As a CM, we don’t have a lot of input on the PCB layouts we receive. Some engineers/software packages output the paste layer 1 to 1, some do an across the board reduction, and some set each part up individually with different reductions per footprint. Thermal pads are also routinely an issue with supplied files applying paste over open, unfilled vias. As such, we have found it preferable to generate new paste layers directly from the copper using a standardized modification profile. That has worked 98% of the time.
Recently, we had a serious bridging issue on a single 0.5mm pitch QFN14. The array files had etch compensation applied and the fabricated PCBs were over-etched slightly. The stencil aperture reduction was done from the adjusted array Gerber, so the stencil didn't gasket properly on the undersized pads.
I would like to know if others have had similar problems and, if so, what changes were made or tools used to prevent them.
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