Abstract:
This invention relates to circuit boards and methods of fabricating circuit boards. A circuit board includes a core layer and a surface layer. The core layer includes a number of fibers and the surface layer has a thickness that is between about 10% and about 30% of the circuit board thickness. Including fibers in the core layer increases the strength of the circuit board. The surface layer is essentially free of fibers and relatively thick. The thickness of the surface layer inhibits the formation of cracks in the circuit board, which improves the reliability of circuits and systems coupled to the circuit board.

Description:
[0001]     This application is a Divisional of U.S. application Ser. No. 10/857,738, filed May 28, 2004, which is a Divisional of U.S. application Ser. No. 09/643,526, filed Aug. 22, 2000, now U.S. Pat. No. 6,757,176, both of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates to circuit boards, and more particularly, to increasing circuit board reliability.  
       BACKGROUND OF THE INVENTION  
       [0003]     Printed circuit boards, which are used in the manufacture of electrical, mechanical, electromechanical, and other kinds of products, provide a substrate for mounting a die on which integrated circuits, such as processors, memories, and amplifiers, are fabricated.  FIG. 1  shows a cross-sectional view of a prior art ball-grid array (BGA) package  100  which includes printed circuit board  103  coupled to board  105  by solder balls  107 , and die  109  coupled to printed circuit board  103  by adhesive  111  and molding compound  112 .  
         [0004]     A longstanding and unsolved problem with printed circuit boards in general and with BGA package  100  in particular is that printed circuit board  100  may develop a crack, such as crack  113 . Crack  113  destroys the structural integrity of BGA package  100 . Once the structural integrity of BGA package  100  is destroyed, unforseen stresses may break or damage electronic connectors, such as electronic connectors  115  and  116 , and as a result, any product in which BGA package  100  is incorporated may malfunction. One method of solving problems associated with printed circuit board cracking and the resulting electronic connection failures is to include a second printed circuit board in the design of an electronic system. The second circuit board is a redundant printed circuit board which mirrors the operation of the primary board, so a system failure occurs only when both the redundant printed circuit board and the primary board fail at the same time. Unfortunately, this solution is very expensive and is cost effective only in systems, such as trains, airplanes, or spacecraft, where the cost of failure is high. For systems in which the cost of failure is low, redundant circuit boards are seldom used.  
         [0005]     For these and other reasons there is a need for the present invention.  
       SUMMARY OF THE INVENTION  
       [0006]     The above mentioned problems with cracking in circuit boards and other problems are addressed by the present invention and will be understood by reading and studying the following specification. A circuit board is described that includes embedded fibers, which strengthen the board, and a surface layer having a certain thickness range that inhibits the formation of cracks in the circuit board.  
         [0007]     The present invention provides, in one embodiment, a circuit board including a core layer and a surface layer. A number of fibers are embedded in the core layer. The surface layer has a thickness which is between about 10% and about 30% of the circuit board thickness. Embedding fibers in the core layer increases the strength of the circuit board. A surface layer thickness of between about 10% and about 30% of the circuit board thickness inhibits the formation of cracks in the circuit board, which improves the reliability of circuits mounted on the circuit board and systems in which the circuit board is embedded.  
         [0008]     In an alternate embodiment, the present invention provides a method of fabricating a circuit board having a circuit board thickness. The method includes forming a core layer including a number of fibers, and forming a surface layer on the core layer. The surface layer has a surface layer thickness that is between about 10% and about 30% of the circuit board thickness and is free of fibers.  
         [0009]     These and other embodiments, aspects, advantages, and features of the present invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art by reference to the following description of the invention and referenced drawings or by practice of the invention. The aspects, advantages, and features of the invention are realized and attained by means of the instrumentalities, procedures, and combinations particularly pointed out in the appended claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is an illustration cross-sectional view of a prior art ball-grid array (BGA) package including a circuit board.  
         [0011]      FIG. 2A  is a cross-sectional view of one embodiment of a circuit board assembly according to the teaching of the present invention.  
         [0012]      FIG. 2B  is a cross-sectional view of an alternate embodiment of a circuit board assembly according to the teaching of the present invention.  
         [0013]      FIG. 2C  is a cross-sectional view of a second alternate embodiment of a circuit board assembly according to the teaching of the present invention.  
         [0014]      FIG. 3  is a block diagram of one embodiment of a computer system suitable for use in connection with the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0015]     In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific preferred embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that logical, mechanical and electrical changes may be made without departing from the spirit and scope of the present inventions. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims.  
         [0016]      FIG. 2A  is a cross-sectional view of one embodiment of circuit board assembly  200 . Circuit board assembly  200  includes die  201 , printed circuit board  202 , and board  203 . Circuit board  202  is coupled to board  203  by solder balls  204 , and die  201  is coupled to printed circuit board  202  by adhesive  205  and molding compound  206 .  
