Through Hole Technology
This technology describes the mounting of leaded components by inserting the leads into holes that are drilled through the PCB. The soldering of the leads of the component is done on the opposite side of where the component sits.
For Plated through holes the capillary suction effect fills the holes with solder making this a very strong soldering connection.
Since the soldering is done from the opposite side of the component, it is most efficient that all THT components sit on the same side of the board.
Soldering is done with a solder wave machine or with a selective solder wave machine or by hand. Another soldering technique is pin-in-paste. Using this technique, one first needs to print the solder paste on the PCB also for the THT component holes. One than places all components and proceeds through the solder wave oven.
Surface Mount Technology
In this technology, the leads of the components are placed on the surface of the PCB and soldered to copper pads that form a component footprint to fit all leads from the component.
Placement of SMT components goes very fast en the soldering is done in a run through reflow oven.
A Mixed component means that it combines both THT and SMT technology through hole and SMD connections to the PCB.
It combines the advantages of the two technologies and is most commonly used for power devices and connectors where the Through Hole leads provide mechanical strength for the part.
PRESS FIT Technology
Press-fit components use solder free technology to connect the part to the bare board. This is mostly used for connectors.
The specially designed leads of these component are pressed into the plated through holes which creates a very strong mechanical joint and good electrical connection.
It is very important that the end diameter of the press-fit holes is in the mid-range of the allowable tolerances as stated by the component manufacturer.
If this is not achieved then the pins will either not go into the holes or the connection will be weak.