Surface Mount Technology

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 significant contributor to D?to?D interconnection costs.

In SMT, the following are important
D-to-D interconnection costs.
Signal integrity and operating speeds.
Device- to-substrate interconnection methods.
Thermal management of the assembled package.

D-to-D interconnection costs have not decreased as much as that of the ICs. A computer-on-a-chip costs less than the surrounding component interconnections. The problem of propagation delay, which is effectively solved at the device level, resurfaces as interconnections between the devices are made.

The modified new IC packages, having greater integration of functions, less in size and weight, and smaller in lead pitch, dictate newer methods of design, handling, assembly and repair. This has given new directions to design and process approaches, which are addresses by SMT.Currently, D-to-D interconnections at the board level are based on ?soldering?-the method of joining the discrete components.

The leads of the components are inserted in the holes drilled as per the footprint, and soldered.In the early decades, manual skills were used to accomplish insertion as well as soldering, as the component sizes were big enough to be handled conveniently. There have been tremendous efforts to automate the method of insertion of component leads to their corresponding holes, and solder them en-mass. The leads always posed problems for auto-insertion. The tendency of Americans against using manual, skilled labour resulted in the emergence of SMT, which inherits with it automation as precondition for success.
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, a matrix of solder balls (BGAs), or terminations on the body of the component.
Components were mechanically redesigned to have small metal tabs or end caps that could be directly soldered to the surface of the PCB. Components became much smaller and component placement on both sides of a board became far more common with surface mounting than through-hole mounting, allowing much higher circuit densities. Often only the solder joints hold the parts to the board, although parts on the bottom or "second" side of the board are temporarily secured with a dot of adhesive as well. Surface mounted devices (SMDs) are usually made physically small and lightweight for this reason. Surface mounting lends itself well to a high degree of automation, reducing labor cost and greatly increasing production rates. SMDs can be one-quarter to one-tenth the size and weight, and one-half to one-quarter the cost of equivalent through-hole parts.
The main advantages of SMT over the older through-hole technique are:
• Smaller components. Smallest is currently 0.4 x 0.2 mm. (.01" x .005" - 01005)
• Much higher number of components and many more connections per component.
• Fewer holes need to be drilled through abrasive boards.
• Simpler automated assembly.
• Small errors in component placement are corrected automatically (the surface tension of the molten solder pulls the component into alignment with the solder pads).
• Components can be placed on both sides of the circuit board.
• Lower resistance and inductance at the connection (leading to better performance for high frequency parts).
• Better mechanical performance under shake and vibration conditions.
• SMT parts generally cost less than through-hole parts.
• Fewer unwanted RF signal effects in SMT parts when compared to leaded parts, yielding better predictability of component characteristics.
• Faster assembly. Some placement machines are capable of placing more than 136,000 components per hour.
Defective surface mount components can be repaired in two ways: by using soldering irons (depends on the kind and number of connections) or using a professional rework system. In most cases a rework system is the first choice because the human influence on the rework result is very low. Generally, two essential soldering methods can be distinguished: infrared soldering and soldering with hot gas.
Benefits and disadvantages of different soldering methods
Infrared soldering:
During infrared soldering, the energy for heating up the solder joint will be transmitted by long or short wave electromagnetic radiation.
• Easy setup
• No compressed air required
• No component-specific nozzles (low costs)
• Fast reaction of infrared source (depends on used system)
• Central areas will be heated more than peripheral areas
• Temperature can hardly be controlled, peaks cannot be ruled out
• Covering of the neighboured components is necessary to prevent damage, which requires additional time for every board
• Surface temperature depends on the component's reflection characteristics: dark surfaces will be heated more than lighter surfaces
• The temperature additionally depends on the surface shape. Convective loss of energy will reduce the temperature of the component
• No reflow atmosphere possible
Conventional hot gas soldering
During hot gas soldering, the energy for heating up the solder joint will be transmitted by a gaseous medium. This can be air or inert gas (nitrogen).
• Simulating reflow oven atmosphere
• Switching between hot gas and nitrogen (economic use)
• Standard and component-specific nozzles allow high reliability and reduced process time
• Allow reproducible soldering profiles
• Efficient heating, large heat amounts can be transmitted
• Even heating of the affected board area
• Temperature of the component will never exceed the adjusted gas temperature
• Rapid cool down after reflow, resulting in small-grained solder joints (depends on used system)
• Thermal capacity of the heat generator results in slow reaction whereby thermal profiles can be distorted (depends on used system)
A rework process usually undoes some type of error, either human or machine-generated, and includes the following steps:
• Melt solder and component removal
• Residual solder removal
• Printing of solder paste on PCB, direct component printing or dispensing
• Placement and reflow of new component.
Sometimes hundreds or thousands of the same part need to be repaired. Such errors, if due to assembly, are often caught during the process, however a whole new level of rework arises when component failure is discovered too late, and perhaps unnoticed until the end user experiences them. Rework may also be used if high-value products require revisions, and re-engineering, perhaps to change a single firmware based component, may revive a once obsolete product. These tasks require a rework operation specifically designed to repair/replace components in volume.
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