Electronics manufacturers at all levels are increasingly concerned with the implications of lead-free processing, and in particular with the relatively high reflow temperatures that lead-free solders generally require. In terms of production, their concerns focus on three areas: the finish on component lead frames, the joints that connect components to the board, and the plating on the board itself.
As the implementation of lead-free solders increases, it is becoming evident that labeling, shipping and managing components may be neither simple nor foolproof. Since many components will exist in two different forms - one intended only for lead-free applications, and the other only for conventional lead applications - what assurance exists that no mix-ups will happen in the supply chain?
Confusion of part types
The question is important, not simply because using the wrong component might cause a failure, but because it might not cause a failure soon enough for routine testing to catch. Suppose, for example, that a component designed for high-temperature lead-free solder goes through a conventional lead line. The solder might be hot enough to hold the component temporarily to the board, and to permit temporary electrical conduction. Still, the component might simply come lose and cause a failure when exposed to shock, vibration, and other environmental conditions.
Conversely, a lead-type component might overheat during the lead-free reflow. Internal damage could occur to the component’s lead-based lead-frame finish, or the connecting solder may flow excessively and cause a weak, temporary solder joint. Both of these situations are more serious than an outright immediate failure, which goes immediately to rework. Both of these situations, furthermore, are likely to occur at some point.
Driving forces for lead-free solders
The United States currently has no legislative requirements demanding lead-free solders in any electronics applications, whether military or commercial. U.S. electronics manufacturers would not be concerned with the lead-free issue if their commercial markets were not global in scope. The biggest driving force at the moment is the European Union’s RoHS (Restriction of Hazardous Substances) law, which requires the removal of lead and several other materials from electronics manufacturing before July 1, 2006.
If only things were that simple. First, the RoHS directive does not call for the complete elimination of lead and other materials; instead, it requires a very low level of the materials within a given system. It might thus be permitted for a complex multiboard system to contain one board that uses lead solder.
Possible exemptions
Second, there will almost certainly be some exemptions to the RoHS directive. In November 2004, the EU’s Technical Adaptation Committee was due to receive a list of 12 product types - servers, internal solder bumps within flip chips, internal solder joints within microprocessor PGA packages, and compliant pin connectors - where exemptions appear necessary because there was no feasible alternative to lead or another banned material. Also on the list were items such as optical glass and light bulbs.
The list suggests a certain lack of trust in the long-term reliability of lead-free solders, but it also poses some intriguing questions. If internal solder bumps within flip chips (i.e., the small bumps between the chip face and the interposer substrate) are permitted to use lead solder, but the larger solder balls connecting the substrate to the board must be lead-free, how does an assembler reflow this assemblage without damage to at least one type of solder?
In Europe and the U.S., component makers typically guarantee their components when handled and reflowed under specified conditions. A lead component is typically guaranteed for exposure to 260 degrees Celsius - the standard highest temperature for lead-free reflow - for a maximum of four seconds, a condition that might occur during lower-temperature lead reflow. But during lead-free reflow temperatures typically reach 260°C for about 30 seconds, far hotter than the guarantee covers.
Against this somewhat confusing background, U.S. electronics manufacturers and the military electronics establishment are preparing in various ways for the advent of lead-free solders. Many - probably most - U.S. component manufacturers and assemblers are already doing the research necessary for them to move into lead-free production with a minimum of delay and expense. Nevertheless, the reality of lead-free electronics has not quite arrived.
DSCC’s role in lead-free
Dave Moore, chief of the document standardization unit at Defense Supply Center Columbus (DSCC) in Columbus, Ohio, says he anticipates the first requests for standards for lead-free electronics to come from one of the DSCC suppliers, or perhaps from one of their customers. “But it hasn’t happened yet. We cover both surface mount and leaded technology. We expect to see the push for lead-free to come first in the surface mount area - probably in chip resistors, chip capacitors and ICs,” Moore says.
When lead-free components begin to arrive at DSCC, will they have part numbers that clearly identify them as lead-free to avoid any confusion with lead parts? As far as DSCC is concerned, the answer will be yes. “It is our intent as we introduce lead-free parts to give those parts different part numbers, Moore explains. “Those numbers are marked on the end item, so those numbers will identify for us the lead item and the lead-free item.”
The maker of the component will be responsible for applying the part number. For very tiny parts - some resistors, for example - the part number will go on the unit pack rather than on the part itself, but the identification will still be clear.
Along with the part numbers will come a standard battery of qualification tests for the new lead-free parts. “These are the tests that the manufacturer must perform when there is a design change, so that we are assured that the risk is low.” The use of lead-free solder is in itself a design change, and is usually accompanied by additional changes to the lead frame and other elements.
In the U.K. and the rest of Europe, high reliability is maintained by reliance on the ISO 9000 series of standards and quality controls, which include labeling and traceability. The standards require proof of performance within an envelope of environmental conditions along with clear identification of material. Separate identification of lead and lead-free components will thus be required.
Commercial route
Suppliers of nonmilitary components are taking a very different view of the best means of identifying lead and lead-free components. For months there have been rumors of suppliers’ unhappiness with the vast increase in part numbers that would be involved.
A survey reported in Electronic News in November revealed the truth behind the rumors - at least as far as commercial, nonmilitary markets are concerned. Of the component suppliers surveyed, 94 percent are designing components that will meet the RoHS requirements, but only 53 percent intend to assign new part numbers to the lead-free components. Forty-two percent made their plans clear not to issue new part numbers. Instead, they will leave the identification of a part as lead or lead-free to the package labeling or to the manufacturing date range.
For OEMs and for contract manufacturers, this is not welcome news. It is simply too easy for labeling with any degree of ambiguity to lead directly to reliability problems - and to large numbers of field failures. Some parts will also inevitably be returned by the user to the distributor, and will sometimes be returned without their original packaging. How will these parts be identified as lead or lead-free?
It seems inevitable that there will be some incidence - probably small - of the use of the wrong component type in commercial markets. Within the U.S., the standards set up by DSCC should work well to keep lead and lead-free components separate in military applications. The fact that no suppliers have yet asked DSCC to qualify a lead-free component tells a great deal about the military’s eagerness to deal with these components. In a few commercial applications, a few newly designed lead-free components have recently survived thermal cycling and other qualification tests better than their lead counterparts, but even numerous success stories of this sort would do little to convince the personnel responsible for supplying and repairing military electronics systems over a period of 10 or 20 years or more.
DSCC has received assurances from its suppliers that lead-based components will be available for years to come. This scenario might change if, for example, lead-free solders become the norm in commercial applications and if a given supplier finds that his military business in lead components is too small for continued attention. Thus, there may be a shifting of suppliers and probably some changes in costs, but probably no lack of supplies, even of legacy components.
The real challenge in military applications will be to ensure that all components are clearly identified at all stages as lead or lead-free. Permitting the two types of components to become mixed in the same reflow environment opens the door for dangerous field failures.
