Abstract:
Methods, memory devices, module boards and systems are disclosed utilizing a non-continuous conductive layer in their circuit board as opposed to a substrate having a continuous length of metal such as copper from one end to the other. By ensuring that a non-continuous conductive layer is no longer present in a substrate, deformation and warping of the substrate or circuit board can be reduced. This can reduce or prevent future errors in processing from occurring due to the tight tolerance required in processing of circuit boards.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. Pat. No. 6,429,385, filed Aug. 08, 2000, and is related to U.S. patent application Ser. No. 09/968,564 filed Oct. 01, 2001. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of circuit board and substrate manufacture and, more particularly, to a non-continuous conductive layer for laminated substrates. 
     BACKGROUND OF THE INVENTION 
     Circuit boards and printed circuit boards (PCB) are commonly used in electronic devices of today. Many electronic devices, such as motherboards, memory devices, video adaptors, network cards and the like are created using circuit boards. 
     Generally, a circuit board is a flat piece of insulating material such as fiberglass, epoxy or phenolic resin, on which electrical components are mounted and interconnected to form a circuit. The flat piece of insulating material forms the substrate. A laminated circuit board is a circuit board in which a conductive layer is laminated onto an insulating layer. 
     Circuit boards or PCBs have multiple conductive paths or interconnects to provide electrical connections among circuit components on the board. FIG. 1A shows a typical laminated circuit board having a copper layer  101  laminated onto a fiberglass layer  102 . This circuit board or substrate has a thickness of 0.010 inches, a length of 7.2 inches and a width of 1 inch. FIG. 1B shows a typical laminated circuit board having a fiberglass layer  102  sandwiched between two metal layers  101 ,  103  such as copper. This circuit board has a thickness of 0.012 inches, a length of 7.2 inches and a width of 1 inch. Circuit boards are commonly used for devices such as memory devices, module boards, video cards, sound cards and the like. 
     The connections between components on a circuit board are typically created by using photolithography. The circuit pattern is drawn, photographed, and reduced to a negative having the desired final size. This negative is called the photomask or mask. Light is passed through the mask onto a substrate having a conductive layer that has been coated with a photoresistive material. Where light strikes the photoresistive material, its composition is changed. In the next step, the photoresistive material not affected by light is washed off. Finally, the circuit board is exposed to an etching solution that eats away the parts of the conductive layer not protected by the photoresistive material, creating the desired circuit pattern on the surface of the circuit board. 
     Standard substrates are used for circuit boards and devices such as memory devices. Minor defects or deformations in these substrates or circuit boards can have a significant impact on further processing of the circuit board which includes attaching components, such as integrated circuits, to the circuit board. Processing requires strict tolerances and even minor deformations can damage equipment or render a circuit board useless. For example, even a {fraction (1/16)}″-⅛″ bow in a substrate for a dual in line memory module (DIMM) can cause problems in processing the module. 
     FIG. 2 shows a typical circuit board after it has been patterned. After a circuit board is patterned, circuit boards typically include lengths of conductive material along each edge of the circuit board, from one end to the other end. These lengths of conductive material are called rails and are shown in FIG. 2 as a first rail  201  and a second rail  202 . The rails  201  and  202  are formed as a result of the patterning. The conductive material can be a metal such as copper. Conductive materials have a property that once they are deformed or bent, they “remember” that deformation. For example, once a piece of copper is bent a certain way, the piece will have a tendency to bend that certain way even after it is bent a different way. Because of the rails, these circuit boards or substrates have a tendency to remember any deformation that they are subjected to. Circuit boards encounter thermal cycling during processing which causes the expansion and contraction of the substrate. This can result in circuit boards that are warped or deformed. 
     What is needed is a way to reduce warping or deforming of circuit boards during processing. 
     SUMMARY OF THE INVENTION 
     A method for fabricating a circuit board having a non-continuous conductive layer is disclosed. A conductive layer is laminated onto an insulating layer. A pattern is etched on the conductive layer to eliminate continuous lengths of conductive material. 
     A method for fabricating a circuit board is disclosed. A conductive layer is formed over an insulating layer. The conductive layer has a first rail area, a pattern area and a second rail area. The pattern area of the conductive layer is patterned. Conductive material from the first and second rail areas is removed. 
     A module board is disclosed. The module board includes a circuit board, a pattern and rails. The circuit board has a conductive layer of a conductive material laminated to an insulating layer. The pattern is etched onto the substrate. The rail is located along a first and second length of the substrate. The rail is an area not etched from the pattern. Conductive material is at least partially removed from the rails to remove continuous lengths of conductive material from the circuit board. 
