Decisions about final finishes are often made from the design
seat.
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
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 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. These questions will
be explored in future Final Finish Forums.
PCD&F
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
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