direct heat stencil,where do you get it? and how to prevent warping?
 

So, lately i found that for smaller chips like pch it is very difficult to reball with nondirect heat stencil. So i decided to try direct heat stencils...and while i am waithing some stencils to arrive i have a few questions:

1. From who do you buy a quality direct heat stencils? i am reading that they can vary in quality and make a big difference...Any trusted sellers?

2. I am reading also the problem of warping (curving) direct heat stencils...Is it because of quality of the stencils? or/and because some preheating is also required? and help and advice here?