AUTOMATED REWORK: METHOD AND ...

This is a preprint of an article whose final and definitive form has been published in the International Journal of Computer Integrated Manufacturing [1996] [copyright Taylor & Francis]; International Journal of Computer Integrated Manufacturing is available online at: http://dx.doi.org/10.1080/095119296131805 To cite this article: N. Geren (1996): Automated rework of printed circuit board assemblies: Methods and procedures, International Journal of Computer Integrated Manufacturing, 9:1, 48-60 To link to this article: http://dx.doi.org/10.1080/095119296131805

AUTOMATED REWORK: METHOD AND PROCEDURES N. Geren and A. Redford Aeronautical and Mechanical Engineering department University of Salford, Salford, M5 4WT, UK

Abstract: In the last few years, although the numbers of rework stations available on the market has grown considerably the process of repair has not been fully automated due to the requirement for flexibility and complex processes unlike PCBA manufacturing which is set up for dedicated process. In order to automatically handle a variety of PCBs each of which may have different fault problems, an autonomous rework system capable of advanced sensing, in process inspection and automatic planning is required. This paper presents automated rework methods and procedure which were developed to extend the capability of a robotic assembly cell to perform rework and in-process inspection of single sided PCBAs for batch sizes of one. It outlines detailed rework procedure for SM and TH components, summarises the hardware requirements and the structure of the cell and evaluates the procedures developed with regard to rework effectiveness.

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AUTOMATED REWORK: METHOD AND PROCEDURES Aeronautical and Mechanical Engineering department University of Salford, Salford, M5 4WT, UK N. Geren and A. Redford

1 INTRODUCTION Since the advent of printed circuit board assembly (PCBA) that began with through hole (TH) components, rework of PCBAs has taken place in PCBA manufacturing.

Product

complexity has made rework and repair difficult and the rework of PCBAs is one of the main problem for PCB manufacturers. Although PCBA has been substantially improved with fully automated, accurate assembly machines, and the use of robots, unfortunately there has not been a significant improvement in rework equipment because the predicted cost of fully automated rework station has made it impossible to justify a fully automated rework cell. Whilst some manufacturer's of reworkmachine, suppliers and researchers have put their efforts into designing more efficient and effective manual rework equipment others have researched into how to improve manual rework efficiency by using advanced techniques and methods (Carrol 1991; Camurati et al. 1989; Strong 1992; Driels and Klegka 1991). Briefly, rework has been traditionally carried out by a group of skilled operators, with the help of various dis-assembly and assembly aids. The problem of rework has not been solved by attempting reworking of PCBAs with existing technology. Rework itself has introduced many extra problems in to the manufacturing environment like repairing unnecessary joints or components, long rework cycle times, the need for talented rework personnel etc. Most important of all as the pitch sizes get smaller, the ability of humans to perform rework at all comes into question. This is currently driving rework equipment manufacturers into providing fine wheel controlled X-Y positioning tables and PC controllers to existing equipment. Recently, the requirement for an automated rework has been specified by Driels and Klegka (1992). There has been a lack of international effort in the development of fully automated rework machines whilst the current defect rates are as high as 35 % (Lo and Goodall 1992; Mangin 1987). This may be sensible as the cost of a fully automated rework cell may be as high as few hundred thousand pounds but this is not the only option. A rework cell may be developed as an extension to a robotic PCB assembly cell where much of the required equipment is already present and this will make it more economic and more easy to justify the investment. Robot applications in PCBA are increasing rapidly due to the need for small batches and quick

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changeover (Hollingum 1991; Warnecke and Wolf 1985). Consequently, the increasing use of robots and vision systems resulting from high accuracy and flexibility requirements have given them a more established place as acceptable technology, and the gradual lowering in their price makes their use more feasible. With the robot's multi-functional ability, it is anticipated that a properly designed cell could be deployed for the assembly, inspection and rework of defected components. At present, robotic assembly cells are not utilised 24 hours a day, and it is envisaged that it may be possible to use the cell for rework in their spare time. Even if this were not so, reducing production to do rework would, because of the high cost of rework, be economic. Further, since the PCBAs being repaired would most likely be assembled by the same cell, problems with component and PCBA feeding, jigs, sensory requirements, grippers, etc. would not usually occur and information about the layout of the board already stored by the cell controller could also be made available to the rework system. This paper presents:



Automated rework methods and procedures which were developed to extend the use of a robotic PCBA cell to perform rework and in-process inspection of single sided PCBAs for through hole (TH) and surface mount (SM) component rework.

 

The results of the automated rework experiments. The improvements necessary to improve rework effectiveness.

