Coating apparatus – Solid member or material acting on coating after application – Heated or cooled member
Patent
1995-05-18
1997-10-21
Czaja, Donald E.
Coating apparatus
Solid member or material acting on coating after application
Heated or cooled member
118106, 118301, 118316, H05K 334, B05C 504
Patent
active
056791550
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to printed circuit boards and more particularly to a method and apparatus for applying solder to the contact points or terminal pads on a circuit board which are to be electrically and mechanically connected to circuit component.
BACKGROUND ART
The electronics industry has become increasingly dependent on printed circuits for the interconnection of electronic components. Initially the electronic components were secured to the boards via leads which extended through holes in the boards. Today many components such as integrated circuits ("ICs") are mounted directly to the surface of the boards, i.e. by soldering the leads directly to terminal pads on the boards.
Technological improvements in integrated circuits have resulted in increased terminal lead densities with the attendant requirement of increased terminal pad densities on the printed circuit boards. The distance between the leads (i.e. pitch) of integrated circuits of the order of 0.008 inches (i.e. 8 mils) are not uncommon. Such a density requires that the spacing between the edges of the circuit board terminal pads be of the order of 4 to 5 mils.
To accommodate surface mounted components, such as ICs, on circuit boards, it is necessary to accurately apply solderable material such as a tin/lead alloy ("solder") or a solder containing paste to the circuit board terminal pads prior to the mounting operation. Once the solder or solder paste has been applied to the terminal pads, the surface mounted components can then be secured on the board by a suitable adhesive or the paste itself with the leads in contact with the terminal pads. The assembly is then heated by means of an oven, heat lamp etc. to a temperature sufficient to melt the solder to form an electrical and mechanical bond between the terminal pads and the component leads.
One method commonly used to apply solder to the terminal pads of printed circuit boards is called dip soldering. In accordance with this method, the circuit board is dipped into a bath of molten solder and removed so that the metalized areas of the board (e.g. terminal pads) receive a layer of solder. However, due to the solder's surface tension, the resulting layer is generally dome shaped, as is illustrated in FIG. 1 of the drawings. Streams of hot air may be used to remove some of the excess solder. However, the depth of the solder on the terminal pads will vary. When surface mounted components, such as IC's with high density lead terminals, are mounted on the board and the solder heated, some of the lead terminals may not be wetted by the molten solder because of the uneven height of the solder on the pads. While the open circuits resulting from the insufficient depth of solder may be correctable, the time and cost of finding and remedying the open circuits is significant.
Another method commonly used to apply solder to terminal pads involves the use of a solder paste which is deposited on the terminal pads by silk screening or mask printing techniques. The surface mounted components are then placed on the board so that their lead terminals are embedded in the paste on the terminal pads. The components are held in place by the surface tension of the paste. The assembly is then heated to melt the solder and evaporate the volatile components and solvents of the paste. The paste, however, is expensive and difficult to apply. In addition, the paste may not accurately follow the contours of the pads resulting in an unacceptable finished product.
More recently two additional methods of applying solder to the terminals pads of printed circuit boards have been disclosed in the prior art, German Patent No. DE 3,810,653 which issued to Dieter & Gretta Friedreich on May 18, 1988, teaches a method for depositing solder on the terminal pads on one side of a circuit board in which a sacrificial solder resist mask is used to border the terminal pads and define the height of the solder. In accordance with the Friedreich patent, the board with the sacrificial mask in place is dipped into
REFERENCES:
patent: 4127692 (1978-11-01), Boynton
patent: 4493857 (1985-01-01), Knigge
patent: 4720324 (1988-01-01), Hayward
patent: 4903631 (1990-02-01), Morris
patent: 5480483 (1996-01-01), Velie
Czaja Donald E.
Jackson Harold L.
Padgett Calvin
Velie Circuits, Inc.
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