Soldering device

Abstract: A device is shown for depositing solder onto a series of conductive surfaces such as the mounting pads of a surface mount integrated circuit board. The device includes a nozzle bead 27 which has an interior bore for receiving an elongate heat source 25. The nozzle head also includes an orifice for receiving solid solder 47 fed within the interior bore to contact the elongate heat source. The interior bore terminates in a solder reservoir for molten solder which is fed within the interior bore to contact the elongate heat source. The molten solder is dispensed through a tip opening 23 to deposit uniform amounts of solder on each pad 13. A source of bleed gas is supplied to the interior of the assembly to protect the component parts and exclude oxygen from the interior of the assembly. A cover gas is also supplied to the solder site to reduce oxidation of the molten solder and reduce the amount of flux required.

Description: The present invention relates generally to soldering devices and particularly to such devices used to deposit solder in a precise manner, as where solder is deposited onto solder wettable contact pads utilized in the semiconductor industry. Solder distribution onto mounting pads for surface mount boards of the type used in the semiconductor industry has generally been accomplished in the prior art through the use of screening processes. In these techniques, art work and screens are fabricated having the solder deposition pattern. Then, a precision alignment process is carried out wherein the solder is screened onto the surface mount pads. The solder paste used for this process requires a substantial cure time and a substantial bake time. Various problems result due to the complexity of the alignment process making the prior art technique relatively time consuming. The prior art technique utilizing screening is further complicated by a requirement that the pattern mixes very fine lead pitch and width surface mount pads along with standard surface mount parts. For example, in the case of tape automated bonding, the pitches vary from about 100 to 500 µm (4 to 20 mils) while, the standard surface mount parts have pitches in the range from about 500 to 1000 µm (20 to 50 mils). Due to the precision nature of the operation, it is common to utilize separate screening steps, one for the very fine lead pitch and width surface mount parts and second for the standard surface mount parts. There is the possibility of damaging the solder deposited in a previous step when multiple screening operations are carried out. Additionally, screening fine line solder represents a problem because the solder paste tends to stick in the openings of the screen as the openings get progressively narrower. The prior art screening systems also presents problems due to the difficulty of performing rework operations. Once a defective part has been removed, there is no commonly available technique for replacing the solder on the board site. While techniques exist to remove the solder after the part has been removed, replenishing the isolated site by screening is not generally feasible. As an alternative to the screening approach, it has been proposed to deposit solder through a nozzle. EP-A-0337149 discloses a solder deposition tool for depositing solder onto a series of conductive surfaces. A heat source is supported within a retaining wall of the tool. Solder is delivered through the retaining wall via a solder feed orifice and is then melted by the heater. In a first embodiment, a melted solder column collects in a reservoir area in the tool head. The shape of the solder tip at the end of the column is maintained using sensors and a pressurised gas, to control the deposition of solder. In a second embodiment, a small volume of melted solder is created as required to collect near the tool tip. A non-wettable blade disturbs the surface tension of the solder to control deposition. SU-A-551857 discloses a device for plating terminals and for soldering pin-conductor type connections for radio components. The device has a heater surrounding a solder reservoir at the solder-feeding end-piece of the device and a covering sleeve mounted on the end piece. An inert gas is fed into the reservoir and the sleeve to prevent oxidation of the solder therein. However because of the high operating temperatures of 340-430°C (650-800 F), nozzles have been found to be subject to degradation. Moreover in order to prevent the formation of oxide skins on the surface of the molten solder amounts of flux are typically applied to the solder site. This has sometimes interfered with the operation of the nozzle in precisely depositing solder and contributed to solder "bridging" defects. It is accordingly an object of the present invention to provide a soldering device which exhibits the advantages of solder placement by nozzle while being less subject than hitherto to the problems referred to above. According to the invention there is provided a soldering device comprising a nozzle head formed of a non-conductive plastics material having a solder feed orifice and having an interior bore sized to receive an elongate heat source for contacting solid solder fed into the interior bore through the solder feed orifice, the interior bore