By George Milad
Uyemura International Corporation
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.
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.
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.
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.
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 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 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 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 roundrobin 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
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.
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.
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.
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.