Is soldering something that other people do but you can't?

Most slot cars are built using solder joints, whether it is simply soldering in oilites or full chassis construction. I would say that soldering is an essential skill for any slot racer – there is nothing worse than having to rely on other people to do your repair work, particularly at busy times in the racing season when their priorities are elsewhere.

The answer is to have a small iron for electrical connections like lead wires and another larger iron for fabrication work. It is a good idea to select an iron, particularly for fabrication work, which has both replaceable and exchangeable tips because some jobs need a larger tip than others. You should be able to obtain replacement tips easily so always buy a well known make where you know you can get spares.

I use a temperature controlled soldering station which has a maximum rating of 60 watts. This is lightweight and comfortable to use. The tips are exchangeable for smaller jobs and the temperature control ensures that the tip temperature does not reduce appreciably during soldering. I also have a fixed output 100 watt iron for large jobs, if required, and a fixed output 15 watt iron for small electrical work.

In the course of working on cars you may have occasion to apply heat in two different places e.g. to remove a motor, so more than one iron can come in useful.

Stands - It is important to get a stand suitable for your soldering iron. This should have a metal coil retainer to keep the tip away from anything that can be melted or that is flammable. The stand should also house a heat resistant cleaning sponge which is used to clean the tip on a regular basis. Cleaning sponges should have hole in the middle which forms a collector for dirt and solder wiped from the tip. The tip is cleaned by drawing it across the sponge so that the dirt and solder falls into the collector in the middle. It is important to clean the tip after every soldering operation as the high temperatures and fluxes involved produce a very corrosive environment which will quickly destroy the tip if it is not kept clean.

Solder - There are many types of solder available, but I will concentrate on two or three. Solder contains lead and tin and is known by it's composition e.g. 60/40 would be 60% lead, 40% tin. Silver solder is also available – this is not made of silver but has a very small (app. 2%) silver content. This makes for a stronger joint but increases the melting point so higher temperatures are required. 

Many people swear by silver solder for fabrications but it can be difficult to work with if using a fixed output iron. It's strength is important though in particular applications.

Solder is also available with a flux core (more about that later) which assists in the soldering process.

With a cored 60/40 solder and high melting point silver solder you can solder anything on a slot car.  

Flux - Flux is required to achieve the chemical cleanliness required to achieve a good joint. When heated, it removes oxides and residues from the metal surface and allows the solder to actually penetrate the metal. In general a rosin type flux is used for electrical connections and an acid type flux (liquid or paste) is required for metal fabrications.

Liquid acid flux is best applied with a very small artist's paintbrush. Paste type flux can be applied with a pointer.

Desoldering - Desoldering is a process whereby soldered joints are melted for disassembly. This is relatively straighforward providing sufficient heat is applied. Solder has a habit is getting into everything so tools are required to completely remove it. A desoldering pump is a handy tool for sucking away the melted solder. These are widely available but make sure you get one with a replaceable tip (and that you can get new tips) as they do get damaged eventually through use. Desoldering wick is also available – this is a copper braid which is used to wick up melted solder. Solder can also be easily removed by sanding with wet and dry or a Dremel type sanding disc.

Tools - Soldering is a process involving metal and heat so holding the components can be painful! Various tools are available for holding components such as tweezers, pointers, third hand tools and forceps! A pair of gloves can be useful and for sure you will eventually improvise several tools of your own. Wet and dry paper is good for achieving mechanical cleanliness.

Try and obtain a solder tip cleaner/tinning agent. This is sold in small tins and contains Salamoniac and solder compound. Alternatively a Salamoniac block can be used with normal 60/40 solder.

If you are starting off with a brand new iron you will need to take a few steps to ensure best results and long tip life. You first need to "tin" or "wet" your iron which involves coating the tip with solder. Tinning is essential for good soldering as it allows the whole tip to work on the joint. It is done as follows:

Plug in the iron and allow it to heat up to full temperature. If you have a cleaning/tinning agent rub the tip in the compound and it will quickly take up the solder and become tinned. Wipe the tip over the cleaning sponge and it will remain bright with the solder. The iron is now tinned and ready for use.

If you are using a Salamoniac block, hold the tip on the block until it fumes (do not inhale these fumes) and simultaneously apply 60/40 solder to the tip. The tip will take up the solder and become tinned. Wipe the tip on the cleaning sponge to remove any residues.

If you iron is going to be on for a long period of time, it is important to keep the tip tinned or wetted with solder. Do this every now and then to ensure good soldering and long life.

It is very important to keep the tip tinned or wetted with solder at all times. Quite simply, if the tip is not tinned you will not be able to solder with it.

