Method of injection molding solder and application thereof

Abstract: An apparatus and method are described for injection molding solder mounds onto electronic devices. The apparatus has a reservoir (46) for molten solder which is disposed over a cavity in an injection plate (34). The injection plate is disposed over a mold (32) having an array of cavities therein into which solder is injection molded. The mold is disposed over a workpiece (50), such as a semiconductor chip or a semiconductor chip packaging substrate. The cavities in the mold are aligned with electrical contact locations on the chip or substrate. The workpiece is heated and the molten solder is forced under gas pressure into the cavity (68) in the injection plate (34) disposed above the array of cavities in the mold. The molten solder is forced into the array of cavities in the mold. The injection plate (34) is advanced to slide over the mold to wipe away the excess solder above the mold at a plurality of wiping apertures in the injection plate. The injection plate is further advanced to a location having a nonsolder wettable surface at which location the injection plate is removed. The mold is then removed to leave solder mounds disposed on the workpiece. The workpiece can be a semiconductor chip, a semiconductor chip packaging substrate or a dummy substrate onto which the injected molded solder adheres such as a polymer layer to form a carrier substrate for a solder mound array which can be subsequently transferred to a substrate such as a semiconductor chip or a semiconductor chip packaging substrate. The apparatus and methods of the present invention can be integrated into an automated manufacturing system for depositing an array of solder mounds onto a substrates.

Description: The present invention is directed to a method for injection molding solder. In the semiconductor electronics industry, semiconductor chips can be mounted in a flip-chip-configuration with solder balls commonly known as C4 therebetween. The C4 provides an electrical connection and bond between a chip contact location and a substrate contact location. As chip circuit densities increase there is an increased demand for chip I/O. Moreover, if a very large chip is used having a very high density of circuits, a very large number of I/O are required for electrical communication with the chip. The I/O provide signal, power and ground to the chip. As the chip I/O increases, C4s which are solder bonded to the chip contact locations must be smaller and more closely spaced. Moreover, the height of each C4 must be distributed within a relatively small height tolerance to insure that all of the chip contact locations will be electrically interconnected to a corresponding substrate contact location through the C4 bonded therebetween. Analogous requirements for electronic connections and solder technology also occur at several levels in electronic packaging. This includes solder connections between 1st and 2nd level packages, or 2nd and 3rd level packages. One example is as follows. To connect between a few chips and a large printed circuit board, there is a small printed circuit board, called an interposer. This requires solder connections both between the chip and interposer, and between the interposer and large circuit board. Another example is as follows. To connect between one or a few chips and a large printed circuit board, there is a ceramic package. This uses wire bonds from chip to package, and an area array of solder connections between package an printed circuit board. This ceramic package and chips is sometimes referred to as a hybrid monolithic module. An exemplary list of workpieces suitable for the present invention are: a chip, a substrate, a small printed circuit board, a large printed circuit board, various other electronic packages for various levels, or other electronic device requiring solder connections. Exemplary workpieces include components made of various materials, including but not limited to: semiconductor, ceramic, organic, metallic, crystalline, polycrystalline, polymeric, glassy or amorphous materials. It is conventional to deposit C4s onto contact locations on a chip or substrate by disposing over the chip or substrate a mask having apertures therein which correspond to the locations of the contact locations on the chip or substrate. The combination of mask and chip are placed into an evaporator which is evacuated. The evaporator contains source materials such as a Pb and Sn. The source is heated to create a vapor of material which deposits upon the mask, filling the apertures therein, and deposits onto the exposed contact locations. Unfortunately, evaporation of C4s do not create very uniform solder heights. Therefore, when there is a very high density of C4s there is a substantial likelihood that a portion of the C4s will not properly electrically interconnect a chip and substrate. Furthermore, solder is evaporated and condensed everywhere in line of sight of the source, leaving orders of magnitude more solder than desired at the