US20110303403A1

US20110303403A1 - Flexible Heat Exchanger

Info

Publication number: US20110303403A1
Application number: US12/814,175
Authority: US
Inventors: Maurice F. Holahan; Eric V. Kline; Paul N. Krystek; Michael R. Rasmussen; Arvind K. Sinha; Stephen M. Zins
Original assignee: International Business Machines Corp
Current assignee: International Business Machines Corp
Priority date: 2010-06-11
Filing date: 2010-06-11
Publication date: 2011-12-15
Also published as: JP6041801B2; DE112011101941B4; GB201214378D0; DE112011101941T5; JP2013534048A; WO2011154316A1; CN103053022B; CN103053022A; GB2494955B; GB2494955A; TW201221043A

Abstract

An embodiment of the invention comprises a method for constructing a heat exchanger for cooling one or more semiconductor components. The method comprises the step of providing first and second planar sheets of specified thermally conductive metal foil, wherein each of the sheets has and exterior side and an interior side. The method further comprises forming one or more thermal contact nodes (TCNs) in the first sheet, wherein each TCN extends outward from the exterior side of the first sheet, and comprises a planar contact member and one or more side sections, the side sections respectively including resilient components that enable the contact member of the TCN to move toward and away from the exterior side of the first sheet, and the side sections and contact member of a TCN collectively forming a coolant chamber. Channel segments are configured along the interior side of the first sheet, wherein each channel extends between the coolant chambers and two TCNs, or between the coolant chamber of a TCN and an input port or output port, selectively. The method further comprises joining the interior side of the second sheet to the interior side of the first sheet, in order to form a sealed flow path that includes each channel segment, and enables liquid coolant to flow into and out of the coolant chamber of each TCN.

Description

BACKGROUND

1. Field of the Invention
The disclosure relates generally to a liquid flow through (LFT) heat exchanger for cooling printed circuit boards (PCB) devices, or other semiconductor devices or components. More specifically, the invention pertains to a heat exchanger of the above type that is very flexible and may be readily adapted for use with semiconductor devices of varying heights or other dimensions.
2. Description of the Related Art
High performance computing systems are using ever increasing amounts of power at higher power densities. As a result, system cooling requirements have become more challenging, and it is necessary to consider solutions that use liquid cooling. Currently available liquid cooling approaches include heat pipe, vapor chamber, and liquid flow through (LFT) solutions. These solutions, however, tend to be quite costly.
In a system that uses liquid cooling, it may also be necessary to place components for removing heat in physical contact with semiconductor devices located on a PCB assembly or the like. However, adjacent semiconductor devices may be of different sizes. Moreover, two semiconductor devices that are of the same type may in fact have a dimension that is different for the two devices, even though such dimension is within the allowed tolerance for both devices. As a result, it may be difficult to provide heat exchanger components that can effectively be adapted to meet the size requirements encountered for these different devices. A thermal interface material (TIM) is typically used by practitioners to perform gap-filling functions (e.g. gels, greases, and thermal putties). However, this limits thermal transfer efficiency. Improvements are therefore necessary in the current state of the art.

SUMMARY

According to one embodiment of the present invention, a method is provided for constructing a heat exchanger for cooling one or more semiconductor components. The method comprises the step of providing first and second planar sheets of specified thermally conductive metal foil, wherein each of the sheets has an exterior side and an interior side. The method further comprises forming one or more thermal contact nodes (TCNs) in the first sheet, wherein each TCN extends outward from the exterior side of the first sheet, and comprises a planar contact member and one or more side sections. The side sections may respectively include resilient components that collectively enable the contact member of the TCN to move toward and away from the exterior side of the first sheet, and the side sections and contact member of a TCN collectively form a coolant chamber. A plurality of TCNs thus formed may accommodate different device heights since each TCN can be formed with varying geometries and each TCN mechanically functions substantially independently. Channel segments are configured along the interior side of the first sheet and/or second sheet, wherein each channel segment extends between the coolant chambers of two TCNs, or between the coolant chamber of a TCN and an input port or an output port, selectively. The method further comprises joining the interior side of the second sheet to the interior side of the first sheet, in order to form a sealed flow path that includes each channel segment, and enables liquid coolant to flow into and out of the coolant chamber of each TCN. The method further comprises a connector means to couple and decouple coolant flow to/from the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view showing an embodiment of the invention, which includes two metal foil sheets of substantially identical dimensions.

