PCB Technical - PCB process allows traditional plug-ins of through-hole components to also go through the reflow furnace process

PCB process allows traditional plug-ins of through-hole components to also go through the reflow furnace process

Through-hole solder paste is to print the solder paste directly on the PTH (Plating Through Hole) of the PCB (Print Circuit Board), and then directly insert the traditional plug-in/through-hole components (DIP insert parts) Insert the electroplated through holes that have been printed with solder paste. At this time, most of the solder paste on the plated through holes will stick to the solder feet of the plug-in parts. These solder pastes will be re-melted after the high temperature of the reflow furnace, and then welded. The parts are on the circuit board.

This method has other names such as pin-in-paste, intrusive reflow soldering, and ROT (reflow of through-hole).

The advantage of this method is that it can eliminate the manual soldering (hand soldering) or wave soldering (Wave Soldering) process, thereby saving labor (Labor), at the same time can improve the quality of welding, reduce the chance of solder short (Solder short).

However, this construction method has the following inherent limitations:
The heat resistance of traditional parts must meet the temperature requirements of reflow soldering. General plug-in parts usually use materials with lower temperature resistance than reflow soldered parts. Because this method requires traditional parts to be reflowed together with general SMT parts, it must meet the temperature resistance requirements of reflow. Lead-free parts must now be able to withstand 260°C+10sec.

The parts are best to have tape-on-reel packaging (tape-on-reel) and enough flat surface can be put on the circuit board(PCB) through the SMT automatic pick and place machine (pick and place machine), if not, consider Send an additional operator to manually place the parts manually. At this time, the required man-hours and quality instability must be measured, because the manual plug-in may touch other parts that have been placed and positioned due to careless operation.

The solder pads of the part body and PCB must have a standoff (elevated) design. Generally, the PIH process will print the solder paste larger than the outer frame of the solder pad. This is to increase the amount of solder paste solder to achieve 75% of the through hole filling requirement. If there is no standoff between the part and the solder pad, go through Reflow The molten solder paste will travel along the gap between the parts and the PCB, forming rainy tin slag and tin beads, which will affect the electrical quality in the future.

Traditional parts are best printed on the second side (if there are two-sided SMT). If the parts have been printed on the first side first, and when SMD is continued on the second side, the solder paste may flow back into the traditional parts, causing the possibility of internal short circuits, especially the connector parts. careful.

In addition, the amount of solder is the biggest challenge of this method. The acceptable standard solder volume of IPC-610 for through-hole solder joints must be greater than 75% of the thickness of the carrier board.

As for the calculation of the amount of solder paste, you can subtract the smallest diameter of the pin from the maximum diameter of the through hole, and then multiply it by the thickness of the circuit board to get it. Remember to x2 again, because the flux in the solder paste accounts for 50%, that is After reflow, that is to say, the volume of solder paste will only be left half of the original printed solder paste.

The required solder paste volume is greater than or equal to (the maximum diameter of the through hole-the minimum diameter of the pin)/2]2*π*the thickness of the circuit board*2.

How to increase the amount of solder? The following methods are provided for your reference:
Reserve enough space near the through hole (PTH) of the circuit board for overprinting.

Discuss with the wiring engineer to leave more space for printing solder paste near the through holes that need paste-in-hole, that is to say, try not to place other pads or other unwanted solder through holes nearby., To avoid short circuit when over print.

It should be noted that the flat space of solder paste printing cannot extend out indefinitely, and the cohesion ability of the solder paste must be considered, otherwise the solder paste will not be able to fully retract the solder pads and form solder beads.
In addition, consider that the direction of solder paste printing must match the direction in which the solder pads extend.

Reduce the diameter of the through hole on the circuit board.
Just like the calculation of the amount of solder paste required above, the larger the diameter of the through hole, the more the amount of solder paste required, but at the same time, it should be considered that if the diameter of the through hole is too small, the parts will be inserted into the through hole.

Use step-up (local thickening) or step-down (local thinning) stencil (steel plate).
This kind of steel plate can forcibly increase the thickness of the solder paste locally, which can also increase the amount of solder paste, so as to achieve the purpose of filling the through holes with solder. However, this steel plate is about 10% more expensive than the general steel plate on average.

Adjust the proper solder paste, the speed and pressure of the printing machine, the type and angle of the squeegee, etc.
These parameters of the solder paste printer will more or less affect the solder paste printing volume, and the solder paste with lower Viscosity (viscosity) will have more solder paste volume.

Add some solder paste.
You can consider using a dispenser to add some solder paste on the Paste-in-Hole pads to increase the amount of solder paste. Since almost all SMT production lines nowadays have no automatic dispensers, you can also consider manual dispensing., But it is necessary to increase the working hours of an operator.

Use solder preforms.