SURFACE MOUNT TECHNOLOGY REPAIR STATION AND METHOD FOR REPAIR OF SURFACE MOUNT TECHNOLOGY CIRCUIT BOARDS

Abstract: A surface-mount-technology repair station and method for replacing components on SMT circuit boards. The station includes a base with a controllable heating element through which deionized air is blown onto the surface of a board opposite to the surface on which the repair is to be effected. This raises the temperature of the entire board substantially but to a point below the melting point of solder. A second heating element through which deionized air is blown is posi­ tioned over the leads of a selected component positioned on the board, that is either to be removed from or secured to the board. The heating of the leads by the second heating ele­ ment causes the solder on these leads to melt, thus, enabling replacement of the component. To remove the component, the operator, using an instrument, such as tweezers, grasps the component and removes it from the board. To install a component, the operator, after melting solder on the leads, discontinues the heating of the component. The method of the present invention includes the steps of heating the surface-mount-technology boards to a sub­ stantially high temperature, and then heating the leads of a selected component to be replaced so that the solder dis­ posed on the leads melts.

Description: Reference is made to the EP-A-0 214 030, invented by Michael J. Kolesar, entitled "Surface Mount Technology Automated Repair System", the disclosure of which is incorporated herein by reference. This invention relates generally to an apparatus and method for the repair of printed circuit board having surface-mounted components positioned thereon. In this invention the equipment for assembling a plurality of electronic components (42) on a circuit board (50) including means (26A.26B.28.30) for manipulating the components relative to the board and processor means for controlling the manipulating means in a predetermined sequence to effect assembly of the components on the board repair apparatus for removing faulty components from boards after assembly comprises : a source of heat ; means (72) for applying heat from said heat source to a faulty component to be removed from a board said heat applying means being carried by and movable with the component manipulating means (26A.26B.28.30) and means for enabling the processor means selectively to activate said heat source said heat applying means and the component manipulating means in a predetermined repair sequence to remove said faulty component from said board. The EP-A-0 214 030 falls under Article 54(3) EPC and does not teach the use of the two heaters as recited in the independent claims. Rather, that patent application discloses a technique suitable for automation for de-soldering using hot air applied to a component on a circuit board. This reference is mentioned in the present application as being a related case. The present invention is an improvement over the invention disclosed in that application. Background of the Invention Electronic circuit assemblies which include a plurality of different electronic components mounted together on modules, often called a circuit boards, are common in computer and other electronic equipment. The boards are usually formed of a fiberglass-filled composition. Each of the electronic components on a board has leads which are electrically interconnected to leads on other components by conductors directly formed on the board. In conventional assemblies of this type, the electronic components are mounted on one surface of the board, and the leads of the components pass through plated holes to the opposite surface of the board where they are soldered, to thereby secure the components to the board, as well as make the required electrical connections. Surface mount technology (SMT) boards differ from conventional boards in that the component leads are soldered to conductive "pads" disposed on the surface to which the particular component is to be mounted. The leads, therefore, need not, and generally do not, pass through holes to the opposite surface of the board. This surface-mount technology is finding increasing usage in the manufacture of circuit board assemblies, because it has marked advantages over prior conventional technology. Specifically, the procedures for producing SMT board assemblies are generally simpler and more economical than the procedures to fabricate conventional board assemblies. Further, SMT board assemblies permit reductions in the sizings of the individual components mounted on the boards and reductions in the sizes of the boards as well. They also permit the unrestricted mounting of components on both surfaces of the boards, thus providing denser assemblies, i.e. with increased circuitry disposed in less space. Technical Digest. Western Electric no. 75, September 1984, page 11, discloses a device for desoldering an IC chip from a printed circuit board, wherein hot air is directed as a first stream to the top of a chip mounted on a board and a second, hotter, stream of hot air is directed onto the leads at the underside of the board. In a typical SMT board repair apparatus, a component to be removed is heated by a hand-held hot air blower to a temperature such that the solder, which joins the leads of the component to the SMT board at solder pads on the board, melts. The component is then removed. In like fashion, components are added to SMT board assemblies by placing the leads, which have a solder paste applied thereto, on the pads of the SMT board and then applying a directed hot air flow over the component and the leads such that the solder paste is liquidified. Renewal of the heat source then permits the solder to solidify and thereby secure the component to the board. An alternative repair method involves the use of a hand-held soldering iron, which functions much like a pair of tweezers. The tips of the iron, when squeezed together, have a shape matching the perimeter of the component to be removed from the SMT board. The tips of the iron are positioned on the leads of the component to be removed and the iron is heated until the operator observes the solder melting sufficiently to permit the component to be removed from the board. For installing components on the SMT board, this procedure