Patent Application
20120236510
This invention relates generally to thermal management in electronic circuits, and more specifically to methods and apparatus for distributing heat in an oscillator system.
Historically, electronic communications systems have relied upon precise clock signals. Without precise clocks, communications systems may be inefficient or even inoperable. One example is the global positioning system (GPS), a space-based system which employs communications signals from satellites to provide location and time information to terrestrial receivers. A GPS receiver uses phase, frequency, and time information from radio frequency signals broadcast by satellites to determine the signals' travel time. A very high precision and high performance clock is used to minimize its Time To First Fix (TTFF) and to maximize performance especially in weak-signal environments. If the clock deviates from a predetermined frequency, then errors in the GPS receiver's calculations will propagate and grow. Other communications systems, including mobile telephone handsets, wireless local area networks (WLANs), wireless broadband, and base stations, also need high precision clocks.
Performance of electronic circuits may vary over temperature, including electronic components/devices in portable communications devices. Piezoelectric crystal oscillators, for example, may be used to generate precision clocks in communications systems, but the piezoelectric crystal's frequency may depend on the temperature. Electronic systems may not only absorb heat from their environment, but also produce heat themselves. Current flowing through active and passive electrical components results in power dissipation and increased temperatures. Greater integration and higher clock speeds result in greater heat generation. This temperature variability in electronic systems may adversely affect the clock signals generated by piezoelectric crystal oscillators and hence the operation of the whole system. Accordingly, there is a need to reliably generate precision clock signals over a range of temperatures.
In the figures, elements having the same designation have the same or substantially similar function. The figures are illustrative only and relative sizes and distances depicted in the figures are for convenience of illustration and have no further meaning.
In the following description, certain details are set forth below to provide a sufficient understanding of the invention. However it will be clear to one skilled in the art that the invention may be practiced without these particular details. In other instances, well-known circuits, control signals, timing protocols, and software operations have not been shown in detail or omitted entirely in order to avoid unnecessarily obscuring the invention.
Generally the frequency at which piezoelectric crystals oscillate will change with variations in temperature. For example, a crystal oscillator exactly on a predefined frequency (or range of frequencies) at 25° C. with a frequency variation of five parts per million (PPM) per degree Celsius change could experience a frequency offset of 25 PPM with only a 5° C. temperature rise. Since temperature effects on a crystal oscillator are, for the most part, consistent and reproducible, circuits may be designed to compensate for the temperature effects on oscillator frequency.
Circuit 120 may include circuitry to compensate for temperature variations. For example, circuit 120 may include a temperature sensor and compensation circuitry which may operate with resonator 130 over a predefined range of temperatures. Oscillator 110, for example, may have an operating temperature range of −40° C. to +85° C. In some embodiments, oscillator 110 has an operating range of −20° C. to +60° C. In operation, circuit 120 may use the compensation circuitry to compensate for temperature effects on the resonator 130.
Resonator 130 and circuit 120 (including temperature sensor and compensation network) together may form a temperature compensated crystal oscillator (TCXO). The compensation network may include capacitors, thermistors, compensating elements (e.g., in series), amplifiers, read only memories (ROMs), low dropout regulator (LDO), divider, and phase-lock-loop (PLL), as well as other circuit elements.
As another example, circuit 120 may include a temperature sensor and an oven controller. Circuit 120 may use the output of the temperature sensor to control an oven. An oven may include a heating element. In operation, resonator 130 may be maintained at a constant temperature, for example, by heating the resonator to a temperature above an expected ambient temperature (e.g., 15° to 20° above the highest temperature to which resonator 130 will likely be exposed). An oven may optionally include a thermally insulated container or enclosure around resonator 130. Resonator 130 and circuit 120 (including temperature sensor and oven controller) together may form an oven controlled crystal oscillator (OCXO).
Other combinations and permutations are possible without deviating from the scope of the invention. Resonator 130 and circuit 120 together, for example, may form a voltage-controlled crystal oscillator (VCXO), digitally-controlled crystal oscillator (DCXO), voltage controlled/temperature compensated crystal oscillator (VCTCXO), as well as other oscillator systems.
Heat source 140 may be one or more components in electronic system 100 which generate heat. Heat source 140, for example, may be a baseband processor for a portable wireless device (e.g., for use in a global positioning system, cellular network, wireless local area network, wireless wide area network, etc.). Heat generated by heat source 140 may affect the temperature of electronic system 100 and in particular the temperature of circuit 120 and resonator 130. Temperature compensation in TCXOs and OCXOs may operate properly when the temperature measured by circuit 120 is substantially the same as the temperature experienced by resonator 130. That is, the amount of compensation provided by circuit 120 for the temperature effect on resonator 130 is based at least on part on the measured temperature. The assumption is that the measured temperature is approximately the same as the temperature of the resonator 130. If the measured temperature, however, does not accurately reflect the temperature of the resonator 130, the compensation provided by the compensation circuit of circuit 120 will not effectively compensate for the temperature impact on the resonator 130. Hence, it is desirable for circuit 120 and resonator 130 to experience substantially the same temperature.
A different temperature between the circuit 120 and the resonator 130 may result, for example, when due to spatial arrangement circuit 120 receives more heat from heat source 140 than resonator 130, or resonator 130 receives more heat than circuit 120. Such an arrangement, for example, may occur when circuit 120, resonator 130, and heat source 140 are arranged on the same plane of a substrate (e.g., printed circuit board) and the circuit 120 and the resonator 130 are located at significantly different distances from the heat source 140.
To facilitate circuit 120 and resonator 130 being heated to substantially the same amount by the heat from heat source 140, embodiments of the present invention include at least one of the components (i.e., circuit 120, resonator 130, and heat source 140) embedded in a substrate onto which the other components may be attached. The other components may be arranged on the substrate in such a manner as to be heated substantially the same amount by the heat from heat source 140. Embodiments of the present invention may also result in a low profile (i.e., height of components attached to the substrate).
Electrical devices 280 may be active and/or passive electrical components, such as resistors, capacitors, discrete semiconductors, small ICs, memory (e.g., dynamic random access memory (DRAM), FLASH memory, etc.), controllers (e.g., touch-screen controller), applications processors, accelerometers, compasses, as well as other components. Circuit 120 may be an integrated circuit (IC) in die form or an IC die assembled in a package. In some embodiments of the present invention, circuit 120 may be an IC die assembled into a chip scale package (CSP) or land grid array (LGA). Resonator 130 may be a piezoelectric crystal or a MEMS resonator mounted in a package such as an LGA.
Package 240 may include package substrate 260 and lid 250, which may optionally be hermetically sealed. Package 240 may be a multi-chip module (MCM) corresponding to an LGA form factor. Package 240 may also be a laminated MCM with encapsulant applied over circuit 120 and resonator 130 (which are positioned side-by-side in package 240), or a system-in-a-package (SiP) with circuit 120 and resonator 130 stacked vertically. Package 240 may also include underfill, thermal gel/paste, and the like. Substrate 260 may be ceramic. Substrate 260 may also be a multi-layer laminated printed circuit board (PCB). Lid 250 may be metal. Lid 250 may also be ceramic or epoxy/plastic, and may include an optional heat spreader.
In some embodiments where the resonator 130 is a MEMS device, resonator 130 may be stacked on the top of circuit 120 using die attach adhesive (not shown). Such a configuration may be referred to as “stacked die.” Interconnection and signal transfer between 130 and 120 may be through bond wires from the pads on 130 to the pads on 120 (not shown). Bond wires may also be used for interconnect and signal transfer from stacked die resonator 130 and circuit 120 to substrate 220. In some embodiments, the stacked die resonator 130 and circuit 120 are assembled in package 240 and package 240 is mounted to substrate 220 as described above. Other combinations and permutations are possible within the scope of the invention. Other packaging technologies may be used.
