Circuit board and structure using the same

According to one embodiment of the invention, a circuit board comprises a conductive layer including a land portion and a line portion connected to the land portion, and; a conductor connected to a surface of the land portion. A planar shape of the connected portion between the conductor and the land portion has a elongated shape along a width direction of the line portion. A part of the connected portion is located within an imaginary region formed by imaginarily extending the line portion toward the land portion.

CROSS REFERENCE TO RELATED APPLICATION(S)

The present application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2009-42841, filed on Feb. 25, 2009, entitled “CIRCUIT BOARD AND MOUNTING STRUCTURE USING THE SAME”, the entirety of which is incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a circuit board and a structure used for electronic devices (e.g., audiovisual devices, electrical appliances, communication devices, computer devices, and the peripheral devices thereof).

2. Description of the Related Art

A structure obtained by mounting or embedding an electronic component on or in a circuit board has been used in electronic devices.

As shown inFIG. 9, a circuit board9includes a plurality of conductive layers91apart from each other in the thickness direction, insulating layers90interposed therebetween, and a plurality of via conductors92electrically connecting the conductive layers91. The via conductors92are formed in the insulating layers90to be apart from each other in a plan view. In the circuit board9, the conductive layers91and the via conductors92constitute a power supply line and a signal line (refer to Japanese Unexamined Patent Application Publication No. 8-116174).

Focusing on a single conductive layer91(other than a conductive layer91′ that is located in an outer layer), each of the via conductors92is connected to an upper surface93and a lower surface94of the conductive layer91. For example, a current flows in the order of the via conductor92(thickness direction), the conductive layer91(plane direction), and the via conductor92(thickness direction) as indicated by an arrow in the drawing. Since a current flows through a path having the shortest distance in a conductive region, the current is concentrated in a portion95(a portion circled in the drawing) in the connected portion between the via conductor92and the conductive layer91, which causes unbalanced current density. Therefore, in the portion95, metal atoms contained in the via conductor92may migrate due to a collision between electrons and metal atoms. This phenomenon is called electromigration. Electromigration decreases the metal density in the portion95(portion in which a current is concentrated) of the connected portion between the via conductor92and the conductive layer91. Consequently, cracking occurs in the connected portion between the via conductor92and the conductive layer91, which sometimes decreases the reliability of the circuit board9.

Electromigration mainly occurs in a power supply line, but it is highly likely to occur in a signal line too if the size of a signal line becomes smaller in the future.

Focusing on a conductive layer91′ that is located in an outer layer, a single via conductor92′ is connected to the conductive layer91′. Even in this case, electromigration occurs in a portion95′ of the connected portion between the conductive layer91and the via conductor92′, which sometimes decreases the connection reliability of an electronic component96to the circuit board9.

BRIEF SUMMARY OF THE INVENTION

An object of the invention is to provide a circuit board and a structure which improve a connection reliability.

A circuit board according to one of the invention comprises a conductive layer including a land portion and a line portion connected to the land portion, and a conductor connected to a surface of the land portion. A planar shape of the connected portion between the conductor and the land portion has an elongated shape along a width direction of the line portion. A part of the connected portion is in an imaginary region formed by imaginarily extending the line portion toward the land portion.

A structure according to one of the invention comprises the circuit board and an electronic component electrically connected to the conductive layer and the conductor in the circuit board.

The circuit board and the structure can improve a reliability of the connected portion between the conductor and the land portion by dispersing a current flowing through the connected portion between the conductor and the land portion.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

A circuit board and a structure according to an embodiment of the present invention are described in detail with reference to the attached drawings.

A structure1shown inFIG. 1is used for electronic devices such as audiovisual devices, electrical appliances, communication devices, computer devices, and the peripheral devices thereof. The structure1includes an electronic component2and a circuit board3.

The electronic component2is a semiconductor device such as an integrated circuit (IC) or a large-scale integrated circuit (LSI), and is mounted on the circuit board3through conductive bumps4such as solder using a flip chip packaging technology. The electronic component2has a base composed of a semiconductor material such as silicon, germanium, gallium arsenide, gallium arsenide phosphide, gallium nitride, or silicon carbide. The electronic component2, for example, having a thickness of 0.1 mm or more and 1 mm or less may be used.

