Abstract:
The present invention is directed to an electronic component assembly and method of manufacture which can be efficiently implemented and which reduces the amount of work required to create a plated through-hole for connecting one circuit to another circuit in an electronic component assembly. During electronic component assembly fabrication, at least a portion of an inner core of the electronic component assembly&#39;s heat sink assembly is replaced with a dielectric “zone.” Once the electronic component assembly is manufactured, connections from one circuit on one side of the electronic component assembly to another circuit on the other side of the electronic component assembly can be achieved using plated through-holes. As such, circuit boards can be connected on opposite sides of the electronic component assembly without having to perform the labor intensive connection of circuits through the use of discrete wires or printed flexes.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to plated through-holes for an electronic component assembly and to an associated method for making the through-holes which eliminates the need to use discrete wires and printed flexes as connectors between circuits on the electronic component assembly. 
     2. State of the Art 
     A circuit element situated on one printed wiring board located on one side of an electronic component assembly is, at times, required to be connected to another circuit element located on another printed wiring board located on the opposite side of the electronic circuit assembly. Connections can be run from one side of the electronic component assembly to the other through the use of discrete wires or printed flexes which are connected to the circuit elements. This is done after the electronic component assembly has been assembled or after each component is attached to the printed wiring boards. As a result, however, more production time is required to connect the discrete wires and printed flexes between wiring boards on opposite sides of an electronic component assembly. Furthermore, the use of discrete wires and printed flexes after the manufacturing of the electronic component assembly requires a high degree of precision labor which is more expensive and has a higher incidence of errors occurring during manufacture. 
     The use of through-holes has been employed to provide communication from one layer of a printed wiring board of an electronic component assembly to another layer. For example, U.S. Pat. No. 3,739,469 (Dougherty, Jr.) describes a method for fabricating multi-layer circuit boards with vias in the internal layers that are concentric with plated through-holes. However, the internal vias are “plugged” with a dielectric pre-peg used during a lamination phase of the fabrication process. That is, excess dielectric material which is a by-product of the lamination process can result in an insufficient amount dielectric material filling the through-hole. This can result in improper plating of the through-hole and incorrect circuit operation. 
     In addition, due to the internal structure of electronic component assemblies, properly placing through holes is difficult. Heat sinks located at the core of the electronic component assembly present problems when forming the through holes. 
     U.S. Pat. No. 5,562,971 (Tsuru et al.) discloses the placement of through-holes in a multi-layer printing board. The through holes are drilled after the formation of the circuit board and insulating layers are laminated by heating under pressure. A conductive layer is then placed over the drilled hole. In this case, there is no heat sink at the core of the electronic component assembly. 
     Electrical circuits contained in the printed wiring boards have a common “circuit ground potential” while the chassis, to which electronic component assemblies are connected, includes a separate “chassis ground potential” . The separate ground circuits are maintained to avoid electrical problems. For example, if the different ground elements of the circuit and the chassis, to which an electronic component assembly is connected, are brought into contact, a ground loop can form between the printed wiring boards which raises the potential of the circuit ground above zero volts and renders the electrical circuits susceptible to noise. The proper operation of circuits which use discrete logic levels can be affected when noise distorts the circuit ground to a value greater than zero volts. 
     Known methods of connecting one printed wiring board to another through the use of connectors are labor intensive and require a high degree of precision to avoid short circuits. Accordingly, an efficient, cost-effective technique to run a connection from one side of an electronic component assembly to the other is needed. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an electronic component assembly and an allocated method of manufacture which can be efficiently implemented and which reduces the amount of work required to create a plated through-hole for connecting one circuit to another circuit in an electronic component assembly. Exemplary embodiments ensure that during electronic component assembly fabrication, at least a portion of an inner core of the electronic component assembly&#39;s heat sink assembly is replaced with a dielectric “zone.” When the electronic component assembly is manufactured, connections from one circuit on one side of the electronic component assembly to another circuit on the other side of the electronic component assembly are achieved using plated through-holes. As such, circuit boards can be electrically connected on opposite sides of the electronic component assembly without having to perform the labor intensive connection of circuits through the use of discrete wires or printed flexes. The plated through holes are made through dielectric zones selectively placed into the inner core of the electronic component assembly. 
     Generally speaking, exemplary embodiments relate to an electronic component assembly which comprises at least first and second electrical circuits and a heat sink assembly connected to the first and second electrical circuits, the heat sink assembly including an inner core having at least a first portion made of a dielectric material having a through-hole from the first electrical circuit to the second electrical circuit, and a second portion made of a material that is different from the dielectric material, wherein the first portion is one of co-cured and bonded with the second portion. 
