15 Rules for Successful Electronics Soldering

Rule #1 – Know the Fundamentals

Here at SRA, we believe anyone can solder! After all, it’s not hard to learn how to melt some solder and make a connection. We do believe it is an art form though, and understanding the science behind it will help you to master the craft. Our aim with this guide is to explain how soldering works, why various reactions occur and how to produce a highly reliable connection by taking these factors into account. This knowledge will serve beneficial for both beginners and experienced technicians alike! For anyone who struggles with soldering, this will help you understand the “why” so you can improve. And for those new to the world of soldering, we hope to eliminate the anxiety entirely by teaching you right the first time. In this guide, we will go over everything you need to know including our best techniques for soldering success!

Not Sure if Soldering is right for your application?

Check out our articles below to find out the differences between soldering, brazing, and welding or learn the facts about soldered versus solderless connections and when to use them. 

Setting Up Your Work Area

The first step in any soldering project is assembling your work area with all the necessary tools for the job at hand. We recognize that soldering is an expansive umbrella that covers a wide range of operations and applications but there are certain tools we consider essential. Our focus here is primarily on electronics handsoldering so this would include things like a soldering iron, solder wire, flux, cleaning materials, and various hand tools. For a rundown of all of our must have tools check out the article below.

Must Have Soldering Tools 

In any case, you’ll want to keep track of all your parts and progress on the build or repair. We recommend having several plastic containers of varying sizes for organizing and storing parts during disassembly and new builds. Be aware, there are ESD safe parts bins available as well for static sensitive components. Scrap pieces of cardboard will also come in handy for creating templates by sticking parts through it, especially keeping track of screws! In many cases, cardboard makes an excellent “solder buddy” to hold parts while soldering too. During tear-downs and assembly, your best friend is definitely your smart phone or digital camera. Take lots of pictures throughout each step, you’ll be glad you did later! Trust us.

Rule #2 – Identify the Construction Method

When it comes to soldering there are few safety concerns that must be addressed. First, working with temperatures of up to 840°F (449°C) introduces risk for burns and fires. Second, electronic devices inherently present a risk of electrical shock. And finally, the soldering process itself produces fumes that are hazardous to breathe in. Don’t worry though, as long as you know the risks you can mitigate them by working safely!  

Fume Extractors (Smoke Absorbers)

The number one safety precaution when soldering is to do so in a well-ventilated space. This could be next to a window where air can circulate. We recommend using a stand-alone fume extractor or one integrated into the iron to redirect to fumes. Whether they be burning flux or plastic the fan will redirect them through the carbon filter and away from your face!

To learn more about the benefits of using fume extractors check out our article here.

ESD Safe Mats 

As part of your setup, we recommend using a heat-resistant rubber mat to protect your work area from any burns and scratches. It may also help keep your components in place and not lost on the ground. If working with sensitive components you will want an ESD safe mat to protect from static charges that are naturally generated. 

Check out our article on ESD for a more in-depth explanation here.

Leaded Solder Exposure

Please note that when using leaded solder alloys, the risk of lead exposure is only through touch. Lead does NOT vaporize (fume) at normal soldering temperatures and so the fumes created by soldering are actually caused by the flux in the wire being burned off. Even though the smoke does NOT contain lead, it is still not good to breathe in and should be avoided.

Protective ESD Gloves

We also recommend wearing gloves when handling leaded solder and be sure to always wash your hands after. Treat it like raw meat and keep it away until its cooked (heated) but don't be afraid of it. Just never eat where you solder.

Protective Eyeglasses

It is also a good idea to wear safety glasses to help protect yourself from molten solder splatter and other debris such as flying component leads and zip ties after being trimmed.

WARNING: Risk of Electrical Shock

Please be aware of the risk for lethal shock when working on electronic devices. Make sure power to device is off and the power cable is physically unplugged from the outlet before opening anything. It is also a good idea to drain any electrolytic capacitors before soldering because these can hold an electrical charge long after the device has been unplugged.

Rule #3 – Choose the Right Solder and Flux

The three essential elements for successful soldering are the solder itself, the flux used, and the heat source. We will be discussing heat in-depth in the next section but for now we will focus on figuring out the right combination of solder and flux for our application. 

