Printed wiring board

Abstract

A printed wiring board (100) includes: two surface conductor layers (111); at least one internal conductor layer; and a plurality of insulating layers (114). The at least one inner conductor layer includes at least one low thermal expansion conductor layer (114) in which the conductor portion is formed of a conductor having a lower coefficient of thermal expansion than the conductors of the two surface conductor layers. At least one of the at least one low thermal expansion conductor layer has, in the conductor portion, two or more regions provided with a plurality of through holes . The two or more regions have mutually different hole densities.

Description

printed wiring board

[0001] The present disclosure relates to printed wiring boards.

[0002] Printed wiring boards are mounted with electronic components that have a different thermal expansion coefficient than the printed wiring board. If there is a large difference in thermal expansion between the printed wiring board and the mounted components, cracks are more likely to occur in the solder joints due to the effects of heat cycles. This shortens the lifespan of the printed wiring board.

[0003] A printed wiring board having a core is known as a conventional technique for reducing the difference in thermal expansion between a printed wiring board and a mounted component.

[0004] Patent Document 1 discloses a package substrate. In this package substrate, a low-thermal expansion metal is used as a core, and build-up wiring layers are laminated on both sides of the core. In addition, a semiconductor chip is flip-chip connected to the package substrate, and multiple through-holes (or slits) are formed in the core.

[0005] Japanese Patent Application Laid-Open No. 2004-253738

[0006] A variety of components with different thermal expansion coefficients are mounted on a printed circuit board. Therefore, it is desirable to adjust the thermal expansion coefficient of the printed circuit board to match each component. It is desirable to have a low thermal expansion coefficient only in the vicinity of low thermal expansion components, but in that case, the difference in thermal expansion on the printed circuit board surface will be large.

[0007] An object of the present disclosure is to make it possible to control the thermal expansion coefficient of a printed wiring board in accordance with the thermal expansion coefficient of each component mounted thereon.

[0008] The printed wiring board of the present disclosure is a multi-layer printed wiring board, comprising: two surface conductor layers formed on the upper and lower surfaces of the printed wiring board; one or more internal conductor layers formed inside the printed wiring board; and a plurality of insulating layers that electrically insulate the two surface conductor layers from the one or more internal conductor layers, wherein the one or more internal conductor layers include one or more low thermal expansion conductor layers whose conductor portions are formed with conductors having a lower thermal expansion coefficient than the conductors of the two surface conductor layers, and at least one of the one or more low thermal expansion conductor layers has two or more regions in its conductor portion where a plurality of through holes are provided, and the two or more regions have different hole densities from each other.

[0009] According to the present disclosure, two or more regions with different hole densities are provided in the low thermal expansion conductor layer of the printed wiring board, making it possible to control the thermal expansion coefficient of the printed wiring board to match the thermal expansion coefficient of each component to be mounted.

[0010] 1 is a partial cross-sectional view of a printed wiring board 100 according to embodiment 1. FIG. 2 is a partial cross-sectional view of a printed wiring board 100 according to embodiment 1. FIG. 3 is a partial plan view of a low thermal expansion conductor layer 112a according to embodiment 1. FIG. 4 is a partial plan view of a low thermal expansion conductor layer 112b according to embodiment 1. FIG. 5 is a partial plan view of a low thermal expansion conductor layer 112 according to embodiment 1. FIG. 6 is a partial cross-sectional view of a printed wiring board 100 according to an example. FIG. 7 is a partial cross-sectional view of a printed wiring board 100 according to an example. FIG. 8 is a partial cross-sectional view of a printed wiring board 100 according to an example. FIG. 9 is a partial cross-sectional view of a printed wiring board 100 according to an example. FIG. 10 is a partial cross-sectional view of a printed wiring board 100 according to an example. FIG. 11 is a partial cross-sectional view of a low thermal expansion conductor layer 112 according to an example.

[0011] In the embodiments and drawings, the same or corresponding elements are denoted by the same reference numerals, and the description of elements denoted by the same reference numerals as elements already described will be omitted or simplified as appropriate.

[0012] First Embodiment A printed wiring board 100 will be described with reference to FIGS.

[0013] ***Description of Configuration*** The configuration of printed wiring board 100 will be described with reference to Figures 1 and 2. Printed wiring board 100 is a printed wiring board having multiple layers (multilayer printed wiring board).

