The European Union WEEE/RoHS directives cause concern in the military and aerospace market as to the availability and reliability of lead-free electronic components.
By Courtney E. Howard
The European Union (EU) has issued two directives that will have a significant influence on the global military and aerospace market.
First, the EU Restriction of Hazardous Substances (RoHS) in electrical and electronic equipment directive (Directive 2002/95/EC of the European Parliament and of the Council), which took effect July 1, prohibits the sale of new electronic equipment containing certain hazardous substances, including lead, cadmium, mercury, hexavalent chromium, polybrominated biphenyls, and polybrominated diphenylethers in the EU.
Second, and expected to take effect on January 1, is the EU Waste Electrical and Electronic Equipment (WEEE; Directive 2002/96/EC) policy, which calls for organizations to take responsibility for recovering and recycling products containing hazardous materials at the end of their useful lives.
The EU is not alone in its environmental legislation with regard to the manufacture and recycle of electronics; it is a global trend. Japan, one of the world’s leading producers of electronic components and printed circuit boards, has been aggressive in its efforts to remove lead from its manufacturing processes and is expected to institute lead-free legislation this year. China is following suit. In the U.S., meanwhile, California legislature approved SB-20, with a compliance deadline of January 2007 that prohibits the sale of electronic products in the state that do not meet EU RoHS standards.
“The component industry is moving all of its parts slowly but surely to be lead-free,” recognizes Andy MacCaig, operations director for Radstone Digital Processing in the United Kingdom. “Even for those countries where there is no lead-free legislation and no requirement to manufacture lead free, while it’s not directly applicable to them, it is going to affect them. The electronic components they need to purchase to make their products are moving to a lead-free finish. All the evidence at the moment certainly suggests that that’s the way the world is going, and that we will be seeing lead-free solder everywhere.”
Getting the lead out
The biggest, although not the only, influence of the RoHS legislation for the electronics industry is the requirement to reduce significantly-nearly to zero-the amount of lead in products, including component finishes and solder. Lead has been used in solder, which is predominantly made of tin, for several decades with good reason. For starters, the addition of lead makes solder softer and more pliable, which is of particular benefit to military and aerospace applications.
“Electronic components and the circuit board that they are soldered to expand and contract at different rates as they heat and cool,” MacCaig explains. “The solder joint connecting them has to be able to accommodate those items expanding and contracting at different rates-it has to link and move without fracturing so that, over lots of thermal cycles, the joint stays intact and doesn’t break.”
A primary concern in moving away from leaded solder is compromising the integrity of the solder joint, which is of keen concern to military systems designers where reliability is paramount. A lead-free solder joint is likely to be far more brittle and intolerant of extreme temperature changes, which are common in military and aerospace environments.
“If you take the lead out, you increase the risk of that solder joint failing due to the expansion and contraction of the parts it is connecting together,” MacCaig says. “That is certainly one of the challenges of moving to lead-free solder-working out exactly what substances and what processes to use to make sure we can get the sort of reliability that we’re used to from a lead-based solder.”
The use of lead-free solder further calls into question the reliability of the overall printed circuit board and its components, not just the solder joints. The move to lead-free products affects the whole manufacturing process, says Doug Patterson, vice president of worldwide sales and marketing at board manufacturer AiTech Defense Systems Inc. in Chatsworth, Calif.
“Lead-free solders require higher processing temperatures, which stress the board during manufacture quite a bit more,” remarks Patterson. “Military customers care about and won’t accept the higher temperature, which puts greater stresses on the boards’ components. Some of our customers are saying, ‘We will not accept anything but tin-lead.’”
Another effect of moving to lead-free solder and components-tin whiskers-is garnering much attention among defense and aerospace systems designers, and rightly so. Tin whiskers have caused documented failures to various satellites, radars, and missiles, including the Galaxy-3 satellite, F-15 jet fighter radar, and Patriot missile.
More recently, officials of the Aerospace Corp. in El Segundo, Calif., debated with NASA leaders about whether NASA should launch the space shuttle Discovery STS-121. Aerospace Corp. executives fear that tin whiskers on the shuttle’s flight-control-system (FCS) avionics boxes will fall onto circuit boards, which would create a failure of the orbiter’s electrical components and the subsequent loss of crew and shuttle. NASA launched STS-121 on July 4.
