Abstract:
An improved polishing apparatus having a dampening system to prevent fluctuations between the planar head polishing device and the circuit panel. The dampening system includes a load actuation arm having a hydraulic-pneumatic cylinder which is controlled by an connected dampening circuit.

Description:
This application is a divisional of Ser. No. 09/169,760, filed on Oct. 9, 1998, now U.S. Pat. No. 6,254,463. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates generally to the chemical mechanical polishing of printed circuit boards, and more particularly, to a planar head dampening system for a chemical mechanical polishing system. 
     2. Related Art 
     The strict process requirements of a chemical mechanical polishing system, used in the manufacture of printed circuit boards, necessitate the use of a compressible polishing device and the application of a low downward pressure, ranging from approximately 0.25 to 10 psi. These requirements are essential to the polishing process due to the delicate nature of the surface contours of the workpiece. Use of a less compressible polishing device or attempting to increase the down pressure could eliminate these desired surface contours or otherwise damage the workpiece. Unfortunately, gravity alone is insufficient to hold the polishing device in contact with the workpiece while polishing. Rather, in the absence of an applied down pressure, the polishing device floats above the workpiece without polishing its surface. Therefore, a polishing system utilizing a planar head polishing device having a low durometer compressible core in conjunction with the application of a low down pressure are currently used in the industry. 
     The requisite down pressure is typically supplied by a compressed air source. However, when the surface of the planar head polishing device was upgraded to a more aggressive material, a destructive phenomenon known as “high frequency cycling” was encountered. High frequency cycling results when the planar head is brought into contact with the workpiece, and due to its coarse polishing surface, the planar head begins to grip the surface of the workpiece and “bite-in.” This causes the compressible inner core of the planar head to deform, forcing it in the opposite direction intended. Due to the compressible nature of the air providing the down pressure, the upward force exerted by the deformed planar head produces a rebounding effect. These fluctuations in contact between the polishing device and the workpiece produce proportionate fluctuations in the planar head impression and the process rate of the workpiece. Additionally, premature failure of the planar head lowering device, inconsistent surface conditions of the planar head and non-uniform planarization of the workpiece surface can result. 
     The high frequency cycling appears to be attributed to the combination of the low durometer inner core of the planar head and the low down pressure. In an attempt to remedy the problem, planar heads composed of different durometer materials were tested. However, these alternative materials created further processing problems and, therefore, offered no solution. In the alternative, various shock absorbers were considered to eliminate the effects of the compressed air. However, none produced a regressive non-compressible system, which would allow the pressure to remain constant throughout polishing. These alternatives were also ineffective since they would be destroyed by the harsh chemical environment of the system. 
     Therefore, there is clearly a need to develop a chemical mechanical polishing device which eliminates the occurrence of high frequency cycling. 
     SUMMARY OF THE INVENTION 
     The present invention provides a device that will overcome the above-identified problems through the use of a hydraulic-pneumatic dampening system. 
     A first general aspect of the present invention provides a polishing apparatus having at least one polishing device and a support platform comprising, a load actuating arm, mounted at a first end to the support platform and at a second end to the polishing device, and a dampening circuit connected to the load actuating arm to prevent fluctuations. This aspect allows for the application of a constant low down pressure of the polishing device, thereby eliminating the occurrence of fluctuations between the planar head and the circuit panel. 
     A second general aspect of the present invention includes a polishing apparatus comprising, a conveyor mounted to a platform to transport a workpiece to a polishing area of the apparatus, and a plate pivotally secured above the conveyor carrying an at least one polishing device rotatably mounted therein. This aspect provides for the apparatus to transport a circuit panel to the polishing area of the apparatus and for lowering the polishing device. 
     A third general aspect of the present invention includes a polishing apparatus having at least one polishing device and a support platform comprising, a conveyor mounted to the platform to transport a circuit panel to the at least one planar head assembly, a plate pivotally secured above the conveyor, a planar head rotatably mounted within the plate, a load actuating arm, mounted at a first end to the support platform and at a second end to the plate, and a dampening circuit connected to the load actuating arm to prevent fluctuations. This aspect provides similar advantages to that of the first and second aspects. 
     A fourth general aspect of the present invention includes a polishing apparatus having a dampening system to prevent fluctuations. This aspect provides similar advantages to that of the first and third aspects. 
