PCB Assembly
Layer Buildup
Online Tools
PCB Design-Aid & Layout
Mechanics
SMD-Stencils
Quality
Drills & Throughplating
Factory & Certificate
PCB Assembly Factory Show
Certificate
Support Team
Feedback:
support@nextpcb.com
Introduction: Welcome to the NextPCB Technical Blog. The world of electronics is full of challenges and opportunities, whether you are designing a multi-layer Printed Circuit Board (PCB) for complex signal integrity or mastering the manufacturing processes of Surface Mount Technology (SMT) and Through-Hole Technology (THT). At the core of these advanced technologies, however, lies an essential foundational skill: soldering. For any electronic engineer, student, or hobbyist, mastering the art of soldering—especially reliably attaching a wire to a circuit board—is a critical step in turning a design blueprint into physical hardware.
This guide is designed to provide beginners with a comprehensive, step-by-step tutorial on soldering wires, covering the entire process from tool selection and material preparation to practical operation and common problem-solving. We will not only show you "how to do it" but also delve into the technical principles behind "why you do it," helping you move from a novice to a proficient user.
Successful soldering begins with proper preparation. This section will detail your essential toolbox and explain the technical considerations behind each choice while emphasizing the paramount importance of personal safety.
The essence of soldering is heat transfer, not merely applying solder. Therefore, a suitable soldering tool is your primary guarantee of success.
A soldering iron is the core tool for soldering. Its function is to transfer heat to the workpiece, melting the solder to form a strong connection. For beginners, it is crucial to choose a soldering iron with adjustable temperature, fast heating, and replaceable tips.
Compared to a standalone iron with a fixed temperature, a soldering station offers precise temperature control, which is especially important when dealing with different components and solder types. For example, lead vs lead-free solder requires different temperature settings, and a good station helps you manage this efficiently. An adjustable-temperature soldering station allows you to easily meet different melting point requirements, significantly improving soldering quality and efficiency.
The soldering tip is the medium for heat transfer, and its shape and size directly affect the soldering result. For BGA soldering or working with small pads, a chisel tip is recommended. Its larger surface area can transfer heat to the joint more effectively, ensuring the pad and wire are heated evenly. A pencil-type tip, with its slender design, is better suited for handling fine areas and components.
Helping Hands or Vise: Stability of the workpiece is crucial during the soldering process. A helping hands tool or a vise can firmly hold the circuit board and wire, preventing "disturbed joints"—unstable connections caused by accidental movement while the solder solidifies. Some clamps also come with a magnifying glass and an LED light, allowing you to see work details more clearly and minimize errors.
Fume Extractor: The fumes produced during soldering are harmful to your health, especially when using leaded solder. A fume extractor absorbs and filters these harmful fumes, ensuring you work in a clean, safe air environment. For enclosed spaces, this is a non-negotiable piece of equipment. Especially recommended when working with solder flux to protect health and ensure clean air quality.
Solder wire is the filler metal or alloy used for joining, which melts at a temperature lower than the metals being connected to form a strong bond. Choosing the right solder wire is key to ensuring a reliable connection.
Traditional solder wire is made from a tin (Sn) and lead (Pb) alloy, with the most common being 60% tin and 40% lead. This alloy has a low melting point (approximately 183°C) and is easy to use, making it a popular choice for many beginners.
However, due to environmental and health concerns, many industries have switched to lead-free alternatives. Lead-free solder is typically tin-based, with added metals like copper (Cu), silver (Ag), or bismuth (Bi) to enhance specific properties. Its melting point is slightly higher, usually between 217°C and 227°C, requiring the use of a hotter iron. For general electronic projects, lead-free tin-copper alloy (Sn-Cu) is widely favored for its balanced performance and cost.
> Recomend reading: 60/40 Vs 63/37 Solder – What are their Similarities and Differences
> The Best Solders for Circuit Boards: A Comprehensive Guide
This is a key difference that beginners must understand. It is strongly recommended that all electronic soldering projects only use rosin-core solder. Rosin is a solid resin, and its residue is non-corrosive, so it does not need to be removed immediately after soldering.
You must avoid using acid-core solder. The acidic substances in this solder are specifically for plumbing and metalwork. Their corrosive properties will eventually eat away at circuit board components, causing permanent damage.
