Lead vs. Lead-Free Solder & PCB Manufacturing

Choosing between lead and lead-free solder isn't simply a matter of deciding which material is easier to work with. The solder alloy you specify can affect processing temperatures, component selection, solder joint characteristics, regulatory compliance, and ultimately how your PCB is manufactured.

For OEMs, the decision often comes down to two questions: What does the application require, and what regulations does the finished product need to meet?

Understanding the differences between lead vs. lead-free solder can help you make that decision before your PCB reaches production.

What Is Lead Solder?

Lead solder, commonly referred to as SnPb solder, is an alloy made primarily from tin (Sn) and lead (Pb). Traditional formulations include 60/40 tin-lead solder and eutectic 63/37 tin-lead solder.

Lead solder has historically been widely used in electronics manufacturing because of its processing characteristics. It flows and wets surfaces well and requires lower processing temperatures than many lead-free alternatives.

However, the presence of lead creates environmental, health, and regulatory concerns. As restrictions on hazardous substances have expanded, lead-free alternatives have become standard for many electronic products.

That doesn't mean leaded solder has disappeared entirely. Some applications may still permit or require its use, depending on the product, market, and applicable regulations.

What Is Lead-Free Solder?

Lead-free solder is a category of solder alloys formulated without lead. Instead, these alloys can contain varying combinations of metals such as:

• Tin
• Silver
• Copper
• Nickel
• Zinc
• Bismuth
• Antimony

The specific formulation matters. There isn't one universal lead-free solder alloy that works for every PCB assembly. Alloy selection needs to account for manufacturing processes, component requirements, expected operating conditions, and applicable standards.

One of the biggest manufacturing differences is temperature. Lead-free alloys generally require higher processing temperatures than traditional tin-lead solder, which affects how the PCB assembly process needs to be controlled.

Lead vs. Lead-Free Solder: What's the Difference?

Both materials perform the same basic job: creating an electrical and mechanical connection between components and the PCB.

How they accomplish that job during manufacturing differs.

These differences don't automatically make one solder better than the other. The right choice depends on the PCB's application, manufacturing requirements, and regulatory obligations.

Processing Temperature

Processing temperature is one of the most significant differences between lead and lead-free solder.

The legacy article notes that 60/40 tin-lead solder becomes liquid at approximately 361°F, while lead-free solder can require temperatures around 422°F, depending on the alloy.

That difference affects more than the solder itself.

Components, PCB materials, and manufacturing processes all need to tolerate the thermal profile used during assembly. When moving from leaded to lead-free manufacturing, a PCB manufacturer may need to adjust reflow profiles and other process parameters accordingly.

You can't simply substitute a lead-free alloy and assume the rest of the manufacturing process stays the same.

Wettability and Solder Joint Appearance

Lead solder typically wets surfaces more readily than lead-free solder, making it easier to achieve the desired solder flow under appropriate processing conditions.

Lead-free solder can behave differently. The finished joint may also look different from a traditional leaded joint, including having a less shiny appearance.

Appearance alone doesn't determine whether a joint is acceptable. The assembly needs to be evaluated against the applicable workmanship and quality requirements.

For OEMs accustomed to the appearance of traditional leaded solder, this distinction is important. A different-looking solder joint isn't necessarily a defective solder joint.

Manufacturing Process

Higher lead-free processing temperatures can influence reflow, wave soldering, and through-hole assembly.

The PCB manufacturer needs to account for the solder alloy, board design, component limitations, preheating requirements, and appropriate thermal profile. Sensitive components need particular attention because they're exposed to elevated temperatures during processing.

Process control becomes especially important when working with assemblies containing a mix of component sizes and thermal masses. A profile needs to provide enough heat to form acceptable solder joints without unnecessarily stressing the PCB or its components.

How Does Lead-Free Solder Affect PCB Manufacturing?

Moving to lead-free solder affects several areas of PCB assembly:

Reflow Soldering

During reflow soldering, solder paste is applied to the PCB, components are placed, and the assembly moves through a controlled thermal profile.

Because lead-free solder generally requires higher temperatures, the reflow profile needs to be appropriate for the selected alloy and assembly.

Heating too little can prevent proper solder formation. Excessive heat can place unnecessary thermal stress on the PCB and components.

A properly developed thermal profile is an important part of repeatable lead-free assembly.

Wave and Through-Hole Soldering

Lead-free materials can also be used for wave and through-hole soldering.

As with reflow, the solder alloy can't be considered in isolation. Board design, component requirements, thermal demands, and the intended use of the finished assembly can all influence the process.

Manufacturers need to establish process parameters that provide adequate solder flow and connection quality while managing the additional heat associated with lead-free materials.

What Causes Soldering Defects?

Whether an assembly uses leaded or lead-free solder, material selection is only one part of solder joint quality.

The legacy article identifies several variables that can influence soldering results, including temperature, vibration, tin whiskers, preheating time, throughput, and operator skill.

PCB design, component condition, solderability, alloy selection, and process control can also affect the final result.

This is why simply specifying "lead-free" isn't enough to define a manufacturing process. The solder alloy needs to work with the board, components, equipment, and process parameters as a complete system.

What Does RoHS Mean for Lead-Free PCB Manufacturing?

One major reason for the electronics industry's move toward lead-free solder is the Restriction of Hazardous Substances (RoHS) Directive.

RoHS restricts the use of certain hazardous substances in electrical and electronic equipment, including lead. As a result, manufacturers selling applicable electronic products into regulated markets need to understand whether their products fall within RoHS requirements and whether an exemption applies.

For an OEM, compliance considerations extend beyond simply asking a contract manufacturer to "use lead-free solder."

Material selection, components, documentation, manufacturing processes, and the intended market for the finished product can all affect compliance requirements.

RoHS requirements should be addressed during the design and sourcing process, not treated as a final manufacturing checkbox.

Is Lead-Free Solder Better Than Lead Solder?

Neither solder should be evaluated in isolation as universally "better."

Lead solder offers favorable processing characteristics, including lower processing temperatures and good wettability. Lead-free solder avoids intentionally added lead and is used extensively to meet applicable environmental and regulatory requirements.

The appropriate option depends on factors such as:

• Product application
• Applicable regulations
• Operating environment
• Component requirements
• PCB design
• Manufacturing process
• Required reliability

For an OEM, the more useful question is: Which solder alloy and manufacturing process are appropriate for this PCB and its end-use requirements?

Choosing a PCB Manufacturer for Leaded or Lead-Free Assembly

Solder selection can affect more of your PCB assembly than it initially appears.

A change in alloy can require changes to thermal profiles, component considerations, process parameters, inspection criteria, and compliance documentation. The original article emphasizes the value of working with a PCB manufacturing partner experienced in RoHS material compliance.

Your contract manufacturer should understand the requirements of your assembly and establish a process that accounts for the solder alloy, PCB design, components, and end-use environment.

The goal isn't simply to manufacture a leaded or lead-free PCB. It's to build the assembly consistently while meeting the requirements established for the product.

Determining which solder alloy and manufacturing process are appropriate can get complicated when regulatory, thermal, component, and application requirements overlap. To discuss the requirements for your PCB assembly, reach out to Matric.

To learn more about other electronics manufacturing regulations that may impact your design, download our free e-book below: