SEALED LIQUID-FILLED MODULE AND METHOD OF FORMING SAME

Abstract: Cooling liquids such as fluorocarbons are enclosed within a module containing electronic components on a printed wiring board. Preferably top and bottom covers are applied to the top and bottom of the board and liquid-sealed thereto. Hot-melt adhesive is used for the seal. Each edge of a cover has an outward-extending flange formed with a peripheral downward turned lip. Heated adhesive is applied to the board in a continuous bead around the flange, the thickness of the bead being greater than the height of the lip. The cover is then squeezed against the board so that the adhesive seals against the board and the flange.

Description: SEALED LIQUID-FILLED MODULE AND METHOD OF FORMING SAME BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a new and improved sealed liquid-filled module and to a method of forming the same. More particularly, the invention relates to a module which is sealed to the top and bottom surfaces of a printed wiring board containing heat emitting components. The module is cooled by liquid such as fluorocarbons within the module. 2. Description of Related Art The use of fluorocarbons to cool printed wiring board components and enclosures for the fluorocarbons to be sealed thereto is known in the art. The present invention, however, provides an improved seal which also can be opened for replacement or repair of components. SUMMARY OF THE INVENTION: High performance cooling technologies for electronic modules often require the use of a liquid. Fluorocarbon liquids, such as FC-72 and HFE-7100 (sold by Minnesota Mining and Manufacturing Company) are typical examples of liquids which may be used in direct contact with the electronic components and the wiring boards. Such liquids have notably low surface tension and hence must be retained with high integrity seals. It is also desirable that the sealing method be low cost, suited to rapid automated assembly, and be reversible so that it is possible to reenter a module to make repairs or replacements. A hot-melt adhesive in conjunction with specially shaped flanges on the edge of the module covers are characteristics of the present invention. The seal is made between the board and the cover using a hot-melt adhesive bead captured in the flange of the cover. The flange is formed with a lip at its perimeter which provides positive location or spacing of the cover relative to the board in a perpendicular direction. The lip may be high enough to accommodate a sufficient thickness of adhesive to provide the required joint flexibility, and wide enough to provide adequate strength in the joint against internal pressure in the enclosure at maximum operating temperatures. The adhesive may be applied to the cover either as a pre-form or may be dispensed manually or robotically onto the flange. Preferably the pre-form is a bead thicker than the height of the lip of the cover. Such construction guarantees that the adhesive contacts the board. Assembly is accomplished by heating the flange and pressing the cover onto the board. A top and a bottom cover may be applied simultaneously. When required, removal is accomplished by reheating the flange until the adhesive remelts, and then pulling the cover off the board. Using an adhesive which is liquid during application makes the joint robust against small surface defects such as scratches or trapped fibers on the board which otherwise might cause a compression loaded gasket to leak. Clips or clamps may be provided which load the joint against the board or opposing cover if it is desired to keep the board in compression under all operating conditions. BRIEF DESCRIPTION OF THE DRAWINGS: The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments of the invention and, together with the description serve to explain the principles of the invention. Fig. 1 is a schematic sectional view of a bottom cover in the course of formation of a bead of hot-melt adhesive thereon. Fig. 2 is a transverse cross-sectional view of a printed wiring board and top and bottom covers applied. Fig. 3 is a view similar to Fig. 2 showing optional use of a clamp. Fig. 4 is a fragmentary sectional view of a modified flange. Fig. 5 is a view similar to Fig. 4 of a further modified flange. DESCRIPTION OF THE PREFERRED EMBODIMENTS Reference will now be made in detail to the preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with the preferred embodiments, it will be understood that they are not intended to limit the invention to those embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the invention as defined by the appended claims. Printed wiring board 11 contains a number of electronic components 12 which are heat emitting. It is necessary to cool the board by circulation of a liquid there around. Typical of such liquids are fluorocarbons such as FC-72 and HFE-7100 produced by 3M. Directing attention to Fig. 1, bottom cover 16 having a bottom panel 17 has upstanding sides 18. At the upper edges of sides 18 are outward turned flanges 19 having peripheral lips 21 projecting upward therefrom. The width of flanges 19 is sufficient to accommodate a squeezed layer of plastic 34 or a pre-formed gasket and the height of lip 21 is sufficient to space flange 19 from the PWB 11 a predetermined distance when the bottom cover 16 is forced against the PWB. As shown in Fig. 1, a continuous bead 31 of a hot-melt adhesive is applied to the upper surface of flange 19, the height of the bead 31 being greater than the height of the lip 21. Typical suitable hot-melt adhesives include Hysol 6009 (polyolefin) and Hysol 7390 (polyamide), both sold by Hysol-Dexter. Other hot melts may be used. Such bead 31 may be pre-formed. In similar manner a top cover 26 having a top panel 27 and having sides, flange and lip 18a, 19a and 21a, respectively, is provided with a second bead 3 la of hot-melt adhesive. The top and bottom covers 26, 16, respectively, are applied, preferably simultaneously, to the top and bottom surfaces of PWB 11. The flanges 19 and 19a are squeezed together so that the beads 31 and 31a flatten and extend outwardly, forming a tight seal against the flanges 19 and 19a. During the squeezing operation it is preferable that the flanges 19, 19a be heated to make the bead 31 or 31 a liquid or at least tacky. The squeezed layers 34, 34a make a liquid tight seal of the covers 16, 26 to the board 11. Hence, in a manner well known to the art, fluorocarbons or other liquids may be circulated within the module to cool the components 12. If it is necessary to open the module, the flanges 19, 19a are heated permitting the cover 16 or 26 to be pulled away from the board 11 providing access to components 12 for repair or replacement. Fig. 4 illustrates a modified flange structure. A raised inner Up 41, slightly lower than outer lip 21b is formed extending upward on flange 19b. Thus adhesive deposited on flange 19b is confined so that it does not extend inwardly of cover 16b. As shown in Fig. 4, lip 41 is formed by extending side 18b upwardly and them bending it outwardly and downwardly in an approximately 180° bend. Other ways to fabricate lip 41 will readily occur to one skilled in the art. Fig. 5 illustrates a still further modified flange. An outer lip is eliminated. A pattern of "dimples" 46 is formed in flange 19c by pressing flange 19c between dies having aligned protuberances and depressions or by molding the flange in such shape or by various other means. The dimples 46 provide the same displacement control as flange 21. This is especially effective if a pre-formed gasket of adhesive is used. In other respects the elements of the modifications of Figs. 4 and 5 resemble those of the proceeding modifications and the same reference numerals followed by subscripts b_ and c, respectively, designate corresponding parts. The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the Claims appended hereto and their equivalents.