What is Solder?

In its simplest definition, solder is merely a mixture of metals, also known as an alloy. Traditionally, electronics solder as always been made with tin and lead in varying amounts. The most common being 60/40 (60 percent tin / 40% lead) and 63/37 (63% tin / 37% lead). These particular metals were originally chosen for their low temperature melting point when combined into an alloy and their superior wetting abilities.

What is Wetting?

The term 'wetting' is used to describe the characteristic of how quickly solder melts and bonds to a given surface. In this reaction, the molecules of solder and plated copper on the board or component bond together to form a new alloy that is mechanically stronger and electrically conductive throughout. When proper wetting has occurred, the solder cannot be scrapped off after solidifying. However, the bond can be undone through desoldering. Which consists of re-melting the solder and removing the wire or component.

Leaded Solder Alloys

The chart below shows some of top alloy choices for lead solder users including the popular 63/37 and 60/40 varieties. We usually recommend the 63/37 alloy over 60/40 because it is eutectic, meaning it melts and solidifies at the same temperature and is therefore easier to work with. The range between liquid and solid states is known as the “plastic range” and since there is virtually none with eutectic solder, it greatly reduces the chance of the joint not getting hot enough for wetting to occur.

Lead-Free Solder Alloys

In recent years, the world has become more cognizant of the dangers of lead and out of this came lead-free initiatives to help limit exposure. Some regions have gone so far as to ban the sale of lead solder alloys to non-business consumers. Almost all consumer electronic devices are now constructed with lead-free solder but many hobbyists and repair shops continue to use leaded varieties for two very good reasons. 
1. Lead alloys are much easier to work with because of their low melting point and wetting properties. Whereas lead-free alloys just can't quite beat the performance. Sometimes requiring higher temperatures to make a proper connection.
2. Its a general rule to match the chemistries of the solder and flux with the project. For example, hobbyists and repair shops working on vintage equipment like ham radios should not be using lead-free alloys for their rework.
That being said, the most popular lead-free alloys in our experience are SAC305 (96.5% Tin / 3% Silver / 0.5% Copper) and 96/4 (96.5% tin / 3.5% Silver). For most lead-free electronics projects, we recommend SAC305 because it out performs the rest by far. In the end, the decision is up to the user but always remember you want to avoid mixing alloys to keep the chemistry consistent for the best results.
Solder comes in a variety of forms for use with many different types of heat sources. Everything from solder wire, to paste, solder bars, preforms (solder that is shaped in specific amounts for repeatable results), and solder balls (tiny spheres of solder used for soldering BGA type chips). For all electronics work with a soldering iron, you’re always going to be using solder wire. More specifically, solder wire with a flux-core. 

What is Flux?

Flux is a chemical agent that aids in the soldering process. When heated, the acids in the flux become active in removing oxides. Its job is to dissolve the surface oxidation on your parts and keep them clean long enough for the solder to do its job. Without flux the solder cannot flow and bond to the workpiece. 

There are a few types of fluxes that are found in solder wire. The most common are No-Clean and Rosin varieties. These are both great choices that do not need to be cleaned off after soldering. Rosin varieties will leave an amber residue but it is non-conductive and actually has some protective qualities for the surface of the parts and board. 
The number one rule here is to NEVER use acid-based fluxes on electronics. While it's true that all flux is acidic to an extent, anything containing zinc-chloride will eat away at electronics. This means common plumbing flux you find at hardware stores. DO NOT USE IT.

Solder Wire Gauge

The final two considerations to make when choosing solder is the gauge (diameter) of the wire and the size (length) of the spool. For general soldering a wire gauge of 0.31” or 0.32” is ideal. For bigger connections, something like 0.62" would be more suitable. Whereas for smaller surface-mount work you’ll probably want to go smaller, around 0.20”.
Be aware that as the gauge gets smaller, the flux-core can become more inconsistent and you may need to apply more flux. This can be done using flux paste or liquid flux in a bottle or flux pen.
Some technicians prefer applying additional flux each time they solder to ensure the highest quality joint possible. A common use case would be using flux paste to coat wires before soldering. By dipping the entire solderable area in flux, you can be certain the solder will be able to flow and stick exactly where you need it to! 

