Described herein are an apparatus and a method for providing an inerting gas during soldering. More specifically, described herein are an apparatus and a method for providing an inerting gas during wave soldering using nitrogen and/or other inerting gas.
Work pieces such as printed wiring boards or circuit boards have increasingly smaller wettable surfaces that need to be solder coated and joined. Typical operations for wave soldering involve a soldering bath through which the printed circuit boards or work pieces to be soldered as transported. A conventional automatic wave soldering apparatus includes a flux application, a preheater, and a solder station that are arranged to process printed circuit boards. The printed circuit boards are transported along a moving track or conveyor with their side edges supported by gripping fingers. Flux may be applied by contacting the board with either a foam or spray of flux. The circuit board is then passed through a pre-heating area in order for the flux to reduce the oxides on the metal surfaces to be soldered. The circuit board is then contacted with single or multiple waves of molten solder in an air or in an inerting gas atmosphere.
The inerting gas atmosphere typically is nitrogen (N2) and/or other inerting gases and is oftentimes called N2 inerting. Soldering within an inert gas and/or nitrogen atmosphere minimizes the formation of dross or oxides on the surface of the solder. The presence of a dross and/or an oxide layer is known to cause skips, bridges, or other defects in solder joints. Proximal to the solder waves—which are produced by the wave soldering apparatus during operation—are porous pipes or tubes which run parallel to the solder wave and are used to transport the inerting gas and/or N2 gas to provide a relatively low oxygen atmosphere, particularly underneath the work piece to be soldered.
For lead-free wave soldering, the value of an inerting gas atmosphere comprising N2 is further increased due to the following reasons. The process temperature by using a common lead-free solder is significantly higher than that of conventional tin-lead solder due to the increased melting points of commonly used lead-free solders. The increase in process temperature promotes dross formation. Furthermore, the cost of a lead-free solder is normally much higher than that of the conventional tin-lead solder, and the economy loss associated with solder waste by dross formation is more significant than that of lead-free wave soldering. In addition, the wetting performance of a lead-free solder is intrinsically poor compared with that of the conventional tin-lead solder. Therefore, the quality of the formed solder joints is more sensitive to the state of oxidation on a lead-free solder surface.
It is well known that inerting in wave soldering can significantly reduce dross formation on the molten solder surface. Reducing dross formation not only saves solder material and lessens maintenance requirement, but also improves solder wetting and ensures the quality of the formed solder joints. To apply an inerting atmosphere in an existing wave soldering machine, one common approach is to insert into the molten solder reservoir a cage-like protective housing with diffusers mounted inside. An inerting gas blanket across the solder reservoir can be formed, thus, reducing the tendency of solder oxidation.
The diffusers are commonly made of porous tubes to introduce an inerting gas such as N2 and/or other inerting gases into the soldering station. The porous tubes, however, become easily clogged by solder splashing or flux vapor condensation during the wave soldering process. Once the diffuser tube is clogged, the efficiency of inerting will be largely reduced. Present methods of cleaning the diffuser tubes such as, for example, using ultrasonic baths filled with cleaning solutions, are extremely difficult and time consuming. The cleaning of these tubes must be performed on a regular basis and can cause physical damage to the tubes. To avoid these issues, the diffuser tubes are typically replaced once they become clogged rather than cleaned. This increases the overall cost to the end-user.
Accordingly, in order to promote the application of inerting by N2 and/or other inerting gases in wave soldering, it is desirable that the apparatus, method, or both fulfill at least one or more of the following objectives. First, it is desirable that the inerting apparatus and method reduces N2 or other inerting gas consumption such as, but not limited to, 12 cubic meters per hour (m3/hr) or less to meet the cost benefits of applying the technology. Second, it is desirable that the inerting apparatus and method reduces the concentration of O2 above the molten solder surface such as, but not limited to, 2500 parts per million (ppm) or less. Third, it is desirable that the inerting apparatus and method uses an apparatus that is simple to install and maintain to minimize retrofitting cost. Moreover, it is desirable that the apparatus or method reduces or eliminates the clogging of the porous diffuser tube to ensure a stable and long lasting inerting performance.
