Pin holes are formed as a result of moisture entrapment. All that is required is a tiny amount of moisture. When the solder comes in contact with the through-hole, component lead, wire or whatever is being soldered, the water boils and it forms a gas bubble that will either escape or be trapped as the solder solidifies.
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.
Involves 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.
Which are the exceptions to the RoHS and WEEE directives?
There are a lot of exceptions to the RoHS and WEEE directives. In order to be sure if one or more exceptions apply to the end product or sub product the directive needs to be consulted carefully. In general the military, air and space electronics are exempt. Some Medical devices are also exempt. Alloys with Pb concentration above 85% are equally exempt.
Where can I get up-to-date web information on WEEE and RoHS directives and progress?
Getting up-to-date information is critical to your company’s transition roadmap. A good place is the web and the following website contains updates originating from the TAC (Technical Adaptive Committee) for the RoHS. The website www.dti.gov.uk/sustainability contains copies of the WEEE and RoHS Directives but also the latest minutes of the TAC meetings. Another useful website in reference to the WEEE directive which includes the EU’s perspective is www.europa.eu.int/comm/environment/waste/weee_index.htm.
What are the new IPC-1066 and IPC-1085 Documents and how can they help you in the RoHS-Lead-free transition?
These IPC documents were issued in January 2005. The IPC-1066 is titled “Marking, Symbols and Labels for Identification of Lead-free and Other Reportable Materials in Lead-free Assemblies, Components and Devices” is a document detailing ways to identify components with lead-free finishes, but it can be expanded to board finishes and solder used for assembly. A letter system from e1 to e9 will identify the various lead-free finishes. This document will be used primarily by component manufacturers in the identification and labeling of lead-free components. This document should be used to train procurement, inventory control and production personnel, so as to create an awareness of the component finishes intended to be soldered. The IPC-1065, Material Declaration Handbook details the hundreds of other controlled chemicals restricted in electronic assemblies and also details approved test methods for their detection. It will be useful if a RoHS banned substance must be tested for.
What are the labeling requirements to indicate RoHS product compliance?
The RoHS Directive doesn’t require any specific label to be put on assemblies or box builds. Although some companies have designed their own label and some are using it, by law it is not necessary. Any product entering the European market will be assumed to be RoHS compliant. The same applies to the lead-free logo; it too is not required. Some manufacturers are using their logos to indicate the product is lead-free but this is usually for marketing purposes.
Do I need Material Declarations for my finished product?
A Material Declaration showing compliancy for your product is not required by the EC law. However, if a product entering the European market is intercepted and found to be non-compliant to the RoHS after July 1, 2006, it will be important to demonstrate that a company has done all that is possible in insuring compliancy. Material Declarations or data from each component used in the assembly will then be required. Keeping Material Declarations for each individual item used in a build is important and can show good due diligence has been exercised. A close relationship with suppliers is essential.
What are the main elements required from a Material Declaration Form for my components, boards, wiring, etc.?
The essential elements a Material Declaration must contain are as follows:
- Compliancy to European RoHS Directive banned substances
- Free of Polybrominated Biphenyls and Polybrominated Diphenyl ethers flame retardants, can be found in some plastic molding compounds and laminates
- Temperature maximum limits for a lead-free soldering process
- New Moisture Sensitivity rating for lead-free assembly
The key is to insure banned substances are not present, but also that the parts are lead-free process compatible. Lead-free soldering when using SAC alloys will require hotter thermal profiles. To insure reliability close attention must also be placed on the maximum temperature the part can see but also the impact of moisture.
What is the definition of “lead-free”? Is there an allowable threshold limit?
The EU RoHS directives defines 0.1 wt% (1000ppm) as the threshold for lead per homogeneous material if not intentionally introduced (i.e. each material prior to soldering).This is defined as a limit for each homogenous material, i.e. component lead, lead plating, glass fibres, plastic moulding, solder, pad finish etc. It is NOT defined at 0.1% by mass of the finished product, or circuit board.
What is the impact of lead contamination on lead-free joints on pull and shear test results?
For small amounts of lead in SAC alloys, no difference was noticed in pull or shear test data. Gintic Singapore Consortium report showed that up to 2% by weight lead in lead-free SAC joints had no discernable negative impact. However, lead-free terminations soldered with lead-free solders are still considered the most reliable. Lead in wave soldering however can result in some fillet lifting to occur.
What are the first steps to take to insure a reliable SMT, wave solder process with lead-free solders?
The knowledge base for Lead-free Assembly is still increasing rapidly. Choosing an alloy, which has been studied carefully is essential. SAC solders would fit this category and lots of data now exists; however this is not the case with others. Choosing an alloy, which does not have substantial historical use or limited data will therefore require substantial investment in reliability testing. Understanding the physical and chemical properties of the lead-free solder alloy is important since many have reduced wetting behavior and higher surface tension. This will enable an engineer to optimize the soldering process to account for these differences and insure a solid solder joint. Knowing the component finishes and board finishes and what can be expected during soldering will enable proper selection of fluxes designed to solder them. Chose a flux system designed for lead-free. After consideration is given to equipment compatibility a DOE should be run to determine the best process parameters to achieve good lead-free solder joints. Proper training will be required since the cosmetics of lead-free joints are different in cosmetics and wetting spread and wetting angles when compared to 63/37 leaded joints.
Is there a standard lead-free solder alloy?
Just as with the lead-based solder alloys, there are several alloys that seem to be the most commonly used. The Solder Products Value Council (SPVC), a group of global solder manufacturers, standardized on SnAg3.0Cu0.5 as the alloy of choice. There are many variations of SnAgCu from 3-4% silver and 0.5-0.7% copper, but the melting temperature only varies in the range 217-220°C.
Does the substantially lower density of lead-free solder compared to tin-lead alloys have any advantages?
The specific gravity, or density in g/cc, of Sn63Pb37 or Sn63Pb36Ag02, the most standard lead-based alloys, is about 8.4. The specific gravity of the lead-free alloys, being mostly tin, is about 7.3, so the same volume of solder would be about 15% less weight for the lead-free solder.
What are some lower temperature lead-free alloys and where would they be used?
Lead-free solder alloys that melt lower than the tin-silver-copper alloys are composed of tin or tin alloys with the addition of bismuth, indium, or zinc, and all have advantages and disadvantages.
- Bismuth alloys can contain a small amount or a large amount of bismuth. Generally, solder alloys with more than about 8% bismuth will be brittle.
- SnAg3.0Bi3.0 melts 213°C
- SnAg3.4Bi4.8 melts 202-215°C
- SnAg2.0Cu0.5Bi7.5 melts 211°C
- SnBi58 melts 138°C
- Indium alloys can contain a small amount or a large amount of indium. Generally, solder alloys containing around 50% indium have little strength, but ductility is good. The high cost of indium is a consideration.
- SnAg3.3In4.8 melts 212-214°C
- SnAg3.0Cu0.5In10 melts 194-200°C
- SnIn52 melts 117°C
- Zinc alloys usually contain less than 10% zinc, but the ease of corrosion minimizes the use of zinc alloys for most applications.
- SnAg3.0Zn15.0 melts 200-202°C
- SnZn09 melts 199°C
Of course there can be alloy compositions that contain bismuth, indium, and zinc.
- SnBi3.0Zn9.0 melts 187-195°C and wets better than SnZn.
- SnBi50.0In2.0 melts 135-137°C with better ductility than SnBi
- SnBi15.0Zn5.0 melts 170-193°C
What are some higher temperature lead-free alloys and where would they be used?
Currently, the high lead-content (>85%) are exempt from banning because there are no viable alternatives. However, there are some possibilities.
- Sn95Sb05 melts 232-240°C, but is not much higher melting than SnAgCu (217°C)
- Sn20Au80 melts 280°C but may be cost prohibitive
- BiAg2.5 melts 262°C
- BiZn2.7 melts 255°C
- BiZn15 melts 255-313°C
- SnAl05 melts 382°C
- Sn65Ag25Sb10 melts 230-235°C
What are the patent issues with using lead-free solder?
With more than 150 patents globally, selection of a lead-free solder alloy must be done carefully so proper license fees are paid for using patented alloys. Some alloys are not patented because of a long history of use and can be used without licensing.
- SnAg, SnCu, SnSb, SnIn, SnBi and SnZn alloys
- SnAg3.0Cu0.5 and SnAg4.0Cu0.5 (depending on the application)
Most other metal combinations for lead-free solder have been patented. The most prevalent patents for SnAgCu alloys include:
- Iowa State University Research Foundation (ISURF) USA patent 5,527,628 covering alloys SnAg(3.5-7.7)Cu(0.9-4.0)
- Senju/Matsushita Japanese patent JP302744 covering alloys SnAg(3.0-5.0)Cu(0.5-3.0)
Kester is licensed globally to manufacture and sell these patented SnAgCu solder alloys, and the licenses pass through to Kester customers to use the alloys.
What is the specification of lead impurity composition in the lead-free solder?
The specification for lead impurity content in lead-free solder content is under 500ppm. There will always be a small amount of lead contamination from the refining process of tin. This value of o.05% is lower than the 0.1% RoHS limit but care must be taken to avoid lead contamination especially during wave soldering.
How is lead-free solder analyzed?
Lead-free solders can be analyzed with spark analyzer and atomic absorption spectrometers. Both spark analyzer and atomic absorption spectrometer provide results equivalent to ICP-AES, however spark analysis is the preferred method.
Is there a lead-free solder of high melting point like Sn10Pb90?
There is no lead-free solder substitute for Sn10Pb90. In addition, there is no problem in using solders with high melting points because they are RoHS exempt if they contain a lead content of greater than 85%.
Is there a lead-free solder with a low melting point?
There are lead-free alloys for solder paste with 35-58% Bi content and others containing a variety of elements such as zinc with a lower bismuth contents, example Sn89Zn8Bi3. Bi is effective to lower the melting point but it also makes solders weak and brittle. Bismuth does lower the melting point and improves wetting but has limitations. Zn oxidizes very quickly and needs extreme caution when used due to its electrolytic corrosion potential. Primarily consumer electronic assemblers use these alloys.
Do the component terminations have to be lead-free?
Component terminations, the surfaces being soldered, must be lead-free. The solder spheres for ball grid arrays must also be lead-free. Internal solder used for construction of the component can contain solder with more than 85% lead.
What lead-free finishes are alternatives for tin-lead finishes?
Component terminations are more consistently solderable if they are pretinned with melted solder. Alternative lead-free solders for tinning are tin-silver-copper, tin-silver, tin-copper, and tin-bismuth. Other metals can be plated onto the terminations, but the same potential problems exist as with printed wiring boards. Component manufacturers seem to have mostly standardized on matte tin plating or palladium-nickel. Electroless nickel under immersion gold is acceptable provided the nickel does not contain more that about 7-8% phosphorous.
What problems might be anticipated with lead-free components?
The main problem is the same as existed with tin-lead coated component terminations, dewetting or nonwetting during soldering. Tin can be plated onto the termination even though the surface being plated is not solderable. Gold and palladium dissolve rapidly into the solder, so the nickel underneath the precious metal must be solderable.
What are the changes in reference to lead-free assembly in the J-STD-020C, dated July 2004?
The IPC/JEDEC J-STD-020C, issued July 2004, entitled Moisture/Reflow Sensitivity Classification for Non-hermetic Solid State Surface Mount Devices details the thermal profiles SMD components must meet to be classified as lead-free process capable. Higher thermal profiles with lead-free in the range of 235-255ºC, may require component re-qualification to new moisture sensitivity limits. This needs to be known and adequate measures taken to avoid moisture issues such as popcorning, delamination and cracking issues during lead-free reflow. This document is also useful to procurement, where lead-free components can be referenced to the requirements set in this standard.
Can I solder leaded components in lead-free wave soldering?
Leaded terminations cannot be soldered in a lead-free wave solder process. Lead-free solder bar will have a small amount of lead when received usually in the range of 0.01 to 0.08%. The RoHS Directive states a maximum of 0.1% lead; it does not take very much lead to surpass this limit. To avoid surpassing this limit, leaded terminations should not be allowed. There is no effective way to reduce lead content except by dilution if lead were to go beyond 0.1%. Also lead contamination can be a contributor to fillet lifting and fillet tearing. Although this is not considered a defect as per IPC-610D, further studies are required to determine the impact on high reliability assemblies. For consumer electronics reliability would not be an issue since most are not exposed to thermal cycling or thermal shock during their use.
Can I solder reliably leaded terminations with lead-free solders in an SMT process?
The amount of lead on SMD terminations can be small often component manufacturers will use 10/90 or 15/85 for the tinning. A small amount of lead will be introduced into the lead-free joint and for small amounts of lead under 2% by weight this doesn’t impact the pull and shear force results when tested. Some assemblers have used a mixed bag of leaded and unleaded SMD’s with no impact to product reliability. Most are in consumer electronics. For the highest reliability a complete lead-free system is preferred. If leaded SMD components must be used, it is recommended to access product reliability and therefore some testing may be required. An important note to that for RoHS compliancy lead must be kept below 0.1% in the joint also, lead in terminations may impact this negatively.
Can I solder lead-free terminations with leaded solders such as 63/37?
Lead-free terminations such as pure tin, silver palladium and tin bismuth have been in use for years. So they are already being soldered with 63/37 solder without much issue. Today more and more components are coming lead-free, where they may have had a leaded finish before. Some component manufacturers are issuing different part numbers some are not; some companies are notifying distributors and assemblers of the change some are not. For the assembler, even if it is not going to lead-free soldering, it becomes important to know what these new finishes are since solderability may change requiring process optimization to maintain product reliability.
How can I check if my components supplier has really delivered lead-free components?
This can be very difficult as not all component suppliers will change part numbers of the components. Some suppliers do mark packaging with some kind of Lead-free symbol. If in doubt some analytical methods exist. These can be destructive or non-destructive. A non-destructive method is to wipe the component lead surface with a cotton bud, place the bud in a reactive chemical. If Lead is present the chemical will color pink to read. The destructive methods consist of SEM/EDX, ICP or AAS measurements.
Is there a universal marking available that can be put on the assembly indicating RoHS compliance and/or Pb-free?
No there is no such universal marking. The RoHS directive does not require any marking of final assemblies. The Soldertec institute and IPC have some advisory guidelines on marking. These can vary from symbols such as crossed out Pb in a red circle to a diamond with RoHS in black letters. A common symbol for the PCB would be the letter e with a number. Each number will indicate an alloy family. No marking would indicate SnPb. More info on www.ipc.org or www.soldertec.org.
How do I ensure that no mix-ups between LF and non-LF soldering materials will occur in the production floor?
A complete inventory list of all current materials should be conducted and the items should be clearly labeled and identified. The production lines should be well dedicated and segregated for leaded and lead-free to avoid any risk of cross-contamination. Kester’s lead-free soldering products are easily distinguishable by the use of different packaging colors for paste and shapes for solder bars. This has helped the customer in implementing a total lead-free solution.
Printed Wiring Boards
Do the printed wiring boards have to be lead-free?
Estimates are that 60-70% of printed wiring boards are tin-lead solder coated, usually by hot air solder leveling (HASL). The solder coatings are applied to the copper surfaces of the printed wiring board to preserve solderability and protect the copper conductors from environmental corrosion. To comply with European Restriction of Hazardous Substances (RoHS) Directive 2002/95/EC, tin-lead solder cannot be used.
What lead-free finishes are alternatives for tin-lead finishes?
There are many proposed alternatives for tin-lead finishes, none performing equal to tin-lead. Changing to lead-free is not going to be as easy as just changing to another coating. Some alternatives are:
- hot air solder leveling (HASL) with lead-free solder, such as tin-silver-copper, tin-silver, or tin-copper
- organic solderability preservatives (OSP) placed on the copper
- immersion tin or bismuth, applied in thin coatings of 1-2 microns, or silver coatings of less than 0.1 micron
- palladium applied electrolessly directly on the copper, or coating the copper with nickel and then palladium
- electroless nickel coated with immersion gold (ENIG)
What problems might be anticipated with lead-free printed wiring boards?
The least amount of problems would be putting no coating on the copper and trying to keep the copper surface clean and active during storage and assembly. The choice of coatings presents different problems.
- Hot air solder leveling (HASL) coatings present a problem common also with tin-lead solder, i.e., non-uniform pads causing problems with placement of surface mount components. Also, the lead-free alternatives do not have the same appearance as tin-lead, generally being dull or grainy.
- Organic Solderability Preservatives (OSP) are not very heat-stable. Though barely being able to withstand one heat of soldering, the OSP coating makes soldering very difficult for a second reflow.
- Immersion tin coatings less than 1-micron directly on copper can result in decreased solderability as the copper-tin intermetallics form. This can lead to the formation of tin whiskers erupting out of the tin coating. A nickel coating under the tin can improve solderability and minimize the tin whisker formation.
- Palladium is deposited on nickel over the copper. This is an expensive coating that is used on components but not often on boards.
- Electroless nickel under immersion gold (ENIG) provides a solderable coating, but is expensive. The gold (about 0.1 micron thick) dissolves instantly in the melted solder, and soldering is done to the underlying nickel. The amount of phosphorous deposited with the nickel determines whether the soldering is going to be reliable or not.
What lead-free solder alloys are recommended for reflow soldering?
The key variable to contend with in selecting an alloy for SMT assembly is the reflow temperature of the alloy. There are component thermal issues to observe in this selection, for example, can the plastic molding compounds sustain the higher temperatures associated with most lead-free solder alloys? Will higher temperatures impact component reliability? The wetting properties of lead-free alloys will differ and is dependent on the surface finishes to be soldered. Presently tin-silver-copper is a popular choice for most SMT assembly applications. These alloys reflow within 217-221 C, and a peak temperature of 235-255 C is adequate to achieve good soldering on most lead-free surfaces such as tin, silver, gold over nickel, and bare copper OSP. There are lower melting options such as tin-zinc and tin-silver-copper bismuth but they may require special flux systems or the leads and boards must be completely free of bismuth to achieve reliable solder joints.
What are the main qualification tests for using lead-free solder paste?
A good selection process for a lead-free paste is essential to a defect free SMT process. Firstly the board finish and component finishes must be determined and the flux chemistry chosen to adequately solder these finishes. Some pastes do well with tinned surfaces but solder poorly bare copper OSP. The solder paste manufacture will normally do extensive testing such as spread and wetting of its particular solder paste in both air and nitrogen atmospheres. This information should be requested. Other critical performance tests are:
- Hot Slump testing, done at higher temperatures such as 180-185C
- Cold Slump testing
- Solder balling test in air and nitrogen using a typical reflow thermal profile
- Wetting to different surfaces, spread testing
- Tack life
- Stencil life
- Residue character
- Pin-testability of the residue
- Cleanability of the residues, especially for water washable pastes.
Will the stencil design or printing process need to be changed for lead-free?
The stencil and print process will not change. Lead-free alloys such as tin-silver-copper do not wet-out completely to the edge of the pads, in some cases a stencil with less reduction or a 1:1 ratio will assist in reducing this effect. A well-designed lead-free solder paste will have excellent tack life and the stencil life will be similar to traditional leaded pastes. Print speeds should not be jeopardized.
In what way does a lead-free thermal profile differ from a traditional tin-lead process?
The main difference when using tin-silver-copper as the paste alloy will be the peak temperature. This alloy melts between 217-221 C; peak temperature will be between 230-255 C depending on the thermal mass of the assembly. It is recommended that the time above liquidus (TAL), be under 90 seconds to avoid charring of the residues; this will also reduce the incidence of intermetallics.
Should a nitrogen atmosphere be used in the reflow process?
The use of nitrogen will depend on the solder paste flux chemistry. New developments in flux chemistries for lead-free pastes do not require nitrogen to achieve good wetting and solder joint integrity. Nitrogen like in the tin-lead system will offer smoother solder joints and better wetting will be gained with the use of nitrogen. Will the higher temperatures needed for melting lead-free solder create more fumes and condensate in the reflow oven? New formulations of no-clean and water washable pastes are designed for lead-free alloys, and therefore decomposition by-products are not significantly more than tin-lead bases solder pastes. If a formulation is not designed for higher peak temperatures in the range of 230-260C as would be expected with tin-silver-copper solder paste, decomposition of flux materials may be slightly more pronounced causing a large amount of condensate to form with the reflow oven and exhaust system.
Will residues from lead-free, water-soluble paste be more difficult to remove?
If tin-silver-copper solder paste is used the peak temperature will be higher, if the flux formulation is not designed for higher temperatures removal may be more difficult. The removal of flux residues may require an assembler to re-evaluate the cleaning chemistry. In some cases increasing the pressure of the cleaning solution, reducing conveyor speeds or the temperature of the solution may promote effective cleaning. The residue’s cleanability may be further degraded with double sided reflow where the residues from the first reflow are further baked onto the board. Selecting a paste with good cleaning properties is essential.
What happens to no-clean flux residues at the higher reflow temperatures?
Since time above liquidus temperatures are higher when using tin-silver-copper, flux residues may exhibit darkening when reflowed in air. A flux system designed for lead-free will not exhibit this phenomena to a great extent. The flux residue will tend to polymerize rendering it harder, increasing the pressures required to make them probable. Pin-testable flux residues remain soft after reflow and will be pin-testable, the flux is designed not to harden at the higher peak temperatures.
Is the appearance of the lead-free solder joints different from those made with tin-lead solder?
Traditional Sn63 solder offers bright solder joints after reflow, tin-silver-copper alloys give dull joints with a slightly crazed surface; this is typical of this alloy and does not suggest poor solder joints. The other difference that will be noticed is the larger contact angles and reduced wetting around the pads, tin-silver-copper doesn’t wet as rapidly and completely as Sn63.
What are the main soldering defects associated with lead-free reflow soldering?
Lead-free can increase the incidence of solder defects and requires a good understanding of the properties of the lead-free alloy and flux system to prevent them. Defects such as bridging, non-wetting, de-wetting and the potential for solder balls can increase. Choosing the correct flux chemistry compatible with the metals to be soldered, and having an optimized reflow profile will prevent the increase in defects. Insuring the solderability of the boards and components by proper storage and handling methods will also enable good soldering with lead-free pastes. If the chemistry is chosen carefully and the SMT process controlled the results in yields will be the same as the Sn63 process.
What other issues can be expected with the conversion to lead-free reflow soldering?
Some of the issues to address before and after the implementation of a lead-free SMT can be summarized as follows:
- Determining process compatible lead-free board finishes
- Determining availability of lead-free components
- Determining thermal compatibility of both boards and components to new thermal profile
- Selecting solder paste chemistry to suit assembly process and the soldered assemblies reliability and operating conditions
- Process optimization and statistical process control development
- Training of operators and line managers to new lead-free process
- Material and logistical control for dual systems, if running both a leaded and a lead-free process
- Defining a proper rework process for lead-free assemblies
- Identifying the lead-free assembly for field service
What lead-free solder alloys are recommended for wave soldering?
Sn96.5Ag3.0Cu0.5 and Sn99.3Cu0.7 will be the alloys recommended for wave soldering. SnAgCu has a faster wetting speed and greater solderability than SnCu. Can I use my present wave soldering machine, and what changes are needed? There are a couple of issues that need to be addressed with the wave soldering machine.
- Convection type preheats will be the best heating method for lead-free wave soldering.
- Due to the corrosive nature of the tin on the solder pot lining, it may need to be replaced or re-lined.
Will there be more dross generated with lead-free alloys? There will roughly be twice as much dross generated with lead-free alloys. SnCu will dross slightly more than SnAgCu. Nitrogen blanketing of the solder pot will reduce dross dramatically. Will my present liquid flux adequately solder with lead-free solder alloys? Fluxes traditional designed for Sn63 soldering may not offer the good wetting and hole-fill required for reliable lead-free wave soldering. The activator package in fluxes for lead-free are more thermally stable and can sustain the slightly higher soldering temperatures. This thermal stability will enable the flux to be active throughout the contact time with solder. No-clean fluxes are particularly sensitive to excesses in solder temperature, water washable fluxes depending on the activators used and also due to the higher content of activators may be suitable with lead-free. However, liquid fluxes are being designed specifically for lead-free wave soldering, these are recommended. Is nitrogen needed at the wave? Nitrogen is not necessary, but may prove beneficial for the following reasons:
- Nitrogen will eliminate most of the dross being generated.
- Nitrogen will improve the wetting and spreading of the solder on the printed circuit assembly.
- Nitrogen will give shinier joints with a lower contact angle.
How are the solder alloy metals and impurities controlled in the pot? Silver and copper tend to not dross out of the solder pot. Depending on the metallization on the board silver (Immersion Silver) or copper (Bare Copper or OSP over Copper) may actually leach into the solder pot. Adding pure tin to the pot will control the alloy. Impurities such as copper, palladium and silver will need to be monitored because they raise the melting point of the alloy. For instance the melting point of a lead-free alloy will increase 25°C for every 1% of copper in the solder alloy. Will the solder joints made with lead-free solder look like those made with tin-lead? Lead-free solder joints will look different than leaded solder joints. They tend to look grainier with a larger contact angle. The surface of the joint may look crazed or even have micro-fractures. This is usually a surface condition caused by the cooling of the joint.
What is fillet lifting, and how is it prevented?
Fillet lifting is caused by a combination of lead from either a HASL board or leaded components plated with tin-lead and a bismuth containing solder alloy. A tin-lead-bismuth rich layer forms just above the intermetallic layer. This layer has a low melting point of 96°C. The rest of the joint cools and contracts but the layer next to the board is still liquid. When the tin-lead-bismuth finally cools and contracts there is not enough solder to fill the gap caused by the contraction. This phenomena can be reduced or eliminated by reducing the amount of preheat added to the board and a higher pot temperature. Thermal shock to the components will need to be considered with this method. A second solution is to quickly cool the board as soon as it comes out of the wave. The obvious prevention is to not use lead with lead-free. Will the same soldering fluxes used for tin-lead work for lead-free soldering? Some tin-lead fluxes do work with lead-free soldering. A better choice may be to evaluate and find a flux designed for lead-free since the lead-free fluxes almost always work for tin-lead soldering. Lead-free fluxes will be primarily water-based (VOC-free) because of their ability to handle the higher temperatures associated with lead-free soldering.
What are the main qualification tests for using liquid flux?
The main qualification tests that will be required will be to determine adequate hole-fill in relation to board and component finishes. No-clean fluxes will be less active and require particular attention, when compared to water washable organic acid fluxes, which will exhibit the best hole-fill. Having optimized preheat temperatures, solder pot temperatures, and flux volume, the conveyor speed may have to be reduced to accommodate the solder wetting when using lead-free solder. The flux will play an important role in maintaining production yields. Lower solids fluxes may require faster conveyor speeds to avoid complete flux burn-off, higher solids fluxes will be more forgiving and conveyor speeds may not have to be reduced.
How do I control the Pb, Cu and Fe concentration in my Lead-free soldering pot? Which steps do I need to make to stay within the limitations for above elements?
In Lead-free wave soldering the Cu, Fe and Pb concentrations need to be followed very closely. Increase of Cu concentration will generate an increase in melting point of the alloy and increase of viscosity of the solder. This will result in slower wetting and reduced hole fill. If i.e. SAC305 is used the volume compensation of the solder pot should be done with SAC300 (without copper) in order to maintain a constant Copper concentration. The Pb concentrations need to be kept below 0.1% wt. If not all components are lead-free the Pb concentration will increase rapidly. As the initial Pb value of the solder bars will be between 0.05% and 0.1% the limit of 0.1% can be achieved rapidly. Only partial removal of the contaminated solder pot will get the Pb concentration back below 0.1% wt. If the Fe concentration increases in the solder pot this indicates that the solder pot is slowly dissolving because of the Pb-free alloy. High Sn concentration (all Pb-free alloys) is capable of dissolving mild steel. If this happens the Fe concentration will increase rapidly and Fe-crystals can be seen on the surface of the solder pot. In this case the solder pot has to be changed or coated and the solder bar has to be replaced. Analysis of the solder pot is done by means of Atomic Absorption Spectrophotometric (AAS) or Inductive Couple Plasma (ICP). Other methods such as X-ray analysis are equally possible. Kester has laboratories capable of doing such analysis including XRF.
For lead-free wave soldering, what is the recommended control limits for impurities such as lead, copper and silver for good control?
The recommended control limits for lead is max 1000 ppm, which is in accordance with the RoHS regulation. There is no industry control limits for Copper and Lead in SnAgCu solder pot. However, it is suggested that for silver impurity, it should be controlled at a tolerance of +/- 0.2%. For copper impurity.it is recommended to control below 1%. It is also recommended that customer monitor the impurity level with the respective yield as the impurities may vary based on the process, board finish and components and then derive a suitable limit for process control.
What is a good method for solder bar addition for lead-free solder bar?
If SnAgCu solder bar is the intended alloy for lead-free, then the recommended lead-free bar for addition to the solder pot should be either SnAgCu or SnAg3 bar depending on the type of board finish used, and process conditions.
How does one minimize solder tearing in lead-free joints for SAC solder after wave soldering and does this affect reliability?
Typically solder tears can be observed when using lead-free solder SAC alloy, particularly in wave soldering and with lead contamination present. This has not been considered as a defect as per IPC-610D specifications. Typically this is a cosmetic effect and some studies have been conducted by the industry that solder tearing does not lead to reported reliability failures. The mechanism for this phenomenon is caused by differences in solder solidification, which is affected by the lead from either the PCB or component lead plating. The differences in solder solidification could result in the solder shrinkage that could result in solder tears visibly noticed at the surface. In certain cases, when the board cooled, the contraction may even result in pads lifting from the surface of the laminate. This was reported in certain studies that lead or bismuth contamination of the alloy close to the joint interface can lead to a difference in solidification rates in the joint. Hence to minimize solder tearing, this may done by super-cooling the board, eg implementing an additional cooling zone when the board immediately exits from the wave. Hence this requires the combination of efforts of the machine vendor.
Is it possible to decrease dross (oxide) generation when using Sn96.5Ag3Cu0.5 solder?
By adding Ge to Sn96.5Ag3Cu0.5 solder, a decrease of dross generation is possible. Ge content (from 0.01% to 0.05 %) is usually effective in reducing oxide formation. Other causes of increased oxides are high pot temperatures, turbulent wave dynamics and the increase of certain metal contaminants such as zinc, iron, and copper.
Is there a lead-free solder that prevents dissolution of Cu?
It is possible to prevent Cu dissolution by making Cu content 2-6% of the lead-free solder. When Cu content of solder is increased, liquidus temperature also rises unfortunately. Higher soldering temperatures could lead to reliability issues for components and boards. Also certain additions of nickel to lead-free solders particularly with SnCu solders can help in reducing solder pot dissolution.
What lead-free solder alloys and fluxes can be used for hand soldering?
Popular lead-free wire solders in use today are Tin-Silver-Copper (melting point 217-221C), Tin-Silver( melting point 221C), and Tin-Copper( melting point 227 C). All three alloys are available with no-clean, water washable or rosin systems and can be drawn down to the finest wire diameters. These alloys have been used for hand assembly on lead-free products and are compatible with lead-free alloys.
Do lead-free solder alloys require soldering irons with hotter tips?
Hand soldering with lead-free solder wire doesn’t necessarily require higher soldering temperatures and tip temperatures from 700-800 F will enable adequate soldering. Operators will notice the wetting will be slightly slower than traditional Sn63 solder, and an incremental increase in contact time may be required to achieve good results. The solder joint finish will look different also the slightly duller finish is however typical of the lead-free solders mentioned above. One consequence of using lead-free solders with high tin content is tip erosion, iron tips may require replacement more regularly.
What are the main concerns associated with reworking lead-free BGAs?
BGA components can see higher temperatures during the de-soldering and soldering process, tin-silver-copper has a melting point of 217-221 C. Excessive localized heating can cause board damage and in the case of component attachment, damage the BGA’s reliability. Excessive heating should be avoided. There are excellent BGA reworking equipment designed for lead-free soldering which prevent this by directing a controlled amount of localized air or nitrogen underneath the component, coupled with good bottom side heating excesses are avoided.
What soldering fluxes can be used for reworking lead-free solder joints?
Lead-free soldering doesn’t differ from Sn63 soldering. Fluxes are available in the no-clean, water washable and rosin formats to suit any hand soldering and rework process. Water washable flux types due to their higher concentrations of activators will solder more effectively, no-clean fluxes are traditionally made with weak organic acids and solder more slowly, and are more prone to de-activation when exposed to excessive heating.
Will there be more smoke or fumes when soldering with lead-free solders?
There are new fluxes for lead-free soldering, designed with heat stable flux systems. These fluxes do not decompose at the slightly higher temperatures that could be associated in a lead-free process.
Is a nitrogen atmosphere needed for hand soldering?
The use of nitrogen-assisted rework is not a necessity if a flux designed for lead-free soldering is used. A good solder manufacturer will insure the flux chemistry remains active at the higher soldering temperatures. The fluxes for solder wire use and flux gels used in rework will have stable activators and resins designed for the specific alloy and the process temperatures they will be used in. Nitrogen use will however, reduce oxidation, enabling the use of less active fluxes and lesser quantities of flux for adequate soldering.
How can I develop a good lead-free hand-soldering process, which will ease the operation?
In a recent study, which appeared in the Lead-free Update by TechSearch International in December 2004, hand-soldering was found to be more problematic to implement when compared to lead-free wave soldering and SMT. The reason could be that hand-soldering is more operator dependent than reflow and wave soldering but also the surface tension in lead-free solders is slightly higher. Wetting or spread is also a little slower when compared to 63/37. To reduce operator issues and reduced wetting proper optimization of the soldering process is key. To avoid issues use a flux content of 2-3% by weight in the solder wire, use a solder tip temperature of 700-800º F. Also Tin-Silver-Copper (SAC) solder will flow more readily than Tin-Copper (SnCu) solder. The main issues encountered with lead-free hand-soldering are cold solder joints, poor wetting and de-wetting. These can be avoided.
Will the fluxes used for tin-lead alloys work with lead-free solder?
Generally the same fluxes will work for lead-free solder. One factor is that the organic flux ingredients begin to decompose and deteriorate above 200°C and more rapidly as the temperature heads up to 250°C. The water-soluble fluxes are more active and more heat-stable than the no-clean fluxes that are designed to self-destruct with the heat of soldering. The VOC-free no-clean fluxes can withstand the heat better than the alcohol-based no-clean fluxes. Heat stability is more evident with solder paste. The water-soluble types are more heat-stable, and the no-clean types for tin-lead solders do not work well with lead-free alloys because of the increased reflow temperature, i.e., 240°C for lead-free compared to 210°C for tin-lead.
What chemistry changes are required for the fluxes designed for lead-free assembly?
Since lead-free alloys have a higher surface tension a good activity flux is required. The formulations change required are new activator systems, which can sustain higher preheat and soldering temperatures. The flux systems will also require new wetting and gelling agents able to sustain these higher temperatures. If new chemicals aren’t added to fluxes a higher incidence of defects would occur. In reflow soldering new resins and gelling agents are used to give the solder paste added hot slump resistance. This property gives the solder paste a lesser likelihood of creating mid-chip balling, solder balls and bridges. In liquid flux formulations new activators are used to give the liquid flux sustained activity after emerging from the wave solder, reducing icicling and bridges. The activators are also more thermally stable to take the higher pot temperatures.
What is the best pre-heat configuration for a Pb-free compatible wave-soldering machine using VOC-free flux?
For VOC-free fluxes or water-based fluxes the pre-heat temperature has to reach 100°C (measured on top side of the assembly) for at least 20-30 seconds. Ideal pre-heat configuration is:
- First zone: calrod, a medium wave heat source that allows the flux (read water) to evaporate without being moved. Any air turbulence in the first zone should be avoided since the liquid flux will be blown under components or between board and carriers.
- Second zone: forced convection. Here the air turbulence will allow the moisture to evaporate, dry the board and activate the flux. The second zone should also take the assembly over the 100°C level
- Third zone: could be any type of pre-heat system. The third zone will maintain the temperature over 100°C without going to warm. It will optimize the activation of the flux, add some heat into the assembly in order to minimize the T when entering the wave