The diameter (or gauge) of the solder wire is also an important consideration. For PCB work, it is recommended to use the thinnest solder wire possible. This helps you precisely control the amount of solder, avoiding the use of too much, which can lead to solder bridges—where solder accidentally connects pads that should not be connected.
The following table summarizes the characteristics of different solder wires to help you make an informed choice:
Soldering is an operation with low safety risks, but neglecting basic principles can still cause personal injury or equipment damage.
Ventilation and Fume Extraction: Soldering fumes, especially those from leaded solder, are harmful to the respiratory system. Always work in a well-ventilated area and use a fume extractor to protect yourself.
Personal Protective Equipment (PPE): Wearing safety glasses is crucial to protect your eyes from solder splashes. Additionally, it's recommended to wear fire-resistant clothing that covers your arms and legs (such as 100% cotton) and to work on a fire-resistant or heat-resistant surface.
Hygiene Practices: After soldering, always wash your hands thoroughly with soap and water to prevent accidental ingestion of harmful substances like lead.
Tool Safety: A soldering iron tip can reach temperatures of up to 400°C. Never touch it directly with your hands. When not in use, always return the iron to its stand and keep it away from flammable objects like cables.
Preparation is the dividing line between successful and unsuccessful soldering. Neglecting these seemingly simple steps can lead to a series of soldering defects.
Keeping your workspace clean, stable, and well-lit is a prerequisite for precise soldering. A clean work area prevents dust or grease from contaminating the pads, while a stable workbench prevents poor connections caused by movement while the solder solidifies. Any slight movement as the solder cools can cause crystallization inside the joint, making it brittle and unstable. Therefore, using a vise or helping hands is not just for convenience but also to ensure the mechanical integrity of the soldered connection.
"Tinning" the iron tip means coating its surface with a thin layer of solder. This simple step is vital for soldering.
The Principle of Tinning: A tinned iron tip becomes shiny, and this layer of molten solder maximizes heat transfer efficiency. When an iron tip is not tinned, its surface oxidizes and turns black from exposure to air, forming an insulating oxide layer that severely impedes heat transfer. An oxidized tip cannot effectively heat the joint, and even prolonged contact can result in an underheated, cold solder joint.
How to Do It: After the iron heats up to its working temperature, gently touch the solder wire to the tip, allowing the solder to melt and cover the entire end. Then, immediately wipe off the excess solder on a wet sponge or brass cleaning ball to keep the tip clean and shiny. Repeating this step before each soldering session and during breaks effectively extends the tip's lifespan and maintains its performance.
Pre-tinning is the process of applying a thin layer of solder to both the end of the wire and the PCB pad before you join them. This step significantly improves the quality of the final connection.
Its importance lies in this: when a pad or component lead is oxidized from air exposure, its surface loses its luster, affecting the solder's "wetting" ability. Wetting is the ability of molten solder to spread and adhere to a metal surface. If the pad's surface has dirt, sweat, or an oxide layer, the solder will not flow evenly, leading to poor wetting and a fragile connection. By pre-cleaning and pre-tinning, you ensure that both surfaces to be joined are in optimal condition, allowing for smooth solder flow and a strong, reliable connection.
This section will break down each step of the wire soldering process, ensuring you can perform the operation accurately and efficiently while understanding the technical essentials behind each action.
Use helping hands or a vise to firmly hold the circuit board and wire in place. This prevents the workpiece from moving during soldering, which avoids defects caused by movement. Make sure the end of the wire is aligned with the pad you need to solder.
Place your tinned soldering iron tip so that it touches both the wire and the PCB pad simultaneously. This is the most crucial step in soldering. The sole purpose of the iron tip is to transfer heat, not to melt the solder wire. Your goal is to get both the wire and the pad to reach the solder's melting point.
This requires finding a delicate balance, the "Goldilocks Zone" of heat. If the iron tip is too hot or you heat it for too long, you could cause the pad to lift off (lifted pads) or damage the component. Conversely, if the temperature is too low or the heating time is too short, the solder will not fully melt, resulting in a cold joint. The correct action is to hold the iron on the joint just long enough for heat to fully transfer, but not for too long. When the joint reaches the required temperature, the solder will "wet" and flow smoothly to the connection point of the wire and pad.
Once you are sure the joint is sufficiently heated, gently touch the solder wire to the intersection of the pad and the iron tip. Never feed the solder wire directly into the iron tip. The molten solder will be instantly attracted by the heat and will flow smoothly onto the pad and wire, forming a smooth, full "volcano" or "Hershey kiss" shape.