is reversed. Both of the above-referenced methods of repair are slow and tedious, and require a significant amount of manual dexterity in their applications. The application of heat necessary to melt the solder can result in damage to the SMT board itself, and the conductors found thereon, in the form of buckling or even delamination. The use of hot air circulation over the board also causes, in some instances, electrostatic discharge damage to components positioned on the board, due to the presence of ions in the heated air. In light of the above, it is an object of this invention to provide an improved apparatus and method for the repair of surface-mount-technology circuit board assemblies. Another object of this invention is to provide an SMT circuit board repair apparatus which is semi-automatic and requires little operator training and only routine operator involvement. Still another object of the invention is to provide an SMT circuit board repair apparatus and method which minimizes the likelihood of damage to the board and to the components on the board. Other objects will be specified in or obvious from the detailed description of the illustrated embodiments of the invention which follows. Summary of the Invention A repair station of the present invention is defined in independent Claims 1 and 8. According to an embodiment of the present invention, a surface-mount-technology repair station for replacing components on SMT circuit boards includes a base with a controllable heating element through which deionized air is blown onto the surface of a board opposite to the surface on which the repair is to be effected. This raises the temperature of the entire board substantially but to a point below the melting point of solder. A second heating element through which deionized air is blown is positioned over the leads of a selected component positioned on the board, that is either to be removed from or secured to the board. The heating of the leads by the second heating element causes the solder on these leads to melt, thus, enabling replacement of the component. To remove the component, the operator, using an instrument, such as tweezers, grasps the component and removes it from the board. To install a component, the operator, after melting solder on the leads, discontinues the heating of the component. Because of the elevated temperature of the entire board, there is a lesser temperature differential between the portions where soldering or desoldering is taking place and other portions of the board, than in prior methods. This lesser differential prevents the delamination and related damage that would otherwise result from differential thermal expansion. Also, deionization of the air that is used to supply heat to the board materially reduces the occurrence of damage due to electrostatic discharge. The method of the present invention is defined in Claim 10. It includes the steps of heating the surface-mount-technology boards to a substantially high temperature, and heating the leads of a selected component to be replaced so that the solder disposed on the leads melts. The method can further include directing deionized air over the board and components thereon. Brief Description of the Drawings The foregoing objects, features, and advantages of the invention will be better understood from the following detailed description taken with the accompanying drawings in which: FIG. 1 is a perspective view of the SMT repair station in accordance with the present invention; FIG. 2 is an enlarged, cutaway view of the transverse arm, with an upper housing thereon, of the SMT repair station of FIG. 1; FIG. 2A is an enlarged view of an orifice plate, positionable in the upper housing of FIG. 2; and FIG. 3 is a partly schematic view of the SMT repair station of FIG. 1. Detailed Description of the Preferred Embodiments With reference to FIG. 1, an surface-mount-technology repair station embodying the present invention has a base 1 with a perforated infrared panel 3 disposed therein. Module holding fixtures 7 supporting an SMT circuit board 63, are positioned on the base 1 and are slidable in elongated slots 9 in the base. An upper housing 21 is slidably positioned on a transverse rod 23. The rod 23 is supported for movement perpendicular to its longitudinal axis by mounting blocks 25. Specifically, each of the blocks 25 has an aperture 27 for receiving and supporting an end of the transverse rod 23. Thus, the housing 21 and rod 23 can be easily displaced to bring the housing over any desired location on the circuit board 63. With reference to FIG. 2, an infrared panel 31 having apertures therein, is mounted in the upper housing 21. A flexible, extendable air line 33 extends through a mounting block 25 and the transverse rod 23 so that air blown through the air line 33 passes through the apertures in the panel 31. The housing 21 is provided with a mounting receptacle 35, which has two elongated U-shaped members 37, sized to receive interchangeable orifice plates, such as the orifice plate seen at 39 (See FIG. 2A). Manual location of the housing 21 with respect to the circuit board 63 is facilitated by a high intensity light source 41 which is carried by the housing 21 and aims a light at the point on the board 63 directly beneath the center of the aperture plate 39. Referring now to FIG. 3, a fan 42 draws air from the atmosphere through the base 1 and the air is directed through apertures in the infrared panels 3 and 31 via an air line 43 and the air line 33, respectively. The lower surface of the board 63 is thus heated by panel 3, both by infrared energy transmission and the air heated by passage through the panel 3. The portions of the upper surface of the board beneath the apertures in the plate 39 (FIG. 2a) are heated by the panel 31, both by infrared energy transmitted and by air flow through the plate 39. Air entering the base 1 is drawn by the fan 42 through a deionizing chamber 55, which deionizes the air passing therethrough in a known manner. As also shown in FIG. 3, a thermocouple 61 is positioned in one of the mounting fixtures 7 so as to contact the board 63. A controller unit 64, to which the thermocouple 61 is connected, operates to control the heat supplied by infrared panel 3 to the board 63 so as to maintain the panel at an elevated temperature, as