With the Europeans WEEE Directive now mandating a phase out of lead in electronic soldering by July 2006 and Japan’s efforts to do the same even sooner, lead-free is rapidly taking on momentum around the world.No different than when no-clean fluxes first emerged into the North American market in the mid-eighties due to the elimination of CFC’s, electronic assemblers were confronted with not cleaning residues off soldered circuits. The use of no-clean liquid fluxes and no-clean solder pastes were considered by assemblers to be an impossible. Cleaning of flux residue was necessary to insure reliability.Today however over 85% of assemblies are soldered using no-clean fluxes. No-clean fluxes have proven to be reliable. There has also been a benefit to the environment by eliminating the cleaning process and its effluents. No doubt lead-free soldering does offer its set of challenges. The lead-free alloys being proposed as the main choices for general assembly are new and less data is available as to their process limits. The two main alloys are variants of Tin-Silver-Copper and Tin-Copper. These alloys have higher melting temperatures and wet metal surfaces more slowly, the joints also look different in that the surfaces are not as reflective as tin-lead joints. The flux chemistries that worked well with a leaded process are not the best fit for lead-free soldering. There are concerns to be addressed with component and board finish compatibility with lead-free solders. There are concerns about the temperature profile components and boards can sustain to not jeopardize their functionality. The process changes needed to achieve adequate wetting and flow characteristics of lead-free solders and the determination of when to use nitrogen assisted soldering have to be determined. As more and more lead-free soldered boards are produced in Asia and Europe, compatible finishes are now available for boards and components. Process modifications such as solder pot finishes for wave soldering equipment to avoid leaching issues are now available. Defining the reflow ovens capabilities for higher temperature reflow profiles is a relatively simple exercise; most ovens will be capable of achieving the necessary thermal demands. Flux chemistries in no-clean and water washable configurations are also available to enhance the wetting process of lead-free without nitrogen use. These new chemistries with innovative activator packages, higher temperature resistant resins enable the spread of tin-silver-copper and tin-copper alloys while reducing voiding and bridging. As good as these formulations are today improvements in their functionality are ongoing and the formulations will be better as time goes on. Lead-free soldering can be a reality today. Looking back about 6 years we have come closer to this reality. Initially as many as 100 lead-free alloy configurations were being considered, today only a dozen or so are being used, with the general alloy being agreed globally as tin-silver-copper and tin-copper alloys mostly for wave soldering assembly. The process parameters for making lead-free soldering effective without reducing yields are well defined today. Revisiting the process variables from component and board finishes to soldering flux selection, to optimization of the individual variables for lead-free solder usage in the process will render a robust lead-free operation. Establishing suitable guidelines for the inspection process as to recognize adequate lead-free soldered joints and creating a suitable rework process will insure the same reliability as we have become accustomed with a tin-lead process. The many companies using lead-free soldering today prepared and executed an effective implementation plan. This does take time and with deadlines fast approaching the time is now to investigate and implement lead-free wave, reflow and rework practices. Lead-free soldering is not a matter of it being feasible; it is more a matter of our time to follow or to lead. Kester has developed best in class soldering products to enable the effective transition to lead-free assembly. From solder pastes to liquid fluxes to solder wire and performs and solder spheres Kester offers the process engineer products designed with reliable lead-free processes in mind. Kester’s continued efforts in developing robust chemistries for lead-free has been the preoccupation of its Global Research and Development Team. Kester also understands that having solid products and the knowledge and know-how to make lead-free a reality go hand in hand. Kester application engineers are certified and able to assist in the technology challenges associated with lead-free assembly. Kester is making lead-free assembly a reality today.
For a list of our Lead-Free products, such as our solder spheres, visit our Products Section and Select Lead-Free in the the drop-down menu.
THE SOURCE FOR LEAD-FREE ASSEMBLY INFORMATION
Kester Helping You Transition Reliably. Practical Information From Industry Experts
The mission of this informative newsletter is to bring the most useful up-to-date and reliable lead-free information to you. This includes but is not limited to the most current lead-free process information, technical lead-free articles pertinent to recent assembly issues, and essential technical guidelines to help achieve solid lead-free processes. The Lead-Free Connection™ Newsletter is a compilation of Kester’s knowledge, experience, and know-how bundled into an easy to read publication that helps assemblers make this transition to lead-free a seamless one on their manufacturing floors. Kester will have contributions from industry leaders to provide readers the most current key issues facing the electronic manufacturing industry. The content provided in the newsletter will be practical, technical, and insightful information for process engineers.
Top 5 Reasons to Read Kester's Lead-Free Connection™ Newsletter
- Up-to-date lead-free process and RoHS information
- Save time and money setting up your lead-free RoHS program
- A great training tool for all your company
- Get only useful technical information
- Get concise reliable information from the global experts, all year long
The following lead-free hot topics will be discussed in future issues of this newsletter:
- Lead-free legislative and global update
- Optimizing lead-free processes
- Lead-free alloy selection
- Flux chemistry selection
- Solderability issues associated with lead-free
- Component and board finishes used in lead-free assembly
- Material compatibility and reliability
- Lead-free reflow soldering process
- Lead-free wave process optimization
- Lead-free RoHS – how to create a company roadmap
- Preventing defects with lead-free
- Component-board considerations for lead-free and RoHS regulations
- Recycling and lead-free alloys
- Reworking lead-free and lead-free hand soldering
- Rework Inspection criteria
Kester and its contributing partners welcome the joys and challenges of being the first in bringing the best Lead-Free practical information to you. Let’s make a Lead-Free Connection™ that will be lasting and rewarding.
- Relatively Low-cost
- Relatively non-hazardous
- No Potential Environmental Problems; Ranking in decreasing order of safety is: Bi > Zn > In > Sn < Cu > Sb > Ag > Pb
- Capable of wetting common lead-free component and board finishes
- Alloys must function with existing flux technologies and desighnations
- Capable of forming a reliable solder joint, free of oxide inclusions and with minimal solder voids
- Compatible with relatively low temperature processing
- Corrosion-resistant and not prone to electrolytic corrosion potential
- Compatible with copper substrates, immersion gold-over-nickel, and a variety of non-lead substrates, as well as leaded substrates
- Alloys must be available in bar, wire, performs, spheres and paste forms
- Metals used in alloying must be available in sufficient quantities
- Low melting temperatures (< 240°C)
- Good electrical conductivity
- Good thermal conductivity
- Easy reparability
- Adequate strength properties
- Alloys must be able to be easily recycled
Summary of Legislation, Regulations and Directions
- Lead has already been banned by law in paint, automobile fuel, food cans, automobile body solders, light bulbs and plumbing solder and fixtures.
- Lead is permitted in solder for electronics: however, the American Industry was asked by the U.S. EPA to reduce the use of hazardous materials. Lead is currently on its list of hazardous materials.
- Recycling of solder in electronic products is possible, but could become a large cost.
- Pressure is mounting from offshore communities to eliminate lead use. NEMI Association has formed a Lead-Free Task Force to investigate alternatives to lead bearing alloys.
- NCMS -found 3 possible replacements for lead-alloys out of 80 considered -No drop-in replacements
- An alloy is considered to be lead-free if it contains < 0.2% (but no official definition exists)
- NEMI at APEX ’00 has named Sn 95.5Ag3.9Cu0.6 (±0.2%) as its choice for a lead-free alloy candidate
- Intends for North American companies to produce lead-free products by 2004
- Total lead elimination by 2004 on a voluntary basis
- Assist in modifying industry standards for Pb-free
Status of the Lead-Free Issue in a Number of States
- California - Updated list yearly of toxic chemicals
- Connecticut - General permit for collecting some recyclables (early 2000)
- Florida - Pilot program of end-of-life for some electronics
- New Jersey - Pilot program for electronic recycling (3 & 6 graders)
- South Carolina - Bill introduced on state wide electronics recycling
Japan and Europe “International Lead-Free Soldering Roadmap Framework”
- Launched at the 2nd Lead-free Summit meeting in November 2002.
- Iinvolves Europe’s SOLDERTEC and Japan’s JEITA (Japan Electronics and Information Technology Industries Association).
- Recommendations: – Manufacturers have a complete inventory of lead-free components by the end of 2004. – The recommendation that industry adopts the use of 0.1weight percentage as a maximum allowable percentage lead in “lead-free” products.
- Agreement on the EU WEEE (Waste Electrical and Electronic Equipment) and RHS (Restriction of Hazardous Substances in Waste Electrical and the Electronic Equipment) Directives.
- RHS ban on hazardous materials confirmed as July 1, 2006. This directive makes lead-free a requirement for products on sale to European Consumers after this date.
- In addition to phasing out lead, the RHS mandates a phase out of: – Cadmium – Mercury – Hexavalent Chromium – Two types of brominated flame retardants.
- Recommends the following schedule for manufacturers. The roadmap suggests that leading manufacturers are expected to conform to these time frames one year ahead of schedule while other manufacturers may reach them 2 years later.
- Components – Some availability of lead-free components since the end of 2001. – Complete line-up of components with lead-free terminations by the end of 2003. – Complete line-up of lead-free components by the end of 2004.
- Assemblies: – Manufacturing of lead-free soldered assemblies began by the end of 2002. – Complete lead elimination from products by the end of 2005.
- The roadmap recommends a solder alloy composed of Sn-Ag-Cu for board assembly. The roadmap recommends that industry leaders develop a system for labeling.
The NEMI consortium in USA recommends SnAg3.9Cu0.6 for surface mount reflow soldering and SnCu0.7 for wave soldering. The JEITA lead-free roadmap in Japan recommends SnAg3.0Cu0.5 for reflow soldering with SnAg and SnZnBi as secondary alternatives. JEITA also recommends SnAg3.0Cu0.5 for wave soldering with SnCu as a secondary alternative. The IDEALS consortium in Europe preferred SnAg3.8Cu0.7 for reflow soldering and SnAg3.8Cu0.7Sb0.25 for wave soldering. SOLDERTEC lead-free roadmap in Europe recommends alloy range SnAg(3.4-4.1)Cu(0.45-0.9) for reflow and wave soldering.The SnAgCu family is the alloy of choice for all regions of the world at present. The true eutectic composition has been argued to be within the range SnAg(3.5-3.8)Cu(0.7-1). NIST has defined the true eutectic composition as SnAg3.5Cu0.9 .In Japan 2/3 of companies use SnAgCu for reflow and wave soldering. For surface mount reflow SnAg, SnZnBi, SnAgCuBi and SnInAgBi are also used to a lesser degree. For wave soldering SnCu and SnAg are also used to a lesser degree. About 3/4 of companies use SnAgCu for hand soldering. The predominant SnAgCu alloy in use in Japan is SnAg3.0Cu0.5 and increasing trend elsewhere as well. Kester is a licensee of ISURF SnAgCu(Bi) patent 5,527,628, Senju-Matsushita SnAgCu(Bi) patent 3027441 and Oatey SnAgBiCu patent 4,879,096. Below is a reference listing of Pb-free alloys in order of melting point. It is not meant to be an exhaustive list and is not meant to preclude the potential use of other alloys.
||Melting Point °C
||Plumbing industry standard in USA; good shear strength and thermal fatigue resistance
||Common low cost alternative for wave soldering
||Predominant alloy in Japan
||NIST determined to be the true eutectic
||Mitsui Metal patent
||Matsushita (Panasonic) patent
||Prone to atmospheric corrosion and oxidation
||Prone to atmospheric corrosion and oxidation
*Note – it is generally agreed that all of these SnAgCu alloys melt at ˜217°C, but published melting range for each alloy varies; the indicated melting range is estimated from NIST phase diagram; in any case NIST determined there will be =0.1% solid material in any of these alloys at 220°C.
Alloy Material Cost vs Sn63Pb37*
||Relative Cost Ratio: Sn63Pb37 = 1
*Note – cost is based on metals market price.
Physical Properties of Lead-free Alloys
- Higher melt point lead-free alternative. SnAgCu family is electronics industry standard which in most cases has shown equal or greater thermal cycle fatigue resistance than SnPb.
- Higher surface tension and poorer wetting than SnPb.
- Ag provides greater strength but less ductility than Pb.
- Cu reduces the melting point of the solder. Cu improves thermal cycle fatigue resistance. Cu improves wettability. Cu retards the dissolution rate of copper from boards and components into the molten solder during soldering.
- Bi reduces melting point of the solder. Bi improves wettability. In the presence of lead from HASL boards or components Bi can greatly reduce thermal cycle fatigue resistance due to the formation of Sn16Pb32Bi52 (MP=95C) which can diffuse along the grain boundaries.
- Low melt point lead-free alternative potentially suitable for some consumer electronics. Low melt point precludes its use for applications where operating temperature is close to 138C.
- Large Bi proportion greatly reduces melting point of the solder, but alloy is more brittle. Bi improves wettability, but is somewhat offset by higher oxidation rate. In the presence of lead from HASL boards or components Bi can greatly reduce thermal cycle fatigue resistance due to the formation of Sn16Pb32Bi52 (MP=95C) which can diffuse along the grain boundaries.
- Small amount of Ag can improve strength and thermal cycle fatigue resistance, assuming the absence of lead.
- Moderate melt point lead-free alternative is only slightly higher than SnPb.
- Zn lowers melting point. Zn exhibits high oxidation rate and is susceptible to atmospheric corrosion. High oxidation rate precludes it use for wave soldering. Stencil life or shelf life of solder paste may be reduced due to reactive nature of zinc.
- Bi further lowers melting point. Bi improves wettability and slightly improves corrosion resistance. In the presence of lead from HASL boards or components Bi can greatly reduce thermal cycle fatigue resistance due to the formation of Sn16Pb32Bi52 (MP=95C) which can diffuse along the grain boundaries.
- Moderate melt point lead-free alternative is lower than SnAgCu.
- Ag provides strength.
- Indium reduces melting point. Indium is a ductile material. In the presence of lead from HASL boards or components indium forms a ternary compound that has a phase change at 114C.
- Bi further lowers melting point and improves wettability. In the presence of lead from HASL boards or components Bi can greatly reduce thermal cycle fatigue resistance due to the formation of Sn16Pb32Bi52 (MP=95C) which can diffuse along the grain boundaries.
The General Rule:
In the component finish selection avoid lead bearing finishes. The lead in the finish will dissolve into the lead-free solder. This will cause the formation of lead intermetallic phases with differing physical properties (such as expansion/contraction differences) and differing melting points. Work is ongoing to determine the long-term effects of small amounts of lead on high reliability electronics. For many consumer electronic assemblies such as mobile phones and household electronics, where the thermal cycling and condition of use are not extreme the inclusion of lead in component finishes has not demonstrated any negative characteristics to the solder joint integrity. As Japan and Europe progress with lead-free assembly numerous component finishes are already available without lead. It is important to work closely with component suppliers to insure the lead finish is lead-free compatible, the component molding plastic is able to withstand the higher temperatures associated with lead-free soldering and also the component’s reliability will not be jeopardized with the higher exposure temperatures.
Some common finishes include:
- More finishes are originating from Asia
- With SnAgCu, some issues need to be investigated
- Patented indium alloy compatible with SnAgCu
- Might be exempt
- Concern about higher processing temperatures, components are available that sustain the higher soldering temperatures.
Component Process Consideration
- Work closely with component suppliers
- Determine component lead-free finish availability
- Select best solderable finish and component finish shelf life
- Select components with compatible molded plastics and the ability to sustain the thermal requirements of the lead-free process
- Material handling logistics, segregate lead-free finished components from leaded components, if using both a lead-free and a leaded assembly process
- Insure the components and component feeders are identified as containing lead-free finishes.
- Train purchasing, receiving and assembly personnel on the handling procedures to avoid confusion between leaded and lead-free components during the transition stage.
- Identify the soldered assembly as lead-free to insure the proper rework of the lead-free components in-house and in the field.
With SnPb soldering processes HASL [SnPb] has been the predominant board finish followed by NiAu and OSP. The following represent the most common board finishes to consider for a lead-free process.
|Electroless Ni / Immersion Au
||Recommended in JEITA Pb-free roadmap and predominant in Japan and EU. Highest cost process, but good corrosion resistance for on-board contact pads and suitable for wire bonding pads for COB designs.
|HASL [SnAgCu or SnCu]
||Recommended in JEITA Pb-free roadmap and commonly used in Japan.
||High volume, lost cost alternative. Commonly used. Solderability more easily degraded by multiple reflows.
||Good solderability and an increasingly significant board finish.
||Commonly used in EU. Presence of carcinogenic thiourea in solutions has limited its use in USA.
Making Lead-free a Reality
Lead-free fluxes used in solder paste, liquid flux for wave soldering, flux gels and wire solder are available today. These flux systems are designed to enhance the soldering process and are formulated to give excellent solder wetting performance with the added thermal stability of the chemistry, required with lead-free assembly. Traditional fluxes used with tin-lead alloys may not be adequate to circumvent the slower wetting of lead-free alloys and the higher temperatures normally associated with lead-free solders. Flux systems specifically formulated for lead-free soldering will require new activator packages and heat stable gelling and wetting agents to avoid solder defects. Due to the slower wetting and higher surface tension of many lead-free alloys, choosing the right flux for lead-free soldering will prevent the increase of solder defects and greatly assist in maintaining production yields. Typical defects, which can show an increase when transitioning to lead-free assembly are detailed below. These defects can be eliminated with proper flux selection and process control.
Potential Defect Increase – Lead-free SMT Assembly
- Bridging – Paste with poor hot slump behavior
- Solder balls – Paste with poor slump properties
- Tombstoning - Thermal differences across board
- Non-wetting - Excessive preheating or inadequate flux activity
- Poor wetting - Poor flux activity or excessive preheating
- Solder Voids - Thermal profile too low, or inadequate flux chemistry
- Solder beading - Paste with poor hot slump or excessive preheating
- Potential Defects Increase – Lead-free Wave Soldering
- Bridging – Flux deactivation during preheating or solder contact
- Icicling – Flux too low in activity or preheating temperature to high
- Solder Balls – Insufficient preheat or flux-solder mask incompatibility
- Insufficient Hole-Fill – Flux activity too low, too low solids, or excessive preheat temperature or too low a contact time with molten solder
Requirements for a Lead-Free Flux:
- Low-activation temperature
- Adequate shelf life
- High activity level
- High reliability
- Residues benign or easily remove with water if the paste is a water washable type
Other Considerations for the Lead-free Flux:
- Is the paste for dispensing or printing?
- Note manufacturers use different types of activators for different alloys
- Select flux carefully to balance activation temperature with thermal profile
- What is the compatibility of the flux with the alloy selected?
- What are the reliability properties (SIR, electro-migration, corrosion)?
Considerations for Lead-free Solderpaste
Important properties to consider during selection:
- Solder Balling Test activity
- Wetting Test , specific finishes and solder atmosphere (air or nitrogen)
- Voiding Potential, lead-free alloys are more prone to solder voids
- Tack Life Over Time
- Stencil life and abandon time
- Cold Slump
- Hot Slump tested to higher temperatures 180-185°C .
- Shelf life Testing
Properties to evaluate in-process:
- Print Speed
- Component Placement
- Examine solder joint formation on a variety of leads and PWB finishes
Properties to evaluate after reflow:
- Thermal Shock
- Thermal Cycling
- Impact Resistance
- Reliability (SIR)
Technical Considerations for Wave Soldering Fluxes Designed for Lead-free Assembly
- Ability to be evenly applicable by spray, wave, or foam applications
- Activator package able to sustain higher preheating temperatures
- Able to be used with a variety of lead-free finishes, bare copper OSP, gold nickel, tin, silver immersion, tin-copper
- Sustained activity, the flux should remain active throughout the contact time with the molten solder, insuring good peel back of the solder
- Low dross potential, the flux must not react excessively with the molten solder as to create large amounts of dross
- The flux must not discolor or char at the higher soldering temperatures associated with lead-free wave soldering
- The flux should not decompose at the higher solder temperatures
- The flux residues must be benign if it is a no-clean flux, and easily washed in hot water if it is a water washable flux type
Technical Considerations for Lead-free Cored Solder Wires
- The flux should not spatter or fume excessively at the slightly higher soldering temperatures associated with lead-free soldering
- The flux should have activator systems designed to solder a variety of lead-free board and component finishes
- The flux must be active enough and remain active enough during tip contact to compensate for the reduced wetting of lead-free alloys
- The flux residues must be benign if it is a no-clean type, or easily removed in hot water if it is a water washable type cored solder
- The residue should not char or darken in color when using slightly higher solder tip temperatures
Lead-Free Reflow Profile Recommended reflow profile for Kester solder paste formulations containing the Sn96.5Ag3.5 and Sn96.5Ag3.0Cu0.5 alloys
K100LD & K100 Case Studies Summary This PowerPoint pictorial examines a successful implementation of K100LD and K100 lead-free alloys at SMT Dynamics and Ayrshire Electronics.
Standard Reflow Profile The recommended reflow profile for Kester solder pastes manufactured with standard tin/lead alloys.
K100 Implementation at SMT Dynamics This paper shows results obtained at a contractor in successful builds using SAC305 for reflow soldering and K100 solder for wave assembly.
Developing a Reliable Lead-free SMT Assembly Process This article offers valuable information to be used during the successful lead-free reflow process implementation.
How do you create a RoHS Compliancy-Lead-free Roadmap? This article describes the various considerations in the transition to lead-free but also RoHS comnpliancy.
How do you ready a solder pot for lead-free solder? This article in designed for those cleaning a leaded solder pot to convert it to lead-free solder. It insures the solder is not contaminated with lead during the switchover.
Implementation of Reliable Lead-free Wave and SMT Processes This article describes the points to consider in transitioning to lead-free wave and reflow soldering.
Lead-free Hand-soldering – Ending the Nightmares Hand-soldering with lead-free solders can be challenging but not if the points in this article are understood.
Lead-free Reliability – Building it right the First Time This article describes the comparison done in reference to lead-free and leaded solder joints. The article also gives insight on the points to improve overall reliability with lead-free solders.
Lead-free SMT Soldering Defects How to Prevent Them This paper shows how avoid soldering defects with lead-free solder pastes. Process control is emphasized to reduce defects.
Lead-free Wave Soldering This article shows the impact to wave soldering with lead-free materials and how to create a reliable defect free process.
Creating Solder Joint Reliability with SnCu Based Solders Tin-copper systems with lead-free wave and selective soldering are a growing option. This paper shows how to implement a reliable process using K100 or K100LD SnCu based solders.
Tin-copper based solder options for lead-free assembly This article details the process changes associated with K100 or K100LD solder systems.
Case Study on the Validation of SAC305 and SnCu Based Solders in SMT Wave and Hand-soldering This paper describes the work done at a contract assembler who implemented a successful process to build over 500,000 assemblies using both Kester K100 and SAC305 solders.
Kester products for lead-free assembly are designed to enhance the wetting properties of lead-free alloys. The chemistries are more thermally stable and offer the engineer the maximum operating window for his process. Optimized flux chemistries results in solid reliability for the process and the assembly; this preserves production yields normally accustomed with tin-lead processes.All products are available in industry standard packages.
For Wave Soldering Operations
- No-Clean VOC-Free – 979
- No-Clean – 959T
- Water Soluble VOC-Free – 2220-VF
For Surface Mount Soldering
- No-Clean – EnviroMark 907
- Water Soluble – R520A
For Dispensing Solder Paste Operations
- No-Clean – EnviroMark 907
- Water Soluble – R520A
For Hand Soldering Operations
- No-Clean - 275
- Water Soluble - 331
- Rosin – 48
- No-Clean - 952D-6
- Water Soluble - 2331-ZX
- Rosin - 186
For Dispensing Solder Paste Operations
Tacky Soldering Fluxes (TSF’s)
- No-clean – TSF 6592
- Water Soluble – TSF 6850