Solder is an essential material used to form both mechanical and electrical connections in printed circuit board (PCB) assembly. But with the variety of solder alloys available with different properties, how do you determine the “best” solder type for your particular PCB application? There is no single answer, as the optimum solder choice depends on factors like:

• Application requirements – e.g. operating temperatures, vibration levels
• PCB design considerations – e.g. pad sizes, pitch between joints
• Process characteristics – e.g. soldering methods used, production volumes
• Reliability expectations – e.g. product lifetime, expected thermal cycles
• Compliance needs – e.g. lead-free requirements, military specifications

By weighing specifications against these factors, PCB designers and production engineers can zero in on the right solder materials for their boards. This article provides guidance on selecting the best solder for PCBs based on different criteria.

What Is Solder?

Solder is a metal alloy that melts at relatively low temperatures and is used to create permanent bonds between electronic components and PCB pads. When heated, solder flows into the joint, and upon cooling, it forms a strong mechanical and electrical connection. The right solder ensures not only a secure physical bond but also optimal electrical conductivity, which is crucial for the proper functioning of electronic circuits.

Read more about:

• Soldering flux paste
• Turnkey PCB Assembly
• PCB soldering iron
• Removing flux from pcb

Types of Solder for PCBs

By Composition

1. Lead-Based Solder (SnPb)

Lead-based solder, typically composed of 63% tin and 37% lead (63/37), has been a staple in electronics for decades. Its low melting point, excellent wetting properties, and reliability made it a popular choice. However, due to environmental and health concerns, its use has been restricted in many applications.

Pros:

• Low melting point (around 183°C)
• Excellent wetting and flow characteristics
• Forgiving to work with, ideal for beginners

Cons:

• Environmental and health hazards
• Restricted use due to regulations like RoHS

2. Lead-Free Solder (e.g., SAC305)

With the implementation of RoHS (Restriction of Hazardous Substances) regulations, lead-free solders have become increasingly prevalent. SAC305 (96.5% tin, 3% silver, and 0.5% copper) is one of the most common lead-free alloys.

Pros:

• Environmentally friendly
• Compliant with RoHS and other regulations
• Good mechanical strength

Cons:

• Higher melting point (around 217°C)
• May require different soldering techniques
• Can be more challenging to work with than lead-based solder

3. Silver Alloy Solder

Silver alloy solders, such as 62/36/2 (62% tin, 36% lead, 2% silver), offer improved mechanical strength and better electrical conductivity compared to standard lead-based solders.

Pros:

• Enhanced mechanical strength
• Improved electrical conductivity
• Good for high-reliability applications

Cons:

• More expensive than standard lead-based or lead-free options
• May still contain lead, depending on the specific alloy

4. Rosin-Core Solder

Rosin-core solder contains a flux core made of rosin, which helps clean the surfaces being soldered and promotes better wetting.

Pros:

• Built-in flux for improved soldering performance
• Reduced need for separate flux application
• Available in both lead-based and lead-free variants

Cons:

• May leave residue that needs cleaning
• Not suitable for all applications, especially those requiring no-clean processes

By Form

1. Solder Wire

Solder wire is the most common form for manual soldering operations. It’s available in various diameters and compositions, making it versatile for different applications.

Pros:

• Easy to use for manual soldering
• Available in many alloy compositions and diameters
• Suitable for both through-hole and surface-mount components

Cons:

• Requires skill for consistent application
• Not suitable for automated processes

2. Solder Paste

Solder paste is a mixture of tiny solder particles suspended in a flux medium. It’s primarily used in surface-mount technology (SMT) applications and reflow soldering processes.

Pros:

• Ideal for SMT and automated assembly
• Allows for precise placement of solder
• Available in lead-free and leaded compositions

Cons:

• Requires special storage and handling
• Typically needs reflow equipment for proper use

3. Solder Bars

Solder bars are large blocks of solder used in wave soldering machines and for replenishing solder pots in automated systems.