of the nozzle head including a solder reservoir for molten solder, the reservoir terminating in a tip opening for depositing molten solder onto a target surface, characterised in that the nozzle head has bleed gas means including a bleed gas port disposed for supplying a bleed gas to the interior bore of the nozzle head to flow past the elongate heat source within said bore to the tip opening to cool the nozzle assembly and exclude oxygen in the surrounding air from entering the tip opening of the nozzle assembly. Bleed gas means are provided for supplying a bleed gas such as nitrogen to the interior bore of the nozzle head and past the elongate heat source to the tip opening to cool the nozzle assembly and exclude oxygen in the surrounding air from entering the tip opening of the nozzle assembly. The bleed gas means can include a bleed gas port provided in the nozzle assembly which communicates with the interior bore of the nozzle head, the bleed gas port being connected with a bleed gas source. The nozzle head is also preferably provided with at least one extension tube mounted thereon adjacent the nozzle tip. The extension tube is arranged to communicate with an internal passageway provided within the nozzle head. The internal passageway can be connected to an inert gas source for supplying inert cover gas to the site to be soldered. The nozzle head is preferably formed from a non-conductive plastics material. In order that the invention may be well understood, a preferred embodiment thereof will now be described with reference to the accompanying drawings. Figure 1 is a schematic, side view of the nozzle assembly of the invention positioned over a conductive surface to be soldered; Figure 2 is an overall perspective view of the nozzle assembly showing the components thereof in exploded fashion; Figure 3 is a side, cross-sectional view of the nozzle mount of the assembly; Figure 4 is a side, cross-sectional view of the nozzle head of the assembly of the invention; and Figure 5 is an isolated view of the extension tube used with the nozzle head of Figure 4. Figure 1 shows a nozzle assembly of the invention designated generally as 11. The assembly 11 is used to deposit solder onto a selected site which is preferably a conductive surface 13. Preferably, the conductive surface 13 is the solder wettable contact pad of a surface mount integrated circuit board. The nozzle assembly 11 includes a nozzle mount (15 in Figures 1 and 3) which is supportable above the conductive surface 13 to be soldered. The nozzle mount 15 has a length ("1" in Figure 3) and has an internal bore 17 sized to receive an elongate heat source. Preferably, the nozzle mount 15 has a polygonally shaped upper exterior 19, approximately square, and a generally cylindrically shaped lower exterior 21 which together define the overall length ("1"). As shown in Figure 2, the elongate heat source is preferably the heated metal tip 23 of a conventional soldering iron 25. The soldering iron 25 is commercially available and can be, for instance, a nickel, iron or palladium coated copper soldering iron connected to a conventional electrical heating source (not shown). The nozzle mount 15 is also provided with at least one port 14 which communicates the internal bore 17 with a source of bleed gas whereby the bleed gas is fed past the elongate heat source to cool the nozzle assembly and exclude oxygen in the surrounding air from entering the interior of the assembly. The bleed gas is preferably a cool, inert gas such as nitrogen or a noble gas such as helium, argon, etc., or a reducing gas such as forming gas. Most preferably, the bleed gas is nitrogen fed to the port 14 at a feed rate on the order of 0.1m³h⁻¹ (5 SCFH) during the time periods when actual soldering is not occurring. The nozzle assembly 11 also includes a nozzle head 27 which has an upper, generally cylindrical interior bore (29 in Figure 4) which joins a lower, converging interior bore 31 provided with a solder feed orifice 33 connected by a slanting internal passageway 35 which passes through the body 37 of the nozzle head 27 to a source of solid solder being fed to the passageway. As shown in Figure 1, the internal passageway 35 can be connected by a tube 39 to a solder wire feed assembly 41. The feed assembly 41 can include, e.g., a motor driven gear wheel 43 and a pressure wheel 45 which are used to deliver a known length of wire solder 47 to the orifice 33. Advancement of the wire solder 47 can be precisely controlled through the use of a stepper motor 49, as will be appreciated by those skilled in the art. Solder fed through the orifice 33 melts on the heated metal tip 23 of the soldering iron and flows to the bottom of a reservoir (81 in Figure 4) formed by the converging interior bore 31 of the nozzle head 27 to form a small solder volume. Mounting means are provided for removably mounting the nozzle head 27 to the nozzle mount 15. As shown in Figures 2 and 3, the mounting means can conveniently comprise a threaded member, such as set screws 51, 53 which are received within mating threaded bores 55, 57 provided in the nozzle head 27. The set screws are engageable with the nozzle mount lower exterior 21 to removably secure the nozzle head 27 to the nozzle mount 15. The nozzle mount 15 