WO2016149269A1

WO2016149269A1 - Comprehensive layout strategy for flip chipping integrated circuits

Info

Publication number: WO2016149269A1
Application number: PCT/US2016/022469
Authority: WO
Inventors: Chenhui JIANG; Lars Bretschneider; Ulrich Keil
Original assignee: Fci Asia Pte. Ltd; Fci Americas Technology Llc
Priority date: 2015-03-19
Filing date: 2016-03-15
Publication date: 2016-09-22

Abstract

In accordance with an example embodiment of the present invention, an apparatus is disclosed. The apparatus includes a substrate and a signal trace at the substrate. The signal trace has a first trace portion with a first trace width and a second trace portion with a second trace width. The second trace width is greater than the first trace width.

Description

Comprehensive Layout Strategy for Flip Chipping

Integrated Circuits

BACKGROUND

Field of the Invention

[0001] The present invention relates to a transmission line, and in particular to a transmission line of a printed circuit board (PCB) .

Brief Description of Prior Developments

[0002] As known in the art, a printed circuit board (PCB) is a plate, or a board, which is used to mechanically support and electrically connect different elements. The PCB thereby contains electrical transmission lines which provide the electric interconnections between the different elements. These transmission lines are also called interconnects and can be present e.g. in form of copper tracks. The transmission lines, or conductive tracks, can be provided within a single layer (single sided PCB) , or more layers (double sided or multi-layer PCBs) of a PCB.

[0003] The transmission lines may be used to convey microwave frequency signals between the electronic components of a PCB. As known in the art, e.g. wave guides or microstrips can be used as electrical transmission lines . The transmission lines are usually provided by laminating copper sheets onto a non- conductive substrate and subsequently etching conductive tracks, pads and other features therein. [0004] Where external electric components are to be placed, the PCB usually has pads, or solder pads, which can be tin-lead, silver or gold-plated copper pads. An integrated circuit (IC) is a small semiconductor wafer on which several electronic components and electronic circuits are provided. Hence, also an IC can feature transmission lines to connect components provided thereon .

[0005] As known by those skilled in the art, several techniques exist for assembling one or more ICs on a PCB. With the so-called flip-chip technique, conductive bumps provided on a surface of an IC are bonded or coupled to respective flip-chip pads provided on the respective surface of a PCB in order to provide electrical interconnections. Such resulting flip-chip assemblies are smaller than traditional systems and feature reduced inductance .

[0006] A High-Density-Interconnect PCB (HDI PCB) is a very compact PCB. HDI PCBs thereby feature among others a fine structuring of the conduction tracks. Especially the flip-chip bonding technology has been widely used in assembling ICs on such HDI PCBs. To increase the compactness of PCBs, solder pads are provided at close distance to each other on a PCB. Care has then to be taken when external contacts are soldered to the solder pads, such that no electrical short circuits arise. For these reasons, so-called solder dams are provided. However, when the pitch of the solder pads reaches about 100 μm, and the dimensions of the pads itself are of tens of μm, the flip-chip process becomes very challenging. [0007] It is therefore an object of the present invention to provide an improved layout strategy for PCBs . It is in particular an object of the present invention to provide an optimized transmission line for connecting solder pads in a more efficient way. Another object of the present invention is to provide an improved structuring of conductive tracks for interconnecting elements on HDI PCBs.

[0008] These and other objects, which will become apparent by reading the following description, are solved by the subject matter of the independent claims.

SUMMARY

[0009] According to the present invention there is provided a transmission line. The transmission line is thereby practicable for the application in PCBs, and in particular for the application in HDI PCBs. However, it will be appreciated that the inventive transmission line can also be used in other applications. They can thus for example be utilized to provide electrical connections on ICs.

[0010] The term "line" is thereby not limiting to a particular structure, and in particular not to a certain shape. It will be appreciated that the transmission line can be present in form of a rather straight line, or curve, or other suitable structure.

