BOC Seminar – National Motor-cycle Museum, Birmingham, UK

Citation

Judd, M. (2001), "BOC Seminar – National Motor-cycle Museum, Birmingham, UK", Soldering & Surface Mount Technology, Vol. 13 No. 3. https://doi.org/10.1108/ssmt.2001.21913cac.002

BOC Seminar – National Motor-cycle Museum, Birmingham, UK

BOC Seminar – National Motor-cycle Museum, Birmingham, UK

Keyword: Lead-free soldering

After more than 50 years of using lead-containing solder, the electronics manufacturing industry is now facing a processing revolution, a challenge which must be quickly overcome, if it is to embrace new technologies without compromising yield or reliability. Driven by threatened legislation in Europe, consumer and marketing pressures in Japan, and global market considerations in the USA, there is a rapidly growing interest in using lead-free solders for assembling printed circuit boards (PCBs).

Whilst lead-free solders avoid the environmental issues associated with lead toxicity, they do bring other problems. When compared with the universally used tin/lead solder, most replacement alloys melt at higher temperatures. Indeed, the melting point of the most widely accepted replacement for conventional SnPb solder, SnAgCu, is about 30-358C higher. In addition, these lead-free materials have poorer solderability, and are more expensive. But it is the necessity of working at higher temperatures that poses the most serious challenges. What effects will the higher soldering temperature have on component durability and processing? Will it affect reliability?

Although lead-free soldering is now accepted as technologically possible, a major concern is the acknowledged narrowing of the process window (the latitude in getting it right), and the effect it might have on yield. Combined with the higher soldering temperatures, this places significant demands on the assembler in terms of tighter process control.

It now appears that these worries can be eased by careful implementation of a technology that has been around for many years, i.e. nitrogen inerting (soldering in an atmosphere of nitrogen rather than air, which effectively displaces oxygen and limits its corrosive effects). We now know that by widening the process window inerting can aid the adoption of lead-free technology with acceptable yields. The value of inerting when soldering lead-free materials was clearly emphasised during the recent UK fact-finding mission to Japan. The mission, sponsored by the DTI and led by SMART, found that nitrogen inerting is currently widely used throughout the Japanese industry, and its use is expected to increase.

But elsewhere arguments continue. Advocates of inerting claim that, apart from widening the process window, it can reduce oxidation and improve the solderability of lead-free systems to a level close to that of SnPb. For wave soldering, it also produces less dross, involves lower flux and solder consumption, and enables soldering to be carried out at lower temperatures, facilitating the use of lead-free solders with melting points closer to that of SnPb. The lower processing temperature is beneficial in several ways, including imposing smaller thermal stresses on the components, and allowing the use of cheaper solders. Its detractors point to the high costs of nitrogen and the purchase (or modification) of ovens capable of maintaining both the necessary low oxygen levels (< 100ppm) and the uniformity of temperature throughout the oven. On another front, the importance of inerting is increasing, as the industry moves towards ever finer pitch product. Inerting gives better yields by reducing the incidence of defects such as shorts and opens.

The good news is that our understanding of the process is improving with more relevant data becoming available. Indeed, there is considerable effort around the world to characterise the process more fully, quantify its advantages, and optimise the combination of lead-free soldering and nitrogen inerting.

For example, work carried out by The National Physical Laboratory (NPL) has thrown light on real benefits of using nitrogen. In wide-ranging studies varying the solder, flux, component termination material, PCB finish, soldering temperature and soldering atmosphere, they have shown that nitrogen inerting can largely compensate for the poor solderability experienced when using lead-free solders. In many cases the addition of an inert atmosphere during lead-free soldering can allow a ~308C reduction in soldering temperature and give the same solderability as soldering with SnPb in air. They have demonstrated that inerting is especially beneficial at low soldering temperatures.

The question of oven suitability is crucial, and it is clear that many existing ovens will not be capable of delivering the tight process control or the uniformity of temperature distribution required by the new lead-free soldering technologies. Inerting and/or new equipment may be necessary.

What the manufacturing industry world-wide is seeking is not only the availability of the inerting gas and of new oven technology, but also an underpinning support for all aspects of the technology. The BOC Group is an acknowledged leader in this respect. It not only supplies the gas, but it is also in the vanguard of developing equipment tailored and optimised for the needs of lead-free soldering. Importantly, it is also responding to the industry's call for underpinning knowledge, by generating such information and sharing it via an innovative new Web site, CATweb.

Lead-free soldering technology is still in its infancy with technical and cost issues posing major challenges for the industry. But it is clear that soldering in a nitrogen atmosphere can overcome many of the technical barriers and provide soldered products comparable with those using conventional lead-containing materials processed in air. Its use is likely to become the norm, if not necessity, for a large section of the industry.

Mike Judd