Plastic and nonmetallic article shaping or treating: processes – Outside of mold sintering or vitrifying of shaped inorganic... – Of electrical article or electrical component
Reexamination Certificate
1999-06-02
2001-04-17
Fiorilla, Christopher A. (Department: 1731)
Plastic and nonmetallic article shaping or treating: processes
Outside of mold sintering or vitrifying of shaped inorganic...
Of electrical article or electrical component
C156S089160, C156S089170, C156S089180
Reexamination Certificate
active
06217821
ABSTRACT:
FIELD OF THE INVENTION
The invention is directed to a method of forming electrical circuitry in and on cofired ceramic tape by applying a thick film metallization composition comprising a boron component to at least one green sheet layer of the ceramic tape and firing the circuit to densify the ceramic, whereby the resulting circuit is distortion-free.
BACKGROUND OF THE INVENTION
In the fabrication of low temperature cofired ceramic circuits (LTCC), ceramic tape serves as the substrate and is cofired to densification with printed circuitry of thick film metallization. If the degree of shrinkage of the metallization is not closely matched to that of the ceramic, the circuit will distort or bow and become unusable. The problem is most severe when (1) the ceramic undergoes crystallization caused by the metallization promoting crystallization of the glasses in the ceramic tape and (2) shrinkage of the metallization composition is different from that of the ceramic tape.
There is a need for a thick film metallization which can be used to fabricate any circuit design and when cofired will result in an undistorted useful LTCC. The present invention utilizes a boron addition to a thick film metallization composition wherein the boron addition minimizes shrinkage of the metallization composition. Therefore, LTCC becomes distortion-free. The metallization compositions are generally known in the art and are described in U.S. Pat. No. 5,782,945 to Gavin, and U.S. Pat. No. 4,101,710 to Sanford and are incorporated herein by reference. Neither patent discloses the use of the composition on a ceramic tape and the shrinkage problem that such a match solves.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is a method of forming electrical circuitry in and on cofired ceramic tape by applying a thick film metallization composition comprising conductive powders selected from the group Ag, Pd, Pt and mixtures thereof and 0.5 to 1.5% boron and 0-4% glass, based on total composition, to at least one green sheet layer of the ceramic tape forming a LTCC circuit and firing the circuit to densify the ceramic, whereby the resulting circuit is distortion-free.
Crystalline or amorphous or combinations thereof of elemental boron powder is added to conventional conductive thick film paste. Large ground planes of the paste may be printed on one side of green ceramic tape layers and after firing and densification, the circuit remains flat without distortion. Without the boron, the circuit will warp or bow usually with the paste layer shrinking less than the ceramic.
It is believed that the boron prevents the distortion by keeping the metal solids in the paste reduced, preventing formation of metal ions which interact with the glass in the ceramic tape to induce uneven shrinkage. The boron ultimately oxidizes to B
2
O
3
at the higher temperature range of the firing cycle by reaction with air. A further feature of the invention is the addition of 0-4% of an additive to combine with the B
2
O
3
to form a more stable structure within the conductor film. The preferred range of additive content is 0.5-2% based on the weight of paste. The additive prevents small beads of glass forming on the surface of the conductive layer. The additive could be an oxide which would react with the B
2
O
3
or a metal which would oxidize to an oxide and react with the B
2
O
3
. For example, the oxide additives may include SiO2, Al2O3 or Cu2O. Elements which can oxidize to form an oxide include Cu, Ni, Fe, Co or Ti. The preferred is Cu. Although, the preferred additive is a non-crystallizing glass, in which the B
2
O
3
would dissolve. The preferred glasses are borosilicates which do not contain modifying ions that may be reduced by boron, such reducible modifying ions would include Pb, Bi or Cd. The preferred borosilicates would contain modifying ions of alkali or alkaline earth elements such as Ca, Na or Li. A most preferred glass used in Glass #1 from the examples which is a borosilicate glass containing Li, Na and K modifying ions.
In addition to the non crystallizing glass additive, oxide powder additives may be added to the paste for the purpose of reacting with the B
2
O
3
formed. A preferred additive is SiO
2
which may be in the amorphous or crystalline form. The preferred range of SiO
2
addition is 1-2% based on the weight of paste. Oxides such as Al
2
O
3
and ZrO
2
have little effect and may increase bowing by increasing glass crystallization.
Generally, the thick film pastes for use in this invention are comprised of finely divided particles of metal particles which may be Ag, Pd, Pt, or mixtures thereof (referred to as “solids”) dispersed in typical thick film medium, for example a cellulosic polmer dissolved in a terpineol type solvent.
The solids are mixed with an essentially inert liquid medium by mechanical mixing using a planetary mixer, then dispersed on a three roll mill to form a paste-like composition having suitable consistency and rheology for screen printing. The latter is printed as a “thick film” on a conventional crystallizable ceramic green tape in the convention manner.
Any essentially inert liquid may be used as the solvent. Examples of organic solvents which can be used are the aliphatic alcohols, esters of such alcohols, for example, acetates and propionate, terpenes such as pine oil, terpineol and the like, solutions of resins such as the polymethacrylates of lower alcohols, solutions of ethyl cellulose in solvents such as pine oil and the monobutyl ether of ethylene glycol monoacetate and dibutyl carbitol. Aresin or combination thereof and a solvent or combinations thereof and various additives as those know in the art compose the organic medium. A preferred organic medium is based on a ethyl cellulose resin and a solvent mixture of alpha-, beta-, and gamma terpineols (generally 85-92% alpha-terpineol containing 8-15% beta- and gamma-terpineols). The organic medium may contain volatile liquids to promote fast setting after application to the substrate.
The ratio of organic medium to solids in the dispersions can vary considerably and depends upon the manner in which the dispersion is to be applied and the kind of solvent used. Normally to achieve good coverage, the dispersions will contain complementarily 50-95% solids and 50-5% organic medium, as described above. The compositions of the present invention may, of course, be modified by the addition of other materials which do not affect its beneficial characteristics. Such formulations are well within the state of the art.
The green ceramic tapes utilized in the invention are conventional tapes as those known in the industry. This would include tapes which contain (1) a crystallizable glass and filler or (2) totally crystallizable glasses with no filler. The preferred tape comprises a castable dielectric composition, based on solids: (a) 25-50 wt. % glass composition comprising based on mole %, 50-67% B
2
O
3
; 20-50% alkaline earth metal oxide; 2-15 rare earth oxide and further comprising, based on mole %, 0-6% alkali metal oxide and 0-10% Al
2
O
3
; (b) 50-75 wt. % refractory oxide; both dispersed in a solution of (c) an organic polymeric binder; and (d) a volatile organic solvent. The castable dielectric composition is used in a method of forming a low loss green tape by casting a thin layer of the dispersion onto a flexible substrate and heating the cast layer to remove the volatile organic solvent.
Within the compositional range of the above described tape, the most preferred solids composition for a green tape is: 45% of a glass composed of mole %: 28.57 CaO, 57.14 B
2
O
3
, 11.43 La
2
O
3
, 1.43 Li
2
O and 1.43 Na
2
O; with 54.8% Al
2
O
3
filler and 0.2% Cu
2
O added for coloring. To 100 g of solids, slip is prepared by addition of 24 g of an acrylic polymer solution of 30% polymer in ethylacetate solvent; 4.8 g of benzoflex plasticizer, 20 g of ethylacetate and 1 g of isopropanol additional solvent. On casting the tape, most of the solvent evaporates leaving the solids, polymer and plasticizer.
A further discussion of t
E. I. Du Pont de Nemours and Company
Fiorilla Christopher A.
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