Surface mount technology is an easiest and prefect form of mounting components in Printed Circuit Boards. It entails making reliable interconnections on the board at great speeds, at reduced cost. To achieve these, SMT needed new types of surface mount components, new testing techniques, new assembling technique, new mounting techniques and a new set of design guidelines. SMT is completely different from insertion mounting. The difference depends on the availability and cost of surface mounting elements. Thus the designer has no choice other than mixing the through hole and surface mount elements. At every step the surface mount technology calls for automation with intelligence. Electronic products are becoming miniature with improvements in integration and interconnection on the chip itself, and device - to - device (D-to-D) interconnections
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.
Compact size of electronic equipment is due to surface mount technology. Surface Mount Technology allows packaging of electronic components so that the overall assembly is compact. In SMT both components and conductive tracer (i.e. connections) are installed on the same side of substrate or surface. Substrate can be ceramic, paper plastic, rigid and flexible PCB’s.
SMT allows production of more reliable assemblies with higher I/O, increased board density, and reduced weight, volume, and cost. The weight of printed board assemblies (PBAs) using SMT is reduced because surface mount components (SMCs) can weigh up to 10 times less than their conventional counterparts and occupy about one-half to one-third the space on the printed board (PB) surface.
SMT also provides improved shock and vibration resistance due to the lower mass of components. The smaller lead lengths of surface mount components reduce parasitic losses and provide more effective decoupling The smaller size of SMCs and the option of mounting them on either or both sides of the PCB can reduce board real estate by four times. A cost savings of 30% or better can also be realized through a reduction in material and labor costs associated with automated assembly.
Surface mount technology was developed in the 1960s and became widely used in the late 1980s. Much of the pioneering work in this technology was by IBM. The design approach first demonstrated by IBM in 1960 in a small-scale computer was later applied in the Launch Vehicle Digital Computer used in the Instrument Unit that guided all Saturn IB and Saturn V vehicles.
Types of Surface Mount Technology
On the bases of their assembly, there are mainly three types of Surface Mount Technolog
Type I : For a single sided type I, solder paste is printed onto the board and components are placed The assembly is reflow soldered and cleaned (if needed). For double-sided Type I, the board is turned over, and the process sequence just described is repeated. This is a full SMT board with parts on one or both sides of the board.
Type II: This is probably the most common type of SMT board. It has a combination of through-hole components and SMT components. Often, surface mount chip components are located on the secondary side of the Printed Board (PB). Active SMCs and DIPs are then found on the primary side. In general practice, only passive chip components and low pin count gull wing components are exposed to solder wave immersion. Multiple soldering processes are required.
Type III: Leaded components are inserted, usually by automatic equipment. The assembly is turned over, and adhesive is applied. Next, passive SMCs are placed by a "pick-and-place" robot, the adhesive is cured, the assembly is turned over, and the wave-soldering process is used to solder both leaded and passive SMCs in a single operation. Finally, the assembly is cleaned (if needed), inspected, repaired if necessary, and tested.
For this type of board, the surface mount components used are chip components and small pin count gull wing components.
Fine Pitch Devices
The need for high lead-count packages in semiconductor technology has increased with the advent of application-specific integrated circuit (ASIC) devices and increased functionality of microprocessors. As package lead count increases, devices will become larger and larger.
To ensure the area occupied by packages remains within the limits of manufacturing equipments, lead pitches have been reduced. This, coupled with the drive toward higher functional density at the board level for enhanced performance and miniaturization, has fostered the introduction of many devices in fine-pitch surface mount packages.
A fine-pitch package can be broadly defined as any package with a lead pitch finer than the
1.27mm pitch of standard surface mount packages like PLCCs and SOPs. Most common lead pitches are .65mm and .5mm. There are even some now available in 0.4mm pitch. Devices with
these fine pitches and leads on all four sides are called Quad Flat Packs, (QFPs).
The assembly processes most dramatically affected by the fine-pitch package are paste printing and component placement. Fine pitch printing requires high quality solder paste and unique stencil aperture designs. Placement of any surface mount package with 25 mils or less of lead pitch must be made with the assistance of a vision system for accurate alignment.
Placement vision systems typically consist of two cameras. The top camera system scans the
surface of the board and locates fiducial targets that are designed into the artwork of the board.
The placement system then offsets the coordinates in the computer for any variation in true board location. The bottom camera system, located under the placement head, views the component leads. Since the leads of fine-pitch components are too fragile to support mechanical centering of
the device, the vision system automatically offsets for variations in the X, Y, and theta dimensions. This system also inspects for lead integrity problems, such as bent or missing leads.
Other manufacturing issues for assembling fine-pitch components on PC boards include:
1. Printing various amounts of solder paste on the 25-mil and 50-mil lands. One stencil thickness
will usually suffice. But stencils may be stepped down to a thinner amount for fine pitch aperture areas to keep volumes lower to prevent bridging.
2. Cleaning adequately under and around package leads,
3. Baking of the packages to remove moisture,. Thin QFPs are susceptible to a problem known
as pop corning where moisture in the plastic can literally explode when heating in reflow or rework and crack the plastic package.
4. Handling of the packages without damaging fragile leads. These challenges are by no means insurmountable. Many equipment choices have already found solutions to these issues.
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