Assembly process Conformal coating

The final step in the soldering process is the removal of flux residues from the completed circuit board. No-clean fluxes, as the name implies, produce residues that need not be removed from the assembly after soldering. The corrosive activators are locked up in a hard, polymerized residue. However, no-clean flux residues can potentially interfere with in-drcuit testing (probes) and inspection, degrade cosmetic appearance, inhibit rework activities, and lessen the adhesion of conformal coatings. In the event that any one of these factors becomes critical, it is best to switch to a cleanable flux than to develop a cleaning process for a no-clean flux. 

The key to any contamination problem is to prevent it from happening. The cost of subsequent operations required to clean a product or rework a product is orders of magnitude more than any cost associated with preventing the contamination from occurring.These contaminants can cause soldering and solder mask or conformal coating problems. There are any number of possibihties for inducing contamination in an assembly environment, such as dirt, dust, machine oils, and process residues however, in many cases, contamination is caused by the human body, specifically the salts and oils on the skin. 

Conformal coating thickness requirements for five types of coatings are listed in Table 52.7. The thickness is commonly measured on a coupon that has been processed with the assembly. 

Any polymerization reaction in which the product of each elongation step can itself also undergo further polymerization. When the same types of bonds and/or conformational states that are present in the reactant(s) are generated within product(s) during elongation, the process is referred to as isodesmic polymerization. Such is the case for the indefinite polymerization of actin, tubulin, hemoglobin S, and tobacco mosaic virus coat protein. See Nudeation Protein Polymerization Actin Assembly Kinetics Microtubule Assembly Kinetics Microtubule Assembly Kinetics... 

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