Intro to Soldering Notes

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  • "If it smells like chicken, you're holding it wrong" -- watch out for the reflex of trying to hold the soldering iron like a pencil.
  • AVOID breathing the fumes from the soldering iron.
  • PROTECT your eyes when trimming leads with the side cutters, they tend to spring away.
  • WASH YOUR HANDS after handling tin/lead solder.
  • Use COLD WATER and soap to wash your hands (hot water opens your pores up).

Class Prep Check List aka Tools & Materials

  • kits
  • batteries
  • spare components
  • solder
    • (Pens to wind solder around)
  • soldering irons
  • power strips
  • lights
  • brass mesh for cleaning iron tips
  • water for sponges for cleaning iron tips
  • desoldering pump (aka "solder sucker")
  • dikes aka diagonal cutters aka diagonal pliers aka side cutters (for trimming leads)
  • wire stripper (depending on kit)
  • screw drivers (depending on kit, if needed then most likely phillips #2)

Additional sometimes-useful items:

  • magnifying glass can be very helpful
  • third hand tool can be very helpful (stand with metal arm & alligator clips)
  • heat shrink tubing & heat gun can be useful if you're soldering a wire splice
  • also see:
    • lineman's splice
    • line t-splice


  • PCB board - (printed circuit board) green plastic circuit board
  • pad - little copper circle on circle board
  • traces - copper (sometimes light green) lines on board covered in solder mask
  • leads - the wires from whatever component

Cathodes and Anodes aka Negative and Positive


  • cathode (negative)
  • anode (positive)
  • cathode is usually shorter on LEDs and transistors and many components
    • but not all!
  • Usually the markings on the PCB will have a circle with one side flattened. The hole on that side is the one that the short/cathode/negative lead goes through.
  • Resistors are not polarized so it doesn't matter which is positive or negative.

Pictures of Tools and Components

TODO: We should scan/photo some of the PCBs, LEDs, etc, from kits we use, and use those images. The images currently used are just examples I found online, and not necessarily copyright free (unless indicated otherwise in the upload description).


Soldering Diagonal pliers 2017 - B.jpg

Soldering Iron

A recommended good soldering iron, the Xtronic #3020 XTS:

Xtronic 3020 xts.jpg

A typical cheap soldering iron:

Electriс soldering iron.jpg


A typical PCB. This image is from an ifixit project kit at: [1](

Ifixit pcb.jpg


LED drawing 01.png


800px-560 ohms 5% axial resistor.jpg

800px-220 ohms 5% axial resistor.jpg

Resistor Color Coding

Resistor are coded:

  • first color is tens digit,
  • second color is ones digit
  • third color is a multiplier (order of magnitude)
  • fourth color is tolerance:
    • gold +/- 5%
    • silver +/- 10%
  • Most resistors you'll see will be gold

There are various resistor color code guides on the internet. Here is one example:


Soldering Irons

Recommended soldering iron is X-Tronic model #3020 soldering iron.


  • a cheap iron costs $20
  • a half-assed iron costs $20-$30
  • a good iron costs $40-$60

Cheap irons are minimalist and have a set wattage/temperature (usually 30-40 watts).

Good irons have feedback circuits to keep the temp steady and usually a sleep mode (a sensor detects when you set it in the rack and turns the heat down automatically, and detects when you pick it up and heats it up again).

Half-assed irons look like good irons but don't have a feedback circuit, just a simple potentiometer and odd little amenities like a sponge tray.

Solder & Flux

Flux is a material that reduces oxidation during the actual moment of soldering (see "More On Oxidation").

Flux comes as either a separate product or as part of the solder (rosin-core solder). Generally HackPgh classes are taught with rosin-core solder, so you don't have to worry about adding flux as you solder.

  • Regular solder is 60% tin, 40% lead (650F).
  • Lead-free solder needs much higher temp (750-800)
  • Flux keeps the contact from oxidizing, also transfers heat.
  • Flux comes separately, also comes in rosin-core solder.
  • Rosin-core solder has a flux core, much more convenient
  • AVOID acid core solder, that's for pipes where the corrosion caused by the acid isn't a big deal.

Also see:


The PCB is a green plastic board, which usually has two different sides. One side has white lettering printed on it, while the other side has copper "pads" connected by lighter green lines called "traces".

The components (LEDs, resistors, diodes, etc) have wire "leads".

Generally you:

  1. Insert the component leads through the holes in the PCB from the printed side.
  2. Bend the leads outward at a 45 degree angle to keep the components from slipping out of the board when you turn it over.
  3. Solder the leads.
  4. Trim the excess lead off with a wirecutter, dike (diagonal cutter aka diagonal pliers aka side cutters)

Some people insert and bend all of the components before soldering them all, then trimming them all. Some people do it one component at a time. Some people do it a few components at a time.

