Coating processes – Nonuniform coating – Mask or stencil utilized
Reexamination Certificate
2001-02-22
2004-05-04
Talbot, Brian K. (Department: 1762)
Coating processes
Nonuniform coating
Mask or stencil utilized
C427S096400, C427S097100, C427S350000, C101S129000
Reexamination Certificate
active
06730358
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
Embodiments of the present invention relate to depositing viscous materials. More specifically, embodiments of the present invention provide methods for depositing paste, in particular solder paste, on a circuit structure.
2. Description of the Prior Art
Chip modules such as multichip modules are widely used in the electronics industry. A typical chip module comprises a semiconductor chip mounted on a circuit substrate. Solder is used to connect the semiconductor chip to conductive regions on the circuit substrate.
In one conventional method for attaching the chip to the circuit substrate, solder paste is selectively deposited onto conductive regions of the circuit substrate using a stencil. The stencil is placed adjacent to the circuit substrate so that portions of the conductive regions are exposed through the stencil apertures. A squeegee pushes solder paste across the upper surface of the stencil and into the stencil apertures. The solder paste passes through the stencil apertures, and deposits onto the conductive region portions.
Problems can sometimes arise during solder stenciling. For example, the solder paste can stick to the aperture walls and may not deposit on the conductive regions. Consequently, some conductive regions may not have enough solder deposited on them, and the uniformity of the solder deposits across the circuit substrate can vary. If severe sticking occurs, the circuit substrate and the stencil are cleaned and the circuit substrate is re-soldered. Re-soldering and cleaning steps increase the time, labor and materials needed to form chip modules, and consequently increase the cost of the modules.
In another conventional method which addresses the solder sticking problem, an air nozzle is aligned with a stencil aperture. Air from the nozzle blows solder through the stencil aperture and onto a conductive region under the aperture. Then, the nozzle moves to another aperture and the process is repeated. While a method, such as this one, may be effective in some instances, a typical circuit substrate may include hundreds of conductive regions. Using a single pressurized air nozzle to deposit solder onto hundreds of conductive regions, one or even a few at a time, is time consuming. Moreover, in a method such as this one, complicated control systems are needed to align the nozzle with each successive stencil aperture.
Therefore, what is needed and what has been invented is an improved method for depositing a conductive paste. What is further needed and what has been more specifically invented is a method which does not possess the deficiencies of the prior art and which is capable of simultaneously dispensing numerous respective solder-paste deposits onto numerous conductive regions.
SUMMARY OF THE INVENTION
Embodiments of the invention provide methods for depositing a conductive paste on a circuit structure. In one embodiment of the invention, a method is provided for depositing a conductive paste comprising aligning apertures in a stencil with conductive regions of a circuit structure; screening conductive paste (e.g., solder) into at least a majority of the stencil apertures in the stencil; and supplying pressurized fluid (e.g., nitrogen or air) to the majority of stencil apertures substantially simultaneously. The method further comprises placing, after screening conductive paste, a pressurized fluid element adjacent to the stencil, wherein the pressurized fluid element includes pressurized fluid outlets which are in communication with the majority of stencil apertures. The pressurized fluid outlets in the pressurized fluid element may form the same pattern as the apertures in the stencil. Furthermore, planar dimensions of the pressurized fluid element are substantially the same as the planar dimensions of the stencil.
Another embodiment of the invention provides a method for depositing a conductive paste comprising aligning apertures in a stencil with conductive regions of a circuit structure; screening conductive paste (e.g., solder paste) into the stencil apertures in the stencil; and inserting an array of pin bodies (e.g., copper or nickel pin bodies) into the stencil apertures to push any conductive paste within the stencil apertures through the apertures. Screening conductive paste preferably comprises screening the conductive paste into a majority of the stencil apertures. Preferably, the number of pin bodies in the array is equal to the number of stencil apertures. The pin body lengths may range from about 50 and about 100 percent of the length of the apertures. The method for depositing a conductive paste further comprises subsequently withdrawing the array of pin bodies from the stencil apertures. The array of pin bodies may be present on a base and is preferably formed by plating. Each of the pin bodies has a length greater than about one-half the thickness of the stencil, and each of the axial crosssectional areas of the pin bodies is from about 80 to about 90 percent of the axial cross sectional area of the apertures.
Another embodiment of the invention provides a pushing element comprising a planar base; and an array of pins, wherein the array of pins are capable of being inserted into the apertures of a stencil to push conductive paste through the apertures. The outer surfaces of the base are preferably entirely metallic. Preferably, each of the pins has a length less than about 1 micron.
These provisions together with the various ancillary provisions and features which will become apparent to those skilled in the art as the following description proceeds, are attained by the methods of the present invention, preferred embodiments thereof being shown with reference to the accompanying drawings, by way of example only, wherein:
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Lee Michael G.
Yamuni Charbel
Fujitsu Limited
Sheppard Mullin Richter & Hampton LLP
Talbot Brian K.
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