You can't hang 'em out to dry; the rinse and dry steps are critical to the aqueous cleaning process.

Q: We solder with a water-soluble flux. After soldering, we use an in-line aqueous cleaner with added chemistry. Even though we have a hot air dryer at the end of the rinse section, I have noticed moisture on some of our more dense assemblies after the cleaning and rinsing process. Should we be concerned about the water left on the assemblies?

A: The short answer to your question is yes. For effective post-solder defluxing of printed wiring assemblies (PWAs), you need to consider the total operation--cleaning, rinsing and drying. With assemblies incorporating high-density PWA designs, cleaning, rinsing and drying the small spaces under and around the components and connectors is often a difficult, but critical, step. The drying capability is often under facilitated, becoming the weak link of an aqueous cleaning system.

When aqueous cleaning of assemblies is required after soldering, the rinse and dry cycles of the cleaning process are very important to the production of clean, uncontaminated PWAs. The rinse cycle must carry away reaction products and leave behind only water of an acceptable and desired purity. The dry cycle must displace and evaporate the residual clean liquid (deionized water).

Typically, an inline aqueous cleaning system incorporates a rough prewash section that dumps to drain, followed by a high-pressure, recirculating wash that adds the appropriate chemistry. After the major cleaning occurs, an airknife squeegees the contaminated water off the assembly. The airknife also helps to irrigate the small spaces and remove the bulk of the cleaning chemistry and residue-laden solution.

Isolated rinse and dry sections follow the airknife module. First, a rough pre-rinse module goes to drain, followed by a recirculating rinse module. A final rinse section should also be in place, incorporating fresh, heated, deionized water to facilitate the rinsing and subsequent drying of the assembly. Next, an airknife removes the bulk of the hot, deionized water before a high-volume, high-pressure hot air dryer module evaporates any remaining water from the assembly. The dryer module may also incorporate an infrared heater.

When cleaning PWAs, high pressure and capillary flow are necessary to get the cleaning chemistry hi the recirculating wash section into the small spaces on the assembly. The cleaning chemistry serves to reduce surface tension, solubilize and mobilize flux residues and other unwanted materials and aids the wetting of the surfaces being cleaned. The airknives following the final cleaning and rinse sections also aid the irrigation of the assembly.

The final deionized rinse cycle relies on hot water, high flow rates and adequate pressure to carry away the solubilized reaction products of the fluxing media. The rinse system must incorporate piping that does not contaminate the final rinse water. The final rinse must use and leave only water of high purity on the assembly and component surfaces.

The line speed in the final rinse and dry section must be slow enough to provide sufficient contact time with the hot, clean rinse water and the drying devices-assuring a complete rinse and drying cycle.

The drying section is critical to the total cleaning process, yet it is often underfacilitized. Drying, the important final step in the process, is dependent on an airknife, infrared heaters and hot air blowers to displace and evaporate the residual water. Adequate thermal energy is required to ensure the effectiveness of the total aqueous clean, rinse and dry process. Although providing adequate thermal energy for drying is a major cost factor, it is absolutely key to the total process. An efficient and effective rinse and dry cycle is important for the initial functionality and long-term reliability of the assemblies.

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