Forming metal preforms and metal balls

A process and tools for forming and/or releasing metal preforms, metal shapes and solder balls is described incorporating flexible molds or sheets, injection molded metal such as solder and in the case of solder balls, a liquid or gaseous environment to reduce or remove metal oxides prior to or during metal (solder) reflow to increase surface tension to form spherical or substantially spherical solder-balls.

CROSS REFERENCED TO A RELATED APPLICATION

This application is cross referenced to U.S. patent application Ser. No. 13/371,431 filed on even date herein entitled “FORMING CONSTANT DIAMETER SPHERICAL METAL BALLS” which is directed to an apparatus and method for forming a plurality of constant diameter spherical metal balls utilizing injection molded metal and unconstrained metal reflow.

BACKGROUND

The present invention relates to tools and processes for forming metal preforms, metal shapes and metal balls useful in microelectronics and more specifically, to injection molded solder and flexible molds which constrain some metal reflow to form metal performs, metal shapes and solder balls which are released or extracted from molds and collected.

BRIEF SUMMARY OF THE INVENTION

In accordance with the present invention, a method for forming metal balls is described comprising filling cavities in a flexible mold with molten metal in an environment inducing surface tension sphering and removing the metal balls from the cavities by mechanical means.

The present invention further describes a method for forming metal shapes comprising:

selecting a substrate capable of bending to a predetermined radius of curvature;

forming a plurality of cavities in the substrate material;

the plurality of cavities having a first shape including cavity walls, the cavities providing a change of shape from the first shape to a second shape upon bending the substrate to a predetermined radius of curvature;

filling the plurality of cavities with molten metal;

cooling the molten metal in said plurality of cavities to form a solid metal of a first shape in respective cavities of the plurality of cavities;

heating the solid metal in the respective cavities in a flux or an atmosphere to reduce or substantially reduce any metal oxides on surfaces of the solid metal;

reflowing the solid metal in the respective cavities;

cooling the reflowed metal to form a solid metal of a second shape in the respective cavities; and

bending the substrate to said predetermined radius of curvature to form the second shape of the plurality of cavities to cause a break in the contact of the solid metal of a second shape in the respective cavities from portions of the respective cavity walls whereby the solid metal of the second shape is released from contact in the respective cavities.

Apparatus for transferring metal solidified in blind cavities in an upper surface of a first flexible tape comprising:

first and second spaced apart rollers for directing a lower surface of the first flexible tape there over;

a third roller positioned between the first and second rollers for supporting the lower surface of the first flexible tape,

fourth and fifth spaced apart rollers for directing a lower surface of a second flexible tape thereover, the second flexible tape having an upper surface having adhesive regions thereon;

the fourth and fifth rollers positioned to position the second flexible tape adjacent the first flexible tape;

a sixth roller positioned between the fourth and fifth rollers to press against the lower surface of the second flexible tape to press the upper surface of the second flexible tape against the upper surface of the first flexible tape;

means for moving the first flexible tape over the first through third rollers in a first direction and at a first speed, and

means for moving the second flexible tape over the fourth through sixth rollers in the first direction at the first speed whereby adhesive regions on the second flexible tape adhere to the metal solidified in the blind cavities in the first flexible tape and wherein the second flexible tape with the metal passes over the fifth roller and separates from the first flexible tape which passes over the second roller.

The present invention further describes apparatus for transferring metal solidified in cavities in an upper surface of a flexible tape comprising:

first and second spaced apart rollers for directing a lower surface of the flexible tape there over;

the second roller positioned to guide the upper surface of the flexible tape to face towards ground,

a transducer coupled to the first flexible tape after the first and second rollers for vibrating the flexible tape whereby the metal in the cavities are vibrated loose from contact and moves away from the flexible tape with the aid of the vibration and gravity.