pad locations. Depending on the low vapor pressure and slow deposition rate of the component metals, evaporation may be very time-consuming, and may expose the workpiece to a long time at elevated temperature. The process duration is further increased by the time to load and unload the process vacuum chamber. Injection molding solder has distinct toxicological advantages compared to sputtering or evaporating solder through a stencil. Many solders include lead or other metals which could cause human and environmental damage if they are not contained. IBM TECHNICAL DISCLOSURE BULLETIN, vol. 29, no. 9, February 1987, NEW YORK US, pages 3817 - 3818, regards an apparatus for injection molding of solder. The known apparatus comprises a reservoir for the solder and a mold having a plurality of cavities. An output orifice of the reservoir is in communication with the mold to permit solder to be injected into the plurality of cavities when applying pressure to the solder. In the cavities solder mounds are formed. After removing excess solder from the mold, the mold with the solder mounds is applied to a surface of a wafer and heated. The mold is removed and the solder mounds remain on the wafer. GB-A-2 138 339 regards applying and securing solder-coated or solderable spheres to solderable or solder-coated terminal pads. Electronic components are located one in each of a number of recesses in a support and a cover plate, with apertures aligned with the terminal pads, is provided to overlie the components on the support. The cover plate shall be not wettable by molten solder. with the cover plate uppermost and in registration with each component spheres are applied to the cover plate which is dimensioned such that one sphere enters each aperture and contacts a terminal pad. Heat is applied to melt the solder on the spheres and/or pads. After cooling, the cover plate is removed, allowing the components with attached spheres to be removed from the support. The spheres provide a stand-off for the components from a circuit board. During injection molding, almost all the solder is directly and usefully applied to the substrate or other product. There is very little wasted solder, and that is contained in the injection molding system, and can be immediately reused for injection molding. Also solder vapors are fully enclosed and minimized. By sharp contrast, sputtering or evaporation spreads a vastly larger amount of solder throughout a process chamber. Only a small fraction penetrates the mask, and is usefully applied to the substrate or other product. To clean the process chamber and to control the vast excess solder, raises questions of worker safety during cleaning, and environmental safety during disposal or recycling. It is an object of the present invention to provide a method of depositing solder mounds having substantially uniform height onto a substrate. The method of the present invention does not require that the workpiece be placed in a vacuum. The method of the present invention allows deposition of solder onto a substrate without requiring that the workpiece be placed in a vacuum. Moreover, using the method of the present invention, a solder bond having higher aspect ratio (mound height/mound diameter) can be easily fabricated by using an injection mold of appropriate shape. Moreover, quite surprisingly it has been found that when molten solder is injected directly onto a polymer surface, the solder mound when solidified adheres to the surface of a polymer. Therefore, a solder mound transfer support surface can be fabricated from a polymer film without an additional adherent layer. An array of solder mounds can be disposed onto the polymer surface using the apparatus of the present invention. The polymer film can then be disposed over a surface having contact locations thereon aligned to the array of solder mounds on the transfer film. Heat is applied which results in the solder mounds wetting the contact locations. The solder is cooled to solidify it. The polymer film is then peeled away leaving the solder mounds on the substrate contact locations. The polymer films peels away since the adhesion of the solder mounds to the polymer film is less than the adhesion of the solder mound to the contact location. These and other objects, features and advantages will become more apparent from the following, more detailed description, the appended drawings and the appended claims. The broadest aspect of the present invention is a method and apparatus, whereby molten solder is injected into a mold which is disposed in contact with a surface to which the molten solder adheres. The mold is removed leaving a solder mound adhering to the surface. In a more particular aspect of the present invention, the solder mound has a predetermined shape. In another more particular aspect of the present invention the solder is solidified before the mold is removed. In a more particular aspect of the present invention, the mold has a plurality of discrete mold cavities arranged in a predetermined array. In another more