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Cleaning surface mounted component assemblies
Cleaning surface mounted component assemblies B. Lambert
Kerry Ultrasonics Ltd., Hitchin, Hertfordshire, UK
1. Introduction
With the technical advances that have taken place in the electronics industry in building up surface mounted components on ceramic substrates or epoxy printed circuit boards there are bound to be a number of unsuspected production problems. Such factors as coping with the higher density of the components, their installation and alignment, evaluating the efficiency of the soldered joint or the difficulties of carrying out reliable quality checks and test procedures, readily spring to mind. One area which is easily overlooked and possibly one where it is necessary to explode a few myths and create a better understanding is the cleaning of the new assemblies. For instance, it is commonly assumed that it is in order to use the same cleaning equipment as used for conventional printed circuit boards, and hope that the cleaned SMCs will pass inspection. Further investigation will soon show that such methods are no longer acceptable and something much more searching is required. 2. The Need to Clean
All solder creams or pastes developed for screen or stencil printing are formulated from finely graded tin/lead powders, mixed with an activating resin flux base, which will fuse at recommended reflow temperatures. The degree of aggression of a particular flux is governed by the intended application such as when dealing with base copper or nickel which may be more difficult to solder. These acidic activators must be removed from the assembly before they go into service, for, with the higher density approach to component layouts and the increasing operational demands, in terms of temperat.ure or humidity, the conditions which may cause chemical breakdown of the flux residues and subsequent circuit failure become very real. The circuits must be absolutely free from residues to ensure that, when using automatic test equipment, reliable electrical contacts are maintained. Equally important, the aesthetic appeal of a clean assembly is more likely to gain customer acceptance. 3. The Value of Ultrasonics
To assess the benefits of using ultrasonic agitation, Du Pont, the makers of'Freon' fluorocarbon solvent, conducted a series of trials using a Forman type 8625 solder paste, fusing with FC70 for 22 to 28 seconds and providing a SMC clearance of 0.003 in. The times required to achieve visual cleanliness were as follows. For solvent cleaning with ultrasonic using Freon FMS at 35~ the time was 1 minute. Using only boiling Freon TMS solvent at 39.7 ~ the time was extended to 14 minutes, a clear demonstration of the cleaning power of ultrasonics. The trial relates to the use of a particular flux paste and time variances associated with different fluxes may be expected. In practice the user should test and select the best flux based on cleaning efficiency as well as overall performance in the soldering system. A most important point is the reference to visual cleanliness. Contamination test trials using the 75/25 volume ratio isopropyl alcohol/deionised water test solution, as specified in 30
MICROELECTRONICS JOURNAL Vol. 17 No. 6 ~ 1986 Benn Electronics Publications Ltd, Luton
MIL-P-28809A, have indicated a satisfactory level of cleanliness with little or no ionic contamination being removed in the test solution (boiling solvent cleaning process - 5 minute cleaning cycle). However, visual examination of the same glass substrate test samples confirmed the presence of flux paste residues between the SMC and substrate interface at a clearance of 0.006 in. or less, thus drawing attention to the limitations of the aforementioned test procedure when applied to SMC assemblies. The ultrasonic cleaning process provided complete visual cleanliness and of course met the ionic contamination requirements of MIL-P-28809A.
Fig. 1 Post-cleaning electronic circuit boards through a defluxer system fitted ~ith a twin-head Autotrans work transporter. It forms part ofa s investment in FMS by British Aerospace at Filton, Bristol.
4. The Choice of Solvents
When evaluating solvents it should be noted that many of these have severe disadvantages such as flammability and high solvency power, causing damage to plastics, insulating materials and paint finishes. Such solvents include alcohol and chlorinated hydrocarbons. Flurocarbon solvents, or to be specific, 1-1-3 trichlorotrifluoroethane, are non-flammable, have dielectric properties and offer maximum chemical compatibility with most materials used in pcb manufacture. For total flux removal a small percentage of alcohol is normally added, producing an azeotropic mixture of the base fluorocarbon. 31
Fig. 2 Programming a typical low-level multi-stage degreasing and solvent drying system with its associated powered feed and exit conveyors.
