Surface Finishing
for Lead-Free By George Milad
Uyemura International Corporation
Southington CT As the world moves forward towards adapting stricter environmental standards, the entire electronic industry needs to re-evaluate its manufacturing practices. Once RoHS (Restriction of use of Hazardous Substances) begin to be enacted by a few countries, innovation will move in leaps and bounds to come up with new lead free (LF) products that meet these requirements. Already the suppliers have a series of products lined up to meet those restrictions. The biggest challenges at the supply end seem to lie with the laminate material. Laminates were always challenged to meet faster speeds and higher frequencies. Now on top of that they have to produce products that can withstand the higher assembly temperatures of lead-free solder alternatives. The SAC alloy is highly adopted and seems to be the obvious choice
for a LF soldering alternative to eutectic (Sn/Pb) solder. SAC alloys
come in a variety of silver concentrations; each with its unique attributes. All SAC varieties require higher assembly temperature. The elimination of eutectic (Sn/Pb) solder, an industry staple for
many decades, will need to be transitioned successfully. Products
that require the highest levels of reliability will be the last to make
the transition. As the industry practices on the lower Class I type products, and builds a new data base, it will be more clear what
direction we need follow for the complete elimination of lead from
high reliability electronic assemblies. For the PWB manufacturer the elimination of lead from their shops
is well on its way. Tin has already replaced tin-lead as an etch resist.
Hot air solder leveling (HASL) remains the largest source of lead in
the board shop. HASL is still the choice of solderable surface finish
for a vast majority of printed circuit boards. LF solder for HASL is available and is in limited use and may see greater implementation
as we move forward. HASL’s dominance as a solderable surface
finish has been gradually eroding as higher technology parts with
finer, smaller pads and BGA’s required a more planar surface finish.
The planarity became crucial for paste stenciling and component placement. Other than the planarity issue, surface finishes are serving other functions other than soldering. Wire bonding, contacting surfaces
and compression connections applications require alternate
HASL finishes. As the technology evolved a series of alternate surface finishes
have been implemented for use throughout the printed circuit
industry. Some finishes are widely used and others are used
for very specific applications. These include: Electroless Nickel based:
- Electroless Nickel / Immersion Gold ENIG
- Electroless Nickel / Immersion Gold/ Electroless Gold ENIGEG
- Electroless Nickel / Electroless Palladium/Immersion
Gold ENEPIG
Finishes on Copper
- Organic Solderabilty Preservatives OSP
- Immersion Silver IAg
- Immersion Tin ISn
- Direct Immersion Gold (DIG)
Mixed Finishes:
- Selective OSP / ENIG, DIG / ENIG
Nickel Based Surface Finishes As can be expected, this class of finishes (ENIG, ENEPIG and
ENIGEG) makes a different intermetallic solder joint than the
non-nickel based finishes. Here a Ni/Sn solder joint is formed,
in contrast to Cu/Sn intermetallic for all the other finishes.
The precious metal cap dissipates into the solder and the joint is
made between the tin from the solder and the Ni-P (all electroless nickels have phosphorous in the deposit, the P content varies from supplier to supplier, as low as 5% and as high as 11%). As the Ni
forms the Ni/Sn intermetallic it leaves behind a phosphorous
enriched nickel band. This P enriched band is a natural component
of this type of solder joint. Ni/Sn intermetallic formation requires
a slightly higher assembly temperature and longer dwell at peak temperature for its formation. EMS providers who successfully
assemble Nickel based finishes understand this well.
These finishes are not the first choice for high (>10MHz) MHz, Rf propagation. The thicker nickel skin is responsible for some signal
loss. In some instances designers have been specifying thicknesses
as low as 50 micro-inches for the electroless nickel layer. As more research is published on this topic, better definition of the
capabilities of these finishes will fall in place. Transitioning to LF assembly may require some modifications to
these finishes. Most electroless nickel baths intentionally use a low
ppm lead-containing stabilizer. The lead in the final deposit is well
below and does not violate RoHS (<1000ppm Pb). The only violation
is the intentional addition. Most suppliers have alternative stabilizers already available to replace the Pb stabilizer. There is an ongoing
effort to exempt electroless Ni from total RoHS compliance.