particular aspect of the present invention, the injection plate is slidably mounted onto a support surface to permit filling the plurality of mold cavities with a substantially uniform amount of molten solder. In another more particular aspect of the present invention the mold plate is formed from a material which is not wetted by molten solder. In another more particular aspect of the present invention, the mold plate is formed from a single crystal silicon which is etched so that the discrete mold cavities have a crystallographically determined shape such as a truncated pyramid. In another more particular aspect of the present invention, molten solder is deposited to directly adhere to a polymer surface. Fig. 1 is a perspective view at a downward angle of and example of an apparatus for carrying out the method of the present invention. Fig. 2 is a schematic side cross-sectional view through workpiece support structure 30 of Fig. 1. Fig. 3 is a top view of an example of a mold plate 32 of Fig. 1 for carrying out the method of the method of the present invention showing the solder cavity array. Fig. 4 shows a cross sectional view along lines 72-72 of Fig. 1 through plate 30, mold 32, injection plate 34, solder reservoir tube 46. Fig. 5 is a top view of injection plate 34 of Fig. 1. Fig. 6, 7 and 8 show a top view of the injection plate on the mold in three different positions. Fig. 9, 10 and 11 show the side view of Fig. 4 for the three different positions of Figs. 6, 7 and 8 respectively, only for the region corresponding to the position of the solder cavity array. Figs. 12, 12.5 and 13 show method steps to separate the solder from the mold to form pyramidal shaped solder mounds on a substrate. Figs. 14-18 show method steps for removing the solder array mold when the molten solder is not easily removed from the mold cavities. Figs. 19-26 show method steps for injecting solder into cavities in a solder mold wherein the mold is used as a transfer means to transfer the solder mound array to a substrate. Fig. 27 shows an automated manufacturing system for disposing an array of solder mounds onto a substrate. Fig. 1 is a perspective view at a downward angle of an example of an apparatus which may be used to carry out the method of the present invention. Apparatus 2 has a base 4 on which there are mounted elongated support columns 6 and 8. End 10 of column 6 is fixedly mounted to surface 12 of base 4 and end 14 of column 8 is fixedly mounted to surface 12 of base 4. Base 4 is shown to have circular shape and columns 6 and 8 are at the extreme ends of a diameter of the circle. This design is exemplary only. Columns 6 and 8 project perpendicularly from surface 12. Cross bar 16 is attached at one end thereof to end 18 of column 6 and at the other end thereof to end 20 of column 8. Structures 6, 8 and 16 form an arch. Disposed under the arch is a support platform 22 which is supported by four legs, only 3 of which, 24, 26, and 28, are visible in the view of Fig. 1. Support structure 22 contains a heater which is used to heat the workpiece as described herein below. Disposed on heater base 22 is workpiece support structure 30 which is shown in cross section in Fig. 2. Disposed on the workpiece support structure is mold 32 which is described herein below. Disposed on mold 32 is injection plate or slide 34, the function of which is described herein below. Between cross bar 16 and injection plate 34 there is disposed a means 36 for pressing the injection plate 34 against the mold 32 and correspondingly the mold 32 against the workpiece support plate 30. Means for pressing 36 consist of a means for attachment 38 to the cross beam 16 and a coil spring 40 which is compressed to press against injection plate 34. Disposed on injection plate 34 is solder reservoir tube 46 which is wrapped with heater coil 44 for heating the solder contained in reservoir tube 46 to the molten state. In contact with reservoir 46, shown for example in the form of a tube, there is gas supply tube 74 which at its top end receives gas through hose 73 and which through a temporary O-ring joint at its bottom end is in communication with reservoir tube 46 to permit gas pressure to be applied at orifice 48 of tube 46. The gas is used to force molten solder in reservoir tube 46 into cavities in mold 32 as described herein below. Fig. 2 shows in cross section substrate support plate 30 having workpiece 50 disposed in a workpiece nest 52 in workpiece support plate 30. The nest 52 height 54 corresponds to the height 56 of the workpiece 50. The nest 52 is to provide an easy means of aligning the workpiece to the apparatus 2 of Fig. 1. The nest 52 is not necessary. The workpiece 50 can be disposed on the surface 58 of the workpiece support plate 30. For simplicity in the remaining Figs. the nest 52 is not shown. Fig. 3 shows a top view of an example of a mold 32 may be used for carrying out the method of the present invention. Mold 32 has a plurality of apertures 60 on the top surface 62 thereof. These aperture extend through to the bottom surface 