MATERIALS FOR USE IN FORMING ELECTRONIC INTERCONNECT

Abstract: This invention provides a device for placing elongate joint-forming preforms such as solder columns on electronic components such as chip carriers using a water soluble retaining member which positions and holds the preforms while they are attached to the electronic component. The retaining member is then dissolved in hot or cold water. The retaining member is constructed of water soluble or water dispersible paper in combination with coatings of or layers of a water soluble material such as polyethyleneoxide. The water soluble retaining member can be a laminate of alter­ nating layers of water soluble paper and water soluble polymeric material.

Description: This invention relates to materials, methods and articles for forming electrical interconnections between electronic components such as attaching chips or chip carriers to other similar components or to supporting substrates such as circuit boards. Solder performs for forming solder joints between electrical components may be located in apertures in a ribbon of plastics or plastics coated paper as described in US-A-3,472,365. Plastics such as pvc, polyvinyl alcohol, or soil polyethylene may be employed. In the electronic, micro-electronics and computer industries it has been found that surface mounting of chip carrier packages and other similar components on printed circuit boards or other components will not withstand thermal cycling and other stresses to which the resulting assemblies are subjected during use. Therefore, it has become desirable to replace surface mounting by connecting one electronic component to another or to a circuit board using elongated joint-forming materials which will provide sufficient flexibility to enable the assembly to withstand thermal cycles and other stresses encountered during use. In commonly assigned US patent and applications serial nos. 509,684, filed 30 June 1983 (corresponding to EP-A-0 133 752) and 610,077, filed 14 May 1984 (corresponding to EP-A-0 133 752), and in UK patent application 2,142,568, the disclosures of which are incorporated herein by reference, there are disclosed devices for providing such interconnection of electronic components using elongated joints between electronic components in an electronic assembly. These devices comprise a retaining member having apertures therein corresponding to the pre-selected points of interconnection and electrically conductive joint-forming material disposed in those apertures. The joint-forming material is an elongate member which can be attached to one electronic component or can be attached simultaneously to opposing electronic components thereby providing an elongate interconnect between the components which will withstand thermal cycles and other stresses. In a typical embodiment, the interconnect device contains columns of solder in the apertures of the retaining member. After the solder columns have been attached to one component or assembled between electronic components, it is frequently desirable to remove the retaining member. It has been recognized that it is desirable that the retaining member be dissolvable so that it can be removed after the joint-forming preforms, such as the solder columns, have been soldered into place on the electronic component. It is an object of this invention to provide an improved retaining member which is dissolvable or removable in water. It is a further object of this invention to provide a retaining member which will provide sufficient strength and stability to provide for accurate placement of the joint-forming preforms, which will withstand soldering temperatures, and which is removable with water in a relatively short period of time. This invention as defined in claim 1 provides in one aspect a structure suitable for holding electrical joint-forming preforms comprising: a layer of paper which is water soluble or water dispersible and a coating on at least one side of said paper, the coating comprising a water soluble polymer, wherein the paper and coating have sufficient dimensional and structural stability whereby the joint-forming preforms can be accurately placed on an electronic component and accurately held in position during attachment of the preforms to the electronic component. In another aspect, this invention as defined in claim 2 provides a laminate structure suitable for holding electrical joint-forming preforms comprising: at least first and second layers of water soluble or water dispersible paper; and a water soluble polymer material which is disposed between said layers of paper and which provides adhesion of said layers of paper to one another and wherein the layers of paper and water soluble polymer material provide sufficient dimensional and structural stability whereby the joint-forming preforms can be accurately placed on an electronic component and accurately held in position during attachment of the preforms to the electronic component. This laminate structure can comprise any number of layers of paper alternating with the water soluble material to provide a desired thickness of the laminate structure. The outside layer of the laminate structure can be the paper or can be the water soluble material on either or both sides. In another aspect, this invention as defined in claim 4 provides a device for connecting electrically conductive components comprising: a retaining member having a plurality of apertures therein spaced to correspond to preselected points on an electrically conductive component; and preforms of electrically conductive joint-forming material