FIG. 2 is a perspective view showing the opposing side of one of the sheets depicted in FIG. 1.

FIG. 3 is a sectional view taken along lines 3-3 of FIG. 2.

FIG. 4 is a schematic view showing TCNs of the embodiment of FIG. 1 in engagement with respective semiconductor devices, to remove heat therefrom.

FIG. 5 is a schematic view showing a modification of the embodiment of FIG. 1.

FIG. 6 is a schematic view showing a further modification of the embodiment of FIG. 1.

DETAILED DESCRIPTION

As will be appreciated by one skilled in the art, the present invention may be embodied as a system or method. Accordingly, the present invention may take the form of an entirely hardware embodiment, an entirely process embodiment (including design, fabrication, assembly and use steps, etc.) or an embodiment combining method and hardware aspects that may all generally be referred to herein as a process or an “assembly” or a “system.”
Embodiments of the invention provide a method and apparatus for removing heat from semiconductor devices or components, such as those on a single module or an entire PCB assembly. Embodiments enhance simplicity, reduce cost, and may be readily adapted for use with multiple semiconductor components that are adjacent to one another, but are of different sizes or dimensions from one another. Embodiments of the invention are also able to adapt to variations in height, or other critical dimension, that can occur among semiconductor devices of the same type.
Referring to FIG. 1, there is shown an exploded perspective view of an embodiment of the invention, which comprises a liquid flow through (LFT) heat exchanger for removing heat from multiple semiconductor devices or components. The heat exchanger can be readily adapted for use with devices that are adjacent to one another, but are of different sizes. FIG. 1 shows two rectangular, substantially planar metal foil sheets 10 and 12, which usefully are of the same dimensions. Thus, the length and cross-section of sheet 10 are equal to the length and cross-section of sheet 12, respectively. Metal foil sheets 10 and 12 are formed from a material that has high thermal conductivity, such as copper, a brass alloy, beryllium copper, (BeCu), aluminum, an aluminum alloy, or stainless steel. However, the invention is not limited thereto.
Each of the sheets 10 and 12 has an interior side, such as interior side 12 a of sheet 12. The interior side 10 a of sheet 10 is shown in FIG. 2. Each sheet also has an exterior side, such as exterior side 10 b of sheet 10. In fabricating the heat exchanger of FIG. 1, sheets 10 and 12 are joined together so that interior sides 10 a and 12 a are maintained in close abutting relationship with each other. It is also useful to join the two sheets so that their respective corresponding corners are aligned with one another, as shown in FIG. 1. However, before the sheets can be joined together, it is necessary to form certain structural or 3-dimensional features in the material of at least one of the sheets. These structural features will be determined by the particular configuration of semiconductor devices with which the heat exchanger of FIG. 1 is to be used, to remove heat therefrom.
Referring further to FIG. 1, there are shown, by way of example and not limitation, thermal contact nodes (TCNs) 14 and 16, which are respectively formed in metal foil sheet 10 and are thus thermally conductive. TCN 14 has a planar contact member 14 a, and TCN 16 has a planar contact member 16 a. Member 16 a extends outward from exterior side 10 b by some amount of spacing, and is supported with respect to side 10 b by side sections 16 b-16 e, which are respectively positioned along the four sides of contact member 16 a. As described hereinafter in further detail, each side section includes a rigid component and a resilient component. The rigid component is firmly joined to exterior side 10 b of sheet 10. The resilient component is positioned between the rigid component and member 16 a, to allow member 16 a to move or flex toward or away from side 10 b, that is, to move along the Z-axis.
Planar contact member 14 a is similarly supported for movement along the Z-axis by side sections 14 b-14 e, which are respectively positioned along the four sides of contact member 14 a. Each side section 14 b-14 e is similar in construction and function to the side sections 16 b-16 e.
FIG. 1 also shows that planar contact members 14 a and 16 a are spaced apart from one another by a particular distance. This indicates that the heat exchanger of FIG. 1, after it has been fabricated, will be used to cool two semiconductor devices that are likewise spaced apart by the particular distance separating members 14 a and 16 a. In addition, FIG. 1 shows that contact member 16 a is significantly larger than member 14 a. This indicates that the device with which TCN 16 will be used is larger, or needs a larger thermal contact surface area, than the device with which TCN 14 will be used.
As stated above, the provision of two TCNs as shown by FIG. 1 is only exemplary, and the invention is by no means limited thereto. More generally, it is to be emphasized that the number of TCNs formed on sheet 10, as well as their respective sizes and positions, can be readily adapted to meet the needs of many different applications for electronic component heat removal. This capability emphasizes the flexibility which is provided by embodiments of the invention. A particular configuration of TCNs, designed for a particular application, can be fabricated by embossing or molding sheet 10, or by using other techniques known to those of skill in the art.