discussed below. Electric current is provided to heat the infrared panel 3 from a source external to the unit through a transformer 65. A manual on-off switch 75 provides operator control for the infrared panel 3, and likewise, a switch 77 provides operator control for the infrared panel 31. The use and function of the above-described unit are as follows: A module, such as the circuit board 63, having surface mounted components, such as integrated circuit chips 85, positioned thereon, is positioned in the movable fixtures 7. The board 63 is heated from underneath by the infrared panel 3. The temperature to which the board 63 is preferably heated to a substantially elevated temperature such that the board has reached its thermal expansion limit over the relevant temperature range. In the case of a board formed of a composition not having such a thermal expansion limit, the board is heated to a substantially elevated temperature, such that the thermal thermal expansion, resulting from the melting of the solder in the leads of the selected component to be replaced, is minimal. This elevated temperature, in any event, should be below the melting temperature of the solder used to secure the components to the board. The board 63 is, thus, for ordinary fiberglass boards usually heated to a temperature approximately between 141°C (285°F) and 149°C (300°F), which is above the "glass transition" temperature of the board, such that only minimal thermal expansion of the board 63 results from the melting of the solder on the leads of the selected component to be replaced. The operator of the repair assembly inserts into the brackets 37 an orifice plate, such as the plate 39, which has apertures corresponding with the leads of a selected component 85 to be removed. The orifice plates, preferably, should be formed of material, such as aluminum, that will not deform from repeated heatings. The upper housing 21 is then positioned by the operator above the selected component 85 to be removed. The high intensity light assists the operator in this task. The upper infrared panel 31 is heated and deionized air is blown through the apertures in the panel. The radiant energy of the panel 31 and the deionized air is directed by the orifice plate 39 to the leads of the component 85. Upon the melting of the solder, usually at a temperature of approximately 260°C (500°F), the component 85 is thereby removable from the board 63. The installation of components is essentially the same. After positioning and heating the board 63, the component 85 to be installed, having solder paste disposed on its leads, is positioned with its leads on soldering pads positioned on the board 63. The operator inserts in the brackets 37 an orifice plate 39, having apertures which correspond with the leads of the component 85. The upper housing 21 is aligned by the operator over the component with the reference beam of the high intensity light 41. The infrared panel 31 is then turned on, and deionized air is blown through the orifice plate 39 to the leads of the component 85. Upon the liquifaction of the solder paste, the heating is stopped so that the component is secured to the board by the subsequent solidification of the solder. It will be recognized and understood that the present invention minimizes damage to SMT circuit boards and the components positioned thereon. The heating of the entire board reduces the likelihood of damage to the board such as possible delamination or buckling resulting from uneven heating. The directing of deionized air over the board and components thereon, also reduces the likelihood of electrostatic discharge damage to the components disposed on the board. The present invention also simplifies manual tasks to be performed by the operators of the device, as compared with those devices of the prior art.

Claims: A repair station for replacing selected components (85) on circuit boards (63), said repair station comprising: A) a base (1) having fixture means (7) for removably mounting a SMT circuit board; C) first heating means (3, 42) disposed in said base (1) to heat said entire circuit board (63) to a substantially elevated, first temperature below that which is capable of melting solder on a plurality of leads of said selected component; D) second heating means (31, 33, 39, 42) to heat the leads of a selected component (85) on said circuit board (63) to a second temperature that is higher than said first temperature, and capable of melting solder on said leads of said selected component to enable the replacement of said component; and E) means connected to said base (1) for mounting said second heating means; F) whereby a lesser temperature differential may be established during heating between the portions of said circuit board where soldering or de-soldering is taking place and other portions of said circuit board, thereby reducing damage from diffential thermal expansion. The repair station according to claim 1, wherein said mounting means comprises: A) a transverse rod (23) having a longitudinal axis and supported on each end by said base (1), such that said rod is movable in a direction perpendicular to said longitudinal axis; and B) a housing (21) carrying said second heating means (31 or 39) and mounted for slidable movement along said rod (23), whereby positioning of said second heating means relative to said selected component (85) is accomplished by movement of said rod (23) and said housing (21). The repair station according to claim 1 or 2, wherein said first and second heating means include respective first and second infrared heating panels (3, 31) for directing infrared energy to said circuit board (63), and said first infrared panel (3) is substantially larger than said second infrared panel (31). The repair station according to claim 3, wherein said first and second infrared panels (3, 31) have apertures for directing deionized air to said circuit board (63), and said repair station further comprises means (33, 42, 43) for directing deionized air to said infrared panels (3, 31). The repair station according to claim 1 or 2, wherein said first heating means includes a first apertured infrared panel (3) for heating a first side of said circuit board (63), and said second heating means includes a second apertured infrared panel (31), and said first panel is