Introduction to Soldering

This write-up is designed to introduce the basics of soldering.

  • after reading this material, you'll take an online quiz

  • after passing the online quiz, you will schedule hands-on instruction

  • once you are comfortable, you will take a hands-on exam

  • after you successfully pass the exam, you will be able to use the basic soldering equipment and materials

Soldering doesn't have to be difficult!

With the amount of time EE and CS students will likely spend soldering, investing 5 minutes to read through this could save you countless hours of frustration at the bench.

What is soldering?

For our purposes, soldering is the process of joining electrical components together by melting solder to make an electrical connection. Soldering should NOT be relied on as a robust mechanical connection.

How does soldering work?

Electronic parts are made with metal coatings on specific surfaces. By putting two or more of these surfaces near each other and adding heat, we can melt the surface coatings together to form a new, continuous, conductive, metal path for the circuit. Because the metal coatings aren't very thick, we also need to add more metal (called solder) to help form a strong electrical connection. This connection is called a "solder joint," and is a mixture of the metals from surface 1, surface 2, and the solder all melted together.

what is solder?

Solder itself is also a mixture (alloy) of different metal elements chosen for their conductivity and low melting temperature properties. Solder comes in a variety of different shapes, sizes, and types.

The most common types of solder

Lead-Free Solder

Called SAC305, it's made up of:

  • 95.5% Tin (Sn), 3% Silver (Ag) , & 0.5% Copper (Cu)

Melts at 220°C (428°F)

Leaded Solder

Called Sn63-Pb37, it's made up of:

  • 63% Tin (Sn), 37% Lead (Pb)

Melts at 183°C (361°F)

What are oxides and why are they REALLY important for soldering?

As you may know, metals can react with oxygen in the air to form OXIDES (like rust). Oxides are usually thermally and electrically insulating, meaning it will be nearly impossible for us to melt all the metals together in order to solder unless we have a method to get rid of the oxides on each of the metal surfaces.

Solder Flux is used to address the oxide problem during soldering. Flux is a chemical that, when heated above a certain temperature, will breakdown the metal oxides formed on the surfaces we want to solder. However, once the oxides are broken down, it is a race against time before they form again (seconds) that is also accelerated at higher temperatures.

One secret to good soldering is proper management of oxides.

If you've ever soldered before, you likely have already used flux. Most solder wire has a hollow "core" filled with it. Since flux is a liquid, it also helps transfer heat between the rigid metal surfaces. cool!

What tools do you need in order to solder?

There are lots of different soldering tools to achieve similar results. At a minimum, soldering requires:

  1. Soldering Station & Iron

2. Soldering Iron Tip

3. Tip Cleaning Sponge

4. Solder Wire (with flux)

5. Soldering Iron Stand

6. Safety Glasses

7. Fume Extractor

8. Holder


Electronic parts come in countless shapes and sizes, but they can generally be grouped into two different categories based on how they are attached to the circuit board:

1. Through-hole components

designed to be placed in holes that pass entirely through the circuit board.

Pros: strong mechanical coupling to circuit board

Cons: takes up room on both sides of circuit board, and is more difficult for automated soldering machines

2. Surface mount components

designed for soldering to pads on the top or bottom of the circuit board.

Pros: pack more contacts/components in smaller area

Cons: less mechanically reliable when surface mount soldering is the sole attachment method

how do you solder? - THROUGH-HOLE COMPONENTS

1. prepare bench

Much like cooking or chemistry, good preparation leads to successful soldering. Collect all the soldering tools listed above and have them in front of you at the bench.

Don't forget to put on your safety glasses!

You must always have safety glasses on while soldering (or be looking through a microscope)!

2. prepare soldering iron

We want the soldering iron to be pre-heating while we continue to the following steps.

  • Before turning on the iron, remove the hand-piece from the holder and ensure there's a tip installed (more on types of tips later), then return it to its holder.

  • Next, turn on the fume extractor and make sure you can hear the fan working (NOTE: it may be built into your soldering bench).

  • Finally, turn on the soldering iron and check that the temperature is set to 340°C (650°F) (consult a lab assistant if it isn't).

Soldering irons get HOT. 🔥 Water boils at 100°C, the iron is over three times that- be careful!

3. secure circuit board

Soldering requires both hands, so your circuit board needs to be securely held in place before working on it- trust me!

  • Start by ensuring you have something to hold your board. This can be a "helping hand" as shown above, a vice, or other means of preventing the board from moving around while you try to solder.

  • Spend the time necessary to put the board in a solid, but comfortable working position. It will pay off in the end.

4. placement

Next, you'll want to place the component you intend to solder. Place just one component before proceeding. You can place more at a time once you know your setup (how you're holding the board, etc...) will work.

