The disclosure is directed to designs of waveguides surrounded by a heat transfer element or a thermal path for a PCB and methods for fabricating the PCB including a combination of waveguides and heat transfer elements.
Printed Circuit Board (PCB) Assemblies may be formed from multi-layer PCBs having Surface Mount Technology (SMT) components of integrated circuits (ICs). As SMT components and ICs require more power in combination with a continuing trend towards miniaturization, thermal management on the PCB assemblies becomes a greater challenge to manage.
PCB assemblies typically have thermal conductivity ranging from 0.25 W/mK to 3 W/mK, which results in a high thermal resistance through the PCB and consequently large temperature variations in the PCB. Typical applications for dissipation of significant power use a thermal coin. Specifically, copper coins are inserted into a PCB to help conduct the heat away from heat sources, such as ICs, die, or other components, to a heatsink underneath the PCB. In the coin manufacturing process, a hole is cut in the PCB and a thermally conductive coin, such as a copper coin, is inserted into the hole. However, the current manufacturing process for producing PCBs with copper coins is labor-intensive and expensive. As such, there exists a need for a more cost-effective method to provide coin-like thermal pathways.
Printed Circuit Board Assemblies (PCBAs) include Printed Circuit Board(s) (PCB) with Surface Mount (SMT) components soldered to the surface of the PCB(s). The SMT components dissipate power. A primary thermal path is through the PCB to a heatsink (HS). Thermal conductivity is a measure of a material's ability to conduct heat and is a material property. The thermal conductivity of typical PCB dielectric materials ranges from 0.25 W/mK to 3 W/mK, which results in a high thermal resistance through the PCB and consequently generates a large temperature delta in the PCBs. The high thermal resistance needs to be lowered to reduce the large temperature delta.
To improve the thermal path through PCBs, thermal vias may be added around the power dissipating components. A conventional way of the thermal vias is accomplished by adding Plated Through Holes (PTHs) under and around the SMT components. The method is referred to as a PTH approach. The PTHs are filled with copper, which has a thermal conductivity of about 390 W/mK which is much higher than that of the PCB, for example, 0.025 W/mk to 3 W/mk. The closer the PTHs can be positioned together, the lower the thermal resistance of the PCB would be. Based on manufacturing design rules, the standard minimum distance between drilled holes or the PTHs is one drill diameter.
When Integrated Circuits (ICs) become more power hungry, coupled with the trend of miniaturization, the thermal management on the PCB becomes a bigger challenge. The PTH approach may not reduce the thermal resistance sufficiently for applications that dissipate significant power. When the thermal vias are not sufficient to improve the thermal path, pre-fabricated copper coins may be inserted into the PCB to improve the thermal path and help conduct the heat away from heat sources, such as integrated circuits (IC), die, or SMT components, among others, to a heatsink underneath the PCB. Inserting a pre-fabricated coin is a labor-intensive manual process. In this process, a hole can be cut in the PCB and the pre-fabricated coin (e.g. commonly formed of copper) is inserted into the PCB, which is referred to as a coin approach. The manufacturing of PCBs with pre-fabricated coins is very labor-intensive and consequently expensive. Usually, the coin approach is cost-prohibitive except for use in low-volume high-performance assemblies. Additionally, there is a tolerance stack-up issue with inserted coins. Each of the coins and the PCB has a thickness with a thickness tolerance. Both the coins and the PCB have thickness variations independently from lot to lot. Another limitation of the inserted coins is that the coins have limited connection or no connection to the PCB ground. Electrical ground connections are critical for radio frequency (RF) and high-speed digital applications.
PCB may include one or more openings through the PCB forming waveguides for mmWave radars. It is desirable to have all components on one side of the PCB and have antennas and feed networks on the other side of the PCB. The trend is to feed RF signals directly from a chip on one side of the PCB, through formed waveguides in the PCB, into a metallic antenna feed-network structure. The antenna may be integrated into the feed-network structure. The waveguide is a structure that guides waves, such as electromagnetic waves, with minimal loss of energy by restricting the transmission of energy to one direction. The interior walls of the waveguide may be made of copper, silver, aluminum, or any metal that has a low electrical resistivity. For example, the interior walls are plated with metal.