The added advantage of using a fluorocarbon solvent cleaning system is the ability to introduce a heat pump thermal recycling system into the solvent circuit. This immediately eliminates the need for electrical limpet heaters for solvent boiling and refrigerated cooling of the reflux coils in the solvent distillation. The cost savings in terms of energy can be as much as 50% and reductions in solvent evaporation losses can be equally impressive. As well as the energy saving benefits, the heat input for solvent boiling provided by heater coils is faster and more even than that obtained from limpet heaters with no risk of hot spots.
5. The Equipment The basic self-contained ultrasonic cleaning systems, comprise two process stages, with the cleaning tank having ultrasonic transducers with an operating frequency of 40kHz. The solvent temperature is controlled at 30~ to ensure correct ultrasonic operation and to maintain an adequate temperature differential compared with the solvent vapour temperature. The vapour stage provides vapour rinsing and flash drying of the pcbs whilst maintaining continuous distillation of the solvent contained in the ultrasonic tank. To remove particulate contamination from the ultrasonic tank, a pump pressurised filtration system can be fitted. The integral heat pump system providing heating and cooling is fitted with automatic temperature control. 32
e 9~s:to~,,~',~,,,~ 9-
D.n,-;ooT ....
Fig. 3 Schematic showing the integration of three p r o g r a m m a b l e w o r k t ra ns port e rs in a FMS for pcb production at British Aerospace.
9
LOAD
I
UNLOAD
7
OW~LL --'~ SL~1RAVEL ~
NORMALT~VEL ~FA.~IR(TURqTRAVEL
Fig. 4 Schematic showing the automated work handling a r r a n g e m e n t for a two-stage ultrasonic degreaser. T h r o u g h p u t in excess of 2500 pcbs a day. Key: 1 - ultrasonic clean 2 - boiling s u m p 3 - vapour zone 4 - reflux cooling coils 5 - heat exchanger 6 - water separator 7 - condensor 8 - compressor 9 - control module.
33
5.1 The value of automation The advantages of using automation for cleaning,degreasing,defluxing or drying pcbs and components, are the opportunities to eliminate manual involvement plus the degree of process control continuity and consistency which can be obtained. Efficient liquid treatment and processing is dependent on establishing optimum cycle times and installing programmable work transporters for process control. The degree of automation employed is largely dictated by workload and obviously if small batch quantity production is involved then programmed work handling need be applied only to progress work through the tanks whilst the presentation of work to the plant can be manual. However, if processing is part of a continuous production sequence or is to be integrated within a FMS complex, then the cleaning plant must be programmed for fully automated operation. 5.2 In-line treatment The basic hardware for both semi- and fully automatic process control of an in-line multistage treatment plant is a microprocessor-controlled work transporter. This can be built either as an overhead or a low-level cantilever-mounted system, depending upon the ceiling height available or the need for access. The latter system lends itself to total enclosure of the process tanks and isolation of the memory control of the transporter head. 5.3 Programming A standard type of memory system provides capacity for several different programmes with manual key switch selection. Using a 'teach and learn' keyboard technique to commit a programme to memory provides fully variable direction control, vertical agitation sequences and adjustable process timing for every stage of a programme. Three operating modes are available: manual, programme and automatic, with digital display to indicate positional data, timing and programme status. Additionally, four auxiliary circuits can be initiated and controlled by the system during any of the four programmes to take care of the secondary functions, such as the triggering of powered feed and exit operations. The options include: 1) Semi-automatic, 2) Fully-automatic, 3)'Pick and Place' (utilising process 'dwell times' to retrieve other baskets in progress), 4) Basket agitation or rotation, 5) Powered conveyors, and 6) Automatic programme selection (interfacing to provide random programme selection).
