Patent Application
20080107867
1. Field of the Invention
This invention relates to low profile bonding two surfaces that may have thermal conductivity, electrical conductivity, or magnetic permeability together with minimal losses occurring across the resultant bonding zone. More particularly this invention relates to low profile bonding in electrical circuit boards employing surface mount technology to heat producing electrical components.
2. Description of the Related Art
Thermal conductivity is a property common to many materials and especially metals. A thermally conductive material is a substance that allows the rapid flow of heat throughout its mass. Many electrical components such as resistors and semi-conductor devices generate considerable amounts of heat during their operation. This heat may result from electrical losses from voltage drops across current carrying conductors. More particularly, power dissipation in watts is equal to the voltage drop across the device times the current flowing through the device in amperes. Voltage drops may take numerous forms including the following:
1) V=IR in resistors where V=The voltage drop across the device, I=The current in amperes flowing through the device, and R=The resistance value in Ohms.
2) Semi-conductor junction potentials. Semi-conductor devices such as diodes have a voltage drop associated with them that is dependent on the particular device. This forward voltage drop typically varies from 0.4 volts to 1.0 volts.
3) Electrical losses in transistor devices such as MOSFETS. Transistors are semi-conductor devices that amplify signals by allowing a small input current to control a much larger output current. MOSFETS are a special type of transistor known as metal oxide semi-conductor field effect transistors. MOSFETS have very high amplification values and can be used as solid state switches for direct current as well as voltage controlled amplifiers. MOSFETS have the following characteristics with respect to voltage drops. They have a specified on state resistance expressed in Ohms and switching losses.
3a) On state resistance refers to the resistance of the device in Ohms to direct current flowing through the device at a specified level when the device is fully switched on. This on state resistance rating is usually specified at a relatively low operating temperature. Generally speaking, as operating temperatures increase so do the on state resistance values of MOSFETS.
3b) Switching losses refer to heat generating electrical losses associated with voltage drops that occur as a result of the device being only partially turned on or partially turned off. When a mosfet or other semi-conductor transistor device is being turned on, a significant voltage drop may occur across the device prior to the device being fully turned on. Likewise, the same situation holds true when switching such devices off. It takes time to turn on and to turn off transistors. During this switching interval The transistor behaves like a resistor and may exhibit appreciable voltage drops. These voltage drops result in switching losses that show up in the form of heat.
3c) Semi-conductor junction losses may also be present in transistors owing to the presence of junctions.
4) Resistive losses in coils and other current carrying devices.
5) Capacitive heating effects in devices involved in high frequency switching.
5a) Sometimes referred to as dielectric loss.
6) Inductive heating effects
6a) Eddy currents induced in conductive materials in the presence of changing magnetic fields.
Many of the elements in the periodic table are metals. Metallic elements such as copper, aluminum, and silver are good conductors of heat and electricity. Because of this they are used in electrical circuitry to carry electric current as well as heat sinks for removal of heat from heat generating components. Of all of the metallic elements, silver and copper are the best conductors of both heat and electricity. Because of the higher cost of silver, copper is commonly employed for this purpose. Aluminum and many of its alloys are also good conductors of heat and electricity. Aluminum is lower in cost than both copper and silver. Unlike copper, aluminum has a strong tendency to form electrically insulating oxide coatings on exposure to air. Because of this, aluminum may not be suitable for many applications requiring the conduction of electric current across two or more contacting surfaces. In addition, aluminum and its associated alloys often present significant difficulties in the soldering process. When ordinary tin lead alloy solder is melted against an aluminum surface, proper flow and surface wet out may not occur. This may be particularly true for use in printed circuit boards where components are soldered into place.
Printed circuits are comprised of numerous electrical components that are soldered into place and electrically connected to each other onto a printed circuit board. The printed circuit board itself often takes the form of a flat planar rigid electrically insulating construction having electrically conductive copper clad laminated on to one or both sides. Copper is often used because it is a good conductor of electricity and lower in cost than silver. Unlike aluminum that is hard to solder to, copper readily accepts melted solder thereby providing good wet out properties.