The circuit board3includes a core substrate5and a pair of circuit layers6on an upper surface and a lower surface of the core substrate5.

The core substrate5electrically connects the pair of circuit layers6to each other while maintains the strength of the circuit board3. The thickness of the core substrate5is 0.3 mm or more and 1.5 mm or less. The core substrate5includes insulating bases50, through-holes51formed in the insulating bases50to penetrate the insulating bases50, through-hole conductors52formed in the through-holes51, and insulating bodies53formed in the through-hole conductors52.

Each of the insulating bases50is a principal part of the core substrate5. The insulating base50can be manufactured, for example, by stacking a plurality of sheets obtained by impregnating a reinforced material with a thermosetting resin and then by curing the thermosetting resin through thermal pressing or the like.

A material composed of, for example, plain-woven glass fiber or resin fiber may be used as the reinforced material. The resin fiber can be formed of a polyparaphenylene benzobisoxazole resin or a wholly aromatic polyamide resin.

Examples of the thermosetting resin include epoxy resins, bismaleimide triazine resins, and cyanate resins.

The insulating base50can also be manufactured from a low thermal expansion resin without using the reinforced material. In this case, the insulating base50may be manufactured from only a low thermal expansion resin or may be manufactured by stacking a plurality of sheets composed of a low thermal expansion resin through adhesive resins.

Examples of the low thermal expansion resin for the insulating base50include polyparaphenylene benzobisoxazole resins, wholly aromatic polyamide resins, wholly aromatic polyester resins, polyimide resins, liquid crystal polymer resins and/or a combination thereof. Among them, a polyparaphenylene benzobisoxazole resin is preferably used. The coefficient of thermal expansion of the polyparaphenylene benzobisoxazole resin is as low as −5 ppm/° C. or more and 5 ppm/° C. or less. With such a low thermal expansion resin, the thermal expansion of the core substrate5itself can be suppressed. The coefficient of thermal expansion is in conformity with ISO 11359-2:1999.

Each of the through-holes51is a portion where each of the through-hole conductors52is formed and penetrates the core substrate5in the thickness direction (Z axis direction). The through-hole51has a cylindrical shape having a diameter of, for example, 0.1 mm or more and 1 mm or less. Such a through-hole51can be formed by well-known drilling.

The through-hole conductor52electrically connects the pair of circuit layers6to each other, and is formed along an inner surface of the through-hole51. The through-hole conductor52is composed of a conductive material such as copper, silver, gold, aluminum, nickel, chromium and/or a combination thereof. The through-hole conductor52can be formed by performing electrolytic plating or the like on the inner surface of the through-hole51.

Each of the insulating bodies53fills the remaining space surrounded by the through-hole conductors52. The insulating body53can be formed by filling the remaining space with a resin material and then by curing the resin material. Examples of the resin material for the insulating body53include polyimide resins, acrylic resins, epoxy resins, cyanate resins, fluorocarbon resins, silicon resins, polyphenylene ether resins, bismaleimide triazine resins and/or a combination thereof. By forming the insulating body53in the core substrate5, the via conductors8described later can be formed immediately on and under the insulating body53. Therefore, the length of a conductive line routed from the through-hole conductor52can be shortened, which can achieve the miniaturization of the circuit board3.

The pair of circuit layers6are stacked on both surfaces of the core substrate5and include a plurality of insulating layers60, a plurality of conductive layers7on the insulating layers60, and a plurality of via conductors8penetrating the insulating layer60and electrically connecting the conductive layers7to each other. The conductive layers7and the via conductors8are electrically connected to each other to constitute a circuit member. The circuit member includes a power supply line and a signal line.