Apparatus for transferring metal solidified in through-hole a flexible tape comprising:

first and second spaced apart rollers for directing a surface of the flexible tape thereover;

a pressurized gas actuator positioned for directing pressurized gas on a surface of the flexible tape and through-hole cavities whereby the metal in the through-hole cavities is loosened and moves away from the flexible tape with aid of the pressurized gas.

DETAILED DESCRIPTION

Referring now to the drawing,FIG. 1shows flexible substrate, mold or sheet12which may be planar or flat comprising a polymer such as a polyimide, polyamide, a glass, a metal, a graphite or a ceramic capable of withstanding 400° C. and which can bend or flex elastically about an axis from a planar or flat position to a predetermined radius of curvature, for example, in the range from infinity to plus or minus 0.025 mm or from 4t to greater than 4t where t is the mold or sheet thickness. Flexible mold12may have an upper surface14and a thickness in the range from 0.012 mm to 12.7 mm. Flexible mold12may have a plurality of cavities16which may be arranged in a two dimensional array18such as a rectangular or square array with rows and columns spaced apart in the range from 0.002 nm to 12.7 mm, respectively. Plurality of cavities16may have a first shape20shown inFIG. 2such as a hemisphere, a flattened hemisphere, or other shape including cavity bottom walls26and side walls28. Plurality of cavities16may change elastically from a first shape20to another shape such as a second shape at times flexible mold12is bent or flexed to a predetermined radius of curvature.

FIG. 2is a cross-section view along the lines2-2ofFIG. 1. InFIG. 2, plurality of cavities16have a bottom wall26and are blind cavities i.e. not a through-hole cavity since there is no opening at the bottom. Plurality of cavities16are spaced apart on a center-to-center spacing in the range from 0.002 mm to 12.7 mm to enable flexible mold material between cavities16to physically support or hold first shape20of cavities16when flexible mold12is not flexed. Plurality of cavities16may have an aspect ratio, depth to width ratio, in the range from ⅓ to ⅔ where the shape is a hemisphere or flattened hemisphere. The depth of cavity16may be in the range from ⅓ to 1 and more preferably ½ the depth of the final metal (solder) ball. The diameter of plurality of cavities16may be in the range from 0.0025 mm to 0.89 mm.

FIG. 3is a cross-section view along the lines2-2ofFIG. 1after cavities16in flexible mold12have been filled with molten solder32by way of injection molding solder using tool34. Tool34which has a reservoir36of solder sweeps solder along upper surface14into cavities16and leaves an upper surface33of molten solder32in plurality of cavities16coplanar with upper surface14of flexible mold12. If molten solder32is in an oxygen environment, a metal oxide or oxide material38will form on upper surface33. Oxide material38may be a uniform layer with a smooth surface and may be thicker than 0.01 μm. Molten solder32is cooled below the melting temperature of molten solder32to form solid solder32′. Molten solder32may be selected from the group consisting of Sn, In, Sn—In, Sn—Pb, Sn—Au, Sn—Ag, Sn—Cu, Ag—Bi, Sn—Ag—Cu, Sn—Ag—Bi, Sn—Ag—Cu—Zn, Sn—Ag—Cu—Bi, Sn—Ag—Cu—Pd, Sn—Ag—Cu—Ti, Sn—Ag—Cu—Al, Sn—Ag—Cu—Sb, Sn—Ag—Cu—Ce, Sn—Ag—Cu—Ge, Sn—Ag—Cu—Mn, Sn—Ag—Cu—La and combinations thereof.