[0011] The transmission line of the present invention comprises a signal trace having a signal trace length and having a signal trace width measured in a direction perpendicular to the signal trace length. Preferably, the signal trace length is measured in longitudinal direction of the signal trace . Accordingly, dimensions of the signal trace can be characterized in terms of a signal trace length and a signal trace width. The signal trace can thereby be made of any material suitable for transferring signals. However, the signal trace is preferably made of copper (the term copper means including copper alloys) . Accordingly, the signal trace allows for signal transfer along the signal trace, thereby providing desired electrical connection between different components of e.g. a PCB or a IC, or between both .

[0012] The inventive signal trace has a first trace portion with a first trace width and a second trace portion with a second trace width. The second trace width is thereby greater than the first trace width. Accordingly, the width of the second trace portion is greater than the width of the first trace portion of the signal trace. In other words, the width of the signal trace varies along the signal trace length.

[0013] The inventive transmission line further comprises a solder pad positioned on the signal trace. As will be appreciated, the solder pad can be provided in any known manner. The geometry of the solder pad can thereby be defined with respect to the application. For example, small rectangular solder pads with an edge length of 50 - 100 μm can be positioned on the signal trace .

[0014] This inventive design of a transmission line with variable width allows for optimizing the performance, in particular with flip-chip bonding technology. Depending on the application, the first trace width and the second trace width can be adjusted to improve the performance of the transmission line and to obtain desired, application-specific characteristics.

[0015] Preferably, the solder pad is positioned on the first trace portion of the signal trace, and the solder pad is preferably not positioned on the second trace portion of the signal trace. Accordingly, the solder pad is positioned on the first trace portion which has a width being less than the width of the second trace portion. If the transmission line is for example part of high-speed data transmission networks, the second trace width can be adjusted relative to the first trace width to obtain a desired value of impedance. As known by the skilled person the characteristic impedance of a signal trace (or e.g. of a microstrip) changes slightly with frequency. For the most part, however, the impedance depends on the dimensions of a signal trace. For example, by increasing the width of a microstrip the characteristic impedance thereof is lowered. Accordingly, by adjusting the first trace width and second trace width, a specifically desired characteristic impedance can be set.

[0016] If the transmission line is employed in power supply applications, the first trace width and second trace width can be adjusted to minimize the parasitic resistance and inductance. Accordingly, the inventive design of a signal trace having variable width allows for optimizing the transmission line characteristics in a flexible way depending on the application. [0017] In a preferred embodiment, the inventive transmission line further comprises a second solder pad positioned on the signal trace, wherein the second trace portion is positioned between the solder pad and the second solder pad. In other words, the solder pad and the second solder pad are each positioned on a respective first trace portion having a first trace width less than the width of the second trace portion, which second trace portion in turn is located in between, i.e. between the solder pad and the second solder pad. Thus, the width of the second trace portion, and thus the transmission characteristics of the second trace portion located between the two adjacent solder pads, can be adjusted mostly independent of the structuring of the solder pads.

[0018] Preferably the first trace portion is having a first trace length and the second trace portion is having a second trace length, wherein the second trace length is being greater than the first trace length. Preferably, the second trace width is twice the first trace width. However, the skilled person understands that the second trace width can also be more or less than twice the first trace width. Depending on the particular application, an optimized second trace width relative to the first trace width can be set.

[0019] In another preferred embodiment, the solder pad has a solder pad width being greater than the first trace width. Accordingly, the solder pad covers the first trace portion of the signal trace. In another preferred embodiment the solder pad has a solder pad width being less than the first trace width. Again, the skilled person understands to choose a proper solder pad width relative to the first trace width depending on the particular application.

[0020] Preferably the solder pad has a solder pad width being less than the second trace width. Accordingly, the width of the second trace portion of the signal trace is greater than the width of the solder pad. This can in particular be advantageous for applications where relatively high currents are passed along the signal trace.

[0021] In a preferred embodiment the inventive transmission line further comprises a solder dam which is positioned on the signal trace. Preferably, the solder dam is positioned on the second trace portion of the signal trace. Said solder dam thereby prevents solder from running from one solder pad to another one, i.e. providing electrical short circuits. Preferably, the solder pads are positioned on the first trace portion of the signal trace, and the second trace portion having a width being greater than the width of the first trace portion is covered by the solder dam. Accordingly, a safe application with e.g. flip-chip technology is enabled, in particular where the pitch between adjacent solder pads is small.

