Solder Medium for Circuit Interconnection

This invention relates to solder medium for connecting electronic circuits and devices.

Modem electronic products including computers, telecommunication equipment, automobile electronics, and consumer electronics require circuit interconnection. The off-chip circuit interconnection and packaging is typically made with solder joints, either by wave soldering or surface mounting technique, as is well known in the art. An increase in circuit interconnection density is desirable for lowering the device cost as well as for miniaturizing the electronic products, which has been the general trend in recent years.

In high or ultra-high density circuit connections, the size of the interconnection contact pads and hence the size, especially the height of solder joints on them, is drastically reduced. As a result, the shear strain which is related to the ratio of the lateral change in substrate dimension (e.g., caused by thermal expansion mismatch) over the solder joint height and accompanying stress on solder are significantly increased. In the widely-used eutectic Pb-Sn solder, it is well known that the solder joint failure by fatigue is caused by the combination of cyclic temperature change and extensive plastic shear strain which in turn causes microstructural coarsening and crack initiation. If the solder joints had high-aspect ratios and were elastically compliant, their elastic bending could have accommodated some of the strain and minimized the amount of plastic strain on solder. However, most present-day solder joints are relatively short (e.g. spherical segment geometry) and, hence, deform plastically with little elastic compliance. In view of these considerations, there is a need for improved solder joint geometry to reduce the shear strain and to enhance the joint reliability during the service of electronic products.

Electronic devices having at least two components with mating contact pads are provided with high-aspect-ratio solder joints between the mating pads. These joints ar formed by placing a composite solder medium containing solder wires in an electrically insulating matrix such that at least two solder wires are in contact with the mating pads, and fusing the wires to the pads. The insulating matrix with remainder of solder wires is then optionally removed from between the said at least two components. The composite solder medium is formed by preparing an elongated body of solder wires in an insulating matrix and cutting off slices of the composite solder medium, the solder wires having a high-aspect-ratio of length to their diameter. Alternatively sheets of the composite solder medium are prepared by magnetically aligning solder coated magnetic particles into columns arranged transverse of an insulating matrix and heating sufficiently to fuse the solder in each column into a continuously conducting solder path.

FIG. 1 is a schematic illustration of a composite solder medium containing solder wires coated with compliant or dissolvable carrier matrix;

FIG. 2 is a schematic illustration of a longitudinally drawn composite solder medium containing the solder wires and compliant or dissolvable carrier matrix, and of a thin slice of the composite cut-off from the elongated composite;

FIG. 3 is a schematic illustration of a composite solder medium containing solder particles embedded in an electrically-insulating, plastically-deformable carrier matrix;

FIG. 4 is a schematic illustration of the composite solder medium of FIG. 3 after the composite solder medium and solder particles are longitudinally deformed and elongated, and of a thin slice of the composite solder medium cut-off from the elongated composite;

FIG. 5 is a schematic cross-sectional view of a portion of an electronic device with a thin slice of the composite solder medium of FIG. 2 or FIG.4 positioned between circuit components of the device for solder interconnection;

FIG. 6 is a schematic cross-sectional view of the electronic device shown in FIG. 5 but in which the carrier matrix is dissolved away after soldering operation;

FIG. 7 is a schematic cross-sectional view of a composite solder medium containing magnetically aligned solder-coated magnetic particles in an insulating matrix;

FIG. 8 is schematic cross-sectional view of the composite solder medium shown in FIG. 7 after the solder is fused together;

FIG. 9 is a cross-sectional view of circuit interconnection using the composite solder medium of FIG. 8; and

FIG. 10 is a cross-sectional view similar to FIG. 9 but in which the matrix material is dissolved away after the soldering operation.

This invention concerns solder-containing composite medium and soldering method using the composite medium that will allow formation in electronic devices of high-aspect-ratio solder joints for improved reliability. One exemplary embodiment of the invention is to create a high-aspect-ratio solder-containing composite medium by embedding solder wires, rods or ribbons in an electrically insulating plastically deformable matrix and then slicing the composite medium cross-sectionally to obtain the high-aspect-ratio interconnection medium with desirable height. The matrix is of a material which is either elastically compliant or can be dissolved away after interconnection. In order to achieve a relatively uniform spacing between the solder wires, a bunch of wires, 11, is coated with an electrically insulating medium, 12, e.g., by dip coating or continuously pulling through a bath of an insulating medium in a liquid form, and curing, semicuring or drying the medium forming a coated bunch, 10, as schematically shown in FIG. 1. Thereafter coated bunch 10 is drawn out into an elongated form, 20, to reduce the diameter of the wires to a desired solder wire diameter. Alternatively, a number of individual coated wires or coated bunches 10 are bundled together with additional matrix liquid, and cured or dryed to obtain a large cross-section composite medium. This large composite medium is then drawn out into elongated form 20 having wires, 21, with a desired solder wire diameter within a matrix material, 22. This procedure may be repeated until the diameter of wires 21 is reduced to a size of 50 .mu.m or less, preferably less than 10 .mu.m.

