Tin-lead components alive and well in the military sector
This article is a response to a story in the July 2007 issue of Military & Aerospace Electronics entitled “RoHS one year later: supplies of leaded solder drying up,” by Keith Gurnett and Tom Adams.
By Chris Reynolds
Editor’s note: This article is a response to a story in the July 2007 issue of Military & Aerospace Electronics entitled “RoHS one year later: supplies of leaded solder drying up,” by Keith Gurnett and Tom Adams.
Contrary to popular opinion, tin-lead termination devices are alive and well, and many devices are readily available from stock. Recently articles have been published that have stated incorrectly that passive components with tin-lead terminations are either not available or will soon become “extinct.” There are reasons for this, and there is some logic behind it, but this is a very significant overstatement.
Despite the perception that the European Restriction of Hazardous Substances (RoHS) has overwhelmed component supply and production assembly technology, today in North and South America more than 79 percent of the solder used is still tin-lead, according to the IPC Solder Quarterly Survey, 2nd Quarter 2007 (www.ipc.org).
In the case of multilayer ceramic capacitors (MLCCs), for example, many of the manufacturers in the world no longer produce parts with tin-lead terminations. This market is dominated by Japanese manufacturers who had already engaged in “lead-free” for their domestic customer base significantly in advance of RoHS.
These suppliers maintain little presence in the worldwide defense industry, however, so today many U.S. suppliers continue to provide tin-lead parts. In other words, despite the overall perception, there is almost no change in defense applications.
The same is true in many other product areas, where Far East manufacturers represent far larger market shares of the global market than they do—or ever have—in the defense and aerospace business. So the scaremongering taking place regarding RoHS dominance and the lack of availability needs to be placed in context and corrected to ensure that perception matches reality. Many manufacturers, and particularly AVX Corp. in Myrtle Beach, Calif., are working hard to ensure that customers who require the more robust and reliable terminations that incorporate lead have a stable and long-term source of supply.
Let’s take a more detailed look at the component-level evolution of “lead-free.” The compliance date for RoHS in Europe was 1 July 2006. All products encompassed by this directive that were on the shelf in EU countries by that date were mandated to contain less than 1000 parts per million (ppm) of five materials (lead, mercury, hexavalent chromium, polybrominated diphenyl ether, and polybrominated biphenyl) and less than 100 ppm of cadmium. To meet this deadline, RoHS-compliant product had to be shipping well before this date.
In the early stages of RoHS evolution, it was recognized that, if all individual components and processes were RoHS compliant, any complex system manufactured with these components and processes would necessarily be RoHS compliant.
Similarly, a component would be RoHS compliant if each individual constituent of that component were RoHS compliant—thus was born the concept of homogenous materials. Although the RoHS directive does not completely define a homogenous material, it does refer to materials that can’t be mechanically disjointed and gives guidance by example—citing a standard IC package—composed of elements including silicon die, gold contact plating, leadframes, wires, etc. The intent is that each of these constituent materials be RoHS compliant in themselves.
By analogy, the homogenous materials of an MLCC would include the dielectric body, electrodes, internal termination, and external plating. For some passive families, ensuring compliance to all six RoHS materials required several changes. Using MLCC as an example, the internal termination material could originally contain traces of cadmium. Although the internal termination material represents only a small fraction of the overall component mass, the termination cadmium content could exceed the low 100 ppm limit, and alternative materials are now used.
In other cases, the dielectric itself could contain traces of lead exceeding the RoHS limit. In the case of some class 1 dielectrics, this is a necessary part of the formulation required to maintain their electrical properties, and is exempt under annex 7 of the directive.
Needless to say, efforts by manufacturers to make passive components RoHS compliant have been huge and largely invisible. They had also, in the most part, been completed several years prior to the RoHS deadline. I say “largely invisible”—but this is really in reference to the “RoHS Five” materials—a term now used for everything excluding lead. In the case of lead content, the situation is just the reverse.
For most military and avionics users, any “fix” to a system that “ain’t broke” is a concern, so changes to the long established tin-lead soldering systems are consequently under close scrutiny.
A solder joint comprises a bond between the printed circuit board (pcb) metallization and component termination using a reflow or wave solder medium. While tin-lead can still be maintained for PCB metallization and reflow/wave, one area of concern is how to source tin-lead plated parts when the commercial trend is to lead-free, so this is a good time to review the history of component plating and review the component level tin-lead options that remain available.