34
Kerry Ultrasonics Ltd., Hitchin, Hertfordshire, UK
1. Introduction
With the technical advances that have taken place in the electronics industry in building up surface mounted components on ceramic substrates or epoxy printed circuit boards there are bound to be a number of unsuspected production problems. Such factors as coping with the higher density of the components, their installation and alignment, evaluating the efficiency of the soldered joint or the difficulties of carrying out reliable quality checks and test procedures, readily spring to mind. One area which is easily overlooked and possibly one where it is necessary to explode a few myths and create a better understanding is the cleaning of the new assemblies. For instance, it is commonly assumed that it is in order to use the same cleaning equipment as used for conventional printed circuit boards, and hope that the cleaned SMCs will pass inspection. Further investigation will soon show that such methods are no longer acceptable and something much more searching is required. 2. The Need to Clean
All solder creams or pastes developed for screen or stencil printing are formulated from finely graded tin/lead powders, mixed with an activating resin flux base, which will fuse at recommended reflow temperatures. The degree of aggression of a particular flux is governed by the intended application such as when dealing with base copper or nickel which may be more difficult to solder. These acidic activators must be removed from the assembly before they go into service, for, with the higher density approach to component layouts and the increasing operational demands, in terms of temperat.ure or humidity, the conditions which may cause chemical breakdown of the flux residues and subsequent circuit failure become very real. The circuits must be absolutely free from residues to ensure that, when using automatic test equipment, reliable electrical contacts are maintained. Equally important, the aesthetic appeal of a clean assembly is more likely to gain customer acceptance. 3. The Value of Ultrasonics
To assess the benefits of using ultrasonic agitation, Du Pont, the makers of'Freon' fluorocarbon solvent, conducted a series of trials using a Forman type 8625 solder paste, fusing with FC70 for 22 to 28 seconds and providing a SMC clearance of 0.003 in. The times required to achieve visual cleanliness were as follows. For solvent cleaning with ultrasonic using Freon FMS at 35~ the time was 1 minute. Using only boiling Freon TMS solvent at 39.7 ~ the time was extended to 14 minutes, a clear demonstration of the cleaning power of ultrasonics. The trial relates to the use of a particular flux paste and time variances associated with different fluxes may be expected. In practice the user should test and select the best flux based on cleaning efficiency as well as overall performance in the soldering system. A most important point is the reference to visual cleanliness. Contamination test trials using the 75/25 volume ratio isopropyl alcohol/deionised water test solution, as specified in 30
MICROELECTRONICS JOURNAL Vol. 17 No. 6 ~ 1986 Benn Electronics Publications Ltd, Luton
MIL-P-28809A, have indicated a satisfactory level of cleanliness with little or no ionic contamination being removed in the test solution (boiling solvent cleaning process - 5 minute cleaning cycle). However, visual examination of the same glass substrate test samples confirmed the presence of flux paste residues between the SMC and substrate interface at a clearance of 0.006 in. or less, thus drawing attention to the limitations of the aforementioned test procedure when applied to SMC assemblies. The ultrasonic cleaning process provided complete visual cleanliness and of course met the ionic contamination requirements of MIL-P-28809A.
Fig. 1 Post-cleaning electronic circuit boards through a defluxer system fitted ~ith a twin-head Autotrans work transporter. It forms part ofa s investment in FMS by British Aerospace at Filton, Bristol.
4. The Choice of Solvents
When evaluating solvents it should be noted that many of these have severe disadvantages such as flammability and high solvency power, causing damage to plastics, insulating materials and paint finishes. Such solvents include alcohol and chlorinated hydrocarbons. Flurocarbon solvents, or to be specific, 1-1-3 trichlorotrifluoroethane, are non-flammable, have dielectric properties and offer maximum chemical compatibility with most materials used in pcb manufacture. For total flux removal a small percentage of alcohol is normally added, producing an azeotropic mixture of the base fluorocarbon. 31
Fig. 2 Programming a typical low-level multi-stage degreasing and solvent drying system with its associated powered feed and exit conveyors.