Traditional printed circuits are made in the following manner. One or more exposed copper surfaces of a printed circuit board is covered with a thin layer of photo resist. Photo resist is a material that changes its solubility to certain developing solutions on exposure to light. Negative photo resists become less soluble on light exposure while positive resists become more soluble on light exposure. A pattern of conductive paths required for the particular circuit in question is made on a transparent plastic film using photographic techniques. This film with its pattern may be referred to as a master. The master is placed over the copper clad board and exposed to light for a set period of time. This may be carried out under the conditions of vacuum in order to maintain good contact between the master and light sensitive photo resist. After exposure, the master is removed and the pattern developed in the photo resist with a suitable developing agent. The circuit board is then rinsed clean from the developing solution and the areas of exposed copper etched away with a suitable etchant. The freshly etched circuit board is then thoroughly rinsed. Photo resist that is now covering the copper pattern on the board is then removed with a stripping agent and the board rinsed clean. The result is a pattern etched board having electrically conductive paths of copper in a suitable pattern for manufacturing a printed circuit board. Holes are then drilled into the board at the appropriate locations for mounting individual electrical components. Individual components are then placed into the circuit board by placing their leads into these holes and soldered firmly into place.
It should be noted that the above described method of manufacture for printed circuit boards is a brief generalization with many possible modifications. It should also be noted that significant detail has been omitted in order to outline the overall process. Certain aspects of the process are well known art and therefore do not require significant elaboration while other aspects more relevant to the present invention will now be explained in further detail.
Etchants suitable for removing copper from developed printed circuit boards are generally water based acid solutions and often contain metal cations (positively charged metal atoms) in an oxidized state. The anions (negatively charged atoms or groups of atoms) may be any number of materials with chloride ion being common. Suitable positively charged oxidized metal atoms include ferric ions and cupric ions. Ferric ions are iron atoms having a +3 charge. Ferric ions are easily reduced to the +2 charge by the addition of one electron to become ferrous ions. Cupric ions are copper atoms having a +2 charge. Cupric ions are easily reduced to the +1 charge by the addition of one electron to become cuprous ions. These electrons come from the exposed copper metal of the board by oxidizing the copper into cuprous ion that dissolves in the acidic water based etchant. In the presence of hydrochloric acid, this cuprous ion rapidly dissolves away thereby efficiently removing unwanted copper from the board. As more and more copper dissolves into the etching solution, the etchant becomes depleted and needs to be replaced. Replacement of etchant represents both a purchasing cost for new etchant and a disposal cost from the waste depleated etchant.
Water based etchants containing cupric chloride along with hydrochloric acid may be extended by employing the following process. When cupric chloride solutions etch copper, the basic reaction can be summarized as follows:
CuCl2+Cu→2CuCl.
The resulting CuCl reacts with HCl to form chlorocuprous acid (a complex of cuprous chloride and hydrochloric acid that is water soluble). Water based solutions of chlorocuprous acid will react with hydrogen peroxide forming cupric chloride and water. The simplified overall chemical reaction can be summarized as follows:
2CuCl+2HCl+H2O2→2CuCl2+2H2O
When etching pure copper clad circuit boards with cupric chloride, it may be desirable to add a small amount of ferric chloride to the mixture. It may also be desirable to maintain and control acid levels by the addition of hydrochloric acid when needed.
It should be noted that because of water losses normally encountered in the etching process (particularly at elevated temperatures) the standard 3% hydrogen peroxide sold in ordinary drug stores will often suffice.
The above described cupric chloride etchant converts copper etched from circuit boards into more etchant. While in theory the etchant could be used forever, in practice, replacement of waste etchant and its subsequent disposal still remains an issue.
Electro-etching employs electricity as the oxidizing agent that drives the dissolving of metals. More particularly, when a positive direct current potential is placed on a piece of metal in a water based ionic solution, there is a strong tendency to oxidize the surface of the metal piece by the direct removal of electrons. Under suitable conditions the metal may be made to cleanly dissolve into the water solution.
Electro-plating employs electricity as the reducing agent that drives the deposition of metals onto conductive surfaces. More particularly, when a negative direct current potential is placed on a piece of metal in a water based solution of positively charged metal ions, there is a strong tendency to plate out metal on the surface by neutralization of metal ions with electrons. Under suitable conditions metals may be plated out in this way.
