating layer 4. For example, an acrylic resin, an epoxy resin, a polyimide resin, or the like can be used in the same manner as the material constituting the adhesive layer (not shown). Among these, an epoxy resin is preferable, and thereby heat resistance and flexibility can be improved.

  The shield layer 7 made of a conductive material formed on the insulating coating layer 4 is formed by printing with a conductive paste (silver paste) or by sticking a conductive shield film, and the characteristic impedance can be controlled. The necessary portions facing the signal wirings 3a and 3b are formed in the opening 7a of the shield layer where the shield layer 7 is not laid as described above.

  FIG. 2 is a partially enlarged view of one preferred configuration of the ground layer 5 described above. FIG. 2 shows a state in which the signal wirings 3a and 3b that require the control of the characteristic impedance described above are superimposed on the ground layer 5 as seen through from a direction perpendicular to the surface. As described above, the ground layer 5 is made of a conductor made of a copper material, and the position of the conductor facing the signal wirings 3a and 3b constitutes a mesh-like conductor portion 5B in which a large number of holes are formed. ing.

  That is, the hole in this embodiment is formed as a parallelogram (diamond) opening by intersecting a plurality of lines in two directions. The plurality of lines in the two directions are preferably inclined with respect to the signal wirings 3a and 3b within a range of 5 to 40 degrees. In addition, the longer diagonal line of the rhomboid opening has a mesh pattern that matches the wiring direction of the signal wirings 3a and 3b.

  The openings described above may have different sizes (opening areas), but preferably have the same opening area. Thereby, the characteristic impedance of the signal wirings 3a and 3b can be controlled with high accuracy.

On the other hand, on the outside of the mesh-like conductor portion 5B, a solid electrode region 5A made of a copper material in which no hole is formed is formed. Signal wiring that does not particularly require characteristic impedance control is arranged facing this solid electrode region 5A.
According to the configuration shown in FIG. 2, the mesh-like conductor portion 5B is configured only in the portion where the characteristic impedance needs to be controlled, and the rest is the solid electrode region 5A, thereby avoiding an increase in the overall impedance of the ground layer 6. be able to.

  In this case, the boundary between the mesh-shaped conductor portion 5B and the solid electrode region 5A is configured to substantially coincide with an opening end 7b in the opening portion 7a of the shield layer, which will be described later, in a direction orthogonal to the substrate surface. It is desirable that

  In order to control the characteristic impedance of the signal wiring, it is effective to provide the mesh conductor portion 5B in the ground layer as described above. However, the characteristic impedance of the signal wiring depends on the line width and the thickness of the insulating layer. Furthermore, it is possible to control the dielectric layer by selecting the dielectric constant of the insulating layer. In the present invention, it is not essential to provide the mesh-like conductor portion 5B.

  In this embodiment, the thickness t1 of the base substrate 2 and the thickness t2 of the insulating coating layer 4 are both 100 μm or less, and the entire laminated structure is applied to a very thin circuit board. Therefore, the distance U between the signal wirings 3a and 3b that need to control the characteristic impedance and the opening end 7b of the opening 7a formed in the shield layer dominates the degree of capacitive coupling between them. Become.

  In this case, when the signal wiring for which the characteristic impedance is controlled is a pair wiring that is differentially transmitted, the distance between the pair wirings 3a and 3b is S as shown in FIG. 1A. At this time, the distance U between the outer sides of the pair wiring and the opening end 7b of the shield layer at the position of the line distance S is set in the range of 3S ≦ U ≦ 20S, more preferably 3S ≦ U ≦ 10S. It is desirable.

FIG. 3 shows a characteristic example in which the ratio of the distance U to the line-to-line distance S, that is, the value of U / S is shown on the horizontal axis, and the characteristic impedance Zo of the signal wirings 3a and 3b is shown on the vertical axis. is there.
In the characteristic example shown in FIG. 3, the line width L of the differential signal lines 3a and 3b / the distance S between the lines S = 100 μm / 100 μm, the interlayer thickness t1 of the insulator layer 2 = 25 μm, the copper of the mesh-like conductor portion 5B. (Conductor) Residual rate = 30%.

