Tom’s Circuits – Selective Soldering for High-quality Through-hole PCB Assembly

January 25, 2018 , in Blog, Tom's Circuits

PC board design trades off the toughness of through hole with the performance, size, and reduced cost of surface mount. Designers often choose through-hole for connectors and heavy components. Selective solder for through-hole components minimizes soldering time and avoids reheating SMT components. Automated selective soldering boosts production speed and repeatability, ultimately reducing costs.

Tempo Automation improves speed and quality with a selective soldering process. Tempo invests in full automation from design upload to final QA.


The selective solder machine creates a small wave of liquid solder that streams upward from the nozzle into a precise pencil-shaped cylinder. The nozzle moves in X, Y, and Z to dispense solder to the through-hole parts. The machine also applies flux. Difficult connections can be preheated with nitrogen, which flows from small vents near the wave. Preheating decreases the dwell time needed for soldering. This results in faster soldering with less dissolved copper, and creates more rugged and reliable circuit boards.

The PCB assembly line has qualified and highly-trained people who can create beautiful hand-soldered joints. Hand-soldering through-hole parts can be problematic, however. Lead-free solder, for example, is still a challenge. The higher iron temperature and higher solder wetting angle of lead-free solder can damage the PCB assembly. Insufficient temperature results in unfilled pins and cold solder joints. The selective-solder process automates through-hole soldering, using preheating as needed to create an accurate temperature profile with the right amount of heat. The result is improved via fill, better repeatability, less board damage, and assemblies that can tolerate more rework and repair.

Improved Via Fill and Solder Repeatability


X-ray view of inconsistent solder-fill

Board X-ray shows the level of solder fill in a through-hole. This enables adjustment of the selective solder process.

The via can be filled to a desired level

Some parts require top-side solder fillets, while others do not. With selective-solder, it is possible to adjust the solder level in a repeatable way.

Controlled Temperature Profile

Preheat enables topside fillets with minimum copper dissolution

For thick PC boards, it is difficult to get solder to flow all the way through the board to create a topside fillet. Manual soldering can be frustrating this way, because too much dwell time on a solder joint will damage both the fiberglass and the copper pads. Lead-free solder is especially difficult due to its higher melting point. Excessive heat also causes PC board discoloration and degrades PC board insulation.

The solution to the problem is to preheat the board without causing oxidation. In the selective solder process, hot nitrogen flows through small vents around the nozzle and selectively preheats the PC board. Preheating reduces the amount of time that the board spends at maximum temperature.

Copper: Soluble in Liquid Solder!

Dissolved copper causes board damage during rework

Dissolved copper is a problem with pads in board rework and repair. The higher melting temperature of lead-free SAC 305 results in more dissolved copper than Pb 63. The copper knee between the barrel and the pad erodes first, creating a weak spot in the structure. When too much copper dissolves, the pad detaches from the via barrel and easily falls off. A controlled temperature profile reduces the time that the solder is in the liquid state and minimizes copper dissolution.

Design for Manufacturability

Follow these selective soldering PCB design guidelines:

  1.  Don’t place SMT parts too close to through-hole pads.
  2. Orient parts close to through-hole pads orthogonal to the nozzle path.
  3. Increase the keepout region for tall parts.

SMT parts on the solder-side of the board should not be too close to the selective-soldered pads. A clearance of 2mm is usually sufficient. For parts this close, it is best to place the SMT parts at a right angle to the direction of the nozzle path. That way, if the solder wave touches the pad, it won’t instantly remove the SMT component.

Tall parts need to be kept further away, because they may interfere with the path of the nozzle. A simple guideline is to keep the distance from the pad greater than the height of the component.

For space-constrained designs, it is sometimes possible to violate the rules, especially if there is time to develop custom fixturing and programming. Reach out to your Tempo Account Executive, or email the team for help with your design DFM questions.

One last thought about the selective solder process—it can also be used for PCB processing in other creative ways such as board rework and unsoldering.


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