disposed in or supported by the apertures; wherein the retaining member comprises at least one layer of water soluble or water dispersible paper coated on at least one side thereof with a water soluble polymer material where the retaining member has sufficient dimensional and structural stability whereby the joint-forming preforms can be accurately placed on an electronic component and accurately held in position during attachment of the preforms to the electronic component. In this device the retaining member may alternatively comprise at least first and second layers of water soluble or water dispersible paper and a water soluble material which is disposed between said layers of paper and which provides adhesion of said layers of paper to one another and wherein the layers of paper and water soluble material provide sufficient dimensional and structure stability whereby the joint-forming preform can be accurately placed on an electronic component and accurate held in position during attachment of the preforms to the electronic component. According to a further aspect of the invention there is provided a method as defined in claim 7. The coated paper or paper laminate useful in the present invention can be made from various combinations of conventional water soluble or water dispersible papers and layers of or coatings of water soluble materials such as water soluble polymers which are known in the art. The resulting coated paper or paper laminate must have sufficient dimensional and structural stability to hold the electrical joint-forming preforms in the desired precision configuration for placement on and connection to the electronic component. The coated paper or paper laminate thus should have sufficient stiffness to hold preforms such as solder columns in appropriate vertical alignment and should be rigid enough to hold those preforms in precise lateral dimensions to correspond to the connecting pads on the electronic components. However, the coated paper or paper laminate holding the preforms should also have enough flexibility to conform to electronic components which are not perfectly flat. The coated paper or paper laminate retaining member of this invention must also be able to withstand soldering temperatures or other conditions used to attach the preforms to the electronic components and be able to withstand those temperatures for sufficient amounts of time without significant deterioration or detrimental change in the properties of the retaining member. The water soluble or water dispersible paper useful in this invention may be selected from those known in the art, such as those described in U.S. patents 3,034,922 and 3,431,166 and referred to in U.S. patent 3,859,125, the disclosures of which are incorporated herein by reference. The paper should be easily dispersible in water and is preferably a paper which is quickly swellable upon contact with water. When the paper is swellable, it is usually more quickly dissolvable and dispersible. It is believed to be generally preferable to use papers made from short fibers because the short fibers are more easily removable from the electronic component after the elongated preforms have been attached to the component. When longer fibers are present in the papers, it may in some instances be more difficult to remove all traces of the paper and clean the electronic components after the preforms have been attached. The paper should be dissolvable or dispersible in either hot or cold water. Standard thicknesses of the water soluble or water dispersible paper may be used in this invention depending on the stiffness and thickness of the retaining member desired for a particular application. For example, paper having a thickness of 0.10mm (0.004 inches) or less can be used as can papers having greater thicknesses, for example 0.18mm (0.007 inches) or greater depending on the properties desired for the resulting retaining member. The water soluble material used to coat or laminate the paper can be conventional water soluble materials known in the art such as polyethyleneoxide, polyvinylalcohol, polyvinylpyrrolidone, polyethyleneglycol and other conventionally known water soluble materials. These materials are illustrated in the above referenced patents and may be selected from standard reference works such as Water-Soluble Synthetic Polymers: Properties and Behavior , Philip Molyneux (CRC Press, Vols. 1 and 2, l984). The water soluble material used to coat the paper or form the paper laminate according to this invention should be able to withstand the soldering temperature or other conditions used to attached the preforms to the electronic component without changing solubility characteristics. In addition, the material should not substantially melt or lose its structural integrity under those soldering or other conditions. However, it will be recognized that the water soluble material used as the coating or used to form the laminates may not be suitable to withstand those conditions or useful by itself but in combination with the reinforcing properties of the water soluble or water dispersible paper may be satisfactory for use according to the present invention. The coated paper or paper laminate according to the present invention may be prepared by conventional coating or laminating methods. For example, the water soluble material may be coated on the paper in liquid or molten form or may be distributed on the paper in powder form then either dried or heated and cooled to cause the water soluble material to adhere to the surface of the paper. A preferred method involves forming the water soluble