I’m assuming this is during putting the balls on the SoC stage and you haven’t got to putting the chip on the board stage yet? but please tell me if I’ve misunderstood.
I’m always a bit weary of doing this, I try to expose CPUs/GPUs/SoC/APUs to as little heat as humanly possible on the die directly, I know other people do it and use preheaters and hot plates and get away with it but it makes me a bit nervous as the Tegra for Switch in particular is very sensitive heat wise. You shouldn’t need to preheat the underside of the SoC and can just use hot air directly to reflow the balls
I gues this is using preformed 0.35/0.4mm balls? and the pads on the SoC were wicked?
If so, in your case I’d leave the stencil in place during reflow, thin layer of flux on the IC prior to placing the stencil and then the balls (you can use a cotton swab as a brush of sorts to get the flux thin) if it’s too thick the balls will want to jump out. then hot air, I use about 390C at minimum air, start at a distance and come in closer when reflow is anticipated. balls can usually be upwards of almost 50% off the pad and provided there is not too much flux on the SoC they will find their home, this is a non issue if you keep the stencil in situe though during reflow.
I avoid using solder paste for the SoC and instead use a dedicated stencil and jig for this which is a bit different to using a direct heat stencil like yours, the jig takes care of stencil alignment (after setup) and it’s fixed, thin flux layer on the SoC, balls are placed in the jig, swirl it around until they find there home, fixed stencil is removed, then the SoC is removed from the jig and the balls are reflown without the stencil (these dedicated 90x90 stencils are way too thick for direct heating)
details here regarding the jig and stencill I use
In the case of the ram I use solder paste (and that’s about the biggest IC vs pad pitch left which I’ll use solder paste on) and on my cut stencil I’ll just align and put slight downward pressure in the center while filling with paste right upto the point of reflow, as I mentioned though, I wouldn’t use this technique on the SoC again, it’s just way to difficult with paste because of the large size.
I personally only keep the stencil in situe on the IC when using solder paste, but you can do the same with balls - but the stencil has to be a direct heat stencil in both cases as they’re thinner than the ones intended for 80x80/90x90 jigs
You’ll get it man, nobody gets this stuff first time, I remember back when I reballed my first CPU, I popcorned the chip, I tore pads, then came a GPU, I killed it with heat, they’re all good learning excercises at least 