[0014] The printed wiring board 100 has two surface conductor layers 111. In Fig. 1, the printed wiring board 100 has a surface conductor layer 111a and a surface conductor layer 111b.

[0015] The surface conductor layers 111 are conductor layers formed on the upper and lower surfaces of the printed wiring board 100. The conductor layers are layers that have portions formed of a conductor (conductor portions).

[0016] The conductor of the surface conductor layer 111 is referred to as a wiring conductor 101 .

[0017] The printed wiring board 100 has one or more internal conductor layers. The internal conductor layers are conductor layers formed inside the printed wiring board 100.

[0018] The low thermal expansion conductor layer 112 and the same type conductor layer 113 are each an internal conductor layer. The conductor of the low thermal expansion conductor layer 112 has a lower thermal expansion coefficient than the wiring conductor 101. The conductor of the low thermal expansion conductor layer 112 is referred to as the low thermal expansion conductor 102. The conductor of the same type conductor layer 113 is a conductor of the same type as the conductor of the surface conductor layer 111.

[0019] In Fig. 1, printed wiring board 100 has two low-thermal expansion conductor layers 112 and two homogeneous conductor layers 113. Low-thermal expansion conductor layer 112a and low-thermal expansion conductor layer 112b are each composed of a low-thermal expansion conductor 102 and an insulating material 103. However, because the portion of low-thermal expansion conductor layer 112 shown in Fig. 1 is a conductor portion, the portion formed of insulating material 103 (insulating portion) is not shown in Fig. 1 for low-thermal expansion conductor layer 112. Homogeneous conductor layer 113a and homogeneous conductor layer 113b are each composed of wiring conductor 101 and insulating material 103.

[0020] At least one of the same type conductor layers 113 is disposed between two low thermal expansion conductor layers 112. In Fig. 1, the same type conductor layer 113a and the same type conductor layer 113b are disposed between the low thermal expansion conductor layer 112a and the low thermal expansion conductor layer 112b.

[0021] The printed wiring board 100 has a plurality of insulating layers 114. The insulating layers 114 electrically insulate the two surface conductor layers 111 from one or more internal conductor layers. The insulating layers 114 are layers formed of an insulating material 103 inside the printed wiring board 100.

[0022] In FIG. 1 , the printed wiring board 100 has five insulating layers 114. The insulating layer 114a is disposed on the inner surface of the surface conductor layer 111a. The low-thermal expansion conductor layer 112a is disposed on the inner surface of the insulating layer 114a. The insulating layer 114a insulates the surface conductor layer 111a from the low-thermal expansion conductor layer 112a. The insulating layer 114b is disposed on the inner surface of the low-thermal expansion conductor layer 112a. The same-type conductor layer 113a is disposed on the inner surface of the insulating layer 114b. The insulating layer 114b insulates the low-thermal expansion conductor layer 112a from the same-type conductor layer 113a. The insulating layer 114c is disposed on the inner surface of the same-type conductor layer 113a from the same-type conductor layer 113b. The insulating layer 114c insulates the same-type conductor layer 113a from the same-type conductor layer 113b. The insulating layer 114d is disposed on the inner surface of the low-thermal expansion conductor layer 112d. The same-type conductor layer 113b is disposed on the inner surface of the insulating layer 114d. The insulating layer 114d insulates the same-type conductor layer 113b from the low-thermal expansion conductor layer 112b. The insulating layer 114e is disposed on the inner surface of the surface conductor layer 111b. The low-thermal expansion conductor layer 112b is disposed on the inner surface of the insulating layer 114e. The insulating layer 114e insulates the low-thermal expansion conductor layer 112b from the surface conductor layer 111b.

[0023] A through hole 119 is provided in the printed wiring board 100. The through hole 119 passes through the printed wiring board 100. The diameter of the through hole 119 is constant inside the printed wiring board 100.

[0024] The through holes 119 are formed of the same type of conductor as the conductor of the surface conductor layers 111, and electrically connect the two surface conductor layers 111 to at least one of the internal conductor layers. For example, the wiring conductors 101 of the through holes 119 electrically connect the wiring conductors 101 of the two surface conductor layers 111 to the low thermal expansion conductors 102 of at least one of the low thermal expansion conductor layers 112.