  • For through-hole components, this means putting the pins of the component through the proper holes.

  • You may need to bend the pins beforehand in order to align properly

  • You can also bend the pins after they go through the holes in order to hold it in place (as shown).

  • If the pins are long, we will trim them AFTER soldering (not before).

5. solder

Assuming you're right handed...

  • Unwind about 6 inches of solder from the spool and hold the wire in your left hand 👈 about an inch or two away from the end.

  • With your right hand 👉, grasp the soldering iron by the foam (or plastic handle) and hold it like you would a pencil (in other words, NOT how you would grab a door handle).

  • Next, gently wipe the old solder off the tip using the brass sponge. This is just to break-loose the oxidation formed on the tip while it was in the holder.

  • Using the solder wire in your left hand, melt a small amount of solder on to the tip to ensure solder will "wet" (AKA stick) to the tip of the iron.

The next few steps should be done no more than a few seconds after wiping the tip. When in doubt, just wipe the tip again.

  • Now place the soldering iron tip onto the pad you intend to solder, making contact with BOTH the pad and the pin. (Remember... you want both surfaces to be hot so all three metals will melt together)

  • After pausing for a moment, use your left hand to feed solder wire into the joint as shown to the right. You want to add enough solder to make it look like the video.

Notice how...

        • clean the surfaces look in the video

        • the solder is fed in one continuous motion

        • the iron is quickly, but smoothly, removed shortly after solder has stopped being fed into the joint.

(More info about what makes a good/bad solder joint in the section below.)

6. Continue soldering

Repeat steps 4 & 5 as many times as necessary. Make sure to re-position the board OFTEN, giving yourself the most comfortable approach possible with the soldering iron.

7. Tin the tip

When taking prolonged breaks between soldering (like repositioning your board) and even when you're finished soldering, it's necessary to prep the soldering iron before shutting down. This is called "tinning the tip."

  • Remove the iron from the holder and melt about an inch of solder wire on to the tip.

  • WITHOUT wiping the tip on the brass sponge, put the iron back in to the holder

8. power down

When you've finished soldering...

  • Tin the tip of the soldering iron

  • Return the soldering iron to the holder

  • Turn OFF the soldering iron

9. clean your circuit board

While soldering, it is common for flux residue to collect around the base of you solder joints.

  • If left on the board for a prolonged time, the flux residue can corrode metal contacts and break electrical connections

  • To clean the flux off the joint, start by putting isopropyl alcohol (IPA) on a paper towel.

  • Put the towel over the solder joint(s) and use a brush to scrub the area through the towel (as shown).

10. wash your hands

You do not want to ingest solder. Period.

Always wash you hands after soldering so you don't transfer solder residue onto your food, desk, etc...

disposing of solder waste

Because of the heavy-metal elements used in solder alloys, it's important that anything with solder on it be disposed of differently than normal trash.

At Stanford, we put ANYTHING with solder on it into a specially marked solder-waste bin. TA's will then tag and dispose of the bins as needed.

  • DO try and be aware of un-needed waste. If a long stretch of wire only has solder on one end, snip the soldered end off and dispose of it properly and then put the wire away in a loose-wire recycling location.

  • For the full EH&S description, click the button above.

how to solder - surface mount devices

Soldering surface mount devices (SMD) is very similar to through-hole devices. But keep in mind:

  1. SMD are a lot smaller. This means they are harder to position and move around on you more easily.

  2. SMD have much higher pins-per-surface-area on the circuit board.

Surface oxides are even more challenging when everything is scaled down to smaller sizes.

Some differences from through-hole soldering...


SMD devices will rarely stay in place on their own while soldering. As a result, the component is typically aligned and "staked" in place.

  • This approach works different depending on the type of SMD package


For soldering SMD resistors, capacitors, diodes, and really anything with less than 5 pins, it's helpful to use the technique shown in the video. To summarize:

  • Melt a TINY bit of solder on to one of the pads

  • Use your left hand to pick up the component with tweezers

  • Put your iron back on the pad you had previously put solder on

  • Carefully "slide" the component into the molten solder

  • Remove the iron, THEN let go with the tweezers (try to be steady!)

  • Solder the remaining pin(s) like normal (touch iron, then add solder)

high pin-count devices

Packages with lots of pins are more difficult to align and are much easier to damage using the resistor/capacitor SMD method.

  • Locate the proper pads on the board for the device and apply a small amount of paste flux to opposing corners (as shown).

  • Place the device and carefully align the pins to match the proper pads on the circuit board. The paste flux will help (somewhat) hold it in place while you nudge it around. Don't worry about getting the paste flux on nearby pads.

  • Now melt a small amount of solder onto the tip of your iron

  • Using tweezers CAREFULLY hold the package in place while you briefly touch your chosen pins with the iron. This will stake the package in place.

