CA1167250A

CA1167250A - Crossover dielectric inks

Info

Publication number: CA1167250A
Application number: CA000388155A
Authority: CA
Inventors: Ashok N. Prabhu; Kenneth W. Hang
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1980-10-17
Filing date: 1981-10-16
Publication date: 1984-05-15
Also published as: FR2492396B1; FR2492396A1; DE3140971C2; DE3140971A1

Abstract

RCA 75,085 IMPROVED CROSSOVER DIELECTRIC INKS
ABSTRACT OF THE DISCLOSURE
Novel thick-film crossover dielectric inks useful in constructing multilayer circuits on suitable substrates are provided. The subject inks comprise a barium magnesium borosilicate glass powder, a pinhole reducing component comprising silicon dioxide, an alkaline earth aluminum borosilicate glass having a high softening point or mixtures thereof and a suitable organic vehicle.

Description

-1- RCA 75,085 CROSSOVER DIELECTRIC INKS
This invention pertains to thick film crossover dielectric inks and their use in constructing multilayer electrical circuit structures on porcelain~coated metal substrates.
BACKGROUND OF THE INVENTION
_ The use of specialized ink formulations to form thick films having various functions on suitable substrates in the construction of multilayer integrated circuit structures is well known in the art. Such technology is of increasing interest in the fabrication of very dense multilayer circuit patterns on various substrates for a wide variety of applications in the electronics industry.
Significantly improved substrates for the fabrication of such circuits are disclosed and claimed in Hang et al., U.S. Patent No. 4,256,796, issued March 17, 1981. The Hang e-t al. substrates are me-tal coated with an improved porcelain composition comprised of a mixture, based on its oxide content, of magnesium oxide (MgO) or mixtures of magnesium oxide and certain other oxides, barium oxide (BaO), boron trioxide (B2O3) and silicon dioxide (sio2)~ The preferred metal is steel, particularly low carbon steel, which may be coated with various other metals such as, for example, copper. The porcelain compositions are applied to the metal core and fired to provide a partially devitrified porcelain coating on the metal core. The coating has a ~ery low viscosity at its initial fusion point and then almost 30 instantaneously obtains a high viscosity due to devitrification. The fired coatinys which are preferred , for hybrid circuit applications have a coefficient of thermal expansion of at least about 110 x 10 7/~C.
While the porcelain coated metal substrates of ~;35 Hang et al. represent a si~nificant improvement over ~previously known substrate ma~erials, they are ",,, . , :
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l -2- RCA 75,085 disadvantageous only in being incompa-tible or poorly compatible with commercially available thick film inks.
In accordance with this invention, improved crossover dielectric inks are provided which are compa-tible with -the Hang et al. porcelain metal substrates.
SUMMARY OF THE INV~NTION
The improved crossover dielectric inks provided in accordance with this invention are based on the system BaO-MgO-B2O3-SiO2. In addition to the barium magnesium borosilicate devitrifying glass powder, the novel inks of this invention contain a pinhole-reducing component comprising silicon dioxide powder, an alkaline earth aluminum borosilicate glass having a high softening point or mixtures thereof, and a suitable organic vehicle.
DETAILED DESCRIPTION OF THE INVENTION
In accordance with this invention, there are provided improved crossover dielectric inks useful in the production of complex, multilayer thick film integrated circuits on porcelain-coated metal substrate boards.
Crossover dielectric films provide insulation between intersecting layers of conductors in a multilayered circuit array. Conventional thick-film dielectric inks generally consist of a mix-ture of dielectric oxide powder and a low softening poin-t glass ~rit wikh a suitable screening medium~
It is generally recognized that films formed from crossover dielectric inks should possess: a thermal coe~ficient o~ expansion reasonably close to -that of the substrate being utilized; good mechanical strength and reheat stabilityi little porosity in general and no open porosi-ty to prevent shorts between the top and bottom conductors; chemical compatibility with the conductor pads; low dielectric constant; high insulation resistance;
and good voltage breakdown characteristics. The crossover dielectric inks of this invention satis~y all of these criteria and, in addition, possess excellent compatibility with the aforementioned Hang et al. porcelain-coated metal c~ircuit boards. The crossover dielectric inks of this ~0 '~,;
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-3- RCA 75,085 invention are likewise compatible with various -types of thick-film inks formulated for the Hang et al.
porcelain-coated metal substrates, thus providing a significant advance in multilayer integrated circuit systems.
The crossover dielectric inks of this inven-tion are uni~ue in that they may be fired in either air or inert environments, e.g. nitrogen. Since most conventional inks are fireable in only one of these environments, the capacity of the inks of this invention to be fired in either atmosphere is partieularly advantageous in -the produetion of highly eomplex multilayer integrated eireuits.
The subjeet dieleetrie inks are based on the system BaO-MgO~B2O3-SiO2 and are speeially formulated so that, during initial firing, a liquid/liquid phase separation oecurs. One of the phases begins to preeipitate from the glass as mierocrystals. These micro-crystals build slowly iIltO polycrystalline bodies which are exceptionally stable during reheating. The resulting films are therefore partially devitrified with the polycrystalline mass eomprising from about 60 to ~bout 75 pereent by volume of the film. The partial devitrifieation of the inks of this invention during firing, in a sense, forms an in situ ceramic. The inks of this invention are formulated to achieve this effect and to minimize pinholes in the thick-films prepared therefrom.
The glass frit eomponent of the inks of this invention consists of, on a weight basis:
a) from about 17 to about 49 pereent of barium ~ oxide;
; ~ b) from about 16 to about 43 percent of magnesium oxide;
e~ from about 13 to about 33 pereent of boron trioxide; and d) from about 8 to about 22 percent of silieon dioxide.
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1 ~4~ RC~ 75,085 The barium oxide may be replaced in par-t by including up to 12 percent by weight, based on the total frit, of lanthanum oxide (La2O3). The partial replacement of BaO wi-th La2O3 modifies the crystal s-tructure somewhat for partlcular applications. The glass composition may additionally con-tain small amounts of a mixture of phosphorous pentoxide (P~O5) and zirconium oxide (ZrO2) which acts to further retard the rate of crystallization.
Such mixture, when present, comprises up to abou-t 6 percent by weight, preferably from about 3 to about 4 percent by weight, of the glass frit. The mixture contains P2O5 and ZrO2 in a weight ratio of between abou-t 1:2 and 1:7, preferably between about 1:3 and 1:4. The glass frit comprises from about 30 to about 70 percent by weight of the total ink composition.
The pinhole reducing component of the novel crossover dielectric inks of this invention consists of silicon dioxide, an alkaline earth aluminum borosilicate

