Abstract:
According to one embodiment of the invention, a method includes providing a printed circuit board having a plurality of optoelectronic components coupled to a first side of the printed circuit board, forming a first clad layer outwardly from the first side of the printed circuit board, coupling an injection molding mold to the first side of the printed circuit board, injecting a material into the mold in liquid form, and after the material is solidified, decoupling the injection molding mold from the first side of the printed circuit board, thereby forming an optical waveguide outwardly from the first clad layer. The method may also include forming a second clad layer outwardly from the optical waveguide, and forming a metal layer outwardly from the second clad layer. In lieu of injection molding, stamping may be performed to form the core layer of the optical waveguide.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates generally to the field of printed circuit board manufacturing and, more particularly, to a method for forming an optical printed circuit board. 
     BACKGROUND OF THE INVENTION 
     Printed circuit boards are used in many applications, including cell phones, stereo equipment, and computers, just to name a few. Nowadays, the components associated with these printed circuit boards include electronic as well as optoelectronic components. The use of optoelectronic components resulted from the continuing increase in clock speed of the electronic components, which resulted in electrical interconnect bottlenecks, especially in the context of long interconnect bus architecture. However, the integration of optical, electronic, and optoelectronic components has been difficult and very manual labor intensive, which results in high manufacturing costs and low volume throughput. 
     SUMMARY OF THE INVENTION 
     According to one embodiment of the invention, a method includes providing a printed circuit board having a plurality of optoelectronic components coupled to a first side of the printed circuit board, forming a first clad layer outwardly from the first side of the printed circuit board, coupling an injection molding mold to the first side of the printed circuit board, injecting a material into the mold in liquid form, and after the material is solidified, decoupling the injection molding mold from the first side of the printed circuit board, thereby forming an optical waveguide outwardly from the first clad layer. The method may also include forming a second clad layer outwardly from the optical waveguide, and forming a metal layer outwardly from the second clad layer. In lieu of injection molding, stamping may be performed to form the core layer of the optical waveguide. 
     Embodiments of the invention provide a number of technical advantages. Embodiments of the invention may include all, some, or none of these advantages. In one embodiment, a manufacturing method achieves high-speed optical communication between different sections of a printed circuit board. This eliminates any bottlenecks that may occur as a result of increasing clock speeds for electrical components. The manufacturing method results in low-cost optical printed circuit board integration that combines both optical, electrical, and optoelectronic components in a single heterogeneous package. The manufacturing method is also particularly suitable for flex circuits that have thin substrates. 
     Other technical advantages are readily apparent to one skilled in the art from the following figures, descriptions, and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the invention, and for further features and advantages, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a perspective view of a flex circuit having optical wave guides formed thereon according to one embodiment of the invention; 
         FIG. 2  is a cross-sectional elevation view of the flex circuit of  FIG. 1  illustrating the forming of a clad layer; 
         FIG. 3  is a cross-sectional elevation view illustrating an injection molding mold for the flex circuit of  FIG. 2 ; 
         FIG. 4  is a cross-sectional elevation view illustrating the injection molding of a polymer to form the optical wave guides illustrated in  FIG. 1 ; 
         FIG. 5  is a cross-sectional elevation view illustrating the flex circuit of  FIG. 4  after removal of the mold; 
         FIG. 6  is a cross-sectional elevation view illustrating the forming of an additional clad layer to the flex circuit of  FIG. 5 ; and 
         FIG. 7  is a cross-sectional elevation view illustrating the forming of a metal layer to the flex circuit of FIG.  6 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a perspective view of a printed circuit board (“PCB”)  100  having one or more optical waveguides  102  formed thereon according to an embodiment of the present invention. PCB  100  is described in greater detail below in conjunction with  FIGS. 2 through 7 . Generally, PCB  100  includes a plurality of electronic and optoelectronic components coupled thereto that are connected to one another by a plurality of metal traces  104  in such a manner as to form one or more electrical circuits. An example of PCB  100  is a mother board for a computer; however, the present invention contemplates PCB  100  representing any circuit board, printed wiring board, flex circuit, or other suitable substrate having electronic and optoelectronic components coupled thereto. PCB  100  may have any suitable size and any suitable shape and may be formed form any suitable material. 
