Hot Melt Conformal Coating?

The Ohio Chapter of the SMTA met on September 18th the meeting was conformal coatings: surface preparation, material selection, application and rework. Jim Stockhausen of Elantas PDG, Inc. presented on material selection. Jim briefly mentioned a class of materials that could be called “hot-melt” conformal coating. These are materials that could be applied either with a dispensing machine or with a hot melt gun. The reason I was interested in this material is that there are times that a spot coating is needed over a specific component or area of the assembly. Dispensing material with a hot melt gun over a specific component could be a better solution than either brushing on a traditional coating or sending the assembly through an in-line conformal coating process.

The hot melt conformal coatings are different that the common hot melt adhesives. One such material is Bectron MR 3406. This coating material is based on polyolefin chemistry in contrast to the conventional polyamide based materials used for adhesives. This results in the coating having better adhesion and absorbing less moisture compared to the adhesive material. The MR3406 is rated to 130C and has low humidity absorption. It is resistant to acids, solvents and fungus growth. It generally meets the IPC-CC-830 tests although it is not rated to this specification because there is no category defined for this material. Rework is easy because it can be softened and peeled away.

I do have a couple of concerns. First, since the material softens at 150C, the temperature at the nozzle must by about 190C-200C. Although the coating cools quickly, is there a danger of thermal shock damage to the component or PCB? Also the adhesion to PCB surfaces with flux residues would need to be tested. I don’t know of any immediate application for this material but it could potentially be used in spot coating applications.