Surface Mount Technology (SMT) is the soldering of Surface Mount Components (SMC) on the surface of the PCB. The circuit board or device thus created is the Surface Mount Device or SMD. SMT takes great advantage over the conventional Through Hole Technology which is the placing of components through the holes of the PCB. SMT components are extremely small in size and are very difficult to identify since most of them are similar in size and shape. Manual assembling is practically impossible since the soldering pads are very small and a magnifier is required to see the solder pads and tracks. Trouble shooting and component replacement is also very difficult and requires extraordinary skill and suitable tools. The SMT is named so because the solder pads and components are present on the surface of the PCB on the same side.
SMT components make the circuit board very compact since the size of these components is too small and have small leads or even they lack leads. These may also in forms with small flat contacts or solder balls to solder with the PCB tracks. The SMT made it easier to produce circuit boards with higher circuit densities and the components can be soldered on both the sides of the PCB.
SMT Design and Assembling
The design of PCB for SMT is special with very small copper tracks and flat tinned solder points. During manufacturing, solder paste containing flux and solder lead is applied to the solder pads through screen printing.
After this process, the PCB passes to the Pick-and –Place machine. The Components are placed in the machine on plastic or paper sheets wound on reels. The Pick and Place machine takes the components from the reels and place them on the PCB by removing them from the reels.
The PCB with the components then passes into a Reflow soldering oven where it first enters in a pre heat zone in which the temperature of the PCB tracks and components rises uniformly. After this, the board enters into a high temperature zone where the high temperature melts the solder paste present in the solder pads and the component leads firmly stick on the solder pads. The surface tension of the molten solder aligns the leads of all the components in place exactly on the solder pads. After the Reflow soldering, heat sensitive devices are soldered using the hand soldering method. In double sided PCB, the reflow soldering is repeated on the other side also using solder paste or glue. If glue is used to stick the components, wave soldering is done later. After completing the soldering, the PCB is washed thoroughly to remove excess flux and soldering balls using Fluorocarbon solvent or flash solvents like Limonene. After completing the whole process, circuit board is tested for any missing component or errors. The Reflow technique includes Hot gas convection method, Infrared lamp method, Vapor phase reflow method etc.
Through hole technology
It is the conventional soldering method in which the components are placed on the component side of PCB and the leads of components pass through the holes. Soldering is done on the copper track side by joining the lead tips with the solder points. This method is called Plated Through Hole or PTH. When compared to SMT, Though hole soldering gives strong mechanical bonding of components with the PCB. Usually the holes for placing the leads are larger than the component leads around 0.48mm. Axial leaded components like resistors, diodes etc are placed by bending the leads at 90 degree at the solder side to give more mechanical strength. The excess leads are then cut using stripper. Radial leaded parts such as Capacitors, LEDs etc are also soldering in the same way.