Abstract:
The present invention relates to surface mount assembly of electronic equipment. More especially it relates to mounting of high frequency electronic components for efficient cooling also with low cost circuit boards. Particularly it relates to efficiently transporting heat and eliminating air gaps in microwave equipment.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The present invention relates to assembly of electronic equipment. More especially it relates to mounting of high frequency electronic components for efficient cooling. Particularly it relates to efficiently transporting heat and eliminating air gaps in microwave equipment. 
       BACKGROUND AND DESCRIPTION OF RELATED ART 
       [0002]    Mounting of flange type packages are known in the art. Flange type packages are known to have good thermal handling capability, but also to not self-align well during reflow soldering and to generally perform poorly at microwave frequencies. A reason to why they do not self-align well is that the restoring action due to surface tension around the solder/air periphery is small relative to the misaligning action caused by gravity and non-planarity/asymmetries. In order to stand the stress from mounting screws, flange type packages need to be robust which is difficult to achieve at reasonable costs combined with fine features and high precision. Self-alignment, fine features and high precision are critical for good microwave performance. 
         [0003]    Packages with fine features, below referred to as fine feature packages, such as BGA (Ball Grid Array) and LGA (Land Grid Array), with many separated pads are known in the art. The thermal performance of these kinds of packages can be acceptable as regards heat transport from the active device to the bottom of the package. 
         [0004]    Various circuit boards are also known in the art. FR4 (Flame Retardant 4) multilayer circuit boards represent one known example type of circuit boards. FR4 circuit boards are essential in almost all modern high volume electronic products. However, they are not particularly well adapted for efficient cooling and high mechanical precision. 
         [0005]    Hard backed boards represent a type of circuit board adapted for efficient cooling and high mechanical precision, however they are available at a substantially higher price than FR4 circuit boards. 
         [0006]    Another known method of achieving heat transport other than through the circuit board is adoption of a convective heat sink. This can be achieved by mounting of cooling flanges on circuit packages or by forcing convection by means of a fan. 
         [0007]    Reflow soldering is a known method of soldering components onto a circuit board. Components are soldered onto a circuit board, which may include earlier soldered circuit boards or parts. 
         [0008]    U.S. Pat. No. 5,019,941 discloses an electronic assembly having enhanced heat-dissipating capabilities. Heat removal is accomplished by utilizing a first metal conductive pad to conduct the heat from an electrical component. The first pad is juxtaposed and thereby thermally coupled by convection to one or more ground pads having plated through holes to a ground plane and heat sink. 
         [0009]    Prior art does not reveal efficient solutions of heat transport away from a board on which a circuit package is mounted without reducing circuitry reliability, particularly it does not reveal heat conductors suitable for surface mounting featuring a thermal path through a board and eliminating or reducing gaps between board and heat sink on the bottom side of the board, rendering low-cost boards available for mounting of high frequency components and applications, e.g. microwave applications. 
       SUMMARY OF THE INVENTION 
       [0010]    A general problem of high frequency electronic equipment is achieving good thermal properties while preserving good electrical performance at reasonable costs and with great equipment reliability. 
         [0011]    Flange type packages, generally, do not perform well electrically at high frequencies (at least not unless used in combination with expensive hard backed boards with milled cavities), while having good thermal properties. Fine feature packages may perform well electrically at high frequencies but impose problems as regards heat transport. 
         [0012]    Convective heat sink generally requires a fan to achieve desired level of heat transport. 
         [0013]    FR4 circuit boards have cost advantages as compared to hard backed boards, while the latter outperforms the former as regards thermal properties. 
         [0014]    Consequently, there is a need of efficiently providing method and means for heat transport and appropriate mounting on circuit boards, not introducing additional requirements on the boards, and preferably not on the packages to be mounted. 
         [0015]    An object of the invention is to increase the thermal transport through a circuit board. 
         [0016]    Another object is to provide a means for arranging heat transport not reducing circuitry reliability. 
