Solder mask Surface preparation

Incoming Copper. The quality of the surface of the copper is very important for achieving good solder mask adhesion. All traces of the etch resist metallization, t icaUy tin or solder, must be removed prior to the solder mask surface preparation process. Solder mask does not adhere well to tin or solder residues or tin-copper intermetallic surfaces. 

In all cases, the mask preparation steps are critical. H the copper is treated well to adhere to solder mask, the interaction from surface finish chemicals is greatly reduced. It is imperative that there is no wedge between the mask and the copper into which the surface finish chemicals can creep. In extreme cases of chemical entrapment, the chemical I eactions are accelerated in the wedge, leading to nonuniform plating. Immersion silver, tin, and OSP can lead to excessive copper corrosion at the solder mask edge and result in electrical opens. This phenomenon is also known as solder mask interface attack. 

Solder mask should be applied to a clean, dry surface. The cleaning step should remove all particulates, oils and greases, and unwanted oxides. In the case of bare copper, this step should provide some tooth to the copper to enhance the mechanical bond of the mask to the copper. The cleaning step is followed by a rinse and drying step that leaves the panels ready for coating with mask. The surface preparation technology chosen is dependent upon the metallization of the circuitry and, to a lesser extent, the base laminate material. 

Copper Circuitry. A large majority of panels produced have solder mask coated over bare copper (SMOBC). The goals of the surface preparation process are to remove all unwanted oxides and contaminants from the copper and to impart a microroughness or tooth to the surface, and to leave a clean, dry surface for coating. 

Mixed Metals. Two good examples of the use of mixed metals are for panels that have been selectively solder-stripped and for panels that have had fingers or contacts plated with gold before the solder mask process. The surface preparation process must be selected based upon the most delicate of the metals on the panel. 

Dry-Film Solder Mask (DFSM) Application. The process for DFSM differs from LPI solder mask only in the application step. Surface preparation, exposure development, and cure are the same as for LPI solder masks. 

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