A practical guide to lead-free assembly from the production floor at Rush PCB UK.

Lead-free soldering has been standard practice for all PCB assembly houses operating in Europe (and to a great extent worldwide) since the introduction of the EU RoHS directive back in July 2006. As regulation has tightened over the last decade or so, the electronics industry has had little choice but to adapt. Moving from the tried and tested tin-lead eutectic formula (63Sn/37Pb) to something less hazardous has been no small feat. Transitioning to SAC alloys (tin-silver-copper) remains a key challenge for production engineers everywhere.

The good news is that 15 years on from RoHS we have had plenty of time to optimise our assembly processes and fine-tune our specifications. With customer demands higher than ever and production/serviceable life expectancy (PSLE) requirements set to increase, how do we prepare for the future of lead-free assembly?

In this article, we cover:

• A brief history of lead-free regulation.
• Five key technical challenges to assembling PCBs without lead.
• Best practices for getting great yields with lead-free soldering.
• Key differences between lead-based and lead-free soldering.
• Why your next project will benefit from lead-free assembly.

RoHS Regulation Explained

So what exactly is the RoHS directive, and why does lead solder suddenly become such a problem?

The Restriction of Hazardous Substances (RoHS) Directive 2011/65/EU was first introduced by the EU parliament in July 2006. RoHS specifically targets electrical and electronic products being placed on the market by restricting seven hazardous materials, lead (Pb) being one of them.

Prior to the regulation, solder paste used for wave and reflow assembly consisted primarily of a tin and lead eutectic alloy. Tin-lead solders have been used since at least the mid-19th century and remain one of the easiest metals to work with due to its low melting temperature (183 °C) and good wetting ability. In order to comply with the new directive, manufacturers replaced lead with less harmful alternatives like silver and copper.

Nowadays, SAC305 (96.5% Sn, 3.0% Ag, 0.5% Cu) is by far the most popular lead-free alloy specified for solder paste. SAC alloys are called eutectic when they meet the following percentages: 99.3% Sn, 0.7% Ag, 0.05%Cu. This has a melting temperature of 221 °C making it ideal for lead-free applications.

During our transition to lead-free assembly at Rush PCB UK, we were able to leverage existing supplier relationships to source lead-free solder paste with a minimum silver content of 3%. It wasn’t as simple as swapping solder paste however; thermal profiles had to be re-engineered, component qualifications carried out, and investments made into reliable high-temperature equipment.

Lead-Free Soldering: Top 5 Technical Challenges

1. Higher Melting Point, Narrower Process Window

Lead-free solder paste has a higher melt temperature of roughly 34 °C compared to SnPb eutectic. This extra thermal load can exceed the glass transition temperature (Tg) of many standard substrates making process control more critical than ever. Many off-the-shelf FR-4 materials have a Tg rating of 130 °C – 170 °C which gives little headroom between maximum reflow temperatures and delamination of the board. For this reason, high-Tg substrates (Tg ≥ 170 °C) are recommended for lead-free assembly.

2. Tin Whisker Growth

Tiny hair-like crystals known as tin whiskers can grow spontaneously from pure tin surfaces. These conductive crystals can reach lengths of several millimetres and pose a significant reliability risk by shorting adjacent circuits. Growth is most prevalent in densely packed assemblies with large areas of exposed tin-plate. Mechanisms for whisker growth are still not fully understood but are thought to be linked with compressive stress in the tin layer. Formation of intermetallics (Cu6Sn5) at the copper-tin interface is one such cause. Conformal coating, nickel barrier layers, and choosing the right PCB surface finish can help reduce tin whisker growth.

3. Brittle Solder Joints / Intermetallic Growth

Lead-free solder forms intermetallic compounds (IMCs) with copper pads during reflow. The primary IMCs you will find at the solder-pad interface are Cu6Sn5 and Ag3Sn. Intermetallic formation is desirable up to a point — we need those metal compounds to create a metallurgical bond between components and PCB. Too much intermetallic growth at the solder interface results in brittle fracture when exposed to shock or vibration. Lead-free SAC solder joints are much more prone to cracking under stress than lead-based equivalents. Brittle fractures are typically the result of thermal or mechanical shock; components exposed to frequent temperature fluctuations or vibration are more likely to fail.

