Why 63 37 Solder is Favored for Prototyping

When it comes to soldering the electronics, the most popular alloy name that comes to mind is 63/37. It is nothing but a combination of 63% tin and 37% lead in 63 37 leaded solder. Yes! To date, we use lead, but why is it used in soldering? We will see in this article. Although there are a lot of developments happening in the solder material, mixed solder is still used everywhere. We will see how the eutectic composition of this alloy provides it with a precise melting point of 183°C, resulting in predictable handling and appropriate performance for prototyping applications. The alloy remains essential for development and testing operations because of its ability to withstand rework and iteration cycles. Although RoHS regulations restrict its use in commercial consumer products. This article explores why the engineers and students will favour 63/37 solder for prototyping. Solder tools are equally important, we have made a overview article on this.

What is 63 37 Leaded Solder?

Composition and Eutectic Behavior:

The eutectic property stands as the primary feature that describes 63 37 solder. The 63/37 alloy exhibits a diverse solid-to-liquid transition at 183°C without entering a semi-solid pasty state, which characterizes lead-free solder materials. The eutectic behavior produces uniform wetting and flow, enabling rapid and reliable joint formation. The alloy provides reliable soldering results by minimizing thermal stress on components during both prototyping and rework procedures.

Common Forms and Flux Options:

The 63 37 solder material is available in three forms: wire (with and without core flux), solder paste, and bar form for wave soldering applications. The three main flux systems contain rosin-based flux, mildly activated rosin (RMA), and fully activated rosin. The combination of rosin-core wire with mildly activated flux works well for hand-soldering and prototyping as it provides good solderability, low cleaning requirements, and controlled residue formation.

Key Advantages for Prototyping

The following advantages make 63 37 leaded solder suitable for prototyping work:

Lower and Sharper Melting Point

The eutectic melting point of 183°C in 63 37 solder reduces thermal stress on components during soldering operations and rework procedures. The low iron temperature required for hand soldering produces fast and dependable joint formation. The solder exhibits a precise melting point, which prevents irregular reflow patterns and partial solidification during assembly rework operations. Although the heating profile is mentioned in the solder datasheet, the solder alloys that do not contain lead have higher melting points by around 40 degrees.

Excellent Wetting and Flow

The wetting and flow characteristics of 63 37 leaded solder exceed those of all lead-free solder alloys. The solder creates smooth fillets, which prevent both solder bridges and tombstoning effects on small electronic components. The uniform fillet formation enables fast assembly work and provides clear visual inspection results. Because the time taken by lead in melting and then forming a joint is way less, that's why the lead-based solder outperforms the non-lead solder materials.

Superior Rework Ability and Ease of Use

The clean melting properties and excellent wetting ability of 63/37 solder enable fast desoldering and reflow operations during the repeated prototyping process. The material allows multiple rework cycles without harming pads or components, which benefits fast design development and hardware testing processes.

Predictable and Repeatable Results

The eutectic properties of this alloy minimize the occurrence of partial solidification when applying localized heat. The consistent melting behavior of 63/37 leaded solder produces uniform joint quality throughout multiple rework cycles in prototype development.

Cost, Availability, and Familiarity

The market offers 63/37 wire and paste at affordable prices, and these remain easily accessible to customers. The handling properties of 63/37 solder make it a practical choice for educational and rapid development settings because many users already understand its behavior.

Limitations and Regulatory Considerations

While 63/37 offers clear prototyping advantages, it contains lead and therefore presents environmental and health considerations:

• Regulation (RoHS): The RoHS regulations enforced by multiple jurisdictions worldwide prohibit the use of lead in all consumer products. The transition from prototype development to mass production necessarily requires lead-free manufacturing methods unless the products fall under specific industrial or medical exceptions. Aerospace is also required in mass production.
• Health & Safety: The toxic nature of lead requires proper safety measures. The laboratory requires proper fume extraction systems, and workers must handle materials using contactless methods followed by hand washing. According to established hazardous waste disposal rules in their area.
• Mixed-assembly risks: The process of solder joints involves unpredictable properties when melted and tested for reliability.

Best Practices When Using 63 37 solder for Prototyping

Soldering Profiles & Temperatures

• Hand-soldering: Hand-soldering requires iron tip temperatures between 320–350°C  when working with 63/37 wire, while using the lowest possible temperature for wetting to reduce thermal stress.
• Reflow (paste): The reflow process for 63/37 solder paste requires a peak temperature of 210–225°C with a liquidus exposure duration of 20–30 seconds. Check the maximum component temperature ratings found in component datasheets before starting the soldering process.

Flux, Cleaning, and Ventilation

Select the correct flux type according to your needs between rosin and RMA for minimal residue generation. The implementation of fume extraction systems during soldering operations protects workers from inhaling flux and solder fumes. The project requires assembly cleaning when it requires precision electronics or surface adhesion maintenance.

Joint Inspection and Testing

The inspection process requires verification of smooth fillets and complete wetting between pads and component terminations, or the absence of cold joints. Basic electrical tests, along with thermal cycling, should be performed on critical prototypes when possible.

Avoid Mixing with Lead-Free on the Same Assembly

The combination of 63/37 solder with lead-free solder on a single printed circuit board requires a validated process for safe assembly. The production house must provide rework procedures for boards that will transition to lead-free, as contamination must be avoided.

When To Choose Leaded 63/37 vs Lead-Free: A Decision Matrix

• 63/37 should be used for: Rapid prototyping, repeated rework, laboratory development, and educational or hobby work that is not expected to conform to the RoHS regulations.
• The selection of SAC305 lead-free solder becomes necessary when a prototype needs to move directly into mass production under RoHS regulations or when product certification demands lead-free assembly.

The selection of 63/37 leaded solder requires verification of product usage, component and manufacturer temperature limits, waste disposal procedures, and a production readiness plan for lead-free transition.

Conclusion

The 63/37 leaded solder alloy remains the selected material because it has a low melting point and excellent wetting properties. A solder beginner found it easy to work with and has consistent joint formation. The soldering process is faster because defects are reduced, allowing designers to perform multiple design changes without putting components or pads. Engineers and hobbyists need to handle 63/37 solder correctly while developing design strategies that lead to a successful transition to lead-free soldering when necessary. The early stages of electronic design benefit from 63/37 solder through its unmatched efficiency when handled correctly and designed properly.