Surface Mount Surface Mount TechnologyTechnology
Research and Presentation:Research and Presentation:
Amy HoppAmy Hopp
Sept. 4, 2003Sept. 4, 2003
CIM ~ Period 2CIM ~ Period 2
Automation/Programmable Processes inAutomation/Programmable Processes in
Table of ContentsTable of Contents
What is SMT?What is SMT?Screen PrintingScreen PrintingAffects of AutomationAffects of AutomationThrough Hole TechnologyThrough Hole TechnologySolderingSolderingComponentsComponentsSurface Mount ComponentsSurface Mount ComponentsOther Main IssuesOther Main IssuesLimitationsLimitationsEquipmentEquipmentThe FutureThe FutureCitationsCitations
What is SMT?What is SMT?
SMT is a process for populating circuit boards.
This is done by mounting components directly to the surface of the substrate.
Then a layer of solder paste is screen printed onto the pads and the components are attached by pushing their leads into the paste.
When all of the components have been attached, the solder paste is melted using either reflow soldering or vapor-phase soldering.
Screen PrintingScreen Printing
Polyester or stainless steel Polyester or stainless steel woven wire mesh is woven wire mesh is stretched over an stretched over an aluminum frame with a aluminum frame with a glued-on photosensitive glued-on photosensitive emulsion.emulsion.
Stencils have etched Stencils have etched openings to match the land openings to match the land patterns on the substrate patterns on the substrate (PCB). (PCB).
Affects of AutomationAffects of Automation
Screens are used mainly because of its lower Screens are used mainly because of its lower cost, faster delivery and reusability.cost, faster delivery and reusability.
Usually the stencil design is a programmed Usually the stencil design is a programmed process for better efficiency and time.process for better efficiency and time.
Hand printing is almost impossible with screens. Hand printing is almost impossible with screens. it is very difficult to align the screen on the work area it is very difficult to align the screen on the work area
which is due to the poor visibility due to the presence which is due to the poor visibility due to the presence of the screen mesh in the openings. of the screen mesh in the openings.
Through Hole Technology Through Hole Technology
Electronic components are inserted through Electronic components are inserted through holes in the PCB prior to soldering. holes in the PCB prior to soldering.
The insertion may be done manually or by The insertion may be done manually or by machine. machine.
SolderingSoldering
Wave soldering was a “drag-and-dip” process.Wave soldering was a “drag-and-dip” process.
Reflow soldering is usually done by reheating Reflow soldering is usually done by reheating the solder (1 method: infra-red soldering).the solder (1 method: infra-red soldering).
BenefitsBenefits CleanerCleaner Much better qualityMuch better quality Less waste of materialsLess waste of materials
ComponentsComponents
In THT, In THT, size of the component is limited size of the component is limited by the packaging requirement.by the packaging requirement.
Leads must rugged enough to survive the Leads must rugged enough to survive the insertion process without damage.insertion process without damage.
Lead diameters, drilling and imaging Lead diameters, drilling and imaging tolerances puts a limit on how closely tolerances puts a limit on how closely leads are spaced, which in turn limits the leads are spaced, which in turn limits the size of component packages.size of component packages.
Surface Mount ComponentsSurface Mount Components
The solder joint is all the more important as it The solder joint is all the more important as it imparts both electrical and mechanical imparts both electrical and mechanical connections unlike the through hole connections unlike the through hole components.components.
Also see much higher temperatures during Also see much higher temperatures during soldering. soldering.
Because of their smaller size, it is sometimes Because of their smaller size, it is sometimes not possible to provide part markings not possible to provide part markings on them.on them.
Other Main IssuesOther Main IssuesFrequency response Frequency response Surface mount packages has reduced component Surface mount packages has reduced component
related parasitic reactants because of shorter related parasitic reactants because of shorter component leads. component leads.
Parasitic reactants affect the performance of high Parasitic reactants affect the performance of high frequency circuits.frequency circuits.
Package Propagation DelayPackage Propagation Delay(time required for an electrical signal to propagate from the (time required for an electrical signal to propagate from the input to the output of the IC)input to the output of the IC)
In surface mount packages there are fewer parasitic In surface mount packages there are fewer parasitic reactants (primary cause of propagation delay) and reactants (primary cause of propagation delay) and they are smaller and more uniform.they are smaller and more uniform.
Other Main IssuesOther Main Issues
Electromagnetic interference Electromagnetic interference
Through hole component leads serve as tiny Through hole component leads serve as tiny antennas that radiate and receive undesirable antennas that radiate and receive undesirable signals. signals.
SMC leads do not penetrate the board and SMC leads do not penetrate the board and thus reduce this problem. Hence sensitive thus reduce this problem. Hence sensitive circuits can often be combined onto a single circuits can often be combined onto a single board, simplifying shield design.board, simplifying shield design.