Under suitable conditions electro-etching and electro-plating may be carried out simultaneously thereby transferring metal from one surface to another. This approach may be used to alleviate a significant amount of cost associated with the purchase and disposal of etching solutions.
Electro-forming involves the patterned deposition of metals from solution onto conductive substrates. This is achieved by masking off areas where deposition is to be avoided and selectively electro-plating the exposed areas of the pattern.
Electroforming may be carried out to produce a desired pattern of raised areas or protrusions for the heat dissipating bonding applications of the present invention. In addition, electroforming operations may be carried out in order to improve heat dissipating properties of copper bonding pads by extending areas of raised surface topography beyond the bonding zone.
Heat generating circuit components present numerous issues with circuit design. Excessive heat generated within individual components has traditionally been alleviated by employing heat sinks. A heat sink is a piece of thermally conductive material that is placed against a heat generating electric circuit component to transfer heat. The heat sink itself may have ridges or other means of increasing surface area for the purposes of improving heat dissipation by the normal processes of convection, conduction, and radiation. Heat sinks are usually made of aluminum and special alloys of aluminum that conduct heat efficiently and are light weight.
Due to the method of manufacture, traditional printed circuit boards have their components in a vertical position. That is to say that when holes are drilled into a circuit board and the leads of individual components are placed into these holes and soldered into place, the components end up in a vertical position. When relatively large heat sinks are placed on these components, a significant overall vertical height often results. The high profile geometry adds significant height and bulk to finished printed circuit boards. This added height of individual components and their attached heat sinks results in a need for taller enclosures and may disrupt air flow patterns from cooling fans.
Because copper has good electric conductivity and is easy to solder to, printed circuit boards usually employ copper. It should be noted that copper is a better conductor of heat than aluminum. Because of this, the copper clad coating employed in printed circuit boards has good thermal heat dissipating properties. The thickness of this copper clad coating is typically one or two ounces of copper per square foot. Two ounce copper clad circuit boards will carry more current than the thinner one ounce per square foot copper clad boards. Because of this, higher copper thickness boards are desirable for use in high power applications. Many of these high power applications employ components that generate considerable heat during use.
Standard printed circuit board construction techniques often involve drilling numerous small holes. These holes are used for placing the leads of individual components prior to soldering into place. Drilling these holes may present problems. For example, many printed circuit boards employ glass composite materials of sufficient hardness to dull drill bits and also contain soft copper that may gum up diamond bits. Because of this, a compromise is often reached between a bit designed for hard materials and one for softer materials. Carbide bits may be employed for this purpose due to their suitable geometry for drilling into soft metals such as copper and having sufficient hardness to drill through glass laminate.
Despite numerous standard configurations for printed circuit boards there remains a need for printed circuit boards having low profile planar surface mounting means for circuit components along with good heat dissipation properties.
It is an object of this invention to provide a bonding means suitable for printed for surface mounting circuit components to circuit boards.
It is a further object of this invention to provide a bonding means having an unusually low profile.
It is a further object of this invention to eliminate component lead holes in printed circuit boards.
It is a further object of this invention to mount circuit components in a low profile configuration on printed circuit boards.
It is a further object of this invention to eliminate large and bulky heat sinks from individual components on printed circuit boards.
It is a further object of this invention to provide printed circuit boards with heat dissipating properties.
Finally it is an object of this invention to provide printed circuit boards having an overall geometry that facilitates removal of excess heat with moving air.
This invention therefore proposes low profile bonding surfaces that may be used to surface mount heat generating electrical components to printed circuit boards. Individual circuit boards may be provided with thermally conductive mounting zones for individual circuit components along with added heat dissipation zones comprised of thermally conductive materials such as copper. Thermally conductive mounting zones may have numerous surface protrusions extending from the surface to facilitate bonding with a bonding agent. These numerous surface protrusions may have an ultra low profile that facilitates the transfer of heat from components and into heat dissipating circuit boards. Additionally, protrusions may extend beyond the bonding zone thereby providing additional heat dissipation. The printed circuit boards themselves may further employ thermally conductive materials such as copper and aluminum within their interior layers thereby turning the entire printed circuit board into one large heat sink.
