Abstract:
Apparatus and a method of cold cross-sectioning soft materials includes providing a chuck attached to a drive motor with a composite plate attached to the chuck and including a heat insulating portion, a heat conducting layer, and a central axially extending duct with a plurality of radially extending conduits in communication therewith, the central axially extending duct is accessible externally for introducing a cooling liquid thereto. A sheet of grinding material is magnetically attached to the composite plate and the drive motor is activated to rotate the composite plate and grinding material. A cooling liquid is introduced into the duct and communicated to the conduits and a lubricant is supplied to the exposed rotating surface of grinding material.

Description:
FIELD OF THE INVENTION 
     This invention relates to apparatus and method for obtaining a cross-section of soft materials using a cold grinding process. 
     BACKGROUND OF THE INVENTION 
     Standard metallographic procedures are normally practiced at room temperature on relatively hard materials such as steel, refractory materials, even aluminum, and typically the samples are of such dimensions that there is little concern about destroying a sample by over polishing. Failure analysis or characterization of electronic materials especially very soft solders and some polymers presents unique challenges. Soft materials are removed rapidly during grinding making it difficult to control end point especially if the area of interest is very small. Soft materials often deform or smear, rendering it impossible to delineate interfaces and boundaries. Chips of harder materials surrounding the soft material or the grinding material itself often become embedded and cannot be removed. Lowering the temperature of the samples to be ground usually hardens the material allowing more controlled grinding and also reduces deformation. Cold polishing techniques are well known for these purposes but tend to be used for specific purposes and on a small scale. 
     Accordingly it is highly desirable to provide apparatus and a method of overcoming these problems. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring to the drawings: 
     FIG. 1 is a simplified sectional view of a composite plate attached to a motor in cold cross-sectioning apparatus in accordance with the present invention; 
     FIG. 2 is a top plan view of a temperature insulating layer of the composite plate of FIG. 1; and 
     FIG. 3 is a top plan view of a temperature conducting layer of the composite plate of FIG.  1 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Turning now to FIG. 1, a simplified sectional view of a composite plate or platen, generally designated  10 , attached to a motor  11  in cold cross-sectioning apparatus  12  in accordance with the present invention. Plate  10  includes a chuck  15  fixedly attached to shaft  16  of motor  11  for rotation therewith. While chuck  15  is illustrated as a circular disk for convenience, it will be understood that any apparatus for attaching plate  10  to motor  11  for rotation is acceptable. Further, as will be understood after a careful study of the following disclosure, the various components of plate  10  may be constructed sufficiently large or thick, individually or as an assembly, so that chuck  15  is incorporated directly into plate  10  and a separate chuck, or other mounting structure is not required. 
     Composite plate  10  includes a heat insulating portion  20  positioned adjacent chuck  15 . In this specific embodiment, heat insulating portion  20  is a circular heat insulating layer of hard plastic, such as polypropylene or the like. Referring additionally to FIG. 2, portion  20  includes a central axially extending duct  21  with a plurality of radially extending conduits  22  in communication therewith. Each of the conduits  22  terminates adjacent the outer periphery of portion  20  for reasons that will be explained in more detail presently. Further, while each of the conduits  22  is illustrated as extending straight along a radius of portion  20  from duct  21  to adjacent the outer periphery, it should be understood that other configurations, such as arcuate, helical, etc., could be used and such configurations would still come within the definition of radially extending conduits. 
     In the specific embodiment illustrated, portion  20  is formed of a plastic and conduits  22  are defined by grooves formed in the upper surface of portion  20 . In another embodiment, conduits  22  are defined by copper tubes positioned in grooves formed in the upper surface of portion  20 . The copper tubes aid in distributing temperatures equally across the entire area of plate  10  and form a more rugged coolant distribution system. While conduits  22  could be formed in other potions of plate  10 , by forming conduits  22  in plastic portion  20 , grooves can be conveniently formed by molding, grinding, etc. Also. portion  20  is formed of heat or temperature insulating material and placed adjacent chuck  15  and/or motor  11  to insulate motor  11  from the remainder of plate  10 , as will be understood from the description to follow. 
     A heat or temperature conductive portion  25  is coaxially affixed to portion  20  so that the lower surface of portion  25  closes the grooves in portion  20  to form complete conduits  22  or contacts the copper tubes, if included. Also a central opening  26  is formed in disk  25  so as to be in communication with duct  21  in portion  20  and, thus, in communication with conduits  22 . In this preferred embodiment, temperature conductive portion  25  is an aluminum disk as illustrated in FIG. 3, which is fixedly attached to portion  20  by screws, etc. Temperature conductive portion  25 , which is in direct communication with conduits  22  conducts heat across the entire area of plate  10 . 
     A magnetic portion  30  is attached to temperature conductive portion  25  opposite heat insulating portion  20 . In this preferred embodiment, magnetic portion  30  is a magnetic disk, such as the commercially available rubberized magnetic sheets. It will of course be understood that magnetic portion  30  could be incorporated into temperature conductive portion  25  as an integral part thereof or as individual magnets embedded in the upper surface. However, for convenience in fabricating magnetic portion  30  of plate  10  a rubberized magnetic sheet can be most easily cut into a disk with a central opening in communication with duct  21 . The magnetic disk is then fixedly attached to the upper surface of temperature conductive portion  25  by an adhesive or the like. 
     A grinding plate  35  is formed by coaxially attaching a disk of grinding paper to a disk of magnetic material, such as steel or the like. Generally, the grinding paper is permanently attached to the magnetic material by an adhesive or the like. A central opening is provided through grinding plate  35  in communication with duct  21  to provide external access to duct  21 . Grinding plate  35  is then magnetically attached to magnetic portion  30  with the grinding surface directed upwardly to provide an exposed rotatable surface of grinding material. As the grinding surface (i.e. the grinding paper) wears or when different degrees of roughness are desired, grinding plate  35  can be easily changed and replaced with new or different grinding surfaces. It should also be noted that grinding plate  35  can be quickly and easily changed without altering the temperature of plate  10 . 
     With all of the components of plate  10  in place as described, a sample of soft material to be cold cross-sectioned is mounted in a holder  40  adjacent to the exposed rotatable surface of grinding material, i.e. the upper surface of grinding plate  35 . A cooling liquid is introduced into duct  21 , in this embodiment from a reservoir  41  by way of a funnel  42  positioned over the central opening in grinding plate  35 . Funnel  42  can be simply suspend above plate  10  by some convenient structure or it can be threadedly engaged in the central opening through grinding plate  35 , magnetic portion  30 , and temperature conductive portion  25 . In this preferred embodiment, the cooling liquid is liquid nitrogen which is introduced to duct  21  at approximately −196° C. As plate  10  is rotated by motor  11 , the cooling liquid is forced outwardly in conduits  22  by centrifugal force so that the entire area of plate  10  is cooled. As the cooling liquid evaporates, nitrogen gas escapes back out of conduits  22  by way of duct  21 , rather than being directed outwardly where it might harm an operator. 
     A lubricant, contained in a reservoir  45  is introduced to the grinding surface by way of a pipe  46 . While a large variety of lubricants are presently available, in this specific embodiment propylene glycol is used because it remains liquid at low temperatures, but unlike most petroleum based lubricants it is non-toxic and can also be used with most plastics with out damage since it is non-corrosive to most plastics and metals. Since propylene glycol is also water soluble and is considered non-hazardous, it can be discharged down an industrial drain, unlike other typical low temperature lubricants based on more toxic petroleum distillates, such as kerosene. 
     Here it should be noted that sample holder  40  can be a commercially available fixture, such as a weighted fixture available from Technology Associated Inc. These individual weighted fixtures are not fixed and can be placed or distributed over the entire exposed surface of grinding plate  35  using simple guides  50  to constrain the movement. In this fashion up to eight samples can be polished simultaneously, whereas current polishers are capable of polishing only one sample at a time. 
     Thus, a new and improved platen and associated apparatus for use in the cold cross-sectioning of soft materials is disclosed. The new apparatus includes a magnetically attached grinding surface for quick change without requiring the temperature of the apparatus to be changed. The apparatus uses a cooling liquid, such as liquid nitrogen, which provides a much lower grinding temperature than was previously achievable. Also, the liquid coolant system is less expensive than the chillers and heat exchangers used in prior art equipment. An environmentally friendly lubricant, such as propylene glycol, is used in the grinding process. Also, because the grinding surface rotates, a plurality of samples can be polished simultaneously and different (e.g. smaller) sample sizes can be used to provide faster polish times. Because of the very high cooling available in the new apparatus, interfacial and microstructural characterization is possible for very soft materials, such as solder, indium bearing material, compliant films, etc. and for multicomponent structures, such as soft materials sandwiched between harder materials. Also, the time required for polishing is dramatically reduced so that failure analysis and characterization of soft materials is speed up along with the development of new materials. 
     While we have shown and described specific embodiments of the present invention, further modifications and improvements will occur to those skilled in the art. We desire it to be understood, therefore, that this invention is not limited to the particular forms shown and we intend in the appended claims to cover all modifications that do not depart from the spirit and scope of this invention.