     In another exemplary embodiment of the present invention, a method for producing a heat sink assembly is disclosed which comprises the steps of: providing an inner core made of a heat sink material, forming a segment of the heat sink material with a first dielectric segment wherein a through-hole is to be established from a first electrical circuit to a second electrical circuit, placing a second dielectric around the inner core, selectively plating the second dielectric with a first electrically conductive foil and plating a conductive material on the first electrically conductive foil, placing first and second electrical circuits on the second dielectric and on opposite sides of the inner core, and making a through hole from the first electrical circuit through the first dielectric segment of the inner core to the second electrical circuit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments, when read in conjunction with the accompanying drawings wherein like elements have been represented by like reference numerals and wherein: 
     FIG. 1 shows a cross-sectional view of an exemplary embodiment of an electronic component assembly of the present invention, with printed wiring boards being located on both sides of a heat sink assembly; 
     FIG. 2 shows a flow chart of an exemplary process for making an electronic component assembly in accordance with the present invention; and 
     FIG. 3 shows a top view of an exemplary embodiment of an electronic component assembly in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a cross-sectional view of an electronic component assembly  100 , wherein a heat sink assembly  10  is located between two printed wiring boards (PWBs)  20  and  30 . One or both of the printed wiring boards  20  or  30  can have one or more circuit elements  70  located on each board. At the center of the heat sink assembly  10  is an inner core  40  composed of a graphite material surrounded by a dielectric layer  50 . 
     The graphite material is chosen based upon several different factors such as thermal conductivity and electrical conductivity. The graphite material of the FIG. 1 exemplary embodiment has sufficient electrical and thermal conductivity to achieve a desired heat sink function, given the heat dissipation of electrical circuits included on the printed wiring boards. Plural sheets of graphite can be combined with a filler resin and pressed to become a composite laminate. An example of a common graphite material used to make the composite laminate is Thermalgraph™, available from Amoco Corporation. Other types of common graphite fiber and filler resins can also be used as the inner core  40  of the heat sink assembly, as can any other thermally conductive material. 
     In accordance with an exemplary embodiment of the present invention, during the fabrication process, to accommodate plated through-holes used to interconnect plural printed wiring boards, one or more dielectric zones  35  can be imbedded within the graphite material of FIG.  1 . The dielectric zones  35  can be placed within the graphite laminate and co-cured with the laminate or bonded to the graphite material after it has been cured. 
     FIG. 2 illustrates a flow chart which depicts an exemplary process by which the electronic component assembly  100  in FIG. 1 can be produced. In step  210  of the FIG. 2 process, the electronic component assembly includes a heat sink assembly  10  on which electrical circuits are placed. The heat sink assembly  10  further includes an inner core having  40  at least a first portion made of a dielectric material. The inner core  40  of the heat sink assembly can comprise of sheets of graphite cut into desired shapes. After a heat sink material and resin have been selected, the inner core  40  of the FIG. 1 heat sink assembly is formed via impregnation. To form the inner core  40  by impregnation, resin is added to the graphite material, such as layered graphite cloth. The graphite cloth and resin are then pressed to formulate the inner core  40  of the heat sink assembly  10 . 
     In step  215  of the FIG. 2 flow chart, to account for a plated through-hole, one or more of the FIG. 1 dielectric zones  35  can be embedded within the graphite material. The dielectric zones  35  can, like the heat sink assembly, be cut from sheets of dielectric materials. The dielectric material can be chosen based upon factors such as electrical conductivity, thermal conductivity and ability of the material to be plated by other conductive materials. For the dielectric material of the zones  35  of this exemplary embodiment, a material which is substantially electrically non-conductive, and which has good thermal conductivity can be chosen. The dielectric zones  35  can be a printing wiring board laminate, such as a cyanate ester laminate, an epoxy glass, kevlar, polyamide, quartz or any other suitable material. The graphite material of the inner core is then co-cured with the dielectric zones  35 , or bonded to previously cured dielectric zone(s)  35 . 
     In step  220 , after the inner core  40  of FIG. 1 has been made via impregnation or lamination, a next layer of the heat sink assembly is applied as a dielectric layer  50  which surrounds the inner core  40 . For example, the dielectric layer  50  can be chosen based on the same factors used to choose the dielectric zones  35 . In addition to being substantially electrically non-conductive and thermally conductive, it is also advantageous for the dielectric material chosen to be easily plateable with material used for electrically contacting the chassis ground. For example, the dielectric material  50  can be plated with a nickel material over a copper layer (e.g., nickel over copper). 
