Component finishes

Components have traditionally used tin-lead coated leads and terminations to provide a solderable finish compatible with tin-lead based solders. It is important that the Lead-Free alternatives do not compromise solder joint reliability. There are many potential Lead-Free component finishes and these include tin, tin-silver-copper, tin-copper, tin-bismuth, nickel-palladium, and nickel-palladium-gold. In the short term it is likely that pure tin will be the preferred alternative although by the time industry has gone completely Lead-Free, there may be an increase in the use of tin-bismuth alloys. For semiconductor lead-frame applications tin-lead plating is being replaced with pure tin, although an underlying nickel layer may be added to reduce the possibility of tin whisker formation. Another popular, highly solderable and durable, Lead-Free lead-frame finish is nickel-palladium-gold, although this is relatively expensive.

It is important to avoid the use of components that have a lead containing finish when using Lead-Free solder alloys as there are still many questions concerning long-term reliability of this combination. However, for applications where the operating conditions are not extreme, the presence of lead in component finishes has generally not been found to have any major negative impacts on solder joint integrity. Until the component supply chain is completely free of lead finished components, it will be important to maintain good segregation of components with and without Lead-Free finishes.

Accelerated shelf life testing of a range of Lead-Free component finishes has shown that when stored in Class Two environments, (which are generally described as indoor environments having no humidity control, such as those found in a warehouse), the parts easily passed surface-mount solderability testing and related board-mount testing after the equivalent of eight years of storage.

Identifying and marking finishes

Identifying which solderable finish has been used on a component may be difficult and as yet no unified approach has been adopted for component marking. JEDEC, has proposed a marking system.

www.jedec.org

Summary

• Components used in Lead-Free assembly need to be able to withstand higher process temperatures.
• Solderable finishes need to be compatible with Lead-Free board finishes solders.