Montana's Ham Radio Forum

And now for an Elmering moment...

Those of you who like to solder probably already know the importance of getting a good, shiny connection. Heat transfer to the wire and contacts being soldered is critical to getting the solder to melt and flow between the two pieces.

A great way to achieve a swift and proper solder joint is to use rosin soldering flux. This is not the "acid flux" used on copper pipe! Acid flux is corrosive and should never be used for electronics. But there is "rosin flux" that is appropriate for electrical work. It is available in paste form in a small tin similar to how plumbing acid flux is sold. The key point is to make sure the container has the words "Rosin" flux on the label. Sometimes you will also see the wording, "Safe for electrical work" also on the label. Rosin soldering flux is also available in liquid form. Many types of solder sold for electrical work have flux in the middle and are labeled as "Rosin core solder". This helps you tell it apart from the plumbing solder that is often labeled as "Acid core solder".

An important tip for using flux is that a little dab works best. Don't over do it and use too much, which will cause a mess. Strip the insulation off the end of a stranded wire, dip it in the flux, then apply your soldering iron and solder. The flux will sizzle and help remove any oxidation or tarnish on the wire. The solder will quickly wick its way up the wire, so be quick to remove the iron before the insulation overheats and begins to melt. It takes practice to learn just the right amount of heat to apply while soldering. Not enough heat and you get a "cold" solder joint with a dull finish that will have high resistance to current. Too much heat and the insulation melts and shrinks back up the wire, leaving an exposed bare wire. Just the right amount of heat results in a shiny joint with a good electrial connection (and no burnt fingers from holding a hot wire too long!)

While most solder sold for radio electronics use has rosin flux in its center, I often find that the small amount is not quite enough to get a quick wicking action from the melting solder. Adding a small dab of soldering flux will make things seem as if they are turbo-charged! The solder will flow and wick into the wire swiftly, allowing you to reduce the amount of time you have the hot soldering iron in contact with the items being joined. Less time spend applying heat means fewer scorched and overheated components and wire insulation!

Another important tip is that when you are finished soldering, use a small stiff brush dipped in isopropyl alcohol to remove any of the brown flux residue that remains. Flux is very sticky and gooey, which attracts dirt and dust. Flux residue can be conductive, which means excessive blobs on a circuit card can bridge across adjacent contacts and short out the connections. This is especially true with IC chips, with their many pins separated by only a fraction of an inch on the circuit board. Leaving behind flux residue bridged across contact pads ranks a close second to using too much solder and bridging across adjacent contacts. Both cause your electronics project to not work properly or even at all. I usually use the cheap silver metal handled brushes sold as "Acid brushes" for applying acid flux to copper pipes prior to soldering them. Use a large pair of scissors to cut the bristles down to 1/4 inch in length. This will give you the stiffness needed to scrub the flux residue away with the help of the isopropyl alcohol. There are small isopropyl alcohol dispensers sold that have a flip lid with a small bowl underneath. Pushing down on the lid will pump the isopropyl alcohol up into the bowl, allowing you to dip your brush without contaminating the whole container with flux residue.

Rosin soldering flux in paste form is sold at Radio Shack (Cat No. 64-022). It costs $6.99 for a small plastic "tin" about the size of a can of dip. Radio Shack used to sell a small 1 oz. tube of liquid flux (Cat No. 64-021), but it has been discontinued and is no longer in stock at our local Radio Shack. To futher emphazise that just a little dab of flux is all you need, I bought one of those small tubes of liquid flux many years ago - and I still have plenty left! So don't let the $6.99 price deter you. If used carefully, it will last you several years (depending on how much soldering you do). Once you get used to using soldering flux, you will appreciate how much easier it makes the task of tinning wires or soldering connections!

A suggestion was made to me that using rosin-core solder would be helpful. This is very true! Rosin-core solder should be used when working on radio electronics. In fact, I use rosin-core solder myself. The rosin flux is inside the center of the solder, so you have both solder and flux together in one convenient package.

However, there are times when I have found the application of a small drop of flux when soldering a connection to be very beneficial. It really speeds up the process. When the hot iron touches the connection, the solder flows quickly and wicks up the strands of the wire like lightning! In applications such as circuit boards with thin traces and small solder pads, the length of time you can leave a iron in contact is greatly reduced. It can be a matter of just one or two seconds. So the additional flux really helps. The downside is the additional clean-up required after the connection is finished, due to the excessive flux residue left behind.

If you have success using rosin-core solder alone, then great! Try that approach first. There is less mess to clean up afterwards. But if it is taking too long to make a good connection, and your soldering iron is working correctly, consider using just a small dab of liquid or paste flux, in addition to the rosin core solder, to speed things up.

A typical soldering iron tip temperature is 700 to 750 degrees. For general purpose work, a 60-watt iron works best. For very small electronic work, a 25-watt iron would be appropriate. For large connections, a 75-watt soldering "gun" is needed. They all operate in the same general temperature range. The difference is in how long they can provide the needed heat. A large connection will require a bigger iron to get the temperature up to the melting point of solder. But a large iron will transfer too much heat to a circuit board, so you have to size the iron to the type of work you are doing.