A more complete appreciation of the invention and many of the advantages thereof will be readily obtained as the same becomes better understood by reference to the detailed description when considered in connection with the accompanying drawings, wherein:
As mentioned earlier printed circuit boards are etched in a pattern required for electrically connecting individual circuit components into a functioning printed circuit. Proper design and subsequent placement of components can be used to provide relatively large areas for mounting of individual heat generating circuit components. These areas may then be masked off prior to etching to leave behind copper pads of considerable size. The individual heat producing components may then be mounted to these copper pads in a low profile horizontal position using a through hole with a nut and screw in the standard manner used to mount a traditional heat sink. Heat sink compound may be employed to provide good thermal contact between the component and copper pad of the printed circuit board. In this way, circuit component may be mounted in a low profile configuration directly onto a copper pad that is part of the printed circuit board itself. The non-etched copper pad portions of the circuit board itself now act as heat sinks. Experimental testing with this method has achieved good results. More particularly, excess heat from switching transistors has been successfully removed by employing one square inch copper pads and mounting the transistors in a planar horizontal configuration as described above. A small fan was employed to blow air over the components and the board to remove the excess heat.
One aspect of the present invention provides a significant improvement to the above described method of removing heat from individual components using copper pads on circuit boards. This method employs thermally conductive bonding agents to attach individual circuit components to their respective heat sink pads thereby eliminating the need to drill mounting holes. The heat removing copper bonding pads may employ special surface geometries that facilitate both bonding as well as the removal of heat. Eliminating holes in printed circuit boards represents a reduction in complexity and tooling costs associated with the printed circuit board assembly process.
One way of eliminating many of the holes in printed circuit boards is to employ the surface mounting technology of the present invention. This may be accomplished by having all components and connections to components on one side of the printed circuit board and mounting individual components into place on one side of the circuit board using a bonding agent. Once the bonding agent cures, component leads may be soldered into place. The result is a low profile circuit board having components firmly attached to the circuit board by both their soldered leads and the bonding agent.
The bonding areas on the circuit board surface for individual components may take numerous forms. Light etching may be used to roughen the bonding surfaces or alternatively, a special bonding surface may be prepared having numerous protrusions extending in an outward direction from the surface of the copper bonding pad. These protrusions may be made of copper or another thermally conductive metal or alternatively may be comprised of a composite material filled with copper powder or other thermally conductive filler.
So far, the heat sinking aspects of the present invention have been limited to relatively small areas of printed circuit boards. It should be noted that surface mounting of circuit components eliminates the need to drill holes through printed circuit boards. Because of this, it is now possible to employ thermally conductive materials into the individual layers of the clad board itself. In this way, the entire circuit board may function as an effective heat sink. In order to achieve this end, copper cloth or other thermally conductive material may be incorporated into the individual layers of the composite board. The exact thermally conductive structure of the board itself will of course depend on the end use application. For example, high frequency applications may require the limiting of capacitive and/or inductive effects in order to reduce capacitive and/or inductive losses. These applications may require the use of more coarse copper mesh and thermally conductive powders and less copper screen and cloth materials. Alternatively, certain applications may require RF shielding. In these instances, copper screen and cloth materials within the printed circuit board itself may provide good RF shielding and heat sinking properties simultaneously.
Copper is one of the best conductors of heat and therefore various forms of this material that are useful for producing heat conductive laminate constructions will now be described in some detail.
1) Copper Wire Mesh. Available from American Art Clay Co., Inc. 4717 W. 16th St., Indianapolis, Ind. 46222.
2) Powdered Copper. Prepared by reducing copper metal from aqueous solutions of copper sulfate using mossy zinc.
3) Powdered copper having a lacquer coating. Prepared by mixing powdered copper with a dilute solution of a lacquer material and evaporating to dryness.
4) Copper Wire Form. Available from American Art Clay Co., Inc. 4717 W. 16th St., Indianapolis, Ind. 46222.
5) Copper Wire Screen. Available from such places as McMaster Carr, This screen material is more open than copper wire mesh and comes in numerous sizes.
6) Copper coated Iron particles. Particles of iron having a thermally conductive copper outer layer. Thin layers of copper metal may be deposited on iron particles by deposition from aqueous copper sulfate solutions whereas thicker layers of copper can be electroplated onto the surface. Provides magnetic properties that may be used to hold circuit components into place during the cure process of bonding agents.
7) Copper foil having numerous holes.
8) Copper foil having etched surfaces for bonding.
9) Copper foil having electroformed surface protrusions.
Copper powder prepared by the interaction of mossy zinc with aqueous copper sulfate solutions may be used to provide good thermal conductive properties to composite circuit board materials. The preparation of this form of powdered copper along with beneficial properties and use will now be explained in some detail.
Zinc is a relatively reactive metal. It is one of the most reactive metals that may be electroplated from water based solutions. Copper is less reactive than zinc. Because of this, if a piece of zinc is placed in a water based solution of a copper salt (such as copper sulfate) the zinc will dissolve into solution and the copper will deposit on the zinc surface. The overall reaction is a displacement reaction and can be expressed as follows:
Mossy zinc is zinc metal that in the form of various small odd shapes that results from pouring molten zinc into water. This form of zinc is a useful form for displacing copper from water based solutions of copper sulfate. The following example will now be given to illustrate the preparation of a suitable form of copper powder for use in the present invention.
0.3 moles (75 grams) of copper sulfate pentahydrate were placed into a one liter glass container. To this were added 500 milliliters of distilled water. The mixture was stirred until about half of the solid dissolved. 20 grams of mossy zinc (slightly over 0.3 moles) were added and the mixture allowed to stand for twenty minutes. The mixture was lightly stirred every fifteen minutes until all of the copper sulfate had dissolved and the resultant solution rendered clear (indicating that no more copper sulfate was present). The copper powder was then collected on filter paper and the residual pieces of zinc removed. The copper powder was then rinsed with one liter of distilled water and allowed to drip dry. The resulting copper paste was placed on fresh filter paper to absorb more water. The semi-dry copper paste was then placed on a piece of fresh filter paper and allowed to air dry to constant weight. The dried copper powder was then removed and weighed. The mass of copper powder was found to be 18.5 grams (0.291 moles). This corresponds to a yield of 97%. The resulting powder was a dark reddish tan color.
The above prepared copper powder was then mixed with epoxy resin in order to determine proper loading density. It was found that 65% loading density resulted in a relatively thin paste and that up to 75% by weight of added copper powder could be mixed in before becoming too difficult to work with. The epoxy resin used was West Systems 105 resin. This was cured with the recommended amount of 205 hardener. West System Inc PO Box 665 Bay City, Mich. 48707. The copper loaded epoxy resin compositions were allowed to cure at room temperature for 24 hours. Inspection of the cured copper loaded epoxy revealed a uniform distribution of copper powder in the mix along with good thermal conductivity. Subsequent surface testing using an Ohm meter revealed good electrical insulating properties to the low voltages used in the test.
It should be noted that in some instances it may be desirable to drill holes through a circuit boards for special purposes. For example, it may be desirable to make a solid electrical connection from one side of the board to the other. In such instances, heat conductive copper within the board itself may be eliminated in select areas prior to assembly of the composite board laminate. In other words, areas of thermal conductivity within the circuit cladding itself may have desirable patterns that leave spaces for the drilling of through holes. Employing patterns of thermally conductive copper within the composite portions of printed circuit boards broadens their use by separating the board into discrete areas of high heat transfer and into discrete areas where it is not desirable to have copper materials embedded within the inner layers of the board.
It should be noted that it may be desirable to employ the surface mounting of components on both sides of the printed circuit board and have areas in the board that are free from internal copper. These areas may be used for drilling holes through the board to make electrical connections from one side to the other without interference from internal electrically conductive copper.
While advantageous in numerous applications, it is to be understood that the heat sinking capabilities of printed circuit boards in their present state may be somewhat limited. It should also be noted that holes must be drilled into circuit boards in order to firmly attach individual components to their etched heat sink pads.