[0025] In FIG. 1, the wiring conductor 101 of the through hole 119 is electrically connected to the wiring conductor 101 of each surface conductor layer 111, the low thermal expansion conductor 102 of each low thermal expansion conductor layer 112, and the wiring conductor 101 of each same type conductor layer 113.

[0026] The thickness of a portion of the low thermal expansion conductor layer 112 is different from the thickness of the other portion of at least one of the low thermal expansion conductor layers 112. In Fig. 2, a portion of each of the low thermal expansion conductor layers 112a and 112b is thinner than the other portion.

[0027] The configuration of the low thermal expansion conductor layer 112 will be described with reference to Figures 3 to 6. At least one of the low thermal expansion conductor layers 112 has one or more regions 120 in its conductor portion. The regions 120 are regions in which a plurality of through holes 121 are provided. In Figure 3, the low thermal expansion conductor layer 112a has regions 120a and 120b in its conductor portion. Furthermore, a plurality of through holes 121 are provided in each of the regions 120a and 120b.

[0028] Region 120 is a region that is located at the same position in the planar direction as one of the component mounting regions of the two surface conductor layers 111. The component mounting region is a region where components are mounted. In Figure 3, region 120a is a region directly below component mounting region a of surface conductor layer 111a. Region 120b is a region directly below component mounting region b of surface conductor layer 111a.

[0029] At least one of the low thermal expansion conductor layers 112 has two or more regions 120 in its conductor portion. The two or more regions 120 have different hole densities. The hole density is also called porosity. In FIG. 3, the hole density of region 120a is different from the hole density of region 120b.

[0030] The printed wiring board 100 has two or more low-thermal expansion conductor layers 112, each having a region 120 in its conductor portion, the regions being located at different positions in the planar direction. In Fig. 4, the low-thermal expansion conductor layer 112b has a region 120c in its conductor portion, in which a plurality of through holes 121 are provided. The region 120c is located directly above the component mounting region c of the surface conductor layer 111b. The region 120c of the low-thermal expansion conductor layer 112b is located at a different position in the planar direction from the regions (120a, 120b) of the low-thermal expansion conductor layer 112a.

[0031] A plurality of through holes 129 are provided in the conductor portion around at least one of the regions 120, with the hole density gradually changing with increasing distance from the region 120. In Figure 5, the low thermal expansion conductor layer 112 has a region 120d in its conductor portion where a plurality of through holes 121 are provided. A plurality of through holes 129 are provided around the region 120d in the conductor portion. The diameter of the through holes 129 becomes smaller with increasing distance from the region 120d. As a result, the hole density around the region 120d gradually decreases with increasing distance from the region 120d. Although the diameter of the through holes 129 becomes smaller with increasing distance from the region 120d in Figure 5, the diameter of the through holes 129 may also become larger with increasing distance from the region 120d.

[0032] At least one of the regions 120 has two or more sections 122. The sections 122 are sections in which a plurality of through holes 121 are provided. The two or more sections 122 have different hole densities. In Fig. 6, the low thermal expansion conductor layer 112 has a region 120e in its conductor portion. The region 120e has a region 122a and a region 122b. The region 122a and the region 122b each have a plurality of through holes 121. The hole density of the region 122a and the hole density of the region 122b are different from each other.

[0033] ***Effects of First Embodiment*** The first embodiment provides a method for controlling the thermal expansion coefficient of a substrate to match the thermal expansion coefficient of a component to be mounted. Two or more regions with different through-hole densities (porosity) are provided in the conductor portion of the low thermal expansion conductor layer 112 of the printed wiring board 100. This makes it possible to adjust the thermal expansion coefficient of the printed wiring board 100 for each region where a component is mounted, thereby reducing the difference in thermal expansion coefficient between the component and the printed wiring board 100. Therefore, it is possible to keep the difference in thermal expansion coefficient below a predetermined value and maintain strain generated in the solder joint of the component mounting portion below a certain level.