Tinning components involves applying a thin coat of solder to the surface to be soldered. This greatly helps in achieving a good joint and should be done whenever possible. It is particularly important for electrical connections. Tinned components can be prepared and stored for later use, as the tinned surface will not corrode.

Tinning of copper and brass can be done with 60/40 rosin cored solder. First of all, clean the component with wet and dry paper to achieve a bright surface finish. Apply heat to the component with the iron and when hot apply solder to the area to be tinned. If possible wipe the area when the solder is molten with a wetted (damp not dripping) sponge. This will remove excess solder and leave a clean tinned surface for later soldering. Tinning of larger components will probably require the use of acid flux to keep the area clean.

Tinning of steel requires the use of liquid acid flux before adding the solder. Mechanically clean the area to be tinned, then heat it up with the iron. "Paint" on a spot of acid flux and then add solder from the iron. The solder will flow over the area which has been prepared. Wipe the suface with a sponge to leave a thin layer of tinning on the component.

Soldering tinned components together is simply a matter of adding heat and a little more solder to flow into the joint. No more flux should be necessary.

People often have trouble keeping the solder to where it is wanted. Solder will only "stick" to surfaces that are mechanically and chemically clean so if you prepare the area well solder should not go outside that area. There are products designed to prevent solder sticking and these can be used if desired - the starch from a cut potato also works, as does oil or grease.

It is not always possible or practical to tin components before soldering. In these cases flux must be applied directly to the joint area before applying the solder.

First of all thoroughly clean the components and assemble dry. Apply some heat to the joint area with the iron (tinned but free of excess solder) to prewarm the joint. Remove the heat and apply a small amount of acid flux with a paintbrush. Put a small amount of solder on the tip and apply it to the joint. The acid flux will sizzle and fume (don't breath in the fumes or let them get into your eyes) and solder should flow into the joint.

It is fairly easy to tell a good solder oint. Have a look at some chassis joints that you know to be good. You will see the solder has flowed into the joint and produced smooth fillets. There are no gaps or pinholes in the joint and the solder is smooth in appearance although the solder may have dulled over time.

You can tell when soldering that the solder is flowing into the right places and that it is "biting" into the metal.

A poor joint is just as easy to spot. The solder will appear in globules around the joint which will have gaps where solder has not flowed. The solder will also be dull and rough (crystaline) in appearance indicating that it has never actually melted fully and has not flowed into the joint.

You can tell when soldering that the joint is not being made properly. The iron will tend to stick to the joint and solder will tend to jump around in beads rather than flow.

I said earlier that there are different types of solder to be used and different types of flux. Here are some examples of what to use and when:

Lead wires: Use 60/40 rosin cored solder to tin the stripped lead wire ends. Tin lead wire clips and motor terminals with 60/40 rosin cored solder after cleaning with wet and dry. Apply a small amount of solder from the iron tip when making the joint – ideal temperature is 380°C. Be careful not to melt the guide flag when soldering – use a small tip if possible.

A good tip: Apply a drop of oil to the wire strands at the point where the insulation starts. This prevents solder from wicking up the wire when tinning and helps keep the end of the lead wire flexible.

Pin tubes: Pretinning is not practical. Clean the joint area on the chassis and the pin tube so that is bright and shiny and assemble dry. Apply a small amount of acid flux around the joint. Apply a small amount of 60/40 cored solder to the joint with the iron and allow it to flow around the joint. Immediately wash away any flux residue from around the joint with mild alkali and a toothbrush.

Pinions: This is everyone's nightmare as a slipping pinion will certainly lose you the race. The secret is good preparation. Thoroughly clean the armature shaft over the length to be covered by the pinion. Brake cleaner is good for this but methylated spirits works well too. Don't use white spirit, as this will leave an oily residue. If the armature is new take the shine off with wet and dry whist spinning the shaft in the motor or in a Dremel. You should also slightly roughen the surface with wet and dry. Now clean the inside of the pinion. If it is new use a needle file or a thin strip of wet and dry to lightly roughen it and remove any machining residues. If it has been soldered previously, drill out the old solder with a 2mm drill at slow speed (don't use a Dremel – it is too fast and could shatter the pinion) and clean it off as best you can.

If your motor has a can ballrace it is important to protect the ballrace from the acid flux. This can be done with a phenolic armature spacer between the pinion and the bearing. The spacer can be sut off later with a sharp knife. If there is no room for a phenolic spacer then a small piece of tracing paper with a hole in will do. Tear it off after soldering and lubricate the bearing.

Apply a small amount of acid flux around the armature shaft and then tin the shaft all around. If the solder does not take all around then clean it, apply more flux and try again. Wipe the end of the shaft with a damp sponge to remove excess solder.

