Electricity: electrical systems and devices – Electrostatic capacitors – With protection or compensating means
Reexamination Certificate
1996-12-23
2001-08-21
Gaffin, Jeffrey (Department: 2841)
Electricity: electrical systems and devices
Electrostatic capacitors
With protection or compensating means
C361S274100, C361S301400, C361S311000, C361S301200
Reexamination Certificate
active
06278601
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to a means by which components, and in particular plastic film capacitors, can be protected from damage when subjected to radiant infrared energy which is used for the purpose of reflowing solder on a printed circuit board.
BACKGROUND OF THE INVENTION
The primary means of attaching components to a printed circuit board has been to have holes formed within the circuit board through which the leads of the components could be placed and solder applied on the bottom side of the board where the leads egressed. The circuit board would then be passed through a solder wave in which the leads would be soldered to the circuit board.
Another method of attaching components to printed circuit boards is using surface mounting techniques wherein either the component is leadless or its leads are formed so that the component can be freestanding. Surface mounting techniques better automate the attachment of components to printed circuit boards since they improve reliability of attaching components to the circuit board and reduce the amount of area that needs to be used on a printed circuit board. When surface mounting, the solder is typically applied as a room temperature solder paste into which the component's terminals are placed and the board is then heated to a temperature at which the solder paste will reflow as liquid solder, thereby attaching the components to the printed circuit board.
The four typical methods by which reflow heat can be applied to a printed circuit board on a production basis are:
1. Conduction—the bottom surface of the board is heated with the heat passing through the board by conduction and reflowing the solder paste.
2. Vapor phase—a neutral liquid material is vaporized, the printed circuit board is placed in the vapor, the latent heat of the vapor is transferred to the board causing the solder paste to reflow and the vapor in transferring its heat returns to being a liquid for subsequent re-heating.
3. Convection—a heated gas, typically just air, but sometimes nitrogen, is gently directed at the printed circuit board and the solder paste is reflowed.
4. Infrared—radiant infrared energy is directed at the printed circuit board and the solder paste is reflowed.
Of the four different methods of surface mounting components, the infrared reflow method offers the highest volume production capability with respect to the cost of the equipment involved.
The primary disadvantage in using infrared reflow is that when components, such as plastic film capacitors, are subjected to radiant infrared energy at the levels being used to reflow solder on printed circuit boards, the components will absorb excessive amounts of energy to such a degree as to physically damage the components thereby causing the circuit board to work improperly. This has required the components to be mounted in a post assembly operation with the soldering being accomplished by conduction or convection heating.
Another disadvantage of using infrared reflow processing with plastic film capacitors is that the infrared radiant energy will typically heat the plastic film capacitors much faster than the joints that are to be soldered. The major reason for this is the higher absorption rate of the plastic film capacitor as compared with the materials forming the joint such as copper or tin.
DESCRIPTION OF THE PRIOR ART
A variety of methods and devices have been developed to protect electronic components from excessive heat associated with radiant infrared energy during infrared reflow soldering. One method suggests using reflective metallized polymeric caps for placement over thermally sensitive components, thereby providing thermal insulation to the components and reflecting part of the infrared radiation away from the components.
Another method suggests using a shroud that is made of molded high temperature plastic with a metal outer layer. The shroud is designed to fit over the component and can be snapped on and snapped off the circuit board Still another method shown in U.S. Pat. No. 4,838,475 discloses placing a metal box like structure over and around the component to be shielded. The metal box has a plurality of apertures formed therein to enable some infrared energy to pass through and reflow solder the device to the circuit board. Covering devices that are placed over the entire component are not cost effective since additional steps would be required in attaching and removing the cover from the circuit board before and after reflow soldering. Also, additional space would be required on the circuit board to allow the covers to be attached, thereby requiring a special design for the circuit board and also increasing the size of the circuit board.
These and other types of devices disclosed in the prior art do not offer the flexibility, cost effectiveness and inventive features of my infrared shielding device. As will be described in greater detail hereinafter, the infrared shielding device of the present invention differs from those previously proposed.
It therefore would be desirable to provide a simple and inexpensive means of protecting components from radiant infrared energy by utilizing a shield which is an integral part of the component rather than an external part of the component, thereby eliminating the need to take special precautions during infrared reflow soldering.
SUMMARY OF THE INVENTION
According to my present invention I have provided a plastic film capacitor shield, the shield being secured to an outer surface of a plastic film capacitor, the shield being sufficient to protect the plastic film capacitor from excessive radiant infrared energy during infrared reflow soldering which could damage the plastic film capacitor.
Another feature of my invention relates to the plastic film capacitor shield described above, wherein the plastic film capacitor shield is a material selected from the group consisting of electrical grade tape, electrical grade plastic, metal tape, and epoxy.
Yet another feature of my invention relates to the plastic film capacitor shield described above, wherein the plastic film capacitor is of a box type configuration having six sides, and the shield is secured to an upper surface of the plastic film capacitor. Various different types of shields can also be used and attached to the capacitor, this would include: having a shield on an upper and lower surface thereof; wrapping and securing a shield to the plastic film capacitor on four sides thereof; securing a shield on all six sides of the capacitor; and securing a combination of different shields to the outer surface of the capacitor.
Still another feature of my invention concerns a method of reflow soldering a plastic film capacitor to a substrate with infrared energy, which comprises the steps of: securing a capacitor shield to an outer surface of the plastic film capacitor; positioning the plastic film capacitor with the capacitor shield on the substrate; and exposing the plastic film capacitor with the capacitor shield to infrared energy, such that the infrared energy is substantially reflected away from the plastic film capacitor by the capacitor shield and the infrared energy causes the plastic film capacitor to be reflow soldered to the substrate.
REFERENCES:
patent: 3592992 (1971-07-01), Costello
patent: 3612963 (1971-10-01), Piper
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patent: 4603373 (1986-07-01), Lavene
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patent: 5196667 (1993-03-01), Gammelin
patent: 5268051 (1993-12-01), Kent et al.
patent: 5371650 (1994-12-01), Lavene
Thermal Component Shrouds from Research Disclosure Aug. 1991 No. 328.
IBM Technical Disclosure Bulletin vol. 20
Gaffin Jeffrey
Illinois Tool Works Inc.
Schwartz & Weinrieb
Vu Phuong T.
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