2 AUTOMATED REWORK TOOLING AND METHODS 2.1 REFLOW TOOLING Studies of manual rework techniques and current developments in industrial assembly robots have indicated that the development of a successful fully automatic robotic rework is very much dependent on the reflow techniques chosen for both type of component. It was found that no existing reflow technique is completely suitable but the iris focused IR and solder fountain reflow methods were found to be most appropriate for SM and TH respectively. Iris focused IR soldering unit: This uses a 150 watts halogen light bulb to develop 1 to 1,2 m short-wave IR light. The light is focused through lenses and the spot size of the heat source is adjusted through an iris ring. The system requires four lenses where each of these cover a group of different sized SM components and there is continuous linear adjustment of the spot size for each lens. The lens changing can be carried out by the manipulator. Currently, this is done manually and the spot size adjustment is automated. Solder fountain:

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This is based on the wave soldering principle and incorporates a set of nozzles through which molten solder is pumped. The wave generated by this is in the shape of a fountain and is controlled by the pump speed and nozzle height and shape. The nozzles are raised and lowered by adjusting screws which

are turned by a screw driving attachment fastened to the

manipulator. Using this reflow technique requires an open frame X-Y-Z positioning table in addition to the

assembly robot (Adept one). The

IR heat source

is located above the

positioning table where the PCBA is fixed whilst the solder fountain is located in the open frame of the table. This arrangement allows defective components to be moved over the solder fountain or under the IR light for reflowing of solder and the components can then be removed by the assembly robot using standard assembly grippers.

2.2 Other Rework Tooling Decisions on reflow methods for TH and SM components allowed analysis and decision making on the other rework tooling necessary for automation, and the solving of related problems. If rework requirements other than reflow methods are to be specified, then in order to achieve fully automated rework, the followings have been considered (see Geren et al. 1992): (1) Preparation of PCBA for rework (i)

Cleaning of PCBA from dust and contamination etc.,

(ii) Removal of conformal coating, (iii) Removal of obstructions. (2) Underside heating, (3) Cleaning of excess solder from pads without damaging the board, (4) Solder cream dispensing, (5) TH and SM defective component fluxing (6) Declinching of TH component legs and removal of TH and SM components, (7) Post desoldering and resoldering cleaning (Cleaning of defective area from flux etc.), (8) Heat control. Since the aim is to automate the rework process, in addition to above sensory, supervisory and other control tools are also necessary. The following propose tools and methods to deal with the above. Preparation of PCBAs for Rework: The cleaning of PCBAs and removal of conformal coatings

are not necessary because the

study aims to rework assembly defects directly after manufacture when they are not covered by dust, other contamination and conformal coatings (PCBAs are coated after the final test of the

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finished good) or subject to field defects which can be mostly dirt. The other problem is the removal of obstructions (i.e. heat sinks, jumper wires, threaded fasteners, other components, etc.). The first three must be removed manually because of their method of attachment to a PCBA. In practice there are relatively easy to remove manually, they represent only a small proportion of board components and in most cases their removal is unnecessary. The rest of the obstructions may be removed by the rework cell, and replaced after the defective component has been repaired. Underside Heating In order to protect the PCBA from localised heat shock and to help reduce delamination and to activate the flux, underside heating is essential in rework. A hot air device was found to be appropriate for this task. Cleaning of Pads from Solder After the defective component has been removed, the pads of SM components must be cleaned of excess solder in order to dispense accurate amount of solder and to place a new component on to the pads without endangering the placement accuracy (the beads and spikes that are left on the pads when a component is removed may cause tilting of the component). This can be achieved using vacuum desoldering iron (V.D.I.). Solder Cream Dispensing Solder cream is essential in SM component attachment. Solder pastes are dispensed in several ways including syringe, pressure-fed reservoirs or guns. An assembly robot can be equipped with necessary tooling which will provide interchangeability and other essential automation requirements like pressure, electricity connections etc. to handle the solder dispenser and dispense solder cream. Through Hole and Surface Mount Component Fluxing Fluxing of the defective component and its location area is essential during rework and must be applied to solder joints of a defective component to aid in heat transfer, to dissolve the oxide on the joints and to allow the molten solder to wet the chemically clean surface. Since rework requires local area fluxing and easy control of the flux coating thickness, spray type fluxers were found to be most suitable for the cell. Declinching of Through Hole Legs and Removal of Surface Mount Components The protruding legs of defective TH components must be removed. The process was achieved using an end mill.

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Surface mount components may be directly removed using suction tools when the solder is molten since adhesives are not used for single sided PCBAs.

