Advantages and disadvantages of SMT

Our portal is called 4SMT, so let’s see advantages and disadvantages of that technology!

The main advantages of SMT over the older through-hole technique are:

  • Smaller components.
  • Much higher component density (components per unit area) and many more connections per component.
  • Components can be placed on both sides of the circuit board.
  • Higher density of connections because holes do not block routing space on inner layers, nor on back-side layers if components are mounted on only one side of the PCB.
  • Small errors in component placement are corrected automatically as the surface tension of molten solder pulls components into alignment with solder pads.
  • Better mechanical performance under shock and vibration conditions.
  • Lower resistance and inductance at the connection; consequently, fewer unwanted RF signal effects and better and more predictable high-frequency performance.
  • Better EMC performance (lower radiated emissions) due to the smaller radiation loop area (because of the smaller package) and the lesser lead inductance.
  • Fewer holes need to be drilled because drilling PCBs is time-consuming and expensive.
  • Lower initial cost and time of setting up for mass production, using automated equipment.
  • Simpler and faster automated assembly. Some placement machines are capable of placing more than 136,000 components per hour.
  • Many SMT parts cost less than equivalent through-hole parts.
  • A surface mount package is favored where a low profile package is required or the space available to mount the package is limited. As electronic devices become more complex and available space is reduced, the desirability of a surface mount package increases. Concurrently, as the device complexity increases, the heat generated by operation increases. If the heat is not removed, the temperature of the device rises shortening the operational life. It is therefore highly desirable to develop surface mount packages having high thermal conductivity.

The main disadvantages of SMT are:

  • SMT is unsuitable for large parts which has high-power, or high-voltage, for example in power circuitry. It is common to combine SMT and through-hole construction, with transformers, heat-sinked power semiconductors, physically large capacitors, fuses, connectors, and so on mounted on one side of the PCB through holes.
  • SMT is unsuitable as the sole attachment method for components that are subject to frequent mechanical stress, such as connectors that are used to interface with external devices that are frequently attached and detached.
  • SMDs’ solder connections may be damaged by potting compounds going through thermal cycling.
  • Manual prototype assembly or component-level repair is more difficult and requires skilled operators and more expensive tools, due to the small sizes and lead spacings of many SMDs. Handling of small SMT components can be difficult, requiring tweezers, unlike nearly all through-hole components. Whereas through-hole components will stay in place (under gravitational force) once inserted and can be mechanically secured prior to soldering by bending out two leads on the solder side of the board, SMDs are easily moved out of place by a touch of a soldering iron. Without expert skill, when manually soldering or desoldering a component, it is easy to accidentally reflow the solder of an adjacent SMT component and unintentionally displace it, something that is almost impossible to do with through-hole components.
  • Many types of SMT component packages cannot be installed in sockets, which provide for easy installation or exchange of components to modify a circuit and easy replacement of failed components. (Virtually all through-hole components can be socketed.)
  • Solder joint dimensions in SMT quickly become much smaller as advances are made toward ultra-fine pitch technology. The reliability of solder joints becomes more of a concern, as less and less solder is allowed for each joint. Voiding is a fault commonly associated with solder joints, especially when reflowing a solder paste in the SMT application. The presence of voids can deteriorate the joint strength and eventually lead to joint failure.
  • SMDs, usually being smaller than equivalent through-hole components, have less surface area for marking, requiring marked part ID codes or component values to be more cryptic and smaller, often requiring magnification to be read, whereas a larger through-hole component could be read and identified by the unaided eye. This is a disadvantage for prototyping, repair, or rework, and possibly for production set-up.