FIG. 4is a cross-section view along the lines2-2ofFIG. 1after blind cavities in flexible mold12are filled with molten solder32as shown inFIG. 3and after reflow of solid solder32′by way of heating in a liquid or gaseous flux environment that eliminates oxide material38on upper surface33. A flux is a reducing agent designed to help reduce or return oxidized metals to their metallic state. One gaseous flux suitable for solder is formic acid (HCOOH) diluted with nitrogen in a bubbler. Another gaseous flux may be forming gas which is a mixture of hydrogen (H2) and an inert gas usually nitrogen (N2) that works well to reduce oxides on metal surfaces33to form metal and water. H2may be in the range from 8 to 25 volume percent in an inert gas. Another gaseous flux may be hydrogen (H2) at 100 percent. A liquid flux, if applied, is removed in a subsequent cleaning step. By raising the temperature of solid solder32′ above the melting point and with oxide material38removed or eliminated, the surface tension of molten solder32will increase and reflow to form spherical, near spherical, or substantially spherical balls42in plurality of cavities16as shown inFIG. 4. As shown inFIG. 4, substantially spherical balls42remain in contact with the bottom wall26or side walls28of plurality of cavities16. Flexible mold12should comprise materials which are hydrophobic and which solder does not wet. While solder does not wet glass or polyimide, solder does form a bond with glass or polyimide that is surprisingly difficult to break causing near spherical solder balls. Further, the formation of or retention of solder oxides should be minimized, since solder oxides make spherical balling of solder much more difficult due to reduced surface tension. Further, metal oxides of solder on surface43of spherical or near spherical balls42may bond to bottom wall26and sidewalls28of cavities16causing near spherical solder balls.

The uniform size, volume or dimensional tolerance of spherical, near or substantially spherical metal balls42such as the volume and diameter corresponds to the uniform size of cavities16in the flexible mold12which determines the volume of metal in substantially spherical metal balls42. The molten metal in the cavities16and reflow of the molten metal is in contact and constrained by the cavity walls26and28. Cavity walls26and28where contacted is a constraining force on the molten metal and any metal oxides thereon. The constraining forces by cavity16and gravity will act to deform metal balls42and are counteracted by the force or magnitude of the molten metal surface tension.

The cross section or diameter dimensions of substantially spherical metal balls42may be different or out of round from one another and within a respective metal ball42depending on the cross section taken. The spherical metal ball out of round dimensions of substantially spherical metal balls42are affected by tolerances of the cavity16dimensions (mentioned above), surface tension of the molten metal, supporting cavity wall26and28contact area (constraining force) with ball42and or metal oxide skin, whether cavity walls26and28are hydrophobic or hydrophilic or under other contact forces, weight of ball42and specific gravity of metal ball42. Surface tension of metal ball42is influenced by metal composition, any metal oxides in or on the surface43of near or substantially spherical metal balls42and flux. The uniform size or volume tolerance of spherical or substantially spherical metal balls42may be less than 16 percent and preferably less than 7 percent. The diameter or cross section dimensional tolerance of spherical, near or substantially spherical metal balls42may be less than 5 percent and preferably less than 2.5 percent.

FIG. 5is a cross-section view along the lines2-2ofFIG. 1as shown inFIG. 4after flexing mold12to extract spherical or near spherical balls42. A mechanical means such as a roller, cylinder or actuator may bend or flex flexible mold or sheet12to a predetermined (positive and/or negative) radius of curvature as shown by arrows46and48. The shape of plurality of cavities16change elastically due to bending flexible mold or sheet12which breaks the contact of spherical or near spherical balls42with bottom wall26and side walls28of cavities16thereby releasing solder balls42. Flexible mold12may be turned upside down during flexing to use the force of gravity to separate spherical balls42from flexible mold12. Once surface43of spherical or near spherical balls42are broken free of contact or bond with bottom wall26and side walls28, various methods may be used to collect the loose spherical or near spherical balls42into a container including gravity as mentioned above, vacuuming, blowing and/or sweeping.

FIG. 6shows a flexible, substrate, mold or sheet52which may be planar or flat having an upper surface54, a lower surface55and a plurality of cavities56. Plurality of cavities56may be arranged in a two dimensional array58such as a rectangular or square array with rows and columns spaced apart in the range from 0.002 mm to 12.7 mm, respectively. Plurality of cavities56may have a first shape60shown inFIG. 7having an upper opening62in surface54and a lower opening64in lower surface55to form through-holes through flexible mold52. Flexible mold52may be a sheet of polyimide of constant thickness capable of withstanding 400° C. and which can bend or flex to a predetermined radius of curvature in the range from plus or minus infinity to 0.025 mm or 4t to greater than 4t where t is the mold or sheet thickness. Plurality of cavities56may change elastically from a first shape60to another shape such as a second shape at times flexible mold52is bent elastically to a predetermined radius of curvature.

