Method of constructing a monolithic block having an internal geometry and the block resulting therefrom

Abstract: A method of constructing a connector housing block which includes a plurality of chambers (16) each for holding a contact member (14) therein, wherein the block is constructed of a plurality of stacked plates (18). Each of the plates (18) is formed with a plurality of apertures (20) so situated on the plates (18) that when the plates (18) are stacked the apertures (20) form the plurality of chambers (16). A solution of epoxy resin and catalyst in a solvent is provided, the plates (18) are coated with the solution, and the solvent is allowed to evaporate from the plates (18) to leave a continuous film of B-Staged resin and catalyst on the plates (18) which does not fill the apertures (20). The plates (18) are then stacked and the resin and catalyst are caused to cross-link and fuse.

Description: This invention relates to monolithic blocks having an internal geometry and, more particularly, to a method of constructing such a block wherein the small size of the internal geometry is unsuitable for conventional molding technology. There are situations where it is desired to produce a monolithic block having an internal geometry which is so small that the fine features thereof cannot be produced through conventional molding technology. For example, our co-pending application (cf. EP-A-0 392 689) discloses a high density connector assembly for an integrated circuit chip carrier which includes a stack of metallic plates having apertures which form chambers for holding planar contact members therein. These spacer plates are relatively thin, illustratively having a thickness of 0.254 mm (0.0100 inch), and a typical aperture is a rectangular slot 0.140 mm by 2.032 mm (0.0055 inch by 0.0800 inch). There are many of these apertures, illustratively equally spaced on 0.381 mm (0.0150 inch) centerlines. The plates are then stacked in alignment to form a block structure, with the stacked apertures forming a plurality of chambers, each of which can accommodate therein a planar contact member. To insulate the plates from the contact members, the plates are coated with an insulating layer of dielectric material so that the stack of plates provides a ground shield around each of the contact members to prevent cross-talk therebetween. Preferably, the plates are made of aluminum which is subsequently anodized or otherwise oxidized to provide a relatively thick layer of oxide on all surfaces of the plates. Although disclosed in connection with a contact socket for a connector assembly, the aforedescribed technique of providing a stack of thin sheets, each with an appropriate geometry representing the dimensions which one would obtain by cross-sectioning a three-dimensional structure in a given plane, has more general application. This technique may be used whenever the fine features of the internal geometry of a desired monolithic block are so small that the traditional technology of molding plastic becomes inadequate due to a limitation of the space between core pins in the mold into which the plastic will flow, provide an adequate packing density, and retain at least some mechanical strength when extracted from the mold. However, it has been discovered that such structures present an environmental problem in actual use, because the plates stacked together in close proximity, and even under pressure, will have a capillary interface between their surfaces. Such capillary interface will trap and hold moisture. In the case of water in liquid form, the holding is accomplished by surface tension. In the case of vapors or very thin film, the holding is accomplished by osmotic pressure. In an electrical connector, entrapped moisture is detrimental because it can be a medium for ion transport, thereby providing an electrolytic path between contacts, which may have a difference of potential in relation to each other. This will result in a leakage path being provided between the contacts which can possibly result in corrosion and an ultimate failure of the connector assembly. Furthermore, if the plates are made of anodized aluminum, there is the additional problem of porosity which is, if not inherent, at least typical in an aluminum oxide coating. The sealing of such pores has traditionally been accomplished by vacuum impregnation or coating with organic fillers. However, this still leaves the capillary interface. It is therefore an object of this invention to provide a method for eliminating capillary interfaces and pores between adjacent plates in a stack of plates forming a block having an internal geometry. The invention consists in a method of constructing a monolithic block having an internal geometry, characterized by the steps of: providing a plurality of relatively thin plates each with an appropriate geometry such that when stacked together in proper sequence said internal geometry is replicated; providing a solution of epoxy resin and catalyst in a solvent; coating each of the plates with said