Each of the insulating layers60is configured to ensure the insulation of a portion other than the circuit member and has a through-hole61. The through-hole61is a portion where each of the via conductors8is formed. The through-hole61can be formed by perpendicularly irradiating the insulating layer60with laser beams using, for example, an yttrium-aluminum-garnet (YAG) laser device, a CO2 laser device, or an excimer laser device. The output energy of the laser beams is set to be, for example, 1.0×10-3 J or higher and 5.0×10-1 J or lower. The irradiation time of the laser beams is set to, for example, 1.0×10-3 sec or longer and 1.0 sec or shorter. By such a method, the laser beams are perpendicularly applied from the upper surface of the insulating layer60and the through-hole61whose lower portion is narrower in width than the upper portion can be formed.

After the through-hole61is formed, desmearing may be performed to remove the residues that adhere to the inner surface of the through-hole61. The desmearing can be performed by plasma treatment or etching treatment. The plasma treatment can be performed, for example, by processing the inner surface using a microwave in an argon gas or oxygen gas atmosphere. The etching treatment can be performed by wet etching with an etching solution. A permanganic acid aqueous solution obtained by, for example, adding 20 g or more and 40 g or less of permanganic acid and 35 g or more and 45 g or less of sodium hydroxide to a liter of distilled water can be used as the etching solution. The etching solution is preferably used after warming, that is, at 30° C. or more and 40° C. or less. In that case, the etching time is set to, for example, 2 minutes or longer and 4 minutes or shorter.

The insulating layer60is preferably formed of a material whose coefficient of thermal expansion is close to that of the electronic component2. The thickness of the insulating layer60after drying is, for example, 1 μm or more and 15 μm or less. The insulating layer60is formed of, for example, a thermosetting resin or a thermoplastic resin. The coefficient of thermal expansion of the insulating layer60is set to, for example, 15 ppm/° C. or more and 80 ppm/° C. or less.

The thermoplastic resin for the insulating layer60needs to have thermal resistance so as to endure heat treatment during reflow soldering, in addition to a coefficient of thermal expansion close to that of the electronic component2. Therefore, the thermoplastic resin for the insulating layer60preferably has a softening temperature of 200° C. or higher. Examples of the thermoplastic resin include polyether ketone resins, polyethylene terephthalate resins, polyphenylene ether resins, and/or a combination thereof.

The insulating layer60may include a filler having a plurality of particles. Since the viscosity of the insulating layer60before curing can be adjusted by providing a filler in the insulating layer60, the dimensional accuracy of the thickness of the insulating layer60can be improved. The particles having a spherical shape whose diameter is, for example, 0.05 μm or more and 6 μm or less can be used as the filler. The particles of the filler is composed of a material having a coefficient of thermal expansion of −5 ppm/° C. or more and 5 ppm/° C. or less. Examples of the material include silicon oxide (silica), silicon carbide, aluminum oxide, aluminum nitride, aluminum hydroxide and/or a combination thereof.

The plurality of conductive layers7constitute the circuit member together with the via conductors8. As shown inFIG. 2, the plurality of conductive layers7are apart form each other in the thickness direction. Each of the conductive layers7is composed of a metallic material such as copper, silver, gold, aluminum, nickel, chromium, and/or a combination thereof, and includes land portions70and a line portion71.

As shown inFIGS. 2 and 3, the land portions70are arranged apart from each other in a plan view and each of the land portions70is connected to a via conductor8. As shown inFIG. 4, the planar shape of the land portion70is larger than that of the via conductor8. Therefore, when the via conductors8are formed, the alignment precision between the land portion70and each of the via conductors8can be increased and the connection reliability between the conductive layers7and the via conductors8can be improved.

As shown inFIGS. 3 and 4, the line portion71includes a linear portion72connected to the land portion70and connects two land portions70to each other.

Such a conductive layer7is formed into a desired shape by forming a conductive film using a well-known film formation method and then by patterning the conductive film. Examples of the film formation method include vapor deposition, chemical vapor deposition (CVD), sputtering, and/or a combination thereof. The patterning can be performed by, for example, photolithography.

The conductive layer7is connected to the via conductors8. As shown inFIG. 2, the via conductor8connects the conductive layers7apart from each other in the thickness direction. The via conductor8functions as an external connection terminal electrically connected to the electronic component2such as an IC via a pad20and a bump4. These via conductors8is composed of a conductive material such as copper, silver, gold, aluminum, nickel, chromium, and/or a combination thereof.

