Lead-free Solder and the Rework Process

Estimated reading time: 10 minutes

In three months, the electronics industry will shift from a proven, highly reliable, lead-containing solder alloy to a variety of lead-free alloys - creating extra work and raising concerns regarding long-term reliability. Despite the challenges that lead-free solders pose, technology will adjust. resulting long-term reliability is still uncertain, and several issues must be considered before implementation.

By Patrick McCall

The electronics industry is facing a shift from a proven, highly reliable, lead-containing solder alloy to a variety of lead-free alloys, which will generate a lot of extra work and raise concerns regarding long-term reliability and quality. The change is drastic and uncertain; and RoHS compliance may prove to be a costly exercise for most manufacturers and OEMs. Still, the list of RoHS exemptions is growing. There currently are 14 exemptions, 8 under review, and 19 pending. These ultimately will take the teeth out of the RoHS Directive, leaving us to wonder how effective it will be when 41 potential exemptions lie in the balance - and more being applied for weekly.

With respect to the portion of RoHS that is driving the move to lead-free solder, the electronics industry uses only 0.5% of the total lead consumed in all manufacturing worldwide. Batteries, munitions, and pigmentation manufacturing consume well over 90%. One has to wonder if the environmental justification will have a positive return on investment (ROI).

Regardless of the challenges associated with lead-free solders, technology has improved to a point where implementation is feasible. However, resulting long-term reliability is still uncertain. There are several issues that must be considered as we move forward with the implementation of lead-free solder. If you haven’t started on RoHS compliance already, you may soon discover that you are running out of time.

Lead-free Solder and Your Process

Lead-free solders generally are comprised of 95% or more tin (Sn). Tin is corrosive to other metals; and lead-free solder lacks many desirable physical properties that have made lead-containing alloys the standard in electronics assembly. Compliance is complicated by the fact that there is no single drop-in replacement for leaded eutectic solder. While there are several lead-free alloys, a few have floated to the top of the acceptable list, including Sn/Cu, Sn/Ag/Cu, Sn/In/Cu, and Sn/Ni/Cu. Some practical differences between these alloys and their lead-containing counterparts include higher melting temperatures; less desirable wetting and spreading properties; they are difficult to work with; and they result in dull, grainy finishes.

Because lead-free solders have higher melting temperatures than solders containing lead, there is an immediate tendency to increase the temperature. Higher operating temperatures do not accelerate the process, and can introduce risk factors. Leaded solders melt at 363°F; and lead-free solders melt at about 425°F. A tool with tip temperatures set at 650° or 700°F will still melt the solder. The issue isn’t temperature; it is the ability of your iron and tip configuration to transfer heat to the work efficiently, and the heater’s ability to keep up with heat loss during the work. High-performance tips are available from several manufacturers, and an evaluation should be done to understand how heat is being transferred efficiently from your existing tips to the work.

Lead-free solder generally does not wet or spread as well as lead-containing solder. When creating a joint, additional dwell time is required to ensure a good joint is made. Turning up the temperature to go faster will only result in a poor-quality joint. Operators must slow down and be patient. When working with lead-free solder, the likelihood of leaving bridges, opens, and insufficient solder defects behind is higher than with lead-containing solder.

With the exception of nitrogen-containing lead-free alloys, the visual finish of the joint is different from what operators are used to seeing. Operators and inspectors will need to be retrained and acclimated to work with lead-free solder.

Because lead-free solders oxidize quickly, more aggressive and longer-lasting fluxes are required to keep surfaces being soldered clean and free from oxidation. Working with no-clean fluxes can be a challenge because their process window often is small. Once they have burned off, oxidation begins to form immediately, resulting in a marginal or defective solder joint. When using lead-free solders, rosin-containing fluxes often are easier to work with than non-rosin-containing fluxes. Using higher tip temperatures also exacerbates oxidation.

Due to the high tin content in lead-free solder alloys, there clearly will be an increase in the likelihood of common defects that had been under control previously. For example, tin whiskers, which can cause bridging, will return as a more serious problem because the average pitch of current component leads is a fraction of what it was years ago.

Without standardization of a single alloy, we are likely to see various alloys in use as companies begin to comply with lead-free initiatives. This creates two problems from a rework standpoint. First, it creates potential compatibility issues between alloys. It will be impossible for a rework technician to visually identify which alloy has been used on a given PCB. Solder alloys used in initial manufacturing and on the benchtop, as well as component lead plating and tip tinning, must be compatible with one another to avoid creating an uncontrolled alloy - the result of mixing several unknown solder alloys in unknown quantities.

It has been suggested that PCBs and components be labeled to track the alloy used. In reality, this would be a huge undertaking and likely would not solve the problem fully as operators will have a variety of solders on their tips and at their workstations. It should be noted that research continues on the compatibility issue and the effects of mixing different alloys. Adjusting an alloy component by 0.5% can change joint wetting and strength characteristics, as well as melting points. Each alloy and melting point provides different joint characteristics and potentially requires different flux chemistries.

Is Existing Equipment Compatible?

There is no doubt that lead-free solders will affect soldering and rework equipment negatively; in particular, tip life (Figure 1). Lead-free solders contain high percentages of tin, which when mixed with the iron (the hard, protective layer on soldering tips), forms an intermetallic compound that wears away more quickly than the iron would by itself, or when used with lead-containing solder. This causes two problems. One is shorter tip life as the protective iron coating is dissolved in the tin. The second is that oxidation forms more quickly, and the high-temperature environment in which the tip works further exacerbates this. Once tin oxides begin to form, the tip loses its ability to wet with solder. If it is not cleaned immediately, it becomes nearly impossible to remove, and the tip must be replaced. Many suppliers have moved from providing a wet sponge to clean the tip to providing an abrasive process (usually metal filings). But this can cause further tip wear-and-tear.

Figure 1. Lead-free rework station.

When using lead-free solders (regardless of alloy), it is important that tips are maintained properly. Tips should be cleaned frequently to remove oxidation before it becomes impossible to remove. Tips should always be tinned when not in use. If the iron will not be used for extended periods of time, the temperature should be turned down to below solder-melt temperature; or it should be turned off. Optional accessories, such as instant temperature setback iron stands, are also available for soldering and rework systems. This option reduces tip temperature when the iron has been idle for a specified amount of time, and the iron is reactivated immediately once it is removed from the stand. This helps preserve tip life.

Many new technologies in tip manufacturing designed to harden the tip to the corrosive effect of lead-free solders have emerged over the last 12 months. Some manufacturers use a coating to extend tip life. Others have changed or modified the protective layer of the iron on the tip, while others have increased the amount of iron on the tip significantly, which doesn’t solve the problem - the iron still wears away, leaving irregular and/or jagged geometries. An additional iron can also affect heat transfer adversely.

Figure 2. Nitrogen farm for a single soldering station.

It is important to verify that your tip supplier is using lead-free solder to tin their tips at the factory. Before using a new tip, it should be heated, and the factory tinning should be wiped off. The tip should be re-tinned with the solder to be used. This process should be repeated several times to minimize alloy contamination. If your tips have been tinned with lead-containing solder, they cannot be used in a lead-free process, as the lead cannot be removed completely and will contaminate any solder in which it comes in contact. To maximize the life of your soldering tip:

1. Use the lowest effective temperatures while soldering.
2. Avoid aggressive fluxes, when possible.
3. Use a properly sized tip. Tips that are too small will wear out, while tips that are too large will wear unevenly. This will change tip geometry, rendering it useless and possibly damaging pads.
4. Tin tips when not in use and after cleaning. A coating of solder will prevent the iron from oxidizing in air, causing tips to lose tinning or wetting capabilities.
5. Feed flux-cored solder wire into the heated work, not the tip. Feeding solder into the tip directly will create pinholes in the iron plating, and the tip will fail quickly. It also causes the flux in the solder wire to be burned off before it can activate and properly prepare the surfaces being soldered.
6. Should tips lose tinning or wetting capabilities, a tip cleaner may be used to restore them.

Does Nitrogen Help?

The use of nitrogen-assisted soldering equipment mitigates some problems associated with using lead-free solders. Nitrogen helps on two fronts. First, it creates an inert environment around the soldering tip, reducing the potential for tip oxidization, which would reduce its ability to transfer heat and hold solder. Second, it assists with the soldering process at the PCB level by purging oxygen from the immediate area, reducing or eliminating the formation of oxidation on the worksite. This not only reduces the amount of flux required, but also helps improve wetting and spreading, and leaves a shinier, less-grainy finish.

Figure 3a. Leaded eutectic solder in normal atmosphere. Figure 3b. Lead-free solder in normal atmosphere. Figure 3c. Lead-free solder in nitrogen atmosphere.

Nitrogen-assisted soldering systems pass nitrogen through or around the heater before it is directed to the worksite, which pre-heats the immediate area and can reduce thermal shock to components and leads. Pre-heating also enables the use of lower, safer, and more-effective soldering tip temperatures. Nitrogen-compatible soldering systems are not significantly more costly than standard counterparts, and many manufacturers offer low-cost, add-on accessories to use nitrogen in the soldering process (Figures 3a, b, and c). The issue to consider is where does the nitrogen come from? Basically, there are two choices. The first is to use bottled nitrogen. A tank supplies the nitrogen where it is compressed and piped to the rework area. A pressure regulator and airflow valve with a cut off is needed to reduce the pressure from the tank and control the airflow to the hand piece. Nitrogen from a tank is pure; however, it is not recommended for use in an enclosed area unless careful consideration has been exercised over air changes. If the nitrogen component increases, and the oxygen component decreases, an unhealthy situation could occur. The other option, which is preferred, is to use a nitrogen farm. Nitrogen farms harvest the gas from a dry, filtered, compressed air supply that is passed through a specialized filter (Figure 2). The other atoms that comprise air are forced through the filter, leaving a pure stream of nitrogen as the filtering product. Nitrogen farms are passive-collection devices, meaning there are no electrical or moving parts, little or no maintenance, low running costs, and they maintain the balance of oxygen and nitrogen in a confined space (as long as the compressed air is pulled from the same space). Once the nitrogen has passed through the hand piece and back into the ambient air, it will recombine with the other elements that create the air molecules before filtering. It can be filtered again without changing the balance between oxygen/nitrogen concentration ratios in the room. Nitrogen farms are available in capacities from single stations - up to 10 and 20 stations.

Conclusion

There are many issues to address when integrating lead-free solders into your processes, and in meeting the RoHS requirements. Many major concerns have been addressed, but undoubtedly others will surface. The best way to ensure you will meet the July 1, 2006, RoHS deadline is to get an early start so that there is time to address every issue and aspect of your operating procedures, existing equipment, and employee training.

Patrick McCall, director of engineering and product services, PACE Inc., may be contacted at (301) 490-9860; e-mail: pmccall@paceworldwide.com.