64 where they end in aperture 66 having larger dimension than aperture 60. The cavity 68 through mold 32 formed by aperture 60 and 66 is shown to have a pyramidal structure. If the material of the mold 32 is silicon these pyramidal structures can be fabricated by pyrocatachol solution which anisotropically etches along various crystallographic planes in <100> silicon. The cavities 68 form the injection mold cavities for the apparatus of the present invention. Cylindrical shape cavities can be formed in metal such as molybdenum by commonly known selective etch methods. Fig. 4 is a side cross section view along the axis 72-72 in direction 70 of Fig. 1. Fig. 4 only shows workpiece support plate 30, mold 32, injection plate 34 and a schematic representation of solder reservoir tube 46 and gas supply tube 74 connected to gas supply hose 73. Through these means compressed air or nitrogen is injected with a low pressure regulator into orifice 48 of tube 46 and temporarily sealed during injection by O-ring 75. Since injection pressures are low, typically a 0-103.4 hPa (0-15 psi) regulator such as manufactured by Modern Engineering Co. will suffice. Injection plate 34 preferably includes Pyrex (TM) or quartz. In the embodiment of Fig. 4, injection plate 34 is shown as a combination of glass plate 76 and polymer film 78 which is disposed on bottom surface 80 of glass plate 76. Fig. 5 shows a top view of injection plate 34. The film 78 has three regions 86, 88 and 90. The dashed lines 92 and 94 represent the imaginary boundaries between these regions. Solder reservoir tube 46 of Fig. 4 and Fig. 1 is inserted into an etched or machined aperture 96 in glass plate 76. The solder reservoir is shown in Fig. 5 to have circular cross section having interior wall 98. As seen in Fig. 4, at the base of the solder reservoir 100 there is an orifice 102 which extends from the base 100 of the solder reservoir tube 46 to the bottom side 80 of glass plate 76. Solidified solder is placed into the solder reservoir tube 46 and by means of heating element 44 of Fig. 1 (which may typically be a Thermofoil (TM) heater made by MINCO) solidified solder is heated to molten solder in the solder reservoir. Because of surface tension and the small cross sectional area of aperture 80, the solder will not spontaneously penetrate into cavity 68. Instead, the liquid solder is forced into cavity 80 by gas pressure means 74. Referring again to Fig. 5, injection plate 34, which may be used for carrying out the method of the present invention has two groups of cavities 80 and 82. These cavities are formed by regions of the polymer layer 78 being selectively removed. The preferred polymer layer is a polyimide, for example, a thin sheet of Kapton tape from which are cut the regions to form cavities 80 and 82. These regions are cut out after the Kapton is adhered onto the bottom surface 80 of glass plate 76. An alternative way of making the cavities 80 and 82 on glass plate 76 is to dispose on one side 80 of glass plate 76 a photo resist material which is selectively developed to form the pattern of cavities 80 and 82. The glass plate can be placed in a glass etchent, such as 50% hydroflouric acid to etch the cavities 80 and 82 directly into the glass plate. These techniques for forming cavities 80 and 82 are exemplary only. It is not necessary for layer 78 to be a polymer. It can be a solder non-wettable metal layer or a dielectric material. Plate 76 does not have to be a glass, it can be another material. Preferably, all surfaces which come into contact with the molten solder should not be wet by the solder, so that the molten solder does not adhere thereto. Figs. 6, 7 and 8 show a top view of the injection plate 34 in three positions corresponding to positioning of the three regions 86, 88 and 90 over the array of cavities 68 in mold 32 as shown in Fig. 4. Figs. 9, 10 and 11 respectively, correspond to a side view of the injection plate 34 showing only that part of the injection plate which is disposed over the array of cavities 68. In Figs. 6, 7 and 8, the array of cavities is shown as rectangle 103 having therein a plurality of dots 104 with each dot 104 corresponding to a cavity 68. In Fig. 6, region 80 which is defined by polygonal shape 81 is disposed over the solder cavity array 103. Referring back to Fig. 4, injection plate 34 is shown in a position corresponding to Fig. 6 and Fig. 9. In Fig. 4, the solder has not yet been injected into cavity 80. The molten solder 106 is forced under gas pressure through orifice 102 into cavity 80 through top opening 60 (as shown in Fig. 4 and Fig. 3) of cavity 68 to fill region 80 and plurality of cavities 68 as shown in Fig. 9. When the plurality of cavities are filled, injection plate 34 is moved in direction 108 as shown in Fig. 7 and Fig. 10. Edge 83 of cavity 80 wipes across the top apertures 60 of cavities 68 wiping away excess molten solder from the top surface 62 of mold 32. As the plurality of apertures 82 pass over the top 62 of the mold 32, the edges 110 (shown in Fig. 4) of the plurality