Abstract:
A screening nest, method of screening green sheets and cleaning the mask and a mask cleaning station. The screening nest includes an electromagnet that clamps the mask to a green sheet on the nest during screening. The mask may be electromagnetically dampened during application and removal. The cleaning station electromagnetically dampens the mask during cleaning and especially during rinsing and drying.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention is related to integrated circuit (IC) chip packaging and more particularly to a screening station for screening paste onto a ceramic green sheet and to a station for cleaning the screening mask between screenings.  
         [0003]     2. Background Description  
         [0004]     Performance and signal density demands in semiconductor chip packaging are forcing Single Chip Modules (SCMs) and, especially, Multi-Chip Modules (MCMs) to become more and more complex. Ceramic module signal density is being increased both by increasing the number of ceramic wiring layers (with one wiring or power layer on each ceramic layer) and by printing narrower and narrower lines on finer and finer pitch. Wiring is printed on an uncured ceramic substrate layer (green sheet), e.g., using a metal mask and screening a paste (molybdenum paste, copper paste, copper/glass paste) pattern onto green sheets to define wiring lines and spaces. Such patterned green sheets are stacked, laminated and sintered to produce a Multi-Layered Ceramic (MLC) product that may include one MLC substrate or, several individual MLC substrates (also known as “ups”) that are separated (e.g., sawn) into individual final products. Misprinted lines or spaces on a single green sheet may ruin the entire product.  
         [0005]     Typically, a green sheet is placed on a nest, with the green sheet then located under the mask. The nest is elevated to raise the green sheet to the mask, tensioning the mask and, hopefully, eliminating any gap between the green sheet and the mask during screening. Unfortunately, a poorly (over or under) tensioned mask distorts the screened pattern. If the mask is not flush (under-tensioned) against the green sheet, the paste may “bleed out” around lines into the gap between the mask and the green sheet. If the mask is over-tensioned, it may stretch out of shape. Also, even if the mask is firmly mated to the center of the green sheet, the mask at its edges may bow upward and lift away from the green sheet.  
         [0006]     Further, since with each screening some paste residue remains behind on the mask, similar to paint remaining on a stencil after stenciling. So, to maintain image quality, each mask must be periodically cleaned, e.g., after each screening. For example, the mask may be sprayed with a Tetra-Methyl Ammonium Hydroxide (TMAH) solution, high pressure rinsed with water and then, dried with pressurized air. Each cleaning causes mask wear, such that the mask may “fatigue” from mask vibration and suffer prematurely broken tabs. While broken tabs may be immediately noticeable, mask fatigue may be more subtle, distorting subsequently printed shapes and that distortion may go unnoticed until well after sintering. Distorted/defective wiring layers degrade module yield and defective/distorted masks must be replaced. Both reduced product yield and frequently replacing masks increases module manufacturing costs and effort.  
         [0007]     Thus, there is a need for extending the life on module wiring masks and improving the image quality of wiring printed on ceramic green sheets.  
       SUMMARY OF THE INVENTION  
       [0008]     It is a purpose of the invention to improve ceramic module wiring quality;  
         [0009]     It is another purpose of the invention to reduce green sheet screening defects;  
         [0010]     It is yet another purpose of the invention to extend screening mask life.  