In particular, a metallic antenna feed-network structure may be mounted flush to the PCB, aligned with the waveguide openings in the PCB. The metallic antenna feed-network structure may face away from the automobile or communication base station and may either form the outer part of the enclosure directly or may have a radome on top, which means that it also serves as part of a heatsink element for the system. The heatsink is a passive heat exchanger that transfers the heat generated by an electronic or a mechanical device to a fluid medium, often air or a liquid coolant, to dissipate the heat away from the device. A metallic antenna feed network mounted to the PCB furthermore spreads the heat in a plane with the PCB and thus provides a larger area for heat dissipation, a heat buffer for pulsed operation, and allows for heat transport back up through to the component side of the PCB in otherwise unused areas to a heatsink on the component side if desired. The heatsink may be used with high-power semiconductor devices such as power transistors and optoelectronics such as lasers and light-emitting diodes (LEDs), where the heat dissipation ability of the component itself is insufficient to moderate its temperature. The heatsink may be designed to maximize its surface area in contact with the cooling medium, such as the aft. Air velocity, choice of material, protrusion design, and surface treatment are factors that affect the performance of a heatsink. The heatsink attachments and thermal interface materials may also affect the die temperature of the integrated circuit and thus affect the performance and lifetime of the chip.
The conventional waveguides are normally routed into the PCB and then the routed walls are plated with a thin layer of copper (e.g., having a thickness of less than 1 mil). The conventional waveguides are separated from the coin. Also, conventionally, the waveguide in the PCB is constructed by drilling closely spaced holes into the PCB and then subsequently plating the walls of the waveguide having an oblong rounded rectangular shape. Since drill bits cannot drill square features or arbitrary features, the waveguides were limited to rectangular shapes with rounded corners.
There remains a need to address the limitations and manufacturing issues of waveguides and heat transfer elements or thermal paths for the PCBs.
In one aspect, a method is provided for forming waveguides in a PCB. The method may include forming an opening in a PCB core comprising a plurality of conductive layers interleaved with a plurality of insulating layers, the opening extending from a first side of the PCB core to a second side of the PCB core. The method may also include filling the opening with metal. The method may also include forming a cavity enclosed by sidewalls by removing a first portion of the filled opening, the cavity extending from the first side of the PCB core to the second side of the PCB core. A second portion of the filled opening is a heat transfer element configured to transfer heat from the first side of the PCB core to the second side of the PCB core. The at least one waveguide is embedded within the heat transfer element and configured for transmitting signals from the first side to the second side.
In some variations, the cavity forms a waveguide configured to transfer signals from a first side to a second side.
In some variations, the removing of a first portion of the filled opening is done by etching.
In some variations, the sidewalls of the formed cavity are plated.
In some variations, the sidewalls are substantially perpendicular to the first side and the second side of the PCB core.
In another aspect, a method for forming waveguides in a PCB is provided.
The method may include forming an opening in a PCB core comprising a plurality of conductive layers interleaved with a plurality of insulating layers. The opening may extend from the first side of the PCB core to the second side of the PCB core. The method may also include filling the opening with metal. The method may also include forming a cavity enclosed by sidewalls perpendicular to the first side and the second side of the PCB core by etching a first portion of the filled opening. The cavity may extend from the first side of the PCB core to the second side of the PCB core to form at least one waveguide. The method may also include pattern plating an extension to the sidewalls of the cavity to form at least one lip having a shape similar to the waveguide. The method may include a lip on one side to aid in interfacing between a chip and the PCB, e.g., as a shield. The method may include a lip on the antenna feed-network structure side that may aid in interfacing between the PCB and the antenna feed-network, e.g., as mating surface and/or for shielding and/or for alignment. A second portion of the filled opening is a heat transfer element configured to transfer heat from the first side of the PCB core to the second side of the PCB core. The at least one waveguide is embedded within the heat transfer element and configured for transmitting signals from the first side to the second side.
