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Final Finish Selection Criteria to Meet PWB Lead-Free Requirements

Final Finish Selection Criteria to Meet PWB Lead-Free Requirements

by Frando van der Pas, Yung-Herng Yau, Karl Wengenroth, Jim Kenny, and Barry Lee Cohen

The printed circuit board industry is moving away from lead bearing hot air solder leveled (HASL) final finishes to alternative materials. To date, the electronics industry has yet to adopt a universal HASL alternative. While the market share of HASL has steadily declined over the last two years, the worldwide use of lead containing HASL materials still exceeded forty percent at the close of 2006. By the year 2008, HASL is projected to decrease to 37%.

Organic solderability preservatives (OSPs), immersion silver, electroless nickel/immersion gold (ENIG) and immersion tin are the key candidates to emerge as viable HASL alternatives. OSPs can claim a higher market share than any of the other HASL alternatives. This is partially due to the OSP's time in the marketplace, compared to other technologies but furthermore the proven highest joint reliability compared to other alternatives. Market share continues to grow with new OSP technologies, with the recent shift to lead-free processing.

Main Drivers

The three main drivers that push the electronics industry to consider HASL alternatives include:

• Cost
• Technology
• New lead-free material to meet RoHS requirements.

The main driver for final finish conversion today is the existing EU RoHS directives and the upcoming China RoHS legislation. These far reaching environmental initiatives have created the need for fabricators to eliminate lead from the process, driving lead containing HASL towards extinction. The impact of RoHS can be felt in PWB design and manufacturing, from laminate selection to final finish choice. These changes cannot be taken lightly because they impact the assembly process as well. When selecting a new process to meet the requirements of RoHS, it is necessary to insure not only that the end product meets the basic lead free requirement but also that the overall process selected will provide ease of use in the manufacturing environment, lowest cost of ownership and the highest assembly yields.

Comparison of RoHS Compliant Final Finish

The use of most well known HASL alternatives; electroless nickel / immersion gold (ENIG), immersion tin, OSPs and immersion silver, seems to be different in each of the world regions. In Europe, ENIG and the immersion tin final finish are generally preferred, although there seems to be an interest shift noticeable towards immersion silver. In the U.S. fabricators favor ENIG and in Asia OSP and immersion silver technologies are being used most extensively. The main reason for this can be tied back to specific OEM preference and specific assembly application.

Immersion silver provides a single surface finish that is, under specific conditions; wire bondable, embodies low contact resistance for key contacts, and can provide metal-to-metal shielding. Although it is a newer finish, there are clear advantages. It is a versatile finish that could replace a great part of the HASL (and even ENIG) market. There have been some historical concerns with silver such as electromigration and sensitivity to oxidation. These limitations have primarily been associated with electrodeposited silver coatings. The latest generation of immersion silver technologies has overcome any lingering concerns about electromigration or solderability issues.

OSP has been a market leader for HASL alternatives worldwide and provides a wide process window for the applicator. Experience and data have shown in the last 10 years that the generation of substituted benzimidazoles are fully compatible with no clean assembly of mixed technology boards, and can withstand more than 3 heat cycles required for this type of assembly. Although nitrogen will help to improve wetting it is not required. OSPs are the lowest cost surface finish available. A new generation of more thermally stabmaterials has been specifically designed to meet the lead-free assembly requirements [1].

Immersion tin has been available on the market for some time and provides a thicker, uniform metallic coating for improved ICT probe life and lubricity for press fit pins. Although the cost is higher and processing of this finish is more difficult than for example OSPs or immersion silver, it seems there is growing interest again for certain applications as fabricators and assembly operations work together to determine which of the available lead-free final finishes are most suitable for their demands. Especially in Europe, the interest in immersion tin as a final finish seems high. Concerns related to whisker formation have been addressed with new additive systems by most of the suppliers of immersion tin chemistries.

Electroless nickel followed by immersion gold is a well-established surface finish that has been used for a number of years. While ENIG has demonstrated excellent solderability, the metallurgy of the solder joint itself comes into question. A benefit of ENIG is that the nickel provides a barrier between the copper and the solder, allowing for less copper erosion and improved through-hole reliability. One of the main drawbacks to this process and other gold and palladium containing processes is the relatively high cost when compared to alternatives due to the precious metals that are used. A secondary consideration for manufacturers is that the chemistry is not as easy to use as the alternatives, requires a high level of routine control and cannot be easily conveyorized for high volume automation purposes.

Lead-free Assembly

The assembly industry is currently evaluating lead-free soldering alternatives. While certain alloys seem to be the choice of specific OEMs, a common industry accepted alloy containing tin, silver and copper is preferred. All of the alloys being used require a higher reflow temperature and yield slower wetting speeds. Paste suppliers have engineered specific flux and paste chemistries to improve the wetting of these new alloys. The alloy type and the temperature of the soldering operation have the greatest influence on solderability, independent of the surface finish. Initial studies indicate that the higher reflow temperatures do not affect the solderability or joint strength of OSP and immersion silver. The higher melting temperatures apparently help penetration of the OSP and wetting of the silver surface even with double-sided reflow. These observations further support the rapid growth that has been seen with these particular final finishes. They exhibit good assembly robustness, increasing the likelihood of higher first pass assembly yields.