[0034] Through holes 121 are formed in the region 120 of the conductor portion of the low thermal expansion conductor layer 112. This changes the elastic modulus of the region 120 and the apparent coefficient of thermal expansion. By changing the coefficient of thermal expansion, it is possible to control the difference in the coefficient of thermal expansion between the region 120 and the mounted components. Furthermore, by changing the porosity of each region 120, it is possible to change the apparent coefficient of thermal expansion for each region 120, thereby making the difference in the coefficient of thermal expansion between the region 120 and the corresponding mounted components in two or more regions 120 the same. Therefore, by changing the coefficient of thermal expansion directly below each mounted component, it is possible to maintain a constant difference in the thermal expansion between the component and the printed wiring board 100 across the entire board. Furthermore, by keeping the difference in the coefficient of thermal expansion between each mounted component and the printed circuit board directly below it below a predetermined value, it is possible to maintain the strain occurring in the solder joints of the component mounted portion below a certain level.

[0035] When components are mounted on both surface conductor layers 111 of the printed wiring board 100, through holes 121 are formed in the respective regions 120 of the two or more low thermal expansion conductor layers 112 to match the thermal expansion coefficients of the components mounted on the respective surface conductor layers 111. This enables adjustment (matching) of the thermal expansion coefficients. The positions of the through holes 121 of the two or more low thermal expansion conductor layers 112 do not need to coincide with each other in the plate thickness direction.

[0036] In a component in which multiple chips are arranged in a single package (e.g., a multi-chip module), the coefficient of thermal expansion differs within the component. When such a component is mounted on the surface conductor layer 111, multiple compartments 122 are provided in the region 120 corresponding to the component. This makes it possible to deal with the difference in the coefficient of thermal expansion within the component.

[0037] The first embodiment provides the following advantages. In a printed wiring board 100 on which multiple different components are mounted, it is possible to control the difference in thermal expansion coefficient between the mounted components and the substrate at a constant level. In a printed wiring board 100 on which multiple different components are mounted, it is possible to control the strain generated at the solder joints of the component mounting area at a constant level. By arranging the low thermal expansion conductor layer 112 near the surface, the effect of controlling the thermal expansion coefficient by the low thermal expansion conductor 102 is enhanced. By electrically connecting the low thermal expansion conductor 102 to the wiring conductor 101 via the through hole 119, it is possible to prevent the low thermal expansion conductor 102 from becoming a floating potential. Furthermore, it is possible to use the low thermal expansion conductor 102 as a heat path. Since the diameter of the through hole 119 is constant within the same through hole 119, it is possible to manufacture a multilayer printed wiring board. When components are mounted on both sides, the positions of the regions 120 of two or more low thermal expansion conductor layers 112 differ in the planar direction, making it possible to control the thermal expansion coefficient to match the components on both sides. By gradually changing the hole density of the through holes 129 toward the region 120, the thermal expansion coefficient can be gradually changed toward the region 120 for adjusting thermal expansion, thereby alleviating stress concentration.

[0038] ***Examples of First Embodiment*** Below, examples will be described relating to the printed wiring board 100. The number of low thermal expansion conductor layers 112 may be one, or may be three or more.

[0039] The number of the same type conductor layers 113 is not limited to two.

[0040] The same type conductor layer 113 may be disposed outside the low thermal expansion conductor layer 112. However, it is desirable that when a component is mounted on the surface conductor layer 111, there is little other material present between the mounted component and the low thermal expansion conductor layer 112. Therefore, it is desirable to dispose the low thermal expansion conductor layer 112 on a layer close to the surface conductor layer 111.

[0041] The layer structure of the printed wiring board 100 may be asymmetric in the thickness direction. However, the low-thermal expansion conductor layer 112, the same-type conductor layer 113, and the insulating layer 114 have different coefficients of thermal expansion and modulus of elasticity. Therefore, when a temperature change occurs, different thermal stresses are generated inside the printed wiring board 100. Furthermore, if the layer structure of the printed wiring board 100 is asymmetric in the thickness direction, the entire board is likely to warp. Therefore, it is desirable that the layer structure of the printed wiring board 100 be symmetric in the thickness direction. In other words, it is desirable that the low-thermal expansion conductor layer 112 and the same-type conductor layer 113 be arranged symmetrically.

[0042] Copper is often used for the conductor layer. Silver paste may also be used. In other words, silver may also be used. The printed wiring board 100 is a laminate of multiple materials. The overall thermal expansion coefficient of the laminate of n types of materials is calculated by calculating Equation (1).

[0043]

[0044] "α" indicates the coefficient of thermal expansion. The subscript s refers to the entire substrate. "Vf" indicates the volume fraction. "E" indicates Young's modulus. "i" indicates one of n types of materials.