of apertures 82 wipe the top surface of the mold and thereby wipe excess molten solder from the top aperture 60 of the cavities 68. Excess solder is accumulated in cavities 82. Fig. 7 and Fig. 10 show the plurality of apertures 82 disposed above the cavity array 104. In passing from Fig. 7 to Fig. 8 and correspondingly from Figs. 10 and 11, the injection plate 34 is moved in the direction of arrow 112 shown in Fig. 8 and Fig. 11 so that region 90 of the injection plate is now disposed over the cavity array 104 as shown in Fig. 8 and Fig. 11. As the injection plate is moved from the positions in Fig. 6 to that of Fig. 7 and to that of Fig. 8, the support plate 22 of Fig. 1 is kept heated so that the solder remains molten as the injection plate is moving over the cavity array 104. After Fig. 8 and Fig. 11 occurs, the apparatus is cooled so the solder solidifies in shapes determined by the cavities 68 of mold 32. As shown in Fig. 12, injection plate 34 is lifted up in direction of arrow 120. The mold 32 is preferably made of a non-solder-wettable material, such as silicon etched as described herein above, so that the mold can be lifted off in the direction 120 leaving solidified solder mounds 122 as shown in Fig. 13 disposed on surface 124 of workpiece 50. It is necessary to separate the mold away from the solder and workpiece. Therefore, Fig. 12.5 shows pneumatic pressure 123, with higher pressure applied to surface 62 and apertures 60 of mold 32, and lower pressure applied to workpiece 50. The solder mound array, which is disposed on surface 124 can be disposed onto an array of solder wettable contact locations on the workpiece 50. The workpiece 50 can be a semiconductor chip or a semiconductor chip packaging substrate. Quite surprisingly, it has been found that when molten solder is directly disposed onto a polymer surface, in particular a polyimide surface such as a Kapton&registered. (Dupont) surface, there is some degree of adhesion between the solder mounds 122 and the surface 124 of the polymer. The solid solder mounds 122 on a polymer film 50 can be used as a solder mound transfer means. The vertical pull-strength has been measured in excess of 250 grams for a larger 4 mm diameter solder preform, which translates to about 1.79 x 10 6 N/m 2 (260 lbs per sq. inch). Alternatively, as shown in Fig. 12, workpiece 50 and mold 32 can be a composite. Workpiece 50 can be made of a rigid solder non-wettable material such as silicon or molybdenum upon which is deposited a dissolvable, patternable material to form mold layer 32. This is processed to produce cavities. An example of such a material is Riston (TM) or other polymers. After injection and solidification, layer 32 can be dissolved away. (Thus Fig. 12.5 does not apply to this embodiment.) More generally, the mold may be sacrificial materials which are dissolvable, friable or otherwise removable. This leaves Fig. 13 with solid solder mounds 122 on solder non-wettable workpiece 50 which can be used as a solder transfer means. Certain generalizations follow from the above description. The above method could equivalently be realized by keeping a fixed injection plate and moving the mold and workpiece. The movement may be linear or circular. In general it is adequate to have various forms of relative motion between the injection plate 34 and mold plate 32. Furthermore, the injection plate and mold plate may remain fixed, and instead introduce a gate-valve, air-knife, or other means to cause wiping or shear on the top surface 62 of the mold 32. Figs. 14, 15, 16, 17 and 18 shows a sequence of steps for removing the solder from the apertures 68 of mold 32 when mold 32 is made of a material which is has side walls which are insufficiently smooth to permit the solidified solder mound to be easily extracted therefrom. Fig. 14 is essentially the same as Fig. 12. In Fig. 15 the workpiece 50 in combination with the mold 32 is flipped 180 degrees with respect to Fig. 14. The surface of the mold 32 which is not disposed in contact with workpiece 50 is disposed in contact with a solder non- wettable material 127, which is disposed on a heat source not shown. Flux 126 is disposed about the periphery 128 of the workpiece 50. As heat is applied the flux wicks into the interface 130 between mold bottom surface 64 and workpiece 50 surface 124. The flux reduces the surface oxide on the remelted solder mounds 136. As shown in Fig. 16, by arrow 138, the workpiece 50 rises away from the mold 32 and the solder non-wettable material 127 which is typically molybdenum. Since the solder is now molten and since it adheres to the surface 124 of workpiece 50 and does not adhere to mold 32, the remelted solder mounds 136 are pulled out of the cavities 68 of mold 32 even if the side walls of the cavities are not smooth. When the remelted solder mounds 136 are fully extracted from the cavities 68 they ball up due to surface tension into spherical shapes as shown in Fig. 17 as 140. As shown in Fig. 18, the mold 32 and non-wettable surface 127 are removed leaving the workpiece 