Cleaning and Rinsing Today's Designs -- Does your facility use tap water in the final rinse phase? Contaminates may be ruining your assemblies.

As with soldering ability, reliable and repeatable cleaning ability requires controlled processes and materials that match the design characteristics of the printed circuit assembly (PCA). Many current PCA designs incorporate smaller packages with increased interconnect density, reduced standoff clearances and overall dense packaging on the substrate. Such designs can present a challenge to some established methods of cleaning.

Cleaning Challenges

Design conditions, along with the possible incorporation of lead-free solders, higher soldering temperatures and more tenacious flux residues, are presenting challenges to the entire PCA manufacturing process. Specifically, the cleaning, rinsing and drying of high-density and high-speed assemblies is of utmost importance. In many respects, this discussion applies to cleaning the bare printed circuit board (PCB) as well.

Mechanical, chemical and thermal energy are critical factors associated with the cleaning and rinsing process, and sufficient processing time is required for these factors to be effective. Such tools are needed to achieve the penetration, soluabilization, mobilization and irrigation required to free various types of contaminates from the PCA and to ensure that adequate rinsing, with deionized water, will remove all contaminants before hot air drying.

Be Sure to Rinse!

Control of the rinse step, in the cleaning process, is critical to make certain that only uncontaminated, deionized water is found on the PCA or PCB and in the holes or blind openings on components. The presence of contaminated rinse water, at the drying stage, may result in erratic operation or total failure in service.

Dragout of spent chemistry and contaminated solution, in the drying phase, is not a major concern in many facilities-many do not understand the cause of foaming in the rinse process. This lack of understanding is a result of improper training and the inattention of process technicians. The significance of the use of high quality deionized water and the importance of the use of airknives and isolation modules, to keep contaminants out of the final rinse and drying section, seems to be lacking in some organizations.

Remember the Basics

New materials and process changes are surfacing, but the basics of cleaning remain the same. Surface tension and flow characteristics are major factors in the process. Many of the spaces, where flux residues and other contaminants can be trapped, may be large enough to be governed by planar flow characteristics-areas where solvents with low surface tension work well.

Tubular flow configuration may exist in smaller spaces, associated with very small components; in larger components, with large numbers of interconnects attached with adhesive; or areas where underfill is used. Under such conditions, capillary flow, or the ability of a fluid to penetrate small openings, is the phenomenon operating. Capillary flow is directly proportional to the surface tension of cleaning and rinsing solutions. Use of higher impingement forces is not always the answer, since these forces may be harmful to some components. Generally, soaking in high solvency solutions is neither desirable, cost-effective or successful in removing residues from small spaces.

Conclusion

The most robust cleaning regime, for densely populated assemblies with very fine-pitch interconnects and small standoff clearances, requires sufficient temperature to assure solvency and mobility of the contaminants. In addition, the cleaning chemistry should possess a high loading capability to assure sufficient fluidity of the contaminated cleaning solution that flows under components in a dense array. Lack of high loading capability may also result in the inability of the solution to dissolve additional residues.

Les Hymes is the owner of Les Hymes Associates, Surprise, AZ; [phone]; e-mail: les.hymes@

worldnet.att.net.

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