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Surface Mount Technology

VIEWS: 4 PAGES: 19

									Automation/Programmable Processes in

  Surface Mount
   Technology
   Research and Presentation:
           Amy Hopp

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

								
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