     Waveguides  102  are formed by any suitable arrangement of optically transmissive material that communicates optical signals as guided waves of energy from an optoelectronic component associated with PCB  100  to another optoelectronic component associated with PCB  100 . For example, a vertical cavity surface emitting laser (“VCSEL”)  106  associated with an optoelectronic component on PCB  100  emits light waves representing one or more electrical signals and these light waves travel through an optical waveguide  102   a  before reaching a photodiode detector  108  associated with another optoelectronic component of PCB  100 , in which the light waves are then converted back to electrical signals. Optical waveguides  102  may comprise multi-mode waveguides or single mode waveguides having any suitable cross-section. One technical advantage of having waveguides  102  formed on PCB  100  is that a much higher bus speed may be achieved through optical communication rather than electrical communication. This increased speed may be significant even when dealing with distances between components on a printed circuit board that may be only a few inches apart. A method of manufacturing waveguides  102  on PCB  100  is described in detail below in conjunction with  FIGS. 2 through 7 . 
       FIGS. 2 through 7  illustrate one method of forming optical waveguides  102  on one side of PCB  100 . The present invention docs, however, contemplate the forming of optical waveguides  102  on both sides of PCB  100 . 
       FIG. 2  is a cross-sectional elevation view of PCB  100  illustrating the forming of a clad layer  200  outwardly from a side of PCB  100 . In the illustrated embodiment, PCB  100  is a flex circuit formed from a polyimide that includes one or more optoelectronic components  202  on one side and one or more electronic components  204  formed on the other side. Although optoelectronic components  202  are shown to be coupled to PCB  100  on one side and electronic components  204  shown to be coupled to the other side of PCB  100 , the present invention contemplates any suitable location for both optoelectronic components  202  and electronic components  204 , including optoelectronics components  202  and/or electronic components  204  embedded within PCB  100 . Also shown in  FIG. 2  is a plurality of metal traces  206  embedded within PCB  100 . Metal traces  206  function to connect components associated with PCB  100 , such as optoelectronic components  202  and electronic components  204 . Although only one “layer” of metal traces  206  is illustrated, any number of suitable electrical interconnection layers may be utilized. PCB  100  also includes one or more alignment holes  208  formed in an outer periphery of PCB  100 . Alignment holes  208 , as described in further detail below, function to align an injection molding mold  300  ( FIG. 3 ) with PCB  100 . Alignment holes  208  may have any suitable shape and any suitable size. For example, alignment holes  208  may be holes formed with a diameter of approximately ten mils. Alignment holes  208  may or may not extend through the full thickness of PCB  100 . 
     Clad layer  200  functions to form a cladding for optical waveguides  102 . In general, a cladding for an optical waveguide prevents light waves from escaping the core during transmission. Clad layer  200  may be formed from any suitable material using any suitable method and may have any suitable thickness. In one embodiment, clad layer  200  is formed from a material that is optically transparent to the wavelength that is desired to be transmitted. As an example, clad layer  200  may be formed from a suitable optical polymer material, such as Lexan®. In addition, a spin coat, spray coat, or other suitable deposition technique may be used to form clad layer  200 . In order to avoid performance degradation of optical waveguides  102 , it is important that clad layer  200  have good planarization. An exemplary surface planarity for clad layer  200  is less than one micron for every five inches of length. 
     In one embodiment of the present invention, a clad layer  210  is formed on the other side of PCB  100 . Clad layer  210  helps to prevent or reduce warpage of PCB  100  and clad layer  200  by balancing any thermal stresses that may occur due to different coefficient of thermal expansions between clad layer  200  and PCB  100 . Clad layer  210 , similar to clad layer  200 , may be formed from any suitable material using any suitable method and may have any suitable thickness. 
       FIG. 3  is a cross-sectional elevation view illustrating injection molding mold  300  according to one embodiment of the present invention. Injection molding mold  300 , which may be formed from any suitable material, couples to PCB  100  by engaging one or more protuberances  302  of injection molding mold  300  with alignment holds  208  of PCB  100 . In the illustrated embodiment, injection molding mold  304  includes a first cavity  304 , a second cavity  306 , and a pair of inlets  308 . The shape of cavity  304  defines the shape of optical waveguides  102 . Cavity  306 , on the other hand, may have any suitable shape. However, if optical waveguides are being formed on both sides of PCB  100  then the shape of cavity  306  defines the optical waveguides being formed on that particular side of PCB  100 . Inlet  308   a  functions to introduce the material used to form waveguides  102  into cavity  304 , and inlet  308   b  functions to introduce the material into cavity  306 . The injection molding process is described in greater detail below in conjunction with FIG.  4 . 
