SMD or Surface Mount Electronic Components for SMT are no different from through-hole components as far as the electrical function is concerned. Because they are smaller, however, the SMCs (surface mount components) provide better electrical performance.
Not all components are available in surface mount for electronics at this time; hence the full benefits of surface mounting on printed circuit board are not available, ad we are essentially limited to mix-and-match surface mount assemblies. The use of through-hole components such as pin grid array for high end processors and large connectors will keep the industry in mixed assembly mode for the foreseeable future.
Availability of Surface Mount Electronic Components
While only a few types of conventional DIP packages meet all the packaging requirements, the world of surface mount packages is vastly more complex.
The package types and package and lead configurations available are numerous. In addition, the requirements of surface mount components are far more demanding. SMCs must withstand the higher soldering temperatures and must be selected, places, and soldered more carefully to achieve acceptable manufacturing yield.
There are scores of components available for some electrical requirements, causing a serious problem of component proliferation. There are good standards for some components, whereas for others standards are inadequate or nonexistent. Some electronic components are available at a discount, and others carry a premium. While surface mount technology has matured, it is constantly evolving as well with the introduction of new packages. The electronics industry is making progress every day in resolving the economic, technical, and standardization issues with surface mount components. SMDs are available as both active and passive electronic components.
Passive Surface Mount Electronic Components
The world of passive surface mounting is somewhat simpler. Monolithic
ceramic capacitors, tantalum capacitors, and thick film resistors form the core group of passive SMD. The shapes are generally rectangular and cylindrical. The mass of the components is about 10 times lower than their through-hole counterparts.
The surface mount resistors and capacitors come in various case sizes to meet the needs of various applications in the electronics industry. While there is a trend toward shrinking case sizes, larger case sizes are also available if capacitance requirements are large. These devices/ components come in both rectangular and tubular (MELF: metal electrode leadless face) shapes.
Surface Mount Discrete Resistors
There are two main types of surface mount resistors: thick film and thin film.
Thick film surface mount resistors are constructed by screening resistive film (ruthenium dioxide based paste or similar material) on a flat, high purity alumina substrate surface as opposed to depositing resistive film on a round core as in axial resistors. The resistance value is obtained by varying the composition of resistive paste before screening and laser trimming the film after screening.
In thin film resistors the resistive element on a ceramic substrate with protective coating (glass passivation) on top and solderable terminations (tin-lead) on the sides. The terminations have an adhesion layer (silver deposited as thick film paste) on the ceramic substrate, and nickel barrier underplating followed by either dipped or plated solder coating. The nickel barrier is very important in preserving the solderability of terminations because it prevents leaching (dissolution) of the silver or gold electrode during soldering. Resistors come in 1/16, 1/10, 1/8 and ¼ watt ratings in 1 ohm to 100 megaohm resistance in various sizes and various tolerance. Commonly used sizes are: 0402, 0603, 0805, 1206, and 1210. A surface mount resistor has some form of colored resistive layer with protective coating on one side and generally a white base material on the other side. Thus the outer appearance offers a simple way to distinguish between resistors and capacitors.