Lead-Free Test and Inspection
Major industrial nations around the world are rapidly moving to eliminate lead from electronics manufacturing processes. Some companies are taking advantage of the situation and using "lead-free" as a major marketing initiative in the consumer market. Others are delaying the inevitable, in spite of the worldwide lead-free legislation.
The move to lead-free solder has an impact on all phases of printed circuit board (PCB) assembly, including test and inspection. Among these numerous technical issues is the impact of lead-free solder on the major test and inspection technologies, including automated optical inspection (AOI), automated X-ray inspection (AXI), in-circuit test (ICT) and functional test.
New Solder Formulations
The upcoming ban on the use of lead has prompted electronics manufacturers and industry organizations such as the National Electronics Manufacturing Initiative (NEMI) and IPC-Association Connecting Electronics Industries to consider several alternatives to the traditional tin-lead solder chemistry. The results have been NEMI's "Roadmap of Lead-free Assembly in North America," the "Lead-Free Hybrid Assembly and Rework Project," and the creation of the IPC 7-32 solder inspectability standards committee.
New lead-free formulations include tin-silver-copper and tin-copper. The majority of the electronics industry seems to be moving toward the tin-silver-copper family of alloys for lead-free soldering. NEMI has recommended an "industry standard" lead-free alloy that is 95.5 percent tin, 3.9 percent silver and 0.6 percent copper ( Sn3.9Ag0.6Cu ± 0.2 percent) for reflow and 99.3 percent tin, 0.7 percent copper (Sn0.7Cu) for wave solder. However, as with any process change, it is important to carefully consider the most appropriate mix for a broad range of applications, along with the logistics and economics of specifying a particular alloy.
Lead-free solder mixes have higher melting points, which can lead to component or board damage. With lead-free solder formulations, melt temperatures rise from 183 C to approximately 217 C for tin-silver-copper, with temperature spikes as high as 260 C. The high temperatures can be somewhat reduced by longer preheat times. Rework temperatures are also affected, with some parts reaching 280 C.
As a result, components need to be qualified for these higher temperatures, and some nonqualified components may actually require hand assembly.
Optical Inspection Issues
Inspecting lead-free solder terminations is fundamentally no different from inspecting a conventional tin-lead joint, because the two are similar in appearance. The key to inspecting either type of solder is an inspection mechanism that can correctly measure the visual attributes of each image type.
That said, there are some differences in the visual appearance between lead-free and tin-lead solder joints that can affect AOI systems. Lead-free solder joints are typically rougher and more striated than corresponding lead joints, due to the differences in phase transition from liquid to solid. Specifically, some of the components that go into lead-free solder solidify at slightly different temperatures-in contrast to conventional lead solder where the entire liquefied alloy solidifies at the same time. The resulting push and pull from these different components in their different phases yields an uneven surface. As a result, the joints may appear slightly dull and uneven.
Lead-free solder also has a higher surface tension when in liquid phase and does not flow as readily as lead solder, causing a slightly different-shaped fillet. These visual differences will probably require that manufacturers recalibrate AOI equipment and software. For example, automatically learned pass levels in some AOI systems may be slightly different for lead-free joints.
This increased surface tension can also result in an increased number of "tombstone" failures, where one end of a component is raised and separated from the PCB while remaining attached to the board at the other end. This is, not surprisingly, a greater problem with smaller, passive components.
If you are currently using human inspectors and considering moving to AOI systems, now may be a logical time to do so because human inspectors will need "recalibration," in the form of retraining and new inspection standards anyway.
Study OKs AOI for Lead-Free
In 2002 the National Physical Laboratory (NPL, Middlesex, UK), the United Kingdom's national standards laboratory, independently evaluated the ability of AOI systems to inspect lead-free solder joints. It then published the results in July 2002 under the title "A Comparison of Automated Optical Inspection Systems for Use with Lead-Free Surface Mount Assemblies." The goal of the project was to determine if lead-free assemblies presented any problems for AOI systems.
The test vehicle used in the study was a single board type developed specifically for the study. A number of boards were produced, some with defects, some defect-free. Each assembly contained nearly 100 components with a total of over 1,400 lead-free solder joints. The component types included 0.4-millimeter-pitch 256-pin quad flat packs, 0.5-millimeter-pitch thin small outline packages, and 0402 resistors. The defect categories included missing components, misaligned components, components of correct size but wrong value, poor quality solder joints, components with wrong polarity, solder bridges and poor component planarity.
The study evaluated AOI systems from six manufacturers. Identical software algorithms were used for lead-free inspection and for inspection of conventional lead-solder assemblies. The study found that the results for lead-free inspections were similar to or better than those performed on PCBs assembled with conventional lead solder. False detect rates were also similar for both sets of results. Test times were unaffected by the lead-free nature of the test vehicle.
Although results did vary slightly among the different machines, the study concluded that most AOI systems can be used to inspect lead-free surface mount assemblies. Some systems using color-based algorithms and systems that rely on single cameras have experienced problems in evaluating lead-free solder joints. But these companies have subsequently rewritten their inspection algorithms to accommodate the changed appearance of the joints. As a practical matter, thanks to the existence of a range of different methodologies, a color image is not needed by an AOI system to analyze a solder joint; a monochrome image contains all the necessary information required for solder-joint analysis. Angled camera systems normally performed better at detecting some J-lead defects, such as bridges and insufficient solder joints.