The added advantage of using a fluorocarbon solvent cleaning system is the ability to introduce a heat pump thermal recycling system into the solvent circuit. This immediately eliminates the need for electrical limpet heaters for solvent boiling and refrigerated cooling of the reflux coils in the solvent distillation. The cost savings in terms of energy can be as much as 50% and reductions in solvent evaporation losses can be equally impressive. As well as the energy saving benefits, the heat input for solvent boiling provided by heater coils is faster and more even than that obtained from limpet heaters with no risk of hot spots.
5. The Equipment The basic self-contained ultrasonic cleaning systems, comprise two process stages, with the cleaning tank having ultrasonic transducers with an operating frequency of 40kHz. The solvent temperature is controlled at 30~ to ensure correct ultrasonic operation and to maintain an adequate temperature differential compared with the solvent vapour temperature. The vapour stage provides vapour rinsing and flash drying of the pcbs whilst maintaining continuous distillation of the solvent contained in the ultrasonic tank. To remove particulate contamination from the ultrasonic tank, a pump pressurised filtration system can be fitted. The integral heat pump system providing heating and cooling is fitted with automatic temperature control. 32
e 9~s:to~,,~',~,,,~ 9-
D.n,-;ooT ....
Fig. 3 Schematic showing the integration of three p r o g r a m m a b l e w o r k t ra ns port e rs in a FMS for pcb production at British Aerospace.
9
LOAD
I
UNLOAD
7
OW~LL --'~ SL~1RAVEL ~
NORMALT~VEL ~FA.~IR(TURqTRAVEL
Fig. 4 Schematic showing the automated work handling a r r a n g e m e n t for a two-stage ultrasonic degreaser. T h r o u g h p u t in excess of 2500 pcbs a day. Key: 1 - ultrasonic clean 2 - boiling s u m p 3 - vapour zone 4 - reflux cooling coils 5 - heat exchanger 6 - water separator 7 - condensor 8 - compressor 9 - control module.
33
5.1 The value of automation The advantages of using automation for cleaning,degreasing,defluxing or drying pcbs and components, are the opportunities to eliminate manual involvement plus the degree of process control continuity and consistency which can be obtained. Efficient liquid treatment and processing is dependent on establishing optimum cycle times and installing programmable work transporters for process control. The degree of automation employed is largely dictated by workload and obviously if small batch quantity production is involved then programmed work handling need be applied only to progress work through the tanks whilst the presentation of work to the plant can be manual. However, if processing is part of a continuous production sequence or is to be integrated within a FMS complex, then the cleaning plant must be programmed for fully automated operation. 5.2 In-line treatment The basic hardware for both semi- and fully automatic process control of an in-line multistage treatment plant is a microprocessor-controlled work transporter. This can be built either as an overhead or a low-level cantilever-mounted system, depending upon the ceiling height available or the need for access. The latter system lends itself to total enclosure of the process tanks and isolation of the memory control of the transporter head. 5.3 Programming A standard type of memory system provides capacity for several different programmes with manual key switch selection. Using a 'teach and learn' keyboard technique to commit a programme to memory provides fully variable direction control, vertical agitation sequences and adjustable process timing for every stage of a programme. Three operating modes are available: manual, programme and automatic, with digital display to indicate positional data, timing and programme status. Additionally, four auxiliary circuits can be initiated and controlled by the system during any of the four programmes to take care of the secondary functions, such as the triggering of powered feed and exit operations. The options include: 1) Semi-automatic, 2) Fully-automatic, 3)'Pick and Place' (utilising process 'dwell times' to retrieve other baskets in progress), 4) Basket agitation or rotation, 5) Powered conveyors, and 6) Automatic programme selection (interfacing to provide random programme selection).
34