Abstract:
Disclosed is a method for reducing screening defects on ceramic greensheets which includes placing additional vias in the kerf that will be eventually discarded during the sizing operation. Also disclosed is a ceramic substrate laminate article with reduced screening defects.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the reducing of defects occurring during the screening of ceramic greensheets and, more particularly, relates to the addition of nonfunctional vias which alleviate certain defects occurring during the screening of ceramic greensheets. 
     Ceramic multilayer substrates are used in the production of electronic substrates and devices. Many different types of structures can be used, and a few of these structures are described below. For example, a multilayered ceramic circuit substrate may comprise patterned metal layers which act as electrical conductors sandwiched between ceramic layers which act as insulators. The substrates may be designed with termination pads for attaching semiconductor chips, connector leads, capacitors, resistors, I/O connector pads, etc. Interconnection between buried conductor levels can be achieved through vias formed by metal paste-filled holes in the individual ceramic layers formed prior to lamination which, upon sintering, will become a sintered dense metal interconnection of metal-based conductor. 
     In general, conventional multilayered ceramic substrates are formed from ceramic greensheets which are prepared by mixing a ceramic particulate, a thermoplastic polymer binder, plasticizers, and solvents. This composition is spread or cast into ceramic sheets or slips from which the solvents are subsequently volatilized to provide coherent and self-supporting flexible greensheets. After blanking and punching, the greensheets are screened with a metallic paste to fill the vias and form wiring patterns on the greensheets. The greensheets are then stacked, laminated and fired at temperatures sufficient to drive off the polymeric binder resin and sinter the ceramic particulates together into a densified substrate. 
     When screening the ceramic greensheets with the metallic paste, defects called “fly-backs” can occur. After completing the screening of the metallic paste, the screening mask is removed. The removal of the screening mask from the screened greensheet can pull the ceramic greensheet away from the backing material due to insufficient paste adhesion between the ceramic greensheet and the backing material which can cause extra metallic paste to be deposited onto the back surface of the ceramic greensheet, resulting in fly-backs. Also, paste smears and oversize vias can occur as a result of the separation of the screening mask from the ceramic greensheet. Fly-backs, paste smears and oversize vias cause the ceramic greensheet to be rejected. 
     To overcome these and other defects occurring during screening, it would be desirable to have a process for reducing such defects. 
     Accordingly, it is a purpose of the present invention to have a method of reducing defects occurring during the screening of ceramic greensheets. 
     It is another purpose of the present invention to have an article produced by a method in which there is reduced screening defects. 
     Sanchez et al. U.S. Pat. No. 5,628,850, the disclosure of which is incorporated by reference herein, discloses a method for producing input/output connections in a ceramic device wherein at least one via is punched at the outer edge of the active area of the ceramic device and then filled with conductive paste. A score line is produced through the center of the via. After sintering, the ceramic device is cleaved along the score line, including through the scored via, to form the ceramic device having an input/output connection. 
     Shinichi Japanese Published Unexamined Patent Application 07-066076, the disclosure of which is incorporated by reference herein, discloses a ceramic substrate laminate having a plurality of through holes of rhombic shape punched therein to prevent cracking of the individual ceramic substrate laminates when they are diced from the ceramic substrate laminate. 
     Yasufumi Japanese Published Unexamined Patent Application 04-169082, the disclosure of which is incorporated by reference herein, discloses a ceramic substrate made by a greensheet having a plurality of through holes filled with a conductive paste and then cut apart through the filled through holes. 
     Taiyo Yuden KK Japanese Patent 01-179389, the disclosure of which is incorporated by reference herein, discloses a ceramic substrate formed by dicing ceramic laminates on a line passing through the through holes. 
     BRIEF SUMMARY OF THE INVENTION 
     One aspect of the invention relates to a method to reduce defects during screening of paste through a mask onto ceramic greensheets, the method comprising the steps of: 
     partitioning a greensheet into a plurality of active areas which will form part of a finished ceramic substrate and kerf which will be discarded in a subsequent manufacturing step; 
     punching a plurality of vias in each of the active areas of the greensheet and punching a plurality of vias in at least part of the kerf of the greensheet; and 
     screening paste into both sets of vias through a mask, the paste-filled vias in the kerf causing increased adhesion between the greensheet and backing sheet. 