Lead-Free is just one aspect of RoHS, but the general trend to “lead-free” or “green” systems predates the RoHS directive by many years. Many Japanese OEMs pioneered the technology in their systems, and as a result most Japanese active and passive component suppliers adopted lead-free terminations.
For high-cost actives, such as integrated circuits with complex leadframe structures, lead-free can include several exotic plating materials. For low-cost, high-volume passives, the common denominator for lead-free was matte tin plating—equally applicable to simple leadframe designs (tantalum chip) and overplated chips (MLCC, resistors etc.).
Another advantage is that matte tin is forward and reverse compatible—it provides a solderable surface that could be used in traditional tin-lead systems or any number of the emerging SAC (tin/silver/copper) reflow formulations or tin copper wave solder systems. When it came to adopting lead-free terminations for RoHS compliance, this was technically straightforward and for many component manufacturers already in place.
Logistically, when it came to implementation, there were several variations on a theme. For mil-spec parts, termination material composition was part of the specification, so tin-lead solder plating was maintained. This also applied to so-called COTSplus parts—“commercial off-the-shelf” parts rated with high-reliability screening.
For commercial passives, most specifications did not specify termination material composition as long as solderable surfaces were maintained—meaning that commercial parts could range from 100 percent lead to 100 percent tin.
There was no “magic switch” to convert a large number of commercial parts, so lines were converted by phasing in new external plating by case size, rating, or dielectric type depending on manufacturing setup. Dates for the completion of the conversion process were listed for all products. As the parts were interchangeable in tin-lead or lead-free systems, this meant that volume manufacturers could get ahead of the curve without requiring changes to their extensive bills of materials.
At the front end of the conversion, several users required matte tin plated parts preferentially for builds for “green” systems well before the RoHS cut-in date, so special part numbers were created for tin plating for those users in advance of full conversion. Similarly, at the back end, there were many military and aerospace users of commercial products who wanted to retain tin-lead, and now special part numbers are available for tin-lead versions of commercial passives.
Although many suppliers posted their plans for commercial parts at the time, they were often overlooked by military and aerospace users. The dialog surrounding the advent of RoHS served to raise awareness, but long after many passive conversion plans were completed.
The new challenge was for passive manufacturers to reverse-engineer many of their new products and offer tin-lead plating options. Often a part number can include a field to designate a termination finish option, but in many cases, AVX has created new series designations to completely differentiate the two types. The company did this because truncation can occur in long part number fields as bills of material are exchanged between different systems; a critical designator at the end of the part number can be lost. Establishing a new series designator at the front of the part number means that there is less chance of confusing termination types.
This aspect has become very important; one other facet of RoHS implementation was a vast amount of data exchange—suppliers are required to provide detailed MDS (Materials Data Sheets) with full ppm information on all content by part type for customers to collate into a materials compliance listing for a finished system. The terabytes of information thus generated represents a large investment by supplier and user alike, and the need to keep lead content out of RoHS products requires continuous monitoring of all base materials used.
Likewise, for tin-lead military and aerospace applications, maintaining a minimum lead content is also under close scrutiny. For the manufacturer, this means that the tin and tin-lead plating processes must be completely separated, which can influence the cost of a commercial part with tin-lead plating given the relatively low volume of the tin-lead production.
In practice, however, both systems are robust to the occasional hiccup. It would take many non-RoHS components to render a system non-compliant if the ppm content were assessed at a system level. Likewise, there is no major issue with using matte tin-plated parts in tin-lead military and aerospace systems.
The majority of high-volume passives have been matte tin terminated now for 10 to 15 years. This represents trillions of solder joints between matte tin plating in tin-lead solder. One concern has been the possibility of tin whiskers, but in practice these surfaces will be alloyed into the reflow or wave media, and they will have the benefit of additional annealing by the solder temperature.
The delineation between tin and tin-lead is here to stay, at least in the short term.
AVX has tested the performance of matte tin terminations in tin-lead and a variety of tin/silver/copper (SAC) and tin-copper systems. Wetting force and wetting time show that most SAC formulations perform well but tin-lead does have the better overall characteristic.
Experts at the U.S. Defense Supply Center Columbus (DSCC) in Columbus, Ohio, are also evaluating long-term joint reliability with SAC systems with good results. However, while military and COTSplus parts remain available with tin-lead plating as standard, and new families of tin-lead-plated commercial products are introduced, tin-lead solder processing looks set to stay in place for mission-critical operations for some years.
Chris Reynolds is product manager at AVX Corp. in Myrtle Beach, S.C. Visit AVX online at www.avxcorp.com.