GB2252554A

GB2252554A - Crystallisable glass powder

Info

Publication number: GB2252554A
Application number: GB9127018A
Authority: GB
Inventors: Friedrich Siebers
Original assignee: Carl Zeiss AG
Current assignee: Carl Zeiss AG
Priority date: 1991-02-08
Filing date: 1991-12-19
Publication date: 1992-08-12
Anticipated expiration: 2011-12-19
Also published as: GB9127018D0; DE4103778C1; JP2566183B2; FR2672588A1; GB2252554B; FR2672588B1; JPH069243A

Description

2?52554 1 CRYSTALLISABLE GLASS POWDER The invention relates to a glass

powder which can be tered glass ceramic containing crystallised to give a sin4L hexagonal cordierite as the principal crystal phase.

As a consequence of its good mechanical and electrical properties, cordierite is highly suitable as a substrate for electronic components, in particular for multilayer circuit boards. Since pure cordierite has a very high sintering temperature at which conductor tracks applied before sintering are destroyed, the starting point is a glass powder which can be sintered at relatively low temperatures and simultaneously substantially converted into a crystal phase.

It is known that glass powders whose composition corresponds to stoichiometrically pure cordierite only have a limited sintering capability. Pressing and firing at temperatures of up to 12000C give sintered glass ceramics which are porous and have low mechanical strength. The poor sinterability is due to premature surface crystallisation of the glass particles, which causes a dramatic increase (several orders of magnitude) in the viscosity, which prevents further sintering of the glass phase. A further disadvantage is that MgOcontaining high-quartz mixed crystal phases initially form. Representatives from this series of mixed crystals are p-cordierite having an MgO:A1203:S'02 oxide ratio of 2:2:5 and a magnesium aluminosilicate (MAS) having a ratio of 1:1:4. The existence of these metastable crystal phases additionally delays crystallisation to give hexagonal cordierite.

2 Glass powders having a modified composition and improved sinterability have therefore already been disclosed. Thus, US Patent 3,926,648 states that the sinterability is improved by addition of 0.5-2% by weight of K20 and/or CS20- However, the sintered elements produced from these glass powders have impaired electrical and dielectric properties and are therefore generally unsuitable for use for electrotechnical and electronic components, in particular if the requirements are high. German Patent 26 02 429 carries out this modification by adding 0.7-5.6 mol% of one or more of the modifying oxides BaO, PbO, SrO and CaO. These oxides are capable of forming mixed crystals in the cordierite structure. However, the low residual glass phase content limits the sinterability of the powder, so that the powders are principally used as coatings on ceramic, glass or glass ceramic articles.

German Patent 29 01 172 describes two different sintered glass ceramics; first a glass ceramic based on p-spodumene as the most important crystalline phase, and secondly a sintered glass ceramic based on cordierite as the most important crystalline phase. The structure of the latter-sintered glass ceramic additionally contains clinoenstatite and in some cases p-cordierite in addition to hexagonal cordierite. This complex structure makes it necessary to observe narrow tolerances for all the process parameters, such as chemical composition, powder properties and sintering programme, in order to achieve uniform structure formation and thus reproducible properties. These sintered glass ceramics therefore require complex-production and are difficult to match to the various processing conditions of various users.

3 US Patent 4,540,671 describes a glass powder which can be sintered to give a sintered glass ceramic and containing cordierite in the sintered state and a high-quartz mixed crystal phase in solid solution. However, the occurrence of the metastable high-quartz mixed crystal phase results in the disadvantages already mentioned at the outset.

An object of the invention is to Provide a glass powder which can be crystallised to give a sintered glass ceramic containing hexagonal cordierite as the principal crystal phase and which has such a composition that it sinters in an air-impermeable manner at a temperature of 970 0 C or less, has good electrical and mechanical properties and allows the user considerable latitude with respect to heating rate, sintering temperature and sintering time in his choice of sintering conditions.

According to the present invention, there is provided a glass powder which can be crystallised to give a sintered glass ceramic containing hexagonal cordierite as the principal crystal phase, characterised by a composition comprising, in mol% based on oxide, of 48 - 61 S'02 0 - 3 ZnO - 16 A1203 0 - 3 CaO 23 - 35 Mgo 0 - 1.5 BaO 0 - 4 B203 0.5 - 12 F (as a 0 - 2.5 P205 substitute for 0) 0.5 - 5 E B203 + P205 In order to obtain a dense sintered glass ceramic, ie without open pores, having a high crystal phase content, the course of sintering and crystallisation is important. In contrast to a compact glass ceramic, which experiences volume nucleation in 4 the interior as a consequence of added nucleating agents such as titanium dioxide or zirconium dioxide, the crystallisation in the preparation of a sintered glass ceramic from glass powder proceeds from the original surfaces of the particles of the glass powder. The dense sintering must be completed substantially still in the glassy state. If premature surface crystallisation occurs, the viscosity of the surface layer of the glass particles increases so much that further dense sintering is greatly hindered if not interrupted. If, by contrast, the ability of the glass to crystallise is suppressed, for example by adding relatively large amounts of modifying oxides, the desired homogeneous, finely crystalline structure having a high crystal phase content can no longer be produced. Coarse-crystalline sintered glass ceramics of low strength which are unsuitable for most applications are then produced.

If the S'02 content drops below 48 mol%, sintering with adequate crystallisation of the hexagonal cordierite phase is not possible below 9700C. If the S'02 content exceeds 61 mol%, metastable magnesium aluminosilicate crystals crystallise preferentially, with the disadvantages described at the outset. The particularly preferred S'02 content is between 51 and 57 M01%.

If the A1203 content exceeds 16 mol% A1203, the temperature necessary for production of a dense-sintered glass ceramic increases, whereas the crystal phase content of the desired hexagonal cordierite phase drops and MgO- and S'02-containing crystal phases (which result in an increase in the dielectric constant) are formed to an increased extent if the A1203 content drops below 10 mol%. The preferred A1203 content is 12.5-15.5 M01%.

The MgO content should be between 23 and 35 mol%. If the MgO content is above this range, the content of hexagonal cordierite is reduced, whereas an MgO content below this range results in an impairment of the sintering properties of the glass powder. The preferred MgO content is 26-31 mol%.

In order to reduce the sintering temperature, the glass powder contains 0 to 4 mol% of B203 and 0 to 2. 5 mol% of P20., provided that the total content of these two compounds is from 0.5 to 5 mol%. The upper limits of these ranges should not be exceeded since otherwise the residual glass phase becomes rife, which results in an impairment of the properties. However, the addition of B203 and/or P205 as a contribution toward reducing the sintering temperature should not be less than 0.5 mol% in total.

Addition of zinc oxide promotes crystallisation of the hexagonal cordierite at low temperatures. However, the zinc oxide content should not be more than 3 mol% since otherwise there is a danger that sintering to give a dense body becomes much more difficult as a consequence of premature surface crystallisation. The preferred zinc oxide content is from 0.3 to 2. 5 mol%.

The presence of CaO likewise promotes crystallisation at low temperatures, but contents above 3 m01% should be avoided since anorthite as an additional and undesired crystal phase occurs even at such low contents.

Addition of BaO improves the homogeneity and stability in the glass melt, but a BaO content of 1.5 mol% should not be exceeded since at higher contents, crystallisation of the glass powder is inhibited and shifted toward unacceptably high temperatures.

An essential element of the invention is a fluorine content of 0.5-12 mol%, preferably 0.5-11 mol%, the fluorine ion replacing 6 a corresponding amount of oxygen ions in the crystal lattice. Surprisingly, it has been found that addition of fluorine allows the crystallisation temperature of the glass powder to be shifted to lower temperatures of down to below 9000C without adversely affecting the sintering operation. This behaviour is in contrast to the crystallisation- promoting additives such as zinc oxide or barium oxide, in which a compromise must always be made between dense-sintering and low crystallisation temperature. The addition of fluorine promotes crystallisation of the hexagonal cordierite phases and suppresses the undesired precursor phases p-cordierite and magnesium aluminosilicate. The structure remains stable for sintering and crystallisation over a broad temperature range. The fluorine content should not exceed 12 moA since otherwise new fluorine-containing crystal phases may occur, adversely affecting the stability of the structure toward changes in sintering temperature. Furthermore, the homogeneity of the glass melt and the hydrolytic stability of the sintering glass ceramic are also impaired. Even addition of small amounts of fluorine result in a significant reduction in the crystallisation temperature while simultaneously reducing the sintering temperature or reducing the sintering times, but the fluorine content should not drop below 0.5 mol% since the effect of the addition of fluorine is not always satisfactory below this level.