Solder Spool Size

As for spool size, consider how much you will need. Solder wire is most commonly found in 1 or 1/2 pound spools which will last many people a long time. You can also get 2- or 4-ounce spools for smaller projects and to test drive different combinations of solder and flux.

Rule #4 – Understand the Art of Heat Transfer

At this point you should have a good idea of what solder and flux combination to use for your application. Let's now take a look at heat and the many factors to consider. Take a look at your project and visualize the outcome you want to accomplish. Start by analyzing the components, specifically their size and makeup. Are they fragile or tough, plastic or metal, big or small. Do they have moving parts like a potentiometer or are they solid? The answers will help inform your approach because they all affect heat transfer in one way or another. 

Managing the Heat Cycle

To do this, you must be able to manage the "heat cycle". Which is simply how quickly the joint heats up, how hot it gets, and how long it stays at that temperature. This is critical because heat and pressure are the two main causes of damage to circuit boards. Both can easily cause the copper pads and traces on the board to lift, leading to a much more difficult and frustrating repair. However, one must also not be afraid of the heat because it is crucial to a successfully soldered joint. Taking it away too soon or “playing paintbrush” with the iron can lead to cold solder joints where the connection did not get hot enough for wetting to occur. 
The time you spend on a joint is referred to as the "dwell time" and it can vary based on a variety of factors. In the coming sections, we're going to touch on everything that affects heat transfer from temperature to tip size. Understanding them will help you master the heat cycle and make the difference between a novice and professional solderer. I'll be referring back to these throughout all of our guides because they help inform basically all decisions of what tools and techniques to use while soldering.

Rule #5 - Use the Right Temperature

You may be thinking that tip temperature is the most critical factor for heat transfer but this is not the case! While it is important, it's only one piece of the heat management puzzle. As long as your iron gets hot enough to melt the solder alloy you're using, you can improvise to get a good result. Check the specifications of your solder to make sure. The right temperature then becomes a balance between heating the joint quickly and giving enough dwell time for the solder to wet into the joint. 

For general electronics soldering, somewhere in the range of 572-662°F (300-350°C) is a good starting point. This can vary based on the size of the components and their makeup. For heating very large parts, you'll need to be in the upper range or higher. Conversely, if the component is smaller and heat-sensitive, you'll want to stay in the lower range. Some soldering stations have preset functions that will let you jump between favorite settings depending on the work you're doing.

Proper tip temperature is most critical for printed circuit boards (PCB) work. You really want to know how hot your iron is getting to properly manage the heat cycle and not damage the board. We highly recommend a soldering station with digitally controlled temperature for this type of work. It may also be helpful to use a heat sink to stop some of the heat from getting to the component. Metal alligator clips or forceps make great heat absorbers that will hold the part as well.
We don’t recommend cranking the temperature up to the highest it will go (Approx. 896°F / 480°C) because the tip will oxidize very quickly. If not careful, you can ruin the tip in a matter of minutes.

Rule #6 - Keep Your Surfaces Clean

All surfaces involved in the soldering process must be free of contaminants and oxides before soldering. This means the surfaces of the work piece, components, and soldering iron tip. Even the thinnest, seemingly invisible layer of oxides can prevent solder from sticking and wetting properly. When you've got a dirty or highly oxidized tip its just not going to transfer heat effectively. 

The level of filth present will dictate how much cleaning you’ll need to do. For new builds you can usually get by with just the flux-core in your solder wire and a well-maintained soldering iron. When reworking old parts, you’ll want to clean them the best you can with isopropyl alcohol, fine sandpaper, or even an ultrasonic cleaner if necessary. Adding a more aggressive flux can also help, when the parts are very oxidized from sitting over the years or in storage. 

Standard Wet Sponge

Abrasive Brass Coils

There are two main ways to clean your iron tip and they are wiping on a wet sponge or plunging into brass coils. There are pros and cons to both but we recommend using a hybrid approach for the best results and convenience. The semi-abrasive brass will get the bulk of the solder residue off without dropping tip temperature, while the sponge more effectively shocks away the acids from the flux. 

