Patent Application
20150382460
A conventional printed circuit board (PCB) may have embedded conductors for transmitting electrical signals, such as radio frequency (RF) signals, as well as for providing power and ground connections. The embedded conductors, which may be part of a conductor pattern, are typically formed of copper disposed on a first dielectric layer, and then covered by a second dielectric layer. The first and second dielectric layers may be referred to as “prepreg,” and the PCB with embedded (copper) conductors between the first and second dielectric layers may be referred to as a “build-up substrate.”
Currently, when the embedded conductors are formed of copper, they are oxidized during the fabrication process to improve adhesion of the copper to the second dielectric layer during and after formation of the second dielectric layer (or, lamination process). Oxidation results in a thin, electrically resistive oxide layer formed on the copper conductors. Thus, using conventional oxidation, the electrical conductivity of the copper conductors can not be increased in standard PCB applications. As a practical matter, the only materials having better electrical conductivity than copper are silver and graphene. However, these materials are generally more expensive than copper, and more difficult to apply, pattern and etch when formed directly on a dielectric layer.
Therefore, there is a need for increased conductivity of embedded conductors in build-up substrates, while maintaining or improving adhesiveness between the embedded conductors and the dielectric layers of the PCB substrate.
The example embodiments are best understood from the following detailed description when read with the accompanying drawing figures. It is emphasized that the various features are not necessarily drawn to scale. In fact, the dimensions may be arbitrarily increased or decreased for clarity of discussion. Wherever applicable and practical, like reference numerals refer to like elements.
In the following detailed description, for purposes of explanation and not limitation, example embodiments disclosing specific details are set forth in order to provide a thorough understanding of the present teachings. However, it will be apparent to one having ordinary skill in the art having the benefit of the present disclosure that other embodiments according to the present teachings that depart from the specific details disclosed herein remain within the scope of the appended claims. Moreover, descriptions of well-known apparatuses and methods may be omitted so as to not obscure the description of the example embodiments. Such methods and apparatuses are clearly within the scope of the present teachings.
The terminology used herein is for purposes of describing particular embodiments only, and is not intended to be limiting. The defined terms are in addition to the technical, scientific, or ordinary meanings of the defined terms as commonly understood and accepted in the relevant context.
The terms “a”, “an” and “the” include both singular and plural referents, unless the context clearly dictates otherwise. Thus, for example, “a device” includes one device and plural devices. The terms “substantial” or “substantially” mean to within acceptable limits or degree. The term “approximately” means to within an acceptable limit or amount to one of ordinary skill in the art. Relative terms, such as “above,” “below,” “top,” “bottom,” “upper” and “lower” may be used to describe the various elements' relationships to one another, as illustrated in the accompanying drawings. These relative terms are intended to encompass different orientations of the device and/or elements in addition to the orientation depicted in the drawings. For example, if the device were inverted with respect to the view in the drawings, an element described as “above” another element, for example, would now be below that element. Where a first device is said to be connected or coupled to a second device, this encompasses examples where one or more intermediate devices may be employed to connect the two devices to each other. In contrast, where a first device is said to be directly connected or directly coupled to a second device, this encompasses examples where the two devices are connected together without any intervening devices other than electrical connectors (e.g., wires, bonding materials, etc.).
As mentioned above, copper conductors, e.g., formed by patterning a copper layer disposed between first and second dielectric layers in a build-up substrate are currently oxidized to improve adhesion to the second dielectric layer during and after lamination. However, according to various embodiments, silver is plated onto the copper conductors in place of the thin oxide layer, effectively wrapping the exposed surfaces of the copper conductors in a silver layer or silver wrap, thereby increasing conductivity. The interface between the silver wrap and the second dielectric layer may be optimized, so that adhesion to the second dielectric layer is improved or not degraded. The silver may be plated onto the copper conductors using an immersion silver process, for example. In applications involving radio frequency (RF) signals, in particular, the increased conductivity may result from substantially all (e.g., about 98 percent) of the RF energy propagating on three skin depths the silver wrap, which is typically much smaller than the total cross-section of the copper conductor. Other skin depths may be incorporated without departing from the scope of the present teachings.
In accordance with a representative embodiment, a PCB includes a wrapped conductor enabling transmission of an RF signal, the wrapped conductor including a conductor core and a conductive wrap disposed on top and side surfaces of the conductor core. The PCB further includes a top dielectric layer disposed on the conductive wrap of the wrapped conductor, at least partially embedding the wrapped conductor. Resistivity of the conductive wrap is less than resistivity of the conductor core, such that a majority of RF power of the RF signal is propagated through the conductive wrap.