Circuit board 58 is shown having board portion 60 along with etched soldering pad portions 62. Also shown are heat sink pads 64 as part of circuit board 58 and are formed during the original etching process. Heat sink pads 64 on circuit board 58 are formed in the same manner as soldering pad portions 62. In this respect, little additional effort is required resulting in the elimination of heat sinks 36 of
The protrusions themselves may be formed from the original bonding pads by electroplating through a patterned mask. This process is known as electroforming. The electroforming process produces protrusions that are part of the copper pad underneath and therefore possess good strength and thermal conductivity.
Other possible methods for producing thermally conductive bonding protrusions include the following:
1) Bonding small copper beads in a pattern on the heat sink pads using electrically conductive epoxy and electroplating over the entire surface.
2) Etching the heat sink pad surface to promote adhesion followed by silk screening on conductive epoxy and then electroplating over the entire area.
3) Etching the heat sink pad surface to promote adhesion followed by using a thermally conductive bonding agent to attach pre-cut copper foil having electroformed surface protrusions.
4) Etching the heat sink pad surface to promote adhesion followed by silk screening a pattern of thermally conductive protrusions.
Thermally conductive bonding pad 66 eliminates the need to drill mounting holes, may be used to hold components into place prior to soldering operations, keeps components firmly attached to their circuit boards, and helps to remove of excess heat.
Circuit board 70 is shown having board portion 72 along with etched soldering pad portions 74. Also shown are heat sink bonding pads 76 having numerous protrusions 78 for attaching circuit components and removing heat. Protrusions 78 extend past the component bonding zone and provide added exposed surfaces for dissipating heat.
The addition of fins to heat sink bonding pads 86 provides a relatively easy way to remove excess heat. This excess heat may dissipate by the natural mechanisms of convection, conduction, and radiation. Alternatively a fan may be placed near the bonding pad to provide additional cooling. Additional cooling may be added by the incorporation of thermally conductive material to the interior portions of the circuit board itself. This aspect of the present invention is shown in
Heat produced from circuit components bonded to bonding pad 104 of printed circuit board 94 is conducted into bonding pad 104 where it spreads out and transfers through top composite portion 100 and into metal mesh portion 98. Metal mesh portion 98 then dissipates this heat throughout the entire circuit board.
The metal mesh used in mesh portion 98 of heat dissipating circuit board 94 may be made from numerous metals including aluminum and copper. The mesh itself may take numerous forms such as WireForm, screen, and woven cloth. The particular metal mesh used will be determined at least in part by the particular application. For example, circuitry running at relatively high frequency and or high alternating currents may benefit from a relatively open mesh such as WireForm in order to reduce capacitive and inductive effects. On the other hand, low frequency circuits requiring a high degree of heat dissipation may benefit from woven copper cloth.
Top composite portion 100 serves to electrically isolate metal mesh portion 98 from bonding pad 104. Bottom composite portion 96 and top composite portion 100 may contain thermally conductive additives such as powdered metals and their oxides. These additives may be employed to further enhance the overall thermal conductive and heat dissipating properties of heat dissipating printed circuit board 94.
When soldering component leads onto their copper pads, rapid removal of heat may result in cold solder joints of poor quality. Adding zones of insulation to printed circuit boards may help to improve solder joints by keeping the heat within the soldered joint long enough to allow proper solder flow prior to its solidification. Numerous materials may be employed to improve heat insulating properties of printed circuit boards. One material is Dicaperl available from Plastic Depot. 2907 San Fernando Blvd. Burbank, Calif. 91504. Dicaperl is a lightweight filler used in composites. It is comprised of numerous hollow glass micro-spheres. This thermally insulating material may be mixed with resin and employed in discrete zones within the internal portions of printed circuit boards.
Employing discrete thermally insulating zones of glass micro-sphere filled composites in printed circuit boards having enhanced thermal conductivity provides easy soldering of component leads. This may be of particular value when employing circuit boards having enhanced thermal conductivity that may be fixedly attached to heat sinks. Insulating zone 148 may be comprised of hollow glass micro-spheres embedded into a polymer resin such as epoxy.
Protrusions 164 are shown as part of metal foil portion 162. This may be accomplished by means of electroforming through a suitable pattern of photoresist.
Heat produced from circuit components bonded to bonding pad 176 of printed circuit board 166 is conducted into bonding pad 176 where it spreads out and transfers through top composite portion 172 and into metal foil portion 170. Metal foil portion 170 then dissipates this heat throughout the entire circuit board.