A preferred glass composition is therefore, on moA based on oxide:- 51 57 12.5 - 15.5 26 - 31 0.5 - 2.5 0.3 - 2.0 1.0 - 3.0 0.3 - 2.5 0.5 - 11.0 Si02 A1203 Mgo P205 B203 E P205 + B203 ZnO, and F (as a substitute for 0).

7 As a further crystallisation-promoting additive, PbO, SrO or Sn02 can be included. The maximum amount of the individual oxide should not exceed 3 mol%. if more than one of these three oxides are used together, the total amount used should likewise be not greater than 3 mol%. Addition of larger amounts than 3 mol% results in premature crystallisation and thus prevention of good sintering. if Pbo is used, it should additionally be noted that the lead oxide may be reduced to metallic lead at elevated sintering temperatures in a reducing sintering atmosphere, which can result in an unacceptable reduction in the electrical volume resistance.

in the production of the glass from which the glass powder is produced, conventional fining agents, such as, for example, Sb203 p AS203 or cerium compounds, in conventional concentrations of up to about 1% by weight, can be added to the glass without adversely affecting its properties. However, they are not absolutely necessary.

The particle size of the sinterable glass powder plays an important part in optimisation Of sintering and subsequent surface crystallisation. In general, good results are achieved using glass powders having a mean particle size of between 1 and 12 pm. If the particle sizes are too small, the large surface/volume ratio causes the glass powder to tend toward premature surface crystallisation, which prevents good sintering. Excessively large particle sizes on the other hand make later crystallisation more difficult, since they result in a relatively coarse crystalline structure having impaired mechanical strength, and give sintered articles of increased surface roughness. Particularly favourable results are obtained if the mean particle size of the glass powder is between 1.5 and 7 pm.

8 The glass powders can be prepared in a manner known per se by melting conventional glass raw materials at temperatures of about 1450-16500C to give a glass of the composition described, then quenching the molten glass by pouring into cold water or onto cooled metal rollers, giving a glass cullet which is converted into powders having the described particle size by known fritting or grinding methods.

The glass powder obtained in this way can then converted into a shaped article in a manner known per se, for example using the forming methods which are customary in the ceramics industry, such as dry pressing, extrusion, injection moulding, calendering, etc. The conversion may be carried out with addition of commercially available organic assistants and/or suitable suspending agents. Thus, for example, extrusion proceeds from a plasticised composition having plastic properties. Dry pressing is carried out by adding pressing assistants and working up to give freeflowing pressing granules. For the production of flexible green films, from which electronic substrates are later produced, calendering from a ceramic slurry has proved successful. If suspending agents are added, a drying process is also necessary before sintering. Any organic assistants present are burnt out during the heating phase by means of suitable temperature control in the sintering programme before noticeable sintering of the glass takes place. Sintering and crystallisation of the glass powder take place at temperatures between about 850 and 9700C, depending on the composition of the glass powder. The sintering times necessary depend on the sintering temperature used and are between about 15 minutes and several hours. Lower sintering temperatures require an extension of the sintering time. For a given glass composition, a similar structure can, if desired, be obtained at a desired lower sintering temperature and an extended sintering time to that which would have been obtained at a high sintering 9 temperature and a short sintering time. A pore-free sintered glass ceramic is accordingly produced by producing a glass having the composition described, comminuting this glass to a particle size of 1-12 pm, shaping the glass particles into the desired shape, heating the glass particles continuously to a temperature of up to 9700C in a mould and holding this temperature until the particles have sintered together, devitrifying the glass, and subsequently cooling the sintered articles. Before the heating and sintering, the shaped articles or precursors thereof can be provided with patterns of electrical conductor tracks, for example by screen printing or the like thereon.