FIG. 7is a cross-section view along the lines7-7ofFIG. 6. Plurality of cavities56are shown with through-holes having upper opening62which is circular having a diameter shown by arrow61and lower opening64which is circular having a diameter shown by arrow69. Lower opening64is smaller than upper opening62. Plurality of cavities56have sidewalls66which are shown as a truncated portion of a cone and/or may be cylindrical. Cavities56may be space apart on a center-to-center spacing in the range from 0.002 mm to 12.7 mm to enable flexible mold material there between to adequately support first shape60of plurality of cavities56when not being flexed. Plurality of cavities56may be formed with an ultra violet laser (UV) and/or eximer laser and may have a wall taper of 4° to 10° shown by arrow53between a vertical axis57and reference line70.

FIG. 8is a cross-section view along the lines7-7ofFIG. 6after molten solder32is injected into respective cavities56, for example, by injection molding solder and solidified in a low oxygen and N2or other inert gas environment63. Flexible mold52is shown positioned on upper surface65of substrate64. Substrate64provides support to flexible mold52and a temporary lower surface to cavities56to permit cavities56to be filled by way of injection molding solder with molten solder32from solder tool34positioned on upper surface54of flexible mold52. Solder tool34moves in a direction to the right shown by arrow35inFIG. 8. Housing67is positioned over flexible mold52and functions to maintain a low oxygen and N2or other inert gas environment63above cavities56and molten solder32. With a low oxygen atmosphere in the range from 10 to 1000 ppm, the upper surface of molten solder32is free or substantially free of oxide material especially at the location where upper surface54and sidewall66meet, join or intersect at the edge of opening62of cavities56. The edge of opening62is initially in contact with molten solder32but is free of metal oxide permitting molten solder32to pull away from upper surface54and sidewall66and ball up due to the surface tension of molten solder32. As shown inFIG. 8, molten solder32in cavities56have a rounded upper surface68as opposed to a flat surface33shown inFIG. 3. Molten solder32is cooled below the melting point of molten solder32to solidify in cavities56as solid solder32′.

FIG. 9is a cross-section view along the lines7-7ofFIG. 6after molten solder32is injected into cavities56and solidified in environment63as shown inFIG. 8and after reflow in a gas environment71of formic acid, forming gas of for example nitrogen (N2) and hydrogen (H2) or 100 percent H2. Molten solder32in flexible cavities56inFIG. 8are shown as spherical or near spherical solder balls72in contact with sidewalls66inFIG. 9. Housing74is shown mounted on the upper surface75of substrate76. Housing74functions to provide a low oxygen atmosphere in the range from 10 to 1000 ppm to prevent metal oxides from forming on solder balls72and/or to remove or substantially remove metal oxides from the surface of solder balls72by means of gas environment71which may comprise formic acid, forming gas of for example nitrogen (N2) and hydrogen (H2) or 100 percent H2. Formic acid, expressed as HCOOH, may be provided by injecting nitrogen into a bubbler containing formic acid which is released through an outlet port to provide a gas environment71comprising nitrogen enriched with formic acid. Spherical or near spherical solder balls72may have no or substantially no metal oxide skin which if present is a uniform layer with a smooth surface on solder balls72where the thickness of the layer is less than 1 micron. Solder balls72should have no or substantially no metal oxide skin so there is minimum adhesion between solder balls72and sidewall66.