substantially larger than said second panel (63); the repair station further comprising a removable orifice plate (39) positioned at the orifice of said housing means (42, 43) for blowing air through said apertures in said first panel for heating said circuit board, and means (42, 33, 39) for blowing air through said second panel and orifice plate for heating said leads of said selected component. The repair station according to claim 1 or 2, further comprising a thermocouple (61) positioned in the fixture means (7) for contacting said circuit board (63); and means (61, 64) coupled to said thermocouple for controlling the temperature to which the circuit board is heated by said heating means. The repair station according to claim 1, further comprising: A) a transverse rod (23) having a longitudinal axis and supported on each end by said base (1), such that said rod is movable in a direction perpendicular to said longitudinal axis; B) a fan (42) disposed in said base; C) said fixture means (7) supporting said circuit board between said base and said rod; D) a housing (21) slideably positioned on said rod; E) an infrared heating panel (31) having apertures therein and mounted in said housing; and F) an air path (33) from said fan to said infrared heating panel for directing air from said fan through said apertures in said panel and to said circuit board, whereby said selected component can be heated by infrared energy from said heating panel and by air heated by said heating panel for liquefying solder on the leads of said selected component. A repair station for replacing selected components on SMT circuit boards (63), said components (85) having leads soldered to said circuit board, said repair station comprising: A) a base (1) with a controllable heating element (3); B) a transverse rod (23) having a longitudinal axis and supported on each end by said base (1, 25), such that said rod is movable in a direction perpendicular to said longitudinal axis; C) a fan (42) disposed in said base; D) mounting fixtures (7) secured to said base for removably supporting said circuit board between said base and said rod; E) a housing (21) slideably positioned on said rod; F) an infrared heating panel (31) having apertures therein and mounted in said housing; and G) an air path (33) from said fan to said controllable heating element (3) and said infrared heating panel, such that air is directed by said fan through said apertures in said panel and through said heating element (3). The repair station in accordance with claim 8, wherein said controllable heating element (3) is infrared panel (3) having apertures therein, and mounted in said base and between said mounting fixtures; and B) an air path (43) from said fan to said lower panel, such that air is directed by said fan through said apertures in said panel, whereby, when a board is disposed in said fixtures, said board is heated by said lower panel. A method of replacing selected components on a SMT circuit board using a circuit board repair station, said method comprising the steps of: A) removably securing a circuit board (63) in a mounting fixture (7) on a circuit board repair station; B) heating said circuit board (63) and components thereon by a first heating means (3) to a substantially elevated, pre-selected first temperature that is below the melting point of solder joining the leads or said components to said circuit board (63); and C) heating the leads of a selected component-to-be-replaced by a second heating means (31) to a pro-selected, second temperature that is above the melting temperature of the solder disposed on the leads of said selected component, whereby the selected component can be replaced. The method in accordance with claim 10, wherein A) said circuit-board-heating step includes activating an infrared panel (3), and directing deionizing air through said panel and to said circuit board, whereby the entire circuit board is maintained at or near said first temperature; and B) said leads-heating step includes activating said second heating means (31), directing deionized air through the activated heating means to thereby elevate the temperature of the air, and directing the air to the leads of the selected components. The method in accordance with claim 10 or 11 A) further including the step of positioning said second heating means (31) relative to a selected component (63) on said circuit board by sliding a housing (21) containing said second heating means along a transverse rod (23) on which said housing is positioned and moving said rod having a longitudinal axis and supported on each end by said base (1,25) direction perpendicular to said longitudinal axis. The method in accordance with claims 10, wherein said first temperature is above the glass transition temperature of the circuit board. The method in accordance with claim 10, further comprising the steps of A) disposing solder paste on the leads of a selected replacement component (85); B) positioning the replacement component (85) on said circuit board (85); and C) discontinuing the heating by said second heating means after the solder paste melts, whereby the solder solidifies and said component is secured to said circuit board. The method in accordance with claim 10, further comprising the steps of A) discontinuing heating by said second heating means (31) after the solder melts; and B) removing the selected component from the circuit board. The method in accordance with claim 10, further comprising the steps of: A) discontinuing the heating of the leads; B) removing the selected component (85); C) positioning a new component (85) with solder paste on the leads on said circuit board; D) heating the leads of the new component to melt the solder paste; E) discontinuing the heating of the leads and said circuit board (63); and F) removing said circuit board from said repair station. The method in accordance with claim 16, further comprising the steps of: A) monitoring the temperature of the temperature of the circuit board; B) controlling the first heating element (3) to maintain the circuit board at the elevated temperature of between 285°F (141⁰C) and 300°F (149°C).

Inventors: BOIS, PHILIP J., KOLESAR, MICHAEL J.
Applicants: DIGITAL EQUIPMENT CORPORATION
Application year: 1987
Main classification: H05K13
IPC codes: B23K1, H05K3, H05K13