Soldering is a fundamental technique for permanently joining metals, widely used both in industry and by hobbyists. This process is divided — based on the melting temperature of the solder — into soft and hard soldering. Each method has specific applications and requires different tools, solders and dedicated substances designed to facilitate soldering work, such as fluxes.
What distinguishes soft soldering from hard soldering? Melting temperature and alloy composition
The fundamental difference between soft and hard soldering lies in the melting temperature of the solder (and the soldering process itself), which directly affects the mechanical properties and thermal resistance of the metal joint.
Soft soldering
This method ensures good electrical conductivity and optimal resistance to mechanical damage in low-temperature installations.
Soldering alloys used in soft soldering have a melting point below 450°C. The most commonly used alloys include:
- Tin-lead alloys (Pb-Sn) – classic solders with good wettability. Once widely used in electronics, their production is now gradually being limited due to lead content. An example is Flux RF800.
- Lead-free tin-copper alloys (Sn-Cu) – popular and compliant with RoHS (an EU directive restricting the use of hazardous substances in electronic and electrical equipment). They have a high melting point (~227°C) and lower fluidity compared to lead-based solders.
- Tin-silver alloys (Sn-Ag) – mainly used in applications requiring higher strength and thermal resistance, such as automotive and some industrial devices.
- Tin-silver-copper alloys (Sn-Ag-Cu) – known as SAC alloys. The most common type in lead-free electronics. Like Sn-Cu alloys, they have relatively low fluidity, but in SAC alloys, silver presence compensates for any inferior soldering properties. An example is the Solder Paste Easy Print /Sn96.5Ag3Cu0.5/.
Hard soldering
Hard soldering uses alloys with a melting point above 450°C (although the conventional upper melting limit is around 900°C, there is no strict maximum temperature in practice). These alloys enable the creation of highly durable and long-lasting metal connections in high-pressure systems and high-temperature heating applications.
Depending on the application method, the following alloy types are commonly used:
- Silver, copper, and zinc alloys (Ag-Cu, Ag-Cu-Zn) – known for their excellent capillary action. Used in refrigeration, HVAC systems and stainless steel pipelines.
- Copper-phosphorus alloys (Cu-P) – popular for joining copper components without requiring flux, as phosphorus acts as both a deoxidizer and flux agent.
- Nickel and cobalt alloys, brass – characterized by high corrosion resistance and the ability to withstand extreme temperatures. These alloys are widely used in aerospace, energy industries and heat exchanger manufacturing.
A commonly used flux for brass soldering and welding is Flux Borax, which enhances soldering efficiency and joint durability.
Soft soldering vs. hard soldering – differences in joint strength and operational durability
Soft soldering
Soft solders provide moderate mechanical strength (typically <50 MPa); however, this is sufficient for many applications, including electronics, automation and lightweight sanitary installations.
These soldering alloys also offer excellent electrical conductivity — an essential property for connecting wires, solder pads and electronic components.
Hard soldering
Hard solders deliver significantly higher mechanical strength (up to 200–300 MPa) compared to soft soldering alloys. They also exhibit superior resistance to vibrations and aggressive substances (e.g., technical gases, acids and saline solutions).
These alloys are used in connections requiring durability and airtightness, especially when exposed to temperature fluctuations, high pressure or dynamic stresses.
Differences in the soldering process – soft soldering vs. hard soldering
Soft soldering
Soft soldering can be performed precisely using simple tools and low energy, without requiring significant heating of each component. The process is quick and does not pose a risk to adjacent materials such as plastics or PCB insulation. Additionally, it can be automated for efficiency.
Hard soldering
Hard soldering requires preheating the entire joint to a temperature exceeding 450°C, which may necessitate the use of a gas torch, induction heating, or an appropriate cooling method. In certain applications, controlled-atmosphere furnaces are used — particularly in vacuum soldering for heat exchanger production.
Proper surface preparation before soldering is essential, along with the use of specialized fluxes that can withstand high temperatures.
