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. n 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 n n n 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 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 n n 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) n 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 n n 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 n Though assembly costs for an automated SMT line are not appreciably different from that for an automatic through hole significant cost reduction occurs at overall system level. Quality n The higher level of automation in SMT results in improved quality and more consistent output. the line,
Limitations Mechanical Strength n n n 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 n n n 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 n n n n 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 n n n 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: 19 mm & 4 mm 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: n n 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. 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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