In another aspect, a PCB may include a PCB core having a first side and a second side opposite to the first side. The PCB may also include a chip mounted on the first side of the PCB core. The PCB may also include a heat transfer element embedded in the PCB core, where the heat transfer element may include a bulk of conductive material and may extend from the first side of in the PCB core to the second side of the PCB. The PCB may also include at least one waveguide comprising a cavity enclosed by sidewalls perpendicular to the first side and the second side of the PCB core. The cavity may extend from the first side of the PCB core to the second side of the PCB core. The at least one waveguide is embedded within the heat transfer element and configured for transmitting signals from the first side to the second side or receiving signals from the second side to the first side. The heat transfer element is configured to transfer heat generated from the chip from the first side of the PCB core to the second side of the PCB core.
In a further aspect, a PCB may include a PCB core having a first side and a second side opposite to the first side. The PCB may also include a chip mounted on the first side of the PCB core. The PCB may also include a heat transfer element embedded in the PCB core, the heat transfer element comprising a bulk of conductive material and extending from the first side of the PCB core to the second side of the PCB. The PCB may also include at least one waveguide comprising a cavity enclosed by sidewalls perpendicular to the first side and the second side of the PCB core, the cavity extending from the first side of the PCB core to the second side of the PCB core. The heat transfer element comprises a first portion connecting to a second portion, the first portion being near the first side of the PCB core, and the second portion being near the second side of the PCB core and extending outward laterally from the sidewalls of the at least one waveguide such that the second portion of the heat transfer element comprises a larger heat transferring area than the first portion of the heat transfer element.
Additional embodiments and features are set forth in part in the description that follows, and will become apparent to those skilled in the art upon examination of the specification or may be learned by the practice of the disclosed subject matter. A further understanding of the nature and advantages of the disclosure may be realized by reference to the remaining portions of the specification and the drawings, which form a part of this disclosure.
The description will be more fully understood with reference to the following figures and data graphs, which are presented as various embodiments of the disclosure and should not be construed as a complete recitation of the scope of the disclosure, wherein:
The disclosure may be understood by reference to the following detailed description, taken in conjunction with the drawings as described below. It is noted that, for purposes of illustrative clarity, certain elements in various drawings may not be drawn to scale.
The first conventional design includes a heat-transferring element on the PCB away from a waveguide formed in the PCB. The first conventional method mechanically forms an opening in the PCB substantially underneath an excitation from a monolithic microwave integrated circuit (MMIC). The mechanically formed opening is plated to have metallic walls and function as a waveguide.
The first conventional design and method have two main limitations. First, the drilled and plated waveguides cannot form the desirable rectangular shapes, thus introducing increased insertion losses or alternatively requiring a larger area for the same performance Second, the plated walls of the waveguides may be thin and may limit the heat transferring capability. Even with the addition of thermal vias or through-vias, the heat transferring capability remains low from the chip on one side of the PCB to the antenna mounted on the opposite side of the PCB.
Alternatively, a second conventional method routes the signals in or on the PCB to a location away from the heat transfer element, and then the signals are brought to the other side via a waveguide transition through the dielectric material of the PCB, which may cause substantial additional losses compared to the first conventional method.
The disclosed technology solves the problems of conventional systems by providing a design that combines the heat transfer and transition of millimeter-wave signals from one side of a PCB to the other side of the PCB. The design implements a waveguide via or a waveguide directly into a heat transfer coin or a fully plated heat transfer element. The disclosed technology allows simultaneously maintaining both thermal resistance and electromagnetic signal losses to a minimum. The thermal resistance remains low when the heat transfer from one side of the PCB to the opposite side of the PCB is efficient. The disclosed technology facilitates co-location of heat transfer and mmWave signal transition from one side to the other side of a PCB, which is different from the conventional technology in which the heat transfer is separated from the signal transfer.