     Other methods, systems and devices are disclosed. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1A is a cross sectional illustration of a typical laminated circuit board substrate having a copper layer laminated onto a fiberglass layer; 
     FIG. 1B is a cross sectional illustration of a typical laminated circuit board substrate having a fiberglass layer sandwiched between two metal layers; 
     FIG. 2 is an illustration of a typical circuit board; 
     FIG. 3A is an illustration of a laminated circuit board according to the present invention; 
     FIG. 3B is an illustration of a laminated circuit board according to the present invention; 
     FIG. 4A is an illustration of a side view of a module board according to the present invention; 
     FIG. 4B is an illustration of a top view of a module board according to the present invention; 
     FIG. 4C is an illustration of a side view of a dual sided module board according to the present invention; 
     FIG. 4D is an illustration of a top view of a dual sided module board according to the present invention; 
     FIG. 4E is an illustration of a bottom view of a dual sided module board according to the present invention; 
     FIG. 5 is a method for fabricating a circuit board having a non-continuous conductive layer according to the present invention; 
     FIG. 6 is a method for fabricating an electronic device according to the present invention; and 
     FIG. 7 is a block diagram of a typical computer system in which the invention may be used. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The term “patterning” refers to one or more steps that result in the removal of selected portions of layers. The patterning process is also known by the names photomasking, masking, photolithography and microlithography. The term “circuit board” refers to a flat piece of insulating material, such as epoxy or phenolic resin, on which electrical components are mounted and interconnected. 
     As stated earlier, FIG. 2 is an example of a standard circuit board  200  used for memory devices such as dual in line memory modules (DIMMs). The circuit board includes a first rail  201 , a second rail  202 , a number of sites  203  and a patterned area  204 . A rail is an area along the edge of a circuit board. The rails  201  and  202  do not generally contain conductive paths, bonding pads or the like. Thus, the rails  201  and  202  do not get etched and conductive material is not removed from the rails  201  and  202 . The rails  201  and  202  include a large amount of conductive material such as copper. The conductive material stretches from one end of the substrate to the other. FIG. 2 only shows 3 die sites or sites, however any number of sites such as  12  can be on a single circuit board. A site is a location where an integrated circuit is attached to the substrate, typically by soldering. The patterned area  204  is the area where electrical interconnects and bonding pads are formed by etching away conductive material. The patterned area  204  is adjacent to the first rail  201  and the second rail  202 . The circuit board  200  of FIG. 2 is susceptible to deformation and warping due to expansion and contraction of the circuit board during thermal cycling. This circuit board has a tendency to remember any deformation that it is subjected to and can cause errors in processing. 
     FIG. 3A is a laminated circuit board  300  according to one embodiment of the invention. The circuit board  300  includes a first rail  301 , a second rail  302 , a number of sites  303  and a patterned area  305 . The patterned area  305  is adjacent to the first rail  301  and the second rail  302 . Even though there are only 3 sites  303  shown in FIG. 3, the invention is not limited to a specific number of sites. The first rail  301  and the second rail  302  have had all conductive material removed and thus, no conductive material remains. The patterned area  305  is the area where electrical interconnects, conductive traces, and bonding pads are formed by etching away conductive material. A conductive material commonly used is copper, however other conductive materials may be used in the circuit board. By having the conductive material removed from the rails  301  and  302 , this circuit board  300  has a reduced tendency to warp or deform. 
     FIG. 3B is a laminated circuit board  300  according to one embodiment of the invention. The circuit board  300  includes a first rail  301 , a second rail  302 , a number of sites  303  and a patterned area  305 . The patterned area  305  is adjacent to the first rail  301  and the second rail  302 . Even though there are only 3 sites  303  shown in FIG. 3, the invention is not limited to a specific number of sites. The first rail  301  and the second rail  302  have had gaps  304  of conductive material removed and thus, no continuous lengths of conductive materials remain. Rather, the first and second rails  301 ,  302  include segments of conductive material between the gaps  304 . The patterned area  305  is the area where electrical interconnects and bonding pads are formed by etching away conductive material. A conductive material commonly used is copper, however other conductive materials may be used in the circuit board. By having the gaps  304  of conductive material removed from the rails  301  and  302 , this circuit board  300  has a reduced tendency to warp or deform. 
     FIG. 4A is a side view of a module board  400  according to one embodiment of the invention. The module board  400  has a top  406  and is fabricated on a laminated circuit board having an insulating layer  405  and a conductive layer  404  laminated onto the insulating layer. For the purposes of describing and defining the present invention, a module board is any electronic device made from a circuit board such as video cards, sound cards, memory devices, mother boards, network cards and the like. The conductive layer  404  is made from a conductive material such as copper. The insulating layer  405  is made from an insulating material such as fiberglass. 
     FIG. 4B is top view of a module board  400  according to one embodiment of the invention. The module board  400  can be any number of electronic devices such as video cards, sound cards, network cards and the like. For this embodiment, the module board  400  includes a pattern area  402  in between two rail areas  401 . Alternate embodiments may have one rail or no rails. The pattern area  402  is etched onto the substrate of the module board  400 . The pattern creates the desired conductive connections, leads and pads on the board  400 . A mask or photomask may be used to etch the pattern so as to remove conductive material. After the pattern area  402  is etched, rail areas  401  along each edge remain and the rail areas  401  have not had conductive material removed during patterning. The rail areas  401  are partially etched or completely etched so that no continuous lengths of conductive material remains from one end of the board  400  to the other. To remove continuous lengths of conductive material from the rail areas  401 , all of the conductive material in the rail may be removed or gaps of removed conductive material in the rail could be created leaving segments of conductive material between the gaps. The module board  400  can be further processed by connecting integrated circuits and devices while having a reduced tendency to deform or warp compared to boards having continuous lengths of such a conductive material. 
