Printed Circuit Design & Fab - July 2008 - (Page 41)

SOLDER MaSk with solder mask as well. Removing the solder mask from high aspect ratio via holes and blind vias can be a difficult and sometimes impossible task. These non-cured residues can create numerous problems, which become especially evident during the component assembly process. Digital inkjet printing, on the other hand, is a selective process that does not fill or coat holes or assembly attachment areas with solder mask at any time, so there is never a chance of residue. Traces and Shoulders. Some experts believe that surface finishing anomalies, such as the “black pad” can be attributed to residue on the copper surfaces from incomplete solder mask development. In the traditional process, conductors are not always symmetrically covered, and the resulting “thin knee” areas can break down, allowing solder to bridge across traces resulting in shorts after the assembly process. Digital inkjet printing, on the other hand, is a selective process the desired thickness of solder mask is selectively applied to the board with a proprietary coating technology that results in a symmetrical coverage of conductors, even traces that result from high copper thicknesses, using less material than the traditional methods, with a corresponding savings in material cost. (FiGurE 3) to the cured solder mask material final properties. According to IPC-T-50, this layer is considered “a heat resistant coating material applied to selected areas to prevent the deposition of solder upon those areas during subsequent soldering.” The chemical, physical and electrical properties of this micron-scale coating are carefully detected and the final material quality should meet the IPC-SM-840D standard. The common UV cured solder mask formulations are always a mixture of various ingredients, composed by several chemical groups, each of them added for specific purpose. The primary ingredients are as follows: resins. A combination of monomers, oligomers and polymers with varying chemical structure and performance, responsible for determining viscosity, basic rheology, adhesion and the dried film chemical and mechanical properties. Fillers. A combination of inorganic particles, various surface treatment performance enhancing ingredients, and organic particles used primarily for ink stability, opacity, color, and the mechanical and electrical properties of the dried film. Surface-active elements. A combination of surfactants including charge properties as well as basic hydrophobic and hydrophilic moieties for ink stability properties via particles stabilization. Additives. Special ingredients such as an antifoaming agent, leveling agent, flame retardant, biocide, etc., are incorporated for both wet ink performance as well as the dried film properties. Each one of the conventional application methods such as screen printing, spray or curtain coating requires different solder mask viscosity i.e. from liquid to past form. In addition, the particle size of the fillers in these formulations often have little effect on the overall performance of the coating. The digital inkjet solder mask printing technology needed to reevaluate the standard solder mask parameters. All the traditional criteria were taken into consideration in the formulation of a new type of solder mask that was tailor made for both the application specifics of inkjet deposition and one that embodied both UV and thermal curing ink properties to facilitate productivity. The demands from inkjet solder mask ink properties are much more complicated than those described above for a traditional solder mask material. The task of optimizing the deposition characteristics and retaining the excellent film properties was paramount. The major differences between the approaches are as follows: Inkjet solder mask ink viscosity. For inkjet technology, the ink viscosity is a major factor. The ink delivery system starts from a main container, continues to a middle container and ends at the print head system. In order to obtain continuous ink flow without blocking a passageway, it is essential to keep the ink at a remarkably low viscosity, usually 10-20 cps at the jetting temperature. This unusually low solder mask ink viscosity is achieved by the careful selection of resins. Maintaining viscosity, while keeping the other properties, such as curing Solder Mask Formulation Challenges The solder mask has a number of functional requirements after application to the PCB. Chemical formulation is key !FiGurE 1. Zero solder mask clearance around pads by digital ink-jet printing technology. JULY 2008 FiGurE 2. Higher registration accuracy and resolution. !FiGurE 3. Selective added solder layer over conductor and holes free of solder by digital ink-jet printing technology. printEd circuit dESign & fAB 41

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Printed Circuit Design & Fab - July 2008

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