Lead Free: RoHS Compliance

06/13/2011

THE LEAD-FREE CHALLENGE
July 1, 2006, was the date when the EU (European Union) Directive on the Restriction of Hazardous Substances (RoHS) went into effect and required that all electronic products sold in Europe be lead free. International OEMs are more sensitive to this issue every day, and recently have begun to put pressure on their suppliers to support them in the development of their lead free strategies. Resistance to this change is all but a memory now as companies throughout the electronics supply chain are being forced to consider the practical steps that they must take in order to make the change to lead free.

For many decades now the electronics industry has been using tin-lead as an electrodeposited contact finish and solder joint material. Environmental concerns regarding the lead content of electrical and electronic assemblies have been increasing steadily over the past few years and the electronics industry has been pressured to eliminate lead from its products and processes. Even though industry sources report that electronics manufacturing accounts for less than 1% of the total lead used industrially, pressure continues to mount for global compliance with regards to lead free solderable deposits.

The "lead-free" movement has its most direct origin from enacted and proposed legislation in Japan and Europe. These legislative activities, combined with the growing expectations for corporate environmental responsibility have had a significant effect on manufactures throughout the electronics supply chain.

The ban of lead in Europe and the Far East threatened U.S. exports to those markets. Consequently, most of the large U.S. OEMs have already enacted their own lead-free strategies. Pure tin has been identified as most cost effective and functional alternative to tin-lead for electroplated component finishes. Several major OEM's both in the international and the domestic market place have chosen pure tin as their standard lead-free electroplated component finish.

These new, pure tin electroplating chemistries have been specifically formulated to provide a whisker resistant, lead-free deposit that provides a drop in replacement for most applications that are currently utilizing tin-lead.

LEAD FREE TIN ISN'T THE ONLY ROHS COMPLIANT FINISH
While many companies may be scrambling to meet this deadline, many more have already made the change. Those companies that have been proactive in terms of implementing their lead free strategies have put themselves in a strong competitive position, as RoHS becomes a reality. It is never too late to develop a lead free alternative.

Several major electronic OEMs in the international market place have chosen pure matte tin, over a nickel underplate, as their standard lead free component finish.

However, tin over nickel is not the only alternative. In a recent paper published by iNEMI (International Electronics Manufacturing Initiative) several different lead free component finishes were discussed along with their relative risks with regard to whisker growth. iNEMI is an industry-led consortium whose mission is to provide cohesive leadership and direction in the global electronics manufacturing supply chain.

The specific paper mentioned above was published by the iNEMI Tin Whisker User Group. This group was formed to develop recommendations for lead free finishes on components that would minimize the risk of failures from tin whiskers in electronic applications. The iNEMI Tin Whisker User Group consists of members from eight large manufacturers of high reliability electronic assemblies. The companies represented by these members are Agilent, Cisco, Delphi, Hewlett-Packard, IBM, Lucent, Sun Microsystems and Tyco Electronics.

While pure matte tin over a nickel underplate is high on the iNEMI list of recommendations, nickel-palladium-gold flash was given their highest recommendation. This plating scheme was developed by Texas Instruments in the late 1980s, as a solution for the lead frame industries yield loss which was due to post plating solder onto the leads.

Nickel-palladium-gold is not prone to whisker growth and the iNEMI Group highly recommends this finish for lead frame applications. However, the use of this finish is not limited to lead frames. In fact, this finish is very versatile and can be used in virtually any solderability application. Of course palladium and gold are noble metals and cost can be an issue, however, depending on the application, the thicknesses of these metals can be kept very low and in many cases the cost impact is minimal. However, in fine gold wire bond applications where nickel-palladium-gold would replace post-plated tin in IC manufacture or where palladium over nickel is replacing aluminum inlay in aluminum wire bond applications, this finish can represent a significant cost savings!

Once again, in those applications where nickel-palladium-gold isn't a practical alternative from a cost stand point, pure matte tin over a nickel underplate is the finish of choice. Recent testing performed by our solution chemistry partners have shown that the whisker risk of these new lead free tin finishes is minimal when they are plated over a nickel underplate. In the iNEMI paper, matte tin over nickel was very highly recommended, second only to nickel-palladium-gold.

In those applications where a nickel underplate can't be used, the best whisker mitigation strategy is reflowed tin. Precision has several applications, primarily in the automotive industry, where lead free tin is reflowed using an economical in-line process. Reflowed pure tin was also high on the iNEMI list of recommendations.

LEAD-FREE REFLOW AND SOLDERABILITY PROBLEMS
The advent of lead-free solders and the lead-free, pure tin plating finishes has resulted in some solderability problems that didn't exist with the old tin/lead soldering systems.

As a result of the push to eliminate lead in electronics most of the solder assembly operations occurring today are utilizing lead-free solder pastes and/or lead-free wave solders. These lead-free solders are usually some combination of tin and silver and sometimes copper. For example, one widely used lead-free solder is 96.4% tin, 3.5% silver and 0.6% copper. By using these lead-free solders you don't lose anything in terms of solderability, however, the melt point of these solders is much higher than eutectic tin/lead solder. The 96.4% Sn/3.5% Ag/0.6% Cu alloy mentioned above for example has a reflow temperature of about 217 degrees C which is 34 degrees higher than 63% Sn/37% Pb, which has a reflow temperature of about 183 degrees C.

What this means is that the solder temperatures and reflow oven profiles have had to increase 34 degrees C in order to insure the same dwell time at temperature as they had when using tin/lead. Since the formation of intermetallic compounds such as tin/copper or tin/nickel occur at a much faster rate with increased temperature, the likelihood of thick intermetallic layers and potential solderability problems has also increased. We have seen oven reflow temperatures as high as 285 degrees C and 260 degrees C is not uncommon.

This is especially true for to applications that require multiple reflow operations such as double sided circuit boards and some chip capacitor assembly operations.

Another common problem that has been commonly seen is a yellow discoloration of the lead-free pure tin plating finishes at these higher reflow temperatures.

by Vince Azzano

my.PPC1904.com
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