Wave Solder Bridging Reduction Through Photo Imaged Solder Mask Gloss Level Control

ABSTRACT This paper will discuss the relationship between the gloss level of a liquid photo-imageable solder mask (PISM) and solder defects created during the wave soldering of printed circuit boards such as solder balls, solder shorts, and solder bridges. The relationship between solder balls and PISM gloss level has been documented previously 1,6 . Industry observations revealed that many of these micro solder balls were attached to the solder mask between adjacent component pins. It was discovered that by selecting a PISM with a non-glossy matte finish, the numbers of micro solder balls left on the PCB surface after wave solder could be significantly reduced. Until recently, it was felt sufficient to merely request that the PCB supplier provide a “matte finish PISM.” Recently a similar correlation between PISM gloss level and wave solder shorts and solder bridges was discovered. The gloss level of a PISM solder mask is, under certain conditions, directly related to the creation of solder bridges during wave soldering. Experiments completed last year by Delphi Delco Electronics have shown that the actual level of matte finish, as measured by a glossmeter (i.e. the amount of reflected light from the surface of the mask at a specific angle), must be controlled. The PISM's gloss level is a critical parameter in the reduction of solder balls, solder shorts and solder bridges particularly when the design or process is not optimized. This paper will provide experimental data on the relationships between gloss level and solder defects. Recommendations for proper PISM gloss level specifications for printed circuit boards will be provided, along with corresponding test methods.