New avenues for wave soldering and lead-free conversion: an electro-magnetic-based technology provides cost-effective lead-free wave soldering.

Several areas still need researching when switching to lead-free solders. So far, most research projects have concentrated on the properties of the new solders and the reliability of the joints, as these factors are of immediate importance. However, far less information has been accumulated from a manufacturing viewpoint. Process parameters, as well as defect rates, are of particular interest to manufacturers. In addition, the capabilities of current process equipment are also important, particularly in wave soldering, to meet the higher temperature and higher tin content requirements of most lead-free solders.

Contrary to many statements found in the literature, the introduction of lead-free wave or reflow soldering is not as straightforward as one is usually led to believe. Both small and large problems will arise that may cause the process engineer to wish for the "good old times" of lead solders.

Lead-Free Challenges

Introducing lead-free wave soldering normally requires the user to, cope with two major changes: a higher tin-content of the solder and a higher process temperature. Whereas eutectic tin/lead (SnPb) solder contains approximately 60 percent tin, the lead-free solders contain more than 90 percent tin. The melting point of the eutectic SnPb solder is close to 180[degrees]C, but the melting points of lead-free solders range from 220[degrees] to 250[degrees]C. Assuming that the solder pot temperature has to increase by about 50[degrees]C, the soldering temperature would be around 280[degrees]C. (1)

A few obvious problems with lead-free solders include: oxidation; residue levels and cleaning; dross; nitrogen; solder pot protection; and cooling.

* Oxidation. The new alloys all need higher process temperatures because their increased tin content increases the tendency to oxidize. Although higher tin contents would lead one to believe that wetting could be improved, this condition is generally not true. As a result, the process window shrinks.

* Residue levels and cleaning. Longer dwell times in preheat and higher preheat temperatures require changes in flux composition, which chemists suggest increase the rosin content and decrease the solvent part. More rosin will help to protect the activators but also will leave more residues. More rosin will also be in the pot. Due to higher temperatures and longer exposure times, the residues will be harder to clean. (2)

* Dross. Although some reports indicate that the generation of dross will not be increased, more trustworthy research indicates that five to seven times as much dross may occur when operating in air. As 90 percent of the cost of bar solder derives directly from metal cost, the higher metal cost will be felt immediately. More dross also means more maintenance.

* Nitrogen. Nitrogen is typically justified for SnPb solder by a cost calculation on dross for SnPb alloys. The quality and reliability are normally bonuses. With higher dross and much higher alloy prices, nitrogen will be a definite necessity for lead-free solders. Alternatives include raising the superheat temperature or increasing flux strength, but these options pose significant process challenges)

* Solder pot protection. The new lead-free alloys have lower density, so bolts, components and tools that used to comfortably swim on the SnPb solder may now sink. This condition can lead to solder contamination or jam the mechanical pumps.

* Cooling. The product's exit from the soldering machine will have to be changed; cooling of the product will require more time or additional cooling means. Assemblies may be hotter and thus cannot be handled immediately.

However, the real surprises come when the new solders are put into existing soldering equipment, assuming that all the lead was actually removed. After a few weeks or months, problems start to happen. Soldering becomes difficult and wave height is no longer achieved. These problems are due to the increased chemical reactivity of tin at high temperatures. Specifically, the increased tin in the new lead-free solder starts to attack the materials of the pot and pump.

Wherever friction and movement between solder and steel are particularly pronounced, the attack is most obvious. Hence, the pump and impeller first show signs of deterioration. Pitting is followed by real corrosion, the pump impeller is "eaten" away and pumping action deteriorates. Then pitting occurs in the pot walls, which may lead to pinholes and liquid solder leaks.

As the tin dissolves the iron in the stainless steel, iron inter-metallics are formed in the solder; contaminants and soldering difficulties then result. To prevent these problems, for example, one company that has used tin/silver (SnAg) solder for several years changes the solder every month, the pump every six months and the pot every year.

Experiments have revealed that, using the above temperature settings, tin/silver/copper (SnAgCu) solder shows iron inter-metallic needles after eight hours of operation in new stainless steel pots. Solder equipment manufacturers had been warned of such occurrences, and they researched new materials for the pots and new coatings for existing steels that would avoid such problems. Alloys do exist that can withstand tin scavenging, but they are extremely expensive and very hard with which to work. Several coatings have been identified that also could offer a solution under certain circumstances.

However, users still face the cost of buying a new soldering machine that often does not even have a convincing track record as manufacturers have generally opted to develop totally new equipment. Mass production with lead-free solders is still relatively rare and mostly limited to companies that worked with "better" solders rather than intentionally targeting lead-free. Because no fundamental change in the concept has been achieved, neither the pumping principle nor the pot volume has changed.

An Alternative Solution

Impeller pumps with pressure chambers or channels are found in most current soldering machines. However, a new soldering machine has been developed that relies on electromagnetic pumping principles. In this machine, an electric current circulated around a magnetic coil induces a magnetic field. The combination of the current and the magnetic force creates the necessary pump action to push solder through the nozzle.

The electromagnetic pump requires a much smaller solder volume, does not produce any vortex around the pump shaft and has no moving parts. This pump produces a solder wave that is flexible yet stable, with a high reach up to 28 mm (Figure 1). The new machine also features a cast iron soldering pot with a coating that is non-conductive and resistant to attack by tin.

[FIGURE 1 OMITTED]

Electromagnetic soldering machines require protection from oxidation as dross greatly reduces the system's performance. When they were first introduced, the electromagnetic machines were equipped with oil dosage systems because their original design was not suitable to nitrogen inerting. However, the use of oil was inconvenient and only relatively narrow wave widths were available, so these first machines were limited in popularity.

With the reintroduction of the lead-free debate a few years ago, electromagnetic soldering systems have generated renewed interest, particularly in Japan where lead free is at the forefront. However, the traditional electromagnetic system has been improved in several ways.

First, a nitrogen inerting system has replaced the messy oil intermix. Second, because the North American market in particular demands wider waves, new solder pots have been developed that offer up to 500-mm-wide flows. The nozzles and nitrogen elements are now made of titanium, but the pot is still made of coated cast iron. Such units are also available as retrofit solder pots. These new electromagnetic systems will work with both plated through-hole and surface-mount components, and they are fully compatible with lead-free solders.

System in Operation

After initial laboratory trials to test the improvements, the new electromagnetic system was tested at an automotive components manufacturer. The system was initially placed on a production line with SnPb solder where it was further tested and fine-tuned. Once the system performed satisfactorily, it was cleaned and moved to a lead-free (SnAgCu) line. Automotive products requiring high temperature resistance were assembled on that line.

After several months in production, the components manufacturer's opinion was highly positive, as the quality of the lead-free solder joints appeared to be excellent. The defect levels under lead-free conditions compared favorably to those of a similar product soldered on a different line with SnPb alloys. Particularly, the use of nitrogen seemed to improve the new system's performance.

Besides reduced maintenance and very little dross, the nitrogen inerting also enhanced wetting. Assemblies that had required additional flux deposit are now soldered with half as much flux and still show significant defect reductions. Although extremely low oxide production was met, the long time assessment of an optimization procedure finally settled on a nitrogen consumption level of around 14 [Nm.sup.3]/h. At that gas flow, the average dross collected was 27 grams per hour (g/h) under lead-free (Figure 2) as compared to 12 g/h initially obtained with SnPb alloys.

[FIGURE 2 OMITTED]

Conclusion

These results have prompted the application of this system as a lead-free retrofit. Accordingly, the system was redesigned to make it retrofittable into almost any existing soldering machine. The small pot size including the electromagnetic pump provides plug-in, high performance capability at a fraction of the cost for a new soldering machine.

Companies that want to start making lead-free product may now do so with their existing equipment by simply removing their solder pot/pump assemblies and replacing them with the retrofit pot assembly. This compact system provides full lead-free compatibility, nitrogen inertion for enhanced wetting and much smaller pot volumes for decreased costs of lead-free alloy fill.

References

(1.) Baggio, T. and K. Suetsugu. 1999. Guidelines for lead-free processing, SMT, September.

(2.) Kenyon, W. 1998. Potential impact of lead-free solders on cleaning, SMT, November.

(3.) Hunt, C., et al. 2002. Evaluation of the comparative solderability of lead-free solders in nitrogen, Proceedings of APEX 2002.

Martin Theriault is global market manager of electronics manufacturing with Air Liquide America Corp., Houston, TX, e-mail: martin, [e-mail]. Armin Rahn is principal with Rahn-Tec Consultants, St. Catherines, Ontario, Canada; e-mail: [e-mail].