The staked pins will likely be damaged if you bump the package right now!

  • Carefully solder a pin or two on the OPPOSITE corner using the normal soldering technique (touch iron, then add solder).

It doesn't take much solder!

  • With the corners staked, you may now proceed with soldering the remaining pins.

On these packages, it's easy to accidentally get solder to "bridge" across multiple pins. If this happens, read the section below on fixing bridges.

tips and tricks

think about how and where the heat is flowing

The heat from the soldering iron won't magically go where you want/need it. If you're having trouble soldering a certain area, think about how the soldering iron tip is making contact.

  • Is there a path for the heat to flow? (called thermal conduction)

  • Is there somewhere ELSE the heat is going?

        • If there's a lot of metal to heat up (heat sink, ground plane, etc...), it can be difficult to get the pin hot enough for proper soldering

  • If used correctly, molten solder as well as paste flux can also aid in transferring heat to the proper places.

solder not wetting the tip?

If the solder melts but doesn't stick, then your tip probably has a thin oxide layer.

  • Try again to melt some solder to the tip, then immediately wipe it off on the brass sponge.

  • Repeat this process a few times to slowly remove stubborn oxide layers

paste flux

As we learned about previously, flux is most commonly used in the form of flux-core solder. However, sometimes we need to add additional flux to break-down the oxides on a solder joint we want to fix.

  • Shown in the video is an example of a bad solder joint (wouldn't we a reliable electrical connection) that is fixed by applying some paste flux and then re-heating the joint with the iron.

  • Like magic, the flux removes the oxides and allows the solder to wet

fixing bridged pins

As shown, paste flux is applied to a group of three bridged pins.

  • Using flux, an appropriately sized tip, and the correct temperature, the solder is melted and surface tension pulls the majority of the excess solder onto the tip

  • Surface tension automatically leaves just the right amount of solder remaining on the pins

soldering header pins

This is a very common activity associated with most development-board purchases. Due to manufacturing costs, it's usually left to the customer to solder the header pins onto the board.

  • Use a breadboard to hold the header pins in place

  • Then feed the pins through your board and solder as explained above

Ta-da! Nice, straight, reliable header pins!

example solder joints (good and bad)

whats a good solder joint?

Bad solder joints can fail when you least want them to! Here are some things that make a "good" solder joint.

  1. For through-hole and SMD components, the solder "wetting angle" (pictured) should never be more than 90°.

2. For through-hole pins, the solder needs to cover at least 270° of the circular pad. In other words, 3/4 of the pin should have solder connecting it to the circuit board pad.


Proper amount of solder, good wetting


Too much solder. Contacting the body of the component


Surface tension can lift small resistors/capacitors


Solder length (D) is the length of (L)


Solder length (D) greater than pin width (W), but less than (L)


Solder length (D) less than pin width (W)

Removing Solder

Mistakes happen! Inevitably, you'll make a solder joint with too much solder, or realize a component isn't placed correctly, and need to remove solder. There are a variety of tools and techniques to "rework" a solder joint:

Braided Solder Wick

A reliable and very precise way to remove solder. The solder wick is made by braiding thin copper wires together.

The general concept of a solder wick is to convince the unwanted solder to stick to the wick more than it wants to remain at the joint. This is easily achieved if we consider...

  1. Where oxides can form. All the surface area from the copper wires creates lots of metal surfaces that will oxidize. So we'll need to make sure extra flux is used.

  2. How heat is transferred. Copper conducts heat quite well, and the whole spool of solder wick will sink heat from our iron if we use it as-is. For best results, we'll need to cut off a piece of solder wick and hold it with tweezers.

Frequently Asked Questions

Q: Isn't lead poisonous?

A: Yes, but only when ingested! That is why ingestion of lead-bearing paint or lead contamination in drinking water is such a big deal since this goes directly into the gastrointestinal tract.

SO WASH YOUR HANDS when you're done soldering and DON'T EAT IN THE LAB!

Q: What about those fumes?

A: A couple of things -

  1. The fumes are from the flux and do NOT contain lead. There is no need to worry about "toxic lead fumes" since there is NO lead in the fumes.

  2. The fumes can be unpleasant or irritating, especially to folks that are sensitive to air-borne particulates.

So USE THE FUME EXTRACTORS and / or WEAR A MASK to reduce your exposure!

Q: Do I really need to wear safety glasses?


Unless you are working under a microscope or wear prescription glasses that serve as safety glasses you should ALWAYS wear safety glasses to protect your eyes.

good job for making it to the end! 🎉

Stanford students, please continue to the online quiz:

Further resources:

Androkavo for (his or her) wonderful soldering videos

IPC-A-610 for info on getting certified