50 as shown in Fig. 18 having a plurality of sphere like solder mounds 142 which are cooled to form solid solder mounds. Figs. 19 through 26, inclusive, show a manufacturing process using the apparatus described herein above to dispose solder mounds onto a substrate such as a semiconductor chip or semiconductor chip packaging substrate. Here the mold-filling steps in Figs. 19-22 are largely decoupled from the solder-transfer steps in Figs. 23-26. Fig. 19 shows a mold 32 having a plurality of apertures 68 therein as described herein above. Mold 32 of Fig. 19 has a bottom surface 144 which has a cavity 146 formed therein adapted for engagement with workpiece 50. Fig. 20 shows anvil piece 160 which is a non-wettable temporary substitute for workpiece 50 of Figs. 9-13. Also shown is mold 32 with the cavity 146 disposed over and in alignment with anvil piece 160 which is in contact with the lower apertures 66 of cavities 68. Fig. 20 shows injection plate 34 with molten solder being injected into cavity 80 as described herein above to fill cavities 68 of the mold 32. In Fig. 21, as described herein above, injection plate 34 is slid over the mold 32 so that wiping apertures 82 pass thereover. As shown in Fig. 22 injection plate 34 and mold 32 have been removed. In Fig. 20 and 21 anvil piece 160 is preferably glass. At the step shown in Fig. 21, an optical observation can be made since all components are optically transparent except for the molten solder. Layer 76 of the injection plate 34 is preferably glass. Layer 78 of the injection plate 34 is preferably a transparent polyimide. The mold 32 is of an opaque material. Since anvil piece 160 is preferably glass, if light is shined from side 152 of anvil piece 160, an optical observation can be made from side 154 of injection plate 34. If light passes there through, it will have to pass through an unfilled aperture 68. If any of the apertures are unfilled, the process flow can return to the step of Fig. 20 to inject again molten solder 106 to fill the unfilled cavity 68. The solder is cooled leaving solidified solder 122 filling cavities 68. In Fig. 22 anvil piece 160 has been removed as shown by arrow 156. In Fig. 23, the mold 32 containing the solidified solder 122 filling the cavities 68 is disposed and aligned over a workpiece 50 having a layer of flux 162 thereon. As shown in Fig. 24, the workpiece 50 is inserted into cavity 146 of mold 32. Thus, the flux layer 162 is brought into contact with aperture 66 of cavity 68 which opens up into cavity 146 of mold 32. The plurality of cavities 68 containing solder 122 can be aligned to predetermined locations on the workpiece 50 such as contact pads on a semiconductor chip. The solidified solder 122 in the apertures 68 of the mold 32 is remelted. The flux layer 162 results in the reduction of a surface oxide on the solder and on any contact pads on the workpiece 50. The molten solder then wets the surface of the contact locations which can be a gold surface. As shown in Fig. 25, the mold 32 rises from the workpiece 50 in the direction 170. When the molten solder comes out of the mold cavities 68, it beads up into a spherical shape as molten solder mounds 140. The solder is allowed to cool to form solid solder balls 142 on workpiece 50 as shown in Fig. 26, which is shown with mold 32 already removed. Some applications need free preformed solder mounds, especially balls with accurately controlled volume. One application is solder balls to rework solder arrays produced by less reliable processes. These balls can be produced with high quality and low cost by the method of this invention, with a slight modification of that shown in Fig 19 - 26. After Fig 22, orient the mold so its second surface (with the wide end of the cavities) is on top. In contrast to Fig 23, do not apply a workpiece. Instead during the process steps shown in Figs. 24-26 the balls form free under the influence of flux, resting upon the wide apertures of the cavities. Once the balls have solidified, and the flux has been cleaned away, then apply an alternative workpiece with low-tack adhesive (such as plastic tape with cured rubber adhesive) which will safely carry an array of individual balls. Alternatively, pour loose balls from the mold into a container. Fig. 27 shows an automated manufacturing process using the methods described herein above with reference to Figs. 19-26. On conveyor track 402 there are disposed a plurality of molds 32 each of which is abutted against an adjacent mold as shown at 406. Each mold has a plurality of cavities 68. Each mold has a central accommodating cavity 146 as described herein above with reference to Fig. 19. Conveyor belt 402 is shown to circulate in a clockwise direction 412. conveyor belt 414 contains a plurality of anvil pieces 160 as described herein above with reference to Figs. 20-22. The anvil pieces provide a surface against which to dispose the mold while injecting molten solder into the mold cavities. Accommodating cavity 146 of mold 32 has dimensions to accommodate the anvil pieces on loop 