By George Milad
Final finish is all about connectivity. It is the starting point to connecting a component to the board, or the board to a device. Connections are primarily made by three methods; soldering, wire bonding or, mechanical contact. In the 1980s and early 1990s, products used simple through holes coated with tin-lead based HASL (Hot Air Solder Leveling) as the dominant finish. Electrolytic nickel followed by hard gold was the coating for insertion tabs and other mechanical contacts.
The introduction of technology advances has forced the industry to seek alternatives to HASL. HASL could not meet the co-planarity requirements to today's product.
Today designers have an impressive line up of surface finish products in use as HASL replacements. The surface finishes available fall into two categories based on the type of intermetallic (IMC) formed at the solder joint, namely Cu/Sn IMC, and Ni/Sn IMC. The Cu/Sn IMC type includes the following final finishes; HASL, organic solderability preservatives (OSP), immersion silver (IAg), immersion tin (ISn) and direct immersion gold on copper (DIG). The Ni/Sn IMC type includes electroless nickel/immersion gold (ENIG), electrolytic nickel gold, electroless nickel electroless and (auto-catalytic) gold (ENAG) and electroless nickel, electroless palladium and immersion gold (ENEPIG).
ENIG is formed by the deposition of electroless nickel-phos on a catalyzed copper surface followed by a thin layer of immersion gold. The IPC-4522, the specification covering ENIG requires 120 to 240 microinches of nickel and 2 to 4 microinches of immersion gold. ENIG is a very versatile surface finish. It is a solderable surface, it is aluminum wire bondable, and it provides an excellent electrical contact surface. It has excellent shelf life, in excess of 12 months, and is easy to inspect by visual methods. ENIG continues to gain market share, particularly after the problem known as "Black Pad" was identified and controlled.
ENAG is a variation on ENIG where additional gold is deposited on top of the immersion gold. The method of deposition is electroless, also referred to as auto-catalytic. Soft gold at 10 to 25 microinches is needed for successful gold wire bonding. This may be achieved by depositing electroless gold on top of the ENIG finish. Alternatively electrolytic nickel with electrolytic soft gold is also used for this application.
ENEPIG is formed by the deposition of electroless nickel (120 to 240 microinches) followed by 4 to 8 microinches of electroless palladium with an immersion gold flash (1 to 2 microinches). Currently there is no IPC specification is available for this finish. ENEPIG is a good soldering surface, a gold wire bondable surface, aluminum wire bondable surface, as well as a contacting surface.
Organic solderabilty preservatives come in different types for specific applications. OSPs are copper specific coatings. All OSPs have the ability to complex the copper surface and create a protective coating, that helps preserve the solderability of the copper during storage and assembly.
IAg is deposited directly on the copper surface by a chemical displacement reaction. The immersion silver processes
DIG is a new finish with great potential as a solderable finish. Direct immersion gold is deposited directly on the copper surface to a thickness of 1 to 2 microinches. The process is a mixed electroless and immersion gold deposition; this gives rise to a very tight non-porous deposit that can resist copper migration into the gold layer. The deposition is slow and requires a high temperature bath.
How does the elimination of lead for RoHS compliance affect a designer’s choice of final finish? Will all these finishes transition successfully into the world of lead-free (LF)? The decision to select one final finishes versus another is often made at the beginning of the supply chain, in the design seat.
See an interview with George Milad, Keeping PWB Manufacturing