     The foregoing and other features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The preferred embodiments of this invention will be described in detail, with reference to the following figures, wherein like designations denote like elements, and wherein: 
     FIG. 1 depicts a cross-sectional view of the deformation of a planar head upon contact with a workpiece, which necessitated the present invention; 
     FIG. 2 depicts a cross-sectional view of a chemical mechanical polishing assembly in accordance with the present invention; 
     FIG. 3 depicts a planar head in accordance with the present invention; 
     FIG. 4 depicts a cross-sectional view of a planar head in accordance with the present invention; 
     FIG. 5 depicts a dampening circuit of the chemical mechanical polishing assembly in accordance with the present invention; and 
     FIG. 6 depicts a cross-sectional view of a hydraulic-pneumatic cylinder in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Although certain preferred embodiments of the present invention will be shown and described in detail, it should be understood that various changes and modifications may be made without departing from the scope of the appended claims. The scope of the present invention will in no way be limited to the number of constituting components, the materials thereof, the shapes thereof, the relative arrangement thereof, etc., and are disclosed simply as an example of the preferred embodiments. 
     FIG. 1 shows a cross-sectional view of a deformation  6  of a planar head  3  when brought into contact with a workpiece  2  carried on a delivery system  1 , such as a conveyor, a robotic armature, a turn table, etc. While planar head  4  shows the ideal contact scenario, planar head  5  depicts the deformation, illustrated by dashed line  6 , that causes planar head  5  to rebound away from workpiece  2 , thereby producing non-uniform planarization. 
     FIG. 2 depicts a cross-sectional view of a chemical mechanical polishing assembly  10 , in accordance with the present invention. A back panel  12  is attached perpendicularly to a platform  11 . A conveyor  13 , or other means of delivery, is mounted to back panel  12 , and carries a workpiece  14  to the various planar head assemblies A, B, etc., to be polished. It should be appreciated that an infinite succession of planar head assemblies may be used in accordance with the invention, and it is not limited to number shown in the figures. A planar head  16 , rotatably mounted about a solid shaft  28  within a plate  15 , is propelled by a pulley  17  via belts  19  and  20 , which are powered by a motor  18 . It should be recognized that chains, ropes, etc., could be used to propel planar head  16 . Further, motor  18  could be placed in alignment with planar head  16 , thereby eliminating the need for pulley  17 . A load actuation arm  21 , designed to raise and lower plate  15  pivotally about a pin P, is attached at one end to platform  11  via a bracket  24 , in this depiction an L-shaped bracket is used, and at the other end to plate  15  via a bracket  25 . Load actuation arm  21  includes a hydraulic-pneumatic cylinder  26 , fittings  22  and  23  located on both ends of cylinder  26 , and an arm  27  slidably mounted within cylinder  26 . 
     FIG. 5 shows a cross-sectional view of the dampening circuit which controls the movement of load actuation arm  21 . To lower planar head  16 , a two position-four way valve  34  is placed in position  34   b . This allows compressed air from supply  35  to apply pressure to a fluid reservoir  31 , which supplies a chamber  35  of cylinder  26  (FIG.  6 ), with a non-compressible medium having a low viscosity, approximately 0.9 to 1.0 centipoises. This draws arm  27  into cylinder  26  and lowers planar head  16 . A flow control area  33  includes a check valve  32  which permits the transmission medium from fluid reservoir  31  to flow unrestricted in one direction only, thereby preventing back flow and maintaining constant pressure in cylinder  26  during polishing. When polishing is complete, two position-four way valve  34  is switched to position  34   a , thereby redirecting compressed air from supply  35  into chamber  36  of cylinder  26  (FIG.  6 ), and allowing chamber  36  to fill with compressed air and raise planar head  16 . It should be appreciated that supply  35  is not limited to the use of compressed air and may be replaced with other acceptable mediums, i.e., hydraulic fluid. 
     FIG. 3 depicts the configuration of planar head  16  having solid shaft  28 . FIG. 4 shows a cross-sectional view of planar head  16 , which includes solid shaft  28 , an inner core  29  and an outer shell  30 . Solid shaft  28  is preferably composed of titanium. Inner core  29  is preferably composed of a low durometer material, i.e., rubber. Outer shell  30  is preferably a polishing paper material. In the alternative, outer shell  30  may be composed of a higher durometer material than inner core  29  in conjunction with a polishing paper. However, these layers are not limited to the use of these specific materials. 
     While this invention has been described in conjunction with the specific embodiments outlined above, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the preferred embodiments of the invention as set forth above are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit ans scope of the invention as defined in the following claims.