Abstract:
A thermally enhanced face-up BGA substrate consists of a metal (copper) core, layers, dielectric layers, conductive through-core and build-up vias. It is a new and simple structure with better thermal performance, resulting in lower cost and better reliability. Moreover, high degree of flexibility in choice of material and layer counts as well as layer thickness allows for a wide range of applications in packaging and high density printed circuit board.

Description:
REFERENCE TO RELATED APPLICATION  
       [0001]    The present application is based on provisional application Serial No. 60/128,948 filed Apr. 13, 1999 entitled METAL CORE SUBSTRATE ENABLING THERMALLY ENHANCED BALL GRID ARRAY PACKAGES. 
     
    
     
       DESCRIPTION OF THE PRIOR ART  
         [0002]    Prolinx C 2 BGA  
           [0003]    The C 2 BGAs are fabricated using a complicated etching donut isolation method, resulting in copper islands on the core that are suspended by some isolation material, and then followed by surface processing and photo via steps. The incurred cost for the complex steps and the resulting complex structure is significant.  
         SUMMARY OF THE INVENTION  
         [0004]    The Ball Grid Array (BGA) is an advanced array package for fine pitch, high pin count semiconductor packaging, which is used normally in a multiple-layer chip-up printed wiring board (PWB) substrate for housing the integrated circuit structure in today&#39;s IC industry. However, the heat dissipation is a major concern with the arrival of high speed CPUs such as the Pentium II &amp; III, as well as high speed graphics, networking, DSP, and programmable logic chips. Better thermal BGA packaging solutions are required to fulfill the need of IC products in the 21 century.  
           [0005]    The object of this invention is to provide a new and simpler PWB structure and method with comparable or better thermal performance, resulting in lower cost and better reliability. High degree of flexibility in choice of material and layer counts and layer thickness allows for a wide range of applications in packaging and high density printed circuit board or PWB. The plated copper vias also allow for better thermal performance and result in better overall thermal performance for the resulting package.  
           [0006]    The processing steps are also ones that have proven to be practical for implementing high density interconnect for packaging applications. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    The above and other objects, advantages and features of the invention will become more apparent when considered with the following description when taken in conjunction with the accompanying drawings in which like reference characters identify corresponding components or elements and wherein:  
         [0008]    [0008]FIG. 1A shows, in section the initial metal core structure incorporating the invention,  
         [0009]    [0009]FIG. 1B shows the metal core structure after the addition of two additional functional layers on respective glass fiber layers, incorporating the invention,  
         [0010]    [0010]FIG. 1C shows a five-layer structure incorporating the invention,  
         [0011]    [0011]FIG. 1D shows the use of a solder mask coating on the structure of FIG. 1C,  
         [0012]    [0012]FIG. 2 illustrates a type  1  via according to the invention,  
         [0013]    [0013]FIG. 3 illustrates a type  2  via according to the invention,  
         [0014]    [0014]FIG. 4 illustrates a type  3  via according to the invention,  
         [0015]    [0015]FIG. 5 illustrates a sectional view of an overall structure incorporating the invention, and  
         [0016]    [0016]FIG. 6 is a perspective view of a finished BGA IC package with a substrate incorporating the invention, mounted on a PWB which can either be of a conventional laminated board or one that incorporates the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]    Structure  
         [0018]    [0018]FIG. 1A shows the structure during the initial metal core  110  forming stage. FIG. 1B shows the structure after the first two build up layers  120  are fabricated. FIG. 1B shows two drilled Plated Through Holes (PTH)  220  in the structure. It can be the complete PWB structure if a 3-layer structure  210 - 110 - 210  is desired, which is simpler and less costly. Please note that the  {fraction (1/3)} oz copper foil 210 and the  5-10 oz copper core  110  are separated with insulating glass fiber prepreg layers  230 . Also the blind or laser drilled &amp; plated via  240  is used for connecting to the core  110  as ground. The five-layer structure in FIG. 1C is intended for more complex and high density applications. FIG. 1C shows two extra build-up layers  310 , with build-up vias  320 , and a PTH  220 . In the following discussion, we will focus on the five-layer structure depicted in FIG. 1C, which includes three distinct types of vias implemented in the invention.  
         [0019]    Type  1  via  220  as illustrated in FIG. 2, is for implementing the majority of the vias in the structure also shown in FIG. 1B, FIG. 1C. It is isolated from the core and is connected to the outer layers through build-up vias  320  that are either laser drilled or controlled depth mechanically drilled.  
         [0020]    Type  2  via, as illustrated in FIG. 3, is a via that is connected with the core, which is typically used as a ground plane. It is implemented by a through hole drill  330  directly on the core  110  and followed by plating, which results in side-wall connection with the core.  
         [0021]    The advantage of the type  2  via is that it provides a direct thermal transfer path from the top layer  310  to the core  110  and then to the bottom layer  350 , ideal for implementing thermal vias in packaging applications.  
         [0022]    The only drawback with type  2  via is that the side-wall plating has interface with various layers including the core  110  and the prepreg  230 , as well as with the interface between the conductive layers  210 ,  310  and the prepreg, which if not properly processed, will contain micro-cracks that allow moisture to penetrate through. The micro-cracks may result in delamination of the interface between the core  110  and the prepreg  230 .  
