Making the critical connection: decisions about final finishes are often made from the design seat.
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 available in the industry all co-deposit an organic anti-tarnish with the immersion silver. The reaction is fast, taking approximately 1 to 2 minutes. This makes this process very conducive to application by a conveyorized method. IPC-4553 specifies the thickness required as 5 to 16 microinches on a pad size of 60 X 60 mils or equivalent.
ISn is deposited directly on the copper surface by a chemical displacement reaction. IPC-4554 specifies a thickness of 30 to 50 microinches. The higher thickness is recommended to ensure adequate pure tin on the surface. A Cu/Sn IMC forms at the interface of the copper and the ISn, and if it works its way to the surface during storage it may compromise solderability. This phenomenon also impacts the shelf life of the finish.
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 Rolls 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. These questions will be explored in future Final Finish Forums.
In the upcoming months the Final Finish Forum will cover related topics from a number of different perspectives to provide additional background on this key interconnection process. Tune in for more from design considerations, through fab applications and into assembly compatibilities. The forum is designed to encourage positive supply chain communication and collaboration.
This month's Final Finish Forum is hosted by GEORGE MILAD. GEORGE MILAD is the national accounts manager of technology at Uyemura International Corporation (UIC) and can be reached at firstname.lastname@example.org.
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|Title Annotation:||FINAL FINISH FORUM|
|Publication:||Printed Circuit Design & Fab|
|Date:||Jun 1, 2008|
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