Post Desoldering and Resoldering Cleaning After the removal and replacement of components, the defective area is usually cleaned by using a decreaser and brush to remove flux and other contaminants. This step can be eliminated from consideration because rework is being carried out in conjunction with assembly and this facility will be available. Heat Control A very important consideration in rework is the safe application of heat to reflow solder. Two programmable PID temperature controllers have been used to perform IR heating and bottom heating process steps using closed-loop temperature regardless of heat load.

control to adjust the process

Fig. 1 illustrates this arrangement. The bottom heater control

mechanism is also used for TH rework.

IR

Input from cell controller Temp. measurement sensor

Output to cell controller

PCBA Temperature controllers Bottom heater Control output

Interface

Input from cell controller Output to cell controller

Temperature feedback

Fig. 1 Schematic of IR and bottom heater control mechanisms. TH solder joint desoldering/resoldering has different requirements to that of SMs. Although preheating requirements can be met using closed-loop bottom heating control, there is a requirement for the detection of the reflowing of solder joints. This is an added complication because the reflow of TH solder joints cannot be detected by sensory tooling. The only option is to perform experimental work to determine the time to reflow each type of joint so that results can be interpreted and the data passed to the cell controller to monitor and control the process. Temperature Measurement: Two temperature measurement sensor requirements were identified. One for bottom heating control, one for IR heating control. For bottom heating control, a contact method temperature

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measurement sensor is used whilst the IR heating (top heating) temperature measurement is achieved using a non-contact temperature sensor. Sensory and Supervisory Control Requirements Sensory requirements: A rework cell should be equipped with all the sensory tooling which an assembly cell has. This will include component verification, placement force sensing, etc. Since, PCBA rework exhibits much more process variability than PCB assembly, factors such as board warping due to weight of assembled components or heat distortion during soldering etc. require additional sensory requirements. The sensory requirements have been identified as: Vision and in-process inspection: 

Fiducial marks on PCBAs must be registered, this is more important role than component assembly because of having to deal with post desoldered PCBAs.



Defective components must be located and checked for type of fault,

access to the

component and obstructions like heat sinks. 

After SM component removal, the post desoldered area must be inspected for missing pads, detached component legs etc.



After all SM pads have been cleaned of excess solder, pads must be checked for solder beads etc. which may endanger component placement.



Solder cream contains volatile substances which may evaporate at the tip of the solder dispenser causing blockages; solder cream flow must be checked by using a reference pad when cell has not been used for a long time.



After solder cream has been dispensed, all the pads of the defective component must be checked for solder cream presence.



New SM component must be registered relative to the manipulator and the target site. This also includes checking of component presence and coplanarity of the legs if it is SOICs or other multileg component.



The end quality must be checked against bridging, misplacement and for other defects. Because of the intrinsically 'unstructured' nature of the PCBA rework environment, in-

process inspection is essential to provide regular information updates. At various points in the rework process, the cell controller has to make decisions on the basis of the uncertain results of the previous actions (i.e. has all the solder been removed?, what is the exact location of the defective component and tools?, etc.). An inspection system with three cameras to check bottom and top side of the PCBAs, and to register new components on robot's end of arm tooling are necessary.

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Other sensory tooling: 

It is necessary to monitor the shear force (for glued SMDs-for double sided PCBAs) and the axial force for (through-hole) component removal exerted on the board against corresponding preset threshold values. This is especially important for TH component removal.



Leg cutting, solder dispensing and solder fountain operations require knowledge of local deflections of a PCBA which may not be flat due to warpage or weight of components .

Supervisory system: Examinations on the rework process has revealed that although most of the activities are carried out in a predefined sequential manner, it often requires the simultaneous activation of a number of piece of equipment and involves the use of process knowledge either stored in the system or acquired during rework. A knowledge-based rework process planning system is necessary to retrieve process information and to invoke task-oriented rework and inspection routines. A major feature of the planning system should be its ability to operate dynamically in ''real time'' prior to and throughout the rework process. In order to cope with the proposed batch size of one rework, the application software should be generic and capable PCBA rework without human involvement. This may require extensive data support which may come from: 

Automatic Testing Equipment (ATE) to determine and identify defective components,



CAD/CAM data specific to reworked PCBAs giving the positions and identification of the defective and the surrounding components.



In-process sensory devices that may supply various piece of information such as type of defect, obstructions, location of post-desoldered defective component etc.



Pre-existing PCBA data pools that may supply various data requirements such as rework procedure and detailed data concerning tool locations etc.

