Elements identifying QFN solder paste printing quality mainly include solder paste, PCB pad, metallic stencil, solder paste printer and manual procedures.
Solder paste includes a a lot more complicated ingredient than real tinlead alloy, that contains solder alloy particles, flux, rheological regulator, viscosity control agent and solvent. Because QFN components are leadless device that contains a big thermal-dissipation pad in the central part, relatively high necessity has been arranged to viscosity and viscosity control technology. Viscosity of solder paste must not be too much since too much viscosity can make it difficult to undergo openings on the stencil. Furthermore, printing traces are imperfect with low viscosity.
Small solder paste particles is, the greater viscous solder paste will be. The bigger amount of particles included, the greater viscous solder paste will be. Solder paste features the best viscosity with round particles and vice versa. With regards to ultra-fine spacing printing, solder paste with slimmer particles needs to be used to obtain better solder paste quality.
Solder paste printing is undoubtedly an elaborate process which has so many specialized parameters each that will bring ahead much damage if they're unsuitably adjusted. Those guidelines mainly include scraper pressure, printing thickness, printing velocity, printing method, scraper parameter, demolding rate and stencil cleaning frequency. When scraper features low pressure, solder paste will neglect to effectively reach underneath of stencil starting and also to fall on pad. When scraper features too big pressure, solder paste will be too slim or even harm stencil. Agreeable thickening of solder paste printing is good to boost QFN components' assembly reliability.
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