US3313084A

US3313084A - Method for encapsulation

Info

Publication number: US3313084A
Application number: US350333A
Authority: US
Inventors: Benjamin G Forman
Original assignee: Columbia Technical Corp
Current assignee: Columbia Technical Corp
Priority date: 1964-03-09
Filing date: 1964-03-09
Publication date: 1967-04-11
Anticipated expiration: 1984-04-11

Description

April 11, 1967 Y e. FORMAN 3,313,034

METHOD FOR ENCAPSULATION Filed March 9, 1964 w wm W Patented Apr. 11, 1967 3,313,084 METHOD FOR ENCAPSULATION Benjamin G. Forman, Massapequa, N.Y., assignor to Columbia Technical Corporation, Woodside, N.Y. Filed Mar. 9, 1964, Ser. No. 350,333 1 Claim. (Cl. 53-22) This invention relates generally to the field of protective coverings, and more particularly to an improved means and method for encapsulating electronic components and assemblies against the deleterious effects of moisture, changes in temperature, changes in atmospheric pressure and other environmental stresses, and the like, and to provide additional mechanical strength as protection against the effects of shock and vibration.

While it is known in the art to provide such protection by dipping the structure to be protected into molten thermoplastic material, it is preferable to avoid subjecting the protected components from the temperature involved in using such a process. In addition, the coating obtained is usually non-uniform in thickness, and sharp corners normally become too thinly coated, while pockets formed between adjacent electronic components become too heavily coated, with a consequent loss in flexibility. It is also known in the art to spray a liquid coating in an evaporable vehicle, and allowing the vehicle to evaporate to leave a protective coat upon the article being sprayed. However, such a process is time-consuming, and normally requires more than a few coats to build up a substantial protective film. It is known in the art that polyesters and epoxies are used to encapsulate printed circuit assemblies. However, repairability is then impossible.

It is therefore among the principal objects of the present invention to provide an improved means and method for the encapsulation of electronic components and similar articles in which the above-mentioned disadvantages have been substantially eliminated.

Another object of the invention lies in the provision of an improved means and method for encapsulation which may require a minimum of time and material in the use thereof.

A further object of the invention lies in the provision of an improved means and method for encapsulation which will produce a protective covering of relatively constant thickness while yet conforming to irregularities in surface or shape of the article enclosed.

A further object of the invention lies in the provision of an improved means and method for encapsulation in which the over-all cost thereof may be of a reasonably low order, with consequent wide sale, distribution and use. In addition, the cost of repair of the item to be encapsulated would be substantially lower.

A feature of the invention lies in the fact that it may be employed in the encapsulation of both planar and relatively non-planar articles.

Another feature of the invention lies in the fact that the electrical connection of an encapsulated object with corresponding means disposed outside of the encapsulating envelope is readily available after the encapsulation has been performed.

These objects and features, as well as other incidental ends and advantages, will more fully appear in the progress of the following disclosure, and be pointed out in the appended claim.

In the drawing, to which reference will be made in the specification, similar reference characters have been employed to designate corresponding parts throughout the several views.

FIGURE 1 is a plan view of a conventional printed circuit board element having attached electronic components mounted thereon prior to encapsulation.

FIGURE 2 is a side elevational view showing a first step in the disclosed encapsulating method.

FIGURE 3 is a plan view showing a subsequent step in the encapsulating procedure.

FIGURE 4 is a schematic view showing a third step in the method.

FIGURE 5 is a schematic View showing a fourth step in the method.

FIGURE 6 is a schematic view showing a fifth and final step in the method.

In accordance with the invention, there is disclosed in FIGURE 1 in the drawing a printed circuit board element, generally indicated by reference character 10, of a type widely used in the electronics industries and related arts. The element 10 is selectively interconnectable with similar elements through means of interlocking plugs or jacks 9 which abut a free edge of a base member 11. The base member 11 is normally formed from resin-impregnated textile materials, glass cloth or paper to obtain high strength consistent With low weight, and mounts a plurality of electronic components 12 which are usually interconnected with a printed circuit on the base member. Although the base member may be of any desired planar or non-planar configuration, it is normally of rectangular shape, being bounded by a pair of

longer sides

13 and 14,

shorter sides

15 and 16, as well as an upper surface 17 and a lower surface 18. Thus, as vmay be seen in FIGURE 2, the device 10 is of generally planar configuration, but includes a plurality of raised shapes of irregular form depending upon the particular electronic component mounted thereupon.

