How smt plays main role in VLSI?

This technology plays a main role in embedded and vlsi technologies. Surface-mount technology (SMT) is a method for constructing electronic circuits in which the components (SMC, or Surface Mounted Components) are mounted directly onto the surface of printed circuit boards (PCBs).
Electronic devices so made are called surface-mount devices or SMDs. In the industry it has largely replaced the through hole technology construction method of fitting components with wire leads into holes in the circuit board. An SMT component is usually smaller than its through-hole counterpart because it has either smaller leads or no leads at all. It may have short pins or leads of various styles, flat contacts. Where components are to be placed, the printed circuit board has flat, usually tin-lead, silver, or gold plated copper pads without holes, called solder pads. Solder paste, a sticky mixture of flux and tiny solder particles, is first applied to all the solder pads with a stainless steel or nickel stencil using a screen printing process. After screen printing, the boards then proceed to the pick-and-place machines, Numerical control pick-and-place machines remove the parts from the tapes, tubes or trays and place them on the PCB. The boards are then conveyed into the reflow soldering oven. They first enter a pre-heat zone, where the temperature of the board and all the components is gradually, uniformly raised.
oThe boards then enter a zone where the temperature is high enough to melt the solder particles in the solder paste, bonding the component leads to the pads on the circuit board.The surface tension of the molten solder helps keep the components in place. There are a number of techniques for reflowing solder. One is to use infrared lamps; this is called infrared reflow. Another is to use a hot gas convection. Another technology which is becoming popular again is special fluorocarbon liquids with high boiling points which use a method called vapor phase reflow. If the circuit board is double sided then this printing, placement, reflow process may be repeated using either solder paste or glue to hold the components in place.
The materials that can be used to manufacture PCB for this technology:
oLaminates (FR-4, etc.)