This application is a 35 U.S.C. §371 national stage application of PCT International Application No. PCT/SE2007/051016, filed on 17 Dec. 2007, the disclosure and content of which is incorporated by reference herein in its entirety. The above-referenced PCT International Application was published in the English language as International Publication No. WO 2009/078767 A1 on 25 Jun. 2009.
The present invention relates to a device, a radio base station (RBS) and an apparatus for decreasing stress on connection points between heat-generating sources and a substrate. The present invention more particularly relates to reduction of stress induced by thermal expansion.
Electronic devices in general and power amplifiers in particular, that are common in many electronic devices such as radio base stations, operate with limited efficiency. Therefore a significant fraction of their power consumption turns into heat. It follows that the thermal design of these devices is important. Such devices are typically air-cooled and their mechanics serve as thermal interface between the hot devices and the air. The heat-generating source, such as a power amplifier transistor or microprocessor, typically attains its thermal contact with the heat-dissipating device by bolting or soldering. The heat-dissipating device can be a cooling flange or heat sink with or without active cooling such as a fan. Heat sinks function by efficiently transferring thermal energy (“heat”) from an object at high temperature to a second object at a lower temperature with a much greater heat capacity. This rapid transfer of thermal energy quickly brings the first object into thermal equilibrium with the second, lowering the temperature of the first object, fulfilling the heat sink's role as a cooling device. Efficient function of a heat sink relies on rapid transfer of thermal energy from the first object to the heat sink, which is designed to efficiently dissipate thermal energy to the surrounding air.
Traditionally, the heat-generating device is soldered to a substrate such as a printed circuit board (PCB) as well. In operation, heat generation will cause both the PCB and the cooler to expand. Since they normally are made of different materials, the magnitude of expansion will be determined by their respective coefficients of thermal expansion. A flange, heat sink, is typically made of metal such as aluminum or copper and will therefore expand more than a PCB which is typically made of plastic laminate or other similar materials. A flange might alternatively be made of an alloy. This difference will impart a force on the connection between the PCB and the cooling flange. Ultimately it will also impart a force on the solder joints between the heat-dissipating device and the PCB that it sits on, since transistors and other heat-generating devices are attached to both the PCB and the cooling flange. As the component work load varies during operation, the temperature will cycle causing variation in the mentioned force, which will lead to stress on the connection points.
Experience has shown that this stress will cause the solder joints to fail prematurely, requiring the soldered component or even the complete device to be replaced. This is a costly process, in addition to the interruption of operation. The stress effect is even more pronounced if there is more than one component with significant heat generation in the device.
Patent document U.S. Pat. No. 6,128,190 A describes a transistor clamp provided to obviate solder joints and stress imparted hereon. However not all components are suited for this solution. Due to their design or for other reasons, some components require soldering or similar fixing methods.
One object of the present invention is to provide a device in which thermal stress on connection points is reduced and thereby service intervals are increased compared to above mentioned prior art.
Another object of the present invention is to provide an apparatus with reduced stress on connection points, which is simple to manufacture without significantly impacting design and production.
Some advantages of the present invention are highly reduced stress level on connection points such as solder joints, increased mean time between failures of an apparatus, higher reliability, less down-time etc. A spin-off effect arising from the present invention is the possibility to use a heat-dissipating device for cooling a variety of heat-generating sources.
An embodiment of the present invention provides a device comprising a larger heat-dissipating part, and at least one smaller heat-dissipating part. The larger part is arranged with at least one cavity for housing the at least one smaller part. The at least one smaller part is adapted to be attached to at least one heat-generating source.
a and 3b illustrate side views of different embodiments of the present invention.
a-4d illustrate examples of different shapes of smaller heat-dissipating parts, according to embodiments of the present invention.
Further on if the device 13 comprises at least one cavity 8 that is bigger than the size of the smaller heat-dissipating part 6 that is intended to be housed in that cavity 8, then the smaller part 6 is more mobile in its cavity 8. This decreases the stress even more since there is some mobility relative to the underlying surface. Additionally, it is possible that each cavity 8 can be adapted to house one or more smaller parts 6.
a and 3b illustrate side views of different embodiments of the present invention. Two different shapes of smaller parts (6a; 6b) are described in
In another embodiment of the present invention the device 13 further comprises a thermally conductive layer 9, such as aluminum or copper, between the at least one heat-generating source 2 and the at least one smaller heat-dissipating part 6. This layer is needed to compensate for thickness variations between different heat-generating sources, and can also be used as part of a fixing means needed to fix the source 2 to the smaller part 6. In yet another embodiment of the present invention, there is another thermally conductive layer interposed between the smaller 6 and the larger 7 heat-dissipating parts, such as thermal paste (not shown in figure). One purpose of using the other thermally conductive layer is to fill up gaps between the heat source 2 and the smaller part 6.
According to yet another embodiment of the present invention the device 13 comprises at least one smaller part 6 and one larger part 7 wherein at least one smaller part 6 of the device is of the same material as the larger part 7. However this is a non limiting embodiment. For example copper can be used for one or more smaller parts 6 while aluminum is used for the larger part 7. Also other materials or alloys are possible to use, like AlSiC (Aluminum Silicon Carbide) as an example. Thus, it is possible according to the present invention to manufacture heat-dissipating devices with one type of larger part 7 and one or more types of smaller parts 6 depending on the needs and current material cost.
a-4d illustrate examples of different shapes of smaller heat-dissipating parts, according to embodiments of the present invention. According to these embodiments the device 13 comprises at least one smaller part 6 which is being adapted to the shape of the at least one heat-generating source 2. This is achieved by one or more of the following: a hollowed-out section of the smaller part 6,
An important benefit achieved by the present invention according to the example mentioned above is that the heat-generating sources are directly bolted or soldered to the smaller heat-dissipating part 6 and are free to move back, forth and sideways in relation to the larger heat-dissipating part 7. The heat source 2 is in this way adapted to follow the substrate, PCB, and “ignore” the larger heat-dissipating part 7 when the larger part 7 and the substrate 1 move relative to each other due to differences in thermal expansion. Pressure is applied on the smaller part 6 by aid of substrate design in such a way that a top surface of the smaller part is typically in same level or higher than a top surface of the larger part 7. The top surface of the larger part 7 is the surface facing the substrate.
It will be understood by those skilled in the art that various modifications and changes may be made to the present invention without departure from the scope thereof, which is defined by the appended claims.
Filing Document | Filing Date | Country | Kind | 371c Date |
---|---|---|---|---|
PCT/SE2007/051016 | 12/17/2007 | WO | 00 | 6/11/2010 |
Publishing Document | Publishing Date | Country | Kind |
---|---|---|---|
WO2009/078767 | 6/25/2009 | WO | A |
Number | Name | Date | Kind |
---|---|---|---|
5426405 | Miller et al. | Jun 1995 | A |
5898128 | Romero et al. | Apr 1999 | A |
6128190 | Hardin et al. | Oct 2000 | A |
6163456 | Suzuki et al. | Dec 2000 | A |
7440282 | Brandenburg et al. | Oct 2008 | B2 |
7864532 | Ehret et al. | Jan 2011 | B1 |
20050224956 | Kao et al. | Oct 2005 | A1 |
Number | Date | Country |
---|---|---|
1 729 342 | Dec 2006 | EP |
1729342 | Dec 2006 | EP |
7-221218 | Aug 1995 | JP |
Number | Date | Country | |
---|---|---|---|
20100271780 A1 | Oct 2010 | US |