Precision solder transfer method and means

Abstract: This invention employs a carrier (10) upon which a thin conductive film (12) has been applied. The conductive film is of a metallic material which exhibits a surface energy such that it is not readily wetted by solder. A patterned mask (14) is disposed on the conductive film, with the mask having openings which expose selected areas (16, 18) of the conductive film. Solder is deposited in the mask openings and is weakly adherent to the exposed areas of the conductive film. The carrier is then disposed over and in re­ gistration with conductive land areas (20, 22) of a circuit carrier (24), such that the solder in the mask openings is aligned with the land areas of the circuit carrier. Subsequently, the carrier and circuit land areas are brought into contact, heated, the solder bonded to the lands, and the carrier is lifted away for subsequent reuse. In another species of the invention, a thin film layer of solder is directly deposited on and weakly adherent to a flexible base layer. Subsequently, tool means which registers with the areas to be soldered, presses the base layer/solder combination against the areas to be coated with solder, thereby causing the solder to adhere to the underlying circuit configuration.

Description: This invention relates to soldering apparatus and more particularly to means for delivering precisely metered amounts of solder to microminiature circuit configurations. It is particularly difficult to apply solder coatings with consistent thicknesses and configurations to circuit boards having conductive lands with close edge-to-edge spacings (e.g. 508 µm (20 mils) or less). Solder screening is not satisfactory as non-uniform amounts of solder often remain after the screen is removed. When solder is reflowed to attach a chip to the lands, a "solder bridge" can occur between adjacent lands causing a potentially catastrophic failure. It has been found increasingly difficult to use a screening process where solder having a thickness of less than 50,8 µm (.002 inches) is either desirable or required. These thicknesses are employed for attaching tape automated bonding (TAB) packages to circuit boards and insure that no solder bridging occurs. TAB packaging is seeing ever wider usage as it can be handled in an automated fashion and is one of the least expensive packaging configurations. It is important therefore, that even with the stringent dimensional requirements placed upon the soldering systems by TAB packages, that the cost of solder application be minimized. The prior art has attempted to solve the precision solder application problem via a number of routes. EP-A-0273131 describes a method and a solder transfer member for applying discrete bodies of solder of predetermined size to the leads of a component for subsequent surface mounting to a substrate. The transfer member is a plate having a non-wetted surface, for example titanium, with an array of cavities matching the component lead pattern, each having a volume corresponding to the desired amount of solder to be applied to the corresponding lead. The method includes placing solder paste on the transfer member and filing the cavities by wiping the plate surface. The component is placed on the transfer member, with the leads contacting the solder paste in the cavities. Reflow of the solder paste bonds to each lead a discrete body of solder having a precisely determined size. To limit wicking of solder on the leads, selective masking may be performed by applying a water soluble mask coating to the leads and removing the mask from selected areas by placing the component against a surface charged with water before placing the component on the transfer member. US-A-4 209 893 provides a solder carrier strip wherein the strip is punched and solder preforms are pressed into the punched areas. This technique, while usable for relatively large circuit configurations, is not usable for the miniature TAB packages for the reason that mechanical punching is too imprecise. US-A-4 354 629 and US-A-4 484 704 both describe a system for joining flat or ribbon cables to connectors. Those patents teach the use of a pair of polymeric webs with a continuous solder preform packaged therebetween. One of the webs has a series of windows disposed in it. The windows are registered over conductors to be soldered, with pressure and heat being applied to cause solder reflow. The partitions between adjacent windows prevent solder bridging. Here again, this technique is suitable for relatively large size conductor/circuit board arrangements; however it is not appropriate for microminiature circuit configurations. For instance, the continuous layer of solder underneath the window divisions would tend, upon melting, to create solder bridging if closely spaced lands were being tinned. Accordingly, it is an object of this invention as claimed to provide a precision solder transfer method particularly suitable to microminiature package configurations. It is another object to provide a precision solder transfer technique which is adapted to employ manufacturing techniques commonly employed for circuit boards and semiconductor devices. The invention provides a solder transfer device as defined in claim 1 and a solder transfer method as defined in claim 5. The present invention employs a carrier upon which a thin conductive film has been applied. The conductive film is of a metallic material which exhibits