Solder is an essential material used in electronics manufacturing to create mechanical and electrical connections between components and printed circuit boards (PCBs). Traditionally solder contained lead, but environmental concerns have driven wide adoption of lead-free solder alloys. However, lead solder still offers some advantages over lead-free alternatives in certain applications. This article compares properties, strengths and limitations of leaded and lead-free solders to help select the optimal type.
Overview of Lead and Lead-Free Solders
Solder is a metal alloy that liquifies at relatively low temperatures making it ideal for electronics assembly. It flows into the joint, then solidifies forming permanent electrical and mechanical soldered connections. Here is a brief overview comparing traditional lead solder versus modern lead-free solder:
Lead Solder
- Contains 30-40% lead alloyed with tin
- Melting point around 180-190°C
- Excellent wetting and flow characteristics
- Relatively low cost
- Toxic – restricted in some regions
Lead-Free Solder
- Replaces lead with silver, copper, bismuth etc.
- Higher melting point, typically 217-220°C
- More difficult wetting and flow
- Higher cost than leaded
- Reduced environmental impact
Both offer trade-offs that make each preferable for certain applications as we will examine in detail.
Why Choose Lead-Free Solder?
Lead-free solder alloys eliminate toxic lead to mitigate environmental and health hazards associated with electronics waste:
Reduced Environmental Contamination
- Lead persits indefinitely and accumulates in the environment when electronics are improperly discarded in landfills. Lead-free solder reduces lead contamination risks.
Improved Worker Safety
- Lead exposure negatively impacts personnel health. Lead-free solder helps protect assembly line staff from lead’s neurological and reproductive toxicity when handled regularly.
Complying With Regulations
- Laws like the Restriction of Hazardous Substances (RoHS) directive ban lead solder from consumer electronics to reduce lead waste pollution. Most electronics OEMs have transitioned to comply.
Avoiding Lead Contamination
- Lead can contaminate food processing equipment and medical devices leading to health risks. Lead-free solder prevents lead leaching.
Minimizing Disposal/Recycling Issues
- Separating and properly recycling leaded waste adds cost. Lead-free products simplify end-of-life disposition.
For most new consumer product designs, lead-free solder is now standard. But in specialized cases, leaded solder still offers advantages.
Why Lead Solder Remains Advantageous
Despite the push towards lead-free electronics, leaded solder provides some ongoing benefits in certain applications:
Lower Melting Point
- Lead solder melts around 180-190°C versus 210-220°C for lead-free. This allows use with heat sensitive components.
Improved Wetting and Flow
- Lead alloys readily wet and spread on copper and component leads. Lead-free solder can require flux assistance.
Reduced Joint Cold Flow Stress
- Softer lead solder reduces shear stresses on thermal cycle. Lead-free joints more prone to tin whiskering.
Lower Cost
- The continued wide availability of tin-lead solder keeps costs modest. Some lead-free alloys utilize expensive metals.
Exemption for Defense/Aerospace
- Mission critical military and avionics applications still permit lead due to reliability concerns.
Repairability
- Leaded solder joints are easier to reheat for component replacement versus lead-free.
Lead solder remains preferable for many high reliability industrial, military, aerospace, and maintenance applications where its unique advantages outweigh toxicity concerns.
Lead-Free Solder Alloys
Lead-free solders most often consist of tin alloyed with metals like silver, copper and bismuth to mimic lead’s properties as closely as possible. Common compositions include:
SnAgCu (SAC) – Tin-Silver-Copper (95.5Sn/3.9Ag/0.6Cu)
- Most popular lead-free alloy
- Good mechanical properties
- Prone to tin whiskering
SnCu – Tin-Copper (99Sn/0.7Cu)
- High tin content improves ductility
- High melting point of 227°C
SnAg – Tin-Silver (96.5Sn/3.5Ag)
- Contains costly silver
- Susceptible to silver leaching
SnBi – Tin-Bismuth (42Sn/58Bi)
- Lowest melting point of just 138°C
- Brittle with poorer thermal fatigue
SnZn – Tin-Zinc (91Sn/9Zn)
- Zinc lowers melting point to 199°C
- Can corrode and weaken over time
SnAgBi – Tin-Silver-Bismuth
- Contains silver for strength and bismuth for ductility
Alloy composition balances properties like melting point, joint strength, ductility, thermal fatigue and cost.
Comparison of Lead and Lead-Free Solder Properties
Here is how typical 63/37 tin/lead solder compares to common 95.5/3.9/0.6 Sn/Ag/Cu lead-free solder for key properties:
| Property | 63Sn/37Pb | 95.5Sn/3.9Ag/0.6Cu |
|---|---|---|
| Melting Point | 183°C | 217°C |
| Electrical Conductivity | 15% IACS | 12% IACS |
| Thermal Conductivity | 35 W/mK | 60 W/mK |
| Coefficient of Thermal Expansion | 29 ppm/°C | 23 ppm/°C |
| Yield Strength | 16 MPa | 41 MPa |
| Elongation at Break | 40% | 40% |
| Creep Resistance | Excellent | Good |
| Wetting Characteristics | Excellent | Fair |
Lead-free alloys compensate for drawbacks like higher melting point and poorer wetting through exact alloy adjustments and flux chemistries.
Mixing Lead and Lead-Free Solder
Ideally all soldering in a given product is done using consistent solder alloy to avoid incompatibility between joints:
- Mixing leaded and lead-free solders risks formation of brittle intermetallic layers that weaken joints
- Using lead-free solder over broader temperature profiles risks melting adjacent leaded solder joints
- Matching solder alloy and flux core is optimal for wetting and flow
For niche cases allowing some mixed soldering, like repairing older electronics, observe these precautions:
- Keep joint types isolated – leaded for repairs, lead-free for new soldering
- Ensure the melting point of existing solder is 5-10°C higher than the applied new solder
- Use solder wick to fully remove previous solder before applying new alloy
- Swap component leads if feasible to avoid soldering dissimilar alloys to the same leads
It is best practice however to standardize on one solder alloy per product during design when possible.
Lead Solder Application Notes
When working with traditional tin-lead solder:
- Choose an alloy blend like Sn60/Pb40 or Sn63/Pb37 suited to the thermal needs
- Specify flux-cored wire or paste for easiest application
- Use the lowest temperature producing good flow to minimize thermal stress
- Allow adequate cool down time since leaded solder remains molten longer
- Employ adequate fume extraction due to lead vapor exposure risks
- Follow safe lead handling procedures to avoid ingestion or skin contact
- Properly label any product containing lead for recycling/disposal
With careful handling, lead solder remains an appropriate choice for high reliability industrial electronics not subject to RoHS regulations.
Lead-Free Solder Application Notes
To successfully apply lead-free solders:
- Select alloy suited to process capabilities, like Sn/Ag/Cu or Sn/Cu
- Use solder specifically formulated for lead-free use
- Increase process temperatures to reach higher melting point
- Adjust dwell times to achieve proper wetting with lead-free alloys
- Use sufficient flux designed to eliminate lead-free processing issues
- Review any component specifications for lead-free soldering suitability
- Follow cleaning processes to remove all lead-free flux residues
With process adjustments, lead-free solders can deliver sufficient joint strength and reliability.
Lead-Free Solder Rework and Repair
Reworking and repairing lead-free solder joints raises challenges:
- Higher melting point makes removal more difficult
- Leaded solder should not contact lead-free joints
- Traces may delaminate if overheated
- Preheating is essential to minimize thermal shock
- Flux must be thoroughly cleaned afterwards
Methods to rework lead-free solder connections:
- Use hot air tool, mini soldering iron, or reflow oven
- Preferentially heat component and junction, not entire board
- For BGAs, use hot air nozzle following profile specifications
- Remove all previous solder first before re-applying fresh solder
With care, lead-free solder joints can be successfully reworked or repaired.
Selecting Lead or Lead-Free Solder
Use lead solder when:
- High reliability is critical – aerospace, military, automotive
- Repairability and long service life are needed
- Heat-sensitive components prohibit higher temperatures
- RoHS regulations do not apply to the product
- Process is already optimized for lead solder
Use lead-free solder when:
- Consumer safety from lead contamination is paramount
- Adhering to RoHS regulations for commercial products
- Establishing an environmentally friendly process
- Heat-resistant components permit higher temperatures
- New process development allows optimizing for lead-free
Evaluate criteria like temperature limits, reliability, regulation, environmental impact and manufacturability when selecting solder alloy. With attention to detail, both leaded and lead-free solder can produce robust joints.
Summary of Lead vs. Lead-Free Solder Trade-Offs
- Lead solder offers a lower melting point and superior wetting and ductility but carries toxicity concerns.
- Lead-free solder eliminates lead risks but requires higher temperatures, precise alloying and well-controlled processes.
- Each option has benefits making it preferable for particular applications subject to different technical and regulatory constraints.
- Electronics assemblies should ideally standardize on consistent lead or lead-free solder, not mix alloys.
- With well-developed processes, both solder alloy families can achieve excellent, reliable solder connections.
Understanding the trade-offs helps designers select the right solder type for each product based on performance, safety, reliability and other requirements.
Frequently Asked Questions
What are the most widely used lead-free solder alloy compositions?
The most common lead-free solder alloys used in electronics are:
- SnAgCu (95.5Sn/3.9Ag/0.6Cu)
- SnCu (99Sn/0.7Cu)
- SnAg (96.5Sn/3.5Ag)
- SnBi (42Sn/58Bi) – for very low melting points
Engineers select different ratios based on properties needed like melting point, strength, cost, and manufacturability.
Is lead-free solder always required? Are any exemptions allowed?
Lead solder remains permissible in:
- Aerospace, military, and high reliability applications
- Maintenance and repair of existing leaded electronics
- Certain high temperature electronics
- Applications exempt from RoHS regulations
If lead contamination is not a concern, leaded solder can still offer advantages.
What can mitigate the higher melting point challenges of lead-free solder?
Methods to conquer lead-free solder’s higher melting point include:
- Adjusting process temperature settings higher
- Lengthening dwell times over joints
- Usingria appropriate soldering iron tip sizes/temps
- Employing profiling techniques like ramp-soak-spike
- Selecting alloy compositions with lower melting points
- Verifying components meet higher temperature needs
With robust process development, reliable lead-free soldering is certainly achievable.
