Latest Developments in Surface Finishing for PWB


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Editor's Note: This article originally appeared in the May 2012 issue of The PCB Magazine.

Surface finish products have been on an evolutionary fast track to meet the complexity of newer designs (lighter, faster, smaller) as hand-held devices like iPads, smart phones, and PDAs continue down the small form factor path. The newest challenges include RoHS compliance and the need for both lead-free and lead-free compatible finishes.

Surface finish is all about connectivity; it is the surface that forms the connection from the board to a device. Today, there is an impressive lineup of surface finish products, including: electroless nickel/immersion gold, electroless gold, electroless palladium, immersion silver, immersion tin, OSP, and DIG (direct gold on copper).

Surface finishes fall into two categories based on the type of intermetallic formed in the solder joint, namely Cu/Sn intermetallic (IMC), and Ni/Sn (IMC). The Ni/Sn IMC requires a higher temperature and extended dwell at temperature during assembly.

Figure 1: Ni/Sn versus Cu/Sn intermetallic (IMC) formation.

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 other finishes.

As shown in Figure 1, the precious metal cap dissipates into the solder and the joint is made between the tin from the solder and the Ni-P. (Note: All electroless nickels have phosphorous in the deposit; the P content varies from supplier to supplier, in a range of 5-11%.) As the Ni forms the Ni/Sn intermetallic, it leaves behind a phosphorous-enriched nickel band, which 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 Ni-based finishes understand this well.

Electroless Nickel Immersion Gold (ENIG)

ENIG is the fastest-growing finish for complex boards. ENIG is formed by the deposition of electroless Ni-P on a catalyzed copper surface, followed by a thin layer of immersion gold. The IPC-4522 ENIG specification cites 120-240 μin of Ni with 2-4 μin of immersion gold.

ENIG is a versatile planar surface finish. It is a solderable surface, aluminum wire bondable, and an excellent electrical contacting surface. Additionally, it has excellent shelf life in excess of 12 months, is easy to inspect, and the thickness is easily verified by non-destructive XRF measurement. ENIG continues to gain market share. With the issuance of the IPC-4552, black pad has virtually been eliminated.

New developments in ENIG include a new generation of immersion gold products introduced to the market in the last couple of years. These new developments were made to minimize corrosion during gold deposition and to control cost. Corrosion control is achieved by minimizing the availability of hydride ion, the primary contributor to nickel corrosion. New-generation immersion gold baths operate in a neutral pH (7.0 - 7.2) in contrast to the acidic pH (4.5 - 5.5) of the previous generation. In addition, newer baths operate at a lower gold concentration, thus reducing by 50 to 80% the initial investment in the process. A new electroless nickel was also developed specifically for flex circuits; its crystal structure does not allow for cracking through the copper underlayer.

Electroless Nickel Immersion Gold Electroless Gold (ENIGEG)

ENIGEG is a gold wire bonding surface finish, whereas ENIG is ideal for aluminum wire bonding. The aluminum wire bonds with the underlying nickel surface. Gold wire needs to bond to the gold surface. The thickness of immersion gold at 2-4 µin is not enough to prevent nickel diffusion to the surface. Thicker, soft gold >25 µin 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), which is challenging because 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. These 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)

ENEPIG is formed by the deposition of electroless Ni (120-240 µin) followed by 4-8 µin of electroless palladium (Pd) with an immersion gold flash (1-2 µin). ENEPIG is the finish with the widest latitude for a variety of applications. Because of its versatility, ENEPIG is sometimes referred to as a universal finish. ENEPIG is suitable for soldering, gold wire bonding, aluminum wire bonding, and contact resistance. In addition, ENEPIG forms one of the most robust solder joints with lead-free SAC-type alloys.

The presence of the thin palladium layer prevents the nickel from migrating to the gold surface making gold wire bonding possible with a thin immersion gold layer. ENEPIG is the most competitive gold wire bonding surface today, and it is gaining market share as its attributes and applications are being understood and as designers are calling it out.

Figure 2: Amorphous electroless palladium (60 nm) forming a diffusion barrier, preventing the nickel from reaching the immersion gold surface.

The versatility of the ENEPIG finish makes it an ideal replacement for the complexity associated with selective finishes manufacturing (see below).

The IPC Plating Committee 4-14 has completed the draft for IPC-4556 ENEPIG Specification. The draft is presently in peer review.

The ENIG, ENIGEG and ENEPIG deposition processes are fairly complex; they require a clean copper surface free of solder mask residues as well as any copper/tin intermetallic (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. Adhering to supplier recommended controls is a must for the successful deposition of these finishes.

Finishes on Copper

Finishes on copper are 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 and organic preservatives volatilize, leaving a clean copper surface for joining, as seen in Figure 1.

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, which helps preserve the solderability of the copper during storage and 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 and 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 basis 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. OSPs are hard to inspect and equally hard to verify.

As the industry progresses towards lead-free SAC alloy assembly, suppliers already have a new generation of high-temperature OSPs that are expected to remain a player in the brave new world of lead-free.

Immersion Silver (IAg)

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.

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 the process conducive to conveyorized processing. IPC-4553-A covers immersion silver and specifies a minimum of 5 µin, but typically 8-12 µin, measured on a 60 x 60 mils pad size. 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 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.

Creep corrosion has been reported and documented for immersion silver. Its occurrence is environment dependant. The following design and assembly considerations would go a long way in eliminating the probability of creep corrosion: 

  • Minimize/eliminate SMD (solder mask defined) pads.
  • Keep test pads apart (minimum of 80 mils).
  • Filled vias must be completely filled.
  • Print paste must cover entire pad.
  • Paste and reflow test pads.
  • Use anti-tarnish overlay.

Immersion Tin (ISn)

The ISn is deposited directly on the copper surface by a chemical displacement reaction. The thickness recommended for ISn is 30-50 µin. 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 lead-free SAC (Sn-Ag-Cu) 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)

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 µin. The process is a mixed electroless and immersion gold deposition, which 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 µin) 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 lead-free 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 phone manufacturers looking for the highest reliability for their mobile products have elected to use two different finishes on the same board, one for soldering and one as a contacting surface. Clearly, the choice for a contacting surface was ENIG. For a solderable surface, OSP, which forms a Cu/Sn intermetallic, was the first choice. DIG is a possible alternative for OSP.

This resulted in two challenges for manufacturers. The first issue was finding a resist to withstand the temperature and dwell in both the electroless nickel and immersion gold; the second is finding an ENIG surface that can withstand resist stripping, acid cleaning, and the 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. Look for new developments in electroless silver on electroless nickel, and electroless palladium direct on copper, with or without immersion gold. These are times of opportunity for nimble companies that are eager to adapt to the ever-changing market demands.

George Milad is the national accounts manager for technology at Uyemura International Corporation.   Prior positions include technical marketing manager at Rehm, director of applications at Atotech, and engineering manager at Automata PWB. He is the recipient of the IPC 2009 President’s award, current chair of the IPC Plating Committee and a permanent member of the Technical Activities Executive Committee of the IPC. Contact George at gmilad@uyemura.com.

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