Process:Lead-free wave soldering: Is process changeover really simple?

Wave-soldering process, compared to reflow process, is subjected to several critical factors.

In July 2006, the changeover to lead-free production comes into effect for the electronics manufacturing industry. There are substantial fines for those who are late in making the change.

However, the process changeover may not proceed as simply as expected. This article examines the problems faced by companies during the changeover to lead-free production.

Fujitsu Siemens Computers' motherboard goes "green"
Fujitsu Siemens Computers, Augsburg, Germany, a manufacturer of professional PCs, workstations, notebooks and servers, as well as system-boards and keyboards, claims to be one of the most modern computer manufacturing centers in Europe. The company manufactures, as well as concentrates on research and development, procurement and production, supply chain management and logistics.
In mid-2002, the company offered limited PC edition with "green" Pentium-4 board. This board contains reduced lead content print circuit boards (PCBs) and lead-free solder (tin-sliver-copper alloy). Most of the halogens and halogen-containing compounds in PCBs have also been replaced and reduced respectively. During this process changeover to lead-free, Fujitsu Siemens used production lines with machines and units that represent different technology levels; and has clearly defined the required quality and machine availability (Figure 1).


Figure 1: Fujitsu Siemens' process engineer, Juergen Felgner (left), and Seho wave solder systems' R&D manager, Markus Walter (right).

Intially, test parameters are established in the soldering machines. This ensures good soldering performance during the initial phase of changeover to the lead-free wave-soldering process. During continuous improvement in solder wave, F-nozzle (Figure 2) is used to achieve optimal results. The F-nozzle produces a defined, turbulent surface of the wave and creates parallel wetting. Optimizing the nozzle has approximated the optimal flow characteristics of the actual wave. The occurrence of solder balls, which may be frequently observed in nitrogen environment, is reduced to a minimum.


Figure 2: F-nozzle which is used to improve solder wave.

Fujitsu Siemens evaluated machine technology positively. "Machine concept has a beneficial effect on maintenance times," comments Juergen Felgner from productions engineering, Fujitsu Siemens Computers. "Access for maintenance and cleaning have improved. All controlling modules include preheating, conveyors and quick acting locks and the units may be exchanged easily and quickly."
Felgner has also seen specific process-technological advantages such as separate fluxer, integrated convection preheating and convective cooling module directly integrated after the solder bath, during soldering machine evaluation. Says Felgner, "These features offer advantages relative to other machines. Lead-free solder alloys require higher processing temperatures. The quick and targeted cooling of the assemblies before exiting the soldering machine affects metallurgical formation of the solder joints and further handling of the assemblies."

"This has been a long improvement process - from the first results which we could gain on the lead-free line -all the way to the achievements of today," explains Felgner. "Particularly, PCB material and components require extensive optimizations to be realized step-by-step."

Siemens drives environmental protection
Siemens in Amberg, Germany, a provider of electronic time relays, communicating engine protection and controlling devices, is convinced that the use of lead in electronic products will end one day, no matter when and how fast the legislator will rule accordingly. The company has looked for competitive advantages by offering lead-free industrial products to the market early.

How does quality change for the process? What is the long-term stability of the entire process? How does the new process affect cost? These have to be addressed quickly in order to identify the impact of lead-free technology on large production environments.

For a long time, the Siemens plant at Amberg has put in efforts in environmental protection. The oxides (dross) and lead-containing dust, which are generated during tin-lead wave soldering, will require special attention and strict observance of the respective safety precautions to protect production personnel from possible health risks.

"The process of conversion is not that simple, because we are using a wave soldering machine, which we have operated with lead-containing solders for the last 10 years. This unit has to be refitted for the lead-free process," explains Thomas Gubisch, who is responsible for Siemens Amberg technological planning. "Generally, a three-metal alloy containing tin, silver and copper is more difficult to control than the traditional two-metal alloy."

One of the most difficult tasks, according to Gubisch, is to establish an optimal thermal profile. Machines will not be the source of the problem as long as the required temperatures can be maintained. The trouble is to qualify components for a maximum temperature of 260¡C. To avoid any reliability risks for the products, the temperature in the lead-free wave soldering process cannot exceed 260¡C, under any circumstances.
A well-defined process, which initially requires time-consuming measurements and research, has to be developed to establish the maximum preheating temperature that will not damage the components, "Step by step, we have to approach the optimal profile for each product," explains Gubisch. "Consequently, we have learnt that the preheating temperature strongly influences the rise of solder in through-holes and the crystalline structure of the joint."

Lead-free solder, specifically tin-silver-copper alloy, which has low specific gravity, exhibits a different flow pattern compared to conventional tin-lead solder. "Many tests are necessary to approach the optimal solder nozzle configuration," remembers Gubish. "We require a solder nozzle, which will create a high wave pressure and at the same time supply sufficient wetting time."

Siemens, similar to Fujitsu Siemens, has its lead-free products processed in a nitrogen wave-soldering machine. "The process window is considerably smaller than when soldering with lead," states Gubisch. "Small tolerances, such as the printed circuit board distance to the solder wave, have an immediate impact upon the soldering quality. The nitrogen atmosphere may partially balance occurring problems and widen the process window. An additional positive side effect of the nitrogen atmosphere is its lower maintenance requirement, particularly in the soldering area, compared to an open machine."

"We have experienced very little oxidization with the nitrogen machine, just some 'black powder'- tin-oxide, between the solder nozzles. During a two-shift operation, the solder bath is cleaned every five to seven days and the solder nozzles are cleaned every month. For such maintenance practice, the accumulated dross amounts to approximately 0.5kg. As a result, using a full-tunnel machine for the lead-free soldering process, has resulted in enormous oxide reduction and consequently, significant savings of expensive material such as lead-free solder.

On its lead-free line, Siemens has used a semi-aqueous no-clean flux, consisting of approximately 40 percent water and 60 percent alcohol, to complete the ecological aspect.

After one year of lead-free mass production, Gubisch remains positive, "The road to an optimal process is not without problems. However, the decision to switch at an early date is correct. Meanwhile, we are also able to optimize our cost. Overall, the production costs for lead-free products are 0.8 percent higher than before the conversion."

Hella's lead-free challenge for the automotive industry
Automotive supplier, Hella in Germany, has succeeded in solving the lead-free soldering problems relatively quickly and without many complications. The quality standards are very demanding. "As much as possible, 0ppm defects are targeted, with a high level of uptime when operating the machines during 3 shifts," comments Volker Kramer, who is responsible for Hella's wave soldering technology.

Hella's reasons for process changeover to lead-free are less influenced by competitive or environmental aspects. The company is dominated by mechanical and thermal stress in its products. For some time, the automotive industry has required higher application temperatures including temperature ranges to 130¡ C, sometimes even to 160¡ C.


Figure 3: Uncoated impeller wheel after six months of use with lead-free solder.

During the process implementation of lead-free wave soldering at Hella, material-related machine parts problems such as de-alloying and leaching are found during the pre-process trials and tests (Figure 3).
Lead-free solder has damaged machine parts severely, particularly those parts in direct contact with the solder alloy such as the solder chimney and solder-wave pumps. Some of these high-grade specialty steel parts are "chewed up" by the constituents of the solder during long tests runs. This abrasion phenomenon has further shown that de-alloying will cause iron enrichment in the solder bath due to the increased diffusion reaction of the higher tin content in the solder, particularly in the quiet areas of the solder nozzle (Figure 4). Materials, which are used in mass production equipment as well as various alternative coatings, are also tested for wear-resistance and leaching. After several tests, a composite coating and special titanium alloys exhibiting positive characteristics are further investigated and subsequently optimized. Finally, they are defined as the standard solution for the solder bath, solder pumps and chimneys, resistant to different lead-free solder alloys.


Figure 4: Iron enrichment observed in the solder bath due to increased diffusion reaction of the higher tin content in the solder.

"The nitrogen atmosphere plays a more important role in the lead-free soldering process, compared to lead-containing soldering process," adds Hella's Volker Kramer. A highly concentrated nitrogen atmosphere is favorable for lead-free processes for these advantages:

Extends the restricted process window for lead-free solder alloys

Gives better wetting for lead-containing solders

Allows operation at lower temperature, which reduces stress on circuit board material and components

Uses less aggressive flux, which minimizes migration risk under moisture condensation conditions

Causes lower oxidation (dross) due to lower oxygen content, and less solder is used.

A nitrogen atmosphere reduces costs in machine maintenance, overheads and solder materials; and increases reliability of the assembly at the same time.

JVC, Matsushita Panasonic: Full or partial inerting?
The advantages of a protective nitrogen atmosphere particularly for lead-free processes should not be rejected off-hand. Nevertheless, there remains the question if it should always be a full nitrogen system - a tunnel machine in which the oxygen-reduced atmosphere does not only exist in the soldering area but extends into the whole preheating zone.

Particularly, small and medium-sized producers of electronic components and producers with only a small amount of leaded components, such as JVC in Berlin and Matsushita Panasonic in Eastern Europe, may hardly be able to change their production accordingly due to the investments involved. These companies have demonstrated that there may be other possibilities such as using equipment without full inerting. JVC in Berlin, a manufacturer of video recorders and camcorders, operates high quality at low cost. The intense competitive pressure from Asia has motivated the conversion to a lead-free production.

In its lead-free conversion, JVC has used a special soldering machine - an open wave-soldering system (Figure 5). "The conversion of the process for our printed circuit board production has encountered hardly any problems," explains Dieter Muessig, manager, production planning, JVC. "The local nitrogen atmosphere satisfies our requirements entirely. Even the maintenance and service hours in the solder bath area are within the limits."


Figure 5: An open wave soldering system with a nitrogen hood ensures local inerting of the solder wave.

However, there is room for improvement with regards to rework, according to Muessig. Repair with lead-free alloys has become costly, as the solder-iron tips, which previously are replaced on a monthly basis, must now be replaced more frequently. "The soldering-iron tip material is no longer resistant against the more aggressive lead-free solders," reasons Muessig. "We expect that the solder tip producers will increase their research and come up with more solder resistant tips."

Matsushita Panasonic had similar negative experience with quickly worn-out solder-iron tips and is not particularly happy about this cost explosion.

As a result of higher melting points of the lead-free solder alloys and the required temperature increase of less than 40¼C into the wave, higher energy is required in the preheating. This allows faster convection and thus the material is subjected to fewer stresses, compared to standard radiation energy. Modular design of the machine is recognized as an advantage as these machines may be upgraded with convection preheating at any time. Modular design in the system also allows separate conveyors for the fluxer and the adjoining preheating area, as well as the possibility to relocate the fluxer module outside of the process chamber. This avoids flux accumulation in the process area and consequently accounts for a substantial reduction of maintenance cost, as the flux residues cannot be transported into the preheating area.

Conclusion
Lead-free production can no longer be avoided. Depending upon application, the costs for the conversion will differ. There is, however, unanimous consent that without a thorough preparatory work for process control, the road into the lead-free age will be bumpy.

You can reach Heike Schlessmann at heike.schlessmann@seho.de; and Markus Walter at markus.walter@seho.de