Compositions – Electrically conductive or emissive compositions – Elemental carbon containing
Reexamination Certificate
1999-01-29
2001-12-11
Gupta, Yogendra N. (Department: 1751)
Compositions
Electrically conductive or emissive compositions
Elemental carbon containing
C252S500000, C252S502000, C252S511000, C252S512000, C252S513000, C252S518100, C338S0220SD
Reexamination Certificate
active
06328914
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to thick-film processes and materials. More particularly, this invention relates to a thick-film paste containing an additive that enables thick film circuit elements to be formed from the paste with reduced thicknesses while maintaining printing performance, including line definition.
BACKGROUND OF THE INVENTION
Hybrid microcircuits are characterized by microcircuit elements, e.g., conductors, resistors and capacitors, that are deposited as thick films on a substrate, such as ceramic. Thick films are typically about 0.5 mil (about 12.5 &mgr;m) or more in thickness, and often formed by screen printing a paste or ink composition that is then dried and fired. Thick-film pastes generally contain an organic vehicle in which is dispersed in an inorganic particulate filler, which is electrically conductive, resistive or dielectric, depending on the circuit element desired. The particulate filler is generally in the form of powdered solids, though in some cases the particulate filler may be dissolved, and becomes a solid during firing. Firing the thick-film paste serves to burn off the organic vehicle and sinter the particulate filler.
To obtain flow properties appropriate for screen printing, thick-film pastes typically contain about 35 to 45 volume percent powdered solids, with the balance being organic vehicle. Depending on the density of the powdered solids, this volume fraction may correspond to about 70 to 90 weight percent solids. Factors that affect the flow properties of a thick-film paste include solids content, the viscosity of the organic vehicle, and the shape and average size of the solid particles. Solids content is particularly critical for achieving a beneficial pseudoplastic property to the rheology of the paste, by which the paste exhibits a relatively high viscosity at low shear rates, and lower viscosities as the shear rate increases. If the solids fraction of the paste is increased significantly above an acceptable range, the paste becomes too viscous and elastic to print well. On the other hand, if the solids fraction is decreased significantly below this range, the paste is not sufficiently tacky and cohesive to print well.
Because most thick-film pastes have a similar solids loading, the dried film thickness of different pastes printed through a given screen will typically fall within a similar range, e.g., about 0.7 to 0.9 mils (about 18 to 23 micrometers) when printed through a 290 mesh screen. The fired film thickness of a paste is generally equal to the dried film thickness times a densification factor, which typically ranges from about 0.5 to 0.8. For some applications, minimizing the fired film thickness of a printed paste is beneficial. However, achieving a significant reduction in fired film thickness without negatively affecting print performance (e.g., line definition) of the paste can be difficult. If the solids fraction of a paste is decreased in order to permit printing of thinner films, the paste will be excessively fluid unless the organic vehicle is either modified or changed to compensate for the lower solids content. However, simply increasing the viscosity of the vehicle to compensate for the reduction of powder can also cause problems. If the organic vehicle lacks a significant elastic component to its rheology, the paste solids will tend to settle out and agglomerate. Increasing the viscosity sufficiently to prevent settling will result in a paste that is too viscous to print. Thixotropic additives that impart a significant elastic component to organic vehicles result in unstable rheology; after resting, the paste will be extremely stiff, but while the paste is being worked the viscosity and elasticity continuously drop as the organic structure breaks down.
An example of an application that is complicated by the circumstances described above is illustrated in
FIG. 1
, which shows a circuit component
10
electrically connected to a conductor
12
with a solder joint
14
. Also shown is a solder stop
16
formed with a conventional dielectric paste. The function of the solder stop
16
is to render part of the surface of the conductor
12
unwettable by the molten solder that forms the solder joint
14
during reflow, and thus defines the solderable areas of the conductor pattern. The component
10
is also shown as being underfilled with an adhesive
18
to promote reliability. In such applications, it is important to minimize obstructions to the flow of the adhesive
18
during the underfill process. Obstruction of the underfill adhesive
18
can result in an incomplete fill under the component
10
, which is difficult to detect and may result in an unreliable part. Therefore, it is desirable that the fired film thickness for the solder stop
16
is as thin as possible while still being sufficient to prevent wetting of the conductor
12
by the molten solder. Theoretically, a solder stop could be less than one micrometer thick and still prevent wetting of the conductor
12
.
Another consideration of solder stop performance is that it should be printed with good definition, since failure to resolve the desired solderable surface on the conductor
12
would result in a non-functioning or unreliable component
10
. Pastes which have sufficient solids loading to achieve the necessary definition typically contain 70 to 80 percent solids by weight, but result in a fired film thickness of about 7.5 to 15 micrometers using standard thick-film processes and tools. This is thicker than necessary for rendering the conductor surface unwettable, and therefore unnecessarily impedes the underfill adhesive
18
without offering functional benefits. Attempts to reduce fired film thickness with pastes having lower solids content have resulted in inadequate line definition.
Similar problems are encountered if attempting to reduce the fired film thickness of thick film conductors, resistors and capacitors. Accordingly, essentially all thick-film processes could benefit from a thick-film paste that can be printed to achieve a reduced thickness while maintaining printing performance, including line definition.
SUMMARY OF THE INVENTION
The present invention provides a thick-film paste for printing thick-film circuit elements, including solder stops, conductors, resistors and capacitors, and a method for using the paste. According to the invention, the paste has a composition that includes an organic vehicle, a filler material that contributes the desired electrical or other material properties to the thick film fired from the paste, and a particulate additive that is insoluble in the organic vehicle and therefore contributes pseudoplastic rheological properties to the paste during printing, similar to that observed with pastes containing only conventional inorganic particulate materials. The additive also preferably evaporates, burns off, sublimates or is otherwise removed from the paste at a temperature lower than the firing temperature of the paste, more preferably not higher than the burn-off temperature of the organic vehicle, and most preferably not higher than the temperature at that which the paste is dried prior to firing. Accordingly, a method for forming a thick-film circuit element with the paste of this invention preferably includes printing the paste, drying the paste such that the additive is removed, and then firing the paste to burn off the organic vehicle and sinter or fuse the filler material.
As a result of the presence of the additive in the paste during printing, desirable line definition can be achieved without an excessive volume fraction of the filler material. Because the additive is eliminated from the paste prior to the paste being completely fired, the additive does not increase the fired film thickness. Accordingly, the present invention provides a very low solids paste that simultaneously achieves the benefits of reduced film thickness and print performance.
Other objects and advantages of this invention will be better appreciated from the following detailed descript
Berlin Carl William
Carter Bradley Howard
Isenberg John Karl
Lautzenhiser Frans Peter
Rama Sarma Dwadasi Hare
Delphi Technologies Inc.
Funke Jimmy L.
Gupta Yogendra N.
Hamlin Derrick G.
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