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Lead-Free Cleaning: Moving from Eutectic to Lead-free
December 31, 1969 |Estimated reading time: 5 minutes
As of July 1, 2006, the WEEE Directive will eliminate the use of lead in solder paste. The industry is intensively Developing solder-paste systems that serve as an alternative to the tin/lead (SnPb) solder paste used. But switching to lead-free paste will increase the need for chemically supported cleaning.
By Umut Tosun
As early as the 1950s, electronic circuit failures were documented due mainly to electrochemical migration of solder connections containing silver. Because of the low silver concentration, the dendrites caused by electrochemical migration (EM) cannot be determined via discontinues in resistance measurements (Figure 1). Temporary failure, however, may occur during the measurement period. The high affinity of silver used to form silver hydroxides and sulfides, respectively, is the reason for the increase in EM (Figure 2). The larger quantities of activators, which tend to be highly hygroscopic in character, are required due to increased reflow temperatures. If they are not partially or fully removed, they can induce moisture films on the assembly. This, in turn, leads to electrochemical migration and the observed formation of dendrites. It seems difficult to encapsulate the high activator concentrations reliably in current lead-free paste formulations, which indicates that no-clean technology has reached its process limit. Many companies have reported that the reliability of the components soldered with lead-free solder pastes can no longer be guaranteed using “only no-clean” methods. Boards are continuously subjected to complex and difficult climatic tests and conditions. Due to the larger quantity of hygroscopic activators, lead-free solder pastes have no sufficient safety margins to pass these tests. Actual assembly cleaning, particularly the removal of increasingly present residues, can contribute to an improvement in reliability under environmental stress, and prevent susceptibility to migration.
Figure 1. Dendrites caused by electrochemical migration.
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Figure 2. Mechanism of dendrite growth.
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Process Changes Caused by Lead-free
Due to the different process conditions required for the use of lead-free pastes, questions regarding the effects of the latter on cleaning processes are being raised frequently (Table 1).
Additional oxidation and polymerization reactions of the flux are possible because of higher reflow temperatures. These reactions cause flux residues to be intensively “baked on” during the soldering process, making them more difficult to remove. To guarantee reliable soldering at higher temperatures, the addition of new flux formulas is necessary with lead-free solder pastes. Solvents with higher boiling points, increased rosin content (solid content) and more aggressive activators to inhibit the oxidation of the solder at higher temperatures are expected to cause an increase in flux residues - increasing demands placed on cleaning processes. A series of tests were conducted with current lead-free solder pastes to determine the effect of the developments in solder paste on cleaning.
Available Cleaning Agents
Cleaning applications commonly used in the electronics industry differ depending on the type of residue to be removed. Generally, a pH-neutral cleaning agent is used to remove solder paste from stencils or misprinted circuit boards, while post-solder flux should be removed using alkaline-based cleaning agents. The three basic cleaning agents are solvent-based, water-based* and alkaline-surfactant-based cleaning agents.
Stencil and Misprint Cleaning: Removing Unsoldered Solder Paste
With the move to lead-free solder pastes, problems in the cleaning and removal of flux residues from soldered assemblies are to be expected due to soldering-process variations. Because of the change of flux formulations, especially with respect to new components or changes in solvents used, resin and thixotropic concentrations, the removal of unsoldered solder paste also may be affected. The lead-free pastes used in this study were tested additionally for the ease of removal from stencils and misprinted circuit boards.
In this test, multiple, stainless steel stencils and PCBs were prepared with the latest lead-free solder pastes available. After an hour of drying time, each stencil, in conjunction with a misprinted board, was cleaned at room temperature in spray-in-air equipment for three to six minutes (Table 2). Subsequently, cleaning tests were repeated at room temperature in ultrasonic equipment. Test substrates cleaned were visually inspected under a microscope (10x) and tested for solder-paste residues. It was shown that all tested cleaning agents removed all lead-free solder pastes (Table 3). Differences were observed in the cleaning times for the different cleaning applications.
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Flux Residue Removal
The removal of flux residues from soldered assemblies is more difficult than the removal of solder paste from stencils and misprints. In cooperation with a reputable local university, various lead-free solder pastes were printed onto standard test substrates and then soldered in a reflow oven at the specific temperature profiles of each respective solder paste. One set of the soldered substrate was cleaned with solvent-based cleaning agents in ultrasonic equipment. Four other substrates were cleaned with water*- and surfactant-based chemistries in standard batch and inline equipment. Cleaning was performed at 122°F for both cleaning chemistries, whereas the surfactant-based chemistries were run at 160°F (Table 4). Subsequent rinsing was accomplished with DI- water before drying the substrates with hot circulated air.
After completing the cleaning trials, the test substrates were visually examined under a microscope (40x) for residues. An ionic contamination test was performed on each substrate. To reveal activator residues from flux not shown by the ionic contamination test, the flux test,** a qualitative color-reaction test, was applied on each individual substrate. This testing procedure detects the carboxylic acid-based activators from the flux, and provides exact information regarding the location and distribution of the acidic impurities (Figure 3).
Figure 3. Flux-test inspection before and after cleaning on Solder Paste B.
The visual inspection for residues, the ionic contamination measurement test, as well as the quantitative color-reaction test exhibited very satisfactory results on most solvent- and water-based* cleaned substrates. Slight levels of remaining residues were detected with less than 5% of the total number of substrates cleaned. The latter were removed completely through a minor adjustment of the previously used process parameters. Testing performed with surfactant-based cleaning agents did provide unsatisfactory cleaning results (Table 5).
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Conclusion
Based on the results of both test series conducted on 40 different lead-free solder pastes, some important conclusions were reached. For the removal of the uncured lead-free solder pastes from stencils and misprints, the results obtained were comparable to those of eutectic pastes.
The removal of flux residues from soldered assemblies with solvent- and water-based* cleaning agents also provided satisfactory cleaning results. Ninety-five percent of examined test substrates were residue-free. For the remaining 5%, which resulted in partial residues, testers were able to remove the latter completely after some minor process adjustments to existing cleaning applications. These studies show that it is possible to remove flux residues easily from lead-free pastes without an additional capital investment. Such findings seem to stand in contrast to other available cleaning agents, as well as other PCB processing steps, which might require costly capital investments during the switch to lead-free soldering.
* MPC-based cleaning agents, Zestron America, Ashburn, Va.** Zestron flux test, Zestron America.
Umut Tosun, application technology manager, Zestron America, may be contacted at (888) 999-9116; e-mail: u.tosun@zestronusa.com.