The apparatus and method described herein fulfills at least one or more of the above objectives for inerting using nitrogen and/or other inerting gases that may be more cost effective and user friendly than comparable methods and apparatuses presently in use.
In one embodiment, there is provided an apparatus for providing an inerting gas during soldering of a work piece comprising: at least one groove on a bottom surface of the apparatus for placing atop of at least one edge of a solder reservoir comprising molten solder; at least one opening on a top surface of the apparatus through which at least one solder wave emitting from the solder reservoir passes through and contacts the work piece as it travels on a moving track; and one or more diffuser tubes comprising one or more openings in fluid communication with an inerting gas source wherein the apparatus is positioned above the solder reservoir and underneath the work piece to be soldered thereby forming an atmosphere and wherein there is substantially no gap between the work piece to be soldered and an apex of the at least one solder wave. In one particular embodiment, the apparatus further comprises a disposable porous sheath surrounding at least part of the length of at least one of the one or more diffuser tubes. In some embodiments, at least one of the one or more diffuser tubes surrounded by the porous sheath is affixed to the bottom surface of the apparatus so that it is positioned within the atmosphere above the solder reservoir. In the same or other embodiments, an optional cover is placed atop of the apparatus through which the work piece travels therethrough wherein the cover further comprises a vent which is in communication with a ventilation system.
In another aspect, there is provided a method for providing an inerting gas atmosphere during wave soldering of a work piece comprising: providing a wave soldering machine comprising: a solder reservoir having molten solder contained therein, at least one nozzle, at least one pump to generate at least one solder wave from the molten solder bath upwardly through the nozzle; placing an apparatus atop at least one edge of the mouth of the solder reservoir wherein the apparatus comprises at least one opening on a top surface, at least one groove that rests atop the at least one edge of the solder reservoir, and one or more diffuser tubes comprising one or more openings in fluid communication with an inerting gas source, wherein at least part of the length of one of the one or more diffuser tubes is surrounded by a disposable porous sheath and wherein the work piece to be soldered and the top surface of the apparatus define an atmosphere and wherein there is substantially no gap between the work piece to be soldered and an apex of the at least one solder wave; passing a work piece along a path so that at least a portion of the work piece contacts the at least one solder wave emitting through the opening of the apparatus; and introducing an inerting gas through the diffuser tubes and into the atmosphere. In one particular embodiment, at least one of the one or more diffuser tubes surrounded by the disposable porous sheath is affixed to the bottom surface of the apparatus so that it is positioned within the atmosphere above the solder reservoir.
a through 2f provide bottom and cross-sectional views of embodiments of diffuser tubes described herein wherein the pores are in the form of one or more rows of longitudinal slots.
a provides a side view of one embodiment of the diffuser assembly described herein comprising a diffuser tube and a protective porous sheath surrounding at least part of the length of the diffuser tube.
b provides a cross-sectional view of the diffuser assembly shown in
a provides a top view of one embodiment of the apparatus described herein.
b provides a top view of another embodiment of the apparatus described herein.
At least one or more of the objectives in the art are fulfilled by the method and apparatus described herein for inerting protection during soldering. The apparatus and method described herein provides inerting protection during soldering, particularly for those embodiments where significant movement and swirling of the solder during soldering of work pieces such as printed circuit boards and increased oxidation of its surface may occur. It is anticipated that the apparatus and method described herein can be used, for example, to retrofit an existing wave soldering machine. In certain embodiments, the apparatus described herein in operation is placed over the solder reservoir and under the moving track or other conveyance mechanism for transporting the work pieces to be soldered. In certain embodiments, there is substantially no gap between the work piece to be soldered and the apex of the at least one solder wave. In other embodiments, there is a gap between the work piece to be soldered and the apex of the at least one solder wave. The one or more diffuser tubes located within the apparatus are in fluid connection to an inerting gas source such as nitrogen, inert gas (e.g., helium, neon, argon, krypton, xenon, and combinations thereof), forming gas (e.g., mixture of nitrogen and hydrogen comprising up to 5% by weight of hydrogen), or combinations thereof to provide an inerting atmosphere. One objective of the apparatus and method described herein is a reduced concentration of oxygen (O2) in the atmosphere defined by the work piece surface to be soldered and the surface of the molten solder contained within the solder reservoir such as, but not limited to, 2500 parts per million (ppm) or less.