[0022] Preferably the length of the second trace portion is in the range of 100 - 200 pm. Accordingly, the width of the second trace portion can be varied along its length. The preferred utilization of solder dams thereby allows for secure soldering despite the short length of the second trace portion, i.e. the relatively short distance between the solder pads provided preferably on first trace portions of the signal trace. [0023] Preferably the signal trace is having a third trace portion with a third trace width equal to the first trace width and a fourth trace portion with a forth trace width equal to the second trace width, and wherein the second trace portion is provided between the first trace portion and the third trace portion, and wherein the third trace portion is provided between the second trace portion and the fourth trace portion. Accordingly, the width of the signal trace is varying in an alternating way.

[0024] In a further preferred embodiment the signal trace has a transition portion which is located between the first trace portion and the second trace portion. Said transition portion has a non-constant width ranging from the first trace width to the second trace width. Accordingly, such transition portion allows for a smooth transition from the first trace portion to the second trace portion, improving the electric characteristics of the transmission line.

[0025] Preferably, the non-constant width of the transition portion is linearly tapered. Accordingly, the width of the transition portion increases linearly, preferably from the first trace width to the second trace width. Alternatively, the periphery of the transition portion is S-shaped. Accordingly, the width of the transition portion increases non-linearly, but still in a rather gradual way. In other words, the transition from the first trace width to the second trace width is not abrupt. The skilled person understands that the transition portion can also have other non-linearly varying widths, which are still providing a smooth transition from the first trace portion to the second trace portion.

[0026] Further preferred, at least one of the first trace portion, the second trace portion and the transition portion is an integral part of the signal trace. Most preferred, all three portions are integral parts of the signal trace, thereby providing homogeneous electronic properties. It is further preferred that the first, second and transition portions are made of the same material, such as e.g. copper.

[0027] It will be appreciated that the inventive transmission line can be utilized in numerous applications. Preferably, it is applied in PCBs, or HDI PCBs for providing interconnections between components, which in turn are preferably electrically coupled with the solder pads. Alternatively the inventive transmission line can also be applied in ICs. Due to the inventive design of the transmission line, improved transmission characteristics can be obtained, even in confined scenarios which are present e.g. with flip-chip technology.

[0028] The skilled person understands that the signal trace can feature several first trace portions, as well as several second trace portions. The signal trace can comprise alternating first and second portions, along at least a certain section of the signal trace. Preferably also other portions may be present, e.g. between the alternating first and second portions.

[0029] The present invention further provides a method for tuning a transmission line. Preferably the transmission line corresponds to an inventive transmission line detailed above. The method thereby comprises providing a signal trace that defines a first trace width and a second trace width, whereby the second trace width is being greater than the first trace width. The method further comprises providing a solder pad on the signal trace, with the solder pad having a solder pad length and having a solder pad width measured in a direction perpendicular to the solder pad length.

[0030] Further on, the method comprises the step of tuning the first trace width, the solder pad width and/or the solder pad length to optimize soldering areas, and/or tuning the second trace width to improve power or ground connections or high-speed signal connections. Accordingly, depending on the application, the transmission line can be tuned to obtain desired characteristics by tuning the first trace width, the second trace width, the solder pad width and/or the solder pad length.

[0031] It is thus possible to optimize the layout depending on the certain application of the transmission line. Besides increasing the performance of the transmission line, also the soldering areas can be optimized. As known by the skilled person, the geometry and structure of the solder pads should be chosen in accordance with the applied soldering technology.

[0032] Preferably the signal trace has a first trace portion with the first trace width and a second trace portion with the second trace width, and the solder pad is provided on the first trace portion of the signal trace. It is thus possible, by varying the width of the signal trace along the transmission line, to tune the performance. This tuning can thereby be performed such that desired structuring of the solder pads can be chosen depending on the respective elements to be coupled thereto, and only the width of the second trace portion is adjusted to achieve preferred transmission characteristics along the signal trace and thus along the transmission line.

[0033] According to the present invention there is further provided a method for manufacturing a transmission line, preferably for application with a PCB or IC. Said method thereby preferably allows for manufacturing an inventive transmission line outlined above. The method thereby comprises the step of applying a metal sheet. Preferably, a metal is cast on a substrate to thereby form the metal sheet covering the substrate at least partially.

[0034] The method for manufacturing further comprises partially etching the metal sheet to form a signal trace. Said signal trace is thereby having a signal trace length and also a signal trace width measured in a direction perpendicular to the signal trace length. The formed signal trace thereby has a first trace portion with a first trace width and a second trace portion with a second trace width_/ whereby the second trace width is being greater than the first trace width.

[0035] Accordingly, by partially etching the metal sheet the inventive structuring of the signal trace can be obtained. The variable width of the signal trace can thereby be adjusted depending on the certain application of the transmission line. [0036] In a further step a solder pad is provided on the signal trace. Preferably the solder pad is provided on the first trace portion of the signal trace, having a width less than the second trace portion. Further preferred, a solder dam is provided on the signal trace, wherein the solder dam is preferably provided on the second trace portion of the signal trace . Accordingly, the solder dam is positioned adjacent to the solder pad and prevents short-circuits during a soldering process.

[0037] It will be appreciated that the inventive design of a signal trace, or of conductive tracks, can be combined with other structuring technology known in the art. In particular, the inventive transmission line and signal trace can be integrated in existing layouts and structures, such that the benefits of the present invention can be exploited .

[0038] The inventive transmission line can feature a signal trace with only two different widths. Preferably, the second trace portion can have an undulating contour. In another preferred invention, the contour of the second trace portion can be circular. The skilled person understands that different shapes of the signal trace come along with different transmission characteristics of the transmission line. Depending on the particular application, the skilled person can choose a suitable shape to benefit from the variable width of the signal trace .

[0039] The term "width" used within the present description refers to a dimension perpendicular to a longitudinal direction of e.g. the signal trace. Accordingly, the width can be measured in-plane of the surface of the substrate, or also be measured out-of- plane. However, it will be appreciated that the variable width of the signal trace refers to a variation of width measured in-plane. In a preferred embodiment, the width of the signal trace varies also out-of-plane in the sense of the present invention, further improving transmission characteristics of the transmission line.

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] In the following various exemplary embodiments of the present invention are described in more detail with reference to the enclosed figures, wherein like parts have the same reference signs.

[0041] Figure 1 schematically illustrates the layout of an inventive transmission line according to one embodiment of the invention, and

[0042] Figure 2 schematically illustrates the layout of an inventive transmission line according to another embodiment .

DETAILED DESCRIPTION

[0043] Referring to Fig. 1, there is shown a schematic view of an apparatus 100 incorporating features of the invention. Although the invention will be described with reference to the exemplary embodiments shown in the drawings, it should be understood that the invention can be embodied in many alternate forms of embodiments. In addition, any suitable size, shape or type of elements or materials could be used. [0044] As can be seen from Fig. 1, there are provided two metal strips 10, or signal traces, which are aligned parallel to each other. On each of the illustrated metal strips there are provided two solder pads 20, which function as flip-chip pads in flip-chip technology. The solder pads 20 are of rectangular shape. The metal strips

10 have thinner portions 11, on which also the pads 20 are provided. The width of the pads 20 is larger than that of the thin portions 11. The thin portions 11 are separated from each other by wide portions 12. As can be seen from Fig. 1, the transition from the thin portion 11 to the wide portion 12 is not abrupt, but rather smooth. Thus, there is no abrupt step in width between the thin portions 11 and the wide portions 12. The thin portions

11 are thus first trace portions having a first trace width and the wide portions 12 are second trace portions having a second trace width, such that the second trace width is greater than the first trace width.

[0045] The wide portions 12 are provided between the solder pads 20, and can have a width being greater than the width of the solder pads 20. Above the wide portions

12 there are provided solder dams 30, preventing solder from creating short-circuits during a later soldering process .

[0046] Preferably the dimensions of the single elements illustrated in Fig. 1 are small, i.e. in the micrometer to millimeter range. The edge length of the solder pads 20 can for example be in the range of 50 - 100 μm. The width of the thin portions 11 is thereby less than the edge length of the rectangular solder pads 20 such that the solder pads 20 cover the thin portions 11. [0047] The length of the wide portions 12 is preferably in the range of 100 - 200 μm. The length of the dams 30 is thus also in said range, since the dams cover only the wide portions 12, but not the thin portions 11.

[0048] As wild be appreciated by those skilled in the art, the desired form of the metal strips 10 or signal traces can be set during the standard manufacturing of PCBs. For example, after a metal sheet mask is applied to a substrate 15, said metal sheet can be etched selectively to thereby create metal strips 10 of the inventive design. Afterwards a solder mask can be applied and again etched in order to form e.g. solder dams.

[0049] The layout 200 presented in Fig. 2 is similar to that of Fig. 1. However, the solder pads 20 have a width being less than the width of the thin portions 11 of the metal strips 10, on which thin portions 11 the solder pads 20 are provided. Further, the thin portions 11 do not feature a uniform width, i.e. they do not have a constant width along their length. The shape of the thin portions 11 is rather oval-like. Nevertheless, the maximal width of the thin portions 11 is less than the width of the wide portions 12, which are again covered by solder dams 30. It will thus be appreciated that the thin portions 11 and wide portions 12 can be of various forms, whereby the widths of the portions are chosen in the sense of the present invention to achieve the desired characteristics . It will further be appreciated that the transition between the thin portion and wide portion can be rather smooth, as e.g. illustrated in Fig. 1, or rather abrupt, as e.g. illustrated in Fig. 2. [0050] Reference chart:

10 metal strip

11 thin portion {first trace portion)

12 wide portion (second trace portion) 15 substrate

20 solder pad

30 solder dam

[0051] Below are provided further descriptions of various non-limiting, exemplary embodiments. The below- described exemplary embodiments may be practiced in conjunction with one or more other aspects or exemplary embodiments. That is, the exemplary embodiments of the invention, such as those described immediately below, may be implemented, practiced or utilized in any combination (e.g., any combination that is suitable, practicable and/or feasible) and are not limited only to those combinations described herein and/or included in the appended claims .

[0052] In one exemplary embodiment, an apparatus is disclosed. The apparatus comprises a substrate, and a signal trace at the substrate, wherein the signal trace has a first trace portion with a first trace width and a second trace portion with a second trace width, and wherein the second trace width is greater than the first trace width. [0053] The apparatus as above wherein the first trace portion comprises a first trace length, wherein the second trace portion comprises a second trace length, and wherein the second trace length is greater than the first trace length .

[0054] The apparatus as above wherein the apparatus further comprises a solder pad on the signal trace.

[0055] The apparatus as above wherein a solder pad width is greater than the first trace width.

[0056] The apparatus as above wherein a solder pad width is less than the second trace width.

[0057] The apparatus as above wherein the apparatus further comprises another solder pad on the signal trace.

[0058] The apparatus as above wherein the apparatus further comprises a solder dam on the signal trace.

[0059] The apparatus as above further comprising a transition portion between the first trace portion and the second trace portion.

[0060] The apparatus as above further comprising a third trace portion and a fourth trace portion, wherein the third trace portion comprises a third trace width substantially equal to the first trace width, and wherein the fourth trace portion comprises a forth trace width substantially equal to the second trace width.

[0061] The apparatus as above wherein the apparatus is a printed circuit board (PCB) . [0062] The apparatus as above wherein the apparatus is a High-Density-Interconnect printed circuit board (HDI PCB) .

[0063] The apparatus as above wherein the apparatus is an integrated circuit (IC) .

[0064] In another exemplary embodiment_/ a method of manufacturing a transmission line is disclosed. The method includes providing a substrate, providing a metal sheet at the substrate, and partially etching the metal sheet to form a signal trace, wherein the signal trace has a first trace portion with a first trace width and a second trace portion with a second trace width, and wherein the second trace width is greater than the first trace width.

