RMA flux affecting surface tension

I have recently started to use RMA flux (customer requirement) for some of our projects. I am finding that some of my BGAs are slightly off. Not enough to be rejected but enough to cause concern. In the past I have been able to rely on the surface tension of the solder to "center" the balls. I'm thinking that the stickiness of the flux is overiding the surface tension of the solder. Have any of you had issues when switching from an OA/NC process to RMA? Have you had better results with one make of flux over the others? I am using Heraus SF3401HT for rework and either Loctite M00140 or Indium SMQ51AC for production. The pads have immersion gold finish. My joints look good otherwise.

reply »