In order to carry out a heat removal function, it is necessary to provide a flow of coolant fluid to and away from each of the TCNs and their respective planar contact members 14 a and 16 a. Accordingly, in addition to forming the TCNs 14 and 16 in sheet 10, a coolant flow channel is also formed therein. More particularly, FIG. 1 shows channel segments 18, 20, and 22 formed in sheet 10. Each of these segments has a semicircular cross section and is convex with respect to side 10 b of sheet 10, that is, each segment extends outward therefrom. Channel segment 18 extends from a channel end 18 a to TCN 16. Segment 20 extends from TCN 16 to TCN 14, and channel segment 22 extends from TCN 14 to a channel end 22 a.
Referring to FIG. 2, there is shown interior side 10 a of sheet 10, that is, the side thereof that is opposite to exterior side 10 b. FIG. 2 further shows that the contact member 16 a and its side sections 16 b-16 e of TCN 16 collectively form a chamber or compartment 24, which can receive and contain liquid coolant fluid. An end 18 b of coolant channel segment 18 is formed to access, or open into, the chamber 24. In like manner, an end 20 a of channel segment 20 accesses or opens into chamber 24.
Referring further to FIG. 2, it is seen that similar to TCN 16, the contact member 14 a and side sections 14 b-14 e of TCN 14 collectively form a chamber 26 that can receive and contain coolant fluid. An end 20 b of channel segment 20 and an end 22 b of channel segment 22 each accesses or opens into chamber 26.
Referring again to FIG. 1, it will be appreciated that when sheets 10 and 12 are joined together as described above, chambers 24 and 26 will be completely enclosed, except at the locations of access to the channel segments. Moreover, the chambers 24 and 26 and the channel segments collectively comprise a system that is enclosed except at channel ends 18 a and 22 a. By using one of the channel ends as an input port and the other as an output port, liquid coolant fluid can be selectively circulated through the channel segments, and through chamber 24 of TCN 16 and chamber 26 of TCN 14.
In joining metal foil sheets 10 and 12 together, laser welding may be used to join regions of sheets 10 and 12 that surround or are proximate to TCNs 14 and 16, and also to channel segments 18-22. This will ensure the formation of very tight seals for the fluid containing chambers 24 and 26 and the channel segments. The edges of sheets 10 and 12 may be joined by means of laser welding, or may alternatively be joined by means of an adhesive, or by a metallurgical process such as soldering.
FIG. 1 further shows small channel segments 28 and 30 formed in sheet 12. Each of these channel segments has a semicircular cross section, and is convex with respect to interior side 12 a, that is, each channel extends away from sheet 10 as viewed in FIG. 1. Channel segments 28 and 30 are positioned to mate with channel ends 18 a and 22 a, respectively, when sheets 10 and 12 are joined together. This provides each of the channel segments 18 and 22 with a circular aperture at its opening. Couplings 32 and 34 are each sized and fitted to a corresponding one of these apertures. The couplings may then be connected to a conventional coolant fluid pump (not shown), with one of the couplings such as 34 selected as the input port and the other as the output port. By operating the pump, liquid coolant fluid is circulated to each of the TCNs, as discussed above, for heat removal applications. The coolant fluid could comprise distilled water, or other fluid used by those of skill in the art to remove heat from semiconductor devices.
Referring now to FIG. 3, there is shown a sectional view taken through metal foil sheet 10, along lines 3-3 of FIG. 2. FIG. 3 thus depicts features of side sections 16 e and 16 c of TCN 16. More particularly, each of these side sections is shown to include a component 36, which is comparatively rigid. That is, when TCN 16 was formed in sheet 10, each of the side section components 36 was constructed so that it would not move in relation to adjacent portions of sheet 10.
Referring further to FIG. 3, there is shown a component 38 attached to each rigid component 36, and also attached to a side or edge of contact member 16 a of TCN 16. In the formation of sheet 10, each component 38 is fabricated in the manner of or to function as a bellows, so that it is capable of flexure or resilience. Contact member 16 a is thus able to move toward or away from sheet 10, i.e., upward or downward or along the Z-axis, as viewed in FIG. 3. In the formation of sheet 10, the resilient components 38 of a TCN are usefully provided with a prespecified spring constant, to permit elements of the TCN to be compressed or elongated within the elastic limit of the sheet 10 material.
Side sections 16 b and 16 d, while not shown in FIG. 3, each comprises a rigid component and a resilient component that are similar or identical to rigid components 36 and resilient components 38, respectively.
Referring further to FIG. 3, there is shown side sections 14 e and 14 c each comprising a rigid component 40 and a resilient component 42. Each component 40 is similar to components 36 and each component 42 is similar to components 38, as described above. Accordingly, contact member 14 a of TCN 14 is able to move along the Z-axis in the same manner as contact member 16 a.
Side sections 14 b and 14 d, while not shown in FIG. 3, each comprises a rigid component and a resilient component that are similar or identical to those shown in FIG. 3 in connection with side sections 14 c and 14 e.
Referring to FIG. 4, there is shown a schematic view that illustrates the use or operation of the embodiment described above to remove heat from semiconductor electronic devices. More particularly, FIG. 4 shows semiconductor devices 46 and 48 mounted on a PCB 44 or the like, wherein contact member 16 a of TCN 16 has been brought into contacting relationship with device 46. Accordingly, heat from the device 46 is transferred to thermally conductive member 16 a, and through the member 16 a to liquid coolant 50 contained in chamber 24. As described above, liquid coolant may be circulated through chamber 24, and thereby removes heat therefrom.
