Finally, we're ready to do some soldering. For our example, we'll use a solenoid and a wire. If you're using a standard iron, plug it in and let it pre-heat for at least 10 minutes. Solder melts at a lower temperature than it needs to flow, and achieve a proper wetting action, so testing the iron by melting solder on it will be misleading.
If using a temperature controlled iron set it at 750° and let it come up to temperature. Wait about 5 minutes after the set temperature is indicated to allow the tip to stabilize. This is the same theory as why a pizza, for example, will cook faster in the oven if you give it a little more time after the temperature is reached before putting the pizza in.
While you're waiting, setup your solenoid and wire so you have a good physical connection and nothing will move while soldering. When working under the playfield you sometimes have to get creative. This is where the plasitc hemostat's or Kapton tape can come in handy. If absolutely necessary you can use locking tweezers to hold the wire, although they will draw heat away from the joint and make soldering more difficult.
Before soldering always clean and tin the soldering tip by applying some solder to both sides and then using a copper tip cleaner. Use the copper sponge frequently while soldering multiple joints, or if there is excess solder on the tip.
This step is optional, but a good idea when you're learning: Add a little flux to the joint, this will help in removing any oxidation and reduce the surface tension of the solder, which will then flow better. Note: If you don't add flux at this point the flux in the solder performs the same function.
It is important to place the iron's tip in the right location and achieve maximum surface contact between the iron and two parts to be joined. In the image below I've placed the soldering tip so it has maximum contact with both the wire and the solenoid lug. (The soldering iron should be pushed tighter against the joint than shown in the picture. I was trying to hold multiple items while taking a photo.)
Wait 2-3 seconds after applying the tip to the joint (the length of time depends on the size of the joint and surrounding material that will draw heat away). Then place the solder against the tip at the point it is touching the wire and lug. As the solder melts a solder bridge will form which conducts heat to the joint more efficiently than simply a mechanical connection. As soon as the solder begins to melt (which should happen immediately if the joint is clean and at temperature) drag the solder to the opposite side of where the soldering iron is placed (you will leave a small trail of solder behind which will expand the solder bridge). Since solder will always flow from cooler to hotter this will ensure the entire joint receives solder.
Note: Do not use the soldering iron tip to melt the solder, you will get a cold joint.
As you add solder it will form a ball and when the solder begins to flow the surface of the ball will briefly, and quickly, shrink (as the solder flows across the joint). Once the solder is flowing, add a little more solder (a couple of seconds maybe) until you have good coverage and you're done. Always remove the solder from the joint before the soldering iron.
If you're having trouble managing the flow of solder, or applying excessive solder, move to a smaller diameter solder. As you gain more experience you will be able to judge the proper quantity of solder based on how much you feed to the joint (1/2", 3/4", etc.). This allows you to move along more quickly when soldering multiple joints.
The entire process, from the time you start pre-heating the joint, should take less than 10 seconds. As you get better, much less than 10 seconds.
Turn the part over and make sure the back side has flowed properly and the wire is soldered completely to the lug. If not, put the soldering iron back on for 2 - 3 seconds and add a little solder, following all of the steps as described above.
If the joint is moved while cooling you risk a cold solder joint. Also do not use compressed air, flux or anything else to cool the joint down faster. Once you're done soldering, and the joint is cool, use a flux remover or naptha/alcohol to clean up any flux residue, and then you should... Inspect the joint using a magnifier.
When you're done, clean the tip, flood it with solder, wipe it again and turn off the soldering iron. This will help protect and prolong the life of the tip. Do not clean the tip by dipping it into flux.
As your skills improve there are a couple of steps you can take to hone your abilities for moving on to working on circuit boards. The first step, if the joint is not heavily oxidized, would be to eliminate the use of additional flux. Note: There will always be times that additional flux is appropriate. Then, if you are using a temperature controlled iron, you can start dialing back the temperature (700 or less is a reasonable goal). You should also see your dwell time per joint reduce from 10 seconds down to, maybe, 5 seconds.
The good news is the same technique works with most board repairs, although we'll be using some different equipment and playing around with temperature a little bit.