Applications of both soldering methods
Soft soldering – precision, conductivity, and versatility at low temperatures
Soft soldering is widely used in industries requiring excellent electrical conductivity, precise assembly and minimal mechanical stress at metal joints.
Consumer and professional electronics
Soft soldering is the standard technique for assembling electronic components, including integrated circuits (SMD, THT), capacitors, resistors and microprocessors on PCBs.
Tin-lead or lead-free solders are commonly used, often in the form of flux-cored solder wire (coated solders). This application method simplifies the soldering process, sometimes eliminating the need for additional flux.
Electrical engineering and industrial automation
Soft soldering is employed in the production of controllers, relays, transformers and connectors, ensuring good conductivity and adequate durability of the connections.
Sanitary and heating installations (up to 110°C)
Joining copper pipes and fittings in water supply systems (hot water installations) or central heating is another common application of soft soldering, particularly using tin-copper or tin-lead solders.
The success of this process depends on maintaining an optimal operating temperature (maximum 110°C). If there is a sudden temperature spike, the choice of flux becomes crucial. A notable flux used for soft soldering in sanitary installations is Soldering Fluid, which improves metal wettability and oxide reduction — ensuring a durable and airtight joint.
Model making, hobby repairs, and DIY projects
Soft soldering is also widely used in handcrafting and repairs of non-ferrous metals and their alloys, such as copper, brass, tin and zinc.
This process does not require extensive tools — an electric or transformer soldering iron is sufficient — making it the most accessible option for home users.
Hard soldering – high durability for extreme working conditions
Hard soldering creates exceptionally strong joints with high mechanical, thermal and chemical resistance. This technique is particularly effective in high-temperature installations and industries such as energy, automotive and aerospace.
Refrigeration, air conditioning and gas systems
Joining copper pipes in air conditioning, refrigeration and gas transmission systems requires soldering with joints that can withstand pressure fluctuations, vibrations and operating temperature changes. Copper-phosphorus or silver-based solders are commonly used in these applications.
Industrial hydraulics and technical installations
In industry, hard soldering is primarily used for assembling and repairing pressure systems involving steel, brass, bronze and copper. This method is ideal for situations demanding resistance to chemically aggressive substances, such as saline solutions, technical glycols and refrigerants like R410A, R32, and R290.
Automotive and aerospace industry
Hard soldering enables durable joining of components exposed to vibrations, fluctuating temperatures and corrosion. In both automotive and aerospace sectors, typical applications include cooling systems, fuel lines, turbocharger components and parts made from hard-to-machine alloys such as nickel superalloys and titanium alloys.
Tool manufacturing, machine parts and equipment
Hard soldering is widely used for joining tool heads and sleeves or embedding carbide inserts into cutting tools (e.g., milling cutters, drills). These industrial tools endure significant mechanical loads, requiring joints with exceptional resistance to shear stress and material fatigue.
Jewelry, industrial electronics and advanced electronic systems
In specialized applications such as jewelry making, precious-metal solders — gold, silver and palladium — are used. These materials must demonstrate high resistance to mechanical and chemical damage while also meeting refined aesthetic requirements.
Interestingly, precious-metal solders also play a crucial role in microelectronics, particularly in soldering microwave systems and other advanced electronic components.
Summary
Soldering may seem like a simple task — but that’s far from the truth. It requires high precision, caution, and most importantly, the right knowledge. To fully understand the intricacies of soldering, you must first grasp the key differences between soft and hard soldering — determined by operating temperature, solder type and applications.
This knowledge will help you avoid potential risks — remember, safety requirements are essential and should always be reviewed before using any equipment, tools or soldering materials!
At AG TermoPasty, we offer a variety of products for soldering and welding — our range is primarily designed for soft soldering (but not exclusively!). We invite you to explore our offer. If you’d like to learn more, feel free to contact us! We’ll gladly help you choose the right fluxes for your industry and needs while answering any questions about soldering techniques.