What Is Solder Made Of? A Guide to Solder Alloys and Compositions

Solder is a metal alloy with a relatively low melting point, used to join electrical components and create permanent conductive connections on circuit boards. The specific composition of solder determines its melting temperature, mechanical strength, and suitability for different applications. Understanding what solder is made of helps you choose the right type for your electronics work.

The Main Metals in Solder Alloys

Every solder alloy centers on a primary metal, with smaller amounts of other elements added to adjust melting point, strength, and wetting behavior.

Tin serves as the base metal in virtually all modern solders. It provides good wetting characteristics, meaning it flows and bonds readily with copper surfaces on PCB pads and component leads. Tin content in solder typically ranges from 60% in traditional alloys to over 99% in simple tin-copper formulations.

Lead has been used alongside tin in electronics soldering for decades. Lead lowers the melting point significantly and makes the solder easier to work with—joints form more slowly, giving you time to position components before the solder sets. However, lead is toxic, and environmental regulations have driven the industry toward lead-free alternatives since 2006.

Modern solders incorporate silver, copper, bismuth, and other elements in small percentages to achieve specific properties. Silver improves mechanical strength and thermal fatigue resistance. Copper maintains joint integrity during thermal cycling. Bismuth lowers melting points for heat-sensitive work.

Traditional Tin-Lead Solder

Before lead-free became the standard for consumer electronics, tin-lead solder dominated electronics manufacturing. Two ratios saw the most use.

The 60/40 composition contains 60% tin and 40% lead by weight. This solder melts in a range from 183°C to 190°C—the solid alloy begins softening around 183°C and becomes fully liquid at 190°C. The relatively wide melting range makes 60/40 forgiving for hand soldering, as the gradual transition from solid to liquid gives you working time to position components. According to electronicsandyou.com, 60/40 solder works well for general-purpose PCB assembly, repair, and DIY projects where ease of use matters more than the lead content.

The 63/37 ratio (63% tin, 37% lead) represents the eutectic point for this alloy system—the precise composition where solid and liquid phases coexist at a single temperature: exactly 183°C. Wikipedia notes that this eutectic blend melts at 183°C, making it ideal for rework on temperature-sensitive components where predictable melting behavior matters. The advantage of eutectic solder is instantaneous transition from liquid to solid during cooling, which reduces the risk of cold joints.

Lead-Free Solder Alternatives

The European Union’s RoHS (Restriction of Hazardous Substances) Directive, which took effect on July 1, 2006, restricted lead in most consumer electronics sold in EU markets. This regulation drove widespread adoption of lead-free solders, which now dominate commercial electronics manufacturing.

SAC305 has become the most widely used lead-free solder alloy. It consists of 96.5% tin, 3% silver, and 0.5% copper. According to pcbdirectory.com, this composition provides high strength and excellent thermal fatigue resistance, making it suitable for the thermal cycling that consumer electronics experience during use. SAC305 melts at 217-220°C—about 35°C higher than traditional tin-lead.

The Sn99.3Cu0.7 alloy (99.3% tin, 0.7% copper) offers a cost-effective lead-free option. Per pcbdirectory.com, this composition melts at 227°C and works well for less demanding applications where budget constraints matter more than premium thermal performance.

Tin-bismuth solder (42% tin, 58% bismuth) melts at approximately 138°C, according to pcbdirectory.com—well below most other solders. This makes it suitable for rework on heat-sensitive components that might suffer damage at higher temperatures.

Why the Shift to Lead-Free? RoHS Regulations

The EU Waste Electrical and Electronic Equipment (WEEE) Directive and RoHS Directive, both adopted in early 2003 and effective July 1, 2006, restricted lead in consumer electronics sold in EU markets. Wikipedia notes that most lead-free solder replacements for conventional tin-lead have melting points 50-200°C higher, requiring changes to manufacturing equipment and soldering technique.

Key Takeaways

• Traditional solder is a tin-lead alloy, with 60/40 (183-190°C melting range) and 63/37 (eutectic at 183°C) being the most common ratios.
• SAC305 (96.5% tin, 3% silver, 0.5% copper) has become the industry standard lead-free solder, melting at 217-220°C.
• Tin-copper alloys like Sn99.3Cu0.7 offer cost-effective lead-free options for less demanding applications.
• The EU RoHS Directive, effective since 2006, restricted lead in consumer electronics, driving widespread adoption of lead-free solders.
• Lead-free solders require higher temperatures and about 2% flux by mass for adequate wetting, according to Wikipedia.
• Choosing the right solder depends on your application: 63/37 for rework, SAC305 for production, Sn-Cu for budget projects.

Frequently Asked Questions

What is solder made of? Solder is a metal alloy, typically based on tin combined with lead, silver, copper, or other elements. Traditional electronics solder is 60% tin and 40% lead (60/40), while modern lead-free solder often contains 96.5% tin, 3% silver, and 0.5% copper (SAC305).

What are the main types of solder? The two primary categories are leaded solder (tin-lead alloys like 60/40 or 63/37) and lead-free solder (tin-based with silver, copper, bismuth, or other additives). Within these categories, specific alloys like SAC305 and Sn99.3Cu0.7 serve different applications.

What is SAC305 solder? SAC305 is an industry-standard lead-free solder consisting of 96.5% tin, 3% silver, and 0.5% copper. It melts at 217-220°C and provides excellent mechanical strength and thermal fatigue resistance for consumer electronics manufacturing.

Why is lead-free solder used? The EU RoHS Directive, effective since July 1, 2006, restricted lead in consumer electronics due to health and environmental concerns. Lead-free solders like SAC305 have replaced tin-lead alloys in most commercial electronics production worldwide.

What temperature does solder melt at? Melting points vary by composition. Traditional 63/37 tin-lead solder melts at exactly 183°C (eutectic). Lead-free SAC305 melts at 217-220°C. Low-temperature tin-bismuth solder melts at approximately 138°C.