material into thin sheets, for example by extrusion, then lightly pressing the thin sheet of water soluble material and the water soluble paper at sufficient temperature to cause the sheet and the paper to adhere. Laminates of the paper according to the present invention can be made in the same way by assembling layers of the water soluble paper and sheets of water soluble material to the desired number of layers and total thickness then pressing the total assembly under an appropriate pressure and temperature to form the laminate. In forming the laminate it has been observed that it is desirable to have a sufficiently thick layer of water soluble material between layers of the water soluble or water dispersible paper to enable the paper to readily disperse in water. It is believed that if the paper layers are too close together and not sufficiently separated by water soluble material, they are more difficult to disperse when the retaining member is placed in water. In this regard, it is generally desirable to use a minimum of 0.05mm (0.002 inches) thick layer of water soluble material between layers of the water soluble or water dispersible paper. The thickness of the water soluble material may be up to 0.38mm (0.015 inches) or even greater depending on the properties of the retaining member desired. The total thickness of the retaining member which is formed from the coated paper or paper laminate according to this invention will depend on the height or length of the electrically joint-forming preforms which are to be placed on the electronic component by using the retaining member. As generally discussed in the above referenced U.K. patent application 2,142,568 it is generally desirable to have the ends of the preforms extending above the surface of the retaining member. For example, when the preforms are 2.5mm (0.1 inches) in height the retaining member will conveniently be about 1.3mm (0.05 inches) in thickness; where the preforms are 1.3mm (0.05 inches) in height, the retaining member will conveniently be about 0.025 inches in thickness. It will be recognized that fillers may be used in the water soluble material that is coated on the water soluble paper or used between the paper layers in the laminate according to the present invention. Fillers may be used to achieve the desired structural and strength properties of the retaining member or may be used to modify the performance of parts of the retaining member during the soldering or other operations used to attach the preforms to the electronic components. For example, if a water soluble material is used in sheets between the laminated layers of water soluble or water dispersible paper layers which tends to melt or liquify during soldering operations, a fibrous or particulate filler can be used in the water soluble material to prevent it from flowing excessively during the soldering operation. The electrically joint-forming preforms useful in the devices according to this invention may be any element which forms an appropriate interconnect between electronic components. These preforms may be wires, solder columns, conductive elastomers, or other materials suitable for the assembly of electronic components. A preferred preform for many interconnect applications is a solder column wrapped with a copper tape as described in Figure 14 of U.K. patent application 2,142,568. Example A retaining member was made using a water dissolvable paper available from Gilbreth International Corp., Bensalem, Pennsylvania, available under the trademark DISSOLVO™ and designation as card stock no. GC-140 which is 0.007 inches in thickness. The water soluble material used was a water soluble polyethyleneoxide available from Union Carbide under the trademark POLYOX™ under the product designation #WSR N-80. The polyethyleneoxide powder was extruded through a sheet extruder at 116°C (240°F) into sheets 0.13mm (0.005 inches) in thickness. The retaining member was produced by laminating three layers of the water dissolvable paper and four layers of the polyethyleneoxide sheets under conditions of light pressure (in the range of about 1-5 pounds per square foot) and moderate temperature (in the range of about 121°C (250°F)) for about 30 seconds and then cooled under the same pressure conditions. The laminate was formed between twin flat belts moving at about four feet per minute through a heating zone two feet in length and a cooling zone about two feet in length. The resulting laminate was about 1.14mm (0.045 inches) in thickness with both surfaces being polyethyleneoxide. The laminate was then cut into approximately one inch squares with the center portion of each square cut out leaving a perimeter strip about 3mm (1/8 inch) wide which was then punched to form 68 apertures in which solder columns 2.5mm (0.1 inches) in height were placed. The solder columns are about 0.56mm (0.022 inches) in diameter formed from a 20/80 solder core wrapped with a copper tape and coated with Sn63 solder. The resulting device is a shape similar to that shown in Figures 4 and 14 of U.K. patent 2,142,568. The device was then positioned on a chip carrier and the solder column preforms were attached to the chip carrier by reflowing the solder at 215°C for one minute. After cooling, the chip carrier was then immersed in boiling water to remove the retaining member. The retaining member dissolved or dispersed and was completely removed from carrier in about 15 minutes in the boiling water leaving the chip carrier with the 68 solder column preforms attached thereto with no remaining paper or polyethyleneoxide materials remaining thereon.