Referring to FIGURE 2 in the drawing, after the device 10 has been suitably cleaned of any dust particles thereon, the same is subjected to a light coating over the entire area thereof with a thin layer of a suitable sealing synthetic resin. I have found polyamid and/or nitrocellulose may be rapidly sprayed to a thickness of one or two thousandths of an inch. In order to assure that all exposed areas, including the surfaces of the electronic components 12, are covered, the coating is preferably app-lied by a spray gun or air brush 20 which permits relative movement between the nozzle and the device 10, whereby the exposed surfaces may be sprayed at different angles. Where the resin is applied in a volatile vehicle, evaporation is almost instantaneous, and the subsequent step described below may he proceeded with almost immediately. It is further possible to coat printed circuit boards with the above-described resins before the assembly of the components. These resins are solderaole. This will allow for complete encapsulation of the components without any adhesion to the components. This enables simple repairs.

Referring to FIGURE 3, as a subsequent step there is provided a bag-like container 23, which may be of cellulose acetate, or any other thermoplastic material which is compatible with the previously applied cellulose acetate spray coating. The bag 23 is preferably of flattened con figuration, of dimensions sufiiciently large to permit ingress of the device 10. In the preferred form, it includes first and

second laminae

24 and 25, respectively, which are sealed along three sides thereof to provide free edges 26 and 27, interconnected

side edges

28 and 29, and an interconnected end edge 30, into which there has been incorporated a short length of metallic or synthetic resinous tubing 31, the outer surface of which is in hermetically sealed relation with respect to the end edge 30. As seen in FIGURE 3, the device 10, after being spray-coated, is conveniently inserted into the confines of the bag 23.

Referring to FIGURE 4, as a subsequent step the free edges 26 and 27 are heat-sealed together to provide a hermetically sealed enclosure, and the tubing 31 is connected to air exhaustion means 32. The now-closed edge 35 resembles the edges 28-30.

213 The now-closed bag 23 is form of infrared lamps or other suitable heating means. As the bag 23 softens under the action of heat, to become thermoplastic, the air exhaustion means 32 is placed in operation, resulting in the drawing together of the first and

second laminae

24 and 25 to closely conform to the precise configuration of the device 10. Where individual wires or conductors stand free from the surfaces of the base member 11 or components 12, under sufficient vacuum the softened material will interconnect beneath the conductors to completely enclose the same.

Referring to FIGURE 5, as a subsequent step, the nowsealed bag 23 may be trimmed to the external configuration of the base member 11, wherein the now-encapsulated device is of no greater appreciable size than prior to encapsulation. As a last step, as seen in FIGURE 6, the encapsulating medium is removed from the jacks 9 to permit the making of proper electrical contact when the device 10 is installed.

The laminae are applied hot, and the outside pressure seals the laminae to the board. Once the bag ".23 has sufiiciently cooled, it may be handled extensively without damage either to the bag 23 or the components 12. Should, however, it become necessary to replace an individual component 12, the same may be reached by merely cutting through the desired first or second lamina 24-25 to the corresponding surface 17-18 of the base member 11, and that individual electronic component disposed directly therebeneath may be conveniently replaced. Normally, such an operation will not disturb the encapsulation of the remaining electronic components, and the newly installed component need merely be re-covered using a suitable resin (not shown).

I Wish it to be understood that I do not consider the invention limited to the precise details of structure shown and set forth in this specification, for obvious modifications will occur to those skilled in the art to which the invention pertains.

subjected to the action of a plurality of

heat sources

36 and 37, which may be in the I claim:

The method of encapsulating electric circuit boards, electronic assemblies, and similar articles of generally planar or non-planar configuration, said boards having a plurality of electronic devices mounted upon at least one planar surfa-cethereof, comprising the steps of:

(a) providing a thermoplastic bag of material compatible withsaid first-mentioned plastic material, said bag having a configuration corresponding generally to that of said board, and having at least one free edge defining an opening, said bag having tubing means extending through said bag;

(b) coating the exposed surfaces of said board and devices with a thermoplastic material, said coating material being compatible and heat-scalable with the thermoplastic material comprising said bag;

(c) inserting said circuit board through the opening defined by said free edge to be completely disposed within said bag;

(d) heating the remaining material comprising said bag to a plastic state;

(e) exhausting the atmosphere within said bag, Whereby atmospheric pressure will cause the material of said bag to flow about said electronic elements and contact the coated surfaces of said boards to form and heat-seal a vacuum enclosure; and (f) severing those portions of said bag extending outwardly of said board.

References Cited by the Examiner UNITED STATES PATENTS TRAVIS S. MCGEHEE, Primary Examiner.