Stay tuned. Data on intermetallic formation with SAC alloys for electroless nickel finishes is beginning to surface; the IMC formed contains Cu, which
is a component of the SAC alloy. ENEPIG at this time seems to be
ideally suited for SAC alloy assembly. Suppliers R & D are busy understanding solder joint reliability and modifying their existing offerings to meet SAC alloy assembly conditions. Electroless Nickel Immersion Gold (ENIG) 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 ENIG Specification-4522 specifies 120 – 240 micro-inches
of Ni with 2 – 4 micro-inches of immersion gold. ENIG is a very versatile surface finish, it is a solderable surface, it
is aluminum wire bondable, and is an excellent electrical contacting surface. It has excellent shelf life, in excess of 12 months, and is
easy to inspect (visual) and the thickness is easily verified by
non-destructive XRF measurement. ENIG continues to gain
market share, particularly after the understanding and virtual elimination of the “Black Pad” issue.
The ENIG deposition process is fairly complex; it requires a clean
copper surface free of solder mask residues as well as free of any copper/tin intermetallics (tin is used as an etch resist and is stripped before ENIG). Solder mask for ENIG plating must be adherent and completely cured (cross-linked) to withstand the high temperature
and prolonged dwell in the electroless nickel bath and in the
immersion gold bath. Today the complexity of the process is well understood by suppliers
and manufacturers. In addition with the issuance of the IPC ENIG specification 4552 that specified only 2-4 micro-inches of immersion gold, the “Black Pad” has virtually been eliminated. The “Black pad” occurred on a compromised nickel surface that is corroded in a
prolonged immersion gold deposition step. However it remains a
possible defect similar to electroless copper voiding, electrolytic
copper cracking, solder mask peel, shorts, opens etc, etc. Electroless Nickel Immersion Gold Electroless Gold ENIG is an ideal surface for aluminum wire bonding. However, with
only 2- 4 micro-inches of gold it is not suitable for gold wire bonding.
Soft gold at 10 – 25 micro-inches 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. Electrolytic nickel/gold requires bussing (electrical continuity) this is challenging as the application of
the finish usually comes after board circuitization. Electrolytic nickel
gold may be applied earlier in the manufacturing cycle as an etch
resist. Such Nickel gold pads wind up with copper sidewalls and a
certain level of undercut from the circuitizing etch process. This is in contrast with electroless gold, which is a post circuitization step after ENIG deposition and totally encompasses the pad and the sidewalls. Electroless Nickel Electroless Palladium
Immersion Gold ENEPIG is formed by the deposition of electroless Ni (120 – 240 uins) followed by 5 – 15 uins of electroless Pd with an immersion gold
flash (1 – 2 uins). ENEPIG is the finish that has the widest latitude
for a variety of applications. Sometimes referred to as the universal finish, it is a good soldering surface, a gold wire bondable surface, aluminum wire bondable surface, as well as a contacting surface. Preliminary indications are that ENEPIG will transition well into
the lead free SAC alloy assembly environment. Finishes on Copper Finishes on copper are all designed to be solderability preservatives. Without exception all these finishes form Cu/Sn Intermetallic solder joints. Metal coatings like silver and direct gold readily dissipate into
the solder, organic preservatives volatilize leaving a clean copper
surface for joining.
As far as solder joint integrity is concerned these finishes should transition very readily into LF alloy assembly. The question here is
how good a solderability preservative will each be after the higher temperature repeated thermal excursions. One would expect modifications to be made as LF begins to take hold. Organic Solderabilty Preservatives (OSP) Organic solderabilty preservatives come in different flavors for
special applications. OSPs are copper specific. 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 during assembly. Most OSPs have thicknesses in the angstrom range and are readily soluble in mineral acids and organic solvents.
This property limits the choice of suitable fluxes. Benzotriazoles are the lowest in thickness sometimes erroneously
called a monomolecular layer. Benzotriazoles fall short if more than
one thermal excursion is needed to complete the assembly process. Benzotriazoles are still in use within that niche market. Imidazoles,
alkyl substituted imidazoles and benzimidazoles are thicker and can withstand multiple thermal excursions. They are the bases of the widespread use of this finish. Although OSPs fill a specific market need, the finish falls short in
many desirable areas, as an organic coating it is not suitable for
wire bonding or as an electrical contacting electrical surface. They
are hard to inspect and equally hard to verify. As the industry progresses towards lead free SAC alloy assembly,
a new generation of OSPs are needed to be able to withstand the
higher assembly temperature. Suppliers already have a new
generation of “High Temperature” OSPs. These OSPs are expected
to remain a player in the brave new world of Lead-free. Immersion Silver: Immersion silver is deposited directly on the copper surface by a chemical displacement reaction. Immersion silver processes available
in the industry all co-deposit an organic anti tarnish with the
immersion silver. The reaction is fast approximately 1-2 minutes
and does not require the relatively high temperatures of ENIG. This makes this process very conducive to conveyorized processing. IPC specification 4553 covers Immersion silver and when issued will
specify 8 – 12 uins on a pad size of 60 X 60 mils or equivalent.
