Removing unwanted residues is a collective undertaking.

Innovative electronics assembly designs strive to increase functionality over smaller surface areas. Advances in cellular telecommunications, broadband, storage, computing, medical, aerospace and defense, automotive and other technology-based industries benefit from continuous innovations in advanced packaging that support small form factors. Rapid innovation stimulates growth in each of these technology areas, enabled by the convergence of the circuit board with advanced packaging technologies. Highly dense assemblies reduce spacing between conductors while yielding a larger electronic field. As products become Pb-free and evolve to levels of higher functionality and smaller size, studies show assembly cleanliness becomes more important.¹ Yet design engineers face technology gaps in meeting needed requirements and desired outcomes.

Miniaturization has a tendency to collapse the process window, reduce tolerances and stress current manufacturing processes. Cleaning methods have been of less concern to engineers because of the successful implementation of no-clean flux technologies. This trend is changing as a result of more active fluxes and localized areas of entrapped flux residue, leading to electrochemical migration and electrolytic corrosion.

Figure 1 illustrates the problem. The images show a component before cleaning, after cleaning (where residue is not noticeable), and after cleaning (the view is from under the part). Projects initiated by cleaning fluid and equipment suppliers aimed to address cleaning process gaps. The chemistry suppliers focused on improved cleaning fluid designs that provide low surface tensions and wetting characteristics.² Research into the problem indicated the cleaning fluid was not penetrating the gap to reach soil under low-standoff (<0.002") components.^3,4 By reducing the droplet size of the cleaning fluid, research scientists hypothesized the cleaner would wet and penetrate tights gaps.⁴


As cleaning fluids have improved, engineers have also pressed the importance of process integration with the cleaning machine. Working closely with the OEMs, designed experiments (DoE) were initiated to study mechanical impingement.⁵ The DoE focused on fluid flow, impingement pressure, nozzle type, directional forces and time. From this work, suppliers gained knowledge for integrating the cleaning process using both innovative cleaning fluids and cleaning equipment to meet the challenge of cleaning highly dense assemblies.

All this work – be it individual or jointly conducted – shares the common aim to eliminate gaps through accelerated deployment of new cleaning technology, improved cleaning equipment designs and cleaning process integration. Cleaning fluid innovations focused on low surface tension fluids provide a second benefit: The fluids had to operate at lower concentration levels.⁴ As such, users gain improved cleaning performance at lower operating costs. Additionally, improved cleaning equipment designs provide fluid delivery for penetrating and removing residues from low standoff components.

References

1. Dirk Ellis and Mike Bixenman, “Applied Research for Optimizing Process: Parameters for Cleaning Pb-Free Flux Residue,” IPC Apex, February 2006.
   
   
2. Abid Merchant and Mike Bixenman, “How New Developments in Hydrofluorocarbon Cleaning Technology Impact Flip-Chip Package Production,” Chip Scale Review, January-February 2001.
   
   
3. Thomas M. Forsythe, New Methods for Evaluating the Cleanliness Beneath Low Standoff Devices,” SMTA Pac Pacific Symposium, January 2008.
   
   
4. Mike Bixenman, “Advanced Cleaning Fluid Design and Process for Cleaning Flip Chip Packages,” IMAPS, October 2006.
   
   
5. Mike Bixenman, “Engineered Cleaning Fluid and Mechanical Impingement Optimization Innovations,” IPC Midwest Conference, September 2007.

Dr. Mike Bixenman is chief technical officer at Kyzen Corp. (kyzen.com); mike_bix@kyzen.com.

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