What is SMT or Surface Mount Technology?

Surface Mount Technology (SMT) is an umbrella term that includes the methods of mounting and soldering electronic components directly onto the surface of a PCB. The components that are mounted can be resistors, capacitors, transistors, integrated circuits, etc., and these components are referred to as Surface Mounted Devices, or SMDs. SMD is an electronic device/component that can be soldered directly onto the surface of a PCB. Surface Mount Technology (SMT) replaced the through-hole technology as it is easier to deploy (no holes required in the board), takes up less space, is quicker and allows for denser, more compact boards.

 What is the difference between Through-Hole and Surface Mount Technology?

Through-hole mounting is the process in which component leads are placed into drilled holes on a bare PCB. The leads are then soldered on the other side of the board to make the connection to the board. This was the conventional way to solder components onto PCB Boards.

SMT has a number of advantages over the through-hole mounting process – It is easier to automate SMT, the component placement process is quicker and the components have better high-frequency performance.

Having said that, it is also quite feasible to incorporate both thru-hole and SMT in the same circuit board. Despite numerous advantages of SMT over thru-hole technology, SMT is not suitable for some components like large transformers and power semiconductors with heat-sink. They are also not great in harsh environments as SMT components are secured only by solder on the surface of the board. In this case, through-hole soldering might be a better option as component leads run through the board, allowing the components to withstand more environmental stress. Through-hole technology is still preferred in military and aerospace products that are exposed to extreme environmental conditions. Through-hole is also useful in test and prototyping applications that sometimes require manual adjustments and replacements.

Advantages of Surface Mount Technology

Surface Mount Technology allows smaller PCB size, higher component density, and easier assembly with the help of automated machines. Since SMT do not require drilling holes, this process reduces cost and along with dramatic reduction in production time.

Other advantages of surface-mounting components include:

• SMT aids the formation of smaller PCB designs by allowing higher component density, providing designers more real estate to work with. 
• In SMT process, components can be placed on both sides of the circuit board, increasing the components per unit area.
• The SMT process is faster to set up for production than its counterpart, through-hole technology. Since, SMT components do not require drilling holes, this process is cheaper as well.
• SMT assembled PCBs can realize high speed signal transmission due to shorter connection length and small delay.
• SMT is the preferred solution for automated assembly with some placement machines capable of placing more than 136,000 components per hour.
• Due to its compact package and lower lead induction, it has a smaller radiation loop area and thereby a better Electromagnetic Compatibility (EMC).

Disadvantages of Surface Mount Technology

Surface mount technology is not a zero-defect process and presents its own set of unique challenges. While components can be placed faster, the machinery required to do so is very expensive. Therefore, even though the board assembly cost gets lower, initial capital investment for the assembly process increases significantly. This increased initial investment may not be suitable for low-volume prototype boards.

SMT also introduces the possibility of misaligning the components, which in the case of through-hole is less likely. In through-hole, once the leads are through the holes, components get fully aligned and cannot move out of position. However, SMT components might get misaligned if not treated with utmost care.

The SMT Process

Surface Mount Technology is used in the PCB Assembly phase. Once a board is fabricated, PCB stencils are used to apply solder paste to the pads on the board where the components need to be placed. Pick and place machines are then used to place the SMDs/components on the PCB. The solder pastes applied acts like a temporary adhesive that ensures a connection.

The board is then passed through a reflow oven where it is exposed to infrared radiation. The exposure results in the melting of the solder paste and forming solder joints. Following this, the PCBs undergo a variety of quality checks which include component alignment and checking for solder bridges. In case the circuit board is double-sided then the process of printing, placement, reflow may be repeated using either solder paste or glue to hold the components in place.