Which way to do it is up to you and what you feel more comfortable with. Some people like getting all of the components set up first, some people find it hard to work around all the leads.

Tinning: Preparing the Soldering Iron

You always want the tip of the soldering iron to look shiny. If it doesn't look shiny, it's time to clean it and re-"tin" it.

Darkness or blackness on the tip indicates oxidation. Oxidation on the tip of the soldering iron interferes with heat flow and is undesirable. (See "More on Oxidation", below.) If the tip looks black or dark, either:

  1. there's no solder on it and the surface is oxidized, or
  2. there is solder on it and the surface of the solder is oxidized.

To prepare the soldering iron tip:

  1. Heat up the iron and then wipe any oxidized solder off on a damp sponge or steel wool or brass wool.
  2. "Tin" the tip of the iron by touching the tip of the solder wire to it and letting a bit of solder melt onto it.
  3. Smooth out the new solder by wiping the tip of the iron a second time on a damp sponge or steel wool or brass wool.

You want the tip to look smooth and shiny.

Re-examine the tip of the iron for oxidation frequently, i.e. every time you pick the iron back up.

Check the tip occasionally if you're doing a large number of solders in a row.


  1. Hold the solder in one hand and the iron in the other (be careful not to pick up the iron like it's a pen or pencil, you'll burn yourself).
  2. Hold the solder near but not actually touching the copper pad.
  3. Touch the soldering iron tip to the copper pad then immediately bring the solder to touch the iron.
  4. Let the solder melt until you have a drop of molten solder. This should take only 2-4 seconds.
  5. Use the iron's tip to move the drop of molten solder around as necessary.
  6. As a general rule of thumb, there should be enough solder to completely cover the pad, but no more than that. Extra solder can to "bridge" the gap between this pad and an adjacent pad, which shorts out the circuit.

Avoiding Mistakes

  • Be VERY careful not to touch the soldering iron to the anything but the copper pad on the PCB, this can damage the traces (the lines of copper or light green that connect the pads).
  • Be careful not to touch the copper pad too long and burn the pad off, this ruins the PCB.

Other common mistakes are:

  • Not enough solder; you should have enough solder to cover the pad, and you should not be able to hold the PCB up to the light and see light through the pad hole.
  • Too much solder, leading to bridging; too much solder can lead to the solder on one pad touching the solder from another pad, causing a short circuit. Most of the time the PCB design will require no pad being bridged to another pad.
  • Putting the component negative/positive leads in backwards (note, you don't have to worry about this with resistors, because they do not have negative/positive sides).

Recovering From Mistakes

If you make a mistake:

  1. Use the soldering iron to liquify the solder.
  2. Use the solder sucker to suck the liquified solder out of the joint.
  3. Start over from scratch.

Some tips on using the solder sucker:

  1. Prepare the solder sucker by pressing the plunger on the top end of the solder sucker down until it clicks.
    1. You can easily do this with both hands, but that means setting the iron down and picking it up again.
    2. It's easy enough to do it one-handed, just press the plunger against the table, your leg, your head, etc until it clicks.
  2. Hold the tip of the solder sucker near but not yet touching the solder joint.
  3. Bring the tip of the soldering iron to the solder joint and hold it a few seconds until you see the solder liquify.
  4. Press the button on the solder sucker to suck out the liquified solder.
  5. Make sure you move the soldering iron off the joint as soon as possible, to avoid overheating it.

Tip: You may find it useful to lay the dikes so the end of one handle pins down the PCB and keeps you from accidentally moving it.

Falstad Circuit

Falstad circuit simulator app: [4](

More On Oxidation

Oxidation is a general term referring to how oxygen atoms tend to interact with other atoms, specifically the oxygen atom tends to pull electrons from the other atom. This process often results in undesirable alterations to the other atom and its chemical reactions. Much food spoilage is related to oxidation.

Heat tends to speed up oxidation, which leads to obvious problems in soldering.

In soldering, oxidation is bad because it introduces impurities into the solder, which can make a weak solder joint.

Also, most soldering is on copper contacts; heating up the copper contact while exposing the copper to oxygen in the air then tends to lead to oxidation of the copper, which again can interfere with the solder joint. Flux helps to prevent this by coating the contact and keeping air from getting to it.

Safety (Again)

  • AVOID breathing the fumes from the soldering iron.
  • PROTECT your eyes when trimming leads with the side cutters, they tend to spring away.
  • WASH YOUR HANDS after handling tin/lead solder.
  • Use COLD WATER and soap to wash your hands (hot water opens your pores up).

And always remember folks, "if it smells like chicken, you're holding the soldering iron wrong."