In accordance with another representative embodiment, a build-up substrate of a PCB includes a first dielectric layer; a copper conductor core disposed on a top surface of the first dielectric layer; and a plated silver conductive wrap disposed on top and side surfaces of the copper conductor core, the plated silver conductive wrap being configured to transmit a radio frequency (RF) signal. A second dielectric layer is disposed on exposed portions of the top surface of the first dielectric layer and the plated silver conductive wrap. A majority of RF power of the RF signal is propagated through the plated silver conductive wrap.
In accordance with another representative embodiment, a method is provided for fabricating a PCB having an embedded wrapped conductor. The method includes forming a seed layer on a first dielectric layer; forming a copper layer on the seed layer; patterning and etching the copper layer and the seed layer to form a copper conductor core; plating silver to exposed surfaces of the copper conductor; and forming a second dielectric layer on the first dielectric layer and the plated silver wrap to form the embedded wrapped conductor.
Referring to
One or both of the first and third conductive layers 110 and 130 may be a ground plane or power plane, for example. Also, although the first and third conductive layers 110 and 130 are referred to as “layers,” it is understood that this term may include conductive “patterns,” indicating the presence of multiple conductors, traces, pads and/or other circuitry, without departing from the scope of the present teachings. Likewise, for purposes of illustration and ease of description, the wrapped conductor 120 is shown as a single conductor, although it is understood that the wrapped conductor 120 may include multiple conductors and/or a conductive “pattern,” as mentioned above, without departing from the scope of the present teachings. The wrapped conductor 120 is embedded within the substrate 105 in that at least the top, bottom and side surfaces in the cross-sectional view are covered by the first and second dielectric layers 101 and 102. The wrapped conductor 120 enables transmission of an RF signal, for example.
In various embodiments, the first and second dielectric layers 101 and 102 (or “prepreg”) may be formed of glass reinforced epoxy, glass reinforced resin and/or organic material, such as FR-4 or polytetrafluoroethylene (Teflon®), for example. The first and second dielectric layers 101 and 102 may be formed of the same or different materials, as long as they adequately adhere to one another to create a durable, integrated substrate 105. The first and third conductor layers 110 and 130 may be formed of metal, such as copper (Cu), for example. The first and third conductor layers 110 and 130 likewise may be formed of the same or different materials. Of course, it is understood that the first and second dielectric layers 101 and 102 may be formed of any other compatible dielectric materials (or combinations thereof), and the first and third conductor layers 110 and 130 may be formed of any other compatible electrically conductive materials (or combinations thereof), without departing from the scope of the present teachings.
The wrapped conductor 120 includes a conductor core 122 and a conductive wrap 125 surrounding the conductor core 122 on three sides. That is, the conductor core 122 is formed on the top surface of the first dielectric layer 101 and the conductive wrap 125 is formed (e.g., electrolytically plated) on exposed surfaces of the inner conductor core 122. The second dielectric layer 102 may then be formed over the outer surface of the conductive wrap 125 of the wrapped conductor 120, as well as over exposed portions of the first dielectric layer 101. Also, the conductive wrap 125 assists in adhering the conductor core 122 to the second dielectric layer 102.
The conductive wrap 125 has a lower resistivity (p) than the conductor core 122, so that the majority of the RF power is conducted through the conductive wrap 125, as opposed to the conductor core 122. For example, the conductor core 122 may be formed of copper, which has a resistivity of about 1.68×10−8 ohm-meter, and the conductive wrap 125 may be formed of silver, which has a resistivity of about 1.59×10−8 ohm-meter. Thus, the silver conductive wrap 125 is about 5.4 percent more conductive than the copper conductor core 122, which increases speed and efficiency of transmitting the RF signal. Meanwhile, the copper conductor core 122 may maintain an impedance of about 50 ohms, which provides interface compatibility with most circuit designs, regardless of the presence of the silver conductive wrap 125.
In the depicted embodiment, thickness 122′ of the copper conductor core 122 is approximately 15 μm, and thickness 125′ of the silver conductive wrap 125 is approximately 1.5 μm. The thickness of silver enables propagation of substantially all RF energy through the silver conductive wrap 125 for RF signals having a operating frequency of about 16 GHz and above, as discussed below with reference to
In various embodiments, the thickness of the conductive wrap 125 is determined based on the portion of RF energy to be propagated through the conductive wrap 125 (as opposed to the conductor core 122) of the wrapped conductor 120. For example, in a representative embodiment, the thickness of the conductive wrap 125 is determined such that the about 98 percent of the RF energy is propagated through three skin depths of the material forming the conductive wrap 125. Skin depth (δ) of a material depends on the resistivity (ρ) of the conductive material (e.g., silver) in ohm-meters and the operating frequency (f) of the RF signal in Hertz, as provided by Equation (1):
δ=√{square root over (2ρ/2πfμ)} (1)
Also in Equation (1), μ is the absolute magnetic permeability of the conductive material. The absolute magnetic permeability μ is equal to μ0×μr, where μ0 is the permeability of free space (4π×10−7) and μr is the relative permeability of the conductive material equal to 4π×10−7 henries/meter, which together may be assumed to be unity.
