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Why Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG)?


The perfect electronic product device should be as
small and light as possible, while containing the
maximum amount of electronic functionality and
operating at the highest possible speed. To meet these
needs, the electronic packaging industry has been
driven to develop more and more advanced packaging
methods, both by increasing the density of integrated
circuits on a single PCB and also by combining multiple
functionalities into single, dense packages.

Increased package and interconnection density have
driven the evolution of assembly methods from throughhole
technology (THT) to surface mount technology
(SMT) and have led an increased use of wire bonding to
attach devices to PCB substrates. Decreased
interconnect pitch and the use of chip scale packaging
(CSP) have enabled the increases in device density,
while multichip modules (MCM) / system in package
(SiP) approaches have allowed integration of
functionalities that would be difficult to produce on a
single wafer substrate.

When the semiconductor industry has concentrated for
many years on increasing the performance of devices
by reduction in critical dimensions, until recently, there
has been less consideration of the fact that devices in
an electronic system must communicate with each other
through the packages that contain them. Large I/O
requirements and signal transmission quality have
emerged as key considerations for the semiconductor
packaging industry, as have the assembly process
requirements and the final finish of PCB substrates
used to achieve reliable interconnections both within IC
packages and for the second level packaging of devices
onto PCB substrates.

This article describes the key factors affecting
interconnection reliability, especially focusing on the
performance of finishes for gold wirebonding

Surface Finish Options for Wirebonding

While electrolytic nickel gold provides excellent gold
wirebonding performance, it suffers from three major
deficiencies, each of which is a major barrier to its use
in leading edge applications:
     • The process cost is high, driven by to the
       relatively high gold thickness required

     • At the higher gold thicknesses normally used,
       solder joint reliability may be reduced due to the
       formation of tin-gold intermetallics. Should the
       finish be combined with a second final finish,
       more suitable for soldering applications,
       additional costs from secondary imaging
       operations are also incurred

       • The electrical bussing required to make
       connections to the features during the plating
       process limit the feature densities that can be

These limitations have provided an opening for
electroless process options. These include electroless
nickel immersion gold (ENIG), electroless nickel
electroless gold (ENEG) and electroless nickel
electroless palladium immersion gold (ENEPIG).

Of these three options, ENIG is not generally
considered to have an acceptable process window for
high reliability gold wire bonding (although it has been
used for some lower end consumer applications) and
ENEG suffers from many of the same process cost
issues as electrolytic nickel gold, with additional
challenges from operating more complex electroless
gold processes.

While electroless nickel electroless palladium immersion
gold (ENEPIG) first emerged in the late 1990's, its
market acceptance was delayed by the very volatile
price of palladium metal around 2000. However, in more
recent years, the market demand has shown strong
growth as users have begun to appreciate the potential
of ENEPIG to address many of the new packaging
reliability needs, while also meeting lead free / ROHS

Apart from the advantage of packaging reliability, the
cost of ENEPIG has now become a positive
consideration. With recent increases in the price of gold
price to levels above US$800 per troy oz, the production
cost of electronic device that required thick gold
electroplating becomes extremely difficult to control.
Since the cost of palladium metal (US$300 per troy oz)
has remained relatively low in comparison to gold, an
opportunity for cost saving by replacement of gold with
palladium is now available.
Comparison of Final Finishes

In the existing market, there are 4 primary lead-free final finishes for PCBs which are suitable for assembly of fine pitch
QFP / BGA devices:
   - Immersion Tin
   - Immersion Silver
   - Organic Solder Preservatives (OSP)
   - Electroless Nickel Immersion Gold (ENIG)

The table below shows a comparison between these four final finishes and ENEPIG. None of the four primary final finish
options is perfect for lead-free assembly, due to a variety of different concerns including multiple reflow cycle capability,
shelf life before assembly and wire bond capability. In contrast, ENEPIG shows substantial advantages combining
excellent shelf life, solder joint reliability, gold wire bondability and usage as a contact surface.

The protection of the electroless nickel interface with the formation of a thin electroless palladium layer, prior to
immersion gold, eliminates the potential for excessive gold attack on the electroless nickel surface.

Table 1 Comparison of Final Finish Performance

Characteristics OSP ENIG ENEPIG Immersion Silver Immersion Tin
Shelf Life
(controlled conditions)
< 12 Months > 12 Months > 12 Months < 12 Months 3 - 6 Months
Handling / Contact with
Soldering Surfaces
Must be avoided Should be
Should be
Must be avoided Must be avoided
SMT Land Surface Planarity Flat Flat Flat Flat Flat
Multiple Soldering Cycles Fair to good Good Good Fair to good Fair to good
No Clean Flux Usage PTH/via fill
No concerns No concerns No concerns No concerns
Solder Joint Reliability Good Good process
control required to
avoid "black pad"
Good Interfacial microvoid
Gold Wire bonding No No Yes No No
Electrical Test Probing Poor, unless solder
applied during
Good Good Good Good
Corrosion Risk after Assembly Yes No No No Yes
Contact Surface Applications No Yes Yes No No
Total Coating Thickness
> 0.15 Au 0.08 – 0.13
Ni 3.0 – 6.0
Soldering :
Au 0.03 – 0.05
Pd 0.05 – 0.1
Ni 3.0 – 5.0
Au 0.07 – 0.15
Pd 0.1 – 0.15
Ni 3.0 – 5.0
0.05 – 0.5 typical 1.0 – 1.1

When we consider the final finish performance in a variety of different assembly methods, it can be seen that ENEPIG is suitable for a wide range of assembly requirements.
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