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Advanced Issues in Assembly: Part 1 Lead Contamination in Lead-free Assembly
December 31, 1969 |Estimated reading time: 6 minutes
Lead-free alloys can lose reliability when combined with lead-coated components or boards. To avoid the problem, reduce the lead-free transition time.
By Karl Seelig and David Suraski
What happens to a lead-free solder joint if it becomes "contaminated" with lead from another source? This question is crucial because during a transition to lead-free soldering it is very likely that tin/lead parts will be used in much of production. In fact, exposure to lead from boards, components and repair operations can occur for years to come.
Heretofore, the presence of lead in lead-free alloys was presumed to be "acceptable," ostensibly because tin and lead are soluble in a lead-free system. Overlooked, however, is that the intermetallic crystalline structures in such systems are insoluble and will precipitate at lead boundaries. Thus, when using a lead-free alloy to solder to Sn/Pb-coated component leads, lead actually can create voids in the solder joint, resulting in a risk of joint failure. For example, with bismuth-bearing alloys, Bi and Pb can form pockets having secondary eutectics of 96°C. This results in probable negative effects on reliability if the assembly is exposed to any thermal stress.
Lead Contaminated Solder Joint Failure DynamicsIt is important to note that when lead contaminates a lead-free solder joint the incursion is not distributed uniformly. Rather, the lead localizes in the last area to cool. This is similar in dynamics to "zone refining," a process for refining high-purity elements whereby as a heat source traverses a billet, the elemental impurities are collected in the liquid phase and are condensed at the end of the billet where they are removed.
Figure 1. A lead-free joint may be contaminated by migrating Sn/Pb-coated component leads during soldering, which will settle at the last area to cool: under the lead at the PCB interface where joint failure is most likely.
Similarly, lead, as an impurity in a solder joint, migrates to the last area of the joint to cool, i.e., under the middle of the component lead at the solder joint/printed circuit board (PCB) interface (Figure 1). Here, the joint forms pockets and the grain structure is disturbed via lead-rich regions with lower melting temperatures. Dewetting is highly probable during soldering, as is the creation of the solder joint failure area.
Figure 2. How a lead sphere from a component lead dissolves into a lead-free solder system (a) during reflow, leading to lead pocket or void dispersion (b) throughout the solder joint.
Figure 2a illustrates how a lead sphere dissolves into an Sn/Ag system during a normal reflow cycle, and 2b is a close-up of how lead pockets are dispersed through the system. Such dispersion is a common part of wetting:
- As the solder wets, the lead dissolves into the joint and concentrates in pockets.
- Next, a eutectic is formed of Sn/Pb/Ag with a melting point of 179°C (vs. 221°C for Sn/Ag or 217° to 218°C for Sn/Ag/Cu). This phase occurs during cooling. The slower the cooling, the larger the pocket that the lower melting temperature alloy will form, a.k.a., a solder joint void.
- As the component heats and cools during the product's life, the void eventually will cause joint failure. Failure rates related to this issue typically occur relatively quickly, i.e., in less than 400 thermal cycles.
Bulk Solder TestingTo determine a lead-free alloy's durability when exposed to lead, an Sn/Ag4.0/ Cu0.5 sample is tested for mechanical reliability after a 0.5 and 1.0 percent contamination of lead. The methodology in the study is as follows: the bulk solder alloy's mechanical strength without lead contamination is tested under low cycle fatigue testing in accordance with ASTM E606. The alloy then is doped with 0.5 percent lead and tested, followed by a doping with 1 percent lead (and tested). The samples then are required to achieve 10,000 cycles without failure to pass the test; results are summarized in the table.
As is seen, Sn/Ag4.0/Cu0.5 alloy passed the requirements. When contaminated with 0.5 percent lead, however, the alloy passed less than 50 percent of the cycles and so failed the test. Further, when contaminated with 1.0 percent lead, the cycles to failure were again reduced by nearly 50 percent another failure. These results are contrary to the assumption that Sn/Ag/Cu bulk alloys are not affected negatively by lead contamination.
Figure 3. Micrograph of a lead-free solder joint fracture resulting from lead contamination and displaying lead pockets.
Joint Failures in the FieldBulk solder strength reductions can affect solder joints as well. Figure 3 is a micrograph of a fracture resulting from lead contamination in an Sn/Ag/Cu solder joint, showing lead pockets. This occurred on an in-field assembly and resulted in a failure. As in Figure 1, the fracture occurred in the middle of the component lead at the solder joint/PCB interface. In actuality, a leading multinational manufacturer recently experienced field failures with a product assembled with Sn/Ag/Cu solder alloy and Sn/Pb-coated leaded components. Samples of the failed solder joints were viewed using a scanning electron microscope (SEM) to determine the possibility of lead or other contamination that could have prompted the failure. Using energy dispersive spectroscopy (EDS) to determine solder joint contamination, lead contamination levels ranging from 3 to 10 percent were revealed. The mating area of the lead-free alloy and Sn/Pb parts is shown in Figure 4. The Sn/Ag/Cu alloy is seen in the lighter areas and the darker Sn/Pb areas surround it.
Figure 4. Micrograph of the mating area of a lead-free alloy and Sn/Pb parts of a solder joint.
In actuality, a leading multinational manufacturer recently experienced field failures with a product assembled with Sn/Ag/Cu solder alloy and Sn/Pb-coated leaded components. Samples of the failed solder joints were viewed using a scanning electron microscope (SEM) to determine the possibility of lead or other contamination that could have prompted the failure. Using energy dispersive spectroscopy (EDS) to determine solder joint contamination, lead contamination levels ranging from 3 to 10 percent were revealed. The mating area of the lead-free alloy and Sn/Pb parts is shown in Figure 4. The Sn/Ag/Cu alloy is seen in the lighter areas and the darker Sn/Pb areas surround it.
Figure 5. The lead-free material comprises the lighter areas and the darker Sn/Pb areas surround it. A micrograph (3,500X) shows a distinct (intergranular separation) phase between a lead-free solder alloy and an Sn/Pb contaminant, a result of poor adhesion.
The failure is an intergranular separation driven by lead in the solder. Figure 5 is a 3,500X photo that shows a distinct phase between the normal grains that cause their separation. The lead forms a ternary alloy of Sn/Ag/Cu that attempts to go to the eutectic at 179°C. This alloy, surrounding the lead-free alloy grains, is the intergranular phase that exhibits the poor adhesion that causes the grain separation.The failure could be the result of a specific heat cycle. By using different heating profiles during assembly, the problem may be minimized (but not eliminated). Accordingly, more solder joints processed using different thermal profiles would have to be investigated.Conclusion There is much interest in lead-free soldering, frequently derived from a fear of legislation and marketing activities. This has spurred a flurry of committee and consortia activity, some of which has been valuable to the industry. However, a pressing question in alternate alloy soldering is that of contamination of the lead-free materials and its effects. As the evidence demonstrates, lead-free alloys can suffer decreased reliability when contaminated with lead from other sources. To avoid problems, the most prudent course is to reduce the lead-free transition period as much as possible. Specifically, when implementing a lead-free solder alloy, it should be accompanied by lead-free component terminations and circuit board coatings. Absent such guidelines, solder joint reliability will be risked.
KARL SEELIG and DAVID SURASKI may be contacted at AIM, 25 Kenney Dr., Cranston, RI 02920; (401) 463-5605; Fax: (401) 463-0203; E-mail: kseelig@aimsolder.com and dsuraski@ aimsolder.com; Web site: www.aimsolder.com.