One of the first steps in the development of any cleaning process is to define and understand the contaminant or “dirt” that needs to be removed. Sounds simple, no? Not so fast! Dirt comes in all varieties and styles. Different kinds of dirt require different cleaning processes. Some dirt is liquid. Oils, coolants, paint, adhesives, and lots of other liquids fall into this category as do fingerprints. Some, but not all liquids are soluble, readily mixing with water, a mixture of water and a saponifier (soap), or water with an emulsifier (an agent which suspends the contaminating liquid as small globules in water). Other liquids, however, are only dissolved by other liquid agents called solvents. Alcohol and acetone are examples of solvents. In order to remove liquid dirt, it is necessary to at least partially dissolve it and flush it away little by little until there is no more left. Solid dirt comes in pieces ranging from smaller than a micron (1/1,000,000 of a meter) up to several inches in dimension. Some solid dirt is soluble, some is not. Soluble solids can be removed by simply dissolving them using processes similar to those described for liquids above. Insoluble solids require more drastic measures. Solid dirt may be held tightly to the surface by adhesion, ionic bonding or magnetism and, in the worst case, may actually be partially attached to or imbedded in that surface. There may also be other contaminants such as oil or adhesives present along with the solid dirt that act as a “glue” to further resist its removal. The trick with solid dirt is to remove or overcome whatever force is holding it to the surface and then mechanically remove it far enough from the surface that it doesn’t re-attach and can be captured and removed. Oh, and while you’re at it, it’s also a good idea to understand where the dirt came from and why it’s there. Keeping a contaminant out of the process is the best way to assure that you don’t have to remove it later. Finally, don’t assume anything! Buffing compound in stick or paste form is NOT the same stuff as the residue it leaves on parts after it has been heated and ground into the buffed surface by the buff. Peanut oil from the bottle is NOT the same stuff that remains on parts after they have been heated to 500F for even a short period of time in a reflow soldering process. See, it’s not really as simple as it seems but the analysis of the dirt is absolutely critical to finding a way to remove it. An improper diagnosis can be fatal resulting in process that doesn’t work and expensive hardware that doesn’t do the job.