Abstract:
A process for forming a heat sink on a semiconductor package at the wafer level stage of manufacture is disclosed. A semiconductor component wafer, prior to separation into separate component packages, is covered on one side with a resin metal foil layer. The resin foil layer is patterned by laser ablation to define the heat sink locations, and then a thermal paste is applied over the patterned layer. The thermal conductive paste is hardened to form the heat sinks. The wafer can then be separated into packages.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority of U.S. Provisional Patent Application Ser. No. 61/543,472 filed Oct. 5, 2011, entitled Wafer Level Applied Thermal Heat Sink, the content of which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE DISCLOSURE 
     1. Field of the Disclosure 
     The present disclosure generally relates to a structure and method for the creation of on-chip thermal heat sinks for active, passive or discrete integrated circuit applications specifically for the application of embedded die in printed circuit boards. 
     2. State of the Art 
     Semiconductor wafers consist of multiple arrays of devices often referred to as chips or die which are later separated into individual discrete devices, a process known as “singulation”. After singulation, these chips are further integrated into a chip package and then mounted onto a printed circuit board during final board assembly for the particular end product. A relatively new and upcoming technique is to combine the chip packaging and the printed circuit board assembly with a process of embedding the chip into a printed circuit board. 
     Assembly of large PWB substrate sizes with multiple embedded die PWB&#39;s in a step and repeat format is desirable to improve economy of scale. It is also desirable to increase component density in order to reduce total package footprint. 
     In many passive, active or discrete semiconductor circuit applications, it is desirable to provide adequate heat sinking of the chip circuitry to ensure optimal chip, and total system performance. Heat sinking allows the chip to perform its function more efficiently at a given power load and allows higher reliability of the chip and the adjacent chip and other devices, since heat generally degrades the performance of most semiconductors. 
     Typically on-chip heat sinking is achieved by electroplating thick metals with good thermal conductivity, such as copper, or a copper alloy, in the desired hotspot region on the chip. This aids heat dissipation into the surrounding package and ambient environment. This process is typically performed on a semiconductor wafer either at the wafer foundry or at the final wafer level packaging supplier. However, a wafer level process offers significant cost advantages as multiple heat sinks can be created simultaneously in the normal wafer level manufacturing process steps. 
     Heat sinks can also be placed discretely with post wafer processing on individual singulated devices, chips or die. Typically, on-chip heat sinks are created by electroplated copper processes. However, increasing the plating area or the plating thickness increases process cost. Thus it is desirable to identify a low cost, high volume alternative process for large surface area structures. 
     Thermal dissipation management by heat-sinking is particularly problematic in the newer embedded chip or die package applications where the chip is encapsulated by the PWB (printed wiring board). Polymeric materials used in the PWB core and subsequent build up layers typically have low thermal conductivity compared to plated metals or applied metal foils. In the embedded chip application, the chip is separated by one or more buildup layers from the external surface of the printed circuit board where air or metal heat sinks can assist in thermal dissipation management. Therefore an embedded chip is susceptible to higher operating temperatures for a given power load. 
     In the encapsulated embedded chip application, it can also be necessary to provide adequate thermal dissipation to both the front side and back side of the embedded component to assist in removing heat to the surface or sides of the PWB. Thus there is a need for a cost effective wafer level process particularly in the embedded chip or die manufacturing process that incorporates increased heat sink capabilities. 
     SUMMARY OF THE DISCLOSURE 
     An embodiment of a process of wafer level chip scale package formation with heat dissipation capability includes: providing an incoming wafer having a back side; applying a resin foil layer to the back side of the wafer; patterning the resin foil layer by laser ablation to form heat sink locations; applying a thermal conductive paste to the back side of the wafer over and on the patterned resin foil layer; and solidifying the thermal conductive paste to form one or more heat sinks on the wafer. 
     Another embodiment further includes forming a thermal via on one or more of the solidified conductive pastes. The process may also include embedding the wafer level chip scale package in an embedded die package. The process may also include applying an outer layer over the wafer level chip scale package in the embedded die package. The process may also include exposing the heat sink of the wafer level chip scale package through the outer layer. The heat sink may be exposed by a via through the outer layer. An external discrete heat sink may be attached to the wafer level chip scale package heat sink to further enhance heat dissipation. 
     The conductive paste is preferably a metal paste. The metal paste preferably may be a copper, tin, or a thermally conductive metal alloy. The resin foil layer is preferably a resin copper foil layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure will be better understood and features and objects of the disclosure, including those set forth above, will become apparent when consideration is given to the following detailed description. Such description makes reference to the accompanying drawings wherein: 
         FIG. 1  is a simplified side view of a conventional wafer level chip scale package with discrete components thereon. 
         FIG. 2A-2E  presents a representation of the buildup process of a conventional wafer level chip scale package and a conventional on-chip heat sink shown in  FIG. 1 . 
         FIG. 3A-3D  presents a representation of the wafer level chip scale package format buildup process in accordance with the present disclosure with the wafer level applied heat sink structure formed on the component back side. 
         FIG. 4  is a schematic sectional view of the final embedded die package with open surface cavity for ambient thermal dissipation. 
