Patent Application
20070178228
The present invention relates generally to printed circuit boards (PCBs). More particularly, the present invention relates to an apparatus and a method for fabricating a PCB without using wet chemicals.
A printed circuit board (PCB) is a flat board or substrate, which is used to support semiconductor chips and other electronic components. The substrate includes a sheet of insulator or dielectric material having a number of conductive traces that are used to interconnect the electronic components. The traces are typically formed on the insulator by laminating a layer of copper foil over the substrate. A photoresist layer is formed on the copper foil. Excess copper is then removed from by chemical etching, leaving only the desired copper traces. Finally, electronic components may be soldered to the traces.
The process may also be repeated to fabricate a multi-layer PCB, which comprises alternating layers of conductive and dielectric material bonded together. To connect one conductive layer to another, holes are drilled through the PCB and plated with a conductive substance. These plated through holes (PTHs) are also commonly referred to as vias. In current circuit designs, thousands of vias may be used to form a complex system of interconnections for circuit elements located in stacks of conductive layers. To improve the speed and performance of a chip, engineers constantly strive to shrink circuit designs so that each chip may support more elements with shorter conductive lines.
Some of the problems with traditional PCB fabrication are inherent in the commonly used processes of drilling, chemical etching, and electroplating. Because drilling is a mechanical process, high precision equipment is required to form the tiny PTHs required. Currently, it is difficult to form a via that is as small as 75 μm in diameter. Since engineers are constantly striving to reduce the size of the circuitry in all electronics to improve performance and speed, this is a limiting factor.
In addition, both chemical etching and electroplating are expensive and difficult to control processes that involve wet chemicals, which may in turn generate hazardous wastes. In particular, it is expensive to electroplate a tiny PTH wall surface. Furthermore, when vias are electroplated, they are typically not completely filled with metal. Because they are not filled, there may be undesirable air bubbles in the via that may expand during heating and crack the substrate and conductive traces.
Accordingly, it would be desirable to have a method and an apparatus to fabricate PCBs without using expensive mechanical or chemical processes. In particular, it would be desirable to improve the resolution of the circuit traces on the PCB in a process that does not use wet chemicals.
The present invention will be readily understood by the following detailed description in conjunction with the accompanying drawings. To facilitate this description, like reference numerals designate like structural elements.
A method and an apparatus for fabricating a PCB are provided. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be understood, however, to one skilled in the art, that the present invention may be practiced without some or all of these specific details. In other instances, well known process operations have not been described in detail in order not to unnecessarily obscure the present invention.
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To fabricate a pattern for the conductive traces of the PCB 10, a light source 16 is used generate a light beam 18 to selectively remove charge from the OPC layer 14 to form uncharged segments 20. By removing charge from selected portions of the OPC layer 14 to form the uncharged segments 20, unaltered charged segments 22 of the OPC layer 14 may be patterned to form an image of the desired conductive traces. Some examples of the light source 16 that are effective for removing charge from the OPC layer 14 include a laser and a light emitting diode (LED).
In one embodiment of the present invention, a programmable laser is used as the light source 16. The programmable laser generates the laser beam 18, which is used to selectively remove charge from the OPC layer 14 and form the uncharged segments 20. By using a programmable laser, the present invention is able to form charged segments 22 having a pitch of less than about 75 μm. If a state of the art laser printer having a low dot size is used, a pitch of less than about 50 μm may be achieved. Alternatively, the laser beam 18 may be directed through a masking plate (not shown) at the layer 14 to form the uncharged segments 20.
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To prepare the printer 32 for depositing the metal powder 24, an OPC layer 38 may be formed on the surface of drum 34. For ease of illustration, the OPC layer 38 is not drawn to scale and is typically only a few microns in thickness. The OPC layer 38 is given an electric charge, for example a uniform positive charge throughout by an ionizer (not illustrated) that is integrated with the printer 32. To determine where the metal powder 24 is to be deposited onto the substrate 10, the light source 36 is used to selectively remove charge from the OPC layer 38. A laser beam 39 is generated by the light source 36 and directed onto selected portions of the surface of the drum 34 to form uncharged sections 40, while leaving charged sections 42 undisturbed. The laser beam 39 is directed at the OPC layer 38 both by the light source 36 and by turning the drum 34. The charged sections 42 of the OPC layer 38 then form a pattern of the conductive traces desired on the substrate 10.
After forming the uncharged sections 40, the positively charged sections 42 attract the negatively charged metal powder 24 such that the negatively charged metal powder 24 adheres to the charged sections 42. The metal powder 24 may be stored in and released from a reservoir (not illustrated). The drum 34 is then used to print the metal powder 24 onto the resin film 12 as the substrate 10 is moved through the printer 32. When the printing process is finished, the substrate 10 is heated in order to melt the metal powder 24 to form the metal layer 28. In some instances, as will be understood by persons skilled in the art, pressure may also be used to aid in forming the metal layer 28.
Referring now to
To form a second metal layer 50, the processes of the present invention described above may be repeated. For example, using the direct paint method described in
One advantage of the present invention is that conductive traces may be formed by metal powder deposition instead of chemical etching and electroplating. Because wet chemicals are not involved, the present invention avoids having to use and control hazardous materials inherent in the traditional process. In addition, because the present invention creates vias by filling drill holes with metal powder, the expensive PTH process is eliminated. Another advantage of the present invention is that it is able to fabricate traces on a PCB having fine geometry (pitch less than about 50 μm), which may be used to build faster and more efficient circuits.
Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention. Furthermore, certain terminology has been used for the purposes of descriptive clarity, and not to limit the present invention. The embodiments and preferred features described above should be considered exemplary, with the invention being defined by the appended claims.