[0045] The overall thermal expansion coefficient of the laminate is strongly affected by the material (layer) with a high elastic modulus, and also by the material (layer) with a high volume fraction.

[0046] The material used for the low thermal expansion conductor 102 is preferably one with a low thermal expansion coefficient and a high elastic modulus. Specifically, a material with a thermal expansion coefficient of 8 ppm / K or less is desirable. Furthermore, a material with an elastic modulus of 200 GPa or more is desirable. ppm means parts per million. "K" means Kelvin. GPa means gigapascal. Desirable materials include, for example, molybdenum (Mo), tungsten (W), or alloys thereof. Additionally, cemented carbide alloys are also exemplified as materials with low thermal expansion and high elastic modulus. Examples of molybdenum alloys include Mo-Ti, Mo-Ti-C, Mo-Zr, TZM, TZC, and Mo-Nb. Examples of tungsten alloys include W-Ag, W-Cu-Ni, W-Fe-Ni, W-Ni-Co, W-Re, and W-Re-Ta. "Mo" and "W" are element symbols. Additionally, composite materials of copper and the above-mentioned metals, such as copper-molybdenum composites or copper-tungsten composites, are also examples of low thermal expansion and high elastic modulus materials. Copper composite materials also have high thermal conductivity. Copper composite materials are preferred materials for the low thermal expansion conductor layer 112 from the viewpoint of heat dissipation. Furthermore, copper composite materials such as those described above are also preferred materials for the low thermal expansion conductor layer 112 because of their high compatibility with the manufacturing process of substrates that use copper as the primary conductor. Other desirable materials include low thermal expansion alloys, negative thermal expansion materials, or composites of these. An example of a low thermal expansion alloy is an invar alloy. Examples of negative thermal expansion materials are bismuth nickel iron oxide and zirconium tungstate.

[0047] The insulating material 103 can be a board material used in printed circuit boards. For example, a material in which glass fiber is impregnated with epoxy resin (glass epoxy) can be used as the insulating material 103. Resins that can be used in place of epoxy resin include PPE, cyanate, PTFE, polyimide, and phenol. PPE stands for polyphenylene ether. PTFE stands for fluororesin. Aramid fiber can be used in place of glass fiber.

[0048] It is also possible to change the thickness of the region 120 of the low thermal expansion conductor layer 112 in order to adjust the thermal expansion coefficient of the conductor portion of the low thermal expansion conductor layer 112. Also, the thickness around the region 120 may be gradually changed with increasing distance from the region 120.

[0049] The following describes an embodiment of the through hole 119. The through hole 119 does not have to be connected to the conductor portion of the low thermal expansion conductor layer 112. In Fig. 7, the through hole 119 is not connected to the conductor portion of the low thermal expansion conductor layer 112a or the low thermal expansion conductor layer 112b.

[0050] The printed wiring board 100 may be provided with through holes 119 connected to the conductor portions of the low thermal expansion conductor layer 112 and through holes 119 not connected to the conductor portions of the low thermal expansion conductor layer 112. In Fig. 8, the through hole 119a is connected to the conductor portions of the low thermal expansion conductor layer 112a and the low thermal expansion conductor layer 112b. On the other hand, the through hole 119b is not connected to the conductor portions of the low thermal expansion conductor layer 112a and the low thermal expansion conductor layer 112b.

[0051] The through hole 119 may be connected to the conductor portion of at least one of the low thermal expansion conductor layers 112, but may not be connected to the conductor portion of the other low thermal expansion conductor layers 112. In Fig. 9, the through hole 119 is connected to the conductor portion of the low thermal expansion conductor layer 112a. However, the through hole 119 is not connected to the conductor portion of the low thermal expansion conductor layer 112b.

[0052] The through-holes 119 may not be provided in the holes provided in the conductor portions of at least one of the low thermal expansion conductor layers 112. In Fig. 10, the through-holes 119 are not provided in the holes (areas surrounded by dashed lines) in the conductor portions of the low thermal expansion conductor layers 112a and 112b.

[0053] The sizes of the holes in the conductor portions may be different between two or more low thermal expansion conductor layers 112. In Fig. 11, the holes in the conductor portions of low thermal expansion conductor layer 112a and low thermal expansion conductor layer 112b are different in size.