Pros:

• Economical for large-scale production
• Suitable for wave soldering processes
• Available in various alloy compositions

Cons:

• Not practical for small-scale or manual soldering operations
• Requires specialized equipment for use

Lead vs. Lead-Free Solder: Key Differences

The shift from lead-based to lead-free solder has been one of the most significant changes in the electronics industry in recent years. Understanding the key differences between these two types of solder is crucial for making the right choice for your PCB projects.

1. Melting Point: Lead-based solder typically melts at around 183°C, while lead-free alternatives like SAC305 have a higher melting point of about 217°C. This difference affects soldering techniques and equipment requirements.
2. Wetting Properties: Lead-based solder generally has better wetting characteristics, making it easier to work with, especially for beginners. Lead-free solders may require more precise temperature control and technique to achieve good wetting.
3. Joint Appearance: Joints made with lead-based solder tend to have a shiny, smooth appearance, while lead-free solder joints often appear duller and slightly grainier.
4. Reliability: While both types can produce reliable joints, lead-free solders may be more prone to issues like tin whiskers in certain conditions. However, advancements in lead-free alloys have significantly improved their reliability.
5. Environmental Impact: Lead-free solders are more environmentally friendly and comply with regulations like RoHS, making them the preferred choice for many modern applications.
6. Cost: Lead-free solders are generally more expensive due to the higher cost of their constituent metals and the more complex manufacturing processes required.
7. Skill Required: Working with lead-free solder often requires more skill and precision, as it can be less forgiving than lead-based alternatives.

Factors to Consider When Choosing PCB Solder

Selecting the right solder for your PCB project involves considering several factors:

1. Type of Project

The nature of your project plays a significant role in solder selection. For hobbyist or prototyping work, ease of use might be a priority, making lead-based or easier-to-work-with lead-free alloys attractive. For professional or commercial projects, regulatory compliance and long-term reliability may take precedence, favoring lead-free options.

2. Materials Being Soldered

Different PCB materials and component finishes may react differently to various solder types. For instance, some lead-free solders work better with specific surface finishes like ENIG (Electroless Nickel Immersion Gold) or OSP (Organic Solderability Preservative).

3. Wire Size

The diameter of solder wire you choose should match the scale of your work. Finer wires (e.g., 0.5mm to 0.8mm) are suitable for delicate SMD work, while thicker wires (1mm to 1.5mm) are better for through-hole components and larger joints.

4. Flux Type

The type of flux in your solder can affect both the soldering process and post-soldering cleaning requirements. Options include:

• Rosin-based flux: Good for general-purpose use, but may require cleaning.
• No-clean flux: Leaves minimal residue, often doesn’t require cleaning.
• Water-soluble flux: Provides strong cleaning action but requires thorough cleaning after soldering.

5. Regulatory Requirements

If your project is for commercial use or will be sold in certain markets, you must comply with regulations like RoHS. This typically necessitates the use of lead-free solder.

6. Cost, Reliability, and Availability

Balance the cost of the solder against its reliability and availability. While lead-free options may be more expensive, they might be necessary for compliance or long-term reliability. Ensure your chosen solder is readily available to avoid production delays.

Solder Paste Deep Dive

Solder Paste Grades Explained

Solder paste is classified into different grades based on the size of the metal particles it contains:

• Type 3 (25-45 μm): Suitable for most SMT applications
• Type 4 (20-38 μm): For fine-pitch components
• Type 5 (15-25 μm): For ultra-fine pitch and micro BGA applications

Finer grades allow for more precise deposition but are generally more expensive.

How to Apply Solder Paste to PCBs

1. Stencil Printing: The most common method for production. A metal stencil is used to apply solder paste precisely to the PCB pads.
2. Manual Dispensing: For prototyping or rework, solder paste can be applied using a syringe or dispenser.
3. Jet Printing: Advanced method using specialized equipment to “jet” tiny droplets of solder paste onto the PCB.