The disclosed technology solves the processing problem with rounded corners of the waveguides by etching the waveguides in the heat transfer element rather than mechanically drilling the waveguide or routing, such that waveguides can have any arbitrary cross-sections. In particular, the disclosed technology etches the waveguides into the heat transfer element in the PCB. The heat transfer element may be a copper coin or preferably a plated heat transfer element. The heat transfer element is also referred to as a thermal path.
The etching method allows the formation of waveguides of any arbitrary shapes. For example, rectangular waveguides can be formed in the plated heat transfer element by etching. The rectangular waveguides can avoid the formation of higher-order modes that can lead to losses. And the rectangular waveguides can avoid the formation of circularly polarized wave transmission, which can be an issue in the antenna radiation pattern formation. In addition, arbitrary shapes can be introduced to enhance energy transfer from a source into the waveguide or to further suppress unwanted modes or limit the frequency range.
In some aspects, the disclosed technology may be used for automotive radars and 5G and 6G millimeter-wave applications, and military radars and guidance products. For example, the disclosed technology may be used in 5G and 6G technologies, which use mmWave and sub-Tera Hz signals, or mmWave and sub-Tera Hertz waveguide antennas.
Plated heat transfer element can be used to transfer heat from one side of the PCB 102 to an opposite side of the PCB 102. The plated heat transfer element allows more flexibility in signal routing. Plated heat transfer element also allows chips to be designed to optimize the heat transfer element, waveguide locations, and waveguide pitch. The plated heat transfer element also allows having different shapes at the interface to chip versus heat transfer elements depending on the needs of heat spreading and signal routing.
As shown above, the first configuration of the PCB assembly 100A has the most heat transfer due to that the area of the heat transfer element 106 is the largest among all the configurations 100A-E. In contrast, the fifth configuration of the PCB assembly 100C provides the most routing among all the configurations of the PCB assembly. The second configuration of the PCB assembly 100B has the second-largest heat transfer due to the larger area of the heat transfer element 106A than the third, fourth, and fifth configurations 100C, 100D, and 100E. The third configuration illustrates a good compromise between heat transferring capability and signal routing when considering the addition of a heatsink to the bottom of the PCB using traditional thermal greases.
The heat transfer element 206 may include the bottom portion 206A and a top portion 206B connecting to the top portion 206B. The side view of the heat transfer element 206 is illustrated in
As illustrated in
The heat from the chip 216 can be transferred from the top side 201A to the bottom side 201B through the heat transfer element 206 outside the waveguide 204A. The heat transfer element 206 may include the top portion 206B which is routed around top signal traces 212A. The top signal traces 212A may extend toward the waveguide 210A more than the bottom signal traces 212B. The bottom portion 206A of the heat transfer element 206 may extend horizontally away from the waveguide 204A more than the top portion 20AB to help spread the heat laterally.
In some aspects, the distance 213 between the vertical plated wall 208 of the waveguide 204A may be small due to the etching approach for forming the waveguide 204A without concern for use of the drilling or milling approach.
The plated heat transfer element approach has also a more conducting area to transfer heat than the filled through-vias and thus is more efficient than the filled through-vias.
Also, the plated heat transfer element approach may be better than the coin approach, e.g., PCB 300C or PCB 300D, for several reasons. First, the PCB 300E does not include any gaps 307 between the heat transfer element 302D and the inner conductive layers 306D or the inner dielectric layer 308D of the PCB 300D for the coin approach as illustrated in
The plated heat transfer element with etched waveguides approach with pattern plated lips provides more copper area to transfer heat than vias and thus is more efficient than the vias 302B, as illustrated in
It will be appreciated by those skilled in the art that many layers in the PCB may vary depending upon applications.
The following examples are for illustration purposes only. It will be apparent to those skilled in the art that many modifications, both to materials and methods, may be practiced without departing from the scope of the disclosure.
The disclosed technology offers significant system performance benefits without adding much cost. In some aspects, the etched waveguides can have higher heat transfer efficiency and lower cost than the drilled waveguides with through-vias or thermal vias. Various designs illustrated in
Heat transfer efficiency and cost factors may vary with various designs. Table 1 lists the comparisons of heat transfer cross-sectional areas and cost factors of various designs. The heat-conducting area or the heat transfer cross-sectional area of the heat transfer element is largely proportional to heat transfer efficiency, depending on where the heat is produced. The cost factor is an estimate based on the number of additional processing steps and the cost associated with the additional processing steps.
As shown in Table 1, the heat transfer efficiency is represented by an area for transporting or transferring heat, in mm2. For example, Design A as illustrated in
As illustrated in
In some aspects, the PCB may include narrower spacing between adjacent waveguides, as there are no concerns associated with mechanical machining or drilling.
In some aspects, the etched waveguides may also have a tighter pitch for the waveguides.
In some aspects, the etched waveguides 502C can have larger x and y dimensions as compared to a drilled/routed waveguide in the same boundary area or pitch and thus a lower cut-off frequency than the drilled waveguides.
Also, as illustrated in
Also, as illustrated in
In some aspects, the etched waveguides may be used for advanced waveguide functions such as coupling between two signals by forming a channel between them. Those skilled in the art will appreciate that many advanced features can be introduced if needed when etching the waveguides as opposed to machining them.
In some aspects, the etched waveguides may also have better control or restriction of higher-order modes.
When mechanically forming an opening, there are limitations in the tool sizes that can be used and costs associated with the operation. For routing out a shape of the waveguide opening, such as a rectangular shape, the smaller diameter of the bit, the slower along the path the drill bit would move, to avoid breaking, i.e., slower speed takes longer time on the router and hence increases cost. The increase in time and cost is exponential as the bit size decreases. Therefore, a typical minimum routing bit has a diameter of 0.8 mm or 32 mils.
Different shapes of the waveguide opening are shown in
An example rectangular shape 600A of the waveguide opening is depicted in
An example mechanical approximation to a rectangular shape or rectangular approximation 600B is depicted in
An example of a mechanical approximation to a rectangular shape without corners or a rectangular approximation 600C is depicted in
A lower cost method of mechanically approximating the rectangular shape is to drill a few bigger, overlapping holes as shown in
The rectangle approximation formed by drilling overlapping holes 600D, 600E, and 600F, with three holes, four holes, and twelve holes, respectively, are worse than the rectangle 600A, the routed rectangle approximation 600B with corner holes, and the routed rectangle approximation 600C without corner holes. The rectangle approximation 600D formed by drilling overlapping holes with three holes gives the best performance among 600D, 600E, and 600F. The additional overlapping holes in emulated rectangle 600E and 600F compared to emulated rectangle 600D do not improve the performance of insertion loss.
In some variations, the waveguide can be lasered to achieve a very good approximation to the ideal shape, but this is even more costly and time-consuming than routing.
Thermal Path and/or Electrical Path
The heat transfer element is also referred to as the thermal path and/or electrical path, which may have different shapes, which can be created by changing the cavity patterns and can be different from the top side to the bottom side, or from the front side to the backside. Examples include creating a thermal path and/or an electrical path having a “T” shape, as illustrated in
In some aspects, the thermal path may be formed by adjoining top and bottom cavities.
In various aspects, the bottom cavity may be identical in size and shape to the top cavity, to form the heat transfer element 302C, as shown in
In various aspects, the filling material in the cavities may be solid-plated copper. In other aspects, the filling materials can be other thermally conductive materials, including but not limited to solid silver, solid gold, and other equivalent materials with similar properties or combinations thereof.
In some variations, the signals have wavelengths that may range from 0.01 mm to 10 mm for TE01 mode.
In some variations, the signals may have frequencies of at least 1 GHz.
In some variations, the disclosed technology may be used for optical applications.