Similarly, FIG. 4 shows contact member 14 a of TCN 14 in contact with semiconductor device 48, to remove heat therefrom and transfer the heat to coolant 50 in chamber 26.
It is to be appreciated that semiconductor devices 46 and 48 shown in FIG. 4 are distinctly different in size from each other. In view of this, TCNs 14 and 16 have likewise been constructed to be different from one another, and each has been adapted to mate with its corresponding semiconductor device. FIG. 4 thus further illustrates the flexibility that can be provided by embodiments of the invention to adapt to different cooling requirements. It is considered that any reasonable number of TCNs and channel segments can be formed in sheet 10, with configurations to meet particular arrangements of semiconductor devices.
To illustrate a further benefit provided by embodiments of the invention, FIG. 4 shows semiconductor device 46 provided with a height mark 46 a. This mark represents the minimum height that device 46 could have, and still be within its prespecified tolerance. FIG. 4 further shows that device 46 exceeds the minimum height requirement 46 a, by an amount Δ. However, by constructing TCN 16 as described above, the resilient components 38 enable contact member 16 a to be adjusted or offset by the same amount Δ, while remaining in close contact with device 46 to provide effective heat transfer. As viewed in FIG. 4, member 16 a is moved upward by the amount Δ, to accommodate the height by which component 46 exceeds its minimum allowable height. At the same time, the resiliency of components 38 prevent device 46 or TCN 16 from being subjected to undue stress, and avoids exceeding elastic limits thereof.
In a modification of the embodiment shown in FIG. 1, one or more TCNs and channel segments, having features similar to those described above in connection with sheet 10, may also be formed in sheet 12. The resulting modified heat exchanger could then be placed between two configurations of semiconductor devices, with one configuration being cooled by the TCN's of sheet 10, and the other configuration by the TCN's of sheet 12.
In a further modification, before or after forming any TCNs or channel segments, the interior sides of both sheets 10 and 12 would be coated with a metal referred to as a barrier metal. This metal does not react with the liquid that is to be used as the coolant fluid. Use of the barrier metal thus reduces interior corrosion of the heat exchanger.
In yet another modification, the cross-sections of one or more channel segments could be made larger than the cross-sections of other sections, to increase the rate at which coolant flows away from a particular TCN. For example, if coolant is flowing from channel end 22 a, through respective channel segments and TCNs 14 and 16 to channel end 22 a, the diameter of channel segment 18 could be made greater than the diameter of segment 22. This would increase the rate at which coolant flowed away from TCN 16, and would thus increase the capacity of TCN 16 to dissipate heat. As an alternative, two or more channel segments could be formed in sheet 10, to carry heat away from TCN 16.
Referring to FIG. 5, there is shown a TCN 52 similar to TCNs 14 and 16. TCN 52 thus comprises a planar contact member 52 a and side sections 52 b-52 e which collectively form a coolant chamber. In a modification of the invention, structure 54, comprising a series of waves, or hills and valleys is formed as part of TCN 52 that is, integral with other components of TCN 52 or in situ. Structure 54 is therefore contained in the coolant chamber of TCN 52, and is formed integral with and supported upon member 52 a.
By placing the structure 54 in the coolant chamber of TCN 52, coolant flowing through the chamber will become quite turbulent. This turbulence, in turn, will cause the fluid to be much more effective in dissipating heat that has been transferred to fluid in the chamber, from a semiconductor device in contact with member 52 a.
Referring to FIG. 6, there is shown a TCN 56 similar to TCNs 14 and 16. TCN 56 thus comprises a planar contact member 56 a and side sections 56 b-56 e which collectively form a coolant chamber. In a further modification of the invention, structure 58, similar to structure 54 of FIG. 5, comprises a series of waves, or hills and valleys. However, structure 58 is formed independently of TCN 56, and is placed into the coolant chamber of TCN 56 after TCN 56 has been formed. Structure 58 causes turbulence of the coolant in the chamber of TCN 56, in like manner with structure 54.
The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.
The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the Claims below are intended to include any structure, material, or act for performing the function in combination with other Claimed elements as specifically Claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.
The invention can take the form of an entirely hardware embodiment, an entirely method embodiment or an embodiment containing both hardware and method elements. In a preferred embodiment, the invention is implemented in process, which includes but is not limited to real components and parts and specific process steps to design, fabricate and utilize the invention.
The description of the present invention has been presented for purposes of illustration and description, and is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiment was chosen and described in order to best explain the principles of the invention, the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.

