How to Tell If Solder Is Rosin Core: Here’s How

When it comes to choosing the right solder for electronics, you have to be sure you’re using the correct core. However, the packaging might be long gone, so here’s how to tell if solder is rosin core.

Generally speaking, rosin core solder is thinner than an acid core and will leave a grease stain and smell sweeter when burned than other solder types, including acid core. Never use acid-core solder on electronics. 

Rosin solder and acid solder are used in two very different areas. For this reason, it is important to be certain of which solder you are using. If the acid core solder, which is meant for plumbing, is used on a circuit board then there is a high chance that the flux residue will damage the board over time. 

How to Tell If Solder Is Rosin Core or Acid Core

Besides the difference in thickness, solder types can be determined by burning a small piece. When doing the burn test, there are two things to look out for. First, you want to burn the solder on a piece of paper so that you can see what kind of stain it makes. 

If burning the solder leaves a greasy stain on the paper, then it is rosin core. Otherwise, the solder is most likely acid core which can be damaging for electronics. However, since this is not a guaranteed test, there is another thing that can help differentiate the two. 

When burning solder to see what its core is it is important to pay attention to the smell. Rosin core solder usually causes light smoke and smells sweet. Acid core solder, on the other hand, has a stronger smell and the acid can cause ocular irritation. 

To the naked eye, rosin core and acid core are hard to tell apart, that is why the most effective tests are usually the burn tests. When burned, rosin core solder not only leaves a greasy stain on paper but also has a sweeter smell than that of the acid core or any other solder—and that’s going to be your big clues.

Why Use Rosin Core Solder

Rosin core is mainly used in electrical circuits; however, it has various other functions. Unlike acid core, rosin core solder contains smaller amounts of flux, which makes it the preferred option for soldering places where flux residue cannot be cleaned. Flux is a chemical cleaning agent that is used before and after soldering.

Acid core solder is mainly used for plumbing and home repairs where the parts are big enough and easily accessible to clean. However, if the flux is not wiped off, it can also corrode. 

Although flux itself isn’t damaging to electronics, if not properly used, the flux residue could cause electrical failure later on. Electrical failure as a result of flux residue is usually because there is flux residue in hard to reach places. 

Rosin core was designed to solder parts that are unable to be cleaned afterward since it has such low flux levels. The acid core, on the other hand, has flux residue that has to be cleaned up afterward to avoid corrosion.

Since rosin core still has some flux there is still some risk of corrosion if not used properly, but there is much less of a risk which makes it better suited for soldering tight spots where cleaning isn’t an option.

How to Use Rosin Core Solder 

When soldering electronics, the risks are higher than if you were soldering pipes or the like. Here are a few tips to make the process easier and help improve your chances of success. 

Acquire the Proper Tools

Before soldering, you need to make sure to grab the right tools to make the job go smooth.

For electronics, you should be using a soldering iron, not a gun. Soldering guns are meant for plumbing jobs and will just make a mess of electronics.

Next, you need to choose the right solder for your job. Keep these tips in mind:

  • Rosin core solder also acts as a flux. Do not add more flux to the rosin core when soldering.
  • Choose the diameter of the solder based on the size of the job. 0.25” and 0.30” are good general-purpose solders, but you can use 0.015 for detailed work.
  • Lead-free solder is environmentally safe, but harder to work with. Don’t use lead-free solder when you are a beginner.

In addition to the proper tools, you also need to grab a few pieces of safety gear:

  • Safety glasses
  • Gloves (optional)
  • Small fan or face mask to avoid inhaling too much smoke

Clean All Surfaces

Once you have all the necessary tools, thoroughly clean all surfaces, including your work space, then prep the work surface. 

If the place you intend to solder is covered in paint or old residue, use a scratch brush to clean the surfaces you will be soldering. Solder only works on bare metal. 

After cleaning off both surfaces, clean the soldering iron:

  • Wipe off the tip with a sponge. One usually comes with your soldering kit.
  • Coat your soldering iron with solder until silver and shiny.
  • Do this each session since the heat causes the tip to oxidize quickly

Time to Start Soldering 

First thing to do when soldering is to make sure to preheat the iron as well as the solder. 

When soldering:

  • Heat both the iron and the solder for about one second. Careful, the soldering iron can reach up to 500-750 degrees Fahrenheit.
  • Apply the soldering iron to one of the surfaces, then the next. Both surfaces need to reach soldering temperatures. 
  • Add the solder to one of the surfaces. Do not put the solder on the soldering iron, instead melt it on the surface itself.

Here’s some more expert tips:

  • Do not use the dribbling method. It’s imprecise and leads to bad connections.
  • Speed is key. The majority of semiconductors are heat-sensitive, so work quick and don’t heat them excessively.
  • Check your work. Make sure it is properly soldered before putting your kit away.

Keep in mind that you might be clumsy or slow in the beginning, but practice makes perfect. Before long, you’ll be a wizard with your iron.

Are There Different Types of Solder?

Acid and Rosin core are the main types of solder, but other types perform different functions. 

Solders can be put into several categories based on alloy materials, core type, application, and much more:

  • Alloy Materials– the proportion of elements used to make the solder (determines solder’s melting point) 
  • Core Type- determines whether the solder is flux-filled or not (acid core and rosin core and self-fluxing solders, unlike solid core)
  • Application– special solders, such as aircraft solder, automotive solder, or general solder

Bottom Line

Despite the similarities in appearance, some tests can be done to determine whether a solder has a rosin core or an acid one—and you need to know, since acid core solder can ruin your electronic project. 

Remember, if you burn the solder and it smells sweet and leaves a greasy stain on paper, then it’s rosin core. Acid core solder has an acrid smell and a bite to it that can cause eye burning and tears, so it’s fairly easy to identify. However, be careful not to breathe the solder’s fumes in directly—always waft towards your face with your hands.

Alexander Berk

A bit about myself: I am a certified international welding engineer (IWE) who worked in different welding projects for TIG, MIG, MAG, and Resistance Spot welding. Most recently as a Process Engineer for Laser and TIG welding processes. To address some of the questions I frequently got asked or was wondering myself during my job, I started this blog. It has become a bit of a pet project, as I want to learn more about the details about welding. I sincerely hope it will help you to improve your welding results as much as it did improve mine.

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