The cell controller has to adjust the system to adapt to the various conditions automatically and the target planning kernel needs to be dynamic based. In addition to the above, the rework cell requires a PC and input/output (I/O) board in order to perform supervisory tasks and to control rework tools remotely.

3 DESIGN AND DEVELOPMENT OF THE REWORK CELL Based on detailed studies of manual rework (procedures, problems, available tooling and methods) along with current technological developments on sensory, robotics, mechanical transfer devices, PLCC control devices, etc. a fully automated robotic rework cell has been

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developed. Most of the equipment used had been designed for manual operations and then needed to be automated using: (1) A stepping motor, a timing belt and two pulleys to adjust the spot size of the infra-red light remotely used in the IR reflow unit. (2) A pneumatic-driven mechanism to raise/lower the position of the milling cutter for the removal of clinched leads. (3) A pneumatic sliding mechanism to enable linear movement of the vacuum desoldering iron. (4) A spring loaded pneumatically raised and lowered thermocouple for bottom heater control. (5) A four-bar linkage mechanism to adjust the orientation of the temperature sensor so that it can be targeted at the required discrete positions underneath the reflow unit. Items (2), (3) and (4) were needed to give clear working space for both the manipulator and the X-Y-Z positioning table. Item (5) was necessary because of the requirement of the IR heat source (each of the lens attachments of the IR heat source requires a different focusing distance to be set). Fig. 2 shows the proposed hardware structure of the rework cell. The structure of the cell may be better explained if it is divided in to four regions as: 1) Interchangeable rework tools, robot manipulator, and assembly devices, 2) Lower rework tooling, 3) Upper rework tooling 4) Control devices. Exchangeable devices, robot manipulator and assembly devices: These are the devices and tooling which are carried, used and operated by the Adept-One assembly robot. They include the Adept-One robot, the solder dispenser, the V.D.I., four IR lenses, grippers and suction tools, a screwdriver, the component feeders and a component discard container. The solder dispenser, V.D.I. and IR lenses are located at one side of the robot, the grippers and suction tools are on the other side and they are interchangeable. The screw driver is mounted on the robot's ''Z'' axis permanently. The component feeders are located around the cell. Lower rework tooling: This includes the open-frame X-Y-Z positioning table where the solder fountain, the bottom fluxer, the leg cutter, the bottom heater, the thermocouple mechanism and a camera are mounted. Upper rework tooling:

This includes the devices which are mounted above the X-Y-Z

positioning table. These include the IR heater, the top fluxer, the non-contact temperature measurement sensor and adjustment mechanism, cameras and other sensory tooling. Control devices: These are located away from the robot's working envelope. These include the Adept's controller,

the positioning table controller, PLCC controllers, the vision camera

controllers, the cell controller PC and other control devices such as solenoids, I/O board etc.

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Although a fully automated system would need

full remote control facilities for the

adjustment of all rework tooling, the adjustment of the solder fountain nozzles and the lens exchange requirements of the IR heat source were not considered as they are straight forward. The V.D.I. was also fixed in a stationary position above the X-Y-Z positioning table where it could have been attached to the manipulator end effector. The rework cell is supervised by a 386 PC, which functions as the cell controller. It is equipped with the input/output (I/0) module and the PC bus indexer card to control the I/0 and positioning control requirements respectively; all the rework tooling, the Adept-one robot, the X-Y-Z positioning table, and the sensory devices (PI 030 vision system) were interfaced to the cell controller. Fig. 3 shows the control hierarchy of the rework cell at the top level.

386 PC

Cell Controller

Mechanical Transfer Devices

Rework Devices

-Robot -X-Y-Z positioning table -IR spot size adjustment step motor

Fig. 3 Control hierarchy of the rework cell.

Sensory Devices -Vision System

Top vision camera

Bottom heater controller

Stepper motor and belt drive

Various grippers and suction tools Top heater cont.

Component kitting magazines Vision monitor and computer

Screw driver

Interchangable solder dispenser and vacuum desoldering iron

Cell controller

Noncontact temp. measurement sensor

IR heat source

Top fluxer

IR lenses

Component checking camera Vision controller

Bottom heater and sensor

Solder fountain

Leg cutter

X-Y-Z positioning table Positioning table cont.

Bottom vision camera

Fig. 2

Lay out of the proposed rework cell's hardware structure.

Bottom fluxer

Robot controller

1 Table 1 Detailed rework procedure for SM and TH components.

Automated SM and TH Component Rework Procedure REWORK 1.I-R1

Operator reads PCBA PCBA design ID.