Tin tactics
In moving to lead-free electronics, component manufacturers must abandon their use of a tin-lead finish on the component legs, designed to ease the process of soldering parts onto the board. Gold, silver, zinc, and other metals can grow whiskers, but tin is perhaps the most susceptible.
“Certain lead-free finishes are very prone to tin-whisker growth,” MacCaig says. “The exact reasons for that are becoming better known, but the phenomenon of tin whiskers is still something of a mystery.”
Tin whiskers represent a crystalline metallurgical phenomenon by which tin grows tiny electroconductive filaments that resemble microscopic hairs. The cause of tin whiskers is not completely understood, even after decades of study, yet experts do know that thermal or compressive stresses encourage their growth. Able to carry a current, whiskers are known to cause shorts in high-voltage circuits and intermittent failures in low-power ones.
Many manufacturers, contractors, subcontractors, and systems integrators are refusing to use pure-tin devices due to the risk of developing tin whiskers, known to cause system failures.
“For the better part of 60 years, we’ve known in the electronics industry that tin whiskers are a natural phenomenon of pure tin, especially when it’s put under mechanical stress,” Patterson says. “You build the lead frame, grow the device, attach the die, cover it with epoxy, and then you bend the leads and cut it; that puts mechanical stresses throughout the lead frame. Those mechanical stresses cause tin whiskers, or molecules of tin, to grow off the device in any different direction, sometimes so thick you can’t even see through it-it’s a forest. If one grows off, it’s so small you can’t see it, except under high magnification, and it can short the device.
“People have put pure-tin, wire-wrapped connectors in their desk drawer and, when they pull them out a couple months later, they are a forest of whiskers-stuff you can’t even see through,” Patterson continues. “And they are strong; you could whack that thing on the table and those fibers stay right where they are.”
Obsolescence concerns
“A lot of non-European organizations, I think, have taken the view initially that this is a piece of EU legislation; they’re not based in the EU, therefore, they don’t need to worry about it,” MacCaig points out. “On the one hand, if you’re not selling products into the European market, then the EU directive isn’t applicable to you. An American company manufacturing in America, selling to American customers has no obligation to be compliant with the European regulations.”
MacCaig admits, however, that the issue at hand is far more complicated.
Military and aerospace systems designers have become increasingly reliant on commercial-off-the-shelf components, given their low cost and ready availability. Commercial component manufacturers, interested in taking advantage of the global market, are dedicated to the production of RoHS-compliant, and therefore lead-free, products and production processes. Most are unlikely to run two product streams-one lead-free for the European market and one that’s still leaded for and military, aerospace, and defense applications. Even if some companies continue to support a non-RoHS-compliant product line, the concern is that the components will cease to be COTS products by definition and will suffer from higher prices and reduced availability.
“As we start developing new products, we start hitting component obsolescence issues,” Patterson says. “WEEE/RoHS has caused some semiconductor vendors to obsolete entire product lines. If they’re selling a million chips a day to the telecom industry and selling 100 chips a day to the military, it doesn’t take a rocket scientist to figure out that they’re not going to maintain two separate product lines with two separate lead frames to support both; it’s economics. They’ll obsolete an entire product and offer you the replacement product, which is the same die but on a different lead frame. If that lead frame is pure-tin, we can’t use it and we have to redesign the product to provide the same level of functionality-there are costs associated with that, and nobody wants to feel those costs passed on.”
Kicking and screaming
The end users in military, space, and commercial aviation environments are likewise concerned, not only about the cost and availability of products but also their safety and reliability.
“Components are becoming an availability issue because manufacturers are shifting to lead-free to meet the needs of their major customers-the major semiconductor market, including cell phones, telecommunications, and computers,” says Vance Anderson, chief of the Microelectronics Design Branch of the DOD’s Defense Microelectronics Activity (DMEA) in Sacramento, Calif. “Most manufacturers are not going to maintain a dual-process line for leaded and lead-free components.”
WEEE/RoHS and similar legislation are forcing changes on the U.S. Department of Defense and related industries, even though no U.S. restriction on these materials exists. Beyond the parts availability issues, military and aerospace personnel are concerned about the effects of mixing lead-free and traditional tin-lead in systems.
Radstone uses the Fischer ED-XRF Fluorescence Spectrometer to determine whether incoming components are lead-free and RoHS-compliant.
“We have a few issues with lead-free,” Anderson says. “One that everybody likes to talk about is tin whiskers, when you have less than roughly 3 percent lead in the finish on the component, which cause failures due to shorts and other problems.”
