Lead-Free Soldering  [ Return towards  Applications  ]

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 factors come into play - toxicity, availability, price, worldwide distribution, wetting ability, reliability, etc. Efforts have 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. Till date, nobody is able to find an ideal combination.

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 more expensive but also heightens its tendency to oxidize dross and react with other metals. It is a myth that with higher tin content, it would improve the process of wetting. Only with nitrogen, can we achieve similar wetting like eutectic tin/lead composition.

The question of inerting becomes even more critical when other solders are considered - indium, zinc, bismuth and antimony. They are just some elements that may be found in our solders in the future. Majority of these 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 the electronic components and packaging industry. There seems to be little doubt that nitrogen "improves" the process. However, its cost-effectiveness and technical necessity are still questionable, due to the difficulties associated in evaluating the benefits of nitrogen that are often process-specific and reliability based.

The introduction of the lead-free debate has intensified the need for accurate and specific information. Initial data and reports published concluded 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 presentation (SMTI - San Jose CA), "Standard SMT equipment can be used to process lead-free. The only unique requirement is a nitrogen-inerted atmosphere for soldering".

Controlling the oxygen level in the atmosphere improves wettability, solders 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.  

Potential issues w/ pb-free	N2 impact & solution
• Excessive dross in Wave Soldering	• N2 reduces dross by 80 % and up.
• Cost Alloy & Dross is 2-3 Sn / Pb	• Less dross means greater savings.
• Reduced Wetting	• N2 reduces oxidation and increases wetting.
• Narrower Process Window	• N2 extra wetting forgives process variations.
• Flux Browning	• N2 reduces flux oxidation and flux baking.
• Cleaning	• N2 avoids flux 'cooking' so that cleaning is much easier.
• Thermal Shock & Popcorning	• Higher T Pb-free reflow will need very dry SMDs. N2 helps keep SMD dry.
• Baking of all SMDs may be necessary	• N2 dry storage keep SMD dry and eliminate the need to bake.

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. Even though the industry is constantly looking for ways to reduce cost, the ultimate decision to use nitrogen should be based on a solid cost-vs.-benefit analysis that goes beyond the unit cost of nitrogen itself. Testing is recommended even for lead-free soldering. Nitrogen is inevitably an added cost to the process, but its overall benefits should outweigh the additional expense of adopting it.