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I am not an expert at soldering but I am good enough for simple tasks. I am trying to solder some wires to a PCB.

If I use some 'old' solder (Radio Shack, 60/40 Rosin-core) I get a result that is acceptable to me. Good flow of solder, shiny result, good connection.

If I use a 'new' solder (63/37 Rosin Core, Flux 1.8%) I do not get anything good at all. The solder gets pasty, does not flow, does not stick.

I am using the same soldering iron, the same temperature (370C) and the same technique. The 'new' solder is thinner (0.8mm) than the old one.

What can I do to correct this? Thanks.

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  • \$\begingroup\$ Try 320C instead. \$\endgroup\$
    – Andrew Morton
    Commented Mar 28 at 19:27
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    \$\begingroup\$ 370 seems really high. You might be burning off the flux before it can work. \$\endgroup\$
    – vir
    Commented Mar 28 at 19:27
  • \$\begingroup\$ That is odd because 63/37 is easier to work with than 60/40. 63/37 melts at a slightly lower temperature and flows easier. 63/37 does not have a plastic stage like 60/40. Sure, you don't have the two mixed up. When you say pasty, it sure sounds like a 60/40 plastic stage. \$\endgroup\$
    – Dereck
    Commented Mar 28 at 20:27
  • \$\begingroup\$ Is this chipquick?? Chipquick is a bit different. \$\endgroup\$
    – Scott Seidman
    Commented Mar 28 at 20:38
  • \$\begingroup\$ You're sure the solder isn't counterfeit right? Some cheap solders cut it with a higher amount of lead which raises the melting temperature. Try adding external flux. \$\endgroup\$
    – DKNguyen
    Commented Mar 28 at 23:37

3 Answers 3

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Likely it is bad quality solder. It's very common that poor quality brands tamper with the tin/lead ratio so that it's more like 50/50. Only buy solder from reputable sources. Rosin core no-clean flux is recommended for general-purpose soldering.

Regarding temperatures:

While it is true that leaded 60/40 solder melts somewhere around 180°C and RoHS solder somewhere around 220°C, that melting point of solder has no direct relation with do with the temperature setting of the solder iron.

You aren't supposed to melt solder with the solder iron, you are supposed to melt it with the heated metal surfaces where the joint is to be formed. Meaning that the solder iron needs to be considerably hotter than 180°C/220°C or it will take forever to heat the surfaces.

Therefore, don't listen to people babbling about "it's too hot", they have no clue about what they are saying. As a rule of thumb, you can use these settings:

  • 250°C beginner setting, leaded solder. Takes longer for the wetting to happen, making it easier to observe. Less chance of damaging components and melting cable insulation etc.
  • 290°C beginner setting, RoHS solder.
  • 310°C professional soldering, leaded solder.
  • 350°C professional soldering, RoHS solder.
  • 350-400°C temporary setting for nasty cases where the surface(s) absorb a lot of heat, like large ground planes, RF connectors etc.

Furthermore, depending on the quality of the iron and where the model measures the temperature (if at all), the set temperature does not necessarily equal the real temperature you get at the tip. For example, some cheap Hakko iron I'm using doesn't work well at all if you set it to 350°C - I have to tweak the setting beyond 400°C for it to work as expected. Whereas on a premium iron, the temperature set is what you actually get.

As the tips age, they tend to get worse over time too, requiring more heat, which will in turn make them age even faster.

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I've run across this exact issue. I had some old radioshack rosin core 60/40 solder I used for many years and worked great. Last year I ran out and got a new brand that was highly recommended on forums and was about 60/40 again, but the new stuff was way worse at flowing and sticking to the metal. I don't have a good explanation for why the new stuff was different, but I can tell you that for me the solution in my case was to:

  • Increase temperature of iron to about 400 C
  • Hold the iron a bit longer on the base metal (ideally at the junction of the two base metals simultaneously) prior to introducing solder so you get good flow. The solder won't flow onto the base metal and make a nice shiny volcano unless the base metal gets hot enough.
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  • \$\begingroup\$ "highly recommended on forums" probably doesn't say much at all, you usually don't know who these people are, what experience they got or what kind of projects they solder. Hobbyists typically think any form of gunk they bought for their first time soldering project is awesome. I'm bad at learning the solder brands myself - just buy it from reputable vendors. Not from Aliexpress or Amazon etc. \$\endgroup\$
    – Lundin
    Commented Apr 2 at 12:58
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If the solder is from a Chinese company and it forms a sludge when "melted", it is likely counterfeit.

Tin is significantly more expensive than lead -- 15 to 20 times more expensive. Fake 60/40 solder has a higher percentage of lead than the advertised 40%.

The melting point of genuine 60/40 solder is \$188^{\circ}\$. At that temperature it melts completely and shouldn't be pasty. A soldering iron of sufficient size at \$200^{\circ}\$ should melt solder. If the soldering iron is not large enough, the solder should melt completely with an iron at some higher temperature, such as \$250^{\circ}\$ or \$300^{\circ}\$. But at whatever temperature is required to melt the 60/40 solder, it should melt completely. You should not need a iron temperature of \$350^{\circ}\$ unless the iron is small, or the solder diameter is large.

Counterfeit solder will be pasty. Part of it will melt, but part of it will remain solid, because the ratio of tin to lead is incorrect.

https://www.youtube.com/watch?v=8cCja0FsYUs

https://www.youtube.com/watch?v=5Ku7I3hA3AA

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  • \$\begingroup\$ Thank you. This is exactly what I am experimenting. I will try to find something better on Amazon. \$\endgroup\$
    – Pascal
    Commented Mar 29 at 12:54

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