solution; allowing the solvent to evaporate from each of the plates to leave a film of resin and catalyst thereon; stacking the plates; and causing the resin and catalyst to cross-link and fuse. When the solvent evaporates from the plates it leaves on each plate a continuous film of resin and catalyst which does not fill any openings in the plate. The film preferably has a thickness of 5µm (about 0.0002 inch). The invention also consists in a method of constructing a connector socket block, said socket block including a plurality of chambers each for holding a contact member therein, characterized by the steps of: providing a plurality of plates, each of said plates being formed with a plurality of apertures so situated on the plates that when the plates are stacked the apertures form said plurality of chambers; providing a solution of epoxy resin and catalyst in a solvent; coating each of the plates with said solution; allowing the solvent to evaporate from each of the plates to leave a film of resin and catalyst thereon; stacking the plates; and causing the resin and catalyst to cross-link and fuse. For a better understanding of the present invention reference will now be made, by way of example, to the accompanying drawings in which:- FIGURE 1 is a semi-exploded cross-sectional view through the connector assembly of EP-A-392 689 useful for illustrating an application of the present invention; and FIGURE 2 is a plan view of a typical spacer plate of the assembly of Figure 1. Figure 1 illustrates the connector assembly disclosed in the aforementioned European patent specification and only as much structure as is necessary for understanding the present invention will be described herein. Thus, the connector assembly shown in Figure 1 functions to connect a pattern of contact pads on an integrated circuit chip carrier, or module, 10 to a pattern of corresponding contact pads on a printed circuit board 12. Connection between the contact pads is made by a plurality of contact members 14 which are positioned in chambers 16 formed by aligned apertures in a stack of spacer plates 18. Figure 2 shows an illustrative spacer plate 18 which has a plurality of apertures 20 in an orderly array. The apertures 20 are typically rectangular, of dimension 0.140 mm by 2.032 mm (0.0055 inch by 0.0800 inch), and are equally spaced on 0.381 mm (0.0150 inch) centerlines. It will therefore be appreciated that the separation between adjacent apertures 20 is 0.241 mm (0.0095 inch). Apertures of that size with that spacing therebetween cannot be molded using conventional molding techniques. Accordingly, the plates 18 are formed of metal. Preferably, the plates 18 are formed of aluminum which is subsequently anodized, or otherwise oxidized, to provide a relatively thick, abrasion resistant dielectric layer. The problem sought to be alleviated occurs when the plates 18 are stacked into the assembly shown in Figure 1, where the aligned and stacked apertures 20 form the chambers 16. The capillary interface between adjacent surfaces of the plates 18 allows moisture to be trapped. Accordingly, what is needed is a method by which the stack of plates can be sealed to totally fill the capillaries while at the same time preventing the sealing material from migrating into the apertures 20 to occupy space which is reserved for the contact members 14. In accordance with this invention, a suitable epoxy resin and catalyst, for example, Hysol Novolac epoxy, in appropriate proportions are dissolved in a solvent, for example, methyl ethyl ketone (MEK), acetone, toluene, or the like. In such a solution, the molecules of both the resin and the catalyst are dispersed, and no cross-linking takes place. The resulting solution is then placed in a suitable container and the plates 18 are immersed individually into the solution and then withdrawn. The wet plates 18 are then allowed to dry, with the solvent evaporating therefrom. This leaves a film of resin and catalyst on each of the plates 18. By choosing a correct ratio of the resin and catalyst to solvent, as the solvent evaporates, a continuous "B-Staged" film of resin and catalyst remains on all the surfaces of the plates 18. As is well known, a B-staged film of resin and catalyst is one in which the resin and catalyst have not reacted together in a cross-linking fashion. Such a film is extremely thin. Preferably, the ratio of the resin and catalyst to solvent is chosen so that evaporation of the solvent from the coated plates 18 leaves a film of resin and catalyst on each of the plates 18 that is 5 µm (approximately 0.0002 inch) thick. This ratio must be experimentally determined for each batch of resin since different batches have different molecular chain lengths which changes the viscosity of the batches. Care must be taken to prevent the film from becoming too thick, because if the film is too thick it can block the apertures 20. An illustrative experimental method for determining a suitable ratio comprises the following steps: 1. A measured amount of epoxy resin and catalyst is placed in a container. 