         [0023]    Type  3  via, as illustrated in FIG. 4, is also a via that connects to the ground plane on the core  110 , and also provides good thermal path to the core  110 . It is implemented by build-up core vias  240  and  320  through laser or controlled depth drill, also shown in FIG. 1B and FIG  1 C. It is good for thermal performance and does not have the reliability drawback as does the type  2  via.  
         [0024]    Materials  
         [0025]    The preferred choice of the metal core  110  is copper C194 foil of 5-10 oz, or 5-15 mils thickness, as shown in FIG. 1A. The liquid to plug the metal core holes  120  can be PHP900 or equivalent materials.  
         [0026]    The inner prepreg  230  is either BT (bismaleimide triazine) or 47N with glass fiber, of 1.5 to 3 mil thickness. The glass fiber ingredient allows for structural enhancement against thermal expansion coefficient mismatch between the metal core  110  and the prepreg material  230 .  
         [0027]    The outer prepreg  340  can be either B.T. or R.C.C. (Resin-Coated-Copper) material, typically used for laser-drilled build-up. The thickness of the outer preg  340  is also within the range of 1.5 to 3 mils. The copper foil  210  used in the non-core layers can be of  {fraction (1/3)} oz thickness, though a wide thickness range is appropriate (   {fraction (1/8)}, {fraction (1/4 )}, or  {fraction (1/2)} oz) for various applications.    
         [0028]    Process  
         [0029]    The following describes a preferred process step sequence, though variations can be adopted by those familiar with the art of printed circuit board and HDI (high density interconnect) fabrication.  
         [0030]    1. Starting with the metal core  110 , drill or etch holes at the through-core vias sites, as shown in FIG. 1B and FIG. 1C, with hole sizes around 25 mils (15 mil to 40 mils is the allowable range for BGA applications). Note that typical panel sizes are 12″×18″, or 18″33 24″, or variations hereof. Black oxide processing is performed on the metal surface for better adhesion to the laminated prepreg layer  210 ,  230   310 ,  250  and  340 .  
         [0031]    Singulation lines at the border of each substrate unit can be pre-drilled or pre-etched, during the first via drill-etch step, for easy singulation in strip or singulated delivery format.  
         [0032]    2. Liquid (PHP900) plug the holes  120  (as the hatched areas shown in FIG. 1A). In the case of a thinner core  110 , such as around 5 mils, the liquid plugging  120  may not be necessary, as the inner prepreg  230  will naturally flow and fill the holes during lamination. For thick cores, it is better to plug the holes first.  
         [0033]    3. Prepreg laminations  230  and  340 . For example, 3 -mil prepregs with ⅓ oz copper foil is used in FIG. 1C. Note that for the reason of maintaining symmetry, one prepreg layer for each of the top and the bottom side is laminated at the same time.  
         [0034]    4. Drill holes for the through vias isolated from the core. The diameter of the drill is about 10 mils laser drill or controlled-depth mechanical drill for vias that are to be connected to the core, with the diameter in the range of 2 mils to 6 mils, as shown in FIG. 1B. Note that the liquid plugging material  120  which is the prepreg that flows into the first drill hole in step 2, isolates the plated vias  220  from the core  110 .  
         [0035]    5. Transfer inner layers  210  and  250  images to form pads and trace circuitry.  
         [0036]    6. Laminate outer prepreg layers  310  and  340  with BT or R.C.C. material. If a 3-layer only structure is desired, the outer prepreg layers  310  and  340  are not needed. With the same principle, if a 4-layer only structure is desired, then the bottom BT or R.C.C. layer is not required.  
         [0037]    7. Form build-up via holes  320  by Laser hole drill or controlled-depth mechanical drill.  
         [0038]    8. Mechanical through hole drill for type  2  via  330  if desired.  
         [0039]    9. Plating copper  310  and  350  for through hole vias and panel plating.  
         [0040]    10. The rest of the steps depend on Ni/Au plating technology and application needs. This includes imaging transfer for outer layers and Ni/Au plating  420 .  
         [0041]    11. Solder mask  410  coating, as shown in FIG. D.  
         [0042]    12. Finishing: Singulating the panel into individual units or into strips for packaging assembly.  
         [0043]    Advantages  
         [0044]    1. Efficient symmetric layer and via structures for high thermal conductance.  
         [0045]    2. Achieving same or better thermal performance comparing to prior art, with mature processing technology and proven materials.  
         [0046]    3. Requires only incremental cost increase for offering better performance than Plastic Ball Grid Array (PBGA).  
         [0047]    Feature of the Invention  
         [0048]    1. New copper-core based structure for chip-up high thermal performance package using 3-layer (core+1-top +1-bottom), 4-layer (core+2-top+1-bottom), and 5-layer (core+2-top+2-bottom). Moreover, 5 or more layers can be built easily. The 3- and 5-layer options are symmetric, with better warpage prevention.  
         [0049]    2. The use of drilling/etching, optional liquid-filled, laminating, drilling, and plating process steps for forming the through core via holes.  
         [0050]    3. The combination of laser blind vias build-up on top of the metal core structure, enabling additional build-up layers for high density applications.  
         [0051]    4. Efficient thermal vias by plating build-up and connecting to the core from both the top and bottom sides.  
         [0052]    5. Singulation lines at the border of each substrate unit can be pre-drilled or pre-etched, during the first via drill-etch step, for easy singulation in strip or singulated delivery format.  
         [0053]    6. Applications: a) Metal core based substrates for thermally enhanced fine-pitch BGAs, and b) Metal core based high density boards such as for high thermal output SDRAM DIMM modules.  
         [0054]    While the invention has been described in relation to preferred embodiments of the invention, it will be appreciated that other embodiments, adaptations and modifications of the invention will be apparent to those skilled in the art.