Next, paint the outside of the pinion with nail varnish. This will stop any solder from getting on the teeth during the soldering operation. When the nail varnish is dry apply a small coating of acid flux inside the pinion and place the pinion on the end of the shaft. Some light pressure will hold it there. Remove all excess solder from the iron tip and lightly press onto the end of the pinion. You will see the flux boil and fume, the nail varnish will burn and the pinion will then slide onto the shaft to the required position under light pressure from the iron.

Buff or scrape off the burnt nail varnish and wash away any flux residue.

Oilites: Again it is impractical to pre-tin these due to the fit up required. Clean the chassis around the mounting hole with wet and dry paper. The oilite only needs to be degreased. Assemble the oilites in the mounting holes on an axle with an equally sized gear on either side. The gear should hold the oilite flange against the chassis plate. Place the assembly on a fat block to ensure the axle is level and you are ready to go.

I prefer to "tack" the oilites in place first with silver solder so that the correct bearing alignment can be achieved before fully soldering in place with 60/40 cored solder. This is done by applying a small amount of flux and then solder to the top of the oilite on the inside of the chassis. The oilite will be fixed later on the outside of the chassis around the flange.

When both are tacked in place, check the axle for level and remove the axle and gear.

To check for alignment put the axle through one bearing and pivot it so that the end rotates close to the opposite bearing. The axle should pivot centrally around the bearing. Re-align the bearing as required and repeat on the other side. When both are aligned they are ready to fix in place.

Apply a small quantity of acid flux around a quarter of the flange to chassis face joint and then apply a small amount of 60/40 solder to the joint. The solder should flow into the joint. The silver solder keeps the bearing in place and does not melt as it has a higher melting point. Do this around the rest of the flange in stages, checking bearing alignment after each operation, until both sides are done.

Motors: Most people try this first time out with insufficient heat being applied to the joint. This happens because, although the iron is hot, heat is lost through the chassis. The secret is to pretin all joints where possible. In this way solder joints can be quickly made with a hot iron onto the joint. Use acid flux and 60/40 cored solder or silver solder to tin the can and the motor mounting plate / braces.

Wipe off excess solder from the can surface so that the motor can sit against the chassis or mounting plate / block. Applying sufficient heat and solder to the joint will result in a good strong joint.

Always wash joints and the surrounding areas thoroughly to prevent later corrosion.

Chassis: Soldering chassis parts together is fairly straightforward and can be tackled when the main soldering techniques have been mastered. The main rule is follow the construction sequence in the instructions. This ensures that assembly is (a) possible and that (b) soldering operations involving a lot of heat are performed first. If this is not done, joints previously made can melt during later operations.

Selective use of high melting point silver solder can help to prevent joints from coming apart when not desired. e.g. pillow block joints can be melted when replacing bearings so a higher melting point solder in the pillow block joint can help avoid this. Always use silver solder for added strength on guide tabs and any joints which may be stressed in a crash.

Guide tabs are the most difficult part to solder properly as they are in an area with the most metal (tab and chassis). The key is pretinning of the chassis surface, both sides of the spacer and the underside of the tab. The next problem is applying sufficient heat to melt all the solder - you'll need at least 60W, a large tip and possibly a second iron! You can tell there is sufficient heat by watching the solder melt throughout the assembly. The components can then slip around quite freely – applying liquid acid flux helps the solder flow. Next problem is keeping everything lined up. Some chassis have locating holes for piano wire which helps line the tab, spacer and chassis up. If these are not provided it's best to do it by eye – you will be surprised how accurately you can line something up by eye. If you use graph paper as a mat this will help.

When all the solder has melted and the components are in the right place, apply even pressure over the the top of the guide tab (a pair of pliers works well and allows even pressure to be applied on either side of the tab) and press down until the solder has cooled. Now check that the tab is level in both directions.

Assembling and soldering up the pillow blocks also requires care, although the soldering operation is quite easy as the components are small and heat up easily. The hardest part is getting everything lined up and held in place before hand.

Generally pillow blocks provide a snug fit for ball races so they have to be perfectly lined up – i.e. you cannot move the bearing around too much to take up misalignment, although if you are using oilites they can be filed to provide clearances. A chassis building jig will help to get them level and square. It is very important to get the pillow blocks as close to vertical as possible. If they are not the ball races will not run square to the axle and this will cause accelerated bearing wear.

Some (laser cut) chassis are cut very accurately and don't require a jig for assembly. Nevertheless it's best to check everything as a matter of course.

Tack things in place first then re-check alignment before soldering everything up. If you do this bit by bit you can correct small errors before it's too late.

Washing off acid flux residue is most important if the chassis is to stay corrosion free. This is very important around pivots and hinges as later corrosion can sieze these up. Add a drop of oil into piviot / hinge joints to keep them free. A light film of oil, grease or vaseline on the chassis will help prevent corrosion.