Surface Mount Technology MT
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Electrical Fan
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14-11-2009, 05:14 PM

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
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.doc   SMT REPORT.doc (Size: 854.5 KB / Downloads: 165)
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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thank you very much sanjay!!!!
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Surface Mount Technology MT

.pdf   F2103 SMT Repair-Touchup-Hand Solder.pdf (Size: 122.72 KB / Downloads: 88)


Surface Mount Technology has been around now for some time, and has proven that the benefits anticipated
are in fact real. The benefits of reduced space, reduced cost, and improved performance (quality, reliability, and
electrical performance) have been proved and continue to fuel the drive to increased implementation of surface
mount technology.
One aspect of surface mount technology that has been difficult to deal with is repair, touch up and replacement.
This is a complex field when all part types are considered. Many of the repair and touch up aspects of this
bulletin will be applicable to all part types, however, some of the very complex repair issues will require even further
evaluation. Use the information as a thought starter and take it from there.
Why and When?
Many of the benefits of surface mount technology are derived from the automation and process control available
with this technology. The tight spacing of parts and the different types of solder joints have caused us all to
relook at what is acceptable and what needs to be repaired. It has been proven that Printed Circuit Assemblies
(PCs) that have not been repaired will have a much higher success rate at final test and in use, compared to
PCAs that have been repaired. Failures of parts and solder joints that have been repaired, and failure of parts
and solder joints in the vicinity of repairs are common.

In General

There are many items to consider when trying to minimize repair and rework. In a very simplistic view, the
total manufacturing cycle for a surface mount product breaks down into four categories, each one of which
should be reviewed with rework and repair in mind.
1. The design and standards for the product. These are the base lines for manufacturing process, equipment,
fixtures, training, inspection criteria, and repair/rework capability in regards to the product. If the
product is designed and the standards selected well, the manufactured product will require little repair
and what repair is required will be successfully performed.
2. The second area is the manufacturing process itself. This includes whatever is necessary in manufacturing
the product to minimize repairs and rework. Design of mounting pads, selection of printed circuit
board location tolerances, placement accuracies, soldering process controls, selection of solder pastes,
operator training, etc; all of these are important to minimize rework. The “Do It Right the First Time” attitude
goes a long way in SMT Technology Assemblies.
3. The third area is the inspection process. Visual inspections are the most prevalent method today, with
“Automatic Optical Inspection” equipment being developed and fine tuned for the more difficult and the
more astute of operations. Which method to use will depend on the product being manufactured and to
some extent on the capital ($) available. It is important to recognize the limitations of each to recognize
these in the standards required of the product and process.
4. The fourth area is the actual rework and repair of the process. This is further broken down into a number
of classifications. Each of the classifications will require different levels of training and different types of
equipment to be used. These classifications are:
a. Touch-up: This is the repair of a solder joint which has excessive solder or which requires more solder.
b. Re-alignment: This is the actual movement of a part either in wet solder paste, wet adhesive, or after

Visual Inspection Standards
The standards for visual inspection and for identifying repair candidates are also very important in
attempting to minimize repair. Inspection and repair standards should be developed under very controlled
conditions. Do not copy someone else’s attempt. Other standards could be used to generate
ideas, however the product being manufactured is unique, so it makes sense that the inspection standards
are equally unique.
a. Ask why a certain condition should be repaired.
b. Are there facts to go along with it?
c. Are the risks of the repair/replacement larger than the risks involved in not repairing?
d. Once deciding on a reasonable standard that makes sense for the product, ask whether the
process can produce product which meet the criteria on a regular basis. (If not, a relook at the
process or the inspection criteria is suggested.)
e. Do not build repair into the process. Repair only by exception and then use history to improve the
process and reduce repair.

The Manufacturing Process
This should be fairly obvious. To minimize the repair the manufacturing process should be optimized
and operated to isolate the cause of repair and correct the situation. We won’t go into a lot of areas for
concern other than to mention that Engineering Bulletins are available from KEMET which discuss the
Wave Solder Process and the Reflow Solder Process and how it can be optimized to minimize the
rework and repair. Other literature is available which will assist in improving the process. The manufacturing
process is the key to minimum repair and rework.

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