Types of Solder: A Comprehensive Guide for Engineering Professionals

Introduction

Solder forms the critical connections in electronic devices, ensuring their functionality and reliability. All types of solder act as connective tissues that glue the electronic components to the PCB substrate, ensuring connectivity to the circuit board. Its low melting point makes it one of the most intriguing elements of PCB design and without a solder, it would be hard to imagine the state of modern electronics. 

While solder is a low melting-point metallic alloy, several types of solder are used according to the nature of the project and PCB. The selection of the appropriate types of solder is a pivotal element for efficient PCB manufacturing and optimized production techniques.  Therefore, understanding the different types of solder is essential for engineering professionals, as these materials significantly influence both the manufacturing process and the durability of electronic products.

What is a Solder?

The word "solder" originates from the Latin "solidare," meaning "to make solid," reflecting its core function.  It refers to a low-melting-point metal alloy that has been used for centuries to join metallic pieces. In the modern age, the word ‘solder’ is typically used for binding electronic components on a printed circuit board. While a solder can have various materials, it combines tin, lead, and optional solder flux to create a strong bond.

The solder is heated with a torch-like tool, which melts the solder at the joint between two metal objects.  This molten solder flows and solidifies, creating a strong connection between the pieces. This technique is commonly used in plumbing to connect pipes and fittings, and in electronics to secure wires to terminals or connectors.

What is Soldering?

Soldering offers a distinct method for connecting metals compared to techniques like brazing and welding. The key difference lies in the temperatures used and the resulting bond strength.  According to the Americal Welding Society, soldering occurs at temperatures below 840°F (450°C), producing a strong connection but one less resistant to stress-induced creep compared to higher temperature methods.

While soldering is primarily used in electronics, it has broader applications.  In essence, soldering joins various metals through melted solder to create a permanent connection which offers

• Electrical conductivity 

• Mechanical strength

Types of Soldering

In a broader context, soldering is divided into two main parts i.e.:

• Soft Soldering - A technique for joining metals using a specially formulated alloy called solder. This solder has a much lower melting point than the metals being joined, typically below 450°C (840°F). The soldering process involves applying heat with a soldering iron to melt the solder, which then flows into the joint between the two metal pieces. Once the solder cools and solidifies, it creates a strong electrical and mechanical connection. 

  • It’s mainly used in the electronics industry where higher temperatures can damage the PCBs and electronic components. 

  • It’s also used in plumbing, particularly for joining pipes and fittings made of metals that wouldn't withstand high temperatures well. 

Fig 1: Solder application for joining copper pipes

• Hard Soldering - Also known as brazing, hard soldering involves solder alloy having a significantly higher melting point typically exceeding 450°C (840°F). This allows brazing to create stronger bonds suitable for applications requiring more durability. The process itself is similar to soft soldering.  Brazing applications can be found in various industries due to the increased strength. It's commonly used in metal fabrication, where strong and durable joints are essential, such as joining pipes in plumbing systems that handle high-pressure or high-temperature fluids or connecting components that may experience significant stress or heavy loads.  

Recommended Reading: Demystifying Soldering Techniques: A Comparison of Wave Soldering and Reflow Soldering

Understanding Solder Composition and Properties

Overview of Solder Materials

Solder alloys are pivotal in determining joint strength, thermal endurance, and electrical conductivity. Key solder materials include:

• Tin-Lead (Sn-Pb) - A eutectic alloy favored for its low melting point of 183°C and excellent flow characteristics.

• Lead-Free Alloys (Sn-Ag-Cu) - Adopted widely due to environmental concerns, these alloys offer strong joint integrity and moderate melting points.

• High-Temperature Alloys (Sn-Ag or Sn-Sb) - Utilized in applications demanding robust thermal resistance, these alloys withstand higher operational temperatures.

Physical and Chemical Properties

This section details the melting points, tensile strength, and thermal conductivity of various solder types, crucial for selecting the right material for specific applications. It also explores the metallurgical aspects and phase diagrams that dictate solder behaviour under different conditions.

Each solder type's properties are directly linked to its performance in manufacturing environments, influencing decisions on its application based on thermal and mechanical demands.

Different Types of Solder

Having seen the chemical composition of solder materials, this section introduces the major types of solders. Primarily, solder types are mainly divided based on:

1. Alloy or Material

2. Core Style

Solder Types by Alloy

1. Lead-Based Solder

Lead-based solder has been around for ages.  This alloy traditionally included a high percentage of lead, often combined with small amounts of tin or antimony. 

• Tin makes the solder flow better and creates stronger, more durable bonds. 

• Antimony, in small doses, also improves durability, especially at higher temperatures.  

The most common type, a 60/40 tin/lead mix (melting around 180-190°C), is widely used in electronics, plumbing, and more. 

However, its low melting point, ease of use, and reliability come at a cost: lead exposure risks. Eventually, the environmental and health concerns have led to restrictions and replacements with safer lead-free alloys.  While these newer options offer similar performance, they can be more complex to work with.