       FIG. 4  is a cross-sectional elevation view illustrating the injection molding of a polymer to form optical waveguides  102 . Any suitable injection molding process may be utilized. As illustrated in  FIG. 4 , an arrow  400  illustrates the injection of a polymer into cavity  304  in order to form waveguides  102  and an arrow  402  illustrates the injecting of a polymer into cavity  306 . Generally, the polymer (or other suitable material) is injected into cavities  304  and  306  in a liquid form and thereafter solidifies. The material used to form waveguides  102  determines the injection temperature for the material, which may be any suitable temperature. In addition, the type of material determines whether or not a curing cycle is required. Typically, once the material solidifies after injection, mold  300  is removed from PCB  100 . This result is illustrated in FIG.  5 . 
       FIG. 5  is a cross-sectional elevation view illustrating PCB  100  after removal of injection molding mold  300 . As illustrated, a core  501  of a particular waveguide  102  has now been formed. Core  501  may take on any suitable form and may be formed from any suitable material, such as any suitable optical polymer. It is important that the material used to form core  501  should have a higher index of refraction than the material used to form clad layer  200 . In a particular embodiment of the present invention, the ends of core  501  have an angle of approximately 45°, as denoted by reference numeral  500 . This enables the light waves being emitted by, for example, VCSEL laser  106  to deflect off of angled surface  500  at a 90° angle. Other suitable shapes are contemplated by the present invention for the ends, as well as the cross-section of core  501 . Also illustrated in  FIG. 5  is a polymer layer  502  that is used to balance any thermal stresses that may occur due to the different coefficient of thermal expansions between core  501  and PCB  100  to prevent or reduce warpage. 
     Also illustrated in  FIG. 5  are contact pads  504  extending outwardly from the edges of waveguides  102 . Contact pads  504  are also illustrated in better detail in FIG.  1 . The waveguides are formed in such a manner as to allow contact pads  504  to be exposed on PCB  100  so that suitable electrical contacts may be made to contact pads  504 . 
       FIG. 6  is a cross-sectional elevation view illustrating the forming of an additional clad layer  600  outwardly from cores  501  in accordance with an embodiment of the present invention. Even though the addition of clad layer  600  to cores  501  is an optional step, it may serve a few useful purposes. First, clad layer  600  may function similarly to clad layer  200  in that it covers cores  501  to prevent light waves from escaping cores  501  during transmission, and second layer  600  may provide protection for cores  501  to avoid any scratches or other imperfections that may deteriorate the performance of waveguides  102 . Any suitable material may be used to form clad layer  600 ; however, the material should have a lower index of refraction than that used to form cores  501 . In addition, any suitable method may be used to form clad layer  600 , such as spin coating, spray coating, or other suitable deposition technique. The planarity of clad layer  600  is not as important as the planarity of clad layer  200 . Also illustrated in  FIG. 6  is a clad layer  602  formed outwardly from polymer layer  502  that functions to balance any thermal stresses that may occur due to the different coefficients of thermal expansion between clad layer  600  and PCB  100  to prevent or reduce warpage. 
       FIG. 7  is a cross-sectional elevation view illustrating the forming of a metal layer  700  outwardly from clad layer  600  in accordance with an embodiment of the present invention. Although the forming of metal layer  700  is an option in the method outlined in  FIGS. 2 through 7 , it may serve a few useful purposes. First, it may provide additional light confinement, and second, it may provide electrical shielding for electrical components associated with PCB  100 . Any suitable metal may use to form metal layer  700  and it may be formed using any suitable method. Also illustrated in  FIG. 7  is a metal layer  702  formed outwardly from clad layer  602 . Metal layer  702  may be used to balance any thermal stresses that may occur due to the different coefficient of thermal expansions between metal layer  700  and PCB  100  to prevent or reduce warpage. 
     Although embodiments of the invention and their advantages are described in detail, a person skilled in the art could make various alterations, additions, and omissions without departing from the spirit and scope of the present invention as defined by the appended claims.