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What is Selective Solder ?

Are you waiting for the right time to use a selective solder? Does your design look like it is coming along, but at some point you just need to take a forklift and push it over the finish line? Do you suffer from the common dilemma of wondering what type of solder you should use for a project, but not knowing whether you are choosing the right type? Or maybe you bought a faulty or defective product that was soldered together with lead-free solder and now it’s not working properly? This article will clear up any confusion.

To build a PCB (print circuit board), you need to undertake two major steps:

  1. Fabrication
  2. Assembly

Fabrication involves the construction of the PCB’s architecture physically. On the other hand, Assembly involves the addition of various components that work with the circuit board. The PCB Assembly stage is the last stage of the PCB processing procedure. At this stage, the PCB should be functioning properly. To attach different components onto the PCB during the assembly stage, we have to solder the component onto the PCB. But what exactly is soldering? What are the different types of solders? Can you define selective solder? What is wave solder? These are some of the questions we shall tackle in this article.

What is Soldering?

Soldering is simply a process whereby one joins two metals together using molten solder.

Solder, which is a metal alloy, is composed of two metal elements:

  • Tin
  • Lead

To create a bond using solder, you have to melt it by heating it till it reaches temperatures higher than its melting point (600 degrees). Then, after melting the solder, apply it between two metal components to create a solder joint.

A solder joint has great conductivity, making it great for creating a PCB joint. What’s more, you can easily get rid of the joint using special desoldering tools.

Soldering Types

There exist three soldering types:

Soft soldering – When you compare it to the other soldering types, you will note that it has the lowest filler melting point (900C– 4500C). Due to the low temperature required to melt this solder, it causes the least thermal stress on metal components. However, it does not create strong solder joints.

Hard soldering – the solder, in this case, contains either silver or brass. To achieve hard soldering, you must use relatively higher temperatures, 450 degrees and above. To achieve such a high temperature, you have to use a blowtorch.

Brazing – To achieve brazing soldering, you must use a metal component with a pretty high melting point. The melting point of this component has to be higher than that of the metal component in soft and hard soldering. You should also note that in this case, you generate the solder joint by heating the metal component, not melting it. Once you heat the two metal components sufficiently, you can insert the soldering metal between them. The soldering metal melts and then cools, creating a perfect solder joint.

What is Selective Soldering?

We mainly utilize selective soldering to solder PCBs assembled entirely or partly using THT (Through Hole Tech). A selective soldering system is an equipment that solders THT lead onto boards or panels. It utilizes a solder reservoir and a system pump to eject molten solder through a special nozzle. Once the solder gets off through the nozzle, it makes contact with the lead, which comes through the PCBs bottom.

The selective solder mechanism is built upon an assembly that moves in multiple directions. The movement functions under program control. The PCB’s operator builds the program that oversees this movement.

Aside from overseeing movement, the program also displays the precise time taken for the solder application to complete. All this helps develop stronger solder joints.

What is even more fascinating about selective soldering is that you can develop the program on your computer. All you need is a solder system software, and you are all set.

Selective solder fills in the gap that exists between automated wave soldering and manual soldering.

Manual soldering consumes a lot of time and is also prone to human error. Selective soldering, on the other hand, is pretty efficient. Moreover, it eliminates almost every mishap that comes up in manual soldering.

Most people confuse selective soldering with wave soldering. However, these two are pretty different. See, wave soldering is the automation of the PCB soldering process. The PCB, in this case, moves over a soldering wave. On the other hand, selective soldering deals with PCBs assembled entirely or evenly using THT.

Selective Soldering Processes

When you compare selective soldering with wave soldering, you will find that selective soldering is much slower. Well, that is because selective soldering follows a sequential soldering procedure. That means that the solder machine, in this case, solders one component and then moves on to another sequentially. It uses a local wave to solder.

However, even though selective soldering is slow, it has some benefits that have made it preferable by many manufacturers. These benefits include:

  1. Time utilization – You will note a huge time difference when you compare selective soldering to manual soldering. Selective soldering uses up less time, while manual soldering consumes a lot of time.
  2. Heating property – The PCB must undergo great heating to achieve a strong solder joint using manual soldering. However, with selective soldering, you do not have to use a lot of heat on the board, which helps minimize board damage.
  3. Flexibility – Since the soldering operator has control over the soldering procedure. They can manipulate soldering variables to gain desired outputs.

During the selective soldering procedure, the operator can:

  • Specify nozzle movement according to the time taken to fill up through holes.
  • Program the solder amount per process
  • Set and program process temperatures
  • Program nozzle movement for proper solder application

With this much control, the operator has better precision in terms of soldering, even if they do not have steady hands. In the past, for one to have great precision in soldering, they had to have pretty steady hands, but that is not the case anymore. Selective solder systems have changed the game.

Other Selective Soldering Benefits

  • Repeatability
  • Utilizes less solder and flux
  • You can easily limit masking to the parts of the PCB that need soldering
  • Customization is possible since you can add your parameters to the soldering program
  • Great for Through Hole Technology cases whereby the boards size prevent the application of selective wave soldering

Selective Soldering Disadvantages

  • The setup is not cheap
  • Selective soldering efficiency is not suitable for large-scale PCB production.
  • It consumes a lot of time when you compare with wave soldering

Selective soldering has the power to provide beneficial processes for varying scenarios. Therefore, when you find yourself in a situation that brings in selective soldering to the equation, you can go for it.


Selective soldering is, without a doubt, pretty beneficial in the PCB assembly process. Having to solder components by hand uses up a lot of time, and it also increases human errors. However, with selective soldering, you have precision and better performance. In addition, mass production within a shorter period is made possible. Hence overall, selective soldering is pretty impressive.