It is noteworthy to point out that most cleaning processes (for OA-flux removal) in the North American market rely on cleaning with DI-water only. Recent market studies suggest that water is beginning to reach its cleaning limitation, favoring the use of aqueous processes. Aqueous is a term that implies the use of aqueous-based chemistry; for example an application concentration of 10% mixed with DI-water. The nature of the ingredients within the aqueous product range varies between vendors and their respective chemical R&D knowledge and product technologies. These are in contrary to cleaning processes with DI-water only or with a solvent, which is used at a concentration of 100%. A solvent does not contain any water at all. After one decade of solvent based processes dominating the precision cleaning market, the current demands are relying on aqueous solutions as their cleaning window has been found to be the widest. A number of reasons can be cited supporting this trend.
Firstly, the increased use of lead-free solder, which require higher soldering temperatures and result in more burnt-in fluxes. These in turn are much harder to remove. DI-water alone has simply a limited, to no ability to solubilize non-ionic residues on the board's surface. Secondly, the cleaning of water-soluble fluxes (especially under components) has also become a lot more difficult. In other words, water with its high surface tension of over 70 dynes/cm cannot effectively penetrate low standoff components. As the standoff heights are decreasing further and component densities are increasing more and more, companies will have to improve their existing cleaning process. Chemistry assisted cleaning can reduce the surface tension to 30 dynes/cm and below. Interestingly, the industry so far has mostly reverted to adjust the DI-water based cleaning process to its respective limits. These limits entail for example an increase in operating temperatures to above 150°F, as well as an increase of the spray pressures, or the reduction of the belt speed to prolong the exposure time. With pure water-soluble fluxes in a eutectic environment such measures can provide sufficient cleaning results. Given the introduction of lead-free however, the solubility of residues in DI-water becomes the limiting aspect. If non-ionic contamination is produced, water alone cannot chemically dissolve such contamination. Much to everyone's surprise, a recent study has actually shown that lower chemical concentrations of only 5% and temperatures of 175°F and 200°F are producing the best cleaning results under components, period. Previously the industry considered 160°F as the highest possible cleaning temperature.
Another often overlooked consequence is that higher pressures and temperatures might allow the water to penetrate low standoff components by forcing water underneath or into the capillary spaces. Unfortunately, the cleaning equipment is often not capable of removing the water during the drying section. To limit the formation of electrochemical migration or leakage currents, it is of utmost importance to verify a dry, water and flux-free environment under components after the cleaning cycles are completed. Cleaning agents on the other hand can be easier rinsed and dried as lower surface tension allows a quick removal. The usage of chemistry in the long run seems therefore to be overall most beneficial. Yes, it's true, there is an additional process cost but the "value added" benefits are considerable. They include, but are not limited to better cleaning through lower ionic contamination, which in turn provides higher product reliability. Recent studies have also demonstrated better bonding and coating results after the introduction of chemistry assisted cleaning. To offset the added cost, users can operate at lower temperatures and with wider process windows one clean not only OA but also RMA and no-clean fluxes. This will become a requirement in the North American market as contract manufacturers are moving to lower volume, higher mix and a significantly more high reliability product. In the end, the introduction of a chemistry assisted cleaning process, will increase your cleaning process window and permit the de-fluxing of all production boards during a single cleaning process.
Despite all valid arguments encouraging the use of aqueous processes, the authors would like to caution interested users as well. Most equipments currently using strict DI-water are not properly plumbed to use chemistry. DI-water machines take the advantage of cascading DI-water tanks from the back to the front. Employing a chemical product in the wash tank would lead to continuous dilution of the recommended application concentration by DI-water. Companies that are strategically planning their capital purchases are therefore well advised to incorporate the mechanical option to run aqueous chemistries. As always, a slightly higher investment will provide significantly more process flexibility in years to come, and might lead to one or two additional contracts.
-Dr. Harald Wack