Analyzing the Debate of Clean vs. No-clean

Estimated reading time: 6 minutes

Most consumer-based products have adapted a true no-clean strategy, primarily due to rapid technological changes within the market. This study illustrates that products manufactured using a no-clean label are not a guarantee of long-term reliability, demonstrates the impact of this, and highlights advantages of fully integrated cleaning processes for no-clean products.

By Umut Tosun, M.S., Ch.E, and Harald Wack, Ph.D.

We all are experiencing new technological advances, on personal and professional levels. Most consumer-based products have adapted a true no-clean strategy, primarily due to advancements.

No-clean products offer paste-specific advantages that have found broad application among high-end electronics industries such as aerospace, avionics, and military and defense. Within these markets, no-clean pastes must be cleaned to eliminate any impact of the low-residue nature, according to J-STD-001, Class 3. The term “no-clean” was chosen as a synonym for achieving identical product quality at lower overall process cost by eliminating cleaning as an integrated process step. Often, products manufactured with a no-clean label are not a guarantee for long-term reliable assemblies.

The missing link between in-field-failure rates and climatic and leakage-current measurements for electronic assemblies have yet to be established. Actual weather conditions are unfortunately not adequate to simulate in-field conditions, and existing micro-climates at particular assembly locations are influenced by site-specific factors. The documentation of micro-climatic conditions for electronic assemblies has only recently been possible due to newly developed sensor technologies. Consequently, there is a lack of available information at this time. In the past, such efforts have been seen in the automotive sector, particularly in areas plagued by high failure rates, such as electronic switches.

Studies on the long-term behavior of no-clean encapsulations show that the integrity of such films can be compromised (Figure 1). This phenomenon depends on the quality of encapsulation during the soldering step, as well as the degree of actual in-field temperature fluctuations (cycling). Some resin systems also become brittle through oxidation reactions, and therefore guarantee protection for a limited period (Table 1).

Figure 1. Encapsulation of organic activators.

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HDI assemblies, particularly in motor vehicles, are used more frequently. The use of high-resistivity components accentuates the sensitivity of these circuits to environmental interferences. High-frequency circuits between 30 MHz and 5 GHz are particularly affected. To maintain signal integrity, these systems require not only an adequate ohmic-insulation resistance, but must also have stable, complex impedance. Parasitic capacitances of contamination can distort the ramp-up of the signal, disrupting integrity and leading to equipment malfunctions (Figure 2).

Figure 2. Influence of high frequency on complex-resistance boards. Flux activator residues can change the impedance of connection surfaces and cause pad geometry enlargements.

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Proof of Cleanliness

Reductions in SIR and capacitive potential that activator residues can build up can be shown qualitatively under a scanning electron microscope (SEM). Imaging such contamination is possible using a test that responds selectively to carbon-acid-based activators. Another direct measurement to determine resistivity values (i.e. of remaining no-clean residues) can be performed through impedance spectroscopy. The surface resistance underneath chip resistors and capacitors can be determined to show the improvements cleaning provides with respect to surface resistivity (Figure 3). For example, during studies, impedance was measured on identical components on five assemblies. These tests were repeated after the cleaning process. Measurements matched, indicating a high level of cleanliness across all assemblies.

Figure 3. Detection of contamination by means of impedance spectroscopy.

This test, in conjunction with other reliability tests, allow assembly behavior to be determined under appropriate climatic conditions to assess the overall benefits cleaning might have on products.

Post-soldering applications, such as the use of protective conformal coatings, should also be included in the clean vs. no-clean manufacturing process discussion. During studies, delamination and electrochemical migration were documented underneath coatings up to 0.4" thick. Consideration must also be given to increasing bleed from within assemblies and components. This can limit the long-term adhesion of coatings and underfill materials. Unfortunately, these critical precipitations are undefined, difficult to characterize or predict, and not monitored. Deteriorated signal integrity will not be explainable or reproducible.

Figure 4. Contaminated IC needles showing conventional flux residues (left) and lead-free flux residues (right).

The no-clean process ideally should encapsulate all soils and residues (or evaporate all critical compounds during soldering) to render them ineffective against corrosion and leakage currents. On the other hand, these hardened films can affect the ability for IC testing (Figure 4). The defect rate of IC measurements decreased significantly with a proper contact on residue-free surfaces. Furthermore, these films often lead to faulty measurements. Contaminated test needles increased needle wear-and-tear, which contribute adversely to overall process costs related to cleaning.

The presence of remaining no-clean flux residues also can impact visibility, especially during the automated inspection of soldered connections where various reflections and contrast impairments are a concern. A lower defect rate (i.e. less rework) is achieved with the use of clean processes.

Introducing a cleaning process for the removal of no-clean flux residues adds to equipment and cleaning-agent costs. Such expenses, can be justified when compared to various cost contributors of a no-clean process. For many electronic manufacturing companies, the consumption of nitrogen (even for modern oven systems) reflects one of the main consumable cost contributors for no-clean processes. In light of lead-free, using nitrogen will be less expendable with promoted oxidation due to higher soldering temperatures.

Cost and reliability considerations aside, other benefits point to an integrated and stable cleaning process. Soldering serves to create soldered and reliable connections. The addition of a cleaning-process step introduces additional flexibility through activated solder pastes and/or fluxes. This results in an extended soldering process window, i.e. shorter soldering profiles and improved tolerances for process fluctuations. Aside from appropriate soldering conditions, cleanliness levels of assemblies must be considered as the second priority for no-clean processes. In comparison, methods with an integrated cleaning process allow for more freedom. Here, the functionality of each process step that increases the output and reduce superfluous rework steps is important.

For Class 3 products, the J-STD 001D stipulates optical cleanliness (20-40×), as well as a rosin content of <258 µg/in2. Ionic contamination values of <10.06 µg/in.2, SIR conformance, and other cleanliness standards are also required. With lead-free, higher amounts of activators and rosins are used, rendering the J-STD001D conformance more difficult to achieve without a fully integrated cleaning process. An overlooked benefit of a clean process is the elimination of any material specification with regard to the actual (no-clean) remaining contamination. By taking advantage of an integrated cleaning process, companies profit from reduced costs and failure rates. This process would result in overall improvement of adhesion of conformal coatings and wire bondability.

Conclusion

With the onset of globalization movements, cost and logistical considerations are becoming more prominent for domestic manufacturing companies to remain competitive. No-clean processes have not only proven themselves effective, but will continue to play an important role. With more experience and knowledge being gathered, we are witnessing numerous high-quality assembly producers reverting to cleaning. The overall benefit of cleaning can realized only by studying the positive effects on production costs, product quality, and long-term climatic reliability. Cleaning also is becoming a requirement due to the increased occurrence of high-frequency technology and the introduction of lead-free solder pastes. Due to the shortcomings of no-clean technologies, the debate of clean vs. no-clean results in one conclusion - cleaning is necessary for critical, highly valuable applications.

For a complete list of references, please contact the authors.

Umut Tosun, M.S., Ch.E, application technology manager, Zestron America, may be contacted at (888) 999-9116; e-mail: u.tosun@zestronusa.com. Harald Wack, Ph.D. is the executive vice president and CEO of Zestron.

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