1. Field of the Invention
This invention relates generally to coating viscous liquid materials onto substrates and, more particularly, to a method and apparatus for screening electrically conductive paste onto ceramic greensheet layers using a paste dispensing system comprising a paste reservoir and a screening nozzle and optionally, a supply cartridge containing the paste which is easily inserted and removed from the system.
2. Description of Related Art
In the manufacture of integrated circuits or printed circuit boards it is often necessary to apply a liquid coating such as a paste to the surface of the substrate. A number of viscous liquid materials are applied to the substrate such as photoresists and electrically conductive pastes and it is important that the liquid material be evenly dispensed to the substrate and that the thickness of the layer be uniform across the surface of the substrate.
One technique for forming layers on a substrate is using a dispensing system comprising a nozzle having an opening through which the paste or other liquid material is dispensed onto the substrate. In general, the nozzle is moved relative to the substrate to form a desired circuit pattern on the substrate. In one method, nozzles produce a direct pattern such as a single line by drawing the nozzle over the surface of the substrate. These processes are typically referred to as nozzle drawing processes. When the nozzle is used in a screening operation, electrically conductive paste or other liquid material is screened onto a ceramic greensheet layer through a mask by drawing the nozzle over the mask surface to provide a desired design on the substrate.
The paste or other coating liquid in the fabrication of a multi-layer ceramic package is generally deaerated. The package is typically made up of as many as 5 to 70 subcomponents (layers) with each of the layers representing a discrete level of wiring in the package. Each layer is basically made up of two materials: a ceramic substrate and a liquid coating material, e.g., molybdenum paste. The ceramic forms the base or substrate and the molybdenum paste is used to personalize or "wire" the layer.
The wiring on each layer comprises interconnection holes known as vias and a distinct wiring pattern on the layer surface. The vias are punched holes in the ceramic sheet that are filled with an electrically conductive material such as molybdenum paste. The vias provide the electrical interconnection in the z direction between the layers and the pattern on the surface transmits signals in the x/y direction of the layer and to the interconnected layers.
The cross-sectional dimensions of the vias and the lines are in the 4 to 6 mil range and it is of critical importance that the feature integrity of the package be maintained. An open/break in a line or via can result in an electrical defect and render the package unusable and it is thus extremely important that the paste or other liquid material be homogeneous, air-free (deaerated) and evenly applied to the substrate and in the vias.
For convenience the following description will be directed to the use of an electrically conductive paste dispensing system in a screening operation using a mask whereby a screening nozzle of the system is contacted with the mask surface and paste forced through the mask to fill vias and signal lines on a ceramic greensheet. It will be understood by those skilled in the art, however, that the paste dispensing system may be used for other liquid coating methods.
Each layer is personalized by means of a paste dispensing system and mask or stencil. The dispensing system comprises a reservoir to hold the paste and a nozzle for the delivery of the paste. The paste is pressurized and flows from the nozzle into and through openings in the mask. The pressure of the paste forces it into the openings in the mask and any corresponding holes in the layer (vias). The assembly is indexed along the mask in order to personalize the entire layer. The holes in the mask define the final pattern on the layer and also ensure all vias are filled.
In the screening operation for ceramic greensheet layers, it is conventional practice to use a paste delivery system comprising a pressurized reservoir holding a specific amount of paste which is sufficient to screen a number of ceramic green sheet layers. The paste is pre-loaded into the reservoir and may be of various formulations and consistencies from a paste to a highly viscous fluid. The paste delivery system is typically stored until it is needed by one of the manufacturing line screeners and the storage of these assemblies can vary from days to weeks. During the storage, the paste homogeneity and its consistency is compromised due to the nature of the paste. The inconsistency of the paste requires frequent visual inspections by the screening operator and leads to interruptions in the screening process.
Another concern with the current state of the art screening operations is that when the reservoir is empty, the assembly must be removed from the tool and replaced with an assembly with a full reservoir. The switching of assemblies thus results in tool down time and operational inefficiencies.
Bearing in mind the problems and deficiencies of the prior art, it is an object of the present invention to provide a method for forming paste and other liquid material patterns (coatings) on substrates such as greensheets and other electronic components using a paste dispensing system whereby the liquid material, e.g., paste, is homogeneous and the system provides uniform coverage of the paste on the substrate.
It is another object of the present invention to provide a paste dispensing system or apparatus to fabricate an electronic component such as an integrated circuit greensheet or printed circuit board using a paste dispensing system in which the paste or other liquid material is maintained in a homogeneous state and the liquid material is evenly and uniformly applied to the substrate.
It is a further object of the invention to provide electronic components such as patterned greensheets made using the paste dispensing system of the invention.
It is an additional object of the present invention to provide a liquid material containing cartridge for use with a liquid material dispensing system which cartridge supplies a fixed amount of liquid material to the system and is then easily replaced with a new cartridge without the downtime associated with conventional dispensing systems.
Other objects and advantages of the present invention will be readily apparent from the following description.