[0054] The positions of the holes in the conductor portions may be different between two or more low thermal expansion conductor layers 112. In Fig. 12, the holes in the conductor portions of low thermal expansion conductor layer 112a and low thermal expansion conductor layer 112b are located at different positions.

[0055] Holes may not be provided in the conductor portion of at least one of the low thermal expansion conductor layers 112. In Fig. 13, holes are provided in the conductor portion of the low thermal expansion conductor layer 112a, while no holes are provided in the conductor portion of the low thermal expansion conductor layer 112b.

[0056] If a hole is formed in the conductor portion of the low thermal expansion conductor layer 112 before the lamination process of the printed wiring board 100, the hole in the conductor portion of the low thermal expansion conductor layer 112 is temporarily filled with the resin that constitutes the insulating material 103 during the manufacturing process of the printed wiring board 100. If a through hole or a hole is formed in the conductor portion of the low thermal expansion conductor layer 112 so as to penetrate the printed wiring board 100 after the lamination process of the printed wiring board 100, the inside of the through hole or the hole becomes hollow. The inside of the hole can then be filled with resin, a composite of a functional filler and a resin, or a metal. Examples of functional fillers include inorganic fillers such as alumina and conductive fillers such as silver-coated copper powder.

[0057] Below, we will explain examples of the low thermal expansion conductor layer 112. In the low thermal expansion conductor layer 112, the hole density of the through holes (121, 129) may be adjusted by the size of the through holes, by the number of the through holes, or by both the size and the number of the through holes.

[0058] When a component is mounted on the surface conductor layer 111a, the region 120, which is disposed directly below the mounted component and in which a plurality of through holes 121 are formed, may be disposed in the low thermal expansion conductor layer 112b, which is far from the surface conductor layer 111a, rather than in the low thermal expansion conductor layer 112a, which is close to the surface conductor layer 111a. However, it is desirable that the distance between the region 120 for controlling the thermal expansion coefficient of the low thermal expansion conductor layer 112 and the component mounting region of the surface conductor layer 111 is small. Therefore, it is desirable that the region 120 for the mounted component be disposed in the low thermal expansion conductor layer 112, which is close to the surface conductor layer 111 on which the mounted component is mounted.

[0059] When a component is mounted on the surface conductor layer 111a, an area 120 located directly below the mounted component and having a plurality of through holes 121 formed therein may be located on each of two or more low thermal expansion conductor layers 112.

[0060] The through holes 121 may be provided throughout the entire low thermal expansion conductor layer 112, and the hole density in the region 120 may be lower than the hole density throughout the entire low thermal expansion conductor layer 112. Alternatively, at least one of the regions 120 may not have through holes 121. In FIG. 14 , the through holes 121 are provided throughout the entire low thermal expansion conductor layer 112. The hole density in region 120f is lower than the hole density throughout the entire low thermal expansion conductor layer 112. Furthermore, no through holes 121 are provided in region 120g. In this way, the thermal expansion coefficient may be changed by reducing the hole density.

[0061] When the positions of the regions 120 in the planar direction are different in two or more low thermal expansion conductor layers 112, at least a portion of the regions 120 may overlap in the planar direction. Also, the region 120 of one low thermal expansion conductor layer 112 may be included in the region 120 of the other low thermal expansion conductor layer 112 in the planar direction, and the entire region 120 of one low thermal expansion conductor layer 112 may overlap in the planar direction with a portion of the region 120 of the other low thermal expansion conductor layer 112. In Fig. 15, a portion of the region 120h of the low thermal expansion conductor layer 112b overlaps with the region 120b of the low thermal expansion conductor layer 112a.

[0062] The shape of the through holes 121 may be circular, elliptical, rectangular, or any other shape. The shape of at least one of the through holes 121 may be different from the shape of the other through holes 121 in the same region 120. Furthermore, the shape of at least one of the through holes 121 in the region 120 may be different from the shape of the through holes 121 in the other regions 120. The through holes 121 may have different lengths in the vertical and horizontal directions. Furthermore, the longitudinal directions of the multiple through holes 121 may be aligned in one direction, or may be irregular in two or more directions. In FIG. 16 , the shape of the through holes 121 in the region 120 is rectangular rather than circular. The through holes 121 have different lengths in the vertical and horizontal directions. Furthermore, the longitudinal directions of the eight through holes 121 in the region 120 are aligned in one direction (vertical direction). In this case, anisotropy occurs in the thermal expansion coefficient of the region 120. Because the hole density is low in the horizontal direction of region 120 and high in the vertical direction of region 120, the thermal expansion coefficient in the horizontal direction is higher than the lateral expansion coefficient in the vertical direction of region 120. If the thermal expansion coefficient of the mounted component is anisotropic, it is possible to adjust the thermal expansion coefficient of printed wiring board 100 to match the thermal expansion coefficient of the mounted component by changing the aspect ratio of through holes 121.