OSP for Lead-free Assembly

OSP processes provide a temporary layer to clean and protect the copper surface from oxidation. During soldering, the organic coating is penetrated, dissolved and chemically broken down by the flux or solder paste flux vehicle and heat of the soldering process. Past experience has provided an understanding of how soldering occurs on an OSP. In general non-thermal cycled OSPs have good wetting properties, almost independent on materials or processes used but thermal cycling results in additional cross linking of the coating making it slightly more chemical resistant and less penetrable by weak organic acids used in standard flux formulations. The profiles used in lead free soldering and subsequently the higher temperatures involved will affect this phenomenon. Therefore a new generation of OSP has been developed and released to withstand higher temperatures and make no clean flux penetration easier, thus resulting in improved solderability.

The industry's next generation OSP have been specially formulated to meet the stringent demands of today's most complex PWB lead-free assembly processes [2]. As shown below, these innovations have been modified to provide a more thermally stable (i.e. higher temperature (HT)) coating without sacrificing the OSP's well-established record of reliability. Compared to existing technologies this new generation of OSPs has the ability to withstand by far more than 3 lead free thermal cycles without significant solderability degradation.

This has been confirmed with wetting balance and solderability testing as well as production experience. Furthermore, this coating is fully compatible with mixed metal finishes technologies such as ENIG/OSP assemblies.

All data gathered using OSP indicate that lead free thermal processing (with elevated temperatures) does reduce the OSP's wetting properties (i.e. larger wetting angle). However, the wetting angles for the lead-free alloys are still well below 90 degrees and consequently should be suitable for lead-free processing. It is important, however, when using the new OSP for lead-free, that the material selection is critical. Many factors should be considered before implementation. Before choosing a supplier, one should consider the amount of thermal processing the parts will be exposed to, the kind of flux or paste that will be used, thermal profiles used, nitrogen or air, etc. All of these factors can then be used to determine the OSP type or supplier as well as the required OSP thickness for that particular process.

Comparison of SAC X0307 and SAC 305 alloys for solderability with a high temperature OSP. Both exhibit good solder fillet with the HT OSP final finish. SAC X0307 exhibited slightly better circumferential wetting.

Immersion Silver Performance

Immersion silver is one of the leading lead free final finish choices for many OEMs in the telecommunications, computer, automotive and consumer electronics industries. As the interest in immersion silver has grown, a number of proprietary plating processes have been introduced. The first immersion silver process was introduced some 12 years ago. There have been numerous studies exploring the capabilities and advantages of the immersion silver plating processes currently available in the marketplace.

Results for lead-free soldering with immersion silver have proven to be compatible with different lead free alloys, even after multiple heat cycles. Not all processes have the same performance. The specific resistance to tarnish that a particular process has should be characterized to insure that the assembly of this final finish is successful. In addition, it has been demonstrated that the control of thickness and the deposition rate of the immersion silver, thus type of process chemistry involved, plays a significant role in the reduction of eliminating microvoids (Steve: link to microvoids.org) and reducing galvanic attack

Summary

The use of alternatives will not only increase but for a great extend will replace HASL as the final finish of choice. Alternatives such as OSPs and immersion silver all provide lead-free, highly solderable, flat, coplanar surfaces that, under production conditions, provide significant improvement in first-pass assembly yields. Matching each coating's benefits to the specific assembly requirements and PCB design can differentiate the HASL alternatives.

This column is based on the article, Selecting the Right Final Finish for RoHS Compliant PCBs , as published in the March 2007 issue of Printed Circuit Design & Manufacture. To download the complete article click here. (Steve: This should take them to...

http://www.enthone.com/docs/FinalFinishesPCD-M_March2007.pdf)

Do you have a Final Finish Question you'd like addressed by our authors?

If so, please email enthonepwb@cooksonelectronics.com and title the Subject Field, "Finish Line." We will address your questions in future issues of The Finish Line, as well as contact you directly with a response.

Jim Kenny is the Global Product Line Manager - PWB Final Finishes, Frando van der Pas is the European Product Manager - Final Finishes and Imaging Technologies, Karl Wengenroth is R&D Manager - PWB Final Finishes, Dr. Yung-Herng Yau is a Senior Scientist, and Barry Lee Cohen is the Director of Communications with Enthone Inc., West Haven, CT USA. Please contact them at enthonepwb@cooksonelectronics.com.

REFERENCES

[1] Next Generation Organic Solderability Preservatives (OSP) for Lead-free soldering

and Mixed Metal Finish PWB's and BGA Substrates , Koji Saeki , Shikoku Chemicals Corporation Chiba Japan Michael Carano Electrochemicals, Inc. Maple Plain, MN, Presented at IPC Printed Circuits Expo SMEMA Council APEX - Designers Summit 2004. [2] Lead free soldering of organic solderability preservatives By F.A.M. van der Pas and K.H. Wandner, Cookson Electronics, Europe PCE - 1st Quarter 2001.