Claims

1. A method for constructing a heat exchanger for cooling one or more semiconductor components, said method comprising the steps of:

providing first and second planar sheets of specified thermally conductive metal foil, wherein each of the sheets has an exterior side and an interior side;

forming one or more thermal contact nodes (TCNs) in the first sheet, wherein each TCN extends outward from the exterior side of the first sheet, and comprises a planar contact member and one or more side sections, the side sections and contact member of a TCN collectively forming a coolant chamber;

configuring channel segments along the interior side of the first sheet, wherein each channel segment extends between the coolant chambers of two or more TCNs, or between the coolant chamber of a TCN and an input port or output port, selectively; and

joining the interior side of the second sheet to the interior side of the first sheet, in order to form a sealed flow path that includes each channel segment, and enables liquid coolant to flow into and out of the coolant chamber of each TCN.

2. The method of claim 1, wherein:

the side sections of at least one of said TCNs respectively include resilient components that collectively enable the contact member of the TCN to move toward and away from the exterior side of the first sheet.

3. The method of claim 1, wherein:

a plurality of TCNs are formed in said first sheet, wherein each TCN has a dimension measured along a Z-axis that is orthogonal to said first sheet, and the Z-axis dimension of one of said TCNs is different from the Z-axis dimension of another of said

4. The method of claim 1, wherein:

the resilient component of each of said side sections comprises a bellows structure.