You will see the solder quickly cover the pad and perfectly envelop the wire. As soon as a small mound or fillet forms, stop feeding the solder immediately.
First, remove the solder wire, and then immediately lift the iron tip from the joint. Do not remove both at the same time, as this could cause the solder to be pulled and deformed.
After soldering, do not blow on the joint. Rapid cooling can cause the solder to crystallize, making it brittle and creating a fragile cold joint or a failed connection. Let the joint cool naturally to ensure its internal structure is solid.
After the joint has completely cooled, a detailed visual inspection is an essential step. A good solder joint should have the following characteristics:
Appearance: The surface should be smooth, shiny, and uniform.
Shape: It should be concave or "volcano-shaped," not a round ball or clump.
Coverage: The solder should completely cover the pad and form a clear and firm connection with the wire, with no exposed copper wire.
If you find any issues, do not hesitate to reheat and refine the joint.
Even when following all the steps, beginners may encounter some common problems. This section will help you identify these defects and provide practical methods for correction.
Appearance: The surface is matte, rough, uneven, or has cracks. Root Cause: The joint was not heated enough, so the solder did not fully melt and properly wet the metal. Or, the workpiece moved while the solder was solidifying. Solution: Reheat the joint and add a small amount of new solder wire if necessary, ensuring the solder fully melts and forms a smooth surface.
Appearance: Solder accidentally connects two or more pads that should not be connected. Root Cause: Too much solder wire was used, or the iron tip was improperly handled, causing the solder to overflow. Solution: Use a solder sucker or solder wick to remove the excess solder. Solder wick is a braided copper wire that acts like an "eraser," soaking up the extra solder.
Appearance: The copper pad on the circuit board peels or lifts off the board's surface. Root Cause: The iron temperature was too high or it was left on the joint for too long, causing the adhesive to fail. Solution: Prevention is key. Use the correct iron temperature and try to shorten the heating time. Once a pad is lifted, it is very difficult to repair and may require a "green wire" method to re-establish the connection.
Appearance: The solder fails to fully cover the pad or component lead and forms an incomplete ball shape.
Root Cause: The pad or wire surface is dirty or oxidized, or there is a lack of flux or insufficient heat.
Solution: Thoroughly clean the pad surface. If the pad is oxidized, you can use a pencil eraser to gently rub it until it regains its shine. Pre-tin the joint before soldering and ensure the iron tip is clean and tinned to maximize heat transfer.
The following table systematically summarizes common soldering defects and their solutions:
The final steps after soldering are equally important.
Removing Flux Residue: While rosin flux is non-corrosive, its residue can become sticky and affect the board's appearance and performance. You can use Isopropyl Alcohol and a stiff-bristled brush to clean the solder joint area.
Using Heat Shrink Tubing to Protect Connections: For wire connections that need insulation and strain relief, heat shrink tubing is a professional method of protection.
A Professional Tip: The use of heat shrink tubing requires strategic planning. A common beginner's mistake is remembering to put the heat shrink on after the soldering is complete. This forces you to cut the wire and start over. Therefore, always slip the heat shrink tubing onto the wire before you start soldering and move it far away from the heat until the soldering is done. Then, slide it over the connection and shrink it with a heat gun or lighter. This simple planning saves you a lot of rework time and frustration, demonstrating the foresight of an experienced maker.
Soldering is not just a technique for joining metals; it is a practical art form that transforms theoretical knowledge into a physical circuit. Through this guide, we hope you have grasped the fundamental knowledge, tools, and techniques required to solder wires to a PCB. From proper preparation and precise heating to the final cleaning and protection, every step is crucial.
We believe that once you master this basic skill, you will be able to explore the broader world of electronics, whether designing complex Gerber files or gaining a deeper understanding of single-layer or multi-layer board manufacturing processes. When you are ready to put your own designs into practice, remember that NextPCB is dedicated to being your reliable partner from prototyping to mass production. We provide high-quality PCB manufacturing and assembly services, ensuring that every one of your projects starts on a solid foundation. Your creativity is limitless, and NextPCB will provide the tools you need to turn those ideas into reality.
Still, need help? Contact Us: support@nextpcb.com
Need a PCB or PCBA quote? Quote now
Printed Circuit Boards
Surface
