PCBs are the heart of modern electronic devices, and their performance and reliability largely depend on soldering quality. The choice of solder plays a crucial role, especially in Surface Mount Technology (SMT) and Through-Hole (TH) component soldering. Selecting the appropriate solder requires careful consideration of the PCB’s physical and chemical properties. This article explores the types of solder and how to choose the right one based on PCB characteristics.

Types of Solder for PCBs and Their Applications

Solder Wire

Solder wire is best suited for manual soldering of through-hole components using a soldering iron, though it can also be used to rework surface-mount devices.
You can choose between solid-core solder wire and flux/rosin-core solder wire, with the latter being the better option.

Solder Paste

Solder paste is a powdered solder mixed with flux, creating a gel-like material ideal for surface-mount components.
The flux’s viscosity keeps the fine solder particles in place, allowing for precise component positioning.

Solder Bar

Solder bars are exclusively used in wave soldering processes. In this method, a machine melts the metal to create a wave of molten solder, which is used to solder through-hole components.

Based on PCB Soldering Types

Soft Soldering

Soft soldering involves heating the filler metal to below 752°F (400°C), making it ideal for soldering delicate electronic components to PCB pads.
These filler metals are often lead-based alloys with low melting points, typically around 621°F (327°C). The overall alloy melting point is usually about 662°F (350°C).

Hard Silver Soldering

This process requires high temperatures, approximately 1472°F (800°C), to melt the filler metal. It typically uses a carbon arc torch or flame for heating.

Brazing

Brazing requires even higher temperatures than hard silver soldering, around 1832°F (1000°C), to solder parts effectively.

Wave Soldering

This bulk soldering process is ideal for attaching through-hole components to PCB copper pads. It involves passing a component-laden PCB over a wave of molten solder.
A pump in the machine generates an upward flow of molten solder, creating a standing wave that contacts the PCB to solder the components.

Reflow Soldering

Reflow soldering is the primary method for attaching surface-mount components to PCBs.
Before entering the reflow oven, the PCB undergoes solder paste application via a stencil printer, followed by component placement with a pick-and-place machine.
The reflow oven then heats the PCB, melting the solder particles in the paste and forming solid solder joints.

How to Choose the Right Solder Based on PCB Characteristics

1. Understand PCB Substrate

The heat resistance of the PCB substrate is a primary factor when selecting solder. For instance, traditional FR-4 materials can endure higher soldering temperatures, while high-frequency materials or flexible substrates may require lower-temperature solder alloys. Knowing the substrate’s thermal tolerance helps in choosing solder with an appropriate melting point.

2. Determine the Soldering Method

The soldering method (e.g., wave soldering, manual soldering, or reflow soldering) affects the type of solder required. Manual soldering typically benefits from solder wire with good flow properties, while automated processes like reflow soldering may require solder paste with specific melting and flow characteristics.

3. Consider Soldering Temperature

The components on the PCB (such as temperature-sensitive SMT components) and the substrate dictate the maximum soldering temperature. Solder selection should align with these temperature limits to prevent overheating and minimize thermal damage to components and the substrate.

4. Evaluate Solder Composition

The alloy composition of the solder determines its mechanical and electrical properties. Applications requiring high mechanical strength may need silver-containing solder, while high-current applications benefit from copper-containing solder for better conductivity. Lead-free solder is widely used due to environmental regulations.

5. Assess Solder Flow and Wetting Properties

The solder’s flowability and wetting characteristics directly impact soldering quality. The compatibility between the solder’s wetting properties and the PCB pad’s plating (e.g., gold, tin, silver, or nickel) ensures proper solder spread, forming uniform and void-free solder joints.

6. Consider Environmental Regulations and Standards

The final application of the PCB may be subject to specific environmental regulations such as RoHS or REACH. Ensure that the chosen solder complies with these standards. For instance, electronic products sold in the EU typically require lead-free solder.

7. Select a Reliable Supplier

  • Quality Certification: Choose solder suppliers with quality certifications to ensure product reliability.
  • Technical Support: Opt for suppliers that offer technical support and after-sales services to resolve potential issues efficiently.

8. Monitor Solder Shelf Life

The alloy composition in the solder affects its shelf life. Alloys with more than 70% lead have a shelf life of two years, while other alloys can last up to three years. Lead-free solder generally has a longer shelf life than lead-based solder. If you plan to use the solder over an extended period, lead-free solder may be a better choice.

What Is the Ideal Solder Diameter for You?

The gauge number determines the diameter of the solder. It is a unit for measuring the thickness of metal sheets and wires.

For hobbyists and enthusiasts, gauges 18, 20, and 21 are the most suitable. Their respective diameters are 1.22 mm, 0.914 mm, and 0.813 mm.

For beginners and amateurs, staying within this range is a good idea. Using solder wires of this diameter minimizes waste.

For Plastic Dual In-line Packages (PDIP) with a 0.1-inch pin pitch, gauge 18 (1.22 mm) is ideal.

Gauge 22 wires, with a diameter of 0.711 mm, are suitable for smaller PCBs.

In SMD and SOIC packages where pins are closer together, the smaller diameter of gauge 22 wires makes them ideal for precise work.

Older circuits typically have much larger pin spacing than modern designs. These circuits also feature larger solder pads.

For these older circuits, gauge 16 wires, with a larger diameter of 1.63 mm, are the best choice.

Comprehensive Solder Selection for PCBs

When choosing solder for a PCB, multiple factors must be considered, including PCB requirements, solder type, solder characteristics, environmental regulations, and supplier reliability. By taking these factors into account, you can ensure the selected solder meets the board’s needs while improving soldering quality and PCB reliability.