Referring toFIG. 3, the via conductors8has a shape that two columns are arranged in a width direction (Y direction) of the linear portion72to partially overlap with each other, and the lower surface of the via conductor8has a smaller area than the upper surface thereof. The lower surface or the upper surface of the via conductor8is connected to the land portion70, and a connected portion80is formed between the via conductor8and the land portion70.

As shown inFIG. 4, the planar shape of the connected portion80is a shape in which two circles are arranged in the width direction (Y direction) of the linear portion72so as to partially overlap with each other. The shape is an elongated shape having a longitudinal direction along the width direction (Y direction) of the linear portion72, and at least part of the planar shape is located in an imaginary region Ra formed by imaginarily extending the linear portion72toward the land portion70. The imaginary region Ra is shown as a hatched portion. Because the planar shape of the connected portion80is a shape elongated in the width direction (Y direction) in such a manner, a current flowing through the linear portion72, the imaginary region Ra, and the via conductor8is dispersed in the longitudinal direction of the connected portion80between the land portion70and the via conductor8, which can reduce the generation of electromigration. As a result, since cracking in the connected portion80can be less likely to occur, the connection reliability between the conductive layer7and the via conductor8can be improved. When the planar shape of the connected portion80is formed such that the length of the imaginary region Ra in the extending direction (X direction) is smaller than that in the width direction, the conductive layer7can be downsized in the extending direction. Consequently, the circuit pattern can be miniaturized in the circuit board3.

The planer shape of the connected portion80has a pair of first long sides81and82that extend in the width direction of the linear portion72. The first long side81closer to the linear portion72includes a plurality of convex portions83projecting toward the linear portion72. In this structure, since a current is dispersed to each of the convex portions83, the current in the connected portion80can be further dispersed. Herein, a concave portion84is formed between the two adjacent convex portions83.

In the planar shape of the connected portion80, the concave portion84is located closer to the center of the imaginary region Ra than the convex portions83. In this structure, since the convex portions83are formed in a region other than the center where a current is easily concentrated in the related art, the current in the connected portion80can be further dispersed.

As shown inFIGS. 4 and 5, the land portion70has a cavity portion73in a first region Rb corresponding to the connected portion80. The cavity portion73includes a second region Rc corresponding to the concave portion84and a third region Rd that is a region other than the second region Rc. The second region Rc is deeper than the third region Rd. Meanwhile, a part of the via conductor8is disposed in the cavity portion73.

Therefore, the part of the via conductor8projects toward the land portion70in the second region Rc corresponding to the concave portion84, whereby the adhesion between the land portion70and the via conductor8can be increased and the connection reliability between the conductive layer7and the via conductor8can be further improved.

The depth of the second region Rc corresponding to the concave portion84is preferably set to be 1.5 times or more and 2 times or less the depth of the third region Rd.

The via conductor8can be formed by applying laser beams to an insulating layer60in two areas that partially overlap with each other in the width direction of the linear portion72using, for example, a YAG laser device or a CO2 laser device to form a through-hole61in which the land portion70is exposed and then by coating the through-hole61with a plating film by electroless plating or the like. When the through-hole61is formed, the cavity portion73is formed in the land portion70due to the irradiation with the laser beams. Furthermore, since the two areas are irradiated with the laser beams so as to partially overlap with each other, the second region Rc that is an overlap region is formed more deeply than the third region Rd. In addition, because the second region Rc is irradiated with the laser beams for a longer time than the third region Rd, the residue of a resin derived from the insulating layer60is appropriately removed. As a result, a plating film can coat the second region Rc where the residue of a resin is hardly left, the adhesion between the land portion70and the via conductor8can be improved.

The whole area of the planar shape of the connected portion80is located in the imaginary region Ra. In this structure, a current in the connected portion80can be further dispersed over the entire connected portion80in the longitudinal direction.

Both ends of the imaginary region Ra in the width direction touch both ends of the land portion70. In this structure, a current in the connected portion80can be further dispersed over the entire connected portion80in the longitudinal direction without increasing the width of the conductive layer7.

In a planar shape of the connected portion80, the length in the transverse direction (X direction) is preferably set to be 20% or more and 50% or less of the length in the longitudinal direction (Y direction).

The present invention is not limited to the above-described embodiments, and various changes and modifications can be made without departing from the spirit and scope of the present invention.

For example, the planar shape of the land portion is not limited to a circular shape and may be any shape. For example, the planar shape of the land portion may be a shape similar to the planar shape of the connected portion between the via conductor and the land portion.

In the cavity portion of the land portion, the second region may have almost the same depth as the third region other than the second region.

The planar shape of the connected portion between the via conductor and the land portion is not limited to a shape in which two circles are arranged in the width direction of the linear portion so as to partially overlap with each other, and may be any shape such as the shapes shown inFIGS. 6A to 6D.

The line portion may further include a curve portion other than the linear portion as long as the linear portion is directly connected to the land portion.

The width of the line portion is not necessarily equal to that of the land portion, and may be smaller than that of the land portion like the configuration shown inFIG. 6E.

The configurations shown inFIGS. 6A to 6Eare described.

The connected portions80A,80B,80C, and80D respectively shown inFIGS. 6A to 6Dare different from the connected portion80in the circuit board3described above in terms of planar shape (refer toFIGS. 1 to 5).

The planar shape of the connected portion80A shown inFIG. 6Ais a shape in which three circles are arranged in the width direction of the linear portion72A so as to partially overlap with each other. In such a connected portion80A, the length in the longitudinal direction can be easily increased. As a result, a current in the connected portion80A can be further dispersed. The connected portion80A having such a shape can be formed by applying laser beams to an insulating layer in three areas that partially overlap with each other and extend in the width direction of the linear portion72A using, for example, a YAG laser device or a CO2 laser device to form a through-hole and then by coating the through-hole with a plating film.

The planar shape of the connected portion80B shown inFIG. 6Bis a rectangular shape whose long sides extend in the width direction of a linear portion72B. In such a connected portion80B, a first long side81B closer to the linear portion72B is a straight line that is parallel to the longitudinal direction. As a result, a current can be dispersed more uniformly on the first long side81B, whereby the current in the connected portion80B can be further dispersed. The corners of the rectangular shape are preferably rounded off. Consequently, the concentration of thermal stress at the corners can be dispersed, which can further improve the connection reliability between the conductive layer and the via conductor.

The planar shape of the connected portion80C shown inFIG. 6Cis a quadrilateral shape having a first long side81C and a second long side82C that each extend in the width direction of a linear portion72C. The first long side81C closer to the linear portion72C is longer than the second long side82C further from the linear portion72C. In such a connected portion80C, a current can be further dispersed on the first long side81C while a region on the side of the long side85C side of the land portion70C is formed so as to have a small area. Preferably, the corners of the quadrilateral shape are rounded off and have a radius of curvature of 2 μm or more and 10 μm or less.

The planar shape of the connected portion80D shown inFIG. 6Dis an elliptical shape with a major axis that extends in the width direction of a linear portion72D.

The connected portions80B,80C, and80D respectively shown inFIGS. 6B,6C, and6D can be formed by forming a through-hole with a desired shape in an insulating layer using, for example, an excimer laser device and then by coating the through-hole with a plating film. With a YAG laser device or a CO2 laser device, the connected portion80D shown inFIG. 6Dcan be easily formed by applying laser beams whose shape is adjusted to an elliptical shape using an aperture.

The line portion71E shown inFIG. 6Eis different from the line portion71(refer toFIGS. 1 to 5) in the circuit board3described above. The line portion71E has a shape whose width is smaller than that of a land portion70E. In such a line portion71E, the circuit pattern can be miniaturized. The imaginary region Ra shown as a hatched portion inFIG. 6Eis a region surrounded by imaginary lines La and Lb that are parallel to the longitudinal direction of the linear portion72E and extend from the side portions of the linear portion72E.

