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.
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.
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.
Capacitors:
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.
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.
2.2 Active components:
There are two main categories of chip carriers: ceramic and plastic. The plastic chip carriers are primarily used in commercial applications. The ceramic packages provide hermeticity and are used primarily in military applications as they are more expensive. We confine ourselves with the plastic packages which are used extensively for nonmilitary applications where hermeticity is not required.
The most common plastic packages used are the discrete transistors known as small outline transistors (SOTs), small outline integrated circuits (SOICs) with gull wings leads, small outline devices with J wings (SOJ), plastic leaded chip carriers (PLCCs) with J leads, and fine pitch devices with gull wing leads known popularly by as minipacks or plastic quad flat packs (PQFPs).
The SOIC is basically a shrink DIP package with leads on 0.050 inch centres. It contains leads on two sides that are formed outward in what is generally called a gull wing lead. It is also called mini flat pack. A typical SOIC package is shown below.
Plastic leaded chip carriers (PLCCs) are almost a mandatory replacement for plastic DIPs, which are not practical above 40 pins because of excessive real estate requirements. The PLCCs come in a lead pitch of 0.050 inch with J leads that are bent under the packages. These packages have an equal number of J leads on all four sides. The J leads in PLCC provide the compliance needed to take up the solder joint stresses and thus prevent solder joint cracking. The outline configuration of PLCC is given below.
Small outline J packages (SOJs) are used for high density (1 to 4 MB) DRAMs. The SOJ packages have J-bend leads like PLCCs, but they have pins on only two sides as shown in the figure. This package is a hybrid of SOIC and PLCC and combines the handling benefits of PLCC packages with the routing space efficiency of SOIC packages. The leads on each side are split between two groups separated by a large center gap. The gap provides a space for traces to pass under the package. However, there are some packages which do not have center gaps.
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2.3 Fine pitch components
The current trend is towards the usage of larger pin count packages. Packages having more than 84 pins become impractical with 50 mil lead centers due larger package area and difficulties in manufacturing. To overcome the difficulties associated with the larger pin count packages, VLSI manufacturers adopted 25 mil center, gull wing package known as the mini-pack. Eventhough the J lead PLCC has been accepted as the industry standard, the fine pitch packages had to be of the gull wing type. As lead thickness and width decrease, J lead packages are harder to manufacture.
Fine pitch packages are also available with leads on all four sides of the package known as quad flat packs (QFPs). The QFP uses a gull-wing lead form, which complicates the automatic handling of the packages since they can not be supplied in plastic tubes like the dual-in-line (DIP) package. Because of this, each package is housed in its own protective compartment and shipped in trays known as “waffle packs” or matrix trays. The major disadvantage of gull wing packages is that they are susceptible to lead damage and distortion of lead planarity during shipping, handling, and placement. Loss of lead planarity in a fine pitch package may be overcome to certain extent if hot bar soldering is used instead of reflow soldering. Also the lower package thickness compounds the thermal problems and therefore the boards should be suitably designed to allow good thermal conductivity through heat spreaders if necessary. Placement and inspection accuracies required will be more demanding for fine pitch components due to closer lead spacing.
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