I've been chipping away on a solder bridging and poor fillet wetting problem for a few weeks now (new to the company) and I found two issues. First, after a review of stencil inventory, I found apertures to be oversized when compared to suggested dimensions for various pitch sizes outlined in a paper I read. My thinking here is this is likely tied to some of the bridging. Second I found pad widths on a particular PWB design were oversized by a factor of 38% when compared with Topline 144QFP suggested dimensions. Presumably a reduction in aperature size combined with an oversized pad will result in less solder density and therefore poor fillet wetting. Is this a correct assumption? Many of the stencils had reduced aperature widths and generally the yield data appears to correlate (unfortunately we don't track failure by reference designator only by CCA #). In the meantime, I've been trying to convince our Engineers to revisit pad size standards. This obviously was not well received and getting action may more painful than root canal surgery. Without supporting data in the form of standards and/or reliability they are not willing to re-spin the design in question or make adjustments in the standards for new designs.
The million dollar question???...Well, can anyone point me to source of comprehensive data on pad geometry design standards? Or if I'm totally off-base on my assumptions on the problems discussed above...please offer any direction you can.
If u don't mind my recommendations. U may want to try to convince them that by using smaller pad size , u save solder paste in the end of the day (provided u are doing mass volumes). It is another way of getting around to ur idea of changing the stencil size. As for stencil pattern, u may want ro try snipping of the corners of the pads for better wetting.