The drawn-out composite 20 is then sliced, e.g. by razor-sharp blade or knife, or any other cutting instruments, into thin wafers or sheets, 23, as shown in FIG. 2. Solder wires are typically mechanically soft and are easily cut by razor blade. The cutting can be optionally carried out at a slightly elevated temperature (e.g. 50.degree.-150.degree. C.) to utilize the extreme softness of solders at these temperatures.

The insulating medium in the composite is desirably mechanically much more softer elastically and plastically than the solder so that the presence of the medium does not exert any excessive stresses on solder during device service, e.g., by thermal expansion mismatch. Exemplary compliant media include elastomers and plastics, such as polyurethanes. Alternatively, instead of the compliant medium, a dissolvable medium can be used, for example, gelatine-,glucose-,fructose-, or honey-based materials, or polyvinyl alcohol, polyacrylamide, polyvinyl pyrrolidinone type organic polymers that are water soluble, or buthylvinyl alcohol, polyvinyl acetate type materials that are soluble in solvent such as toluene or acetone. Optional additional materials and fillers may be added for various purposes such as viscosity control, plasticizing, etc., for example fine silica or alumina particles. The dissolvable medium is maintained as a solid throughout the stages of slicing of the composite, and subsequent soldering interconnection process, but is dissolved away after soldering is completed, as shown in FIG. 6, so that each of the vertical columns of high-aspect-ratio solder in the joint can be flexible without mechanical constraints between them.

Another way of creating the high-aspect-ratio solder-containing composite medium is to use solder particles, 31, dispersed in a plastically deformable matrix, 32, e.g. thermoplastic polymer, plasticized rubber, clay type material with appropriate mechanical strength forming a composite structure, 33. Heat-softenable glass, salt or ceramic material may also be used, but these rigid materials are less preferable unless they are easily dissolvable. Composite structure, 33, shown schematically in FIG. 3 is then extruded (or rolled), into an elongated structure, 43, so that the embedded solder particles become elongated into "wires " (or ribbons), 41, as shown in FIG. 4. It is desirable that the mechanical strengths of the solder and the matrix medium are properly matched by choice of materials and/or the choice of processing temperatures, for example, so that the matrix is mechanically comparable to or slightly stronger than the embedded solder in order to achieve the elongation effect during extrusion. The mechanical softness of the solder can be significantly increased if necessary by carrying out the deformation at a higher temperature, e.g., close to or even above the solder melting temperature which is typically in the range of 100.degree.-250.degree. C. depending on the solder alloys. The extruded composite with the elongated solder is then sliced to a thin slice or sheet configuration, 44, for interconnection use.

The final diameter and density of the elongated solder wires in the composite depend on the initial size and volume fraction of the particles in the precursor composite of FIG. 3 as well as the amount of deformation by extrusion or rod drawing. The use of the particle-shaped solder in the precursor is more advantageous than the use of wire-shaped solder since fine solder particles of 20-40 .mu.m diameter are commercially available more easily than the wire-shaped solder of similar diameter, and can also be present in a random distribution manner without the need for wire alignment, thus, allowing the production of small diameter wire structure by deformation with less complicated processing. If a 20 inch square composite, e.g., 33, containing 30% by volume of 25 .mu.m diameter solder particles is extruded into a final shape of 2 inches square in cross-section, the average elongated particle diameter is about 2.5 .mu.m and its length .about.1670 .mu.m. When sliced into sheets and used for interconnection of 1 mil.times.1 mil contact pads, the number of parallel conduction paths (the number of high-aspect-ratio solder wires) per pad is more than 30. For the slice thickness of .about.10 mil (.about.250 .mu.m), the length to diameter aspect ratio of the solder wires in the slice is about 100: 1. A similar analogy about the diameter deformation ratio is applicable to the wire-containing structures shown in FIGS. 1 and 2 of the drawings.

The preferred range of the slice thickness, such as for slices 23 or 44 shown in FIGS. 2 and 4, respectively, is 1-500 mil and more preferably 5-100 mil. The preferred aspect-ratio of the solder wires on the interconnected devices is at least 2 and preferably at least 4 and even more preferably at least 8. The number of parallel conduction paths in the solder joints is at least 2 per contact pad and preferably at least 4. The diameter of each of the elongated solder wires is preferably less than 50 gm and most preferably less than 10 .mu.m. This is accomplished by repeated elongation deformation of the composite containing solder particles or wires and bundling the resultant wires or rods followed by additional deformation. This process may continue until the final composite form includes wires only a few nanometers thick. The processing or handling of fine wires with diameter less than about 100 nanometers has to be conducted very carefully, desirably in an oxygen-free atmosphere, as they could be pyrophoric.

The slices of the composite medium are then placed between the mating pad surfaces of devices to be interconnected, and soldered, as shown in FIGS. 5 and 6. As is shown in FIG. 5, a slice, 51, of the composite solder medium, such as slice 23 or 44 shown in FIGS. 2 and 4, respectively, is placed between contact pads, 52, of circuit component, 53, and contact pads, 54, of another circuit component or mother board, 55. Solder slice 51, includes solder wires, 56, embedded in the compliant medium or matrix, 57. Appropriate flux may be used to improve the solder wetting.