a surface energy such that it is not readily wetted by solder. A patterned mask is disposed on the conductive film with the mask having openings which expose selected areas of the conductive film. Solder is deposited in the mask openings and is weakly adherent to the exposed areas of the conductive film. The carrier is then disposed over and in registration with conductive land areas of a circuit carrier, such that the solder in the mask openings is aligned with the land areas of the circuit carrier. Subsequently, the carrier and circuit land areas are brought into contact; heated; the solder bonded to the lands; and the carrier is lifted away for subsequent reuse. For a better understanding of the present invention, together with further objects, advantages and capabilities thereof, in the following preferred embodiments of the invention are described with reference to the accompanying drawings, in which Fig. 1 is a sectional view of the base layer after a conductive film has been placed thereon; Fig. 2 is a showing of the structure of Fig. 1 after a patterned mask has been placed thereon; Fig. 3 is a side view of an electroplating bath for deposition of solder in the mask openings; Fig. 4 is a side sectional view of the solder transfer web of this invention in registration over a circuit carrier; Fig. 5 is a side sectional view of the circuit carrier after removal of the solder transfer web; Fig. 1 depicts a first embodiment of the solder delivery system of this invention. In one embodiment substrate 10 is a polymeric carrier or tape, preferably comprised of a polyimide which will withstand the operating temperatures encountered with this invention. Preferably, carrier 10 is similar to a 35 millimeter film strip with sprocket holes which enable its forward and/or backward movement under precise control of a sprocket. The polymeric carrier could easily be configured as individual carriers having the desired pattern or as groups of individual carriers in various planar configurations. Carrier 10 is metallized with a thin layer 12 of a metal which will not be wetted by a molten tin/lead solder. In specific, conductive film 12 should exhibit a surface energy such that it cannot experience a strong surface bonding interaction with a solder deposit but should sustain a weak mechanical bond with solder deposited thereon. Metals such as niobium, aluminum and chromium all exhibit this particular requirement. In a preferred embodiment of the invention, both carrier 10 and metallization layer 12 are sufficiently transparent that registration features on an underlying circuit board can be recognized therethrough. A very thin layer of niobium (e.g. on the order of 50 nm (500 Angstroms) thickness) will satisfy this requirement. A preferred method for deposition of the niobium is via sputtering such that a relatively constant thickness film can be produced which is adherent to underlying tape 10. Referring to Fig. 2, a mask 14 has been emplaced over conductive film 12 leaving areas 16 and 18 thereof exposed. Mask 14 allows solder to be selectively deposited onto areas 16 and 18 while preventing its deposition elsewhere. Mask 14 may be either permanent or non-permanent. If it is non-permanent, it may comprise a dry film photo resist which is removed prior to solder transfer. A permanent solder mask is preferred, however, as it is re-useable and less costly in terms of processing steps. The solder mask must be able to withstand solder reflow temperatures (e.g. 350°C) and provide the necessary lithographic tolerances and stability. A preferred mask is a polyimide which is built up to the thickness of the desired solder layer. The mask may be patterned by a variety of known techniques. For instance, the polyimide layer may be deposited, cured and then etched using a normal photoresist mask to clear away the portions where solder is desired to be deposited. Alternatively, a photoresist may be directly placed onto conductive film 12, areas opened to expose the areas of conductive film 12 where solder is not desired and subsequently, a polyimide electro-phoretically deposited onto the exposed conductive areas. As schematically shown in Fig. 3, the assembly of Fig. 2 is placed in an electroplating bath which contains a proper ratio of tin and lead ions such that when electroplating is started, proper proportions of ions are deposited onto the exposed areas of conductive film 12. Preferred plating thicknesses are in the range of 0.0127mm to 0.0635mm (.0005 to .0025 inches) of solder in exposed areas 16 and 18. In Fig. 4, the solder transfer film has been turned over and aligned with fluxed copper lands 20 and 22 on circuit board 24. The alignment of the solder transfer film and the circuit board is straight-forward as the transfer film is transparent and may be aligned automatically with registration features on the underlying circuit board. The transparency feature is useful in the initial setup and adjustment of the mechanisms which controls the relative orientation between the transfer film and the underlying circuit board. Thus, assuming that a sprocketed film is being employed, adjustments can be made to the placement of the circuit board with respect to the film and the registration visually checked through the film. Once the solder transfer film is in place over circuit board 24, the assembly is placed in a reflow unit wherein solder inserts 26 and 28 are brought to the melting point and are caused to adhere to