         [0017]     Circuit board assembly  200  is sometimes referred to as a ball-grid array, but the present invention is not limited to ball-grid array circuit board assemblies. Any circuit board assembly that includes a circuit board, such as printed circuit board  202 , which is described in more detail below, is suitable for use in connection with the present invention. For example, a circuit board assembly in which a first circuit board is coupled by edge connectors to a second circuit board is suitable for use in connection with the present invention.  
         [0018]     Die  201  includes an integrated circuit, such as a memory circuit, a processor, an amplifier, or an application specific circuit (ASIC). Memory circuits suitable for use in connection with the present invention include but are not limited to dynamic random access memory (DRAM) circuits, static random access memory (SRAM) circuits, erasable programmable memory (EPROM) circuits, and electrically erasable programmable memory (EEPROM) circuits. Processor circuits suitable for use in connection with the present invention include but are not limited to microprocessors, digital signal processors (DSPs), and reduced instruction set computing (RISC) processors. Amplifier circuits suitable for use in connection with the present invention include but are not limited to operational amplifiers, differential amplifiers, and power amplifiers. Application specific integrated circuits (ASICs) suitable for use in connection with the present invention include telecommunication circuits, such as telecommunication interface circuits. Die  201  is coupled to printed circuit board  202  by adhesive  205 , which is selected to maintain a low stress interface between die  201  and circuit board  202 . Any adhesive that has a coefficient of thermal expansion which maintains a low stress interface between die  201  and circuit board  202  during the heating and cooling of circuit board assembly  200  is suitable for use in connection with the present invention.  
         [0019]     A board  203 , such as a circuit board, provides a substrate for mounting a number of circuit boards, such as circuit board  202 , and other electronic or electro-mechanical components. In one embodiment, board  203  is a computer system circuit board including a processor. Board  203  may also provide a substrate suitable for securing circuit assembly  200  to a larger package or other housing, such as a cabinet or case. Board  203  is fabricated from materials commonly used in the fabrication of circuit boards, such as polymeric composite materials. One example of a polymeric composite material is phenolic. However, board  203  is not limited to polymeric composite materials, and other materials, particularly insulating materials, may also be used in the fabrication of board  203 .  
         [0020]     Solder balls  204  provide a number of signal paths to electronically couple circuit board  202  to board  203 . The number of signal paths permit communication between components mounted on circuit board  202  and on board  203 . Any material that is a good conductor is suitable for use in fabricating solder balls  204 . Tin, gold, copper, silver, and alloys of tin, gold, copper, and silver, are examples of materials suitable for use fabricating solder balls  204 .  
         [0021]     Circuit board  202 , as shown in  FIG. 2A , provides a substrate for mounting die  201  and coupling signals to board  203 . In one embodiment, circuit board  202  is formed from a polymeric composite material. Circuit board  202  is coupled to die  201  by adhesive  205 .  
         [0022]      FIG. 2B  is a detailed cross-sectional view of one embodiment of circuit board  202 . In this embodiment, circuit board  202  includes core layer  208  and surface layer  209 , which is a first layer or a first resin layer. Circuit board  202  may also include slot  211  for routing conductive connectors, such as conductive connectors  207  and  208  shown in  FIG. 2A , from above circuit board  202  to below circuit board  202 .  
         [0023]     Core layer  208  includes one or more fibers  213  embedded in an insulator. Core layer  208 , in one embodiment, is fabricated from a resin and has a thickness  212  of between about 0.006 inches and about 0.012 inches. The one or more fibers  213 , in one embodiment, are glass fibers having a diameter of between about 0.0005 inches and about 0.001 inches. In an alternate embodiment, the one or more fibers  213  are woven fibers, such as woven glass fibers, and have a diameter of between about 0.001 inches and about 0.002 inches.  
         [0024]     Surface layer  209 , in one embodiment, is fabricated from a resin that is free of fibers. In an alternate embodiment, surface layer  209  is fabricated from a resin that is essentially free of fibers. Surface layer  209  is essentially free of fibers when any fibers embedded in surface layer  209  do not significantly increase the likelihood of cracking in circuit board  202 . The inventors discovered that cracks in circuit board  202  frequently occur on the surface of circuit board  202  at stress concentration points located above fiber hills, such as fiber hills  215  and  217 , and that cracks are less likely to occur at stress concentration points located above fiber valleys. In board assembly  200 , stress concentration points occur along an edge of an interface between two surfaces, such as along the edge of adhesive  205  at the interface between adhesive  205  and circuit board  202 , as shown in  FIG. 2A . The inventors also discovered that cracks at stress concentration points were most likely to occur during the solder reflow process, when temperature gradients are formed in board assembly  200 . Once a surface crack, such as surface crack  219  intersects with a fiber, such as the one or more fibers  213 , the crack proceeds along the interfacial interface between the one or more fibers  213  and the material in which the one or more fibers  213  is embedded. According to the teachings of the present invention, the inventors also discovered that by increasing thickness  210  of surface layer  209  to between about 10% and 30% of circuit board thickness  221 , the likelihood of crack formation on the surface of circuit board  202  is decreased. Therefore, in the novel circuit board of the present invention, the thickness  210  is preferably between about 10% and about 30% of circuit board thickness  221 .  