VISION

CELL CONTROLLER

serial number and 2.I-C1 3.I-C2 4.I-C3 5.I-C4

6.I-R2

9.I-R3

12.I-R4

Operator changes the fixture if Necessary. 7.I-C5 8.I-C6

Operator confirms changing. Cell controller prompts the operator to place the PCBA and secure it.

10.I-C7 11.I-C8

Operator confirms placement and initiates rework. Cell controller formulates positioning table and initiates vision for fiducial inspection. Cell controller initiates fiducial recognition procedure. Cell controller alters the board fiducials relative to cell datum. Cell controller formulates positioning table and vision procedure for component inspection. Cell controller gains pre-desoldered item position, fault confirmation and defect type.

Operator places and secures PCBA.

Positioning table moves to inspection area for fiducial recognition.

12.III-I1

Vision locates fiducials.

12.II-C9 13.I-C10 14.I-C11

15.I-R5

Operator types in PCBA serial number and PCBA design ID. Cell controller gains A.T.E. information and checks reparability. Cell controller gains some CAD/CAM data. Cell controller prompts type of fixture to be fixed by operator.

Positioning table locates underneath the camera.

defective

item

15.II-I2

SM REWORK PROCEDURE

Vision locates component position, confirms fault and clear access.

15.III-C12

TH REWORK PROCEDURE

2 SURFACE MOUNT REWORK PROCEDURE 16.II-R6 17.II-R7 18.II-R8 19.II-R9 20.II-R10 21.II-R11 22.II-R12 23.II-R13 24.II-R14 25.II-R15 26.II-R16 27.II-R17 28.II-R18 29.II-R19 30.II-R20 31.II-R21

32.II-R22

33.II-R23

34.II-R24 35.II-R25

Positioning table locates the item underneath the top fluxer. Positioning table / fluxer fluxes component.

16.I-C13

Cell controller plans overall rework procedure.

17.I-C14

Robot exchanges lenses. Robot attaches pick and place head. Positioning table moves to IR heater. IR heaeter adjusts spot size and temperature sensor angle. Temperature controllers and heaters are initiated for desoldering. Positioning table moves to removal/ replacement point. Robot removes defective item. Positioning table locates defective item on inspection area. Positioning table moves defective item to top fluxer. Positioning table / fluxer fluxes pads.

18.I-C15 19.I-C16 20.I-C17 21.I-C18

Cell controller formulates positioning table for fluxer positioning and controls fluxer. Cell controller instructs robot to exchange IR lens. Cell controller decides type of pick and place head. Cell controller sends positioning table to IR heater. Cell controller adjusts spot size and temperature sensor angle.

22.I-C19

Cell controller initiates removal step.

23.I-C20

Cell controller moves positioning table to removal /replacement point. Cell controller determines successful removal of component. Cell controller instructs vision and positioning table for postdesolder inspection. Cell controller gains post-desolder inspection results and decides next step. Cell controller formulates positioning table for fluxer positioning and controls fluxer. Cell controller initiates pad cleaning process.

Positioning table and V.D.I. are initiated for pad cleaning. Positioning table / V.D.I clean pads from excess solder. Positioning table locates the pads on inspection area. Robot attaches solder cream dispenser positioning table locates solder quantity test area on dispensing point. Robot / solder dispenser dispenses solder cream to dispensing point, positioning table moves to inspection area. Positioning table locates pads on solder dispensing area - robot / solder dispenser dispenses solder cream. Positioning table locates the pads on inspection area for solder cream inspection. Robot exchanges its solder cream dispenser with component gripper.

25.III-I3

Vision inspects post-desoldered area for soldering defects.

24.I-C21 25.I-C22 26.I-C23 27.I-C24 28.I-C25 29.I-C26

30.III-I4

Vision inspects pad cleaning quality and determines warping.

30.I-C27 31.I-C28

32.III-I5

34.III-I6

Vision checks solder cream ball existence on the target pad.

Vision inspects position and defects of solder cream.

Cell controller formulates and controls positioning table and V.D.I. for solder removal. Cell controller instructs vision and positioning table for pad cleaning quality and warping check. Cell controller gains pad cleaning results and local height of the PCBA surface.

32.I-C29

Cell controller formulates and controls solder dispensing on a target check point.

33.I-C30

Cell controller initiates solder dispensing and controls it.

34.I-C31

Cell controller formulates solder cream inspection.

35.I-C32

Cell controller gains solder cream inspection results and decides option. Cell controller interacts / operator types in component presence on the kiting magazine.