The metal foil used in portion 170 of heat dissipating circuit board 166 may be made from numerous metals including aluminum and copper. The foil itself may take numerous forms such as having holes or surface protrusions. The particular metal foil used will be determined at least in part by the particular application.
Top composite portion 172 serves to electrically isolate metal foil portion 170 from bonding pad 176. Bottom composite portion 168 and top composite portion 172 may contain thermally conductive additives such as powdered metals and their oxides. These additives may be employed to further enhance the overall thermal conductive and heat dissipating properties of heat dissipating printed circuit board 166.
When soldering component leads onto their copper pads, rapid removal of heat may result in cold solder joints of poor quality. Adding zones of insulation to printed circuit boards may help to improve solder joints by keeping the heat within the soldered joint long enough to allow proper solder flow prior to its solidification. Numerous materials may be employed to improve heat insulating properties of printed circuit boards. One material is Dicaperl available from Plastic Depot. 2907 San Fernando Blvd. Burbank, Calif. 91504. Dicaperl is a lightweight filler used in composites. It is comprised of numerous hollow glass micro-spheres. This thermally insulating material may be mixed with resin and employed in discrete zones within the internal portions of printed circuit boards.
Employing discrete thermally insulating zones of glass micro-sphere filled composites in printed circuit boards having enhanced thermal conductivity provides easy soldering of component leads. This may be of particular value when employing circuit boards having enhanced thermal conductivity that may be fixedly attached to heat sinks. Insulating zone 220 may be comprised of hollow glass micro-spheres embedded into a polymer resin such as epoxy.
Numerous methods may be employed to hold individual components in place during the cure of the bonding agent. Numerous methods may be employed including pressing down with silicone rubber sheets, the implementing of various fixtures, and the spring action of previously soldered leads. Of particular interest is the use of magnets for holding individual circuit components into place during the cure cycle of bonding agents. Numerous magnet types may be used including ceramic and rare earth. A piece of steel or other magnetic material may be placed on the back side of the circuit board for this purpose.
Another option for magnetically holding individual circuit components into place involves the addition of magnetic materials to the printed circuit boards themselves for this purpose. Numerous forms of magnetic material may be used including steel sheet, iron wire and cloth made from iron and its magnetic alloys. Of particular interest is the addition of powdered iron into the composite layers of printed circuit boards. The powdered iron may by coated with copper, or alternatively may be mixed with copper to further improve thermal conductivity. Other magnetic materials including magnetic iron oxide may be used as well.
Heat produced from circuit components bonded to bonding pad 250 of printed circuit board 240 is conducted into bonding pad 250 where it spreads out and transfers through top composite portion 246 and into metal mesh portion 244. Metal mesh portion 244 then dissipates this heat throughout the entire circuit board.
Magnetic section 252 is shown located directly below circuit component 254. Magnetic section 252 is composed of a magnetic material such as powdered iron composite. Magnetic section 252 may be used to hold circuit component into place with permanent magnet 256 during the bonding agent cure cycle. Once the bonding agent is cured, permanent magnet 256 may then be removed.
Special attention will now be paid to the ultra low profile bonding aspects of the present invention and their resulting advantageous properties with respect to thermal dissipation.
The hemispherical protrusions of
Electroforming involves plating of metal through a patterned mask that covers a conductive surface. In short, metal is deposited in a pattern by the process of electroplating. Electroplating and electroforming operations are well known art and therefore significant detail of these operations be given here.
Undercut zone 290 while providing interlocking properties to bonding agents often results in a relatively small cross sectional area of attachment between top portion 286 of base portion 288 and lower curved portion 292 of hemispherical protrusion 284. This may result in a zone of high thermal resistance, and/or electrical resistance, and/or low magnetic permeability.
The hourglass shaped protrusions illustrated in
The hourglass shaped protrusions illustrated in
Other possibilities include laminating, exposing, and developing multiple layers of photo-resist to build up a pattern having internal voids having an hourglass shape and electroforming to replace the void with a suitable material such as copper.