The pad size was specified because the thickness of the deposited
silver varies with pad size, the smaller pads plate thicker than
the ground plane areas. Immersion silver can be measured using XRF equipment. The
proper setup of the equipment is critical for reproducible results. The primary use of IAg is as a solderabilty preservative. During
assembly the immersion silver dissipates into the solder and allows
the formation of a Cu/Sn intermetallic. Occasional voiding in the
solder joint was reported. The IPC is presently conducting a round
robin study to determine if excessive silver thickness is a contributor
to that phenomenon and to set upper thickness limitations. The immersion silver is an active surface and readily combines
with sulfur from the environment. Silver sulfide tarnishes the
surface and creates doubt about the integrity of the finish at
inspection. Some suppliers are presently offering an anti-tarnish
post deposition step to protect the surface from the environment. Proper packaging of IAg finished boards is critical to control
sulfurization. The key in packaging is to minimize contact of the
surface with the environment and to ensure all materials used
in packaging and during storage are sulfur free. Issues and fears of dendrite growth and electro-migration related
failures for modules with IAg were also dealt with in an IPC
committee setup in conjunction with UL laboratories and
proven to be a non-issue. Al indications are that IAg will transition readily into LF assembly.
This is to be expected since the SAC alloy contains a relatively
high percentage of silver in the alloy Immersion Tin The ISn Immersion is deposited directly on the copper surface by
a chemical displacement reaction. The thickness recommended for
ISn is 30-50 uins. The higher thickness is recommended to ensure adequate pure tin on the surface. Thickness verification of ISn is
done mostly by XRF; however, this method does not differentiate between the different IMCs and pure tin. Immersion tin forms IMCs (Cu3Sn and Cu6Sn5) with the underlying copper. As the IMC
works its way to the surface solderability is compromised. This
phenomenon also impacts the shelf life of the finish. Another issue with ISn is its propensity to form whiskers at room temperature. ISn whiskers do not grow as a result of exposure to
heat, vacuum, pressure, humidity or bias voltage. They grow
naturally over time, which would seem to indicate, that the primary source is Cu6Sn5 migration stress. Whisker length has been
reported to be significant with whiskers in vias being measured
at 150 microns. Whiskers of smaller length have been recorded
growing off the edge of SMT pads as well. Immersion tin is a suitable minimum risk selection that has been successfully used by some companies. It is a viable lead-free finish
option for some PCB applications. How this finish will survive high temperature assembly associated with LF SAC alloy remains to be
seen. The solder joint IMC should not be a problem, however the
higher temperature excursion could accelerate the IMC formation compromising the solderability of the surface. Direct Immersion Gold: 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 1-2 uins. 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.
With an electroless gold (10 – 15 uins) overlay, the finish is also
gold wire bondable. DIG does not have any of the limitations of the other non-nickel
surface finishes. It is expected to transition readily into LF assembly conditions. The finish is in direct competition with OSP, IAg and ISn. Mixed Finishes Selective OSP/ENIG and DIG/ENIG Cell phones or mobile phones manufacturers looking for the
highest reliability for their highly mobile products, have elected
to use 2 different finishes on the same board, one for soldering
and one as a contacting surface. The choice for a contacting
surface was clearly ENIG. For a solderable surface OSP, which
forms a Cu/Sn intermetallic, was the first choice. DIG is a
possible alternative for OSP. Two challenges faced manufacturers; the first was finding a resist
that will withstand the temperature and dwell in both the electroless nickel and immersion gold and an ENIG surface that can withstand
resist stripping, acid cleaning, micro-etching necessary for surface preparation for OSP or DIG. A modified ENIG is available for this specific application. Modifications
are made in the phosphorous content of the Ni to be more chemical corrosion resistant. The immersion gold is also modified to give tighter less porous deposit to better protect the underlying nickel during processing.
Conclusion Surface finish has always been an active area in PWB manufacturing with new developments every few years. Evolution was needed to
meet the requirements of new technologies, smaller pads, high frequency signals, controlled impedance, wire bonding etc. The
next evolution cycle seems to be driven by regulation and not by
leading edge technology. With the new regulations associated
with RoHS and WEEE, one would expect to witness an influx of renovations as well as creative modifications to existing concepts.
These are times of opportunity for nimble companies that are
eager to adapt to the ever-changing market demands. There is no one surface finish that fits all applications. Designers
choices will increase again to accommodate LF assembly conditions.
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