To learn how to use these methods to properly clean your tip check out our article

5 Ways To Maintain Your Soldering Iron Tip

Please DO NOT file your tip down to “clean” it. This practice dates back to when soldering tips were simply bare copper. Modern tips are plated for extended life and performance. Aoyue soldering tips for instance, are constructed of a copper core that is then plated with iron, chromium, nickel, and metal alloy for strength. When you file the plating off, you dramatically reduce tip life by compromising its ability to melt solder and for solder to stick to it. This is soldering tip abuse folks.

Rule #7 – Tinning Your Tip is Key

The best way to keep your tips working perfectly aside from cleaning them before and after each joint, is by keeping them tinned with fresh solder. Tip tinning is the practice of adding solder or Tip Tinner (a solder like mixture) to the tip for protection and optimal heat transfer. 

Pre-Tinning the tip before soldering helps to jump start the heat transfer process as the flux cleans the tip and gets the solder flowing before you even touch the joint. 

While Tinning before returning the iron to its holder will create a protective coating on the tip. The fresh Solder or Tinner on the tip forms a gas tight seal so oxidation caused by sitting in the open air is dramatically reduced. 

For protection, Tip Tinner is the better option because it does not contain any flux acids that will eat away your iron tip. 

Any solder residue that is left on the iron after coming off a joint should always be cleaned off. If the iron is smoking in the holder that means there is a concentration of flux still on it that needs to be cleaned off. Again, the acidity will eat away at the plating of the tip reducing its life. 

How to Tin Your Tip

When using your iron for the first time, you will want to perform a renewing cycle of tinning and cleaning until the tip has a nice coating of fresh solder built up and accepts solder easily. Add solder or Tinner all around the tip and clean with your coils/sponge. Repeat until the tip is shiny in appearance and able to melt solder quick and easy. Finish by adding solder or Tinner and returning to its holder. 

Oxidation can form over night as well, so still do this at the start of each day before your soldering session for best results. 

To get an in-depth explanation on different cleaning and tinning methods checkout our article 

5 Ways To Maintain Your Soldering Iron Tip

Rule #8 - Pay Attention to Surface Mass

What we mean by this is the size and surface area of the parts being soldered but also of the iron tip itself. When choosing a soldering iron tip, you want to match the size to the work you’re doing. The shape is in part up to your preference but also what fits best into the joint and provides the greatest surface area for heating. In general, the shorter the tip is, the more efficient it will be because it is closer to the heating element and therefore more evenly distributed. The simple physics here is that larger components will take longer to heat and smaller tips will take longer to heat said components. 

Small tip + Big Mass = slow temperature rise
Big Tip + Small Mass = Fast temperature rise

Most soldering irons will have interchangeable tips that can be quickly swapped out or replaced when worn out. There are two main types of tips which are the standard type that sit on the heating element and the cartridge type that plug into the iron handle. Cartridge type tips actually have the heating element built into their long body. The advantage is that they can be changed very easily while working. Both standard and cartridge tips come in many different shapes and sizes, each with their own purpose or application. The most common are the pointy shape known as "conical tips" which are great all-around workhorses for general soldering use. When you've got bigger components or more surface area to heat "bevel tips" come in handy with their flat edge. Make the job easier and more efficient heat transfer. 

Rule #9 - Make Sure Your Iron is Powerful Enough

The job of every soldering iron is to generate and store heat. When you touch an iron to a mass such as a component or circuit board, it will start losing its heat. The larger the mass you're trying to heat or the longer it sits there, the harder the iron must work to recover the heat loss. This is known as the rate of recovery and the heating capacity of the iron is measured by its wattage rating. In order to last longer and heat larger areas without dropping temperature, more wattage is needed. 

Having a higher wattage means that the iron has a larger heating element or “Thermal Bank” to draw from, before the temperature starts dropping. In a worst case scenario, the iron could actually get stuck to the work piece from being drained of its heat supply and allowing the solder to solidify. 