Example

In this Example, a correlation between the planar shape of a connected portion and the current density in the connected portion was investigated.

The current densities for connected portions80A′,80B′,80C′, and80D′ respectively shown inFIGS. 7A to 7Dwere calculated through simulation.

A planar shape of a connected portion80A′ between a land portion70A′ and a via conductor is a shape in which two circles are arranged in the width direction of a linear portion72A′ so as to partially overlap with each other. In the planar shape of the connected portion80A′, the length in the width direction of the linear portion72A′ is 88 μm and the diameter of the circles is 48 μm. The diameter of the land portion70A′ and the width of the linear portion72A′ are each 120 μm.

A planar shape of a connected portion80B′ between a land portion70B′ and the via conductor is a rectangular shape having a long side81B′ that extends in the width direction of a linear portion72B′. In the planar shape of the connected portion80B′, the long sides81B′ has a length of 88 μm and the short side has a length of 48 μm. The sizes of the land portion70B′ and the linear portion72B′ are the same as the sizes of the land portion70A′ and the linear portion72A′ shown inFIG. 7A.

A planar shape of the connected portion80C′ between a land portion70C′ and a via conductor is an elliptical shape having a major axis that extends in the width direction of a linear portion72C′. In the planar shape of the connected portion80C′, the major axis has a length of 88 μm and the minor axis has a length of 48 μm. The sizes of the land portion70C′ and the linear portion72C′ are the same as the sizes of the land portion70A′ and the linear portion72A′ shown inFIG. 7A.

A planar shape of a connected portion80D′ between a land portion70D′ and a via conductor is a circular shape. In the planar shape of the connected portion80D′, the diameter of the circle is 48 μm. The sizes of the land portion70D′ and the linear portion72D′ are the same as the sizes of the land portion70A′ and the linear portion72A′ shown inFIG. 7A. This via conductor corresponds to a conventional via conductor.

Assuming that1ampere of direct current flows from a land portion70D′ to another land portion70D′ as shown inFIG. 8, the current density along the rim of a planar shape of the connected portion80D′ between the via conductor and the land portion70D′ of the conductive layer was calculated. The current densities along the rims of the connected portions80A′,80B′, and80C′ were also calculated in the same manner as that of the connected portion80D′. The current densities were calculated using commercially available software “Ansoft Q3D Extractor”.FIGS. 7A to 7Dshow the results. In each of the drawings, the abscissa indicates the length L along the rim of the connected portion80A′,80B′,80C′, or80D′ from a reference point P and the ordinate indicates the result of the calculated current density.

In the connected portion80A′,80B′ and80C′ shown inFIGS. 7A to 7C, a portion having high current density extends over a relatively wide area. In contrast, in the connected portion80D′ having the conventional shape shown inFIG. 7D, a current becomes concentrated in a portion opposite the reference point P.

The maximum current densities calculated for the connected portion80A′,80B′, and80C shown inFIGS. 7A to 7Cwere smaller than that calculated for the connected portion80D′ shown inFIG. 7D.

Accordingly, in a case that the planar shape of a connected portion has a long shape along the width direction of a linear portion, the concentration of a current in the connected portion between the land portion and the via conductor can be reduced.

As shown inFIG. 7A, in the connected portion80A′, a region having a high current density was separated into two portions. Thus, the maximum current density calculated for the connected portion80A′ shown inFIG. 7Awas even smaller than that calculated for the connected portion800′ shown inFIG. 7C.

This is because a current in the connected portion80A′ can be dispersed to each of the convex portions83A′ because of convex portion83A′ formed in the connected portion80A′.

As shown inFIG. 7B, in the connected portion80B′, a current in a region having a high current density was dispersed over a certain range. Thus, the maximum current density calculated for the connected portion80B′ shown inFIG. 7Bwas even smaller than that calculated for the connected portion8C′ shown inFIG. 7C.

This is because a current in the connected portion80B′ can be uniformly dispersed through the first long side81B′ due to a long side81B′ parallel to the longitudinal direction of the planar shape of the connected portion80B′.