  When the distance U between the signal wirings 3a and 3b and the opening end 7b of the shield layer 7 is below a certain level, the capacitive coupling between the signal wiring and the shield layer is increased, and the characteristic impedance of the signal wiring is reduced. It becomes difficult to control. That is, as shown in the characteristic diagram of FIG. 3, when the value of U is less than 3S, the capacitive coupling increases rapidly, and this must be avoided.

  On the contrary, when the value of U exceeds 3S, the capacitive coupling of the shield layer 7 to the signal wirings 3a and 3b that need to control the characteristic impedance decreases rapidly, and the signal wiring of the mesh-like conductor portion 5B is reduced. Impedance control accuracy can be improved.

  The same measurement was attempted under the condition that the thickness t2 of the insulating coating layer 4 was 100 μm or less. However, when the value of U was less than 3S, the capacitive coupling of the shield layer 7 to the signal wiring was abrupt. It has been verified that the characteristic impedance of the signal wiring increases and the characteristic impedance of the signal wiring decreases, and it is recognized that the results are almost the same as the characteristics shown in FIG.

On the other hand, if the value of U is set to a large value, the degree of influence on the setting of the characteristic impedance of the signal wiring is reduced, but on the other hand, the shielding effect is reduced and cannot be overlooked in the EMI countermeasure.
Therefore, in order to ensure the shielding effect for the signal wiring, the value of U is desirably set to 20S or less, preferably 10S or less.

  Therefore, by setting the relationship between the line S between the pair of signal wires and the distance U between the outer sides of the pair wires and the opening end 7b of the shield layer within the above range, the characteristic impedance of the signal wires can be reduced. It is possible to provide a circuit board capable of achieving both adjustment and EMI countermeasures.

  When the signal wiring is a pair wiring that is differentially transmitted, the distance S between the both outer sides of the pair wiring and the opening end 7b of the shield layer is set as a parameter using the line spacing S of the pair wiring as a parameter. It is reasonable to set and manage.

  The reason is that the distance between the pair wirings affects the characteristic impedance of the signal wiring next to the shield layer. Therefore, the distance from the signal wiring to the opening end of the shield layer with respect to the distance between the lines is reduced to the characteristic impedance. This is because the magnitude of the impact is determined. This can be considered that a virtual GND plane exists at the center of the distance between the lines of the pair wiring, and when the area of the side surface of the signal line is also taken into consideration, the distance of the signal wiring is next to the shield layer. Since it is easily determined that the characteristic impedance is affected, the characteristic impedance of the signal line is determined by the coupling between the wirings, and the shield layer beyond the opening end is separated so as not to affect the characteristic impedance. Can be described in relation to the distance between the signal lines.

FIG. 4A shows a laminated structure of a circuit board according to the second embodiment of the present invention, and FIG. 4B is a plan view showing signal wiring portions on the insulator layer shown in FIG. 3A at almost the same scale. is there.
The configuration shown in FIGS. 4A and 4B shows an example in which the signal wiring that requires control of the characteristic impedance is a pair wiring that is differentially transmitted, as in the example shown in FIGS. 1A and 1B. .

  In FIGS. 4A and 4B, parts that perform the same functions as the parts shown in FIGS. 1A and 1B already described are denoted by the same reference numerals, and therefore detailed description thereof is omitted. The distance U between the outer sides of the pair wiring and the opening end 7b of the shield layer has the above-described relationship using the line spacing S of the pair wiring as a parameter.

  In this embodiment, a linear guard pattern 8 having the same potential as the ground layer 5 is formed along the outer sides of the signal wirings 3a and 3b whose characteristic impedance is controlled. It is arranged on the top. And the center part of the width direction of the linear guard pattern 8 is comprised so that it may be located in the opening end 7b of the said shield layer 7. FIG.