         [0017]    Further, it is an object to eliminate or reduce gaps between a circuit board and a heat sink, not requiring clamping means in the immediate vicinity of an electric component. 
         [0018]    It is also an object to provide a means suitable for surface mounting and reflow soldering. 
         [0019]    A further object is to provide a means for heat conduction along a circuit board suitable for automatic mounting. 
         [0020]    Additionally, an object is to economically reduce thermal resistance from a heat source to a heat sink. 
         [0021]    Finally, it is an object to provide a method of reliably assembling circuit board, heat conductor and heat sink. 
         [0022]    These objects are met by method and means of heat conductor for surface mount assembly. 
         [0023]    Preferred embodiments of the invention, by way of examples, are described with reference to the accompanying drawings below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1  illustrates a side view of an example heat conductor, soldered to a circuit board according to the invention. 
           [0025]      FIG. 2  monitors a side view of an example heat conductor clamped to the circuit board according to the invention. 
           [0026]      FIG. 3  shows a top view of an example heat conductor according to the invention. 
           [0027]      FIG. 4  illustrates a schematic flowchart for mounting of an example heat conductor according to the invention. 
           [0028]      FIG. 5  depicts from above an example board with electronic components and heat conducting plates according to the invention. 
           [0029]      FIG. 6  shows a side view of the example board illustrated in  FIG. 5  connected to a heat sink according to the invention. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0030]    As described in the background, there are high frequency flange type packages available. However, they are comparatively expensive and not suitable for surface mount assembly. Further, they require a metal backed board with cavities, which increases costs additionally. 
         [0031]    Fine feature packages can have sufficiently fine features and precision to perform well at microwave frequencies at relatively low costs. Even if the heat transport from the active device to the bottom of the package is at an acceptable level, the path through the board on which they are mounted has a much greater resistance than for the flange type package. Further, present solutions of leading heat through the board rely on vias with limited cross section area and there are also various restrictions on putting vias under the packages. A further problem of prior art solutions is that even if the heat is brought to the side of the board opposite to the side on which the package is mounted, the heat needs be transported to a heat sink. This is a true problem as when fixing the board to the heat sink gaps may occur between the board and the heat sink, unless the board is clamped to the heat sink. Such gaps contribute to undesired thermal resistance. 
         [0032]    If screws are used to clamp the board to the heat sink, they cannot be put in the immediate vicinity of the package, since this would reduce reliability of the circuitry, e.g. there would be a risk of solder joint fractures and board bending. 
         [0033]    Also, generally for fine feature packages the path through (through the board and into the heat sink) has generally much higher thermal resistance than for flange type packages. 
         [0034]    A problem of convective heat sink as a solution to problems in prior art is that in order to give a significant improvement forced convection would be required. 
         [0035]    Problems of FR4 circuit boards relate particularly to limited thermal conduction in the board and limited thermal conduction to a heat sink. 
         [0036]    According to the invention a thermal path is added to the fine feature packages, thereby thermal properties are improved in relation to prior art solutions while preserving desired electrical properties. Also according to the invention, it is applicable for FR4 circuit boards and does not require hard backed boards as flange type packages normally do. 
         [0037]      FIGS. 1 and 2  illustrate a side view, and  FIG. 3  a top view, of an embodiment according to the invention. A heat conductor &lt;&lt;CONDUCTOR&gt;&gt; is shaped into a profile similar to that shown in  FIG. 1  and plated with a surface finish suitable for soldering. Preferably it is provided for mounting as any other component in, e.g., T&amp;R or on a tray. The heat conducting plate &lt;&lt;CONDUCTOR&gt;&gt; is formed with opposite legs to be soldered &lt;&lt;SOLDER&gt;&gt; to an essentially flat circuit board. Preferably the conductor has been pre-shaped such that when put onto a circuit board, as illustrated in  FIG. 3 , three soldering points are in contact with the circuit board. With only three points or legs in contact with the circuit board, it is guaranteed that all points or legs can be in contact with the board, even if the circuit board or conducting plate is skewed to some small extent. This cannot be guaranteed with more than three soldering points or legs. The pre-shaping is preferably such that there is a small gap between a central, and essentially flat, part &lt;&lt;C&gt;&gt; of the conducting plate &lt;&lt;CONDUCTOR&gt;&gt; and the circuit board. This gap is eliminated when one or more screws clamp conducting plate, circuit board and heat sink. 
