Implementing Lead Free.

Electronics manufacturers worldwide are converting to a lead-free soldering process for a variety of reasons including environmental concerns and compliance with impending regulations. The development of practical lead-free alloys and flux chemistries, as well as more sophisticated soldering equipment, have made lead-free soldering a process whose time has come, and one that can be implemented successfully at the production level.

Successful implementation of lead-free soldering requires careful planning and rigorous process monitoring to ensure quality and process control. For some time, a diverse team of engineers has been developing a practical, robust methodology for lead-free process implementation, which has resulted in a five-step lead-free soldering program to help those who are responsible for implementing lead-free wave or reflow soldering. The five-step program is based on modern techniques such as problem-solving/process-improvement models (plan, do, check, act), quality engineering, design of experiments and statistical process control. The goal was to create a repeatable lead-free soldering process that will deliver consistent high yields.

This program begins where most lead-free papers stop, at the point of beginning to actually solder lead free. Many recently published papers discuss legislation, marketing, temperature and alloy selection issues, but generally do not describe how to actually implement the process. What must be done and investigated before the first lead-free soldered boards leave the assembly line? How can the new process be controlled and improved after implementation?

Today, knowledge of lead-free soldering is based on feasibility studies of several consortia, wetting balance tests and university investigations. Very little knowledge is based on real lead-free soldering in the production environment. Most companies that have been soldering with lead-free alloys for two years or more are automotive or other special applications, where assemblies are subjected to temperatures in excess of 150 degrees C. The experiences of these companies teach us more about issues such as solder contamination, yields, parts wear, costs, board warpage and process control aspects, whereas feasibility studies and other studies help us with material selections.

The 5 Steps Program Overview

- Step 1: Select the right materials and equipment from alloy and flux selection to necessary equipment capabilities.

- Step 2: Define the process. Define process parameters and conduct Taguchi experiments and data analysis.

- Step 3: Develop a robust process. Collect and analyze data, do the homework and get the "bugs" out of the process.

- Step 4: Implement lead-free manufacturing. Begin production, monitoring the process carefully and making needed changes throughout the process.

- Step 5: Control and improve the process. Constant follow-up, monitoring and data analysis keep the process under control.

Step 1: Select The Right

Materials and Equipment

Arguably, no alternative alloy exists to tin/lead (SnPb) with the same melting point, reliability and process-friendly characteristics; no drop-in solution currently exists. Consequently, whichever alternative lead-free alloy is chosen, its implementation will affect the entire product and process, including the selection of new fluxes, board finishes and components.

In the process of lead elimination, we find three distinct areas of attention:

-Selection of lead-free solder paste (reflow) and alloy (wave). The materials and technologies to utilize them are available currently.

- Elimination of halogenated flame retardants from board materials with lead-free board finishes. Board material must be able to withstand higher temperatures.

- Components' lead-free finish material is dictated by the lead-free solder alloy/paste materials selection. Plastics and other component materials should have a higher heat resistance.

Alloy Selection

The first selection is the alloy. Numerous lead-free alloys are available today including patented ones. Use the following criteria to select an alloy:

- Research by universities and consortia. Very useful information can be found in research work available on the Internet and in papers:

- IPC Roadmap, A Guide for Assembly of Lead-Free Electronics; www.leadfree.org

-The DTI report Lead-Free Soldering and the Update 2000; www.npl.co.uk/ npl/ei/news/pbfree.html

- Improved Design Life and Environmentally Aware Manufacturing of Electronics Assemblies by Lead-Free Soldering; IDEALS, Marconi Materials Technology Ltd., Philips CFT, Siemens AG, Multicore Solders Ltd., Witmetaal BV and NMRC 

- Road Map 2000 for Commercialization of Lead-free Solder, ver.1.2, Japan Electronic Industry Development Association.

- Experiences from other companies already producing lead-free products. Although most companies do not mention defect numbers, they can provide information about solder contamination, process adjustments, solder temperatures, parts wear and flux residues and their contamination factors.

- An excellent guide to commonly recommended lead-free solders for reflow and wave can be found at www.Pb-Free.com, in the FAQ section.

-The availability of elements like indium and gold is limited; therefore, costs are high (> $150 per kg). Price and availability issues reduce the field of candidate elements to tin (Sn), silver (Ag), zinc (Zn), antimony (Sb), copper (Cu), bismuth (Bi) and cadmium (Cd). Cadmium is toxic and, therefore, is banned from the list. Antimony is also toxic, but only at temperatures above 630 degrees C (its melting point), when toxic fumes are emitted. Bismuth has recycling and fillet lifting issues. Tin is most suitable, exhibiting low cost and excellent physical, electrical and thermal properties. However, other elements must be added to lower tin's melting point to an acceptable level.

- Wetting balance tests indicate the solderability of available alloys. However, wettability in wave soldering is a function of solder temperature, contact time, flux, use of nitrogen and wave configuration.

- Melting point will also be one criterion for alloy selection, because the process temperatures, peak and time above liquidus for reflow and solderpot temperature and contact time for wave soldering will very much depend on melting temperature. The lower the melting point, the wider the process window, which makes designing a capable process much easier.

- Thermal properties of the alloy are critical. For example, automotive requirements range from -40 degrees to 150 degrees C in the engine compartments. In contrast, some consumer applications only have to deal with ambient temperatures of minimum 0 degrees and maximum 60 degrees C. This narrow range allows some consumer electronic suppliers to use small amounts of bismuth to reduce melting temperatures.

These issues all reduce the list of suitable alloys (Table 1). The final recommendations for alloy selection are as follows. Two alloys are suitable for reflow and wave soldering: SnAg3.5 and SnAgCu (Ag>3.5 percent). For SnAgCu, several patented alloys and compositions are available.

For consumer electronics, SnAgBi is a good alternative. SnCu0.7 can be used in wave soldering if cost is a main consideration and the used materials are compatible with higher temperatures, because the solder temperature must be at or above 270 degrees C to achieve sufficient through-hole penetration.

Flux Selection

Because the wetting characteristic of lead-free alloys is not as good as tin/lead at lower temperatures, a good flux is very important. Temperatures in lead-free processing have increased, whereas the fluxes that are used today for lead-free soldering are the same as used for tin/lead.

Flux's main job (flux/vehicle in solder paste also) is to remove and prevent oxidation. For both wave and reflow processes, activation temperature and activation time during heating and soldering must be reformulated for use with lead-free alloys. For the reflow process, the flux should be able to withstand the higher peak temperatures (up to 250 degrees C) at the same time above liquidus. For wave soldering, the flux must be able to withstand 130 degrees C preheat and 280 degrees C solder liquidus temperatures for approximately 3 seconds of contact time.

Water-based fluxes with no volatile organic compounds (VOCs) are the primary candidates, but solder balls and voids must be reduced. New technologies such as dissolving rosin in VOC-free, water-based fluxes are currently being investigated.

Requirements that the selected flux must meet include the following:

- comply with appropriate legislation and be environmentally friendly 

-must enhance solderability and be compatible with all materials used in the lead-free process

- must have an acceptable cost.

In general, a VOC-free, water-based flux is recommended. The minor objections related to these materials should be resolved in the very near future.

Board Material and Finish

For lead-free soldering processes, the focus is on FR-4 halogen-free material. Halogen-free boards are desirable because the current European Community draft proposes to phase out halogenated flame retardants by 2004. Again, the new materials must be able to withstand the requisite higher temperatures. They must also exhibit no warpage or delamination of the layers at these higher temperatures. To meet these requirements, board materials should have higher glass transition temperature (Tg) numbers; 150 degrees C and higher. Halogen-free boards are already available for 1.33 the price of conventional FR-4.

Most printed circuit boards in today's soldering processes have a tin/lead coating on the pads, as well as in the through holes. This coating is applied using hot air solder leveling (HASL).

Organic solderability protectants (OSPs) are an alternative finish, which are currently available in the same price range as HASL. OSPs are easily processed and are relatively free of ionic contamination. However, some concerns exist, particularly low storage life, and excess handling can create process reliability problems. Multiple soldering processes and soldering at higher temperatures can create problems with regard to oxidation, but an inert atmosphere may help.

Electroless nickel/gold (NiAu) is another promising alternative. Nickel/gold is superior to other finishes with regard to shelf life. It is also quite resistant to damage and free of ionic contaminants. Nickel/gold is preferred for higher temperatures and multi-soldering processes. The costs are approximately 25 percent higher than OSP.

Electroplated pure tin exhibits excellent solderability and corrosion resistance. However, electroplated tin is prone to whisker growth. Heat fusing or reflow is used to eliminate tin whiskers. Whiskers are promoted by internal stress. Important factors include grain structure and organic impurities in the deposits. Electroplating tin chemistry is currently robust and well-designed. Tin whisker growth seems to be under control with current technologies.1

Component Material and Finishes

Several electronic components are already available with lead-free finishes. Alternatives are pure tin or tin/copper. In general, the changeover to lead-free finishes for components is expected to be the last part of the total lead-free process conversion and certainly the most difficult step of lead-free implementation. Component manufacturers have concerns about going lead free with respect to reliability and higher process temperatures.

Current technology for certain components, such as ball grid arrays (BGAs) and relays, is not so advanced that all of these components can meet the new requirements for heat resistance of 260 degrees C for 10 seconds. New molded components with solutions to popcorning and delamination need to be designed and qualified. Compounds used for these components should also be halogen-free. Higher temperatures can cause warpage with connectors and BGAs. Most companies complain that lead-free components are not available for the market, as component manufacturers appear to want to produce lead-free products for higher volume demand.

Are Selections Compatible with Legislation?

Once all selections for the lead-free materials have been made, the selected items should be reviewed and compared with legislation in the countries in which they are intended to be used. Are all materials compatible with the higher soldering temperatures and do they meet customer specifications? If all materials are within specification and the price is acceptable, then the machine configuration should be reviewed.

Wave Soldering Machine Configuration

The main difference between tin/lead and lead-free soldering processes is the increased melting point of the lead-free alloy. Lead-free alloys require higher preheat temperatures; the thermal shock at the entry of the first wave may not exceed 100 degrees C. VOC-free, water-based fluxes are recommended for lead-free soldering because they can meet these new higher temperature requirements and are also environmentally friendly.

Due to the different thermal properties and the viscosity of water, flux suppliers do not support foam fluxing for VOC water-based fluxes. Therefore, the fluxing machine must be retrofitted with a spray fluxer if a VOC water-based flux is used. If it is not, a more robust, heat-resistant flux must be selected for foam fluxing; the foam tube and density control unit have to be verified for this flux.

Spray fluxing a VOC-free, water-based flux requires some special attention. First, the atomizing air should be increased (with approximately 0.2 bar) to achieve the finest possible droplets. A special small fluid cap might be required to spray an amount of flux, which is almost one-third of the amount of a no-clean, alcohol-based flux. The spray pattern can be inspected with a glass plate or water-sensitive paper. The through-hole penetration of the flux can be examined by a spray flux uniformity measurement system.

The topside temperature of the board will increase to 110 degrees to 130 degrees C at the end of the last zone, to limit the thermal shock at the entrance of the first wave. The best way is to heat up the board as quickly as possible to 100[fraction one-quarter]C. Then, continue with hot air (forced convection) for optimal evaporation of the water in the through holes, underneath components and between pallets and boards. Depending on the type of production, the third preheat zone can be hot air, infrared lamps (special for mixed production) or calrod heaters.

For these VOC-free fluxes, all water must be evaporated before entering the first wave to prevent spattering. Proper preheating requires at least a heating length of 1,800 mm for conveyor speeds of 120 cm/min and higher. Particularly for high speed soldering (>180 cm/min), a preheating length of 2,400 mm is recommended. If high throughput is important and decreasing conveyor speed is not allowed, a longer preheating unit and higher solder temperatures may be required.

Solderpot temperatures will increase, depending on the alloy. For AgSnCu with a melting point of 217[fraction one-quarter]C, the solder temperature will be from 250[fraction one-quarter] to 270[fraction one-quarter]C. For high melting alloys like tin/copper, the solder temperature will be 260[fraction one-quarter] to even 280[fraction one-quarter]C. In general, the higher the solder temperature, the better the solderability, and the shorter the contact times. The contact time is defined by the conveyor speed and the wave former configuration and settings. Solderability can be improved by using nitrogen, a stronger flux or a "smart" wave (Figure 1).

The main differences between tin/lead and lead-free heating profiles are shown in the increased preheat and solder temperatures (Figure 1). If the peak soldering temperature is limited, the contact time will invariably increase to achieve good wetting.

Reflow Soldering Machine Configuration

For reflow soldering, the implementation of lead-free alloys impacts temperatures and, consequently, the heating profile. Increasing the melting temperature means that the process window becomes narrower, because both time above liquidus and the maximum allowed temperature of 250 degrees C (to prevent component damage and board delamination) do not change (Figure 2). A key distinction between the tin/lead and lead-free profiles is higher process temperatures and, therefore, a smaller process window.

Board warpage becomes an issue if temperatures increase, and the materials used in the process become more critical. A proper board support or gripper system might prevent board warpage.

Heat transfer becomes more important as the process window shrinks. Three principles of heat transfer can be applied in a reflow process. A choice should be made between radiation, hot air/nitrogen convection or vapor phase. Lamps (radiative) will not meet lead-free requirements, and throughput in a vapor phase process is too low. Thus, hot air convection is the most suitable way to reflow lead-free solders.

Using nitrogen is recommended for OSP finishes. An inert atmosphere will prevent oxidation and improve wetting. However, use of an inert gas blanket also adds cost to the process and may introduce other possible defects such as tombstoning.

Process specifications such as oven configuration, heating length and cooling length will be defined by such factors as required machine throughput, board dimensions, distance between boards, required exit temperature and temperature profile based on the solder paste and component specifications.

Heating zone setpoints are determined by the profile for the product. Several peak zones may be required. Try to achieve the smallest temperature differences at the entry of the peak zone as possible. The goal is to keep the board above liquidus temperature long enough for the coolest spot on the board to solder properly and to not exceed the maximum limitations for the hottest spot on the board.

The melting point of the selected alloy plays a dominant role in this process. Therefore, alloys with higher melting points, such as SnCu (227 degrees C), will probably not be used for reflow. Alloys like SnAgBi (melting range of 200 degrees to 216 degrees C), SnAgCu (217 degrees C) and SnAg (221 degrees C) are recommended.

The long-term reliability of a surface-mount solder joint is affected by the cooling rate of the solder during solidification. In the lead-free process, even more attention should be paid to cooling aspects because of the complexity of the components and substrates. Cooling has a direct and profound effect on the following:

- time above liquidus

- solder joint grain structure

- protection of OSP coatings

- product exit temperature

- material damage

- residual stresses in solder.2

Reflow equipment should be selected that offers multiple options and flexibility in terms of controlled cooling, which is essential in lead-free soldering where temperature control is critical. In general, a lack of standardization exists in lead-free soldering and also with regard to cooling.

The remaining four steps will be described in individual articles in the next four issues of Circuits Assembly.

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References

1. Zhang, Y., et al. An Alternative Surface Finish For Tin/Lead Solders: Pure Tin. Murray Hill, NJ: Lucent Technologies Bell Laboratories.

2. Hall, W. Cooling Parameters in Reflow Soldering. Stratham, NH: Vitronics Soltec Corp.

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Dr. Denis Barbini is the advanced technologies manager with the Surface Mount Laboratory, Vitronics-Soltec, Binghamton, NY; e-mail: [e-mail]. Gerjan Diepstraten is a senior process engineer with Vitronics Soltec BV in The Netherlands; e-mail: [e-mail].

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