2. A measured amount of solvent is added to the epoxy in the container. 3. The contents of the container are thoroughly mixed. 4. The thickness of a test piece, such as a plate or rod, is measured. 5. The test piece is dipped into the solution in the container. 6. The test piece is removed from the container and the solvent is allowed to evaporate. This takes place in a few seconds. 7. The thickness of the coated test piece is measured to determine the film thickness. 8. If the film thickness is too great, more solvent is added to the container and the preceding steps are repeated until a satisfactory film thickness is obtained. Since the amounts of epoxy and solvent to obtain the desired film thickness is known for that particular batch, subsequent mixtures with the appropriate ratio can be made. 9. If the film on the coated test piece is too thin, the testing is repeated with a fresh mixture. The criteria for determining if the film is too thin is that the test piece is not completely coated. A requirement is that the film be continuous on the test piece, so that it is continuous on the plates 18. The coated plates 18 are then stacked in correct numbers, sequence, and alignment to each other in a fixture, where they are subjected to pressure and heat to permit the B-Staged material to cross-link and fuse, converting the stack into a monolithic structure. The pressure applied to the stack of plates 18 must be sufficient to bring adjacent surfaces of the plates into intimate mutual contact so that substantially all voids between the plates are filled with the resin and catalyst film. The heat is for the purpose of cross-linking the epoxy, which is of a type that doesn't cross-link at room temperature. Depending upon the epoxy used, the temperature required for cross-linking ranges from 121°C to 232°C (about 250°F to about 450°F). In the case of Hysol Novolac epoxy, a temperature of 177°C (approximately 350°F) has been found to be suitable. The aforedescribed method accomplishes two functions simultaneously. The first function is the filling and sealing of the pores in the cases in which the stack is made of oxidized aluminum plates and also providing a filler of the capillaries between the plates. The second function is the mechanical fusion of the plates to create a monolithic block structure. Accordingly, there has been disclosed an improved method of constructing a monolithic block having an internal geometry.

Claims: A method of constructing a monolithic block having an internal geometry, characterized by the steps of: providing a plurality of relatively thin plates (18) each with an appropriate geometry such that when stacked together in proper sequence said internal geometry is replicated; providing a solution of epoxy resin and catalyst in a solvent; coating each of the plates (18) with said solution; allowing the solvent to evaporate from each of the plates (18) to leave a film of resin and catalyst thereon; stacking the plates (18); and causing the resin and catalyst to cross-link and fuse. A method of constructing a connector socket block, said socket block including a plurality of chambers (16) each for holding a contact member (14) therein, characterized by the steps of: providing a plurality of plates (18), each of said plates (18) being formed with a plurality of apertures (20) so situated on the plates (18) that when the plates are stacked the apertures (20) form said plurality of chambers (16); providing a solution of epoxy resin and catalyst in a solvent; coating each of the plates (18) with said solution; allowing the solvent to evaporate from each of the plates (18) to leave a film of resin and catalyst thereon; stacking the plates (18); and causing the resin and catalyst to cross-link and fuse. A method according to claim 1 or 2, wherein the step of providing a solution of epoxy resin and catalyst in a solvent includes choosing a ratio of resin and catalyst to solvent so that when said solvent evaporates from each of the plates (18) there remains on each of the plates (18) a film of resin and catalyst 5µm (approximately 0.0002 inch) thick. A method according to claim 1, 2 or 3, wherein the step of causing the resin and catalyst to cross-link and fuse comprises applying pressure to the stacked plates (18) sufficient to bring adjacent surfaces of the plates (18) into intimate mutual contact so that substantially all voids therebetween are filled with resin and catalyst. A method according to claim 4, including the step of heating the stacked plates (18) sufficiently to cause crosslinking of the resin and catalyst. A method according to any preceding claim, wherein the step of coating each of the plates (18) comprises placing the solution in a container, immersing each of the plates (18) in the solution in said container, and removing each of the plates (18) from the solution.

Inventors: GRABBE DIMITRY G, GRABBE, DIMITRY G.
Applicants: WHITAKER CORP, THE WHITAKER CORPORATION
Application year: 1990
Main classification: H05K7
IPC codes: H01R43, B32B7, H05K7