         [0011]     The present invention relates to a screening nest, method of screening green sheets and cleaning the mask and a mask cleaning station. The screening nest includes an electromagnet that clamps the mask to a green sheet on the nest during screening. The mask may be electromagnetically dampened during application and removal. The cleaning station electromagnetically dampens the mask during cleaning and especially during rinsing and drying. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The foregoing and other objects, aspects and advantages will be better understood from the following detailed description of a preferred embodiment of the invention with reference to the drawings, in which:  
         [0013]      FIG. 1A  shows an example of a preferred embodiment mask and nest arrangement according to the present invention;  
         [0014]      FIG. 1B  shows a more detailed assembly view of a preferred embodiment nest for a green sheet with a single up;  
         [0015]      FIG. 1C  shows an assembly view of a multiple up location preferred embodiment nest;  
         [0016]     FIGS.  2 A-C show an example of tensioning a mask against a green sheet located on a nest according to a preferred embodiment the present invention;  
         [0017]     FIGS.  3 A-C show a comparison of a prior art nest with a mechanical dampener disposed on a mask with two alternate preferred embodiment mechanical dampened nests according to the present invention;  
         [0018]      FIG. 4  shows an example of a mask cleaning station according to a preferred embodiment of the present invention. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0019]      FIG. 1A  shows an example of a preferred embodiment mask  100  and nest  102  arrangement according to the present invention. Primarily, the mask  100  is of a magnetic or ferromagnetic material (e.g., nickel) and the nest  102  includes a switchable magnetic field, e.g., one or more electromagnet embedded in the nest  102 . As is typical of metal masks used in printing green sheets, the preferred mask  100  is an electroformed metal mask of a patterned copper core electroplated with nickel and held in any suitable frame  106 , preferably of non-magnetic material. A green sheet  108  is located on the upper surface of the nest  102 . The copper core does not exhibit magnetic properties while the ferromagnetic plating material (nickel) can be temporarily magnetized in a magnetic or an electromagnetic field. Thus, by activating the electromagnet the mask  100  is magnetically clamped to the green sheet  108  during screening for a high quality image; and thereafter, quickly and cleanly magnetically separated when screening is completed to maintain image quality.  
         [0020]      FIG. 1B  shows a more detailed assembly view of a preferred embodiment nest  110  for a green sheet with a single up. In this example, a nest top  112  has been removed from a nest bottom assembly  114 , exposing the electromagnet  116 . In this example, the electromagnet  116  is embedded in epoxy and patterned to match the single up configuration. Also in this example, the nest bottom assembly  114  includes an electromagnetic dampener  118 , e.g. located in the center and buried in epoxy. Thus, electromagnet  116  clamps the mask to the green sheet during screening and the electromagnetic dampener  118  dampens mask vibrations when the nest  110  is raised and, optionally, when the nest is lowered after screening is complete.  
         [0021]      FIG. 1C  shows an assembly view of a multiple-up location preferred embodiment nest  120 . In this example, the nest top  122  includes four up locations  124  or stations for screening multiple patterns simultaneously. So, for example, a 44 mm final product size may be made as a 4 up 185 mm green sheet. In yet another example, (not shown) the same final product size may be made as a 9 up configuration in a larger 215 mm green sheet. Since after lamination and sintering, the ups are cut into individual ups of the final size (44 mm), the metal mask has a solid kerf between the locations  124 , i.e., the printed pattern is blank between the locations  124 . So, in this example the nest bottom assembly  126  includes an electromagnet  128  configured for four locations, also embedded in epoxy. Thus, for a multiple up nest, the electromagnet  128  is patterned to match the particular ups configuration to better seal the mask during the screening process, forcing a gasket around each up, i.e., in the kerf locations. Also in this example, a single electromagnetic dampener  118  is located in the center and buried in epoxy.  
         [0022]     FIGS.  2 A-C show an example of steps in tensioning a mask (e.g.,  100  of  FIG. 1A ) against a green sheet  108  located on a nest  102  according to a preferred embodiment the present invention. First in  FIG. 2A , the mask  100  is positioned such that the nest  102  holding the green sheet  108  is located beneath the mask  100 . Then, in  FIG. 2B , the nest  102  is elevated to bring the mask  100  in contact with the green sheet  108  on the nest  102 . Thus, although the mask may be in contact with the green sheet  108 , the electromagnet has not been switched on or activated and so, the mask is not taught as indicated by the non-linear, rippling in the mask  100 . However, since the preferred mask  100  is a ferromagnetic material it acts as a magnet when it is in a magnetic field, i.e., by activating the electromagnet, e.g.,  116  in  FIG. 1B . So as shown in  FIG. 2C , once the electromagnet is switched on or activated, the mask  100  becomes taught and firmly holds in place flush against the green sheet  108  at the surface of the nest  102 . Thus, under the magnetic field the mask  100  conforms to and mates with the green sheet surface to prevent paste bleed out. Also, since the mask  100  is not being stretched to tension it, mask deformation is reduced to minimize pattern shifts otherwise occurring during screening. The electromagnetic field may be modulated to assist mask application/removal by selectively attracting and repelling the mask  100 . Thus, the magnetic or electromagnetic field (e.g., from electromagnet  116  in  FIG. 1B ) and ferromagnetic plating cooperate to securely hold the mask  100  in place and reduce mask deformation and/or vibrations. Once screening is complete, the dampener (e.g.,  118 ), if included, further reduces mask vibrations post screening and also reduces mask-vibration induced fatigue.  
