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SMT Assembly

SMT or surface mount technology is a method applied to solder SMD (surface mount devices) components to PCBs that ultimately result in compact and small electronic products. The SMD components are smaller, light weight and can be directly placed and soldered on the same side of the PCB.

In figure 1 an Atmega-8 with 28-pin dip package and 32-pin TQFP package is shown. The SMD component is much smaller as compared to the TQFP package IC and requires smaller height and area to solder.

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Figure 1: Comparison of Atmega-8 28-pin and TQFP-32 pin package IC.

In figure 2 an assembled PCB is shown with DIP and equivalent SMD packaged ICs. The SMD components assembled PCB is much smaller than its predecessor DIP packages.

 

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Figure 2: Assembled PCB with DIP and equivalent SMD components

The SMT technology components have replaced their DIP counterparts due to their size and comparable prices. However, it is possible to use both of the technologies on the same board.

The SMT production process i.e., soldering of SMD components is also fast and lowers the soldering cost due to less soldering pin area.  But it causes increased defects due to miniaturization of components and increased pin density per unit area of package. In that case the defect detection and troubleshooting is more costly. The PCB design and fabrication is also tricky and time taking for the SMT devices.

The SMT assembly has various advantages over the conventional DIP methods:

  1. A higher component  and pin count is possible for signal routing that reduces PCB real estate
  2. Components can be placed and soldered on both sides of the PCB
  3. Due to smaller pins and track length PCB parasitic becomes lower such as parasitic capacitance and parasitic inductance
  4. Better EMC/ EMI performance
  5. Lower cost of soldering materials
  6. Because the components have lower height and smaller footprint are size the produced products are more smart, light weight, noise free and portable

Although there are many advantages of the SMT assembly but there are some disadvantages of this method:

  1. Fault tracing is hard to find specially in the multilayer boards
  2. Manual assembling, prototyping and component level repairing is much difficult such as  in case of QFN, BGA packaged ICs
  3. Needs specialized tools for soldering and re-work like microscope, fine soldering tips etc.
  4. PCB manufacturing, prototyping tools and accessories are costly
  5. Solder joints are more prone to dry-joint
  6. Components during desoldering may be permanently deranged due to very small contacts and extra heat application
  7. Extra adhesive composites potting is necessary for components which face frequent mechanical stress such as connectors, heat sinks, and large devices.
  8. For large productions auto pick and place machines outperform and preferred over manual assembly but they are much costly.

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Figure 3: Auto SMD pick and place machine