2~) glass having a high softening point or mixtures thereof, with mixtures being preferred. The presence of this component in the subject inks helps slow the rate of crystallization and, more importantly, provides a physical aid to entrapped gas bubbles which result from 26 decomposition or volatilization of the organic vehicle during firing so that they may reach the surface of the ink and escape into the environment. By aiding the gas to ~escape more rapidl~y and exerting a retarding effect on crystallization, this component virtually eliminates through pinholes and significantly reduces pinhole density in films in e~cess of 1 mil thick, formed from the inks of this invention.
The terminology "high softening point" with regard to the glass utilized as a pinhole reducing component in the context of this invention means an alkaline earth aluminum borosilicate glass having a softening point in excess of 700C. A preferred glass is based on the system BaO-CaO-B2O3-Al2O3-SiO2 with a particularly preferred glass having, on a weight basis, 45 ~: ~

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~-5- RCA 75,085 to 55 percent of BaO, 6 to 15 percent of CaO, 10 to 20 percent of B~03, 6 to 13 percent of Al203 and 5 to 15 percent of Si~2. As stated above, it is preferred -to S utilize mixtures of the alkaline earth aluminum borosilicate glass and silicon dioxide as the pinhole reducing component. While -these two may be mixed in any proportions, approximately e~ual parts by weight are preferred. The pinhole reducing component comprises from about lO to about 30 percent, preferably from 15 to about 25 percent, by weight of the total ink composition.
The ink formulations of this invention may also contain a minor amount of a conventional colorant oxide such as the oxides of chromium, cobalt, nickel and the like. The inks of this invention con-tain up to about 5, preferably from about 0.1 to about 1.0, weight percent of a colorant oxide.
The solid components of the subject ink formulations have an average particle size of be-tween about 1 and 5 ~m. They are thoroughly mixed, preferably in a ball mill or other suitable grinding apparatus with an aliphatic alcohol, preferably isopropanol, for from 8-48 hours. The alcohol is then evaporated and the solids combined with from about ~0 to about 50 percent, preferably from about 30 to about 40 percen~, by weight, of a suitable organic vehicle to make the ink. The organic vehicles are selected to give screen printing characteristics to the inks and to burn off cleanly during firing in nitrogen or air, i.e. without leaving a carbonaceous ~esidue.
The organic vehicles are binders such as, for example, cellulose derivatives, particularly ethyl cellulose, synthetic resins such as polyacrylates or methacrylates, polyesters, polyolefins and the like. In general, conventiona`l vehicles utilized in inks of the type described herein may be used in the subject inks.
Preferred commercially available vehicles include, for example, pure liquid polybutenes available as Amoco H-25, Amoco H-50 and Amoco L-100 from Amoco Chemicals .
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I ~6- RCA 75,0~5 Corporation, poly n-butylmethacLylate available from E. I.
duPon-t de Nemours and Co., and -the like.
The above resins may be utilized individually or in any combination of two or more. A suitable viscosity modifier can be added to the resin material if desired.
These modifiers can be solvents such as those conventionally used in similar ink compositions, e.g. pine oil, terpineol, butyl carbitol acetate, an ester alcohol available from Texas Eastman Company under the -trademark Texanol and the like or solid materials such as, for e~ample, a castor oil derivative available from NL
Industries ~mder the trademark Thixatrol. Regardless of the vehicle utilized, it is important -that dispersion and homogeneity of the solids in the vehicle be maximized.
For -this reason, mixing is usually carried out in conventional apparatus which disperses the mixture and subjects it -to a shearing force at the same time.
Generally, the higher the shearing forces, the better the resulting dispersion.
The crossover dielectric inks of this invention are applied to the substrate board, preferably the porcelain-coated metal boards of Hang et al., by conventional means, i.e. screen printing, brushing, spraying and the like with screen printing being preferred. Generally, as is conventional in the art, two individually dried and fired layers of dielectric are ; utilized to minimize the potential for through pinholes.
The coating of ink is then dried in air at 100-125C for about 15 minutes. The resulting film may be fired in air or nitrogen at temperatures from 850 to 950C for from 4 to 10 minutes. The dielectric films thus obtained have good mechanical strength and good reheat stability. When utilized in multilayer circuits, they have demonstrated excellent isolation between layers of copper conductors.
In addition to the porcelain-coated metal boards of Hang et al, the crossover dielectric inks of this invention are compatible with and can be utilized with conventional circuit boards, e.g. aIumina circuit boards.

