Surface Mount Technology：.ppt

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