Lead-Free PCB Assembly: RoHS Compliance, Alloys & Process Guide
Everything you need to know about lead-free soldering — from RoHS exemptions to alloy selection and reflow profile optimisation.
Why Lead-Free Assembly?
The European Union's RoHS Directive (Restriction of Hazardous Substances, 2002/95/EC and its recast 2011/65/EU) restricts the use of lead (Pb) in electrical and electronic equipment sold in the EU. Similar regulations exist in China (China RoHS), Japan (J-MOSS), South Korea, and other major markets. As a result, the vast majority of consumer and commercial electronics manufactured today use lead-free solder alloys.
Common Lead-Free Solder Alloys
| Alloy | Composition | Melting Point | Best For |
|---|---|---|---|
| SAC305 | Sn 96.5% / Ag 3% / Cu 0.5% | 217–220 °C | General SMT (industry standard) |
| SAC405 | Sn 95.5% / Ag 4% / Cu 0.5% | 217–220 °C | High-reliability applications |
| SnCu0.7 | Sn 99.3% / Cu 0.7% | 227 °C | Wave solder (cost-effective) |
| SnBi58 | Sn 42% / Bi 58% | 138 °C | Low-temperature assembly (heat-sensitive components) |
| SN100C | Sn 99.25% / Cu 0.7% / Ni 0.05% | 227 °C | Wave solder (good joint appearance) |
Kingdta uses SAC305 as the standard lead-free alloy for SMT reflow and SN100C for wave soldering. Both are fully RoHS and REACH compliant.
Lead-Free vs Leaded Solder: Key Differences
Lead-free alloys have a higher melting point than traditional Sn63/Pb37 eutectic solder (183 °C). This has several process implications:
- Higher peak reflow temperature: Lead-free reflow typically peaks at 245–260 °C, compared to 210–220 °C for leaded. This places greater thermal stress on components and PCB laminates.
- Narrower process window: The difference between the liquidus temperature and the maximum component temperature rating is smaller, requiring tighter oven profile control.
- Dull joint appearance: Lead-free joints are typically duller and grainier than leaded joints. This is normal and not a defect indicator — IPC-A-610 explicitly addresses this.
- Higher susceptibility to tin whiskers: Pure tin and high-tin alloys can grow tin whiskers over time. SAC305's silver and copper additions mitigate this risk significantly.
RoHS Exemptions
RoHS includes specific exemptions for applications where lead-free alternatives are not yet technically viable. Key exemptions relevant to electronics include:
- Exemption 7(a): Lead in high-melting-temperature type solders (Pb ≥ 85 %) — used in power electronics
- Exemption 7(c-I): Lead in solders for servers, storage, and network infrastructure (expiry under review)
- Exemption 15: Lead in solders to complete a viable electrical connection between semiconductor die and carrier within integrated circuit flip-chip packages
Medical devices (Annex IV) and monitoring and control instruments (Annex III) have separate exemption schedules. If your product falls under an exemption, you must document this in your technical file.
Mixed Assembly: Lead-Free Boards with Leaded Components
Occasionally, a design requires a component that is only available with leaded (SnPb) plating — typically legacy or military-grade parts. Assembling these on a lead-free board requires care: the leaded component terminations will alloy with the lead-free solder paste during reflow, creating a mixed alloy joint. This is generally acceptable for Class 2 applications but requires careful reflow profile optimisation to ensure complete melting of both alloys.
Kingdta's Lead-Free Process
All Kingdta production lines are equipped for lead-free assembly. Our standard process uses SAC305 paste with a nitrogen-atmosphere reflow oven for optimal joint quality. We maintain separate storage and handling for any leaded components to prevent cross-contamination. All finished assemblies are accompanied by a RoHS and REACH compliance declaration upon request.
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