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Process Control For Lead Free Wave Soldering

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Once the decision has been made to convert a tin-lead wave solder process to lead-free, the need for changes in the existing process control procedures becomes an important step to success. There are a number of changes to be made in how to control a lead free wave soldering process. The key areas of concern are element concentration control, equipment corrosion inspection, and dross reduction control. These controls will require close cooperation between the production personnel, maintenance, and the solder supplier.

The ability to control element concentration is very important to achieve successful lead-free soldering. In the tin-lead process, the wave solder pot is typically analyzed every 6 months. The key elements reviewed are tin, copper, silver, and gold. The concern with tin is that it preferentially drosses out faster than lead so over time the tin content will drop and pure tin has to be added to the pot to maintain the correct tin level. Copper, silver, and gold are carefully monitored because these are the surfaces coming in contact with the solder and will dissolve into the bath. If they exceed recommended maximum levels, it could have a detrimental effect on the solder joints. Excess copper can cause increased defects due to the increase in the pasty range of the tin-lead not allowing the solder to completely drain from the joints before solidification occurs. Increased silver content can have the same effect. If the gold concentration is too high, it can cause brittle joints.

The control of elements in a lead-free wave solder process is similar to tin-lead with different concerns. Instead of being a major component in the alloy, lead is now a major contaminant. There will be two major lead contaminant potentials that have to be carefully controlled. The one that will affect all wave applications is lead contamination from component plating. Lead-free wave soldering has been in high volume production use for more than five years and leaded components had to be used due to the lack of lead-free components for all parts required. Depending on the percentage of components with lead plating, the lead level may increase to a level the industry considers too high for reliable joints. Testing of the tin-copper-nickel alloy has indicated that the alloy can tolerate lead levels of 0.2% for double-sided boards and 0.5% for single-sided boards. The graph below illustrates the defect rate increase in the SN100C allow when lead contamination was 0.2% and 0.6%.

Unfortunately, these levels will not be allowable under the current interpretation of the RoHS requirements. This new interpretation will only allow a maximum of 0.1% lead contamination in solder joints for products offered for sale in Europe after July 2006. This low level will be a concern with many production lines due to lead plated components. The chart below follows the lead content in two representative wave solder lines in high volume production for the last 10 months. Despite the use of some components with lead plated leads, the lead content in the pots have stayed steady. Pot 1 will have to be diluted to bring it back down below 0.1% before July 2006 but the lead content should be controllable with the greater availability of lead-free components in the next 2 years.

The other major lead contamination concern will only affect companies running both tin/lead and lead-free wave solder lines. The concern is the accidental inclusion of 63/37 bar into the lead-free solder pot. With a production limit of 0.1% and the typical lead content of new lead-free bar at up to 0.05%, this allows for very little accidental inclusion of 63/37 bar into a lead-free pot. The following calculation can be used to determine how much 63/37 bar can be accidentally added to a lead-free bath before it reaches the RoHS limit.

X = P(1-10L)/369
Where
X= Amount of 63/37 bar added to reach 0.1% limit
P= weight of lead-free solder in pot
L= weight percentage of lead in lead-free pot
As an example, only 1.4 lbs. of 63/37 bar will contaminate a 1000 lb. pot with an initial lead content of 0.05%!

The 0.1% limit allows very little 63/37 contamination in a pot and so it will be very important that proper controls are in place to eliminate this possibility. The industry is considering different bar box markings/colors, and different bar shapes to assist companies in their control processes.

In addition to lead, copper will also have to be carefully controlled. Unlike 63/37, copper is an integral ingredient in all lead-free alloys being considered for wave soldering. These alloys are utilizing the tin/copper eutectic as the base alloy which contains copper at approximately 0.7%. If the copper is allowed to increase above 1.0%, there is a chance that the tin/copper intermetallic may precipitate out and collect at the bottom of the pot as it has a higher density than the lead-free alloy. It then has a chance of interfering with the solder pump. Most solder suppliers are issuing recommendations for maximum copper contents during production and as an example, SN100C has a recommended copper content range of 0.5%- 0.85%. The chart below illustrates the copper range for two lead-free wave solder pots being used in high volume production for the last 8 months. The copper content has been easily maintained within the recommended limits. All lead-free processes will utilize a two alloy system for maintaining the copper content. The target alloy will be used to fill the pot. Top-up solder will be either the target alloy or a copper reduced version as the copper dissolving into the pot from the soldering process may be adequate to keep the copper within the recommended limits. This will require close monitoring of the solder pot and it is recommended that the solder pots be tested at least monthly to determine the trend for the copper levels.

CUSTOMER INTERVIEW

SAMSUNG ELECTRO-MECHANICS (KOREA)

Adopting SN100C for the tuners that have the top share in the world market

Samsung Electro-Mechanics is a leading manufacturer of a wide range of electronic components in Korea. They provide many kinds of electronic products, such as multi-layered circuit boards, chip components, devices for mobile communications, computers, audio/video equipment, and set top boxes. On top of that, they have approximately 22% market share in the world for tuners. They were certified ISO14001 in 1996, and take their environmental policy seriously. They adopted lead-free solder in August 2000 for power components, and aim to change all their products to lead-free including solder joints by the end of 2004.

Mr. Sean Yoo, the manager of material research lab at their R & D center had started to research lead-free solder in 1994. He developed lead-free solder alloy and had it patented. Netsurfing the data of lead-free solders in the world on the web, he recognized the existence of “SN100C” in the end in 2000. Studying the properties and the reliability through the evaluating tests, he decided to adopt SN100C for the production lines of tuners and adopters in June 2002.

“The tin-silver-copper alloy is identified as the main option for wave soldering by many people. But concerning the cost, the reliability of the solder joint and the patent situation, we choose SN100C for wave soldering”, said Mr. Yoo. “In commercial production, the reliability of solder joints of SN100C is the same as that of tin-lead. The productivity of this alloy is further than that of any other lead-free alloy. Someone says that lift-off fillets occur when through hole boards are soldered with SN100C, but that problem was cleared by our manufacturing technology.” He notes.

Their factories are going to a broad step-by-step basis; manufacturing lines such as tuners are exported to Chin a and Thailand. “The lines at Tenjin, China will be started this year. We will also adopt SN100C for those lines.”

SN100C has been patented by Nihon Superior and is offered worldwide through licensed manufacturers including FCT Solder for North America. We offer SN100C for wave solder, lead tinning applications, and as solder paste and SN100CL for lead free hot air solder leveling (HASL) of P.C. Boards.

We will be including in future newsletters, many of the advantages our current customers have discovered in switching to SN100C as their lead free alloy of choice and we welcome your comments and questions. We look forward to working with you to make your transition to lead free as successful as possible.

For more information about our products, please visit our web sites at www.finelinestencil.com for stencil products and www.fctassembly.com for solder products.

All files for stencils, templates and PCB carriers should be sent to sales@finelinestencil.com to assure immediate attention.

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