Although the composition of the glass powder according to the invention differs from the stoichiometric composition of cordierite, high crystal phase contents having a hexagonal cordierite structure are nevertheless produced by mixed crystal formation; crystal phases not comprising cordierite or the residual glass phase are kept relatively small. This formation of mixed crystals furthermore allows the incorporation of superstoichiometric amounts of MgO and S'02 at the expense of A1203 without impairing the crystallinity. However, the sinterability of the glass powder is improved and the amount of modifying oxides necessary is reduced.

The sintered glass ceramics produced from the glass powder according to the invention are impermeable to air and therefore allow the encapsulation of sensitive electronic components. The low dielectric constant means that metallised electronic substrates manufactured therefrom have little delay in the signal transmission time. The low sintering temperature of a maximum of 9700C allows metallisation with highly conductive metals, such as copper, silver, silver/palladium and gold, to be employed. The thermal expansion coefficient of the sintered glass ceramic can be matched to that of silicon, so that mechanical stresses between the substrate material and the silicon semiconductor wafer due to different thermal expansion coefficients are minimised. In addition to compact sintered glasi ceramic substrates, multilayer wiring substrates with internal conductor tracks can also be produced. Sintered glass ceramic parts produced from the glass powder according to the invention are distinguished by low thermal conductivity, high heat resistance and good thermal shock behaviour. The high electrical volume resistance and the high dielectric strength allow use for electrical insulation purposes. The glass powders are furthermore suitable for coating or for grouting between ceramics and are stable even at high temperatures.

The invention is illustrated in greater detail with reference to Examples. The results are collated in Tables 1-4. Tables 1 to 3 show the chemical compositions whilst Tables 2 to 4 show the physical data of the examples. of the physical data, the mean particle size (d.,) was measured by means of a laser granulometer. The sintering was carried out at the temperature indicated, a uniform heating rate of 30C/min being used. The following Xray reflections were used for determining the crystal phase contents: 102 reflection for hexagonal cordierite; 011 reflection for MAS; 120 reflection for forsterite; 610 reflection for magnesium aluminosilicate (JMS 35-310). In some cases, the sintered glass ceramics still contained small amounts of enstatite modifications. Due to the large number of possible modifications and the low contents, it was necessary to carry out a qualitative estimate here. The determination of hexagonal cordierite (JWDS File No 13-293), p-cordierite (JUDS File No 14-249) and forsterite (JWDS File No 34-189) was carried out against fully crystallised standards. The comparison standard for the MAS phase (JWDS 27-716) also contained other crystal phases, so that the amount data here are 11 affected by a somewhat greater error (in the order of about 20%). The dielectric constant c and the loss angle tan 6 were measured at 250C and a frequency of 1 MHz. At higher frequencies in the Gigahertz range, the dielectric constant and the loss angle drop significantly. Differential thermoanalysis was employed to estimate the crystallisation temperatures. Since this method requires a high heating rate (here SOC/min), the true crystallisation temperature in standard sintering programmes is systematically lower. However, it is possible to give an indication of the effect of chemical composition on the crystallisation temperature. The Table shows in each case the beginning of the crystallisation peak and crystallisation maximum. These values allow the crystallisation temperatures of various sintered glass ceramics to be estimated relative to one another.

Examples 1-4 (Tables 1 and 2) show the effect of the fluorine content on the sintering temperature and the crystallisation behaviour for otherwise identical compositions. Example 1 is used for comparison. The glass powders having the compositions indicated in Table 1 were prepared by melting an appropriate glass batch at 16000C. The melt was quenched between two watercooled metal rollers and ground to the particle size indicated in Table 2. Free-flowing press granules were obtained from the glass powder by granulating the powder in a manner known per se. in an intensive mixer with an aqueous solution of polyethylene glycol. Shaped articles for sintering experiments were produced by dry pressing under a pressure of 800 bar. The sintering time was I hour at the sintering temperature indicated, and the heating rate was 30C/min. The temperature was held at 3100C for 1 hour in order to remove polyethylene glycol from the sintered article.

Examples 2-4 show very clearly, in comparison to Example 1, the 12 extraordinarily positive effect exerted by the fluorine content. It can be seen from Table 2 that fluorine shifts the crystallisation temperature toward lower temperatures. The crystallisation of hexagonal cordierite is promoted, and the other crystal phases are suppressed. The structure is stabilised, and the crystal phase content changes only a little in a broad temperature range; thus, the content of hexagonal cordierite in Example 3 is virtually constant at sintering temperatures between 910 and 9700C. The stabilisation of the structure by means of the fluorine content also means that the thermal expansion coefficient for the compositions according to the invention changes only little at sintering temperatures between 910 and 9700C. In contrast, Example 1, carried out for comparison without fluorine, shows greatly varying crystal phase contents and a varying thermal expansion coefficient. All the sintered articles according to the invention are sintered at the sintering temperatures so as to be impermeable to air.

Examples 5-13, using further glass powder compositions according to the invention, are summarised in Tables 3 and 4. Examples 11-13 show the effect of the crystallisation-promoting additives SrO, PbO and Sn02.

Example 14

Flexible green films having a thickness of 200 pm were produced by the process below from the glass powder of Example 8. Organic assistants and an organic solvent mixture were added to the glass powder in a ball mill, and the mixture was homogenised giving a pourable slurry. The batch comprised 53% by weight of glass powder, 37% by weight of an azeotropic solvent mixture of trichloroethylene and ethanol, 5% by weight of polyvinylbutyral as binder, 4% by weight of dioctyl phthalate as plasticiser and 1% by weight of menhaden fish oil as thinner. The slurry was 1 13 degassed and poured onto a moving continuous belt. The casting shoe and the belt were separated by a narrow adjustable gap via which the thickness of the films is regulated. After passing through a drying zone, the flexible film can be removed from the casting belt.

Several layers of these green films were stacked one on top of the other and pressed in a laminating press at a pressure of 2 0.5-3 kN/ cm at a temperature of 900C to give a uniform composite. The laminates obtained in this way were sintered on a flat firing support at a temperature of 9300C for a sintering time of I hour. Due to the relatively high contents of organic assistants, a heating rate of 20C/min was generally used. In the region of the burn-out temperature of the organic assistants, ie in the temperature range from 2200C to 3300C, the heating rate was reduced to 1OC/min. The laminates were then treated at 3300C for two hours in order to complete the removal of the organic assistants. Heating was then continued at 2 0 C/min until the desired sintering temperature was reached. The structure and properties of the sintered glass ceramic substrate obtained in this way agree with the values also obtained on sintering compacts using the same temperature programme.

In all Examples, the specific electrical volume resistance at room temperature was greater than 1013 n _ CM, ie the material was a very good electrical insulator.

Table 1:

Example:

Glass powder compositions (mol%) 1 S'02 A1203 Mgo P205 B203 F (as substitute for 0) 0 1 54.0 15.1 28.4 2.0 0.5 2 3 4 -A I- 54.0 15.1 28.4 2.0 0.5 54.0 15.1 28.4 2.0 0.5 54.0 15.1 28.4 2.0 0.5 4.0 6.6 10.6 T 2.

phase contents and material pcopeffies of the sinlered 9Uum owarnks obtained Example Mean Sinter- Crystal phase contents (% by weight) Dielectric properties Thermal DTA powder Ing torn- g MAS hex cord M9IN silicate Forsterite Othera (1 MHz, 2CC) expansion Beginning Peak maximum particle peratuic tan 6 coefficient of peak dso ([t-) (sinter- (1o4) (20-3WC) (OC) (OC) ing time a (10'6/K) 1 h) T. (OC) 1 28 890 59 < 10 - 915 994 28 910 54 22 2 10 333 28 930 38 39 2 to - 28 950 18 53 2 10 2,8 970 to 57 2 10 5.2 16 2.25 2 2,4 890 45 36 a 905 9W 2.4 910 24 55 5 1.82 2.4 930 5 68 < 5 2.4 950 < 5 70 - - 2.4 970 < 5 76 5.1 16 1.86 3 24 890 18 61 - - 885 922 2.4 910 5 70 1.84 2.4 930 5 71 2.4 950 < 5 71 2.4 970 < 5 75 5.1 19 2.01 4 2.3 890 31 45 - 8W 897 2,3 910 17 53 2,89 2,3 930 11 58 23 950 5 70 2.3 970 < 5 71 5.1 28 2.74 ILn Table 3: Glen m compositions (mol%) Example