The apparatus and method described herein is intended to be placed atop of a solder reservoir containing molten solder that is maintained at or above (e.g., up to 50° C.) the solder's melting point. The apparatus described herein has an internal volume that sets atop of the solder reservoir thereby defining an atmosphere between the work piece to be soldered that is conveyed in one direction on a moving track above the solder reservoir and the molten solder surface. In certain embodiments, the work pieces are supported by a moving track or conveyor fingers at side edges and the fingers pass through the solder wave(s). In other embodiments, the work pieces are supported on pallets, fixtures, or frames as they are conveyed through the wave soldering machine. The solder reservoir has one or more nozzles therein that project one or more solder waves that are generated by a solder pump. The solder pump is typically a variable speed pump that allows the end user to control the flow of solder from the solder wave(s) and raise or lower the apex or crest of the solder wave(s) to suit processing conditions. The one or more solder waves contact the surface of the work piece to be soldered through one or more openings in the top surface of the apparatus described herein. During this process, the apparatus includes one or more diffuser tubes comprising one or more openings, apertures, slots, perforations, or pores that are in fluid communication with an inerting gas source such as N2 that pass through the interior volume of the tube and out through the opening or pores of the tubes into the atmosphere. In doing so, the under surface, front edge, back edge and side edges of the work piece are uniformly blanketed by the inerting gas as the work piece passes through the solder wave(s).
In certain embodiment of the apparatus and method described herein, the size of the apparatus placed atop the solder reservoir is minimized to intensify the inerting efficiency around the moving solder waves. In this or other embodiments, the static molten solder surface, or area outside of the footprint of the apparatus in the solder reservoir, can be covered by a high temperature material that can withstand the temperature of the molten solder contained within the solder reservoir.
The apparatus and method described herein comprises one or more diffuser tubes comprising an interior volume and one or more openings which can be, but are not limited to, pores, holes, slots, vents, apertures, perforations or other means that allow for the passage of nitrogen and/or other inerting gas within the interior volume of the tube and out through the openings of the tube. The openings may be arranged in one or more lines, may be staggered, or may have any other regular or random arrangement. The openings may be of any suitable size to provide a sufficient flow of inerting gas, and their size may vary based upon a variety of factors, including the flow rate of inerting gas, the size of the interior volume to be inerted, and the dimensions of the diffuser tube, among others. In one particular embodiment, the tubes are porous and comprise an average pore size of about 0.05 to about 0.5 microns (μm), preferably about 0.2 microns, to provide a laminar flow of inerting or N2 gas out of the porous tube. In another embodiment, the tubes comprise one or more parallel rows of slots to provide a laminar flow of inerting or N2 gas out of the diffuser tube. For example, the openings may be arranged in a line along the bottom of the diffuser tube, such that inerting gas flowing out through the openings is directed downward onto the top surface of the solder in the solder reservoir. In another embodiment, the openings may be arranged in two parallel lines offset from the bottom center line of the diffuser by from about 0 to 45° in each direction, or by about 30° in each direction, so as to direct inerting gas outward and down as it flows out from the diffuser tube into the atmosphere above the molten solder in the solder reservoir. In such embodiments, the lines of openings may be separated from one another by about 30° to about 120°, or by about 60° or about 90°. In certain embodiments, the openings may be slots each from about 0.3 to about 1.5 mm in length, preferably from about 0.5 to about 1.0 mm in length. The slots may be separated by from about 0.5 to about 5 mm, preferably by about 1 mm.