Other Main IssuesOther Main Issues
Cost Cost Though assembly costs for an automated Though assembly costs for an automated
SMT line are not appreciably different SMT line are not appreciably different from that for an automatic through hole from that for an automatic through hole line, significant cost reduction occurs at line, significant cost reduction occurs at the overall system level. the overall system level.
Quality Quality The higher level of automation in SMT The higher level of automation in SMT
results in improved quality and more results in improved quality and more consistent output.consistent output.
LimitationsLimitations
Mechanical Strength Mechanical Strength The solder joints of surface mount The solder joints of surface mount
components become both the electrical and components become both the electrical and mechanical contacts to the board. mechanical contacts to the board.
The amount of solder available is generally The amount of solder available is generally much less than at a through hole joint; so its much less than at a through hole joint; so its mechanical strength is less. mechanical strength is less.
Since leads do not penetrate the board, they Since leads do not penetrate the board, they cannot reinforce the joint. cannot reinforce the joint.
LimitationsLimitations
CTE Mismatch CTE Mismatch The coefficient of thermal expansion for The coefficient of thermal expansion for
ceramic component body is less than that of ceramic component body is less than that of the organic printed wiring board.the organic printed wiring board.
The difference in expansion over temperature The difference in expansion over temperature must be entirely absorbed within the solder must be entirely absorbed within the solder joints. joints.
For leadless components, over the full For leadless components, over the full product temperature range, large stresses product temperature range, large stresses can be built in the joints, causing eventual can be built in the joints, causing eventual failure. failure.
EquipmentEquipmentVitronics XPM Series Reflow OvensVitronics XPM Series Reflow Ovens
Maximum Operating Temp. 350º C Maximum Operating Temp. 350º C Windows 98® Operating System Windows 98® Operating System Data Logging Data Logging Cell Fan Speed ControlCell Fan Speed Control Nitrogen Reduction Apertures Nitrogen Reduction Apertures Individual Zone and Controlled Exhaust Individual Zone and Controlled Exhaust Closed Loop Conveyor Speed Closed Loop Conveyor Speed Over-temperature Switches Over-temperature Switches Swing Arm for Computer Swing Arm for Computer Pentium Computer w/ VGA Monitor Pentium Computer w/ VGA Monitor Auto Start/Stop Auto Start/Stop 2000 (+) Recipe Storage 2000 (+) Recipe Storage Light Tower Four Color Light Tower Four Color Electromechanical Power Hood Lifts Electromechanical Power Hood Lifts Closed Loop Temperature Control Closed Loop Temperature Control
EquipmentEquipmentDEK 265 Horizon Screen PrinterDEK 265 Horizon Screen Printer
Windows NT operating system with touch Windows NT operating system with touch screen monitor screen monitor
Programmable telecentric single-camera Programmable telecentric single-camera vision alignment system vision alignment system
Motorized stencil alignment Motorized stencil alignment Automatic rail width adjust with Automatic rail width adjust with
programmable board stop programmable board stop Magnetic board support tooling pins: 19mm Magnetic board support tooling pins: 19mm
& 4mm diameter & 4mm diameter Software controlled closed loop squeegee Software controlled closed loop squeegee
pressure pressure On board event loggingOn board event logging SPC data collection capability SPC data collection capability SMEMA machine interfaceSMEMA machine interface Operator/maintenance documentation on Operator/maintenance documentation on
CD-Rom with on-board help CD-Rom with on-board help 1 year machine warranty (parts and labor) 1 year machine warranty (parts and labor)
The FutureThe Future
Most aims towards the future in SMT is Most aims towards the future in SMT is geared towards:geared towards:
To evolve solder, to create a newer, better To evolve solder, to create a newer, better alloy composition to improve performance. alloy composition to improve performance.
the capability to create PCBs smaller and the capability to create PCBs smaller and lighter (eventually to the nano scale).lighter (eventually to the nano scale).
CitationsCitations
http://www.tutorialsweb.com/smt/smt.htmhttp://www.tutorialsweb.com/smt/smt.htm
http://http://www.engr.iupui.edu/~solaiyap/smt/radhika/smtintwww.engr.iupui.edu/~solaiyap/smt/radhika/smtintro.htmlro.html
http://smt.pennnet.com/Articles/Article_Display.cfhttp://smt.pennnet.com/Articles/Article_Display.cfm?Section=Articles&Subsection=Display&ARTICm?Section=Articles&Subsection=Display&ARTICLE_ID=183179LE_ID=183179
http://http://www.uic.comwww.uic.com//
http://smtinfo.net/http://smtinfo.net/
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