More particularly, for purposes of illustration, the material forming the conductive wrap 125 is silver, and curve 210 depicts the thickness of silver required to conduct at least 98 percent of the RF energy through three skin depths of silver at corresponding RF frequencies. Representative first and second thicknesses 235′ and 235″ of the silver conductive wrap 125 are depicted as corresponding dashed lines in
When the RF frequency is known, the conductive wrap 125 may be designed more precisely to have a thickness capable of propagating about 98 percent of the RF energy of the corresponding RF signal within three skin depths, thus conserving the amount of material used to form the conductive wrap 125. Curve 210 would change based on factors such as the type of metal conductor (and corresponding resistivity) and/or frequencies of the RF signal. Also, the RF signal may operate over a range of potential frequencies, and the operating frequency for determining the skin depth of the conductive wrap material is the lowest frequency of the range of potential frequencies.
According to various embodiments, the PCB 100 may be fabricated using various techniques compatible with semiconductor processes. A non-limiting example of a fabrication process directed to representative PCB 100 is discussed below with reference to
In step S311 of
In step S312, a seed layer 421 is then formed on the first dielectric layer 101, also shown in
In step S313, a dry film pattern 422 is formed on the seed layer 421 using photo-lithography, for example, where the dry film pattern 422 includes opening 423 to enable eventual formation of the conductor core 122. In step S314, copper is plated up on exposed surface(s) of the seed layer 421 through the opening 423, as shown in
In step 317, conductive wrap 125 is applied to exposed surfaces (top and sides) of the conductor core 122, as shown in
Second dielectric layer 102 is formed on the first dielectric layer 101 and the conductive wrap 125 of the wrapped conductor 120 in step 318, as shown in
In step 319, third conductive layer 130 is formed on the second dielectric layer 102, also as shown in
Referring to
The wrapped conductor 520 is partially embedded within the substrate 505 in that at least the top and side surfaces in the cross-sectional view are covered by the dielectric layer 502, while the bottom surface of the wrapped conductor 520 (corresponding to the conductor core 522, discussed below) is exposed. The wrapped conductor 520 enables transmission of an RF signal, for example.
In various embodiments, the dielectric layer 502 and the wrapped conductor 520 may be formed of the same materials discussed above with regard to the first and second dielectric layers 101 and 102, and the first and third conductor layers 110 and 130 in
The conductive wrap 525 has a lower resistivity (p) than the conductor core 522, so that the majority of the RF power is conducted through the conductive wrap 525, as opposed to the conductor core 522. For example, the conductor core 522 may be formed of copper, which has a resistivity of about 1.68×10−8 ohm-meter, and the conductive wrap 525 may be formed of silver, which has a resistivity of about 1.59×10−8 ohm-meter. In the depicted embodiment, thickness 522′ of the copper core 522 is approximately 15 μm, and thickness 525′ of the silver conductive wrap 525 is approximately 1.5 μm. This thickness enables propagation of substantially all RF energy through the silver conductive wrap 125 for RF signals having a operating frequency of about 16 GHz and above, as discussed above with reference to
In various embodiments, the thickness of the conductive wrap 525 is determined based on the portion of RF energy of the RF signal to be propagated through the conductive wrap 525 (as opposed to the conductor core 522) of the wrapped conductor 520. For example, in a representative embodiment, the thickness of the conductive wrap 525 is determined such that about 98 percent of the RF energy is propagated through three skin depths of the material forming the conductive wrap 525, as discussed above with regard to the conductive wrap 125.
In various embodiments, the thicknesses and/or materials of the various layers may vary to provide unique benefits for any particular situation or to meet application specific design requirements of various implementations, as would be apparent to one skilled in the art.
The various components, materials, structures and parameters are included by way of illustration and example only and not in any limiting sense. In view of this disclosure, those skilled in the art can implement the present teachings in determining their own applications and needed components, materials, structures and equipment to implement these applications, while remaining within the scope of the appended claims.
