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Soldering in Electronics Active components

Introduction to SMT

 1. Benefits of SMT
 2. SMT packages commonly used
           2.1 Passive components 
           2.2 Active components 
           2.3 Fine pitch components
 3. Component package shipping methods
 4. Component procurement guidelines

1. Benefits of SMT

- reduction in package size resulting in greater functionality in the same board area

- reduction in weight. Mobile and handheld electronic items such as video camera, cellular telephone are examples which have low weight and high performance.

- reduction in noise, this is primarily due to smaller electrical paths compared to leaded components. This feature is very useful in RF and microwave circuits where low noise contribution is mandatory and is a design feature.

-higher operating speed resulting from shorter interconnect distances

2. Component packages commonly used:

Surface mount components are functionally no different from leaded components. SMCs (Surface Mount Components) are mounted on the surface of the PCB, the solder joint is all the more important as it imparts both electrical and mechanical connections unlike the through hole components where the component leads sit in the plated through holes which provide certain amount of mechanical strength when soldered.

The surface mount components also see much higher temperatures during soldering. They must be designed with this requirement in mind, Because of their smaller size, it is sometimes not possible to provide part markings on them. If the devices get mixed up, they must be positively identified or thrown away.

SMCs may be broadly divided into passive and active components.

2.1 Passive components:

Ceramic capacitors, tantalum capacitors, thick film resistors form the core group of passive components. The shapes are generally rectangular and cylindrical ( MELF: Metal electrode leadless face). When using SMC passive components, Nickel barrier underplating is desired to prevent dissolution/leaching of silver or gold electrode during soldering.

Resistors:

There are two main types of surface mount resistors.: thick film and thin film. Thick film resistors are constructed by screening resistive film on a flat, high purity Alumina substrate surface. In thin film resistors the resistive element is sputtered on the substrate instead of being screened on. Glass passivation is done above the resistive element.

The resistive layer on the top surface dissipates the heat and should always face away from the substrate surface. The passivation layer is very brittle and should not be probed with hard points such as test probe points during testing. Resistors come in sizes of 0805, 1206, 1210. 0805 represents a resistor component 0.08inch long and 0.05inch wide (apx). The thickness of resistive components is approx. 0.028 inch.

                                   

Chip Resisitor: Line Diagram

 A sample of actual chip resistor

Though there is no standard colour, the 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. Capacitors have generally the same colour (usually brown) on either side.

Ceramic capacitors are multi layered and have improved volumetric efficiency. In the multilayered ceramic capacitor, the electrodes are internal and are interleaved with ceramic dielectric. The alternate electrodes are exposed at the ends and connected at the end termination. The capacitors also come in same sizes as that of resistors, but the thickness of caps is generally double that of resistor components. They come in the ranges from 1pf to .47 uf.

Ceramic capacitors are highly rugged. However, ceramic capacitors are prone to cracking during soldering. The primary causes of cracking in ceramic capacitors are thermal shock during soldering and poor quality control by the vendor. Thermal shock can be minimized by gradually preheating the board before reflow.

For capacitors, the dielectric can be ceramic or tantalum. Surface mount tantalum capacitors offer high volumetric efficiency and high reliability. The capacitance values for tantalum capacitors vary from 0.1 to 100 uF and from 4 to 50 V dc.

A sample of actual chip capacitors shown:

Tubular passive components:

The cylindrical devices known as metal electrode leadless faces (MELFs) are used for resistors, jumpers, ceramic and tantalum capacitors and diodes. They are cylindrical and have metal end caps for soldering. MELFs are cheaper than the rectangular devices. Like the conventional axial devices, MELFs are color coded for values.

Soldering in Electronics Active components
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