[0063] *** Supplementary Note on First Embodiment *** First embodiment is an example of a preferred embodiment and is not intended to limit the technical scope of the present disclosure. First embodiment may be implemented in part, may be implemented in combination with at least one of the embodiments, or may be implemented in combination with other embodiments or other embodiments.

[0064] 100 Printed wiring board, 101 Wiring conductor, 102 Low thermal expansion conductor, 103 Insulating material, 111 Surface conductor layer, 112 Low thermal expansion conductor layer, 113 Same type conductor layer, 114 Insulating layer, 119 Through hole, 120 Region, 121 Through hole, 122 Partition, 129 Through hole.

Claims

1. A printed wiring board having multiple layers, comprising: two surface conductor layers formed on upper and lower surfaces of the printed wiring board; one or more internal conductor layers formed inside the printed wiring board; and a plurality of insulating layers electrically insulating the two surface conductor layers from the one or more internal conductor layers, wherein the one or more internal conductor layers include one or more low thermal expansion conductor layers having a conductor portion formed with a conductor having a lower thermal expansion coefficient than the conductors of the two surface conductor layers, at least one of the one or more low thermal expansion conductor layers has two or more regions in its conductor portion with a plurality of through holes, and the two or more regions have different hole densities from one another.

2. A printed wiring board having multiple layers, comprising: two surface conductor layers formed on upper and lower surfaces of the printed wiring board; a plurality of internal conductor layers formed inside the printed wiring board; and a plurality of insulating layers that electrically insulate the two surface conductor layers from the plurality of internal conductor layers, wherein the plurality of internal conductor layers are composed of two or more low thermal expansion conductor layers having conductor portions formed with a conductor having a lower thermal expansion coefficient than the conductors of the two surface conductor layers, and one or more same-type conductor layers having conductor portions formed with the same type of conductor as the conductors of the two surface conductor layers, at least one of the one or more same-type conductor layers is disposed between any two of the low thermal expansion conductor layers, and at least one of the two or more low thermal expansion conductor layers has one or more regions with a plurality of through holes in its conductor portion.

3. The printed wiring board according to claim 1 or 2, which is made of a conductor and has through holes provided therein for electrically connecting said two surface conductor layers to at least one of said internal conductor layers.

4. The printed wiring board according to claim 3, wherein the conductor of the through hole electrically connects the conductor of the two surface conductor layers and the conductor of at least one of the low thermal expansion conductor layers.

5. The printed wiring board according to claim 3 or 4, wherein the diameter of said through holes is constant inside said printed wiring board.

6. A printed wiring board according to any one of claims 1 to 5, comprising two or more low thermal expansion conductor layers each having the region in the conductor portion, the positions of the region in the planar direction being different from each other.

7. A printed wiring board as claimed in any one of claims 1 to 6, wherein a plurality of through holes are provided in the conductor portion surrounding at least any of the regions, the hole density of which gradually changes with increasing distance from the region.

8. A printed wiring board according to any one of claims 1 to 7, wherein at least one of said low thermal expansion conductor layers has a thickness in one portion different from the thickness in the other portion.

9. A printed wiring board as claimed in any one of claims 1 to 8, wherein at least any one of the regions has two or more sections each having a plurality of through holes, and the two or more sections have different hole densities from each other.

10. A printed wiring board as claimed in any one of claims 1 to 9, wherein the position of each of said areas is the same in the planar direction as one of the component mounting areas of said two surface conductor layers.

11. The printed wiring board according to any one of claims 1 to 10, wherein the shape of each of the through holes is a circle, an ellipse or a rectangle.

12. The printed wiring board according to any one of claims 1 to 11, wherein each of the through holes has a different length in the vertical and horizontal directions, and the longitudinal directions of the plurality of through holes are aligned in one direction.

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