Automated X-Ray Inspection Issues
With lead-free solder ball joints, we can expect an increase in voids, because of the more aggressive fluxes that must be used and their subsequent outgassing. Not only that, longer reflow cycles provide more time for smaller, harmless voids to coalesce into larger ones.
Fortunately, lead-free solders have similar densities to lead-based solder, allowing cracks and voids in the solder joint to be easily detected. Copper, tin and silver are still dense materials and therefore, like lead, impede X-rays. Some recalibration of the X-ray system may be required, but all the X-ray inspection companies-whether they produce manual X-ray or automatic X-ray inspection systems-have concluded that lead-free has no major inspection issues.
This does not mean, however, that there isn't still plenty of opportunity for continued research to better characterize good joints, monitor the assembly process and, most importantly, analyze the structural integrity of joints.
Impact of Lead-Free Solder on ICT
Although tin alloys are the lead-free choice, tin has demonstrated a "whisker" phenomenon-small protrusions of metal that "grow" out of a solder joint or pad. These whiskers can become large enough to short across two lands and can carry sufficient current to cause equipment malfunction. This can be found easily with an in-circuit test. However, tin whisker growth can take time and may be a long-term reliability issue. In fact, whisker growth often occurs well after the solder has cooled if the joint is put under stress. Efforts are underway in organizations such as NEMI to minimize this phenomenon with different tin alloys.
To help with the reflow process for lead-free, higher than normal amounts of flux are often used to help counter potentially higher oxidation rates at higher operating temperatures. In a no-clean environment, this can have a detrimental effect on fixture performance with increased contact resistance and a buildup of contamination on probe tips. As a result, assemblers will have to increase maintenance of fixtures and maybe change to more aggressive probe tip styles. At the same time, though, more aggressive probe tips may conflict with the brittle nature of lead-free solder and cause damage. The brittle nature of lead-free solder also means that more care will have to be taken to limit the flexing of boards in the test fixture.
Excess flux can also have a detrimental effect on radio frequency (RF) circuits, such as modem PCBs with high-speed serial communication protocols. This may put more pressure on the functional test to detect RF-related problems due to board contamination.
Rework and Repair Issues
A final area to consider is the impact of lead-free solder on rework and repair. Higher temperatures are needed to melt lead-free alloys. If you have larger components on a board, you may need to preheat the component because of the larger heat dissipation. With the introduction of lead-free solder and the elimination of certain fire-retardants within the bare PCB, the higher temperatures required for rework may damage the component or board. These problems are being investigated by the NEMI lead-free hybrid assembly and rework project. Many companies may find that they need to minimize or eliminate defects from the PCB assembly line, and the "zero-defect" line may become a reality. Also there will be increasing demands for accurate diagnostics for device- and pin-level failures and an elimination of any false fails from the inspection systems.
This article is based on a presentation at the 2004 APEX Designers Summit in Anaheim, CA.
The state of lead-free legislation varies dramatically in different parts of the world. In fact, Western Europe is almost unique in having taken definitive action limiting the use of the lead in electrical assemblies. In its Waste Electrical and Electronic Equipment (WEEE) and Restriction on Hazardous Substances (RoHS) directives, the European Union (EU) is calling for the abolition of a number of substances by July 1, 2006, including lead, cadmium, hexa-chromium and a number of flame retardants. Unfortunately, the directive at this point is just that, a directive. The vast majority of the union's member states have not done the work necessary to give it the force of law, even though there was an August 2004 deadline for doing so. As a result, the situation is a fairly chaotic one with regard to specific guidelines and definitions.
In the United States, it's a free-for-all with regard to lead-free at the federal level. However, in 2003 the State of California passed the Electronic Waste Recycling Act, or SB 20 legislation, which is similar but not identical to the EU directives. Recently, the state approved SB 50 legislation to further hone SB 20 and make it easier to implement.
At this point, there are no restrictions on the use of lead in electronic assembly in China. But that could soon change. In June 2002 China enacted the Clean Production Promotion Law, which took effect at the beginning of 2003. Currently China's Ministry of Information Industry is working on restrictions for the use of hazardous substance similar to the EU's RoHS directive, with a focus on consumer electronics. A final draft of the proposed legislation is expected in early 2005. Whatever the government eventually decides to pass, it will likely go into effect in July 2006.
In Japan, there are strict laws governing waste disposal and the recycling of manufactured goods. But the actual trend toward the use of lead-free solder is largely voluntary and not mandated by government. One look at the green policies for major electronics manufacturers like Sony and Panasonic, and it is apparent that these are companies are serious about minimizing the environmental impact of their products. Sony, for example, has been lead-free in accordance with its own company policy since 2002. In fact, "green" electronics have proved to be very popular among Japanese consumers, with the result that Japan leads the world in the consumption of lead-free solder paste.
In South Korea, government and industry have established a voluntary program to phase out the same chemicals proscribed by RoHS. Over 300 companies representing approximately 95 percent of South Korea's electronics production have signed on. South Korea has also passed legislation making manufacturers responsible for recycling their products as they enter the waste stream.
For more on lead-free legislation worldwide, visit the IPC-Association Connecting Electronics Industries legislation and regulation Web page at http://leadfree.ipc.org.