420. Loop 420 rotates in a counter clockwise direction as indicated by arrow 422. As loop 418 rotates in a clockwise direction, and as loop 420 rotates in a counter clockwise direction a plurality of anvil pieces 160 come into engagement with the accommodating cavities 146 of the molds 32. The cavities 68 extend from the base 147 of the accommodating cavity 146 through to the opposite surface 62 of the molds 32. At location 427 an injection molding apparatus as described herein above and schematically shown as element 34 is disposed over a anvil piece 160 engaged with a mold 32. Apparatus 34 forces molten solder into the plurality of cavities 68 of mold 32 at location 427 as described with reference to Figs. 20 & 21. As circulating 418 and 420 loops advance, the anvil piece 160 and mold 32 are pulled away from each other as shown in position 429 and described with reference to Fig. 22. Also, as described herein above, the anvil piece can be a glass substrate to which the solder mounds do not adhere strongly. Between locations 427 and 430 the apertures 68 of molds 32 are filled with solidified solder 122. Loop 432 has a conveyor belt 434 which picks up a workpiece 50 as indicated by arrow 438 at location 440. Conveyor 434 rotates in a counter clockwise direction so that surface 51 of workpiece 50 engages with the accommodating cavity 146 of mold 32 and so that surface 51 of workpiece 50 contacts surface 147 of the base of the accommodating cavity 146 at location 444 as described with reference to Figs. 23 & 24. Between locations 444 and 430 heat is supplied. Flux is either supplied as described with reference to Fig. 15, or the workpiece 50 has on surface 51 a layer of flux 162 as described with reference to Figs. 23-25. The reflowed solder 140 adheres to solder wettable contact locations on surface 51 which correspond with the locations of the solder filled cavities 68. Between locations 430 and 446, molten solder 140 solidifies into solder balls 142. At location 446 the workpiece 50 is pulled away from the mold 32. As described with reference to Fig. 26, the solidified solder is pulled out of the mold as solder balls 142 shown at location 452. At location 454 the workpiece 50 is removed as indicated by arrow 456 from conveyor 434 to be a finished workpiece 450 having a plurality of solder balls 142 on the surface thereof. In general, this apparatus and method is applicable to a wide variety of solder alloys and melting points. For each alloy, the materials of the apparatus must be chosen to withstand its melting temperature, and to have suitable non-wetting / wetting properties. This invention has been practiced successfully with many alloys including: SnPb (eutectic 183 C), BiSn (eutectic 138 C), BiPb (eutectic 124 C), InSn (eutectic 118 C), BiIn (eutectic 109 C), InBi (eutectic 72 C), BiPbInSn (liquidus 69 C). Many embodiments have been repeatedly achieved. A partial list is as follows. The apparatus and method of Fig 6 through Fig 11 was used to inject solder into a molybdenum mold. Then the apparatus and method of Fig 14 through Fig 18 was used to transfer this to a workpiece that was a small ceramic wiring substrate with metal terminal pads. The result was a nicely uniform array of good quality solder balls upon pads upon ceramic. The solder was 63% Sn, 37% Pb, and the geometry was: ball diameter = 0.250 mm, pad diameter 0.150 mm, array pitch = 0.450 mm, mounds in array = 1738. An apparatus for carrying out the method illustrated in of Fig 1 through Fig 13 was used with a silicon mold illustrated in with pyramidal cavities. The workpiece was a polyimide sheet with gold metal pads. The result was a uniform array of solder pyramids with good adhesion to the pads. The geometry was approximately: pyramid width = 0.175 mm, pyramid depth = 0.175 mm, width of injection aperture at apex of pyramid = 0.012 mm, array pitch = 0.225 mm, mounds in array = over 1000. In another example, the workpiece was an uncoated piece of polyimide sheet, and the same mold was used. The resulting solder pyramids had significant adhesion to the workpiece, with good uniformity and quality. An array of free solder balls was fabricated by the apparatus and method of Fig 19 through Fig 26, with modifications as described in the above specifications. This array was picked up by a tape with low tack adhesive. The same silicon mold was used. The resulting balls had notably uniform solder volume. In summary, an apparatus and method for injection molding discrete mounds of solder in predetermined locations on a workpiece has been described. The workpiece, in particular, may be a semiconductor chip or a semiconductor chip packaging substrate having on a surface thereof an array of electrical contact locations onto which solder mounds, as described herein, are to be disposed. An automated apparatus and process for depositing the solder mounds onto such substrates has been described using intermeshing conveyor belt loops to provide a completely automated apparatus and process for disposing solder mounds onto electronic devices.

