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Resistance Soldering

Craftsman Tool Chest: 5
Mastering techniques for Resistance Soldering

By George Riley/photos by the author

The technique of soldering has been the preferred method for attaching non-ferrous metals to one another since the earliest days of metalworking. Far from becoming an obsolete craft, solder literally binds together our modern electronic world. From our computers to our automobiles we are surrounded by items produced in whole or in part by the process of soldering. In the model railroading hobby learning how to solder proves to be useful in many of its facets that range from electric wiring to constructing and detailing our metal models.

Basic soldering involves heating the metals to be attached so that a soft lower melting point metal (solder) will flow between the two and once cool create a mechanical bond between them. The basic tools are a heat source in the form of an electrically heated iron, gas fueled torch, electric resistance unit, or commercial wave oven, solder of which there are a wide number of types for varied applications and flux to allow the solder to flow and aid in cleaning the two metal surfaces.

For model construction the most useful of the different techniques is the resistance soldering unit. A transformer housed in the power supply unit drops household voltage down to a safe low voltage, high amperage current that when a circuit is created on a grounded work piece causes that piece to instantly heat up, quickly melting the solder and creating a clean, solid bond. The piece will only be heated when the circuit is complete. An additional benefit of resistance soldering is that it will create a joint with a minimum of solder allowing the user to use millimeters of a roll of solder rather than inches. This reduces the amount of clean up needed to finish a presentable project.

Since nearly all resistance units allow the user to regulate the output to the pieces being attached the system can be used to solder nearly any metal adding to their versatility. The only items that one should not use resistance soldering to assemble are circuit boards with electronic components. The current output by these units could potentially damage delicate circuitry and therefore is not worth the risk.

Resistance Soldering

The Equipment
There are several hand pieces available that allow the user to tailor their unit to the type of soldering that needs to be done. The two most useful for model building are:

The single electrode handpiece: This tool with its separate grounding clamp is the most similar to commonly used soldering irons and will be the easiest to master if one has any previous experience. The handpiece uses a replaceable carbon electrode similar to the tip of a conventional iron; however, this is where the similarity ends. Most users of regular irons will use the heated tip to pry and move pieces around, the resistance handpiece should not be used in that way since the carbon electrode is brittle and easily broken. In addition the electrode cannot be "tinned" and will not hold solder; it is solely used to heat the metals being bonded. Otherwise the handpiece can be used to tin materials, make joints, or perform any other function that conventional electric soldering iron can accomplish.

The tweezer handpiece: This handpiece combines the ground and positive electrode in a single hand-held unit. This tool allows the user to hold items in place and is ideal for applying small detail parts to models. The tweezer handpiece should always be used with a foot pedal so that current can be cut off to allow the joint to cool while still holding the part in place. While somewhat more difficult to master, this tool is a great aid to fine detail application.

The power supply: Any of the handpieces will work well with the standard resistance soldering unit power supply. Units featuring up to 250 watts output are available to the hobbyist. The power output can be adjusted for the thicknesses and types of metals being soldered. The setting for most normal brass soldering is between 40-60% power, while 20-30% works well when attaching white metal. These power requirements may be different between users and projects due to the many variables involved.

The foot pedal: Nearly all premium resistance sets include a foot pedal to interrupt the current to the handpieces. Not only is this a "failsafe" to assure safe operations of the unit, but also allows the user to use the handpiece to hold the item being attached while the joint cools and hardens.

Resistance Soldering

The Project
To demonstrate some of the features and uses of the American Beauty Resistance Soldering Outfit a Backwoods Miniatures On30 Vertical Boiler Steamer Kit was used. This is a complete locomotive kit featuring photo etched brass and die-cast with metal parts. Since the kit uses two different types of metal; .064 silver bearing electronic solder was used to attach brass to brass and Tix low melting point solder was used to attach the white metal parts to both white metal and brass pieces.

Use the proper power setting
Resistance soldering works best when joints are made quickly. Set the power setting so that the solder melts and the joint is finished in less than ten seconds if at all possible. The power supply is designed to provide power for no more than twenty seconds at a time. After this length of time it will need an equal amount of time to bounce back. Operating continuously or without a recovery period may cause damage to the unit.

Resistance Soldering

Clean it up!
Surprisingly, even shiny brass will have a thin film of oxide on it. Since resistance soldering relies on a clear electric path to operate it is best to carefully clean all areas that will be soldered. Fine sand or emery paper, files and fiberglass brushes will make short work of any oxides on the metal.

Resistance Soldering

A little goes a long way
You will find that good joints require very little solder. Clip short pieces off the roll and place on the joint with tweezers. Once the flux flashes off the solder will quickly make the joint.

Resistance Soldering

Clean any of the residual flux off with a fiber class brush and detergent and water. An ultrasonic cleaner will make short work of clean up as well.

Resistance Soldering

Ground those small parts
Grounding small pieces can be a challenge. One easy inexpensive way around this dilemma is to clamp the pieces to a sheet of copper clad circuit board with alligator clamps. Once the parts are fixed in place the entire sheet is grounded making soldering each of the parts a breeze.

Resistance Soldering

Working with large parts and white metal
Soldering dissimilar metals or metals of different thicknesses can be a challenge. One faced in building the locomotive was to attach thick white metal buffer blocks to the front and rear striker plates. The brass was first cleaned and then tinned with Tix low melting point solder. The pre-cleaned buffer block was fluxed then held in place with a spring clamp. The probe heated the back of the work until the solder melted.

Resistance Soldering

White metal has a much lower melting point than brass. This is made even more confusing since it seems that not two model casters use the same mixture in their work. To solder white metal first find out the relative melting point of the material. This can be done using a casting sprue that was attached to one of the parts. Dial up the power level until the piece begins to melt, then turn back the setting a couple of notches. Now test the solder to see if it will melt and flow at this setting. If the solder works there should be no problems working with the white metal parts. Keep in mind that larger or heavier pieces will have a slightly higher melting point. So adjust accordingly.

Resistance Soldering

Finishing up
The finished model needed very little clean up due in large part to the clean solder joints that I was able to make with the resistance unit. Additionally, some potentially complicated attachments, particularly, those that needed to be sweated were made quickly with out a lot of excess heat usually associated with the process. The model is almost ready for the paint shop with just a quick bath in the ultrasonic cleaner to wash off any flux residue.

The Resistance Soldering Kit (Item #80462A), Tix Solder (Item #17110), Fiberglass Brushes (Item #14259), and Ultrasonic Cleaner (Item #82413) is available from Micro-Mark.

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