Any ranges cited herein are inclusive. The terms “substantially” and “about” used throughout this Specification are used to describe and account for small fluctuations. For example, they can refer to less than or equal to ±5%, such as less than or equal to ±2%, such as less than or equal to ±1%, such as less than or equal to ±0.5%, such as less than or equal to ±0.2%, such as less than or equal to ±0.1%, such as less than or equal to ±0.05%.
Having described several embodiments, it will be recognized by those skilled in the art that various modifications, alternative constructions, and equivalents may be used without departing from the spirit of the invention. Additionally, many well-known processes and elements have not been described to avoid unnecessarily obscuring the invention. Accordingly, the above description should not be taken as limiting the scope of the invention.
Those skilled in the art will appreciate that the presently disclosed embodiments teach by way of example and not by limitation. Therefore, the matter contained in the above description or shown in the accompanying drawings should be interpreted as illustrative and not in a limiting sense. The following claims are intended to cover all generic and specific features described herein, as well as all statements of the scope of the method and system, which, as a matter of language, might be said to fall therebetween.
This patent application is a continuation-in-part of U.S. patent application Ser. No. 17/162,773, entitled “Printed Circuit Board Assemblies with Engineered Thermal Path and Methods of Manufacture,” filed on Jan. 29, 2021, which claims the benefit under 35 U.S.C. § 119(e) of U.S. Patent Application Ser. No. 62/968,807, entitled “Printed Circuit Board Assemblies with Engineered Thermal Path and Methods of Manufacture,” filed on Jan. 31, 2020, each of the foregoing applications is incorporated herein by reference in its entirety. This patent application is also a continuation-in-part of U.S. patent application Ser. No. 17/543,512, entitled “Devices and Methods for Forming Engineered Thermal Paths of Printed Circuit Boards by use of Secondary Layers,” filed on Dec. 6, 2021, which claims the benefit under 35 U.S.C. § 119(e) of U.S. Patent Application Ser. No. 63/123,400, entitled “Devices and Methods for Forming Engineered Thermal Paths of Printed Circuit Boards by use of Secondary Layers,” filed on Dec. 9, 2020, each of the foregoing applications is incorporated herein by reference in its entirety. This patent application is also a continuation-in-part of U.S. patent application Ser. No. 17/225,491, entitled “METHODS FOR FABRICATING PRINTED CIRCUIT BOARD ASSEMBLIES WITH HIGH DENSITY VIA ARRAY,” filed on Apr. 8, 2021, which is a continuation of U.S. patent application Ser. No. 16/435,174, entitled “METHODS FOR FABRICATING PRINTED CIRCUIT BOARD ASSEMBLIES WITH HIGH DENSITY VIA ARRAY,” filed on Jun. 7, 2019, which claims the benefit under 35 U.S.C. § 119(e) of U.S. Patent Application Ser. No. 62/814,776, entitled “METHODS FOR FABRICATING PRINTED CIRCUIT BOARD ASSEMBLIES WITH HIGH DENSITY VIA ARRAY,” filed on Mar. 6, 2019, and also claims the benefit under 35 U.S.C. § 119(e) of U.S. Patent Application Ser. No. 62/837,637, entitled “METHODS FOR FABRICATING PRINTED CIRCUIT BOARD ASSEMBLIES WITH HIGH DENSITY VIA ARRAY,” filed on Apr. 23, 2019, each of the foregoing applications is incorporated herein by reference in its entirety.
Number | Date | Country | |
---|---|---|---|
62968807 | Jan 2020 | US | |
63123400 | Dec 2020 | US | |
62814776 | Mar 2019 | US | |
62837637 | Apr 2019 | US |
Number | Date | Country | |
---|---|---|---|
Parent | 16435174 | Jun 2019 | US |
Child | 17225491 | US |
Number | Date | Country | |
---|---|---|---|
Parent | 17162773 | Jan 2021 | US |
Child | 17939786 | US | |
Parent | 17543512 | Dec 2021 | US |
Child | 17162773 | US | |
Parent | 17225491 | Apr 2021 | US |
Child | 17543512 | US |