Patent Application
20150282299
1. Field of the Invention
Embodiments of the present invention generally relate to an electrical interconnect component, and in particular, to an electrical interconnect component including an electrical trace having a construction for suppressing a skin effect, thereby increasing bandwidth.
2. Description of the Related Art
Electronic devices, such as tablets, computers, copiers, digital cameras, smart phones, control systems and automated teller machines, among others, often employ electronic components such as dies that are connected by various interconnect components. The dies may include memory, logic or other IC device.
In general, interconnect components are subject to various electrical effects that cause issues such as distortion and attenuation. One such electrical effect is known as a skin effect. The skin effect is a frequency-dependent attenuation effect that affects higher frequency components to a greater degree than lower frequency components. More specifically, current at a higher frequency flows through a narrower portion of the interconnect component than current at a lower frequency, thus reducing the effective cross-sectional area, and thereby increasing the resistance experienced. The skin effect also induces a frequency dependent phase delay into signals, which undesirably causes dispersion.
Therefore, a need exists for an improved interconnect component construction that reduces frequency-dependent effects related to the skin effect.
A method and apparatus are provided for suppressing the skin effect in an electrical trace. Advantageously, the construction of the electrical trace suppresses effects in the electrical trace that are related to the skin effect. The effects include frequency-dependent attenuation and frequency-dependent phase delay effects. The improvements described herein generally improve bandwidth, and reduces dispersion, jitter, and bit error.
In one embodiment, an electrical interconnect component of an electronic device is provided. The electronic device has a characteristic data rate. The interconnect component includes a dielectric surface and an electrical trace disposed on the dielectric surface. The electrical trace has a thickness selected in response to the characteristic data rate and independent of any relationship to a width of the electrical trace.
In another embodiment, an electronic device having a characteristic data rate is provided. The electronic device includes an electrical interconnect component. The interconnect component includes a dielectric surface and an electrical trace disposed on the dielectric surface. The electrical trace has a thickness selected in response to the characteristic data rate and independent of any relationship to a width of the electrical trace.
In another embodiment, a method of communicating via an electrical trace is provided. The method includes transmitting a signal having a characteristic data rate through the electrical trace. The electrical trace has a thickness selected in response to the characteristic data rate, the thickness selected independent of any relationship to a width of the electrical trace.
In another embodiment, a method of selecting a thickness for an electrical trace is provided. The method includes determining a characteristic data rate. The method also includes determining a frequency based on the characteristic data rate. The method further includes determining a skin depth based on the frequency. The method further includes determining a thickness for the electrical trace based on the skin depth.
Other features will be recognized from consideration of the Detailed Description and Claims, which follow.
So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.
To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements of one embodiment may be beneficially incorporated in other embodiments.
Embodiments of the invention generally provide an electronic device comprising an electrical interconnect component that includes an electrical trace. The electrical trace has geometric characteristics that serve to suppress the skin effect over a large band of frequency components. More specifically, the electrical trace has a thickness that is less than a skin depth for a particular chosen frequency component. By making the electrical trace have a thickness that is less than the skin depth, the current flows through substantially the entire cross-sectional area of the electrical trace for all frequencies up to the chosen frequency component, which reduces the effects associated with the skin effect.
Turning now to
The chip package 110 includes a plurality of IC dies 114 connected optionally by a silicon-through-via (TSV) interposer 112 to a package substrate 122. The chip package 110 may also have an overmold covering the IC dies 114. The interposer 112 includes circuitry for electrically connecting the dies 114 to circuitry of the package substrate 122. The circuitry of the interposer 112 may optionally include transistors. Package bumps 132, also known as “C4 bumps,” are utilized to provide an electrical connection between the circuitry of the interposer 112 and the circuitry of the package substrate 122. The package substrate 122 may be mounted and connected to a printed circuit board (PCB) 136, utilizing solder balls 134, wire bonding or other suitable technique. The PCB 136 is mounted in the interior of a housing 102 of the electronic device 100.
The IC dies 114 are mounted to one or more surfaces of the interposer 112, or alternatively, to the package substrate 122. The IC dies 114 may be programmable logic devices, such as field programmable gate arrays (FPGA), memory devices, optical devices, processors or other IC logic structures. Optical devices include photo-detectors, lasers, optical sources, and the like. In the embodiment depicted in
The electrical components of integrated chip package 110, such as the dies 114, communicate via traces formed on electrical interconnect components. The interconnect components having the traces can include one or more of the PCB 136, the packages substrate 112 and optional interposer 112, among others components. Careful construction of the traces as described below reduce the skin effect as compared to conventional traces utilized on conventional electrical interconnect components, thereby allowing the electronic device 100 having the integrated chip package 110 to perform more efficiently as compared to conventional designs.
Electrical signals traveling through the conductors 202 are affected by an electrical phenomenon known as the skin effect. The skin effect is a phenomenon whereby an alternating current tends to flow predominantly through a skin region of a conductor as opposed to the central, interior region of the conductor. This effect is dependent on the frequency of the alternating current. A higher frequency current flows within a smaller, i.e., shallower, skin region. In other words, higher frequency signals flow through a relatively shallower region of a particular conductor as compared with lower frequency signals, which flow in a region extending comparatively deeper into the central, interior region of the conductor.
Because the skin effect restricts the current flow of a signal at a particular frequency to an area of a conductor consistent with the skin depth for that particular frequency, the resistance experienced by the signal at the particular frequency increases as the frequency increases. More specifically, resistance is defined as follows:
R=μL/A
where ρ is the resistivity, L is the length of the conductor, and A is the cross-sectional area of the conduct. As the skin depth decreases, the effective area through which current flows is reduced, and thus the resistance increases.
