Solder Ball Attach

We are attaching 0.50mm diameter SAC 305 solder balls onto 0.38 diameter Au plated pins placed at a 0.80mm pitch (pins are pressed into a plastic insulator). A tacky flux is then stenciled onto the pins using a stencil with.a 0.38mm diameter aperture and 0.13mm thick SS.There can be up to 400 balls placed on a 0.80mm pitch on a 20X20 grid. After solder ball transfer the BGA’s are reflowed using a convection oven. The peak temp of the reflow profile is 237C, with time above liquidus (217C) at 59 seconds. The entire cycle is a approximately 3.5 minutes. The issue we are having is that after reflow many of the solder balls have not centered themselves onto the Au plated surfaces. This does not happen when attaching 63-37 eutectic balls. To solve this issue, we use a small brush to literally paint more flux onto the cooled solder balls, then reflow the parts a second time. After the re-fluxing and re-re-reflow the parts look awesome. Has anyone ever encountered this?

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