GB2234461A

GB2234461A - Desoldering method and apparatus for integrated circuits

Info

Publication number: GB2234461A
Application number: GB8915757A
Authority: GB
Inventors: Rodney John Browning
Original assignee: AUTOMATED INTEGRATED DIAGNOSIS; AUTOMATED INTERGRATED DIAGNOSI
Current assignee: AUTOMATED INTEGRATED DIAGNOSIS; AUTOMATED INTERGRATED DIAGNOSI
Priority date: 1989-07-10
Filing date: 1989-07-10
Publication date: 1991-02-06
Anticipated expiration: 2009-07-10
Also published as: GB8915757D0; GB2234461B

Abstract

A method of removing an integrated circuit (1) or other component from a printed circuit board (2) comprises the step of placing an upstanding screen (3) around the periphery of the component (1), blowing hot air from a hot air source (6) into the region adjacent the component and once the solder has melted, removing the screen with the integrated circuit engaged therewith from the printed circuit board. The open base (5) of the screen forms a tight fit over the pins of the integrated circuit. <IMAGE>

Description

Desoldering method and apparatus for integrated circuits This invention relates to a method and apparatus for removing an integrated circuit from a printed circuit board on which it is mounted by means of a plurality of soldered joints.

Typically an integrated circuit is mounted on a printed circuit board by means of discrete soldered joints formed between a plurality of pins extending from side edges of the integrated circuit and electrically conducting tracks on the printed circuit board. In the event of failure of an integrated circuit it is often desirable to repair the apparatus of which it forms part by replacing the integrated circuit concerned. In order to remove an integrated circuit from a board it is desirable to soften all the soldered joints substantially simultaneously and this is awkward and time-consuming with known techniques.

A problem with using a single point soldering iron for this purpose to melt each joint in turn, is that the solder tends to re-solidify relatively quickly and joints are then reformed. A number of tools are available for use with a single point soldering iron which attempt to solve this problem by removing solder from each joint as it is melted, for example by suction, however this is not entirely satisfactory and is still time-consuming.

Systems are also known in which hot air is directed onto the integrated circuit to melt the soldered joints, the integrated circuit being subsequently lifted from the board by means of a suction head. However known such apparatus are complex and costly, and do not always successfully operate to simultaneously soften the joints.

Viewed from one aspect the present invention provides a method of removing an integrated circuit or other component from a printed circuit board, comprising the steps of placing an upstanding screen around the periphery of the integrated circuit, the base of the screen forming a tight fit over the pins of the integrated circuit, blowing hot air into a region adjacent the integrated circuit defined by the screen to melt substantially simultaneously the solder forming each of the plurality of soldered joints, the screen acting to shield other regions of the printed circuit board from the heat, and once the solder has melted removing the screen with the integrated circuit engaged therewith from the printed circuit board.

There is thus provided a method in which substantially all of the soldered joints between the integrated circuit and the printed circuit board can be melted simultaneously. The operation is quick and straight forward to perform as compared with known techniques. The screen which extends around the periphery of the integrated circuit serves to shield adjacent components from the source of heat and also assists in channeling the heat to the integrated circuit and distributing the heat relatively evenly around the periphery of the integrated circuit.

The screen is adapted to tightly engage the integrated circuit and the two are conveniently removed together from the printed circuit board once the solder has melted. No costly suction equipment is therefore required, the integrated circuit being lifted up with the screen. There is an interference fit between the screen and the pins around the periphery of the integrated circuit so that the screen can be clipped securely over the integrated circuit. In this way there is direct heat transfer between the screen and the pins to be desoldered which substantially improves the desoldering process as compared with known hot air techniques. The screen acts as a heat sink and helps evenly to distribute the heat around the pins.

The screen is therefore advantageously made of a heat conducting material such as a metal and is of tubular construction having a cross-section at least at one end conforming to the shape of the integrated circuit. The screen may be slightly funnel shaped,#i.e.

opening out at its upper end to assist the ingress of hot air. Once the screen is fitted over the integrated circuit, hot air is then be blown towards the integrated circuit from the open end of the screen, for example by a conventional proprietory hot air blower having an electrically operated fan and heater unit.

In a preferred method the hot air blower is first directed towards the centre of the screened integrated circuit, and is then directed around the periphery of the integrated circuit. This has been found in practice to be an effective way of heating joints around the edges of the integrated circuit more evenly.

The hot air blower may be switched off automatically or manually by an operater for example after a predetermined time has elapsed, or once a temperature sensor or sensors indicate that a predetermined temperature has been reached. The temperature of the solder must be high enough for all the joints to have melted, but if adjacent components are allowed to become too hot they may be damaged.

A hot air blower such as a fan and heater unit may have only one temperature setting, or the temperature of the hot air may be variable for example so as to allow heating to be slowed as the final predetermined temperature is reached. Feedback for example from a thermocouple or thermocouples adjacent the integrated circuit may be used to control the temperature of the hot air as well as other factors such as the direction in which the hot air is blown and distance of the hot air outlet from the integrated circuit. More than one source of hot air may be provided and each may be controlled individuallly.

Viewed from a further aspect the invention provides a desoldering screen for integrated circuits, the screen being of box like configuration and having an open base adapted to be fitted tightly over the periphery of an integrated circuit and an open top portion for the ingress of hot air in use. The side walls of the screen preferably taper outwardly so that the screen is generally funnel shaped.

The invention also provides desoldering apparatus for integrated circuits comprising a screen as described above in combination with a hot air source.