36.I-C33

3 37.II-R26 38.II-R27 39.II-R28 40.II-R29 41.II-R30 42.II-R31

Robot picks up and locates the component on the camera. Pos.table moves to removal/replacement point and robot replaces the new item. Positioning table locates the component on IR heater. Temperature controllers and heaters are initiated. Positioning table locates the item on inspection area. Positioning table moves to unloading/ loading point and robot releases its gripper.

37.III-I7

41.III-I8

Vision inspects component presence and coplanarity of the legs.

Vision inspects post resolder joint quality.

37.I-C34 38.I-C35

Cell controller instructs robot and vision for component inspection. Cell controller formulates replacement of new item.

39.I-C36

Cell controller prepares resoldering process.

40.I-C37 41.I-C38

Cell controller initiates resoldering process. Cell controller initiates post resolder inspection.

42.I-C39

Cell controller decides acceptability of rework quality and stops rework. Cell controller prompts operator for the next rework.

43.I-C40

THROUGH-HOLE REWORK PROCEDURE 16.I-C13 17.I-C14

Cell controller plans overall rework procedure. Cell controller initiates leg cutting procedure.

19.I-C15 20.I-C16

Cell controller coordinates positioning table for leg cutting. Cell controller formulates positioning table for bottom fluxer positioning and controls fluxer.

Bottom fluxer fluxes component. Positioning table moves to bottom heater.

22.I-C17

Cell controller formulates positioning table for pre-heating and controls bottom heater.

Bottom heater preheats the component. Positioning table locates the item above solder fountain.

24.I-C18

Cell controller initiates component removal.

25.II-C19

Cell controller determines successful removal of component.

28.I-C20

Cell controller interacts / operator types in component presence on kiting magazine. Cell controller formulates positioning table for bottom fluxer positioning and controls fluxer.

17.II-R6 18.I-R7 19.II-R8 20.II-R9

Positioning table locates the item above the cutter. Cutter moves up and revolves. Positioning table / cutter cuts the leg. Positioning table moves to bottom fluxer.

21.I-R10 22.IIR11 23.I-R12 24.IIR13 25.I-R14 26.I-R15 27.I-R16

29.IIR17 30.I-R18 31.IIR19 32.IIR20 33.I-R21 34.I-R22

Robot grips component. Robot / solder fountain removes component. Positioning table moves to safe position.

Positioning table moves to bottom fluxer. Bottom fluxer fluxes reworked area. Robot picks up a new component - positioning table moves to solder fountain. Positioning table / solder fountain desolder holes. Robot replaces the new component - positioning table moves to safe position. Robot parks tooling - positioning table moves to nloading point.

29.I-C21

31.I-C22

Cell controller initiates component replacement.

32.I-C23

Cell controller determines successful replacement of new component.

33.II-C24

Cell controller terminates TH rework.

4 AUTOMATED REWORK PROCEDURE This section describes the rework procedure for TH and SM component rework. Fig 4 illustrates the core rework procedure that automated rework methods were developed from. The core consists of predefined rework steps as it is seen in the figure. SM REWORK Defectiv e PCBA

Flux defecive area

Preheat target area

Reflow defective component by IR

Remove defective item by robot

Clean pads from excess solder by V.D.I

Dispense solder cream by solder disp.

Place new component by robot

Preheat target area

Reflow new component by IR

TH REWORK Defectiv e PCBA

Cut protruding legs of target component

Flux defecive area

Preheat target area

Reflow defective item by solder fountain

Remove defective item by robot

Flux defecive area

Resolder holes by solder fountain

Place new component by robot

Repaired PCBA

Repaired PCBA

Fig. 4 Core of the automated rework procedure. A brief summary of the reworking of a defective component: A PCBA with defective component(s) is placed on a fixture which is positioned in the centre of the X-Y table. The X-YZ positioning table manipulates the target location of the PCBA to various locations where upper or lower rework tooling are located to perform individual rework requirements such as cutting of the TH component legs, solder dispensing, IR heating, pad cleaning, etc; and rework is carried out sequentially. In this arrangement, the function of the manipulator is to remove and replace the components, exchange the lenses of the IR heat source by the use of suitable grippers or end effectors, carry the solder dispenser and V.D.I. to dispense solder and clean pads respectively and adjust the solder fountain using the screwdriver.

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When considering

rework automation which replaces

human senses with remotely

manipulated mechanical, electronic and pneumatic devices etc. the core needs expansion in order to group

the functional controller and task related activities in a procedural order.

