THT ASSEMBLY
The assembly processes in the electronics sector
THT technology (Trough Hole Technology) concerns the assembly and soldering of all the components that are connected to the Printed Circuit Board (PCB) crossing it from one side to the other through metallized holes.
This is the traditional fixing system with which electronic soldering was born, it is a solid and reliable method but the possibility of miniaturization of this component is limited. For this reason, it has been supplanted over time for almost all the logical or passive functions but it remains an important technology in power applications and connections.
The assembly process of THT components can be very varied because there is less standardization of formats than in the SMT world.
Therefore, upstream of an automatic soldering process there may be both manual and automatic component positioning phases.
The phases of THT automatic assembly are:
Mechanical fixing before soldering
Some components require mechanical anchoring to the PCB before soldering through specific mechanical components such as screws, rivets, turrets etc.
These are components that must be fixed to keep them in position during the wave soldering process or components that require a mechanical retention greater than that guaranteed by the soldered joints.
In the first case the screws can be removed after soldering, in the second case it remains as the final component of the assembly.
In both cases this processing is normally performed before the positioning process of the other components to avoid handling the board with screwdrivers or other tools with all the components already positioned.
Automatic positioning of THT components
In some cases it is possible to assemble some THT components with special machines that insert them in the correct position according to a program loaded in the machine.
These are typically passive components such as resistors, capacitors and diodes that can be purchased in tape and are therefore suitable for being equipped on an automatic machine.
It is also possible to mount integrated modules in DIP format using special loaders; in any case the capacity of the machine is limited to relatively small components. The components are fixed to the PCB with a Cut & Clinch system; once the component has the terminals inserted in the holes, they are cut to size and bent to prevent them from coming out.
In recent years the diffusion of these machines has decreased because the passive THT components have been largely supplanted by those in SMD format which offers greater miniaturization and leaves open the possibility of mounting the components on both sides of the board.
If this process is used, it normally occurs before the mechanical fixings to facilitate the passage in the loading systems of the machine.
Manual positioning of THT components
The manual in-line positioning system is used for all components not suitable for automatic positioning.
The assembly tables are aligned along the loading line of the wave soldering machine, where the operators place the components on the PCBs in the correct positions according to the prepared drawings.
The components were previously prepared divided by type and position, cut to the necessary length so as not to subsequently have to intervene with the soldered joint Once the positioning is complete and the board has been checked, the operator fixes it on a tray that is carried from the loading line to the entrance of the machine and then inside where the soldering processes begin.
At the end of the process, the same tray returns along the lower part of the line where the operator retrieves the soldered board and submits it to the necessary process checks.
Automatic wave soldering
Once the printed circuits have been completed with all the THT components required and positioned on the trays, the actual soldering cycle takes place.
The wave soldering machine consists of an automatic inlet line that drags the trays to the entrance of the machine, here the boards receive the flux and the thermal profile begins with the pre-heating phase which serves to activate the flux and to raise gradually the temperature of the board before passing over the solder bath.
Automatic sensors signal the arrival of the tray near the soldering pot and, at that point, an internal device raises the liquid in the form of a wave that laps the lower side of the printed circuit and forms the soldered joints.
Transit times affect the thermal curve and adjustments to the inclination of the line with respect to the alloy bath are also essential.
If all the parameters are set correctly, all the components are soldered simultaneously both on the side in direct contact with the alloy and inside the holes for capillary rising.
After the alloy bath, the product completes its cycle by returning to the discharge line by completing the last phase of the thermal cycle, cooling, during the return journey.