FIG. 10is a cross-section view along the lines7-7ofFIG. 6after molten solder32is injected into cavities and solidified in environment63as shown inFIG. 8, after reflow in a gas environment71of formic acid, forming gas of for example hydrogen (H2) and nitrogen (N2) and 100 percent hydrogen (H2) as shown inFIG. 9and after blowing gas77on through-holes on lower side55of flexible mold52to loosen and extract spherical solder balls72. InFIG. 10, housing74and substrate64shown inFIG. 9have been removed. Air or gas77such as N2is blowing at lower surface55of flexible mold52and into lower openings64of cavities56as shown by arrows78to easily loosen and remove spherical or near spherical solder balls72from contact with sidewalls66and from through-hole cavities56.

FIG. 11is a schematic view of conveyor belt or tape100and an adhesive tape102which are brought together for extraction or transfer of non-reflowed metal (solder)104,106and108from blind cavities110,112and114on conveyor belt or tape100to adhesive tape102. Conveyor belt or tape100passes over rollers116,118and120. Conveyor belt or tape100also has empty blind cavities122and124. Conveyor belt or tape100moves in a clockwise direction shown by arrow126. Adhesive tape102moves in a counter clockwise direction as shown by arrow130. Adhesive tape102passes over rollers132,134and136. Adhesive tape102is pressed against non-reflowed metal (solder)104by roller134which may be soft or compressible to apply pressure over a larger area against non-reflowed metal (solder)104in cavity110in conveyor belt100and roller118which may be hard or non-compressible. Non-reflowed metal (solder)104was loosened by passing over roller116. Conveyor belt or tape100and adhesive tape102move in the same direction and at the same speed when passing between rollers118and134. Adhesive tape102adheres to an upper surface of non-reflowed metal (solder)104and extracts non-reflowed metal (solder)104from blind cavity110as conveyor belt or tape100separates from adhesive tape102via rollers120and136. Previously transferred non-reflowed metal (solder)138from blind cavity122and non-reflowed metal (solder)140from blind cavity124are shown adhered to adhesive tape102.

FIG. 12is a schematic view of a conveyor belt or tape144and a vibration transducer146for extraction of non-reflowed metal (solder)147-153from blind cavities155-161, respectively, on conveyor belt or tape144. Conveyor belt144passes over rollers164and166and moves in a clockwise direction shown by arrow168. Non-reflowed metal (solder)148and150are initially loosened when passed over rollers164and166as conveyor belt or tape144moves. Vibration transducer146moves up and down transverse to or against conveyor belt or tape144as shown by arrow170to loosen and remove non-reflowed solder172from blind cavity174and non-reflowed metal (solder)176from blind cavity178as conveyor belt or tape144moves passed vibration transducer146. Non-reflowed solder172and176move away from conveyor belt or tape144as shown by arrow180due to vibration or motion from vibration transducer146and by gravity.

FIG. 13is a schematic view of a conveyor belt or tape184and pressurized gas186for extraction of non-reflowed metal (solder) preforms188-194from through-hole cavities196-202, respectively, on conveyor belt or tape184. Conveyor belt184passes over rollers204and206and moves in a clockwise direction show by arrow208. Non-reflowed metal (solder) preforms189and191are initially loosened when passed over rollers204and206as conveyor belt or tape184moves. Pressurized gas186impinges against through-hole cavity208in conveyor belt or tape184as shown by arrow212to loosen and remove non-reflowed metal (solder)214from through-hole cavity208and non-reflowed metal (solder)216from through hole cavity218. Non-reflowed metal (solder)214and216move away from conveyor belt or tape184as shown by arrow220due to pressurized gas186and by gravity.

InFIGS. 1-13, the structures therein are not drawn to scale.

While there has been described and illustrated an apparatus and methods for forming metal (solder) preforms, metal shapes and metal (solder) balls using flexible molds with either blind or through-hole cavities, injection molded metal such as solder, and in the case of solder balls, a liquid flux or a gas environment to reduce or remove metal oxides prior to or during metal or solder reflow to induce surface tension sphering of metal or solder balls, it will be apparent to those skilled in the art that modifications and variations are possible without deviating from the broad scope of the invention which shall be limited solely by the scope of the claims appended hereto.