In these or other embodiments, the tubes are in fluid communication with an inerting gas source that supplies the inerting gas such as, for example, N2 through the interior volume of the tube and out through the openings or pores of the tubes into the area defined by the surface of the molten solder in the reservoir and conveyed work pieces. Gas flow through the diffuser tube described herein may vary, but is typically in the range of from about 0.5 to about 8 m3/hr.
As previously mentioned, the apparatus described herein comprises a housing that contains an interior volume within which one or more diffuser tubes are located. In certain embodiments, the tubes may be located between the plurality of solder waves, at the board entrance side of the solder reservoir, at the work piece exit side of the solder reservoir, perpendicular to the direction of the solder wave, or combinations thereof. In these embodiments, there is substantially no gap between the surface of the work piece to be soldered and that of the solder waves. In certain embodiments, one or more of the tubes, such as those embodiments wherein one or more of the tubes resides between a plurality of soldering waves, may further comprise one or more disposable porous sheaths or tubes surrounding at least part of the length of the diffuser tube. The sheaths may be formed from any suitable non-hazardous (i.e., ROHS-compliant) material that allows for periodic, simple, and cost-effective removal and replacement of the sheaths as they become coated or clogged with molten solder and/or flux residue during normal operation. For example, the sheaths may comprise a woven fiberglass or fiberglass-type material, alone or in combination with other compositions. In these or other embodiments, the sheath material selected should maintain its integrity at or above the molten solder temperature commonly used in lead-free wave soldering process (e.g., up to about 260° C.). In certain embodiments, at least one of the one or more diffuser tubes surrounded by the porous sheath is affixed to the bottom surface of the apparatus so that it is positioned within the atmosphere above the solder reservoir. The use of one or more disposable porous sheaths to surround at least a portion of the length of the diffuser tube protects the diffuser tube and avoids the problems associated in the prior art with solder splashing, immersion, and/or contacting the diffuser tube with the solder bath. In certain embodiments comprising a center diffuser tube and one or more side diffuser tubes, only the center diffuser tube is at least partially surrounded by a porous sheath as described herein. In alternative embodiments, the center diffuser and one or more of the side diffusers are at least partially surrounded by a porous sheath as described herein.
In one particular embodiment of the apparatus and method described herein, one or more of the plurality of diffuser tubes, such as, but not limited to, the center diffuser tube in between a plurality of solder waves, and/or one or more of the protective sheaths surrounding at least part of such diffuser tubes may comprise a non-stick coating. An example of a non-stick coating is polytetrafluoroethylene (PTFE) coating, which may be found under the trademark Teflon® non-stick coating (Teflon is manufactured by DuPont, Inc. of Wilmington Del.). In these or other embodiments, the non-stick coating selected should maintain its integrity at or above the molten solder's temperature commonly used in lead-free wave soldering process (e.g., up to about 260° C.). In a more particular embodiment, the non-stick coating is comprised of Thermolon™ non-stick coating, an inorganic (mineral based) coating which is manufactured by Thermolon Ltd. of South Korea, and which can maintain its integrity at 450° C. and avoids generating toxic vapor at elevated temperatures. In embodiment wherein the center porous tube resides between one or more pairs of soldering waves, the dissolved flux in the solder reservoir can directly contact the center diffuser surface located between the 1st and the 2nd waves due to a continual dynamic movement of the molten solder. When the liquid flux on diffuser surface is evaporated or thermally decomposed, solid flux residue may be left on the diffuser surface, thus causing diffuser clogging. To remedy this, a non-stick coating or a porous sleeve or sheath or a slotted metal shell coated with a non-stick coating may be applied to the diffuser tube or may cover at least a portion of the diffuser tube. It is believed that the addition of a non-stick coating, or a porous sheath, or a slotted metal shell coated with a non-stick coating to at least one of the porous diffuser tubes may prevent the clogging of the porous tube such as the center tube by solid flux residue. The non-stick coating can also be applied to at least a portion of the internal surface of the apparatus or the internal surface of the top cover, to allow for ease of cleaning.