         FIG. 5  is a schematic sectional view of the final package shown in  FIG. 4  with further vias formed through an overlayer with an additional external heat sink structure installed. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are set forth in order to provide a more thorough disclosure. It will be apparent, however, to one skilled in the art, that the art disclosed may be practiced without these specific details. In some instances, well-known features may have not been described in detail so as not to obscure the art disclosed. 
     Embodiments in accordance with the present disclosure enable enhanced heat sinking. This allows adjacent components to be placed closer together with reduced thermal interaction in embedded PWB (printed wiring board) electronic package applications. Furthermore, this improved heat sinking at chip level can allow for chip shrinkage while maintaining the same chip performance. 
       FIG. 1  Illustrates a conventional active, passive or discrete semiconductor chip scale package in cross section. The chip scale package has typical front side circuitry  100  and exposed front side contact pads  110  for electrical interconnection. Excess heat produced by the front side circuitry must be dissipated by conduction through the bulk semiconductor material  120  and then finally dissipated by dedicated heat sink structures  130 . 
       FIG. 2A-2E  illustrates the conventional electronic chip scale package buildup process with dedicated heat sink structures formed on the component backside to produce the head sink structures  210  on the package back side. In this case, the final heat sink structure  210  is formed by a typical electroplating process. 
       FIG. 3A-3D  Illustrates wafer level formation of an electronic chip scale package with dedicated heat sink structures formed on the component backside in accordance with the present disclosure. 
     In the operation of  FIG. 3A , the incoming fabricated wafer level chip scale package is shown prior to separation into discrete chip packages. The chip scale package  301  has contact pads  302  already formed on the front side thereof. 
     In the operation of  FIG. 3B , a resin layer  303  and a copper foil  304  are deposited on the back side of the wafer level chip scale package  301 . This composite layer called a resin copper foil (RCF) application layer. 
     In the operation of  FIG. 3C , the RCF layer  303 ,  304  is patterned by laser ablation. 
     In the operation of  FIG. 3D , a copper (Cu) paste is printed between the RCF patterning. This Cu paste hardens and forms the wafer level heat sinks  330  in the finished chip scale package after separation of the wafer into discrete chip scale packages (singulation). 
     Thus the heat sink structure  330  is formed using a resin copper foil application process. A resin coated copper foil  320  is applied and subsequently etched to define the heat sink features. The copper heat sink is then formed by an application of a copper paste  330  that has an integral binder in the paste that solidifies during a post-cure process and forms a permanent copper heat sink. 
     The process applied in  FIG. 3A-3D  may similarly be applied to the front side of the incoming wafer  301 / 302 . Therefore, although not illustrated, the description of the operations  3 A through  3 D above described applies equally well to the front side of the wafer  301 / 302 . 
       FIG. 4  Illustrates a chip scale package  405 , formed by the process just described, used in an embedded die package. The chip  405  is mounted onto the PWB core  400  and adhered thereto via a pre-preg adhesive layer  410 . Electrical interconnects to the chip scale package  405  through the PWB are formed by means of vias  420  and routings  430 . 
     A typical resin coated copper foil is applied to form the PWB inner and outer layer(s)  440 . These layers  440  may include structures to connect components  460  as shown. Cavities  445  in the back side outer layer  440 , created by conventional laser ablation, expose the on-chip heat sink regions  450  formed by the process described in  FIG. 3   a - 3   d . These exposed heat sink regions  450  aid thermal dissipation to the package environment. The additional surface mountable componentry  460  is mounted to PWB outer surface  440  in a conventional manner. Additional circuitry  470  can be created within the PWB outer layers  440 . 
       FIG. 5  Illustrates an alternative embedded die package which includes further heat sink capabilities. The chip backside is fully encapsulated by an additional PWB outer layer  500 . Copper filled vias  510  have been created in the layer  500  to contact the on-chip thermal heat sinks  450  allowing thermal dissipation through the layer  500  to the surface of the PWB outer layer  500 . A optional surface mountable heat sink  520  is connected to the vias  510  for additional heat dissipation. 
     A method for low cost, high yielding wafer level applied thermal heat sinks is disclosed herein utilizing patterning of resin coated copper foil and copper paste combined with wafer level processing to produce wafer level chip scale packages. The present disclosure provides a means to apply a variety of heat sink designs, thickness and geometries without adding significant process complexity or cost. 
     The chip in accordance with the present disclosure contains integrated electronic circuitry and pads used for electrical interconnects to a PWB substrate or other external circuitry. Further, the chip contains dedicated heat sink features used for on-chip mass or local thermal dissipation. In particular, the chip contains a resin copper foil layer used in patterning to form the heat sink feature and to assist with surface adhesion in the final packaging format. 
     Various modifications and alternatives to the disclosed embodiments will be apparent to those skilled in the art. The process described herein is applicable for other than chip scale packages. The process may also be applied to an incoming flipchip package, system-in-package, embedded chip structures, stacked chip packages, and other multi-die, multi-discrete 3D packages. The incoming wafer level package shown in  FIG. 3A-3D  is merely exemplary. Accordingly, all such alternatives, variations and modifications are intended to be encompassed within the scope of and as defined by the following claims.