Solder Paste vs. Solder Mask

It’s important not to confuse solder paste with solder mask:

• Solder Paste: A mixture of tiny solder particles and flux, used to create solder joints.
• Solder Mask: A thin lacquer-like layer applied to the PCB to prevent solder from bridging between closely spaced solder pads.

Solder Paste Storage Tips

1. Store at recommended temperatures (usually 0-10°C) to maintain viscosity and prevent separation.
2. Allow paste to reach room temperature before use to prevent moisture condensation.
3. Mix gently before use to ensure uniform consistency.
4. Keep containers sealed when not in use to prevent contamination and drying.

How to Use Solder Paste

1. Apply paste to PCB pads using a stencil or dispensing method.
2. Place components on the paste deposits.
3. Reflow the assembly using a reflow oven or hot air station, following the recommended temperature profile for the specific paste.
4. Allow the assembly to cool naturally to room temperature.
5. Inspect joints for quality and touch up if necessary.

Safety Precautions When Soldering

1. Ventilation: Work in a well-ventilated area or use a fume extractor to avoid inhaling harmful fumes.
2. Eye Protection: Wear safety glasses to protect your eyes from solder splatter.
3. Temperature Control: Use soldering irons with temperature control to prevent overheating.
4. Clean Workspace: Keep your soldering area clean and free from flammable materials.
5. Hand Protection: Use heat-resistant gloves when handling hot PCBs or components.
6. Lead Safety: When using lead-based solder, wash hands thoroughly after soldering and avoid eating or drinking in the work area.
7. Fire Safety: Keep a fire extinguisher nearby and never leave a hot soldering iron unattended.

Common Soldering Issues & Solder Selection Fixes

1. Cold Joints: Often caused by insufficient heat. Use a higher temperature or switch to a solder with a lower melting point.
2. Solder Bridges: Can occur with excess solder. Use a finer gauge solder wire or adjust your technique.
3. Dry Joints: May result from insufficient flux. Consider using a solder with a higher flux content or apply additional flux.
4. Component Damage: Often due to excessive heat. Use a lower temperature or a solder with a lower melting point.
5. Poor Wetting: Can be caused by oxidized surfaces or inadequate flux. Ensure proper cleaning and consider a solder with more active flux.
6. Tin Whiskers: More common with pure tin solders. Use alloys with small amounts of other metals (like silver or copper) to mitigate this risk.

Top 10 Solder Brands for Circuit Boards

1. Kester
2. AIM Solder
3. MG Chemicals
4. Chip Quik
5. Indium Corporation
6. Alpha Assembly Solutions
7. Multicore
8. Maker’s Guide
9. SRA Soldering Products
10. Sn100c

Each of these brands offers a range of solder products suitable for various PCB applications, from hobbyist to professional use.

Where to Buy High-Quality Solder

1. Electronics Supply Stores: Retailers like Digi-Key, Mouser, or Newark offer a wide range of professional-grade solders.
2. Online Marketplaces: Amazon, eBay, and AliExpress carry many solder options, but be sure to verify the authenticity and quality of the products.
3. Local Electronics Shops: Many carry a selection of solders suitable for hobbyist and professional use.
4. Direct from Manufacturers: Some solder manufacturers sell directly to consumers or businesses through their websites.
5. Specialty PCB Suppliers: Companies that specialize in PCB prototyping and small-scale production often sell high-quality solders as well.

When purchasing solder, always check for proper certification and compliance with relevant standards, especially if you’re working on commercial or regulated projects.

In conclusion, choosing the best solder for your PCB project involves considering multiple factors, from the type of project and regulatory requirements to specific soldering needs and personal preferences. By understanding the different types of solder available and the key considerations in selecting the right one, you can ensure better results in your PCB assembly process. Remember to prioritize safety, quality, and compliance with relevant standards to achieve the best outcomes in your electronic projects.