An embodiment of the invention will now be described by way of example only with reference to the accompanying drawings wherein: Figure 1 shows a perspective view of an integrated circuit, a screen and part of a printed circuit board and a hot air blower; and Figure 2 shows a perspective view of a rework station.

Figure 1 shows an integrated circuit 1 mounted on a printed circuit board 2 (part only shown). The spaces on the printed circuit board adjacent the integrated circuit will generally be occupied by additional integrated circuits or other components. A plurality of metal pins extend downwardly from the periphery of the integrated circuit 1 as shown in Figure 1. A discrete soldered joint is formed between each pin and a conducting track provided on the surface of the printed circuit board. Screen 3 is box-like in shape having four walls formed of metal sheet, and having an open top 4 and an open base 5. The cross section of the base 5 conforms to the shape of the integrated circuit 1 and is dimensioned so that there is an interference fit between the two components.The cross-section of the top 4 of the screen is slightly greater in this case so that hot air is funnelled downwardly by the screen from hot air blower 6 (part only shown). The hot air blower is also provided with sections of reduced internal cross-section forming a nozzle approaching the air outlet 7. Only one hot air blower is used in this embodiment but it would be possible to use a plurality of blowers, or a single blower having a plurality of outlets. The hot air blower may be one of various types available on the market such as that manufactured by Stienel of West Germany.

Figure 2 shows a rework station 10 which incorporates a hot air blower 6 and a moveable table 11 on which a printed circuit board may be placed for work to be carried out. A microscope 12 is provided through which the workpiece may be viewed if desired.

The hot air blower 6 incorporates a variable heater and a two-speed fan (neither shown). In this embodiment the heater control 13 is manually operable such that hot air emerging through nozzle 7 may be at a temperature between 150-5000C. The hot air blower is provided with a manually operable on/off switch 14 and a timer with controls 15.

In order to remove the integrated circuit 1 from the printed circuit board 2 the printed circuit board is placed on table 11 and screen 3 is clipped over the integrated circuit 1. The hot air blower 6 is moved over the integrated circuit 1 and is lowered so that nozzle 7 extends into the top opening 4 of screen 3.

The hot air blower is switched on for a predetermined time, the timer being set so that after the predetermined time has elapsed the hot air blower is automatically switched off. The hot air blower may be directed at the same area of the integrated circuit during the entire period, or preferably it will be directed at the centre of the integrated circuit surface for a first predetermined time and then moved around the periphery of the integrated circuit for a second predetermined time. This method is found to be more effective in that there is a more even distribution of heat over the surface of the integrated circuit and a shorter period of time elapses before all the soldered joints have melted. Screen 3 shields surrounding components from the heat so that their soldered joints are not melted and they are not damaged. In addition, because the screen acts as a heat sink it aids the more even distribution of heat.

Once all the soldered joints have melted the screen 3 is lifted from the printed circuit board, for example using a pair of tweezers. Because of the interference fit with the integrated circuit 1 the screen remains attached to that component, and the screen and integrated circuit are lifted from the printed circuit board together.

In a more complex embodiment, the blower 6 will be switched off when a predetermined temperature is detected by thermocouples located close to the integrated circuit. If required the setting of the heater control may be automatically varied depending on the thermocouple readings, and the position and orientation of the hot air blower may be adjusted automatically as appropriate.

Claims

CLAIMS:

1. A method of removing an integrated circuit or other component from a printed circuit board, comprising the steps of placing an upstanding screen around the periphery of the integrated circuit, the base of the screen forming a tight fit over the pins of the integrated circuit, blowing hot air into a region adjacent the integrated circuit defined by the screen to melt substantially simultaneously the solder forming each of the plurality of soldered joints, the screen acting to shield other regions of the printed circuit board from the heat, and once the solder has melted removing the screen with the integrated circuit engaged therewith from the printed circuit board.

2. A method as claimed in claim 1 wherein the screen is formed of a heat conducting material.

3. A method as claimed in claim 1 or 2 in which the screen is of a tubular construction, having a crosssection at least at one end conforming to the shape of the integrated circuit.

4. A method as claimed in claim 3 in which the upper end of the screen has a greater cross-section than the base of the screen.

5. A method as claimed in any preceding claim in which hot air is first directed toward the centre of the screened integrated circuit and is then directed around the periphery thereof.

6. A method as claimed in any preceding claim wherein the hot air supply is terminated after the elapse of a predetermined time.

7. A method as claimed in any preceding claim wherein the hot air supply is terminated once a predetermined temperature has been reached.

8. A method as claimed in any preceding claim wherein the temperature of the hot air is variable.

9. A method as claimed in any preceding claim wherein the temperature adjacent the integrated circuit is monitored and is used to control at least one of the temperature, direction and position of the hot air source.

10. A desoldering screen for integrated circuits for use in the method of claim 1, being of box like configuration and having an open base adapted to be fitted tightly over the periphery of an integrated circuit and an open top portion for the ingress of hot air in use.

11. A desoldering screen as claimed in claim 10 which is formed of heat conducting material.

12. A desoldering screen as claimed in claim 10 or 11 wherein the cross-section of the upper end of the screen is greater than the cross-section of the base.

13. A desoldering apparatus comprising a desoldering screen as claimed in any of claims 10 to 12 in combination with a hot air source.

14. A method of removing an integrated circuit or other component from a printed circuit board, substantially as hereinbefore described with reference to the accompanying drawings.

15. A desoldering screen substantially as hereinbefore described with reference to the accompanying drawings.

16. A desoldering apparatus substantially as hereinbefore described with reference to the accompanying drawings.