The DMEA has tested lead-free assemblies, to which most commercial manufacturing processes are moving. Its results show that they do not perform as well as traditional tin-lead systems in a military environment, and they may not survive the accompanying extreme environments and long product cycles.
“We use our systems at a much higher temperature range, both hot and cold, than the commercial industry,” Anderson says. “We also keep the products around for much longer. You get rid of your cell phone about every two years; we don’t get rid of our weapons systems and replace them every two years. They can last 20, 40, even 60 years in service. Those things set our systems apart from the commercial arena.”
The longevity of products, and the need to repair them periodically, lends to another area of concern-the combination, whether consciously or unknowingly, of tin-lead and lead-free products.
“Some of the materials being used as replacements are not at all compatible with tin-lead,” Anderson says. “For example, if you get bismuth in a tin-lead solder joint, it becomes very brittle. There are configuration-control issues with how you do repairs, what components you replace, and what solders you’re using in the repair methodology. It’s causing quite a concern, especially if you keep in mind the military is one of the few industries that still repairs circuit cards. Most of the high-reliability programs, the space programs in NASA, for example, are absolutely demanding that we still use traditional tin-lead solder.”
Still, Anderson and others at the DMEA are running several test programs to evaluate new lead-free solders in the military environment. While the manufacturers are performing tests on and qualifying solders, they do not test according to the temperature extremes and the product-life durations necessary for military use. A great deal of testing is still required to qualify these solder replacements for the mil-aero environment; yet they are learning more each day, Anderson says.
Compliance and pure-tin testing
The EU RoHS directive does not require manufacturers to include specific verbiage or a certain label or logo on product assemblies or packaging. The assumption is made, therefore, that any product entering the European market is RoHS compliant. U.S. customers, however, prefer to know precisely what materials are incorporated in each product.
“Some component manufacturers have switched to pure-tin lead frames, and they have not marked their packaging as having switched,” Patterson mentions. “It’s a complicated and serious issue. A whole industry has popped up around testing components on their content.”
“The quality of the information that we get from component suppliers is variable,” MacCaig admits. “Some give us more info than we could possibly want and allow us to confirm that they really are supplying us product that is compliant and that we can use. We have manufacturers at the other extreme that give us very little data and do not tell us when they move from a leaded to a lead-free finish on products. People need to be careful to make sure that what they’re buying is what they think they’re buying. We’re having to work very hard to do just that.” To that end, Radstone has purchased equipment for incoming product inspection to analyze whether they are compliant with the ROHS regulations.
“We already know of lead-free parts in our supply system, even though our contracts specify that they should not be supplied that way,” Anderson says. “There could be problems in repair and manufacturing because of that. You have to manage your parts and your processes. If you do nothing, you’ll be fighting fire drills and dealing with problems. You really have to know what is going into a system.”
Options and offerings
With the EU RoHS directive in effect, the industry has a finite list of options. The first option is to continue using leaded solder. Some companies have elected to buy all the parts they can while they are still available to complete their production requirements.
“We can take a part from a manufacturer that is supplied to us lead-free, and refinish it ourselves to allow us to continue to solder it with a leaded solder,” MacCaig says, offering up another choice, although it adds extra process steps, extra time, and extra cost to the product.
“The third option is to move to lead-free product, but even that isn’t as easy as it sounds,” MacCaig continues. “We can’t necessarily get all the parts lead-free for some of the products we currently have in production. We have this slight conundrum where we cannot always get the parts lead-free to build a lead-free board, but we also cannot get all the parts leaded to build a leaded board. Still, we are able to refinish an unleaded part to be leaded, and go the other way as well, if needed.”
Some viable options for replacing tin-lead solder exist today, and the industry in general continues to work on alloys of tin-including palladium, gold, and nickel-to keep pure-tin content down below 97 percent to prevent whiskers from forming.
“Unfortunately, all those packages are really expensive now,” Patterson says. “Palladium is roughly $1,000 a gram. The market usage and demand is very low. The whole precious-metals industry has just absolutely exploded over the past four years in terms of inflation, making it even more difficult. Even so, this market will settle down, the requirements will settle down, the solution will be found, and life will go on.”
Work in progress
In the meantime, much of the industry wonders if the WEEE/RoHS legislation and others like it will have a negative effect on the industry as a whole, and whether the bodies implementing such policies fully understand the ramifications and implications that they have worldwide.
