solder black stains after reflow

Hi Guys,recently i am facing an issue. Refer to the attached photo. This PCB is high RF speed, gold immersion single layer. After reflow, my solder seems to have black stains on it. I tried with SAC305 paste water soluble and no-clean, both results are the same. the max temp of the oven is at 235 Degrees C. i am suspecting on the porosity of the gold finish. Do you guys have any ideas on how to solve?

thanks

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