Detailed representation of the rework procedure is quite difficult because of the unstructured nature of the PCBA rework environment and regular sensory information updates. However, the automated SM and TH rework procedure may be simply illustrated as a flow chart in order to illustrate the primary logic and flow of rework. Fig. 5 and 6 present the SM and TH rework flow diagrams respectively. As seen from Figs. 5 and 6, rework activities are carried out in a predetermined manner even though some steps are repeated if the first attempt fails. This feature at least provides a good opportunity for describing a step by step rework procedure assuming that all the decisions that take place in both types of rework are positive and the defective component is not obstructed. Detailed rework procedures were formulated and outlined in two flow charts as shown in Table 1. Rework, inspection and cell controller activities are grouped in separated columns but listed in a sequential manner to illustrate their interdependent relationships. In addition, these procedures also provides detailed information concerning activities of all rework tooling, devices and control systems.

2 PCBA with defective SM component

Manual rework

Locate fiducials

Inspect defects, confirm s fault and clear access

Second Level Sorting out

Defective component obstructed ?

Yes

Yes

Remove obstructing SM item(s) firstly

Obstruction SM ?

No

No

Apply flux

Proceed TH removal

Preheat

(Rem ove TH obstruction(s))

Yes

E R R O R

R E M O V A L

Heat and remove SM component

No

Inspect post desoldered area

Pads m issing ?

Stop rework Component rem oved/pads okey ?

No

Yes

R E C O V E R E D

No

All SM obstructions and SM defective items removed ? Yes

(Starts replacement from defective SM)

Apply flux No Yes

Is it third attem pt for cleaning ?

Clean pads from excess solder

Stop rework B Y O P E R A T O R

Inspect pads for rem oval of solder No

All clean ? Yes

R E P L A C E M E N T

Dispense solder cream No Yes

Is it third attem pt for dispensing ? Stop rework

Inspect pads for solder cream dispensing

No

All dispensed ? Yes

Pick, orient and register new SM com ponent Discard and pick new one No

Good component ? Yes

Place component

Preheat and heat

Inspect for success Yes No

Good ?

Was it SM ?

Yes

SM Defect repaired Any obstructions removed ? Good PCBAs

Fig. 5

Flow chart of SM component rework procedure.

No

Proceed TH replacement Yes

(Replace TH obstruction(s))

3 PCBA with defectiveTH component

Manual rework Locate fiducials

Inspect defects, confirms fault and clear access

Second Level Sorting out Yes Obstruction TH ?

Yes

Remove obstructing TH item(s) firstly No

No

Cut legs

Proceed SM removal

Apply flux

(Remove SM obstruction(s)) No Yes

E R R O R

Defective component obstructed ?

R E M O V A L

Preheat

Is it third attempt ?

Heat (exposure to solder fountain)

Stop rework

Detect removal force and remove component

Yes

R E C O V E R E D

Removal force exceeding threshold value ? No

All TH No obstructions and TH defective item(s) removed ? Yes

B Y

(Starts replacement from defective TH)

Apply flux

O P E R A T O R

R E P L A C E M E N T

Pick new item

Heat (exposure to solder fountain)

No Yes

Stop rework

Detect placement force and place item

Is it third attempt ?

Yes

Placement force exceeding threshold value ? No

Yes

Place component Was it TH ?

No

Proceed SM replacement TH Defect repaired

(Replace SM obstruction(s))

Yes

Any obstructions removed ? No Good PCBAs

Fig. 6 Flow chart of TH component rework procedure.

Not installed :

4

5 EVALUATION OF REWORK PROCEDURE Evaluations of the above rework procedures and the rework methods developed for both types of components were made by running the cell using some demonstration components. The components chosen were: Through hole: DIP-20s and 5 pitch resistors or capacitors Surface mounts: All types of rectangular chips (0805, 1206, 1812, 2220 etc.), SOTs, SOICs and PLCCs. Four different types of PCBAs which were loaded with the above components were used for the evaluation of the chosen rework procedures and methods. The components were removed and replaced by the rework cell under the supervision of the cell controller. TH Rework The rework quality assessment of TH components were made based on visual acceptability standards (IPC-R-700 and IPC-A-610) only; no electrical tests were carried out. Rework tests revealed that successful removal and replacement depended on the cutting of clinched legs as close as possible to PCBA surface and the design of the grippers. Based on the visual quality criteria, reworked TH components complied with acceptable rework qualities but the automated rework experiments revealed that one more sensory requirement is necessary after TH component removal because PCBAs change their shape once they are exposed to solder fountain during component removal. Local deflection must be determined

before

component replacement for more reliable operation. The additional steps necessary for TH rework are (see Table 1) : 28. II- R16a Positioning table moves to inspection area 28. III-I3 Vision system determines local deflection.