[0065] The method above wherein a metal is cast on the substrate to form the metal sheet.

[0066] The method above further comprising providing a solder pad on the signal trace, wherein the solder pad is on the first trace portion of the signal trace, and wherein the solder pad comprises a width less than the second trace width.

[0067] The method above further comprising providing a solder dam on the signal trace, wherein the solder dam is provided on the second trace portion of the signal trace.

[0068] In another exemplary embodiment, a method of tuning a transmission line is disclosed. The method includes providing a signal trace comprising a first trace width and a second trace width, wherein the second trace width is greater than the first trace width, providing a solder pad on the signal trace, wherein the solder pad comprises a solder pad length and a solder pad width, and varying the width of the signal trace along the transmission line to tune the performance of the transmission line.

[0069] The method above further comprising varying the width of the first trace width and varying the solder pad width to optimize soldering areas.

[0070] The method above further comprising varying the solder pad length to optimize soldering areas.

[0071] The method above further comprising tuning the second trace width to improve power or ground connections or high-speed signal connections.

[0072] It should be understood that the foregoing description is only illustrative of the invention. Various alternatives and modifications can be devised by those skilled in the art without departing from the invention. Accordingly, the invention is intended to embrace all such alternatives, modifications and variances which fall within the scope of the appended claims .

Claims

CLAIMS What is claimed is:

1. An apparatus comprising: a substrate; and a signal trace at the substrate, wherein the signal trace has a first trace portion with a first trace width and a second trace portion with a second trace width, and wherein the second trace width is greater than the first trace width.

2. The apparatus of claim 1 wherein the first trace portion comprises a first trace length, wherein the second trace portion comprises a second trace length, and wherein the second trace length is greater than the first trace length.

3. The apparatus of claim 1 wherein the apparatus further comprises a solder pad on the signal trace.

4. The apparatus of claim 3 wherein a solder pad width is greater than the first trace width.

5. The apparatus of claim 3 wherein a solder pad width is less than the second trace width.

6. The apparatus of claim 3 wherein the apparatus further comprises another solder pad on the signal trace.

7. The apparatus of claim 1 wherein the apparatus further comprises a solder dam on the signal trace.

8. The apparatus of claim 1 further comprising a transition portion between the first trace portion and the second trace portion.

9. The apparatus of claim 1 further comprising a third trace portion and a fourth trace portion, wherein the third trace portion comprises a third trace width substantially equal to the first trace width, and wherein the fourth trace portion comprises a forth trace width substantially equal to the second trace width.

10. The apparatus of claim 1 wherein the apparatus is a printed circuit board (PCB) .

11. The apparatus of claim 1 wherein the apparatus is a High-Density-Interconnect printed circuit board (HDI PCB) .

12. The apparatus of claim 1 wherein the apparatus is an integrated circuit (IC) .

13. A method of manufacturing a transmission line comprising: providing a substrate; providing a metal sheet at the substrate; and partially etching the metal sheet to form a signal trace, wherein the signal trace has a first trace portion with a first trace width and a second trace portion with a second trace width, and wherein the second trace width is greater than the first trace width.

14. The method of claim 13 wherein a metal is cast on the substrate to form the metal sheet.

15. The method of claim 13 further comprising providing a solder pad on the signal trace, wherein the solder pad is on the first trace portion of the signal trace, and wherein the solder pad comprises a width less than the second trace width.

16. The method of claim 13 further comprising providing a solder dam on the signal trace, wherein the solder dam is provided on the second trace portion of the signal trace.

17. A method of tuning a transmission line comprising: providing a signal trace comprising a first trace width and a second trace width, wherein the second trace width is greater than the first trace width; providing a solder pad on the signal trace, wherein the solder pad comprises a solder pad length and a solder pad width; and varying the width of the signal trace along the transmission line to tune the performance of the transmission line.

18. The method of claim 17 further comprising varying the width of the first trace width and varying the solder pad width to optimize soldering areas.

19. The method of claim 18 further comprising varying the solder pad length to optimize soldering areas.

20. The method of claim 17 further comprising tuning the second trace width to improve power or ground connections or high-speed signal connections.