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EPISODE 4"How does soldering iron work?"


  • Hikaru
  • Yotchan
  • Mr.Tanaka

Now, let's start the repair.

First, I should remove the plastic insulation of the lead wire. (Photo 1) What do I do next?

Photo 1

You should suck up the solder from the motor terminal. Put the desoldering braid onto the terminal and press it with the soldering iron on the braid. (Photo 2) Be careful not to hold the soldering iron down for too long. Otherwise, the motor's plastic parts will melt.

Photo 2

This makes me nervous. Am I doing it right?

You're doing a good job, Hikaru.

Thanks! Now, I'll wrap the lead wire around the terminal (Photo 3) and solder the wire to the terminal (Photo 4). How is that?

Photo 3

Photo 4

I'm afraid there's a problem with this.

What's wrong?

This type of soldering condition is called "Cold-Joint", an example of unacceptable soldering. (Photo 5) The surface is rough and has no brightness.

Photo 5

But the wire is now connected to the terminal.

It looks the solder connects the wire and the terminal but the connection will come off if it is left in this condition.

Really? What did I do wrong?

Hikaru, you melted the solder with the soldering iron before soldering the wire, didn't you? (Photo 6)

Photo 6

Yes, I did. I know that the soldering iron is a tool for melting solder.

The soldering iron is a tool for heating the target metal up to the temperature at which solder melts and spreads over the metal. Hikaru, your method cannot heat up the target metal sufficiently, so the solder does not spread. Good soldering spreads the solder so that it looks similar to the base of Mount Fuji.


Solder must wet and spread like a water drop spreading on the glass surface. The Good/No Good judgment of soldering depends on the wettability and spreadability of the solder.

Wettability and spreadability!?

Yes. Heat up the target metal and then feed the solder. The wet and spread solder will be absorbed into the target metal and the atoms of the metal will be linked with the atoms of the solder. This is the principle of soldering.

You mean a metal soaks into another metal? You must be kidding!

At the atomic level, there are many apertures in metal, like a sponge. Solder is soaked into this "sponge" to link metals.

I see. There are a lot to learn about soldering. I should not apply solder to the metal, but make the solder soak into the metal. Mr. Tanaka, could you show me a good example?

OK. But I am not an expert either. First, heat up both the lead wire and the terminal, and then feed the solder. After feeding an appropriate amount of solder, stop feeding the solder. After the solder spreads well, pull the soldering iron from the joint. How is that? (Photo 7)

photo 7

That's really great.

After finishing work, let's put the tools away. To put the soldering iron away, first thoroughly wipe off the solder adhering to the soldering tip with the cleaning sponge and then feed fresh solder to the soldering tip. (Photo 8) This can prevent the soldering tip from being oxidized, ensuring a long service life. (Photo 9)

Photo 8

Photo 9

I see.

Then, be careful not to put the solder and the soldering iron within a child's reach. After putting the tools away, clean your hands thoroughly. Solder contains lead and it is not good if lead enters the body.

The soldering iron is a tool that can be used by anyone but it must be handled with care. Now, I'm getting interested in soldering.

Hikaru, did you repair the car?

It has just been repaired. Let's try to run the car.

OK. I'll start. Look!

It's so fast!

Yes. Thank you, Hikaru.

Hikaru, why don't you come to my home next time? I have an interesting handicraft kit.


Examples of common soldering failures