can be supported in any convenient fashion over the conductive surface 13 to be soldered. In the embodiment of Figure 1, the nozzle mount 15 is secured to a circular attachment plate 59 by means of mating bores 61, 63 (Figure 2) which are adapted to receive engagement means such as screw connectors. The attachment plate 59 is, in turn, supported by means of a yoke assembly 65 (Figure 1) from a robot assembly 67. It will be understood, however, that the nozzle assembly 11 could be supported in a variety of fashions, depending upon the particular soldering application. Preferably, the nozzle mount 15 and nozzle head 27 are formed from a non-wettable, plastic material. A preferred material is the polyimide sold under the trade name VESPEL by E.I duPont deNemours & Company. This material is lightweight, easily machinable and, to some extent chemically resistant. However, the material is degraded rapidly at high temperatures for long periods of time. At operating temperatures above 260 to 320°C (500-600 F), dimensions, thermal conductivity and other critical properties of the VESPEL material change. In order to maintain proper operating temperatures for the soldering iron 25 and to protect the surrounding nozzle mount 15, an insulating sleeve (69 in Figure 3) is preferably received within the internal bore 17 of the nozzle mount 15 to separate the heated metal tip 23 of the soldering iron 25 from the plastic components of the nozzle assembly. As shown in Figure 3, the internal bore 17 of the nozzle mount 15 is defined by generally cylindrical sidewalls, the internal diameter of the sidewalls decreasing from an upper extent 71 to a lower extent 73 to form an internal shoulder 75. The insulating sleeve 69 has a stepped external diameter sized to be received upon the internal shoulder 75 in the nozzle mount interior bore 17. The insulating sleeve 69 is preferably formed from a high temperature machinable ceramic material having low thermal conductivity and high temperature resistance. A suitable material is commercially available under the trade name MACOR. As shown in Figures 1 and 2, the attachment plate 59 supports the nozzle mount 19 and includes a mounting collar 77 for mounting and orienting the soldering iron 25 along a vertical axis 79 coincident with the internal bore 17 of the nozzle mount and the interior bore 29 of the nozzle head 27. One or more soldering iron centering screws are received in threaded bores 80, 82 for properly aligning the heated metal tip 23. The assembled nozzle head and nozzle mount support the heated metal tip 23 of the soldering iron 25 with the heated metal tip 23 contacting the insulating sleeve 69 rather than the plastic material of the nozzle mount. As shown in Figure 4, the converging interior bore 31 of the nozzle head 27 terminates in a solder reservoir (generally illustrated at 81) for molten solder which is fed within the lower, converging interior bore 31 from the orifice 33 to contact the heated metal tip 23 of the soldering iron. The converging interior bore 31 terminates in a tip opening 83 for depositing solder from the solder reservoir 81 onto the soldering site. A replaceable insert assembly 85 is received within the tip opening 83 of the nozzle head 27. The insert assembly 85 includes a generally cylindrical, wettable wall region 87 for contacting the molten solder passing from the solder reservoir 81 out the tip opening 83. Preferably, the insert assembly 85 includes an outer ring 89 of non-wettable material and an inner ring 91 of wettable material which is sized to be press-fit within the outer ring 89. The nozzle tip opening 83 is generally cylindrically shaped and is sized to receive the insert assembly in press-fit fashion. As best seen in Figure 4, the tip opening 83 decreases in internal diameter to form a stop or shoulder region 93 to facilitate positioning of the replaceable insert assembly 85. The wettable ring 91 can be formed from, e.g., nickel, while the outer ring 89 can be of a plastic similar or identical to the plastic of the nozzle head 27. As also shown in Figure 4, the body 37 of the nozzle head 27 can also be provided with one or more internal passageways 95. These passageways are located within the wall section of the body 37 between the interior bore 31 and the exterior of the nozzle head 27. Passageway 95 includes an inlet opening 97 provided in the upper section 19 of the nozzle head 27 which communicates by means of angled passageway 95 with an outlet opening 99 in the lower portion 21 of the nozzle head. Preferably, a generally cylindrical extension tube 101 is received within a counter bored opening 103 which connects the tip region 105 of the nozzle head 27 with the outlet opening 99 of the internal passageway 95. The extension tube 101 includes a circular, side opening 107 in the region of the nozzle tip 109. It will be understood that, in some applications, the extension tube 101 will be a straight piece of tubing having an open end in the region of the nozzle tip 109, rather than having the circular, side opening 107. In the embodiment of the tool shown in Figure 2, twin nozzles 104, 106 located at the front of the nozzle assembly are used to supply an inert cover gas during soldering operations. One or more of the internal passageways 95 