“Fundamentally, I don’t have a problem with this lead-free issue; in the long run, it’s probably a good idea,” notes Patterson. “But setting an unrealistic timetable for the world to convert from a technology that has been in place for 60 or 70 years and expecting the industry to react in a year’s time is patently unrealistic and totally unreasonable.”
Doubtless, the changeover to lead-free solder and component finishes in the military and aerospace markets will take time. A wealth of suppliers to the electronics industry, even major entities and European businesses, are struggling with the transition. Some, unable to make the leap, have requested exemptions
On the user side, Anderson admits that everyone is in the process of learning about new tin-lead solder replacements. “We still have a long way to go before we catch up to the 60 or so years of experience that we have with tin-lead solder,” acknowledges Anderson. “There is no single solution-a solution that a satellite application takes is going to be different than that of a commercial radio. The truth is I’ve been hoping that somebody invents the perfect lead-free replacement solder-but unfortunately, it just hasn’t been invented yet. There are advantages and disadvantages to all of them. The keys are to become educated on the issue, to have a controlled transition plan in place, and to know how it affects your system. Look at the safety, reliability, and mission requirements of your system and ensure that these changes don’t attack them. We’re going to continue to build reliable, supportable systems, how exactly we’re going to do that is still yet to be determined.”
“It’s a very hard time for a lot of people because we don’t know how this issue is going to settle out, and when,” Patterson says. “That’s what people are searching for-answers.
“The bottom line is we are not going to ship unreliable products out to the field, out the troops-period,” Patterson insists. “No ifs. No ands. No buts, and it’s not just us. There’s no cost-effective solution out there yet today, but there will be. Hang in there. We’re all in this together, and we’re busy working on the solution together.”
Radstone advances reliability of lead-free products, processes
More often than not, a discussion about lead-free products and processes in the electronics industry will turn to the phenomena of tin whiskers. Lead-free products suffer an increased risk of tin whiskers over tin-lead solutions. Nevertheless, it is possible to produce lead-free products that have no bigger risk of developing tin whiskers than their leaded counterparts, says Andy MacCaig, operations director for Radstone Digital Processing in Towcester, England. Radstone Digital Processing has been working on safe and efficient lead-free options for more than 18 months.
“I think a lot of people assume that lead-free and tin whiskers are directly linked, and a lead-free product must be more susceptible to tin whiskers,” MacCaig says. “That’s not necessarily true, if the right steps have been taken to mitigate that risk. We work closely with our suppliers, from the start of a design process all the way through to the manufacturing process, to make sure the risk of tin whiskers is as small as we can make it, and certainly no greater than a leaded product.
“The thing that’s important to us and to our customers is that we can demonstrate that we have the required level of reliability of those products in mil-aero applications,” MacCaig continues. “It’s about validating our lead-free processes, and making sure we have the qualification data available to prove that a product is reliable.”
Radstone takes umbrage with manufacturers who test their products at room temperature, evaluating before and after a temperature cycle. Under those circumstances, the test results might not reveal solder-joint fractures that cause a product to fail only when it’s at an elevated temperature. As a result, Anderson and other Radstone personnel make a point of testing the product the entire time that it being temperature cycled.
The first board samples that the Radstone team temperature-cycled with a lead-free process, roughly 18 months ago, withstood only 10 thermal cycles from -40 to +85 Centigrade before solder joints fractured-not nearly the level of reliability required. Company representatives worked with various solder manufacturers on the exact composition of the solder paste and on the soldering profile.
“It’s really about optimizing the mix of materials that make up that lead-free solder alloy,” Anderson says. “We now have a process that can withstand well over 1000 thermal cycles and still be reliable.”
Resources
AiTech:
www.rugged.com
Best Manufacturing Practices:
www.bmpcoe.org
Defense Microelectronics Activity (DMEA):
www.dmea.osd.mil
Europa, Gateway to the European Union:
www.europa.eu.int/comm/environment/waste/weee_index.htm
NASA Goddard Space Flight Center:
http://nepp.nasa.gov/whisker/background/index.htm
Radstone Digital Processing:
www.radstone.com/home/rad_digital-processing.aspx
U.K. Department of Trade and Industry:
www.dti.gov.uk
University of Maryland’s Center for Advanced Live Cycle Engineering (CALCE) Electronic Products and Systems Center:
www.calce.umd.edu/lead-free