         [0038]    With reference to  FIG. 1 , according to a preferred mode of the invention a shaped heat conducting plate is soldered close to an electrical component &lt;&lt;IC&gt;&gt;, e.g. an IC (Integrated Circuit), to be cooled. The heat conducting plate is soldered &lt;&lt;SOLDER&gt;&gt; to the circuit board &lt;&lt;BOARD&gt;&gt; close to the electrical component &lt;&lt;IC&gt;&gt;. Preferably, the shaped heat conducting plate and the IC are surface mounted on the circuit board &lt;&lt;BOARD&gt;&gt;. The amount of metal of the heat conducting plate and its thermal conductivity contributes to the heat transport. The use of a separate heat conducting plate also contributes to the invention being mechanically robust and reliable. The heat conducting plate is preferably formed such that the central part &lt;&lt;C&gt;&gt; of the heat conducting plate is separated a small distance, typically in the range of hundreds of micrometers. 
         [0039]    With reference to  FIG. 2 , due to the shape of the plate and it being soldered along opposite sides of the plate &lt;&lt;CONDUCTOR&gt;&gt;, the mechanical stress on the solder joints is relaxed and a central region of the plate &lt;&lt;CONDUCTOR&gt;&gt; can be clamped close to the circuit board &lt;&lt;BOARD&gt;&gt; without causing solder joint fractures. With at least one solder point an un-broken heat conducting path &lt;&lt;PATH&gt;&gt; is formed. Further, a preferred flat surface area at the center of gravity of the plate allows for automatic mounting, e.g. by means of vacuum nozzles. 
         [0040]    With reference to  FIG. 3 , the heat conducting plate &lt;&lt;CONDUCTOR&gt;&gt; is preferably wing shaped &lt;&lt;WING&gt;&gt; to further reduce the mechanical stress while essentially preserving thermal properties. To further reduce the mechanical forces when the plate is pressed towards the board by the mounting screws, there are preferably one or more profiled or bent zones of the heat conducting plate. The one or more zones preferably comprise grooves &lt;&lt;GROOVE&gt;&gt;, along which the heat conducting plate easily bends, e.g. when screws, bolts or corresponding devices &lt;&lt;SCREW&gt;&gt; through mounting holes &lt;&lt;MOUNTING HOLE&gt;&gt; are tightened, pressing the heat conducting plate &lt;&lt;CONDUCTOR&gt;&gt; towards the circuit board &lt;&lt;BOARD&gt;&gt;, as illustrated in  FIG. 2 . To maximize flexibility the grooves are preferably arranged along parallel axes. The relaxed mechanical forces minimize the risk of the circuit board being bent during soldering and minimize the risk of solder joint fracture during and after the soldering operation. Also preferably, the heat conducting plate &lt;&lt;CONDUCTOR&gt;&gt; has more than one hole &lt;&lt;MOUNTING HOLE&gt;&gt; for mounting screws, bolts, or corresponding devices &lt;&lt;SCREW&gt;&gt;. By mounting the heat conducting plate with more than one screw, bolt, or corresponding device, more than one screw can be mounted before tightening the screws, whereby the mechanical stress from torsion and shear stress on the solder joints is relaxed when fixing the circuit board and the heat conducting plate to a heat sink. Also, by use of multiple thermal paths through the board &lt;&lt;BOARD&gt;&gt; the thermal resistance is reduced. To further reduce thermal resistance, screws, bolts or corresponding devices &lt;&lt;SCREW&gt;&gt; are preferably made of metal with high thermal conductivity. 