conductive lands 20 and 22. As aforestated, since conductive film 12 is not wettable by solder, no residue is left thereon and the film may be reused. The final circuit assembly, after removal of the solder transfer film is shown in Fig. 5. The solder transfer film may then be replated and used again. The film can be stored and does not require expensive screening equipment. Furthermore, by varying the thickness of polyimide mask layer 14 and controlling the solder plating process, the precise amount of solder to be put in place can be well controlled. EXAMPLE The process shown in Figs. 1-5 was carried out using as a starting material, a Kapton polyimide sheet, .0508mm (0.002 inches) thick (Kapton is a trademark of the DuPont Corporation, Wilmington, Delaware). The material is first sputtered clean in an oxygen atmosphere with a background pressure of ten µm at 500 Watts rf for five minutes. It was then sputter cleaned in argon with a background pressure of 5 µm at 500 Watts rf for five minutes. It was then coated with a layer of Niobium to a thickness of 20 nm (200 Angstroms), using a DC sputtering system. A second Kapton film having a thickness of 0.0254mm (0.001 inches) was coated with an epoxy modified acrylic adhesive, also 0.0254 mm (0.001 inches) thick. The second film was then mechanically punched to yield openings coincidental with lands to be coated. The two films were then hot press laminated at 130°C for 45 minutes at 2,87 10⁻¹ N/mm² (3 tons/sq. ft). Solder plating was then done on a commercially available system using a Solderex 60/40 Sn/Pb plating bath. (Solderex is a trademark of the Sel-Rex Corporation, a division of OMI International, 75 River Road, Nutley, New Jersey 07110). Current density was maintained at 21 mA/sq. cm. for 35 minutes yielding 0.0508mm (0.002 inches) of solder. A suitable board with conductive lands coincident to the solder mask was coated with Alpha 611 RMA (rosin, mildly activated) flux (611 RMA is a trademark of Alpha Metals Inc., 600 Route 440, Jersey City, New Jersey 07304). The solder transfer film was then held by a tool having both vacuum and nozzles capable of delivering heated nitrogen. The solder transfer film was brought into contact with the board to be coated while allowing nitrogen heated to 350°C to blow on the back of the film for 10 seconds. Vacuum was applied by the tool and the solder transfer completed. As an alternative to the second Kapton film, a Vacrel permanent resist film having a thickness of 0.0508mm (0.002 inches) could be applied to the first Kapton sheet with Niobium already in place. (Vacrel is a trademark of the DuPont Corporation, Wilmington, Delaware). The composite could then be exposed, for 10 seconds using a glass mask to coincide with lands to be soldered coated. The films are then allowed to sit for 30 minutes before being developed in a 1,1,1 trichloroethane and then dried with nitrogen. A blanket exposure for 2 minutes followed by a bake at 150°C for 30 minutes will yield a permanent mask from the Vacrel material.

Claims: A solder transfer device comprising: a base layer (10); thin conductive film means (12) of metallic material exhibiting a surface energy such that it is not readily wetted by solder, disposed on said base layer; a patterned mask (14) disposed on said conductive film means, said mask having openings which expose selected areas (16, 18) of said conductive film means; and solder (26, 28) disposed in said openings and weakly adherent to said exposed areas of said conductive film means. The solder transfer device as defined in claim 1 further comprising: means (31) for disposing and registering said solder transfer device over circuit means (24), such that said circuit means are in alignment with said solder pattern and; means (32) for effecting transfer of said solder to said circuit means. The solder transfer device as defined in claim 2 wherein said base layer (10) and conductive film (12) are transparent. The solder transfer device as defined in one of the preceding claims wherein said conductive film (12) is selected from the group consisting of niobium, chromium and aluminum. A solder transfer method with a carrier including a polymeric base layer (10), a thin conductive film (12) of metallic material adherent thereto, which conductive film is non wettable by solder and a mask layer (14) exposing a pattern of discrete areas (16, 18) of said conductive film, the method comprising: disposing said carrier in an electroplating bath solution of lead and tin in proper ratio to comprise solder; and applying a voltage between said conductive film and an electrode in said electroplating bath to electroplate said lead and tin onto said thin conductive film wherever said conductive film is exposed by said mask layer. The method of claim 5 further comprising: registering said carrier over a substrate containing conductive areas, and bringing said solder pattern into contact with said conductive areas; and transferring said solder pattern from said carrier to said conductive areas. The method of claim 6 wherein said transfer step includes the application of heat to effect reflow of said solder pattern onto said conductive areas.

Inventors: BICKFORD HARRY R, GREBE KURT R, KOVAC CAROLINE A, PALMER MICHAEL J, BICKFORD, HARRY R., GREBE, KURT R., KOVAC, CAROLINE A., PALMER, MICHAEL J.
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1989
Main classification: H05K3
IPC codes: B23K35, H05K3, B23K3, H01L21, H05K13