Fig 2: Lead Solder releases toxic fumes

2. Lead-Free Solder

Lead-free solder replaces traditional lead-based solders in electronics (PCBA) and plumbing.  It’s a safer lead-free option which is much less toxic, protecting workers and minimizing environmental impact.  These new solders are typically a mix of tin, silver, copper, and trace of bismuth or zinc. Lead-free solder has a higher melting point than lead solder. 

• The most common type is tin-copper with a touch of other metals, melting around 217°C.  

• A SAC (tin-silver-copper) solder offers great performance but with a higher price tag due to the silver content.  

Lead-free solder requires more precise and controlled soldering techniques due to the higher temperatures. Moreover, an erroneous solder connection can be difficult to fix, requiring a cleaner work environment to avoid impacting the bond.

Recommended Reading: Solder Reflow: An In-Depth Guide to the Process and Techniques

3. Lead Alloy Solder

Originally, solder was made mostly from lead with small amounts of other metals like tin or antimony mixed in. These additives improve how the solder flows (wetting) and form strong bonds, or make the solder joints more rust-resistant.

A common type is 63% tin and 37% lead solder, which melts at a precise temperature (183°C) - ideal for controlled manufacturing.  Changing this ratio creates a wider melting range, making it trickier to use in real-world production settings.

4. Silver-Alloy Solder

Silver-alloy solder is often used in silver brazing. It operates at considerably higher temperatures due to silver's high melting point (ranging from 300°C to 420°C depending on the alloy).  These solid-core rods require a more complex and aggressive flux typically formulated with alkali metal fluoroborates, tetraborates, boric acid, and organic binders.  

These white pastes or powders, activated before use, create a protective layer that excludes oxygen, removes oxides, and promotes smooth solder flow

Solder Types by Core

5. Rosin-Core Solder

It’s a less common solder type but a viable option for its ease of use. It solder combines the metal alloy with a core filled with rosin, a natural pine tree resin and the original flux used in soldering.  Flux acts like a mild acid, cleaning slight tarnish from the joining metals, allowing the solder to flow smoothly and create a strong bond.  

It also helps remove impurities for a clean and shiny finish, contributing to strength, corrosion resistance, and aesthetics.  The convenience of having both solder and flux in one makes rosin-core popular, especially for tasks like electrical connections in electronics, circuit boards, and small-scale soldering projects. 

6. Acid-Core Solder

Acid-core solder is a special material for plumbing applications, particularly soldering copper pipes and fittings.  Unlike rosin-core, it uses a powerful acid flux (typically zinc chloride or ammonium chloride) to aggressively clean oxide layers from copper surfaces.  This allows the solder to flow effectively, penetrate the joint deeply, and form a strong, clean connection.  

The strong acidic action is crucial for effective copper soldering in plumbing, as it cuts through the oxidation that would otherwise prevent a good bond.  However, this very aggressiveness makes it unsuitable for electronics, where it can damage delicate components.

7. Flux-Core Solder

Flux-core solder is a broader category encompassing any solder with internal channels filled with flux to aid in creating clean and efficient joints. The flux itself can vary:

• Natural Rosin - While rarely used alone now, rosin often forms the base with additional cleaning agents and isopropyl alcohol for dilution.

• Organic Flux - Suitable for electronics, this flux is gentle enough not to harm sensitive components. However, it might not be strong enough for heavily oxidized copper surfaces.

• Inorganic-Acid Flux - More aggressive than organic flux, it tackles tough oxidized copper but leaves residues that can damage electronics.

8. Solid-Core Solder

Solid-core solder simply refers to solder wire without any internal flux. This allows users to apply the specific flux needed for the job.  Most solder alloys, from traditional lead solders to modern lead-free options, come in solid-core form to cater to diverse applications requiring different flux types.

Advancements in Solder Technology

Recent Innovations

Solder technology has evolved rapidly, with new compositions and techniques enhancing both safety and performance. Key innovations include:

• Refined Lead-Free Solders - These new alloys offer enhanced wetting and lower melting points, facilitating easier application and stronger bonds.

• Reinforced Composite Solders -  By embedding metals like copper, these solders gain superior strength and resistance to thermal fatigue.

• Nanotech-Enhanced Solders - The integration of nanoparticles leads to finer grain structures, improving thermal cycling endurance and minimizing voids.

These advancements represent a leap forward in solder technology, delivering safer and more reliable solutions for modern electronics.

Environmental and Health Considerations

The transition to lead-free solder is primarily motivated by its lower health and environmental risks compared to lead-based alternatives:

• Lead-Based Solder - Known for its neurotoxicity, lead solder poses significant health risks, such as neurological impairments, and environmental challenges, including soil and water contamination.

