Soldering

Intro Stuff

Soldering

Soldering is a necessary skill for most experimental physics labs. The goal is to join two conductors electrically and mechanically by 'gluing' them together with molten metal. Soldering is not difficult, but requires some simple practices and tricks for best results.

This page serves as the Advanced Project Lab's primer on soldering. It is not a complete description, and you should supplement your knowledge from other sources. Since soldering is a basic skill, there are many detailed lessons/instructions online (for instance, here, here, and here, if the links still exist)

Tools of the trade

• Soldering iron: A tool for applying heat to parts through conduction. We have several soldering stations in the lab. Most are as simple as being a piece of metal that is made hot using an adjustable resistive heater, but we also have a fancy station which senses the thermal load using an RF signal and adjusts the heat accordingly.
• Solder: This is the metal alloy which is melted to join objects together. The spools we have in the lab contain lead, and basic safety should be observed: try not to inhale fumes as you are soldering, and thoroughly wash your hands before touching your face or eating any food. Melting point is around 400F, and the solder also contains flux, which helps it flow and adhere to hot metallic objects.
• Wire strippers: This tool cuts to a specific depth to expose the ends of insulated wire in preparation for soldering. There are two types of strippers in the lab: fixed size (which will have a series of holes for stripping various size wire gauges), and adjustable (use a sliding screw to change the depth of the cut).
• Heatshrink tubing: To insulate wire after soldering, we wrap exposed leads with a shrinkable plastic. Remember to put tubing over the wire before soldering, slide it into place, and heat with a heat gun to shrink.
• Heat gun: Basically an overpowered hairdryer. Use it to quickly heat things up, but be careful because it can melt plastic in a hurry. It should only take a few seconds to activate the heatshrink tubing.
• Soldering gun: This can be used as a soldering iron. Its main advantages are fast heat-up and cool-down time (so you have on-demand heat), and that it can deliver large amounts of heat (use for joining heavy connections if a normal soldering iron isn't working).
• "Helping hands": These things are nice.

Often times you'll need to undo some soldering or rework previously soldered materials. The following are helpful in this endeavor

• Solder wick: An effective way to remove solder from a piece, solder wick is a strand of braided copper wire. The idea is that the braid has a much larger surface area than the piece you are removing solder from. Thus, as the solder wick is heated along with the existing solder and conductors, solder will preferentially 'stick' to the braid and be wicked away. Place the braid on top of the existing solder and then the iron tip on top of that (the idea is to heat up the existing solder through the braid so that you know it's hot enough to attract the solder; sometimes touching a bit of fresh solder to the iron helps expedite this process). This tool is good for removing final remnants of solder to restore the shape and size of the original conductor (e.g., remove excess metal from a flat surface).
• Desoldering pump: Colloquially known as the "solder sucker," this tool uses a spring-loaded mechanical plunger to create a small vacuum that pulls molten solder away from a piece. It is especially useful for removing solder joints on a printed circuit board (PCB). To use, load the pump by pushing in the plunger, heat the joint you want removed, quickly place the end of the pump over the molten solder, and push the button to release the plunger. Take care to avoid melting the plastic tip of the pump as you work near a hot soldering iron tip. Solder will gather and solidify inside the pump body, so you should periodically push the plunger all the way in and remove accumulated solder from the metal tip that's inside.
• Rosin flux: As mentioned above, flux is matrixed within the spool of solder to help it 'flow' toward conductive surfaces. The flux is no longer within the metal, but is instead pushed to the surface or onto exterior objects. When reworking an old joint, the previous flux may have been removed (through cleaning, evaporation, or just rubbing off). This can cause remelted solder to seem 'gummy' and make it hard to make connections. Adding additional rosin flux helps effectively decrease the viscosity of the molten solder and makes solder bridging (see below) less of an issue.

Mantainance and cleaning

Having a well-maintained soldering iron tip helps deliver heat in a more accurate fashion. These tips can degrade quickly if proper care is not taken. First and foremost, remember that this is a piece of heated metal and it is more ductile than at room temperature; please don't use the tip to pry or bend the pieces your are working on, since this can deform and even break the tip of the iron.

To keep the tip nice while your are working, drag it across a moistened sponge to clear debris and globs of solder. The steam shocks extra bits off the iron and restores a smooth, hot surface to enable efficient soldering. Before even starting on your piece, it is common to apply a bit of solder to the tip of the iron (this is called 'tinning') and then drag it across the sponge to establish a nice, fresh surface. It is also helpful to do this at the end of your session to cover up any exposed surfaces of the tip and reduce the potential for corrosion of the metal. Try to get into the habit of using the sponge every time you pick up and replace the iron in the cradle ('tinning' the tip should only be necessary at the beginning and end of a soldering session).

Basic Soldering

The goal is to heat the two objects you want to join, then apply solder and let it flow between the objects, and then allow them all to cool together. A good solder joint should look shiny and have minimum surface area

There are two main failure mechanisms in soldering:

• Solder bridge: This is when solder joins objects which were not meant to be joined by forming a 'bridge' across the space between the conductors. This occurs when soldering in cramped spaces, and is sympotmaic of too much solder being applied or an inaccurate placement. Simply use some solder wick or the desoldering pump to remove excess solder and reconnect. If you struggle to make the connection you want without bridging to other nearby conductors, try using some rosin flux to help keep that solder in place! (This reduces viscosity and encourages solder to go to conductors rather than forming a bridge between them; the only conductors which will be connected are the ones in physical contact.) Solder bridging is easy to identify using a multimeter to test for conductivity.
• Cold-solder joint: This is a more nefarious problem than solder bridging because it might not be noticeable for some time (eve YEARS later!). The problem arises if one of the conductors is not heated to above the solder melting temperature when the joint is made.

Practice soldering activities

practice stripping wire, using heatshrink tubing, and creating usable product, DB9 connector

BNC connection voltage divider box

To work with perfboard and and combine everything together!