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1 -7- RCA 75,085 The following Example further illustrates this invention, it being understood that the invention is in no way intended to be limited to the details described therein. In the Example all parts and percentages are on a weight basis and all -temperatures are in degrees Celsius, unless otherwise stated.
EXAMPLE
Crossover dielectric inks were prepared from the following formulations:

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I ~9- RCA 75,085 *The vehicle for Ink No. 1 was a mixture of 28.57 and 14.29 percent~ respect~ively, of the liquid polybu-tenes Amoco H-25 and~L-100 available from Amoco Chemical Co. In Ink No. 2, a mixture of 10.00 percent Amoco H-50, 16.67 percent Amoco L-100rand 6.66 percent of the ester alcohol Texanol available from Texas Eastman Company was the vehicle. In the remaining ink formulations, the vehicle was a 13 percent solution of Elvacite 2044 ln pine oil.
Elvacite 2044 is a poly(n-butylmethacrylate) resin available from E. I. duPont de Nemours and Co.
**Chromium oxide In the above inks, the devitrifying glass powders had the following formulations:
lS Formula (Percent) Ingredient I II
MgO 27.8341.05 BaO 37.1618.22 B2O3 20.4523.63 SiO2 14.5613.25 Zr2 ~~~~~2.89 P2O5 0.96 ~ The high softening point glass had the following composition by weight percent: BaO (49.28 ), CaO (12.01), B2O3 (14.923, Al2O3 (10.92), and SiO2 (12.87).
The solid ingredients were mixed in a ball mill with sufficient~isopropyl alcohol to thoroughly wet them or a period of 12 hours. The alcohol was evaporated and the solids were combined with the organic vehicle. All ingredients were first hand mixed, then mixed on a 3 roll mill~to obtain a smooth paste suitable for screen ~; printing~. Additional vehicle was added to replace loss during mixing and assure proper rheology.
Copper conductor inks were printed and fired onto~a porcelain steel substrate of the type disclosed by Hang et al. The copper inks~were air dried at 125 for about 15 minutes, then fired in nitrogen at about 850.
Samples of each of the a~ove inks~were printed over the 40~

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1 -~10- RCA 75,085 copper electrodes through a 200 mesh s-tainless steel screen, 1 mil emulsion thickness. The crossover dielectric inks were dried in air a-t 125 for 15 minu-tes and fired in nitrogen in an Inconel Muffle, 5 zone, BTU
Transheat belt furnace with a belt width of 6 inches at a peak temperature of 900. Time at peak temperature was about 4-8 minutes. Total cycle time was about 30-45 minutes. A second dielectric layer was printed and fired over the first. Finally, the top copper elec-trode was printed, dried and fired. The fired parts were tested for electrical shorts between the top and bottom copper electrodes, dielectric constant, dieleetrie loss, inswlation resistance and breakdown voltage of the dielectric films.
Ink No. 1 was unaceeptable because of pinholes which produced shorts between the copper electrodes. Ink No. 4 had some blistering in the films. Inks 2 and 3 had significantly decreased pinholes in comparison to Ink No.
1. Inks 5 and 6 demonstrated exeel1ent results in all tests.

