A two-phase study to assess how to completely clean underneath leadless devices following soldering with Pb-free water soluble and Pb-free no-clean pastes.

Ed.: Part 1 was published in the August issue.

Phase 2 methodology. In this phase of the study, eight pastes were evaluated: four water-soluble lead-free (Pastes A, B, C and D) and four no-clean lead-free (Pastes E, F, G and H). Additionally, Practical Components’ PCB007 test vehicle was used. In this case, boards and components were provided to RIT, where they were screen-printed and reflowed. Following reflow, the assembled boards were returned to Zestron and cleaned within 24 hr. of reflow in an Aquastorm AS200 inline cleaner using the dynamic surfactant and optimized wash parameters from Phase 1 (see Table 6, previous issue). Cleanliness assessment followed, including visual inspection and ion chromatography, each conducted at Zestron. Regarding visual inspection, the same protocol used in Phase 1 was employed for Phase 2.
In total, 24 test boards were assembled or three with each paste type. Of the three board sets, two were used for ion chromatography analysis and one for visual inspection. Each board was populated with the component quantities as detailed in Table 7. For reflow, RIT employed a six-zone industrial oven. Refer to Table 8 for the reflow zone set points and Figure 13 for the documented reflow profile.

Following reflow, visual inspection using a Keyence VHX-1000 microscope confirmed standoff height at 2 mil (Figure 14).

Phase 2 results: visual inspection. Of the 24 boards assembled for Phase 2, eight boards (one with each paste type) were visually inspected on the surface, as well as underneath the components utilizing the same protocol as that used in Phase 1, following the cleaning process. Optimum cleaning parameters identified in Table 6 were held constant throughout Phase 2 trials. Using these parameters and maintaining the wash temperature at 145°F, all water-soluble pastes were fully cleaned on the board surface underneath all components. However, as suspected with the no-clean pastes, MLF-68 components reflowed with Pastes F and H, as well as all the dual-row components, required 155°F wash temperature to achieve full cleanliness.

The determination to increase wash temperature was made during the visual analysis. Since multiple boards were prepared for each paste, and each board was cleaned individually, wash temperature adjustments were made as required for subsequent trials.

Thus, for the no-clean pastes, the following observations were made:

Pastes E and G:
Completely cleaned underneath MLF-20, MLF-44 and MLF-68 components using process parameters determined in Phase 2.
Dual-row MLF-156 components were completely cleaned underneath at 155°F wash
temperature.

Paste H:
Completely cleaned underneath MLF-20 and MLF-44 components using process parameters determined in Phase 1.
Completely cleaned underneath MLF-68 and dual-row MLF-156 components at 155°F wash temperature.

Paste F:
Completely cleaned underneath MLF-20, MLF-44 components using process parameters determined in Phase 1.
Completely cleaned underneath MLF-68 components at 155°F wash temperature.
Slight residues remained under one of three dual-row MLF-156 using 155°F wash temperature.

Table 9 shows wash temperature requirements for full cleanliness underneath all components.

Surface residues were completely removed on all boards with all paste types. Figures 15, 16, 17 and 18 are representative of the cleanliness level achieved underneath for all four component sizes.

Phase 2 results: ion chromatography. Sixteen boards (two for each paste type) were assembled for ion chromatography analysis. Ion chromatography analysis was performed to characterize ionic residues on the board surface in terms of anions (fluoride, acetate, formate, chloride, nitrite, bromide, nitrate, phosphate, sulfate), cations (lithium, sodium, ammonium, potassium, magnesium, calcium) and weak organic acids. If contaminants are present on electronic assemblies and bare boards, they contribute to electrochemical failures when mixed with moisture and applied voltage.³

Ion chromatography testing was performed according to IPC-TM-650, method 2.3.28.3 Test equipment used enabled analysis of anions, cations and weak organic acids. All boards tested resulted in contamination levels well below IPC limits. Ion chromatography results are detailed in Tables 10, 11, 12 and 13 (Appendix).

Conclusion

Effective cleaning of flux residues underneath low-standoff components is difficult and is made more so as component surface area increases. However, as this study has proved, this challenge can be met with an appropriate cleaning agent and optimized cleaning process.

Flux type is certainly a critical factor, and water-soluble flux residues are more easily cleaned compared to the no-clean flux residues.

For water-soluble flux residue, cleaning agent concentration, wash exposure time and spray configuration proved less critical for achieving full cleanliness underneath the components.

For the no-clean flux residue, higher cleaning agent concentration, increased wash exposure time and the eight spray bar manifold with the intermix nozzle technology resulted in better cleaning results underneath all QFN components. In several cases (depending on paste type and component configuration), an increase in wash temperature was required as well in order to fully clean underneath the QFN.

As indicated in Table 9, slight residues remained under one of six dual-row MLF-156 components. However, it should be noted that all boards subjected to ion chromatography testing resulted in contamination levels well below the standard limits.

Although specific no-clean paste types, as well as the MLF size, influence the process parameters required to clean underneath components, this study confirms through visual inspection underneath all QFN types, as well as with ion chromatography results, that a dynamic surfactant cleaning agent and optimized inline cleaning process can fully clean the surface underneath low-standoff QFNs.

References
2. Harald Wack, Ph.D., Umut Tosun, Joachim Becht, Ph.D., Helmut Schweigart Ph.D., “Why Switch from Pure DI-Water to Chemistry?” SMTA International Proceedings, October 2009.
3. IPC-TM-650, Test Methods Manual, Method 2.3.28, Ionic Analysis of Circuit Boards, Ion Chromatography Method, May 2004.

Acknowledgments
Special thanks to ERSA North America for providing the 10 zone ERSA HotFlow 3/20 reflow oven, Rochester Institute of Technology for populating and reflowing test boards, Speedline Technologies for providing the Aquastorm 200 inline cleaner, and StenTech for providing the stencils for this study.

Ed.: This article was originally published in the Proceedings of SMTA Penang in April 2013, and is republished here with permission.

Umut Tosun is application technology manager; Naveen Ravindran is application engineer, and Michael McCutchen is the former vice president Americas and South Asia at Zestron; u.tosun@zestronusa.com.