PCB Tech - A new way to play PCBA: through-hole components

PCB through-hole solder paste

Past-In-Hole (PIH) is to print solder paste (Solder paste) directly on the PCB (Print Circuit Board, circuit board) plating through holes (PTH, Plating Through Hole), and then put the traditional The plug-in/through-hole component (DIP insert parts) is directly inserted into the plated 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 are reflowed The high temperature of the soldering furnace will re-melt, and then the parts will be welded on the circuit board.

This method has other names called "pin-in-paste", "intrusive reflow soldering" and "ROT, reflow of through-hole) "Wait.

The advantage of this method is that it can eliminate the process of hand soldering or wave soldering and save labor hours. It can also improve the quality of soldering and reduce the chance of solder shorts. .

However, this construction method has the following inherent limitations:

1. 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 the PIH method requires traditional parts to be reflowed together with general SMT parts, it must meet the temperature resistance requirements of reflow. Lead-free parts are now required to be able to withstand 260°C for 10sec.

2. It’s better to have tape-on-reel packaging (tape-on-reel) and enough flat surface to put it on the circuit board (PCB) through the SMT pick and place machine. If it doesn’t work, just It is necessary to consider sending an additional operator to manually place the parts manually. At this time, the required working 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. .

3. 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 part and the PCB, resulting in excess tin slag and tin beads, which will affect the electrical quality in the future.

The part body and PCB soldering pads must have a standoff (elevated) design

4. 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 of IPC-610 for through-hole solder joints must be more than 75% of the thickness of the carrier board, and some require 50%. (Please refer to the figure below, please refer to IPC-610 section 7.5.5.1 for detailed specifications)

Traditional parts are best printed on the second side (if there are two-sided SMT) Traditional parts are best printed on the second side (if there is two-sided SMT)

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 ≧ [(Maximum diameter of the through hole/2)2－Minimum diameter of the pin/2)2]* π * thickness of the circuit board * 2

How to increase the amount of solder through holes? The following methods are provided for your reference:

1. Reserve enough space near the PCB through hole (PTH) for overprinting.

Discuss with the Layout Engineer (PCB layout engineer) to make more space for printing solder paste near the through-holes that need paste-in-hole. Solder through holes are required to avoid short circuits during overprint.

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. (We will discuss this again when we have the opportunity)

2. Reduce the diameter of the through hole on the circuit board.

Just like the above [Calculation of the amount of solder paste required], 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 part will be inserted into the through hole. .

3. Use step-up (partial thickening) or step-down (partial 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, thereby achieving the purpose of filling the through holes with solder. However, this steel plate is about 10% more expensive than ordinary steel plates on average.

4. 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.

5. Add some solder paste.

You can consider using a dispenser to add solder paste on the Paste-in-Hole pads to increase the amount of solder paste. Since almost no automatic dispensers are available for PCB production lines nowadays, manual dispensing can also be considered., But it is necessary to increase the working hours of an operator.

6. Use solder preforms