SM Rework Assessments were made based on the visual quality acceptance criteria of SM PCBA rework too. As all individual steps of the rework were successfully accomplished by the individual rework tooling, the final result of the component rework quality was very good. Even when the same component was removed and replaced three times on the same PCBAs, no visual damage was seen on the PCBA. As a conclusion, the quality of the automated SM rework is satisfactory and acceptable to visual standards. In the light of results it is possible that the specification for sensing in general and vision in particular could be downgraded. e.g. Pad cleaning and solder paste dispensing are

5

so effective that the need for inspection of these is questionable. These is especially useful since the in-process inspection of the dull solder cream and shiny solder joints poses difficult problems.

6 CONCLUSION Even though various rework methods and techniques could be applied to automated PCBA component

rework

as

part

of

a

robotic

PCBA

cell,

IR

and

solder

fountain

desoldering/resoldering methods were chosen for SM and TH component rework respectively because of their technical superiority. Appropriate rework procedures were developed based on these methods. The procedures developed proved to work well and can be considered to be more effective than alternative rework methods because of their suitability to automation, effectiveness, reliability and most importantly cost effectiveness. The advantages found for the chosen rework methods are: SM Rework



All SM components are desoldered and resoldered without needing any auxiliary heads such as nozzles, heating heads etc.



The desoldering/resoldering method also allows utilisation of current pick and place end effectors that are originally used for PCBA so for PCBA component rework no special extra grippers are necessary.

 

Because of the above , the cost of a rework cell is reduced. Heating of the whole of defective component will enable adhesives to be softened if double sided PCBAs are reworked so it should possible to rework double sided PCBAs.



Blowing or disturbing nearby components is eliminated when using a IR heat source unlike the hot air/gas method.

TH Rework



TH components are desoldered and resoldered by using the same source, no extra tooling is necessary.



Removal and replacement are fast.



The reworked area looks good and is almost indistinguishable from the original. The current design occupies 54 % of the robot envelope and indicates that robotic rework is

possible as an extension to a PCB assembly. This will make the economic justification of a rework cell much easier.

6

References Camurati, P., Mezzalama, M., and Prinetto, P. (1989), "Knowledge-Based Systems as an Aid to Computer-Aided Repair", Microprocessors and Microsystems, Vol. 13 No. 7, September 1989, pp. 457-461. Carroll, P. (1991), "How to Build a Universal AOI Rework Station", Printed Circuit Fabrication (U.S.A.), Vol. 14, Pt: 8, August 1991, pp. 28-30. Driels, M. R. and Klegka, J.S. (1991), "Analysis of Alternative Rework Strategies for Printed Wiring assembly manufacturing Systems", IEEE Transactions on Components, Hybrids, and Manufacturing Technology, Vol. 14, No.3 September 1991, pp. 637-644. Driels, M. R. and Klegka, J.S (1992), "An Analysis of Contemporary Printed wiring Board Manufacturing Environment in the U.S.A.", The International journal of Advanced Manufacturing Technology", Vol. 7, Number 1, pp. 29-37. Geren, N. Chan C.H. and Lo E.K. (1992), ''Computer Integrated Automatic PCBA Rework'', Integrated Manufacturing Systems, Volume 3, number 4, pp 38-43. Hollingum, J. (1991), "Robot System Builds Customised Printed Circuit Boards", Assembly Automation, Vol. 11, No. 3, pp. 21-23. IPC-A-610, "Acceptability of Printed Board Assemblies", The Institute for Interconnecting and Packaging Electronic Circuits, 7380 North Lincoln Avenue, Lincolnwood, Illinois 60646, U.S.A. IPC-R-700C, "Suggested Guidelines for Modification, Rework and Repair of Printed Boards and Assemblies", The Institute for Interconnecting and Packaging Electronic Circuits (IPC), 7380 North Lincoln Avenue, Lincolnwood, Illinois 60646 U.S.A., Revision C, January 1988. Lo, E. K. and Goodall, A. J. (1992), ''Automatic In-process Inspection During Robotic PCBA Rework", Journal of Electronic Manufacturing, Vol. 2, 1992, pp. 55-60. Mangin, C. H. (1987), "Minimising Defects in Surface Mount Assembly", Electronic Packaging and Production, October 1987, pp. 66-67. Strong, J. (1992), "A Common Sense Approach to Rework & Repair", Surface Mount Technology, Feb. 1992, pp. 55-57. Warnecke, H. J., and Wolf, H. (1985), "Robotic Insertion of Odd Components into Printed Circuit Boards", Assembly Automation, Nov. 1985, pp. 198-201.