         [0025]      FIG. 2C  is a detailed cross-sectional view of an alternate embodiment of circuit board  202 . In this embodiment, circuit board  202  includes core layer  223 , first surface layer  225 , and second surface layer  227 . Core layer  223  is located between first surface layer  225  and second surface layer  227 .  
         [0026]     Core layer  223  includes one or more fibers  231  embedded in core layer  223 . Core layer  223 , in one embodiment, is fabricated from a resin and has a thickness  229  of between about 0.006 inches and about 0.012 inches. Core layer  223  also has greater mechanical strength than first surface layer  225  or second surface layer  227 . The one or more fibers  231 , in one embodiment, embedded in core layer  223 , are glass fibers having a diameter of between about 0.0005 inches and about 0.001 inches. In an alternate embodiment, the one or more fibers  231  are woven fibers, such as woven glass fibers, and have a diameter of between about 0.001 inches and about 0.002 inches.  
         [0027]     First and second surface layers  225  and  227 , in one embodiment, are fabricated from a resin that is free of fibers. In an alternate embodiment, first and second surface layers  225  and  227  are fabricated from a resin that is essentially free of fibers. First and second surface layers  225  and  227  are essentially free of fibers when any fibers embedded in first and second surface layers  225  and  227  do not significantly increase the likelihood of cracking in circuit board  202 . The inventors, in addition to discovering the source of cracks in circuit boards having a single surface layer, have also discovered that cracks in a circuit board having two surface layers, such as circuit board  202  shown in  FIG. 2C , frequently occur on one of the surfaces of circuit board  202  at stress concentration points located above fiber hills, such as fiber hills  233  and  235 , and that cracks are less likely to occur at stress concentration points located above fiber valleys. In board assembly  200 , which is shown in  FIG. 2A , stress concentration points occur along an edge of an interface between two surfaces, such as along the edge of adhesive  205  at the interface between adhesive  205  and circuit board  202 . The inventors also discovered that cracks at stress concentration points were most likely to occur during the solder reflow process when temperature gradients are being formed in board assembly  200 . Referring again to  FIG. 2C , once a surface crack intersects a fiber, such as one of the number of fibers  231 , the crack proceeds along the interfacial interface between one of the number of fibers  231  and the material in which one of the number of fibers  235  is embedded. The inventors also discovered that by increasing the thickness  237  of surface layer  225  to between about 10% and 15% of circuit board thickness  241  and the thickness  239  of surface layer  227  to between about 10% and 15% of circuit board thickness  241 , the likelihood of crack formation on the surface of circuit board  202  is decreased. Therefore, first surface layer thickness  237  is preferably between about 10% and about 15% of circuit board thickness  241  and second surface layer thickness  239  are preferably between about 10% and about 15% of circuit board thickness  241 .  
         [0028]      FIG. 3  is a block diagram of one embodiment of a computer system  300  suitable for use in connection with the present invention. System  300  comprises processor  305  and memory board assembly  310  including one or more circuit boards shown in  FIGS. 2B and 2C  according to the teachings of the present invention. Memory board assembly  310  comprises memory array  315 , address circuitry  320 , and read circuitry  330 , and is coupled to processor  305  by address bus  335 , data bus  340 , and control bus  345 . Processor  305 , through address bus  335 , data bus  340 , and control bus  345  communicates with memory board assembly  310 . In a read operation initiated by processor  305 , address information, data information, and control information are provided to memory board assembly  310  through busses  335 ,  340 , and  345 . This information is decoded by addressing circuitry  320 , including a row decoder and a column decoder, and read circuitry  330 . Successful completion of the read operation results in information from memory array  315  being communicated to processor  305  over data bus  340 .  
       CONCLUSION  
       [0029]     A number of circuit boards, circuit board assemblies, and methods of fabricating circuit boards and circuit board assemblies have been described. The circuit boards include a number of layers including a core layer and one or more surface layers. The core layer includes a number of embedded fibers to increase the strength of the core layer. Cracking is reduced in the surface layer or layers by fabricating the surface layers with a thickness sufficient to reduce the probability of stress concentration points inducing cracks during the heating and cooling of the circuit board. The methods described for fabricating a circuit board include forming a core layer including a number of fibers, and forming a surface layer on the core layer, such that the surface layer is free of fibers and has thickness that is between about 10% and 30% of the circuit board thickness. Cracking is reduced in the surface layer or layers of circuit boards fabricated using the methods of the present invention.  
         [0030]     Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement which is calculated to achieve the same purpose may be substituted for the specific embodiment shown. This application is intended to cover any adaptations or variations of the present invention. Therefore, it is intended that this invention be limited only by the claims and the equivalents thereof.