In practice, electronic system 200 may be a subassembly in a larger assembly (not shown). The surface 270 of substrate 220, devices 280, and package 240 may be covered by a metal lid or plastic/epoxy encapsulant 290. The metal lid or plastic/epoxy encapsulant 290 may facilitate handling of the electronic system 200 by automated manufacturing machines (e.g., pick and place machine) during assembly of the larger assembly. In some embodiments, the combined height h of substrate 220 and metal lid or plastic/epoxy encapsulant 290 may be 1 mm or less. For example, substrate 220 may be 400 μm or less thick, and package 240 substantially covered by metal lid or plastic epoxy encapsulant 290 may be 400 μm or less tall, resulting in a combined height h of 1 mm or less. In some embodiments where resonator 130 is a MEMS resonator, package 240 may be omitted, and circuit 120 and resonator 130 may be coupled to surface 270 of substrate 220, reducing height h further.
In operation, heat generated by heat source 140 spreads through printed circuit board 220. In some embodiments of the present invention, substrate 220 may include a heat conducting plane or layer 230 that may be disposed between heat source 140 and a surface 270 of substrate 220. The heat conducting plane or layer 230 may contribute to heat distribution in substrate 220. The heat conducting plane or layer 230 may be a layer of metal, such as copper, and may be substantially solid (with vias) or comprised of signal traces. Heat from heat source 140 may propagate through substrate 220 to package 240, and within package 240 to circuit 120 and resonator 130. Accordingly, circuit 120 and resonator 130 in package 240 may be positioned on a surface 270 of substrate 220 to be heated substantially the same amount by heat source 140 embedded within substrate 220.
For example, in some embodiments of the present invention, package 240 is approximately centered above heat source 140. In the embodiment illustrated with reference to
As may be readily understood by one of ordinary skill in the art, different combinations and permutations are possible within the scope of the present invention. Assembly 210 is depicted in two dimensions such that package 240 may appear to be positioned along one dimension (i.e., left-right). However package 240 may be positioned in two dimensions over surface 270 of substrate 220. Package 240, for example, may be positioned on a surface 270 of substrate 220 off-center from heat source 140 embedded in substrate 220. Heat conducting plane 230 may transfer heat approximately uniformly on the same horizontal plane to both circuit 120 and resonator 130. It is desirable for the package 240 to be positioned so that circuit 120 and resonator 130 in package 240 are heated substantially the same amount by heat source 140.
Circuit 120, for example, may be an IC in die form or an IC die assembled in a package. In some embodiments of the present invention, circuit 120 may be an IC die assembled into a CSP or LGA. Resonator 130 may be a piezoelectric crystal mounted in package 240. Package 240 may be an LGA including package substrate 260 and lid 250, which may optionally be hermetically sealed. Other combinations and permutations are possible within the scope of the invention. For example, other packaging technologies may be used in place of or in addition to those described above. In other embodiments, resonator 130 may be a MEMS die coupled to surface 270 of substrate 220 and package 240 may be omitted.
Heat source 140, optional heat conducting plane 230, and metal lid or plastic/epoxy encapsulant 290 are analogous to that of
In some embodiments of the present invention, package 240 is approximately centered above circuit 120. Although shown in
As can be readily understood by one of ordinary skill in the art, different combinations and permutations are possible within the scope of the present invention. For example, assembly 310 is depicted in two dimensions such that package 240 may appear to only be positioned along one dimension (i.e., left-right). However package 240 may be positioned in two dimensions over surface 270 of substrate 220. Package 240, for example, may be positioned on a surface 270 of substrate 220 off-center from circuit 120 embedded in substrate 220. Heat conducting plane 230 may transfer heat approximately uniformly on the same horizontal plane to both circuit 120 and resonator 130. It is desirable for the position of package 240 is that circuit 120 in substrate 220 and resonator 130 in package 240 be heated substantially the same amount by heat source 140.
In operation, heat generated by heat source 140 may propagate through substrate 220 to circuit 120 and resonator 130. In some embodiments of the present invention, substrate 220 may include a heat conducting plane or layer 230 that may be disposed between heat source 140 and circuit 120 and resonator 130. The heat conducting plane or layer 230 may contribute to heat distribution in substrate 220. Accordingly, circuit 120 and resonator 130 may be positioned in substrate 220 to be heated substantially the same amount by heat source 140.