In yet another embodiment of the apparatus and method described herein, the apparatus further comprises an optional cover mounted on the moving track thereby forming a tunnel for the work pieces to pass therethrough. The optional cover further comprises a ventilation hole that is in fluid communication with the ventilation exhaust of the wave soldering machine that allows for the collection of flux vapor from the atmosphere underneath the cover. In one embodiment, the optional cover is made of a single layer metal cover with a center hole connected to the ventilation exhaust of the machine. In another embodiment, the optional cover is made of double layer metal sheets, and the double layer space is connected to the furnace ventilation exhaust, thus forming a boundary gas trap. In one particular embodiment, the distance between the two layers of metal sheets can range from about ⅛″ to about ¼″. When a work piece or circuit board is passing underneath the cover, flux vapor generated inside the soldering area can be collected through the boundary trap, while air surrounding the solder reservoir can also be trapped in the double layer space, thereby ensuring good inerting performance. For the case when there is no work piece or circuit board on top of the solder reservoir, the inerting gas generated from the plurality of diffusers in the inerting apparatus can be sucked into volume underneath the double layer space of the cover, thereby forming a boundary inerting gas curtain to minimize air from entering into the volume.
Perforations 20 are designed so that gas flow is either narrowly directed, for example, with circular holes as shown in the embodiment of
a, b, c, d, e, and f further illustrate diffuser configurations in which the perforations 20 are in the form of one or more rows of longitudinal holes or slots.
In some embodiments of the present invention, at least one of the one or more diffuser tubes, such as, but not limited to, the center diffuser tube between a plurality of solder waves, can further comprise a protective covering or sheath that surrounds at least a portion of the length of the diffuser tube. An example of such an embodiment is provided in
a and 4b provide top views of embodiments of the apparatus 30 described herein. Referring to
Further benefits of apparatuses and methods according to the present invention include reduction in manufacturing and material costs, improved solder joint quality, and simplified transition to lead-free soldering technology. With regard to manufacturing and material costs, reductions of 20-40% in solder consumption, 40-90% in dross formation, 10-30% in flux consumption, and 70-80% in equipment maintenance have been observed, along with lower costs for post assembly board cleaning, reduced board defects and reworking, and higher productivity uptime. A further benefit of the apparatuses disclosed herein is that they can easily be scaled up or down and can be configured to fit solder pots having a variety of different dimensions.
Various terms have been defined above. To the extent a term used in a claim is not defined above, it should be given the broadest definition persons in the pertinent art have given that term as reflected in at least one printed publication or issued patent. Furthermore, all patents, test procedures, and other documents cited in this application are fully incorporated by reference to the extent such disclosure is not inconsistent with this application for all jurisdictions in which such incorporation is permitted.
Certain embodiments and features of the invention have been described using a set of numerical upper limits and a set of numerical lower limits. For the sake of brevity, only certain ranges are explicitly disclosed herein. However, it should be appreciated that ranges from any lower limit to any upper limit are contemplated unless otherwise indicated. Similarly, ranges from any lower limit may be combined with any other lower limit to recite a range not explicitly recited, and ranges from any upper limit may be combined with any other upper limit to recite a range not explicitly recited. Further, a range includes every point or individual value between its end points even though not explicitly recited. Thus, every point or individual value may serve as its own lower or upper limit combined with any other point or individual value or any other lower or upper limit, to recite a range not explicitly recited. All numerical values are “about” or “approximately” the indicated value, and take into account experimental error and variations that would be expected by a person having ordinary skill in the art.
While the foregoing is directed to embodiments of the invention and alternate embodiments thereof, various changes, modifications, and alterations from the invention may be contemplated by those skilled in the art without departing from the intended spirit and scope thereof. It is intended that the present invention only be limited by the terms of the appended claims.
Number | Date | Country | Kind |
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201410043162.3 | Jan 2014 | CN | national |