Claims: A laminate structure having sufficient dimensions and structural stability for holding electrical joint-forming preforms and for accurately placing the preforms on an electronic component and accurately holding the preforms in position during attachment of the preforms to the electronic component comprising: a layer of paper which is water soluble or water dispersible and which has a coating on at least one side of said paper, the coating comprising a water soluble polymer, and which has a plurality of apertures therein for receiving and holding said preforms. A laminate structure having sufficient dimensional and structural stability for holding elecrical joint-forming preforms and for accurately placing the preforms on an electronic component and accurately holding the preforms in position during attachment of the preforms to the electronic component comprising: at least first and second layers of water soluble or water dispersible paper; and a water soluble polymer material which is disposed between said layers of paper and which provides adhesion of said layers of paper to one another and wherein the laminate of paper layers and water soluble polymer material have a plurality of apertures therein for receiving and holding said preforms. A laminate structure according to claims 1 or 2 wherein the water soluble material is polyethyleneoxide, polyvinylalcohol, polyvinylpyrrolidone, or polyethyleneglycol. A device for connecting electrically conductive elements comprising: a retaining member having a plurality of apertures therein spaced to correspond to preselected points on an electrically conductive element; and preforms of electrically conductive joint-forming material disposed in or supported by the apertures; wherein the retaining member comprises at least one layer of water soluble or water dispersible paper coated on at lest one side thereof with a water soluble polymer material where the retaining member has sufficient dimensional and structural stability whereby the joint-forming preforms can be accurately placed on an electronic component and accurately held in position during attachment of the preforms to the electronic component. A device according to claim 4 wherein the retaining member comprises: at least first and second layers of water soluble or water dispersible paper; and a water soluble material which is disposed between said layers of paper and which provides adhesion of said layers of paper to one another and wherein the layers of paper and water soluble material provide sufficient dimensional and structural stability whereby the joint-forming preforms can be accurately placed on an electronic component and accurately held in position during attachment of the preforms to the electronic component. A device according to claims 3 or 4 wherein the water soluble material is polyethyleneoxide, polyvinylalcohol, polyvinylpyrrolidone, or polyethyleneglycol. A method for providing electrically conductive components with elongate interconnect means suitable for connection with other electrically conductive components comprising positioning an electrically conductive component and a device in appropriate contact, wherein the device comprises: a retaining member having a plurality of apertures therein spaced to correspond to preselected points on the electrically conductive component; and elongate preforms of electrically conductive joint-forming material disposed in or supported by the apertures; wherein the retaining member comprises at least one layer of water soluble or water dispersible paper coated on at least one side thereof with a water soluble polymer material where the retaining member has sufficient dimensional and structural stability whereby the elongate joint-forming preforms can be accurately placed on an electronic component and accurately held in position during attachment of the elongate preforms to the electronic component; and causing the elongate preforms to attach to the electrically conductive component.

Inventors: LOPEZ, ARTHUR WILLIAM, JR., WONG, GEOFFREY B.
Applicants: RAYCHEM CORPORATION (A DELAWARE CORPORATION)
Application year: 1986
Main classification: H05K3
IPC codes: H01R43, B23K35, H01R4, B32B27, H05K3