Introduction

Lead-free solder is increasingly being used in the electronics industry as a replacement for traditional tin-lead solder due to health and environmental concerns over lead content. Lead-free solder alloys have different melting temperatures and other properties that require modified soldering processes compared to tin-lead. Understanding the suitable temperature curve for lead-free solder is essential to achieve good solder joint quality. This article provides an overview of lead-free solder alloys, discusses key considerations in determining the optimal soldering temperature profile, and provides guidance on setting up the temperature curve for common lead-free alloys.

Lead-free Solder Alloys

The most common lead-free solder alloys used today include:

• Tin-silver-copper (SAC) alloys
• Tin-copper (SnCu) alloys
• Tin-silver (SnAg) alloys
• Bismuth-containing alloys

Tin-silver-copper (SAC) alloys are the most popular lead-free solder choice, with SAC305 (3% silver, 0.5% copper) being widely used. SAC alloys have a melting point around 217°C, compared to 183°C for tin-lead.

Tin-copper (SnCu) alloys with 0.7% copper content have a melting point of 227°C. The higher copper content improves mechanical properties.

Tin-silver (SnAg) alloys are typically composed of 3-4% silver. They have good solderability but a higher melting point around 221°C.

Bismuth-containing alloys such as Sn-Bi and Bi-Ag solder have melting points ranging from 139-200°C. They can be useful for lower temperature applications.

Key Considerations for Lead-free Soldering Profile

Choosing the right soldering temperature profile is critical to producing good solder joints with lead-free alloys. Some key factors to consider include:

• Higher melting temperature – Lead-free alloys have higher melting points than tin-lead, so higher peak temperatures are required. The profile must reach suitable temperatures for alloy melting.
• Wetting time – Longer wetting times may be needed to allow lead-free solder to fully spread and wet the joint surfaces.
• Cooling rate – A controlled cooling slope is important to form the desired solder joint microstructure. Slow cooling can lead to excessive grain growth.
• Soaking zones – Soaks below peak temperature allow solder to fully melt and stabilize across the joint.
• Minimize thermal shock – Care must be taken to minimize thermal stresses on components during soldering.
• Flux activation – Ensure the profile enables soldering fluxes to properly activate to remove oxides.
• Alloy composition – Adjust profile based on the specific melting point, wetting and other characteristics of the alloy used.

Recommended Temperature Profile for SAC Alloys

For common SAC305 and similar SAC alloys, the following soldering temperature profile guidelines apply:

• Ramp-up rate: 1 – 3°C/second up to 150°C, then 1°C/second thereafter
• Soak zone: 30-90 seconds above 183°C (melting point of tin-lead)
• Peak temperature: 240-250°C
• Time above liquidus: 30-100 seconds
• Cooling rate: 2-6°C/second

This profile should allow SAC alloys to fully melt and wet the joint surfaces without excessive thermal exposure. The soak between 183-217°C helps stabilize melting across the joint prior to peak temperature. A peak temperature of 240-250°C gives a reasonable margin above the 217°C melting point of SAC solder. Cooling should be gradual, between 2-6°C/second if possible.

Here is an example SAC305 reflow profile:<img src=”https://i.postimg.cc/xTQMz9qJ/Reflow-Profile.png” alt=”Example SAC305 Reflow Profile” width=”400″>

Figure 1: Example reflow profile for SAC305 lead-free solder

Some key points on this SAC reflow profile:

• 30°C/second initial ramp then reduced rate after 150°C
• Soak for 60 seconds above Sn-Pb melting point of 183°C
• Peak temperature of 245°C with 45 second time above liquidus
• Cooling slope of -4°C/second

This provides a good baseline from which to optimize the profile specifically for a given assembly.

Profile Considerations for Other Alloys

The optimal profile settings will vary based on the specific lead-free alloy in use. Here are some guidelines for other common alloys:

SnCu solder

• Peak temperature: 250-260°C
• Extended time above liquidus: 60-120 seconds
• Slow cooling at 3°C/second or less

The higher copper content requires higher peak temperatures and extended time above liquidus.

SnAg solder

• Peak temperature: 240-255°C
• Rapid cooling at >6°C/second
• Shorter soak time: 30-60 seconds

Faster cooling can minimize grain growth with SnAg. Peak temperatures account for higher 221°C melting point.

Low-temperature alloys

• Peak temperature just above liquidus temperature
• Short time above liquidus
• Rapid cooling

For low-temp alloys like Sn-Bi, minimize time/temperature exposure to prevent damage.

Implementing the Soldering Profile

To implement the desired lead-free soldering profile in a production environment:

• Select a soldering method that allows temperature profiling, such as reflow oven, vapor phase, or hot air.
• Choose programmable equipment that allows flexible profile adjustment.
• Use thermocouples and profiling tools to set up, monitor and optimize the profile.
• Program the ramps, soaks and cooling rates into the process settings.
• Adjust based on monitoring data to account for thermal mass, load variations, etc.
• Verify joint quality through cross-sectioning and testing. Tweak profile as needed.

Maintaining tight process controls is also important to keep the assembly ramping through the profile consistently. Factors like board placement, conveyor speed and zone settings impact thermal transfer.

Potential Defects from Improper Profiles

Failure to follow suitable lead-free soldering temperature profiles can result in solder joint defects including:

• Poor wetting – When peak temperature or time above liquidus are insufficient to fully melt the solder. This can leave gaps or voids.
• Disturbed joint morphology – Fast cooling can set the solder structure before equilibrium is reached, while slow cooling can cause crystal growth. This can weaken joints.
• Solder balling – When solder forms spherical beads instead of conforming to joint surfaces, due to excessive solder activity.
• Component damage – Excess heat exposure during soldering can damage temperature-sensitive components.
• Voiding – Gas bubbles trapped during solidification of solder, as a result of high peak temperatures or improper cooling.

Careful temperature profiling minimizes these and other soldering defects. Cross-sectioning and inspection of solder joints provide feedback to further optimize the profile settings.

Frequently Asked Questions

What is the optimal peak temperature for lead-free SAC alloys?

240-250°C is recommended as a peak temperature range for SAC lead-free alloys. This gives a suitable buffer above the 217°C melting point while minimizing excessive thermal exposure.

How long should the soak time be when soldering with SAC alloy?

A soak time of 30-90 seconds above the tin-lead melting point of 183°C is typical for SAC solder. The soak allows flux activation and stabilization prior to reflow. Extended soaks aren’t necessary.

How fast can I cool SAC solder joints?

For SAC305, a cooling rate between 2-6°C/second is normally recommended. Faster cooling risks disturbed joint microstructure while slower cooling causes excessive grain growth.

Should I use different profiles for soldering leaded vs lead-free PCBs?

Yes, different profiles should be used due to the higher melting points of lead-free solder. Lead-free profiles require higher peak temperature, longer time above liquidus, and controlled cooling.

How can I tell if my lead-free soldering profile needs adjustment?

Inspect solder joint quality – signs like poor wetting, voiding or cracks indicate a needed profile adjustment. Thermal profiling tools can also indicate issues with ramps or soak durations.