     A second aspect of the invention relates to an article with reduced screening defects comprising: 
     a greensheet partitioned into a plurality of active areas which will form part of a finished ceramic substrate and kerf which will be discarded in a subsequent manufacturing step, and each of the active areas of the greensheet and at least part of the kerf having a plurality of paste-filled vias. 
     A third aspect of the invention relates to an article with reduced screening defects comprising: 
     a greensheet laminate comprising a plurality of greensheets, each of which is partitioned into a plurality of active areas which will form part of a finished ceramic substrate and kerf which will be discarded in a subsequent manufacturing step, and each of the active areas of the greensheet and at least part of the kerf having a plurality of paste-filled vias. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the invention believed to be novel and the elements characteristic of the invention are set forth with particularity in the appended claims. The Figures are for illustration purposes only and are not drawn to scale. The invention itself, however, both as to organization and method of operation, may best be understood by reference to the detailed description which follows taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a plan view of a prior art ceramic greensheet. 
     FIG. 2 is a plan view of a prior art ceramic substrate laminate. 
     FIG. 3 is a plan view of a ceramic greensheet according to the present invention. 
     FIG. 4 is a plan view of a ceramic substrate laminate according to the present invention. 
     FIG. 5 is a plan view of a second embodiment of the ceramic greensheet according to the present invention. 
     FIG. 6 is a plan view of a second embodiment of the ceramic substrate laminate according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings in more detail, and particularly referring to FIG. 1, there is shown a prior art ceramic greensheet  10 . Ceramic greensheet  10  is partitioned into a plurality of active areas  12 . When the ceramic greensheet  10  has been stacked with other like ceramic greensheets, sized and sintered, each of the active areas  12  will form the basis for a functioning ceramic substrate suitable for receiving an electronic device such as an integrated circuit device. As shown in FIG. 1, ceramic greensheet  10  will be used to manufacture four ceramic substrates. For manufacturing efficiency, modern day ceramic substrates are made in such multiples, called “ups”. 
     Each of the active areas  12  contains vias and/or wiring circuitry. As shown in FIG. 1, active areas  12  only contain paste-filled vias  15 . 
     A plurality of similar ceramic greensheets would be formed and stacked in conventional manner to form the ceramic substrate laminate  20  shown in FIG.  2 . Ceramic greensheet  10  shown in FIG. 1 is the top layer of ceramic substrate laminate  20 . During a subsequent “sizing” operation, ceramic substrate laminate  20  will be cut or sawn along lines  14  and  17  into four smaller ceramic substrate laminates  22 ,  24 ,  26 ,  28 . When sintered, ceramic substrate laminates  22 ,  24 ,  26 ,  28  will form fully functioning ceramic substrates. The area  16  between the lines  14  and the area  19  between lines  17  and the edge  21  of the ceramic substrate laminate  20  is called the kerf and is discarded as a result of the sizing operation. 
     There is typically no screening of paste for vias, conductive lines or any other structures in the kerf since the kerf is discarded during sizing. The present inventors have discovered, however, that due to the lack of screened paste between active areas  12 , defects such as fly-backs, paste smears and oversized vias can occur on the individual greensheets as a result of screening. The present inventors have further found that the number of defects increases with the number of ups. 
     Accordingly, the present inventors have proposed adding paste-filled vias in the kerf area to decrease the defects normally occurring during screening. 
     Referring now to FIG. 3, there is shown a ceramic greensheet  30  containing active areas  32 . According to the present invention, however, between the active areas  32  are a plurality of paste-filled, non-functional vias  38  wholly contained in the kerf  36 . As in the prior art ceramic greensheet, the active areas  32  contain vias and/or conductive wiring lines. In the particular embodiment shown in FIG. 3, the active areas  32  contain only vias  35 . 
     Ceramic greensheet  30  is then stacked with other similar ceramic greensheets in conventional manner to form ceramic substrate laminate  40 . Each of the ceramic greensheets in ceramic substrate laminate  40  would contain the paste-filled vias  38  similar to ceramic greensheet  30 . The ceramic substrate laminate  40  is then sized by cutting or sawing along lines  34  and  37  to form smaller ceramic substrate laminates  42 ,  44 ,  46 ,  48 . The kerf  36  (i.e., the area between lines  34 ) and vias  38  would be discarded. Kerf  39  (i.e., the area between lines  37  and the edge  41  of the ceramic substrate laminate  40 ) would also be discarded. Kerf  39  does not contain any vias. 
     The inventors have found that placement of vias  38  between the active areas  32  has substantially reduced or eliminated the previously-described screening defects. 
     The vias  38  are most preferably wholly contained within the kerf  36  so that when the kerf is removed after cutting or sawing, the vias  38  are entirely removed, thereby removing any electrical or appearance effect the vias  38  might otherwise have on each of the smaller ceramic substrate laminates  42 ,  44 ,  46 ,  48 . 
     Referring now to FIGS. 5 and 6, there is shown a second embodiment of the ceramic greensheet and ceramic substrate laminate shown in FIGS. 4 and 5, respectively. In extreme cases, it may be necessary to place paste-filled nonfunctional vias along the edge of the ceramic greensheet, such as vias  52  on ceramic greensheet laminate  30 ′ shown in FIG. 5 (and in the kerf  39  shown in FIG.  6 ), in addition to paste-filled vias  38  in the kerf  36 . Preferably, vias  52  should be wholly contained within kerf  39 . If vias  52  are utilized, their position on each ceramic greensheet should be staggered with respect to the ceramic greensheet above and below it to avoid excess thickness at the edges of the sintered substrate. 
     Then, when the ceramic substrate laminate  40 ′ is cut or sawn along lines  34  and  37  shown in FIG. 6, vias  52  will be discarded along with kerf  39 . 
     The non-functional vias  38 ,  52  in the embodiments shown in FIGS. 3 to  6  should be evenly or uniformly spaced on the ceramic greensheet  30 ,  30 ′. The actual number of nonfunctional vias  38 ,  52  would depend on the size of the ceramic greensheet, the size of the via and the active pattern area. As an example, for a 185 mm ceramic greensheet, the present inventors have found that there should be a minimum of one hundred (100) 6 mil vias in the kerf  36 . The actual number and size of non-functional vias  38 ,  52  for any given ceramic greensheet application can be easily optimized through routine experimentation by a person skilled in the art given the teachings of the present invention. 
     In addition, while the nonfunctional vias can be located only in the kerf  36  or in both the kerf  36  and kerf  39 , the nonfunctional vias should not be located only in kerf  39  as this could lead to screening defects. 
     The advantages of the invention will be more apparent after reference to the following example: 
     EXAMPLE 
     Four hundred (400) 185 mm. ceramic greensheets were screened on the same screening apparatus with the same masks except that two hundred (200) of the ceramic greensheets had one hundred (100) 6 mil vias in the kerf (similar to that shown in FIG. 3) and the other two hundred (200) ceramic greensheets did not. In the ceramic greensheets that had the vias in the kerf, the vias were uniformly spaced with approximately fifty (50) in each of the X and Y directions. 
     After screening each of the ceramic greensheets with a metallic paste, the ceramic greensheets were inspected for defects. Of the two hundred (200) ceramic greensheets without the kerf vias, one hundred twenty (120) failed because of fly-back defects. Of the two hundred (200) ceramic greensheets with the kerf vias, only two (2) failed and these failures were for reasons other than fly-back defects. 
     According to the present invention, 99% of the ceramic greensheets were screened without fly-back defects while only 40% of the prior art ceramic greensheets could be screened without fly-back defects, clearly demonstrating the efficiency of the present invention. 
     It will be apparent to those skilled in the art having regard to this disclosure that other modifications of this invention beyond those embodiments specifically described here may be made without departing from the spirit of the invention. Accordingly, such modifications are considered within the scope of the invention as limited solely by the appended claims.