Standard solder, often referred to colloquially as soft solder, is historically defined by its most common composition used for joining metals in electronics and plumbing. Based on the provided reference, the most common mixture is a 60/40 blend of tin and lead.
Understanding the 60/40 Tin/Lead Blend
This specific alloy ratio became prevalent due to its favorable properties for soldering, particularly its relatively low melting point and good wetting characteristics.
Here's a breakdown of the composition and properties:
- Composition: 60% Tin (Sn) / 40% Lead (Pb)
- Melting Point: Around 180-190°C
- Colloquial Name: Soft Solder
Why This Blend?
The combination of tin and lead in this ratio offered specific advantages:
- Tin's Role: Tin is chosen for its lower melting point, which is crucial for joining components without damaging them. It also provides good wetting properties, allowing the solder to flow smoothly and adhere to the metal surfaces.
- Lead's Role: Lead is included to inhibit the growth of tin whiskers. Tin whiskers are crystalline structures that can grow from pure tin surfaces and cause short circuits in electronic devices. Lead effectively prevents this phenomenon.
- Strength: The reference notes that the higher the tin concentration, the better the tensile and shear strengths of the solder joint. While 60/40 is common, alloys with higher tin content might be used where greater mechanical strength is required.
Historical Context
The 60/40 tin/lead solder was the industry standard for decades, especially in electronics assembly. However, due to environmental and health concerns surrounding lead, lead-free solders have become the new standard in many applications, particularly consumer electronics, following regulations like the RoHS directive.
Despite the shift to lead-free alternatives, the 60/40 mix remains a significant historical standard and is still used in various applications where lead is permitted or preferred for technical reasons (e.g., certain high-reliability or aerospace applications).