     Also, the dielectric material for both the dielectric zones  35  and the dielectric layer  50  can be selected in accordance with a temperature value, T g , which is the temperature at which the material changes physical properties by becoming soft or liquefying. The dielectric material&#39;s T g  value can be selected high enough that the material does not melt or become soft during lamination processing when the electronic component assembly is being formed. For example, where a dielectric material has a relatively low T g  value, higher temperatures (i.e., temperatures above the T g  value) cause a change in material property that results in a softening of the dielectric layer  50  or delamination of the dielectric material  50  down to the inner core  40 . The dielectric material  50  is applied to the surface of the inner core  40  by, for example, lamination, or any other suitable technique, such as any coating technique. The dielectric material  50  can be applied through a coating technique to avoid bonding of a conductive foil to the dielectric laminate. The T g  value of dielectric materials which are applied through coating are typically lower than dielectric materials that are applied through lamination. Alternately, the dielectric layer  50  can be formed during formation of the inner core  40  via the impregnation. For example, sheets of dielectric material can be placed on top of the graphite cloth subsequent to embedding the dielectric zones  35 . Then resin can be added and the assembly pressed to produce a composite impregnated inner core  40  having dielectric zones  35  and dielectric layer  50 . 
     In step  230 , after the FIG. 1 dielectric material  50  shown in FIG. 1 has been formed, a conductive foil  45  of FIG. 1, such as copper, can be bonded to the dielectric material  50  and subsequently plated (e.g., with copper) to cover the edges of dielectric layer  50 . 
     In step  240 , portions of the FIG. 1 conductive foil  45  can be selectively removed through patterning and etching. For example, photoresist can be deposited over the conductive foil and selectively patterned to remain over areas where the foil is to be retained, using known photolithography techniques. The heat sink assembly  10  is irradiated with light and the conductive foil  45  is removed, via chemical etching, in any areas where the photoresist was not exposed to the light. In an exemplary embodiment, the foil  45  can be retained around the edges of the heat sink assembly as shown in FIG.  1 . 
     In step  250  of FIG. 2, a layer of the FIG. 1 conductive material  60  (e.g., nickel, gold) is plated onto the heat sink assembly  10  which now includes the patterned foil  45 . The type of conductive material  60  is chosen based upon, for example, the electrical conductivity, corrosion resistance, durability and/or the ability to adhere to the foil. The conductive material  60  is placed on top of the patterned foil formed in steps  230  and  240  of FIG.  2 . Depending on the manner in which plating is performed (e.g., electroplating), the conductive material  60  can adhere to conductive foil and not to the dielectric material of the heat sink assembly. 
     In step  260  of FIG. 2, the FIG. 1 printed wiring boards  20  and  30  are placed onto the heat sink assembly through another lamination process. The conductive material  60  applied in step  250  and the conductive foil  45  applied in step  230  are, in an exemplary embodiment, prevented from directly contacting conductive areas of the printed wiring boards or the inner core of the heat sink assembly by any desired spacing. 
     In step  270 , one or more through-holes  55  of FIG. 1 are drilled after the printed wiring boards  20  and  30  have been attached to the assembly through lamination. When the one or more through-holes are drilled, the drill penetrates through a dielectric zone  35  of the inner core  40 . Each hole is drilled from a printed wiring board on one side of assembly  100 , through a dielectric zone  35 , to a printed wiring board on another side of the assembly  100 . 
     In step  280 , another layer of conductive plating  65  (e.g., copper, nickel, gold, rhodium or other suitable material) can be applied to the surface of the printed wiring boards  20  and  30  and through-holes  55  to establish a circuit ground within the electronic component assembly  100 . The conductive foil  65  can then be selectively removed for placement of individual components, such as circuit components  70 , on the printed wiring board in accordance with known methodology. 
     The drilling of a through-hole  55  allows for a connection between the two printed wiring boards  20  and  30 . Provided there is no connection with the chassis ground, there will be no shorts and, as a result, the electronic component assembly  100  can efficiently operate. 
     FIG. 3 shows a top view of the electronic component assembly. As shown in FIG. 3, the conductive material  60  surrounds the periphery of the electronic component assembly  100 . A connector  90  is attached to a lower part of the electric component assembly  100 . The shell of the connector  90  is chassis ground. The circuit elements  70  and plated through-hole(s)  55  of the printed wiring board  20  are maintained at circuit connection. 
     It will be appreciated by those skilled in the art that the present invention can be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed embodiments are therefore considered in all respects illustrative and not restricted. The scope of the invention is indicated by the appended claims rather than the foregoing description and all changes that come within the meaning and range and equivalence thereof are intended to be embraced within.