     FIG. 4C is a side view of a dual sided module board  409  according to one embodiment of the invention. The module board  409  has a top  410  and a bottom  420 . The module board  409  is fabricated on a laminated circuit board comprised of a top conductive layer  430  laminated onto an insulating layer  431  laminated onto a bottom conductive layer  432 . For the purposes of describing and defining the present invention, a module board is any electronic device made from a circuit board such as video cards, sound cards, memory devices, mother boards, network cards and the like. The conductive layers  430  and  432  are made from a conductive material such as copper. The insulating layer  431  is made from an insulating material such as fiberglass. 
     FIG. 4D is top view of a dual sided module board  409  according to one embodiment of the invention. The module board  409  can be any number of electronic devices such as video cards, sound cards, network cards and the like. The module board  409  includes a pattern area  412  in between two rail areas  411 . The pattern  412  is etched onto the substrate of the module board  409 . The pattern creates the desired conductive connections, leads and pads on the board  409 . A mask or photomask may be used to etch the pattern so as to remove conductive material. After the pattern  412  is etched, rails  411  along each edge remain and the rails  411  have not had conductive material removed during patterning. The rails  411  are partially etched or completely etched so that no continuous lengths of conductive material remains from one end of the board  409  to the other. To remove continuous lengths of conductive material from the rails  411 , all of the conductive material in the rail may be removed or gaps of removed conductive material in the rail could be created. The module board  409  can be further processed by connecting integrated circuits and devices while having a reduced tendency to deform or warp compared to boards having continuous lengths of such a conductive material. 
     FIG. 4E is a bottom view of a dual sided module board  409  according to one embodiment of the invention. The module board  409  includes a pattern area  422  in between two rail areas  421 . The pattern  422  is etched onto the substrate of the module board  409 . The pattern creates the desired conductive connections, leads and pads on the board  409 . A mask or photomask may be used to etch the pattern so as to remove conductive material. After the pattern  422  is etched, rails  421  along each edge remain and the rails  421  have not had conductive material removed during patterning. The rails  421  are partially etched or completely etched so that no continuous lengths of conductive material remains from one end of the board  409  to the other. To remove continuous lengths of conductive material from the rails  421 , all of the conductive material in the rail may be removed or gaps of removed conductive material in the rail could be created. The module board  409  can be further processed by connecting integrated circuits and devices while having a reduced tendency to deform or warp compared to boards having continuous lengths of such a conductive material. 
     FIG. 5 is a method  500  for fabricating a circuit board having a non-continuous conductive layer according to one embodiment of the invention. A conductive layer is laminated onto an insulating layer at block  501 . The conductive layer can be composed of a conductive material such as a metal such as copper. The insulating layer can be of a material such as fiberglass. A pattern is then etched into the conductive layer of the circuit board at block  502 . This removes some of conductive material and creates the electrical connections and bonding or connecting pads on the circuit board. However, rails have not had any conductive material removed and contain conductive material stretching from one end of the circuit board to the other. The rails are etched to remove conductive material from the rails  503 . The conductive material may be completely removed from the rails or sections or gaps of conductive material may be removed so that no continuous lengths of conductive material remain. 
     FIG. 6 is a method  600  for creating an electronic device according to one embodiment of the invention. A first conductive layer is laminated onto an insulating layer at block  601 . A second conductive layer is laminated onto the opposite side of the insulating layer at block  602 . Conductor patterns for a number of die sites are etched into both conductive layers at block  603 . Bonding pads are also etched into the conductive layers  604 . Rails having continuous lengths of conductive material remain on the edges of both conductive layers because they have not had conductive material removed from etching. The rails are then etched to remove the conductive material, at least partially such that no continuous lengths of conductive material remain on the rails  605 . A continuous length of material is a strip of material that completely goes from one end of the circuit board to the other. The circuit board then continues processing and integrated circuits are attached to the die sites  606 . 
     FIG. 7 is an illustration of a computer system  712  that can use and be used with embodiments of the present invention. As will be appreciated by those skilled in the art, the computer system  712  would include ROM  714 , mass memory  716 , peripheral devices  718 , and I/O devices  720  in communication with a microprocessor  722  via a data bus  724  or another suitable data communication path. These devices can be fabricated according with the various embodiments of the present invention. 
     Having described the invention in detail and by reference to preferred embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims. For example, support and guidance structures used in packaging often have continuous layers of conductive material. Gaps or portions of conductive material can be removed from support and guidance structures to reduce deformations caused by the memory effect.