The as-sliced composite solder of FIG. 2 or FIG. 4 has its matrix surface flush with the cut ends of solder wires, rods, or ribbons. This configuration is not always amenable to easy soldering operation. Therefore, it is desirable to make the ends of the solder wires to protrude, e.g., by slight dissolution of the surface of the matrix if it is made up of dissolvable material, or by addition of solder material onto the exposed ends of the solder wires in the sliced sheet, e.g. by electroplating, electroless plating of solder, or dip-coating in a molten solder bath similarly as in wave soldering, so that the ends of wires, 56, protrude above the surface of the compliant medium or matrix. During the interconnection soldering of the mating devices, the protruding solder will melt and either completely wet the mating surfaces of contact pads, 52 and 54, if the molten solder wetting contact angle is low (e.g., 37Pb-63Sn eutectic solder on Cu surface or 87Sn-8Zn-5In solder on Au-finish surface) or only partially wet the surfaces of these pads if the contact angle is high (e.g., Sn-Zn-In solder on Cu surface). The degree of desired wetting is adjustable by proper choice of solder, pad surface, flux and other parameters.

The subdivided nature of the solder material on each soldering pad is an added beneficial feature. The thin elongated solder geometry allows easier elastic bending than thicker solder joint, and induces a stress state close to plain-stress condition than a plain-strain condition so that higher fracture toughness is exhibited by the solder material. The fact that there are multiple paths for electrical conduction is an added advantage in that the chance of catastrophic failure (no electrical conduction) by the mechanical fracture of the solder joint is minimized. This is especially important for area-array solder interconnection of mating devices where the inspection or repair of solder joints is rather difficult.

Yet another way of creating the high-aspect-ratio solder-containing medium is by using magnetic field alignment in combination with solder-particle fusion. As reported by S. Jin et al in J. Appl. Physics, Vol. 64, p. 6008 (1988), the use of vertical magnetic field creates a laterally-separated, vertical chain-of-sphere arrangement of magnetic particles in a viscous matrix. By coating magnetic particles, 71, see FIG. 7 with solder, mixing solder-coated magnetic particles, 72, with either compliant or dissolvable liquid matrix material, 73, exposing the mixture to a magnetic field and solidifying, curing or semicuring the matrix liquid, a structure, 74, (schematically illustrated in FIG. 7) is obtained. This structure provides an anisotropic, z-direction-only electrical conductivity with in-plane isolation. In order to minimize lateral shorting, the volume fraction of the particles in the matrix is kept preferably less than 40% and even more preferably less than 25%. The solder coating is applied to magnetic particles, such as iron, nickel, cobalt, iron-nickel alloys, or conductor-coated soft ferrite particles, such as to produce a solder-to-iron volume ratio sufficient upon fusion to form a self-supporting solder column. For example, 5 .mu.m thick electroless coating of solder on 7 .mu.m diameter iron particles gives a solder-to-iron volume ratio of about 4:1.

The composite structure of FIG. 7 does not provide continuous high-aspect-ratio solder conduits since the vertically arranged solder particles are only in physical contact. Therefore, the composite structure is heated to either melt or diffusionally sinter the solder particles so that they are fused into a high-aspect-ratio configuration, 81, as illustrated in FIG. 8. This process of fusion can be accelerated by optionally applying compressive pressure (e.g. by weight) or high vertical magnetic field that will try to bring the core magnetic particles closer. After the fusion process, optionally part of the matrix material may be dissolved away forming a fused structure, 82, or alternatively more solder may be added to the ends of each vertical column to make them protrude beyond the matrix surface for easier soldering onto the devices, as discussed earlier. Electroplating or dip-coating in a molten solder bath (such as used in wave soldering) can be utilized for this purpose. The density of individual magnetically aligned solder paths is selected similarly to that in the case of solder wires. Namely, there should be at least two solder paths per pad.

Fused structure 82 is then placed between pads, 91 and 92, of mating devices, 93 and 94, respectively, and solder interconnected as illustrated in FIGS. 9 and 10, using a flux if desired to promote surface wetting. The matrix material can be left on if compliant (such as elastomer), or can be removed if it is dissolvable (such as gelatine), leaving a high-aspect-ratio solder structure shown in FIG. 10.

Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention is not limited, in its broader aspects, to the specific details, representative devices, and illustrated examples shown and described. Accordingly, various modifications may be made without departing from the spirit and scope of the general inventive concept as defined by the appended claims and their equivalents. An example of such modification is the use of the high-aspect-ratio interconnection, such as shown in FIG. 6 or 10 of the drawings, as a regular solder interconnection. In such a case, the solder wires or fused structures are further fused or collapsed after the matrix is removed to form the interconnection. For such use, the aspect ratio of the wire does not need to be as high as in the multiple wire path interconnection. Also, the high-aspect ratio interconnections may be used for construction of multilayer or multi-step interconnected structures utilizing various heights of the inventive medium. For example, chips may be attached in a mother board with a 20 mil solder joint height and then the mother board may be interconnected to another board by a higher, e.g. 100 mil high, solder joints.