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Claims

-11- RCA 75,085 CLAIMS:

1. A crossover dielectric ink suitable for forming an intermediary film between two or more conductor films on a substrate comprising:
a) from about 30 to about 70 percent by weight of a partially devitrifying barium magnesium borosilicate glass powder;
b) from about 10 to about 30 percent by weight of a pinhole reducing component selected from the group consisting of silicon dioxide, an alkaline earth aluminum borosilicate glass having a softening point above about 700°C, and mixtures thereof; and c) from about 20 to about 50 percent by weight of a suitable organic vehicle.

2. A crossover dielectric ink in accordance with Claim 1, wherein said partially devitrifying glass consists essentially of:
a) from about 17 to about 49 percent by weight of barium oxide;
b) from about 16 to about 43 percent by weight of magnesium oxide, c) from about 13 to about 33 percent by weight of boron trioxide;
d) from about 8 to about 22 percent by weight of silicon dioxide;
e) from 0 to about 12 percent by weight of lanthanum trioxide; and f) from 0 to about 6 percent by weight of a mixture of phosphorus pertoxide and zirconium oxide in a weight ratio of from about 1:2 to about 1:7.

3. A crossover dielectric ink in accordance with Claim 2, wherein said glass contains from about 3 to about 4 percent by weight of said mixture of phosphorus pentoxide and zirconium oxide.

-12- RCA 75,085

4. A crossover dielectric ink in accordance with Claim 1, wherein said alkaline earth aluminum borosilicate glass is a barium calcium aluminum borosilicate glass.

5. A crossover dielectric ink in accordance with Claim 4, wherein said barium calcium aluminum borosilicate glass consists essentially of:
a) from about 45 to about 55 percent by weight of barium oxide;
b) from about 6 to about 15 percent by weight of calcium oxide;
c) from about 10 to about 20 percent by weight of boron trioxide;
d) from about 6 to about 13 percent by weight of aluminum oxide; and e) from about 5 to about 15 percent by weight of silicon dioxide.

6. A crossover dielectric ink in accordance with any of Claims 1, 4 and 5, wherein said pinhole reducing component is a mixture of silicon dioxide and said alkaline earth aluminum borosilicate glass.

7. A crossover dielectric ink in accordance with Claim 1, wherein said ink contains up to about 5 percent by weight of a colorant oxide selected from the group consisting of chromium oxide, cobalt oxide and nickel oxide.

-13- RCA 75,085

8. A process of forming a multilayer electrical circuit on a circuit board comprising:
a) applying and firing on said board a conductor ink thereby forming a first conductor film;
b) applying and firing on the portion of said conductor film which will intersect a second conductor film in said circuit, the crossover dielectric ink of Claim 1 thereby forming a crossover dielectric film on said portion of said first conductor film; and c) applying and firing a conductor ink on said board thereby forming a second conductor film having said crossover dielectric film interposed between said first and second conductor films.

9. A process in accordance with Claim 8 wherein said circuit board is a porcelain-coated metal circuit board.

10. A circuit board having on a portion of the surface thereof a conductor film, said conductor film having on a predetermined portion a coating of a crossover dielectric ink comprising:
a) from about 30 to about 70 percent by weight of a partially devitrifying barium magnesium borosilicate glass powder;
b) from about 10 to about 30 percent by weight of a pinhole reducing component selected from the group consisting of silicon dioxide, an alkaline earth aluminum borosilicate glass having a softening point above about 700°C, and mixtures thereof; and c) from about 20 to about 50 percent by weight of a suitable organic vehicle.

11. A circuit board in accordance with Claim 10, wherein said board is porcelain-coated metal.

12. A circuit board in accordance with Claim 11, wherein said metal is steel.

-14- RCA 75,085

13. An electronic assembly comprising a circuit board having a multilayer circuit thereon, said circuit containing two or more layers of conductor film, each pair of layers of conductor film having interposed between them at their intersecting portion a layer of crossover dielectric film formed by applying and firing a crossover dielectric ink comprising:
a) from about 30 to about 70 percent by weight of a partially devitrifying barium magnesium borosilicate glass powder;
b) from about 10 to about 30 percent by weight of a pinhole reducing component selected from the group consisting of silicon dioxide, an alkaline earth aluminum borosilicate glass having a softening point above about 700°C, and mixtures thereof; and c) from about 20 to about 50 percent by weight of a suitable organic vehicle.

14. An assembly in accordance with Claim 13, wherein said circuit board is a porcelain-coated metal circuit board.

15. An assembly in accordance with Claim 14, wherein said metal is steel.