Claims: WHAT IS CLAIMED IS: 1. A cover for a printed wiring board module comprising a panel, sides for said panel, a peripheral flange normal to said sides, and means formed on said flange to restrain outward movement of adhesive positioned on said flange. 2. A cover according to Claim 1 which said means comprises an upstanding member extending around said flange. 3. A cover according to Claim 2 in which said member comprises a peripheral lip. 4. A cover according to Claim 3 in which said lip elevates said flange from a supporting surface a pre-determined distance. 5. A cover according to Claim 4 which further comprises an inner upstanding lip extending around said flange vicinal an inner edge of said flange. 6. A cover according to Claim 1 in which said means comprises a plurality of dimples upstanding from an upper surface of said flange. 7. A cover according to Claim 1 which further comprises a continuous bead of adhesive on said flange. 8. A cover according to Claim 7 in which said adhesive is a hot-melt material. 9. In combination, a cover according to Claim 7 which further comprises a printed wiring board, said lip engaging a first surface of said board, said bead being distorted to seal said board to said cover around the periphery of said flange in a liquid-tight manner. 10. In combination, the combination of Claim 9 which further comprises a second cover having a second panel, second sides for said second panel, a second peripheral flange normal to said second sides, a second upstanding lip around said second flange, a second continuous bead of adhesive on said second flange higher than said second lip, said second lip engaging a second surface of said board, said second bead being distorted to seal said board to said second cover around the periphery of said second flange in a liquid-tight manner. 11. A method of forming a liquid-tight module comprising providing a cover according to Claim 1 , applying a bead of hot melt adhesive to said flange, melting said adhesive, squeezing said cover to a first side of said printed wiring board while distorting said bead, said adhesive sealing said cover to said board in a liquid-tight manner. 12. A method according to Claim 11 which further comprises providing a second cover according to Claim 1, applying a second bead of said adhesive to the flange of said second cover, squeezing said second cover to a second side of said board while distorting said second bead, said second adhesive sealing said second cover to said board in a liquid- tight manner. 13. A method according to Claim 12 which further comprises applying a plurality of clips to the flanges of said covers to deter displacement of a cover. 14. A method according to Claim 12 in which both covers are applied substantially simultaneously. 15. A method according to Claim 11 which further comprises circulating coolant within said module between said cover and said board and wherein said board includes heat- emitting electronic components. 16. A method according to Claim 15 in which said coolant is a fluorocarbon liquid. 17. A method according to Claim 1 1 in which said adhesive provides strength to the seal of said cover to said board against internal pressure applied in an enclosure between said cover and said board. 18. A method according to Claim 11 in which said bead is pre-formed. 19. A method according to Claim 1 1 in which said adhesive is dispensed onto said flange. 20. A method according to Claim 11 in which said cover has an upstanding lip around the periphery of said flange, said bead of adhesive being thicker than the height of said lip, said cover being squeezed until said lip engages said first side of said board.

Inventors: DAVIDSON HOWARD L, DAVIDSON, HOWARD, L.
Applicants: SUN MICROSYSTEMS INC, SUN MICROSYSTEMS, INC.
Application year: 1999
Main classification: H05K5
IPC codes: H05K5, H05K7