         [0041]    When fixed to the heat sink, the mounting screws, bolts or corresponding devices &lt;&lt;SCREW&gt;&gt; through the heat conducting plate will transport heat, as illustrated in  FIG. 2  by a dashed arrow &lt;&lt;PATH&gt;&gt;, from the press side of the screw heads &lt;&lt;SCREW&gt;&gt; to the heat sink &lt;&lt;SINK&gt;&gt;, and the electrical component will be cooled. The one or more screws contribute to a low resistance path formed by the screws, or corresponding devices. The heat does not need to be transported to the sink by the board material. Due to the clamping force from the screws, there is no or negligible air gap &lt;&lt;AIR-GAP&gt;&gt;, which could deteriorate performance, between the layers formed by the heat conducting plate &lt;&lt;CONDUCTOR&gt;&gt;, the circuit board &lt;&lt;BOARD&gt;&gt; and the heat sink &lt;&lt;SINK&gt;&gt; in the vicinity of the heat conducting plate and its mounting screws &lt;&lt;SCREW&gt;&gt;, as illustrated in  FIG. 2 , and the dominating component of heat transport is due to the stress from the screws, bolts or corresponding devices. 
         [0042]    An advantage of the invention is that it does not require thermal gap fillers, of e.g. thermal pads or thermal grease, to achieve low thermal resistance. The invention, however, does not exclude use of thermal gap fillers. Among the drawbacks are risk of contamination by thermal grease that leads to poor electrical contact or poor solder ability, risk of air entrapment, additional process steps of inserting thermal grease or thermal pads introducing extra processing time and cost, and cleaning and applying of new grease when rework is required. Gap fillers may also increase thermal resistance and require high mounting precision. 
         [0043]    The heat conducting plate is preferably made of deformable or flexible metal with high thermal conductivity. The ease with which the heat conducting plate deforms, particularly along the grooves &lt;&lt;GROOVE&gt;&gt; and the extensions/wings &lt;&lt;WING&gt;&gt;, allows the extensions/wings &lt;&lt;WING&gt;&gt; to adjust such that the center portion of the heat conductor approaches the board during screw mounting, as shown in  FIG. 2 . 
         [0044]    The invention is particularly well suited for electrical components, e.g. ICs, that allow a good thermal connection from the semiconductor die of the integrated circuit to the side of the package. This can be achieved, e.g., with LGA packages, which have signal pads on only two sides. 
         [0045]    Influence on positioning of an electrical component, irrespective of its reflow self alignment, from a heat conducting plate arrangement according to the invention can be eliminated by ensuring that solder from the pads for wing of heat conducting plate and package, respectively, will not come into contact. This can be ensured, e.g., by putting a line of solder-stop separating the pads. Thereby a continuous copper plane can be used for maximum heat conduction. 
         [0046]    Among the advantages of the invention are, as intimated above, improved thermal performance without degraded electrical performance in microwave frequency range and without adding new requirements on the circuit board or on the soldering process. 
         [0047]    When mounting onto a heat sink &lt;&lt;SINK&gt;&gt; it has also been mentioned above the preference of installing more than one screw &lt;&lt;SCREW&gt;&gt; into the heat conducting plate &lt;&lt;CONDUCTOR&gt;&gt; prior to tightening the screws. 