Additionally, the phase delay of signal at a particular frequency is also affected by the skin depth. More specifically, the phase delay t is proportional to the square root of LC, where L=Lint Lext, and C is capacitance. Lint is the internal inductance and Lext is the external inductance. As the skin depth decreases and current flows in an increasingly smaller region, internal inductance increases. Thus, as the frequency increases and skin depth decreases, the phase delay t increases.
In general, any particular signal transmitted via electrical devices such as the electronic device 100 described above has a frequency spectrum that includes a fundamental frequency and harmonic frequencies, as is generally known to those of skill in the art. Each frequency component of a signal is affected by the skin effect to a different degree. In other words, a low frequency component of a frequency spectrum for a particular signal has an associated skin depth that is greater than a skin depth associated with a higher frequency component of the particular signal. Because different frequencies are present in any given signal, those different frequencies are affected by the increased resistance and increased phase delays differently.
In order to improve signal quality by increasing bandwidth and reducing dispersion, an electrical trace having a thickness that is based on characteristic skin depth is provided herein. The characteristic skin depth is based on a desired or “characteristic” data rate of the electronic device in which the electrical trace is located. The “characteristic” data rate is the intended data rate of signals that flow through the electrical trace. A characteristic data rate may be associated with any portion of an electrical device (e.g., a single trace, a group of traces, the entire device). For an electrical device having a higher characteristic data rate, the thickness of the electrical trace is lower than an electrical device having a lower characteristic data rate.
In various embodiments, the “frequency of interest” (i.e., the frequency for which the related skin depth is calculated) is dependent on various characteristics. In one embodiment, the frequency is the highest frequency of system response that supports distortionless transmission of an incoming signal. In some embodiments, such a frequency is approximately equivalent to 0.35/RT, where RT is the rise time for the signal. In another embodiment, the frequency is below this frequency, and thus the skin depth does not penetrate the entire trace at all frequencies that are transmitted for a particular data rate.
As described above, the highest frequency of system response is based on the data rate intended for the electrical trace. More specifically, every data rate is associated with a minimum signal frequency for transmitting data at that data rate. The Nyquist theorem states that this signal frequency is one half of the data rate. Thus, a signal frequency of 28 gigahertz (GHz) is associated with a data rate of 56 gigabits per second (Gbps).
In some embodiments, the skin depth for the electrical trace is calculated as the skin depth associated with a frequency equivalent to one half of the data rate as described above. With such an electrical trace, frequency components of a signal up to the described frequency would flow through the entire cross-sectional area of the electrical trace, as the trace thickness would be less than the skin depth for all such frequencies. Thus, frequency-dependent signal attenuation would be generally unaffected by the skin effect up to that frequency.
In other embodiments, the skin depth is not calculated for the minimum frequency for supporting the particular data rate, but is calculated for a frequency substantially below that frequency. With the skin depth calculated for such a frequency, many, but not all, frequency components of a signal, up to the minimum frequency for supporting a particular data rate, are considered as being unaffected by the skin effect. Higher frequencies closer to this minimum frequency are, in fact, affected by the skin effect and thus are affected by some amount of frequency-dependent attenuation. One benefit of calculating the skin depth for a frequency below this minimum frequency is that overall attenuation for spectral components below this minimum frequency is lower than if the skin depth were calculated for the minimum frequency. Thus, these embodiments provide a trade-off by allowing frequency-dependent attenuation at the higher frequencies, while reducing overall attenuation at lower frequencies.
The skin depth referred to above describes the portion of an electrical trace through which current flows. Thus, because typically, current flows through both top and bottom skin regions, the thickness of the electrical trace is two times the calculated skin depth.
In some embodiments, the trace may have specific dimensions based on a characteristic data rate. More specifically, for a characteristic data rate of 56 gigabits per second and below (56 Gbps), the trace should have a thickness of approximately 1 μm. For a characteristic data rate of 28 Gbps and below, the trace should have a thickness of approximately 2 μm. For a characteristic data rate of 10 Gbps and below, the trace should have a thickness of approximately 5 μm.
In some embodiments, trace thickness is determined with additional reference to trace surface roughness. More specifically, because surface roughness represents variation in thickness of the trace, the full extent of possible thicknesses, including surface roughness is taken into account. The thickness of the electrical trace, without taking into account roughness effects, is referred to herein as the “nominal thickness.” In some embodiments, the trace has a width that is chosen to create a particular characteristic impedance and a low reflection transmission line. In some embodiments, the thickness of the trace is substantially the same over the entire length of the trace.
Although described above as being within the package substrate 122, electrical traces having characteristics described herein may be within other interconnect components of an electrical device, such as the interposer 112, PCB 136 or other components of the electronic device 100. In further embodiments, wires within a wire bond package coupled to the electronic device 100 may have a diameter calculated to match the skin depth as described above. In yet other embodiments, other electrical traces or wires in other electrical devices may be fabricated with the techniques described above.
As described above, the electrical trace 501(2) illustrated in
The electronic device including electrical trace of an interconnect component described above advantageously suppresses effects in the electrical trace that are related to the skin effect. The effects include frequency-dependent attenuation and frequency-dependent phase delay effects. The improvements described herein generally improve bandwidth, and reduces dispersion, jitter, and bit error.
While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.