• Lead-Free Solder - Offers a safer alternative, significantly reducing toxic exposure and environmental pollution. Compliance with international standards like RoHS (Restriction of Hazardous Substances) underscores its global adoption.

This shift not only aligns with global health and safety regulations but also marks a critical step towards sustainable manufacturing practices.

Solder Applications and Case Studies

Electronics Manufacturing

The selection of solder in electronics manufacturing is critical, with specific types chosen to meet the demands of high-frequency and high-power devices. Low electrical resistance and high thermal fatigue resistance are essential for high-frequency applications, making Sn-Ag-Cu (SAC) alloys a standard choice for their mechanical and electrical properties.

Fig 3: Electronic components soldered on a PCB using Surface-mount technology

For high-power electronics, which generate significant heat, solders must maintain integrity at elevated temperatures. Bismuth-containing solders are often employed for their ability to endure thermal stress without failure.

A case in point is the use of sintered Silver solders in LED lighting, where they mitigate thermal impacts, preventing color shift and prolonging lifespan by ensuring low operating temperatures and strong joint formation.

Suggested Reading: SMT Manufacturing: Everything You Need to Know

Aerospace and Automotive Applications

Solder selection in aerospace and automotive industries is critical, driven by stringent technical requirements:

1. Thermal Resistance - Solder must withstand extreme temperature variations typical in these sectors, ensuring component safety and functionality.

2. Mechanical Strength -  High-strength solders are essential to cope with the constant vibrations and shocks experienced in these environments.

3. Longevity and Reliability - The solders are chosen for their proven durability and reliability, crucial in applications where failure can lead to catastrophic outcomes.

In aerospace, silver-bearing solders are favored for their superior thermal fatigue resistance, vital for the safety and longevity of flight-critical electronics. Automotive applications, particularly near engine components, often use high-lead solders known for maintaining structural integrity at high temperatures.

Navigating Challenges and Considerations

Common Challenges in Soldering

Effective soldering is essential for electronic assembly integrity, yet several challenges can compromise joint quality:

• Joint Reliability - Inadequate soldering leads to fragile joints. The problem can be fixed by ensuring a thorough surface cleaning with flux and complete solder flow into joints.

• Solder Fatigue - Joints may crack or fail due to stress from thermal cycling. Therefore, designers must opt for solders designed for high fatigue resistance and manage thermal loads.

   Fig 4: Cracked or broken solder joints due to fatigue

• Thermal Cycling -  Frequent temperature changes accelerate component wear. So, conducting extensive thermal cycling testing is essential to validate joint durability under expected service conditions.

These strategies are critical for ensuring the reliability and longevity of soldered connections in electronic devices.

Safety and Handling Guidelines

Handling solder, particularly lead-based types, requires adherence to strict safety protocols to minimize health risks:

1. Proper Ventilation - Always work in well-ventilated areas to avoid inhaling fumes, which can be toxic, especially when working with lead-based solder.

2. Use of Personal Protective Equipment (PPE) - Wear protective gear such as gloves, safety glasses, and masks to prevent skin and eye contact and inhalation of fumes.

3. Hygiene Practices -  Wash hands thoroughly after handling solder, especially before eating, drinking, or touching your face to prevent ingestion of harmful substances.

4. Safe Storage - Store solder in a cool, dry place away from children and food items to prevent accidental contact or ingestion.

5. Disposal Considerations - Dispose of solder waste and related materials according to local environmental regulations to prevent environmental contamination.

Fig 5: A technician wears rubber gloves to ensure soldering safety

These guidelines are essential for ensuring the safety of individuals using solder in various applications, particularly when dealing with lead-based products.

Conclusion

The type of solder dictates its use and efficiency for wide-ranging applications. This article has detailed the critical aspects of solder types, their applications, and technological advancements, with a historical background of different solder materials over centuries.  Hence, it provides a guideline for selecting the appropriate solder for optimal performance and reliability in engineering projects.FAQs

1. What are the main differences between lead-based and lead-free solders?

Lead-based solders have lower melting points and are easier to work with but pose health and environmental risks. Lead-free solders comply with modern safety regulations and are less harmful.

2. How do I choose the right type of solder for my project?

Evaluate the thermal and mechanical demands of the application, along with compliance needs. For high-temperature environments, opt for solders with higher melting points.

3. What are the safety considerations when using solder?

Maintain good ventilation, use protective gear to prevent exposure to harmful fumes, and practice proper hygiene to avoid contamination.

4. What is the impact of solder alloy composition on joint reliability?

Alloy composition determines key properties like melting point and strength. Appropriate selection based on the specific application requirements can greatly improve joint durability and performance.

References

1. Sintered Silver for LED Applications | SpringerLink 

2. 8 Different Types of Solder | Xometry

3. Types of Solder – A Thomas Buying Guide (thomasnet.com)

4. Types of Soldering: A Detailed Guide - ElectronicsHub USA

5. What are the Different Types of Solder? (cadence.com)