Claims: A method of injection molding comprising: providing a workpiece having a workpiece surface; providing a reservoir (46) for a solder (106) having an output orifice (102); providing a mold (32; 332; 332') having at least one cavity (68; 368; 368') extending from a first surface to a second surface of said mold (32; 332; 332'); disposing said second surface of said mold onto said workpiece surface; providing an injection plate (34) having a first injection plate surface, a second injection plate surface and an injection cavity (80) at said second injection plate surface; disposing said second injection plate surface on said first surface of said mold (32; 332; 332'); heating said solder to become molten solder (106); bringing said output orifice (102) of said reservoir (46) into fluid communication with said injection cavity (80); disposing said injection cavity (80) at a first location over said at least one cavity (68; 368; 368'); applying pressure to said molten solder (106), forcing said molten solder into said injection cavity (80) and therefrom into said at least one cavity (68; 368; 368') of said mold, whereby a molten solder mound (122; 136; 140; 142; 380; 306; 308) is formed in said at least one cavity (68; 368; 368') on said workpiece surface; moving said injection plate (34) from said first location to a second location, whereby molten solder on said first surface of said mold (32; 332; 332') in the vicinity of said at least one cavity (68; 368; 368') of said mold is wiped away by an edge (83) of said injection cavity (80); and removing said mold (32; 332; 332') from said workpiece (50; 350) surface leaving a solidified solder mound on said workpiece surface. mound (122; 136; 140; 142; 380; 306; 308) formed in said cavity (68; 368; 368') of said mold is left on said surface. The method of claim 1, characterised in that said solder (106) contains a material selected from the group consisting of Pb, Sn, Bi and In. The method of claim 1 or 2, characterised in that said workpiece (50; 350) is selected from the group consisting of a semiconductor chip, a semiconductor chip packaging substrate, a packaging substrate for plural semiconductor chips, a polymer body, a ceramic body, or a metallic body. The method of claim 3, characterised in that said workpiece is a polymer body (50) comprising solder mounds (122; 136; 140; 142; 380; 306; 308) adhered directly to said polymer body. The method of claim 3 or 4 characterised in that said polymer body (50; 350) is of a polyimide. The method of anyone of the claims 1 to 5, characterised by heating said molten solder (106) in said at least one cavity (68; 368; 368'). The method of anyone of the claims 1 to 6, characterised in that said injection plate (34) is provided with at least one wiping cavity (82) comprising an edge (110) at said second injection plate surface wiping away molten solder (106) at said first surface of said mold in the vicinity of said at least one cavity, when said injection plate (34) is moved from said first to said second location.

Inventors: FERENCE THOMAS GEORGE, GRUBER PETER ALFRED, HERNANDEZ BERNARDO, PALMER MICHAEL JOHN, ZINGHER ARTHUR RICHARD, FERENCE, THOMAS GEORGE, GRUBER, PETER ALFRED, HERNANDEZ, BERNARDO, PALMER, MICHAEL JOHN, ZINGHER, ARTHUR RICHARD
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1993
Main classification: H01L21
IPC codes: B23K1, B23K3, B29C45, H05K3