  According to the configuration described above, the upper surfaces of the signal wirings 3a and 3b that require control of the characteristic impedance are opened by the opening 7a of the shield layer, but the signal wiring 3a viewed in a cross section as shown in FIG. 4A. , 3b, and the upper and lower sides are surrounded by a sufficiently close ground, so that the attenuation of the EMI shield characteristic can be minimized.

Further, FIG. 5A shows a laminated structure of a circuit board according to the third embodiment of the present invention, and FIG. 5B is a plan view showing the signal wiring portion on the insulator layer shown in FIG. 5A at almost the same scale. It is.
The configuration shown in FIGS. 5A and 5B is obtained by further adding the configuration requirements to the above-described second embodiment shown in FIGS. 4A and 4B. The ground layer 5 and the guard pattern 8 include: The guard pattern 8 and the shield layer 7 are connected through an opening 4 a formed in the insulating coating layer 4. The guard pattern 8 and the shield layer 7 are connected through a via hole 2 a formed in the insulating base 2.

As shown in FIG. 5B, via holes 2a connecting the ground layer 5 and the guard pattern 8 are formed at a plurality of locations along the longitudinal direction of the guard pattern 8, and both are connected via the via holes 2a. Has been.
The openings 4a formed in the insulating coating layer 4 are also formed at a plurality of locations along the longitudinal direction of the guard pattern 8, and the guard pattern 8 and the shield layer 7 are formed through the openings 4a. And are connected.

  The above-described configuration can be suitably employed in a portion where the circuit board is not bent or the like. According to this configuration, a gap between the grounds on both outer sides of the signal wiring is filled, and the ground layer 5 functions as a ground potential. The mechanical coupling between the guard pattern 8 and the shield layer 7 is strengthened, and it can contribute to further improving the EMI shield characteristics.

  Note that the ground layer in the above-described embodiment may be configured to be applied with a reference potential of the circuit, or the operation power supply of each device may be superimposed. Therefore, the potential applied to the ground layer is not particularly limited.

  The circuit board according to the present invention can be used for a printed wiring board, a flexible printed wiring board, a multilayer flexible printed wiring board, and the like, and can be suitably used particularly for a circuit board on which a device operating in a high frequency band is mounted.

1 Circuit board 2 Base substrate (insulator layer)
2a Via hole 3a, 3b Signal wiring 4 Insulation coating layer 4a Insulation coating layer opening 5 Ground layer 5A Solid electrode region 5B Mesh-like conductor 6 Insulation coating layer 7 Shield layer 7a Shield layer opening 7b Open end 8 Guard pattern

Claims (5)

A circuit board for controlling characteristic impedance of the signal wiring by arranging a signal wiring via an insulator layer with respect to the ground layer and the ground layer, and controlling capacitive coupling between the ground layer and the signal wiring. Because
An insulating coating layer is formed on the signal wiring disposed in the insulator layer, and a shield layer made of a conductive material is further formed through the insulating coating layer, and the insulating coating facing the signal wiring is formed. A circuit board characterized in that the layer is formed in an opening of a shield layer on which the shield layer is not laid.   The signal wiring for which characteristic impedance is controlled is a pair wiring that is differentially transmitted, and when the distance between the pair wirings is S, both outer sides of the pair wiring and the open end of the shield layer The circuit board according to claim 1, wherein the distance U is set in a range of 3S ≦ U ≦ 20S.   The circuit board according to claim 2, wherein the insulating coating layer between the signal wiring and the shield layer has a thickness of 100 μm or less.   A linear guard pattern having the same potential as that of the ground layer is disposed on the insulator layer along both outer sides of the pair wiring that is differentially transmitted, and the width direction of the linear guard pattern is 4. The circuit board according to claim 1, wherein a central portion is configured to be positioned at an opening end of the shield layer. 5.   5. The ground layer and the guard pattern are connected through a via hole, and the guard pattern and the shield layer are connected through an opening formed in the insulating coating layer. Circuit board.

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