In certain instances it may be possible to directly etch hourglass shaped protrusions into a metal surface by controlling the etch process itself. A developed copper clad board having photo-resist remaining in a pattern of circles in theory should etch out cylindrical shaped protrusions. However, if during the etch process a greater supply of etchant is available to the central portions than the top and bottom portions, the cylindrical shape may be modified significantly. More particularly this modification would tend to result in protrusions having an hourglass shape. In addition, metals having different etch rates may be plated in subsequent layers. Etching these constructions may result in different etch rates from top to bottom thereby modifying the overall protrusion geometry.
Good heat transfer characteristics of bonding substrate 354 may be realized by paying close attention to materials and their configuration. For example, copper is a good material choice for both thermally conductive bonding surfaces. Thin film FR-4 laminate of about 0.005″ in thickness has minimal thickness and therefore adds only minimal thermal resistance. Such material is available from Injectorall Electronics Corp located at 110 Keyland Court, Bohemia, N.Y. 11716. thin film FR-4 material is available in 0.005″ thickness with 0.5 ounce copper cladding on both sides. Electroplating copper onto this cladding to a thickness of 0.005″ on both sides followed by electroforming copper protrusions from about 0.005″ to 0.010″ in height would result in useable substrates for numerous applications. The exact configurations employed may of course depend on the particular applications employed.
The finished thermally conductive bonding laminate material may then be laminated with a thick photo-resist material, and subsequently exposed through a master, developed, and etched. Once etched, the board may then be stripped of undeveloped photo-resist and thoroughly cleaned. Electrical components may then be bonded into place and later soldered. The flexible circuit board may now be bonded on the back side to a rigid heat sink. By soldering component leads to their etched pads prior to bonding the construction to a heat sink, soldering may be made easier. Attempting to solder component leads to etched pads after heat sink bonding has taken place may require large inputs of heat energy in order to achieve good melting and subsequent wet out of solder.
Thermally conductive bonding agent 402 may be comprised of a mixture of a liquid bonding agent such as epoxy blended with a material that enhances thermal conductivity. For example liquid epoxy bonding agents may be blended with powdered copper to enhance thermal conductivity. Copper powder prepared by the interaction of mossy zinc with aqueous copper sulfate solutions may be used to provide good thermal conductive properties to composite circuit board materials.
1) Bonds wire 406 securely into place.
2) Provides a good electrical connection to wire 406.
3) Provides an electrical connection capable of handling high current values.
4) Provides an electrical connection capable of dissipating substantial amounts of heat.
5) Improves the overall quality of soldered connections by increasing the available interfacial surface area between the circuit board and solder.
Laminated and bonded construction 420 may be employed in numerous applications including the bonding or permanent magnets. It should be noted that improved properties of magnetic permeability may be realized by reducing the overall thickness of the bonding zone by properly spacing and interposing trapezoidal protrusions 424 extending from surface 426 of top portion 428 with trapezoidal protrusions 432 extending from surface 434 of bottom portion 436. This particular configuration is shown in
The low profile geometry offered by bonded construction 440 may be desirable for use in bonding applications requiring exceptional thermal conductivity, and/or electrical conductivity, and/or magnetic permeability.
It should be noted that the low profile thermally conductive bonding surfaces of the present invention provide a means for surface mounting individual circuit components otherwise unfit for surface mounting. This may be advantageous in that it provides a greater choice of circuit components that may be surface mounted in particular while at the same time providing a means for removing excess heat.
Those skilled in the art will understand that the preceding exemplary embodiments of the present invention provide foundation for numerous alternatives and modifications. These other modifications are also within the scope of the limiting technology of the present invention. Accordingly, the present invention is not limited to that precisely shown and described herein but only to that outlined in the appended claims.
This application claims the benefit of and priority of U.S. Provisional Patent Application Ser. No. 60/856,971, entitled “Heat Dissipating Circuit Bonding Construction” by Fred Miekka, filed on Nov. 6, 2006 and U.S. Provisional Patent Application Ser. No. 60/897,325, entitled “Low Profile Interlocking Bonding Construction” by Fred Miekka, filed on Jan. 25, 2007.
| Number | Date | Country | |
|---|---|---|---|
| 60856971 | Nov 2006 | US | |
| 60897325 | Jan 2007 | US |