For example, in some embodiments of the present invention, circuit 120 and resonator 130 are approximately centered below heat source 140. In the embodiment illustrated with reference to
In embodiments of the present invention, circuit 120, resonator 130, and heat source 140 are embedded in substrate 220. Circuit 120, resonator 130, and heat source 140 may occupy the same horizontal plane. As depicted in
As may be readily understood by one of ordinary skill in the art, different combinations and permutations are possible within the scope of the present invention. Assembly 410 is depicted in two dimensions such that heat source 140 may appear to be positioned along one dimension (i.e., left-right). However heat source 140 may be positioned in two dimensions over surface 270 of substrate 220. Heat source 140, for example, may be positioned on a surface 270 of substrate 220 off-center from circuit 120 and resonator 130 in substrate 220. Heat conducting plane 230 may transfer heat approximately unifomrly on the same horizontal plane to both circuit 120 and resonator 130. It is desirable for circuit 120 and resonator 130 in substrate 220 to be positioned so that circuit 120 and resonator 130 are heated substantially the same amount by heat source 140.
As another example, circuit 120 and heat source 140 may be included in the same integrated circuit die (not depicted). In some embodiments, the combined circuit 120 and heat source 140 work in conjunction with resonator 130. The combined circuit 120 and heat source 140 may be coupled to surface 270 of substrate 220 or embedded in substrate 220. Resonator 130 may also be coupled to surface 270 of substrate 220 or embedded in substrate 220. It is desirable for resonator 130 to be arranged so that circuit 120 (in the combined circuit 120 and heat source 140) and resonator 130 are heated substantially the same amount by heat source 140 (in the combined circuit 120 and heat source 140).
In some embodiments of the invention, the arrangement of the resonator 130 and the circuit 120 may result in an encapsulated package that has a lower profile compared to conventional arrangements, for example, the resonator 130 and circuit 120 stacked within the package 240 that is attached to a surface of the substrate 220. For example, the embodiment illustrated in
First layer 510, for example, may be a dielectric material with a layer of metal foil bonded on one side. Second layer 520 may be a dielectric material and may include a mechanically- and/or chemically-created opening for embedded component 525. Third layer 530 and fourth layer 530 may be a dielectric material having a thin layer of metal foil bonded on one side. The dielectric materials of the first layer 510, second layer 520, third layer 530, and fourth layer 540 may be cured (i.e., core) or uncured (i.e., prepreg) fiberglass-epoxy resin, such as FR-4, CEM, BT-Epoxy, polyimide, Teflon (polytetrafluoroethylene), and the like. The metal foil may be copper foil.
Various combinations and permutations may be used without deviating from the scope of the present invention. The substrate may have a different number of (metal) layers (e.g., 2-24 layers). In some embodiments of the present invention, the substrate includes six layers. Although only one embedded component 525 and one conventionally mounted component 560 are depicted in
Antenna block 610 may be designed for a specific frequency or range of frequencies. Antenna block 610 may be omnidirectional. RF receiver/transmitter, block 620 may include a low-noise amplifier (LNA), band-pass filter (BPF), and mixer. In some embodiments, RF receiver/transmitter block 620 includes only one of a receiver or transmitter (e.g., a GPS receiver may only include a receiver). Baseband and logic block 640 may include a digital signal processor (DSP), memory (e.g., SDRAM), memory management unit, input/output (I/O), and the like. TCXO block 630 may also, for example, be an OXCO and/or VCTCXO. In some embodiments, baseband and logic block 640 may be combined with a portion of the TCXO block on one integrated circuit die. In these embodiments, an oscillator (e.g., crystal or MEMS oscillator) may be used in conjunction with the one integrated circuit die. Microcontroller block 650 may include an interrupt controller, microcontroller, programmable I/O, etc. The microcontroller in microcontroller block 650 may be connected to the memory management unit in baseband and logic block 640.
From the foregoing, it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Such modifications are well within the skill of those ordinarily skilled in the art. Accordingly, the invention is not limited except as by the appended claims.