         [0048]    Mounting is preferably performed in two or more steps, schematically illustrated in  FIG. 4 . The preferred steps include soldering of the heat conducting plate to the circuit board &lt;&lt;S 1 &gt;&gt;. Electrical components are preferably mounted in the same reflow process &lt;&lt;S 1 &gt;&gt; as the heat conducting plates. They can also be soldered in a second reflow process, before or after soldering of the heat conducting plates. Mounting the circuit board to a chassis/heat sink, is preferably performed in one step, however illustrated in three distinct steps &lt;&lt;S 2 &gt;&gt;, &lt;&lt;S 3 &gt;&gt;, &lt;&lt;S 4 &gt;&gt; in  FIG. 4  for reasons of clarity. One or more circuit boards with mounted components and one or more heat conducting plates are positioned in relation to mounting holes on heat sink, and possibly also to a chassis where it is installed &lt;&lt;S 2 &gt;&gt;. Mounting screws or corresponding devices are positioned, and possibly loosely tightened in more than one hole of each heat conducting plate according to the invention &lt;&lt;S 3 &gt;&gt;. Preferably all mounting screws are positioned prior to tightening the screws &lt;&lt;S 4 &gt;&gt;, thereby fixing the heat conducting plate, the circuit board and the heat sink. When essentially simultaneously mounting the board in a chassis also screws holding the circuit board to the chassis are tightened. 
         [0049]      FIG. 5  illustrates a top view of an example board with electronic components &lt;&lt;e 1 &gt;&gt;, &lt;&lt;e 2 &gt;&gt; and heat conducting plates &lt;&lt;c 1 &gt;&gt;, &lt;&lt;c 2 &gt;&gt;, &lt;&lt;c 3 &gt;&gt; positioned in the vicinity of components &lt;&lt;e 1 &gt;&gt; and &lt;&lt;e 2 &gt;&gt; and mounted as described above. In the figure is illustrated for the example board that one conducting plate &lt;&lt;c 1 &gt;&gt; is arranged for conducting heat from an electronic component &lt;&lt;e 1 &gt;&gt; and more than one conducting plate &lt;&lt;c 2 &gt;&gt;, &lt;&lt;c 3 &gt;&gt; are arranged for conducting heat from an electronic component &lt;&lt;e 2 &gt;&gt;. In order to maximize cooling, each heat conducting plate &lt;&lt;c 1 &gt;&gt;, &lt;&lt;c 2 &gt;&gt;, &lt;&lt;c 3 &gt;&gt;, is positioned with two of their preferably three legs facing the electrical component &lt;&lt;e 1 &gt;&gt;, &lt;&lt;e 2 &gt;&gt; to be cooled primarily. When more than one heat conductor can be put around a package, the thermal resistance is further reduced. With both legs soldered to the board two un-broken heat conducting paths are formed thereby approximately halving the thermal resistance related to each plate. The illustrated heat conducting plates &lt;&lt;c 1 &gt;&gt;, &lt;&lt;c 2 &gt;&gt;, &lt;&lt;c 3 &gt;&gt; comprise extensions/wings, in preferred more than one direction, in order to maximize cooling. Each mounting plate in the example realization is illustrated mounted with two screws &lt;&lt;sc 11 &gt;&gt;, &lt;&lt;sc 12 &gt;&gt;, &lt;&lt;sc 21 &gt;&gt;, &lt;&lt;sc 22 &gt;&gt;. 
         [0050]      FIG. 6  illustrates a side view of the example board according to the invention illustrated in  FIG. 5  with heat sink &lt;&lt;s&gt;&gt; to which the heat conducting plates &lt;&lt;c 1 &gt;&gt;, &lt;&lt;c 2 &gt;&gt; and the board &lt;&lt;B&gt;&gt; are clamped by screws, bolts or corresponding devices &lt;&lt;sc 11 &gt;&gt;, &lt;&lt;sc 12 &gt;&gt;, &lt;&lt;sc 21 &gt;&gt;, &lt;&lt;sc 22 &gt;&gt;. 
         [0051]    In this patent application acronyms such as FR4, BGA, LGA and IC are applied. However, the invention is not limited to equipment with entities with these acronyms, but holds for all equipment similar in nature. 
         [0052]    The invention is not intended to be limited only to the embodiments described in detail above. Changes and modifications may be made without departing from the invention. It covers all modifications within the scope of the following claims.