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).
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
As a first practice practice soldering project, let's connect five wires on a single row of a female DB9 connector. See this video for an interesting way to achieve a connection like this.
- Cut five pieces of wire about 2 inches each and strip about 0.25" off each end. Turn on the soldering iron to allow it to heat up while you're preparing these.
- Use the small electronics vise to hold the DB9 connector (with the solder cups accessible) and set up the helping hands to hold the wire in place.
- With the wire inserted into the solder cup, place the tip of the soldering iron so that it contacts both the cup and the end of the wire. Hold it there for about a second, then melt a bit of solder by touching it to the cup/wire junction. You can melt the solder on the iron itself, but this is not optimal because you want to know that the pieces themselves are hot enough to cause solder to flow. After some solder melts, it should wick into and fill the cup. Remove the iron (and unmelted solder). After it cools, check that the cup is filled and that the solder is fused to the wire (you want to see a shiny finish with a miniscus-like tension between all pieces the solder connects; a big blob/droplet means you've used too much solder). NOTE: Try not to apply too much heat with the soldering iron! The cups can heat up enough to melt the plastic in the DB9 and you'll have to discard the connector. You should really only touch the soldering iron to the piece for brief amounts of time.
- Repeat above step for all other wires.
- Check that you have made good connections. Look for solder bridging, and double check that there aren't any unexpected shorts between conductors by using a multimeter in conductivity mode (set to measure resistance and give an audible beep when a connection is made between the two leads). Cut and strip a sixth wire to use as a probe on the opposite side of the DB9 connector. Use alligator clips to connect and test for conductivity between each lead and the output of the DB9.
- Apply heatshrink tubing to insulate the connections. Cut lengths long enough to cover all exposed conducting and slide the tubing over each lead up to the DB9. You should choose tubing with a diameter just larger than the wire/solder joint since it can only shrink a finite amount. Get out the heat gun and use it to shrink the tubing onto the connectors. You should be able to observe shrinking within several seconds. Again be careful about applying too much heat and melting the plastic in the DB9! Note that if you were making a real cable, you'd have to plan ahead and slide heatshrink tubing over the conductors before you soldered them in place. Extra care is then needed to avoid shrinking the tubing via soldering iron heat (typically sliding it an inch or so away from the soldering location is enough, but if you heat up the conductor too much you can shrink the tubing so that it won't slide over the joint you'd like to insulate in the end; practice makes perfect).
- Install the DB9 connector shell. This is a two-piece metal-body which clamps around the DB9 and allows it to be securely screwed into a mating connector (think the screws of a VGA monitor connector on the back of a PC). Various sized rubber grommets give strain relief to the connections you just made. Choose an appropriate size (just large enough to fit the five conductors you connected), slide your conductors through the grommet, and insert it into the groove in one half of the connector shell. The shell has four screws (two for clamping the two halves together and two to attach mating connectors) and two nuts, and before tightening the clamping screws, you should insert the mating screws into the holes made at the joining seam of the two halves. Once tightened, the mating screws should slide freely but remain encapsulated in the shell.
- Now do the reverse! After removing the connector shell, use a razor blade to slice and remove the heatshrink tubing. Desolder the connections by heating up the joint and pulling out the wires. Use the desoldering pump to remove solder from the inside of the solder cups (effectively prepping the DB9 to be soldered again in the future). Again try not to apply too much heat (especially while using the desoldering pump) since we'd like to reuse this connector in the future!
BNC connection voltage divider box
A printed circuit board (PCB) is handy for several reasons, but primarily it (1) minimizes radio frequency pick-up by reducing the size of inductive loops formed by leads of circuit components, and (2) allows convenient mounting of circuit components so you don't have to creatively assemble them in free space. Soldering to a PCB is even easier than the connector we made in the previous section. Here one of the conductors you are trying to connect is the 'pad' surrounding a hole on the PCB (this is how connections are made to 'wires' in the plane of the board). You want the solder joint to look like a volcano, with the base at the pad and the pin of the piece you are soldering in place sticking out of the neck of the volcano. A cold solder joint is fairly obvious because the poor connection breaks the symmetry of the volcano; you can easily identify a joint that is not connecting to the base or the pin (see figure).
As some simple practice, let's make a divide by 10 voltage divider using some perfboard (refer to ... in wiki).
- Pick two resistors to use in the voltage divider that give a gain of ~1/10; for clarity, refer to the larger resistor as R1 and the smaller as R2.
- Get a small piece of perfboard