6 7 8 9 11 12 13 8'02 A1203 M90 P205 B203 ZnO C&O Bao Sro Sn02 PbO 53.5 13,8 29.0 1.0 1.2 1.5 52,7 13.6 28,6 1.2 0.9 1.5 1.5 52.7 M6 28.6 0.6 1.5 1.5 1.5 52.7 13.6 28.6 1,2 09 1.2 1.5 0.3 55.0 13.0 28.5 1.1 0.9 1.5 55.0 13.0 28.5 1,1 09 1.5 55.0 13.0 28.5 1.1 0.9 - 1.5 55.0 13.0 28,5 1.1 0.9 1.5 55.0 13.0 28.5 1.1 0.9 1.5 F (as substitute for 0) 4,8 3.0 3.0 2.9 0.7 0.9 0.9 0.7 0.8 1 1 ' 1 1 4:. 10 Cr phase conh mid material pcopea of ithe untwed glass em obu Example Mean S4nhis Crystal phase contents 1%, by weight) LIChichIC properties Thermal DTA powder ing tom. p MAS box cord Mg AI silicate Foistfitter Others 41 MHz. 25"C) expansion Beginning Peak maximu particle petalure tan a coefficient of peak size (strifer. (104) (20.3OWC) CIG) (C) Oing time cl, 1pm) ing time (10 6/K) 1 h) T, (%) 31 910. -C % 76 -Z 1 213 890 925 3; 930 78 1 1 31 970 at 2 54 17 225 6 35 910 2 63 930 973 930 83 7 970 70 3 54 38 30 7 38 910 58 3 920 975 38 930 so 3 38 970 64 54 26 280 a 31 910 60 6 920 944 31 930 68 5 31 970 72 2 53 28 256 9 33 930 59 2 5 b) 9M 990 33 970. . 77 2 b) 52 22 2.97 33 930 67 2 < 5 923 952 33 970 74 2 ' 5 53 15 2.19 11 32 930 15 44 3 5 C) 9W 1013 32 970 6 55 5 < 5 C) 55 14 3.55 12 31 930 67 2 < 5 9M 973 31 970 71 2 < 5 55 24 271 13 33 930 22 42 to 925 975 33 970 15 48 9 53 13 240 a) - very little onstatitis b) - vovy little anorthile C) - very little high sanidine m -j 1 18

Claims

I. A glass powder which can be crystallised to give a sintered glass ceramic containing hexagonal cordierite as the principal crystal phase, characterised by a composition, in mol% based on oxide, comprising:- 48 61 S'02 0 - 3 ZnO - 16 A1203 0 - 3 CaO 23 - 35 Mgo 0 - 1.5 BaO 0 - 4 B203 0.5 - 12 F (as a 0 -
2.5 P205 substitute for 0) E B203 + P205 2. A glass powder according to Claim 1, having a composition comprising:- 51 - 57 12.5 - 15.5 26 - 31 0.5 - 2.5 0.3 - 2.0 1.0 - 3.0 0.3 - 2.5 0.5 - 11.0 S'02 A1203 Mgo P205 B203 E P205 + B203 ZnO and F (as a substitute for 0).
3. A glass powder according to Claim 1 or 2, characterised by a total content up to 3 moA of one or more of the oxides PbO, SrO and Sn02.
4. A glass powder according to any preceding, characterised by a mean particle size of from 1 to 12 pm.
5. A sintered glass ceramic containing hexagonal cordierite as the principal phase, when produced from a glass powder as claimed in any preceding claim.

Z Z.