DFM 101: Final Finishes ImmSn

Estimated reading time: 1 minute

Introduction

One of the biggest challenges facing PCB designers is understanding the many cost drivers in the PCB manufacturing process. This article is the latest in a series that will discuss these cost drivers (from the PCB manufacturer’s perspective) and the design decisions that will impact product reliability. 

Final Finishes

The primary purpose of a final finish is to create electrical and thermal continuity with a surface of the PCB. Final finishes provide a surface for the component assembler to either solder, wire bond, or conductively attach a component pad or lead to a pad, hole, or area of a PCB. Another use for final finishes is to provide a known contact resistance and life cycle for connectors, keys, or switches.

There are a number of final finishes in use in the industry today, including:

• ENIG (electroless nickel, immersion gold)
• ENIPIG (electroless nickel, immersion palladium, immersion gold)
• ENEPIG (electroless nickel, electroless palladium, immersion gold)
• ImmAg (immersion silver)
• ImmSn (immersion tin)
• Sulfamate nickel/hard or soft gold (electrolytic nickel/gold)
• HASL (hot air solder leveling)
• SnPb (63/37 tin/lead)
• LF (lead-free)
• OSP (organic solderability preservative)

Final finishes are primarily application driven, so there are a number of considerations that should be part of any decision to choose a final finish:

• Lead (Pb) tolerant or lead-free (LF) process
• Shelf life
• Flatness
• Lead or ball pitch
• Wire bondability
• Lead insertion
• Solder joint integrity
• Corrosion resistance
• Potential problems
• Cost

Lead-free finishes are considered RoHS-compliant (< 0.1% BW of finish, for Pb, Hg or Cd) with the single exception of tin/lead HASL. The RoHS-compliant finishes include the following:

• ENIG
• ENIPIG
• ENEPIG
• ImmAg
• ImmSn
• Electrolytic nickel/gold
• LF HASL
• OSP

Lead-free PCBs require that the standard HASL surface finish cannot be used. There is still a significant amount of discussion on what the long-term surface finishes will be. Currently, the immersion silver and OSP surface finishes are the most prevalently specified surface finishes for solderable PCBs. Immersion tin is the prevalent surface finish for press-fit backplanes. Please contact your PCB fabricator for current information on where industry specifications are heading.

To read this entire article, which appeared in the November 2022 issue of Design007 Magazine, click here.