can also be connected to a source of soldering flux by means of a supply tube (110 in Figure 1), whereby flux supplied through the extension tube opening 107 can be applied to a solder site adjacent the tip opening 83. Preferably, a plurality of passageways and extension tubes 101 (Figure 2) are spaced about the circumference of the tip opening 83 in order to merge the application of inert cover gas and flux at the solder application site. In operation, the nozzle assembly 11 is used to deposit solder onto solder wettable contact pads 13, with solder being deposited in substantially uniform amounts on each pad. The reservoir 81 of the nozzle head comprises a plenum for the solder fed through the orifice 33 and melted by the heated tip 23 of the soldering iron 25. After a sufficient amount of solder is fed, a small ball of molten solder becomes extended below the nozzle tip. The nozzle tip 109 is supported over the soldering site and maintained at a sufficient, predetermined distance above the substrate so that contact between the pad and nozzle tip does not occur. The wettable ring 91 and soldering tip 23 provide a predetermined surface tension which causes a discrete amount of solder to be deposited upon the soldering site as the assembly 11 is moved over the substrate. During actual soldering operations, selected ones of the extension tubes 101 and internal passageways supply an inert cover gas to the solder application site. During those time periods when solder is not being applied, an inert bleed gas is flowed through the interior of the nozzle assembly to protect the components thereof and exclude oxygen from the nozzle interior. An invention has been provided with several advantages. The nozzle assembly of the invention merges flux and solder in the vicinity of the nozzle tip to prevent contamination of the molten solder and reduce the amount of flux needed. One or more internal passageways and extension tubes permit flux and/or an inert cover gas to be provided at a focal point adjacent the nozzle tip where solder is being applied. The extension tubes can be used to apply flux to the soldering site before and after the soldering process. By flowing an inert bleed gas through the interior of the assembly, the useful life of the assembly is prolonged. Incidences of solder bridging are also reduced.

Claims: A soldering device comprising a nozzle head (27) formed of a non-conductive plastics material having a solder feed orifice (33) and having an interior bore (31) sized to receive an elongate heat source (23) for contacting solid solder fed into the interior bore (31) through the solder feed orifice (33), the interior bore (31) of the nozzle head including a solder reservoir (81) for molten solder, the reservoir terminating in a tip opening (83) for depositing molten solder onto a target surface (13), characterised in that the nozzle head has bleed gas means including a bleed gas port (14) disposed for supplying a bleed gas to the interior bore of the nozzle head to flow past the elongate heat source within said bore to the tip opening to cool the nozzle assembly and exclude oxygen in the surrounding air from entering the tip opening of the nozzle assembly. A device according to claim 1 for depositing solder onto a series of conductive surfaces, including a nozzle mount (15) supportable above the conductive surface (13) to be soldered, the nozzle mount (15) having a length and having an internal bore (17) sized to receive an elongate heat source, said nozzle head (27) being removably mounted on the nozzle mount (15) and having an interior bore (29) sized to receive a portion (21) of the length of the nozzle mount, thereby allowing the elongate heat source (23) to extend at least partly within the interior bore of the nozzle head for contacting solid solder fed within the interior bore through the solder feed orifice (33), said bleed gas means being arranged to bleed gas to the internal bore of the nozzle mount past the elongate heat source, and to the interior bore of the nozzle head to cool the nozzle assembly and exclude oxygen in the surrounding air from entering the nozzle assembly. A device according to claim 1 or claim 2, further including cover gas means (95, 97, 99) for supplying a cover gas to the target surface site to be soldered to inhibit oxidation of molten solder during deposition. A device according to claim 3 wherein the cover gas means includes at leat one internal passageway (95) provided in the body of the nozzle head (27) isolated from the interior bore (31) thereof. A device according to claim 4 wherein there are a plurality of said internal passageways (95) for supplying cover gas and soldering flux to the solder application site. A device according to claim 2 including an insulating sleeve (69) received within the internal bore (17) of the nozzle mount (15) for separating the nozzle mount (15) from the elongate heat source (23).

Inventors: HAYDEN TERRY F, HICKS CHRISTOPHER A, LEDERMANN PETER G, NGUYEN ALVIN D, STEINBACH STEPHEN C, YU STANLEY K, HAYDEN, TERRY F., HICKS, CHRISTOPHER A., LEDERMANN, PETER G., NGUYEN, ALVIN D., STEINBACH, STEPHEN C., YU, STANLEY K.
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1991
Main classification: B23K3
IPC codes: H05K3, B05B1, H01L21, B23K3