Generally speaking, a basic AC voltage iron of 40-60W is ideal for electronics. Anything higher wattage on a fixed temp iron will get too hot to solder accurately. If the temperature is digitally controlled, a higher wattage rating becomes an asset though because it will stay at the set temperature but have more power in reserve if needed.

Rule #10 - Use the Contact Patch Advantage

The contact patch refers to the surface area between the iron tip and the joint while soldering. If you only use the very tip top of the soldering iron to solder, you will get poor heat transfer. By using the broad side, you can increase the surface area or "contact patch" resulting in superior heat transfer. This means you can get in and out faster because you spend less time heating the joint. 

In order to better understand this concept, try a dry run without heat so you can observe the surface areas that touch while soldering. Another masterful way to increase heat transfer is by using solder to expand the linkage. When you add solder wire between the iron tip and the joint you are adding mass to both. Not to mention the flux, that is cleaning the surfaces, aiding in the heat transfer and allowing the solder to stick.

Rule #11 – Place Your Components Neatly

Almost as important as soldering itself is how you install the components to be soldered. In terms of through-hole devices (THD) I’m talking about the art of lead forming. Sometimes out of the bin or box, your components can have bent or even mangled leads. When forming leads, it’s nice to start with the leads as straight as possible. Our favorite tool for this is long needle-nose pliers that have serrated jaws. ESD Safe doesn’t hurt either.

To learn about the many types of electronic construction you'll come across check out our article

7 Types of Electronic Construction: THT, SMD, Point-to-Point, and More

Rule #12 – Add Solder and Heat Simultaneously

Now we’re ready to finally solder our first connection! Many guides teach soldering in such a way that is NOT conducive to the fastest heat transfer possible. They often will tell you to apply the iron at the joint and hold for 1-2 seconds. Then add solder from the opposite end, making sure to not touch the iron, and when the solder starts melting you know the joint has reached temperature. The problem with this approach is that it takes too long to heat the joint, so you run a greater risk of damage. 

Instead we recommend adding the solder and heat at the same time which will kick start the heat transfer process. To do this, take up the solder spool approximately 3 inches from the end. This gives you enough wire to work with without getting burnt and allows for great control over it.

Next, position the solder wire right on the joint where the component lead(s) meet the pad or lug. The end of the solder wire should not extend too far past the joint.

Now bring your iron in and press down right on top of the solder. Let gravity do the work as if you’re balancing the iron on the joint. Remember hard pressure can cause damage to parts and especially circuit boards. In this fluid action, you are effectively tinning the iron tip, starting heat transfer to the joint, and making the heat transfer quicker by increasing the thermal linkage between the iron and the joint with the solder wire.

Add more solder as need to the opposite side of the joint to fill it in. Then as soon as you have enough take away the wire and wait 1-2 seconds to make sure all the solder has been completely melted and wetted to the joint.

Finally, remove heat and wait for solder to cool and solidify. I do not recommend blowing on the joint to speed up this process because you could interrupt the wetting and create a cold solder joint. 

In order to simply this guide we have broken it up into separate soldering operations so the instruction can better match your application.

Solder Wires and Cables

Solder THD to PCB

Solder SMD to PCB

Rule #13 – Inspect and Evaluate the Joint

You should now have a perfectly formed joint with the solder vacuumed (wetted) down into a cone shape on the component lead. Clean the iron, tin with solder or Tinner, and return to holder. Sometimes you can solder several joints consecutively if you’re quick about it but if there’s a lot of acid built up and the iron is smoking a lot, you’ll want to clean it asap. Oxidation can occur very rapidly and the iron will lose its effectiveness. Finally, If you’ve got long component leads sticking out, trim them with flush cutters so you have just a little bit visible above the solder cone.

Cleaning Flux Residues

Once finished, you can clean the flux residue left over off the board if wanted. No-clean and Rosin-based fluxes generally do not need to be cleaned off after soldering. But any fluxes with aggressive acids will require cleaning with isopropyl alcohol or Ultrasonic Cleaner to stop them from eating away at the components and board.

For a step-by-step guide of how to clean a PCB with an Ultrasonic Cleaner check out our article and video

9 Easy Steps to Clean a Circuit Board with an Ultrasonic Cleaner

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