5. The method of claim 1, wherein:

one or more TCNs are formed in the second sheet, wherein each TCN formed in the second sheet extends outward from the exterior side of the second sheet, and comprises a planar contact member and one or more side sections.

6. The method of claim 1, wherein:

the cross-section of one or more of said channel segments is made different from the cross-section of one or more other channel segments in order to cause said coolant to flow out of the coolant chamber of at least one of said TCNs at a different rate than it flows out of the coolant chamber of another of said TCNs.

7. The method of claim 1, wherein:

selected structure is placed in a given coolant chamber, to cause turbulence of coolant flowing through the given coolant chamber.

8. The method of claim 1, wherein:

said TCNs and channel segments are formed in the first sheet by means of an embossing process.

9. The method of claim 1, wherein:

a laser welding process is used to join said first and second sheets together at regions that respectively surround each of said TCNs and each of said channel segments.

10. The method of claim 1, wherein:

prior to forming said TCNs and configuring said channel segments, the interior sides of said first and second sheets are each coated with a selected barrier metal that does not react with said coolant.

11. The method of claim 1, wherein:

said channel segments and coolant chambers collectively define a path of flow for said coolant from said input port to said output port.

12. Heat exchanger apparatus for cooling one or more semiconductor components, said apparatus comprising:

a first planar sheet of specified thermally conductive foil that has an exterior side and an interior side, wherein one or more thermal contact nodes (TCNs) are formed in the first sheet, each TCN extending outward from the exterior side of the first sheet and comprising a planar contact member and one or more side sections, the side sections respectively including resilient components that collectively enable the contact member of the TCN to move toward and away from the exterior side of the first sheet, the side sections and contact member of a TCN collectively forming a coolant chamber, and a channel segment is configured along the interior side of the first sheet, wherein each channel segment extends between the coolant chambers of two or more TCNs, or between the coolant chamber of a TCN and an input port or an output port, selectively;

a second planar sheet of said specified thermally conductive foil that has an exterior side and an interior side; and

means for joining the interior side of the second sheet to the interior side of the first sheet, in order to form a sealed flow path that includes each channel segment, and enables liquid coolant to flow into and out of the coolant chamber of each TCN.

13. The apparatus of claim 12, wherein:

the resilient component of each side section comprises a bellows structure.

14. The apparatus of claim 12, wherein:

the cross-section of one or more of the channel segments is made different from the cross-section of at least one or more other channel segments, in order to cause said coolant to flow out of the coolant chamber of one or more of said TCNs at a different rate than it flows out of the coolant chamber of another of said TCNs.

15. The apparatus of claim 12, wherein:

selected liquid flow turbulence structure is placed in a given coolant chamber, to cause turbulence of coolant flowing through the given coolant chamber in order to increase the thermal transfer efficiency of the TCN.

16. The apparatus of claim 12, wherein:

17. The apparatus of claim 12, wherein:

prior to forming said TCNs and configuring said channel segments, the interior sides of the first and second sheets are each coated with a selected barrier metal that does not react with said coolant.

18. A method for constructing a heat exchanger for cooling one or more semiconductor components, said method comprising the steps of:

configuring channel segments along the interior side of the first sheet, wherein each channel segment extends between the coolant chambers of two or more TCNs, or between the coolant chamber of a TCN and an input port or output port, selectively;

joining the interior side of the second sheet to the interior side of the first sheet, in order to form a sealed flow path that includes each channel segment, and enables liquid coolant to flow into and out of the coolant chamber of each TCN;

an input coolant connector joined to said input port for receiving coolant from a coolant circulating mechanism; and

an output coolant connector joined to said output port for returning coolant to the coolant circulating mechanism.

19. The method of claim 18, wherein:

a first one of said TCNs is adapted to contact a first semiconductor component, and a second one of said TCNs is adapted to contact a second semiconductor component, wherein said first and second semiconductor components are adjacent to each other, and have respective height dimensions that are different from each other.

20. The method of claim 18, wherein: