Lead-Free Soldering  [ Return towards  Clean Processes  ]

• Why go Lead-free ?

Legislation :
• Lead Reduction
• End of Use

Market advantage:
• Differentiation - Ahead of Competitors
• Green Awareness in Consumers

Technical advantage:
• Joints support higher temperature
• Some alloys show greater reliability

The problem of soldering without lead is an intricate one. Many different aspects play into it: toxicity, availability, price, worldwide distribution, wetting ability, reliability, and many more. Much effort has gone into finding a replacement for 63/37 Sn/Pb, not only because of its lead contents (most specialists agree that the danger posed by lead in the solder is rather limited) but because there is a general need for a ‘better’ solder. However, so far nobody seems to have been successful in finding an ideal combination that could replace (as a drop-in) the solder of 5,000 years ago.

The few potential choices (tin plus some copper; tin plus some silver; tin plus some copper and some silver, etc.) all need higher process temperatures and thus may not really work without good inerting. The increased amount of tin in the solder not only makes them somewhat more expensive (lead is cheap!) but heightens the tendency to oxidize, dross and react with other metals. Although the higher tin content would lead one to believe that wetting could be improved, this is not generally true. Only under nitrogen do we generally achieve similar wetting as for the eutectic tin/lead composition.

The question of inerting becomes even more critical when other solders are considered: indium, zinc, bismuth and antimony are just some elements that may be found in our solders in the future. Most such solders show very bad wetting properties in air and some of them are even marginal under nitrogen.

Using nitrogen to inert the atmosphere during soldering of conventional Sn / Pb alloys has become a generally accepted practice in electronic assembly. There seems to be little doubt in the minds of the user that nitrogen "improves" the process. However, its cost-effectiveness and technical necessity are still questioned very frequently. A reason being that the benefits of nitrogen are difficult to evaluate since they often are process-specific and reliability based.

The introduction of the lead-free debate has intensified the need for accurate and specific information on that topic. Initial data and reports published all came to the conclusion that lead-free soldering with nitrogen is highly recommended. The need for a controlled atmosphere in lead-free soldering is even greater than with traditional Sn/Pb alloys. As Mr. Bill Trumble from Nortel Networks expressed in a paper presented (SMTI - San Jose CA): "Standard SMT equipment can be used to process lead-free. The only unique requirements is a nitrogen-inerted atmosphere for soldering". Controlling the oxygen level in the atmosphere improves wettability, solder joint appearance, and impacts the level of board residues. Better wetting could even allow lead-free process temperatures to be close to current peak reflow temperatures as long as the thermal demand of the product is not a significant issue. This could eliminate component thermal shock issues altogether.

• Issues w/ Pb-free vs. N2 solutions

Despite these findings, a nitrogen process should, however, not be adopted without fully understanding its benefits and limitations. As the pressure to reduce cost in the industry is continuously increasing, the ultimate decision to use nitrogen should be based on a solid cost-vs.-benefit analysis which goes beyond the unit cost of nitrogen itself. Testing is recommended for that purpose even for lead-free. Nitrogen is inevitably an added cost to the process, but its overall benefits normally should outweigh the additional expense for a user to adopt it.