Automotive PCBs and aerospace applications are a prime example where lead-free SAC alloys are used but stringent reliability requirements limit their application.

4. Poor Wetting Ability

Lead-free solder simply does not wet like tin-lead. Surface tension is significantly higher which means solder will not spread across pads and component leads as easily as before. Insufficient solder fillets, solder bridges, and head-in-pillow defects are commonplace when hand-optimising lead-free assembly parameters. Specific flux formulations become even more important when working with SAC alloys. No-clean, halide-activated pastes work best for us at Rush PCB UK. A nitrogen environment is also beneficial for achieving high-quality leads.

5. Component / Substrate Compatibility

Something that is often overlooked during the transition to lead-free assembly is that not all components are created equal. Maximum body temperature ratings are specified for all components, many of which do not exceed 217 °C. Simply put, exposing components to temperatures outside of their specification, even for a couple of minutes, can cause irreparable damage and affect long-term reliability. Parametric shift, bond wire lift, and internal delamination are all possible outcomes of over-temping sensitive devices.

Best Practices When Working Lead-Free

– Reflow Oven Profiling

Good thermal profiling is by far the most important aspect of assembly yield. A standard SAC305 reflow profile looks something like this:

• Preheat (ramp 1–3 °C/s) to ~150 °C
• Thermal soak (between 150–200 °C) for ~60-120s
• Reflow (TaL — time above liquidus) >217 °C for ~45-90s
• Peak temperature between 235–250 °C
• Cooling (ramp down ~3–4 °C/s)

We tend to use thermocouple test boards to profile new assemblies ensuring ΔT across the PCB doesn’t exceed ±5 °C.

– Moisture Sensitivity Level (MSL) Management

Higher reflow temperatures required for SAC alloys significantly increase the risk of ‘popcorn’ cracking if components are not correctly stored or baked. All components with an moisture sensitivity level (MSL) of 3+ should be kept in dry cabinets or vacuum sealed moisture barrier bags. Component floor life starts from the day of packaging and will vary from manufacturer to manufacturer. Most commercial components have a floor life of 168 hours (1 week). If the floor life has been exceeded, a moisture bake at 125 °C for the times stated in the datasheet is required before assembly.

– PCB Surface Finish

Surface finish selection is crucial when working lead-free for many reasons:

• Solderability and shelf life.
• Long-term joint reliability.
• Component compatibility.

Here are our thoughts on the most common PCB finishes:

• ENIG: Excellent wetting properties and solderability. Provides flat surface ideal for fine-pitch components. Lead-free assemblies are also less prone to tin whisker growth thanks to the nickel barrier layer. However, gold-nickel plating can result in black-pad if not applied correctly.
• Lead-Free HASL: One of the cheapest and most robust finishes around. Uneven surface can cause issues with fine-pitch QFP/BGA components.
• OSP: Inexpensive and provides a flat surface but has very limited shelf life. Performance degrades rapidly after first reflow.
• Immersion Tin: Good flatness and solderability but prone to tin whisker growth without additional barriers.

We recommend discussing finish options with your fabricator during their design-for-manufacture (DFM) review stage.

– Wave Soldering

Wave soldering with lead-free solder also requires higher bath temperatures (~260 °C) than traditional SnPb processes. More dross, erosion of stainless steel/cast iron components in the solder pot, and special attention to flux formulations all add to the operational costs. Selective soldering systems allow for much more controlled soldering of through-hole components on boards with heat-sensitive SMDs on the top side.

Lead-Based vs Lead-Free Assembly Summary

Read More: Surface Mount Technology (SMT) vs Through-Hole (THT): Explained!

Why Rush Can Help with Your Next Lead-Free Project?

The challenges of lead-free assembly should not deter you from migrating away from legacy hardware. Doing so will only improve your product’s reliability and allow access to markets that require RoHS compliance. Yes, the process is more demanding. Inspection is more rigorous (X-ray for BGAs! ), and incoming material specifications are more robust. But who doesn’t like a challenge?

At Rush PCB UK we’ve invested in industry-leading convection reflow ovens with onboard nitrogen generation. Our AOI/X-ray inspection machines are calibrated and automated, and we even store moisture sensitive components in a climate-controlled facility. All of our production engineers are trained to IPC standards and we offer full traceability on all jobs.

Got questions? Get in touch with our team today or request an instant quote now!