         [0023]     FIGS.  3 A-C show a prior art dampened nest  130  with a mechanical dampener  132  disposed on a mask  134  as compared with two alternate preferred embodiment mechanical dampened nests  102 ′,  102 ″ according to the present invention with reference to the examples of FIGS.  1 A and  2 A-C. So, in the prior art example of  FIG. 3A , a weight stack  136  on the mechanical dampener  132  applies a downward force to the mask  134  to dampen vibrations from raising the prior art nest  130  to tension the mask  134  and from lowering the nest  130  to release that tension. However, if the dampening weight  136  is too heavy, it stretches the mask  130 ; if it is too light, it provides very little dampening. By contrast however, the centered-buried electromagnetic dampener  118  of  FIG. 3B  only applies force when it is active, i.e., clamping the mask  100  when the nest  102 ′ is being lowered. Similarly, the top-centered electromagnetic dampener  118 ′ of  FIG. 3C  adds little weight to the mask  100 ′, but with power being supplied  137  to activate it during separation, the magnetic force actively pushes the mask  100 ′ up and away from the green sheet  108  during separation and assists in clamping when the nest is up. Further, by slidably locating the top-centered electromagnetic dampener  118 ′ on the center post  138 , it may be raised and lowered on the post  138  as desired for an optimized dampening position.  
         [0024]      FIG. 4  shows an example of a mask cleaning station  140  according to a preferred embodiment of the present invention. The mask  100  passes vertically though the cleaning station  140 , from up to down in this example. High-pressure cleaner dispensers  142  direct TMAH at either side of the mask  100 . An electromagnet  144  is fixed at one side just below rinse dispensers  146  and is activated through electrical connection  147 . Preferably the rinse dispensers direct clean de-ionized water on the mask  100 . Finally, a blower  148  above the rinse dispensers  146  blows hot air on the cleaned mask  100  to dry it. Thus, activating the electromagnet  146  during the cleaning, in particular during the high-pressure air dry, dramatically reduces mask vibrations that would otherwise occur. Since these vibrations can cause mask fatigue and break flexing mask features (tabs), thereby, making the mask  100  unusable; decreasing or eliminating mask vibrations (by magnetically immobilizing the mask during cleaning) reduces mask fatigue and extends mask life.  
         [0025]     Advantageously, since a preferred embodiment mask is magnetically clamped to the green sheet during screening, the mask pattern is more faithfully screened onto the green sheet. Further, since magnetically clamping the mask to the green sheet virtually eliminates any gap between the mask and the green sheet, the paste cannot bleed out to avoid associated bleed out defects, frequently found in final prior art product. Further, mask life is extended because mask vibrations are minimized before and after screening, which minimizes associated mask fatigue and damages it, i.e., breaking tabs. Thus, associated defects to the screened pattern are minimized as well. Additionally, mask life is further extended because mask vibrations are minimized during cleaning where mask fatigue and damage (i.e., from broken tabs), has plagued prior art masks and mask cleaning procedures.  
         [0026]     While the invention has been described in terms of preferred embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the appended claims. It is intended that all such variations and modifications fall within the scope of the appended claims. Examples and drawings are, accordingly, to be regarded as illustrative rather than restrictive.