Honestly, I wouldn't try to solder my own BGA's. I know this doesn't directly address your issues, but hear me out.
It takes a lot of work and effort to solder a BGA. There's a lot of trial and error. A lot of messed up test boards. But then it's soldered. Now what?
Now you have to prove that it's soldered correctly. For that you need one or more of the following: JTAG test (US$10k, never has 100% coverage), optical inspection (US$20k for the equipment), or X-Rays (US$500k). The cost of doing these tests is too much for the normal hobbyist, and is even beyond many small companies.
Skipping those tests, you proceed with debugging your PCB. And let's say that the BGA is a complex CPU. Inevitably you'll find a bug. The CPU will randomly crash. Is it your software, your electrical design, or the soldering on the BGA that's causing the problem? Debugging this, in light of some possibly problematic soldering, is going to be terrible. It will add a lot of time to your debug process, possibly months, and you'll loose a lot of hair on your head. And then you can repeat this for the next major bug.
Without confidence that your soldering is perfect, you will always have this dark cloud over your head. Every little bug that shows up "could be a BGA soldering problem". This is made worse if you have multiple engineers working on the same PCB since the software guy will be questioning the hardware guy, etc.
Then, even if the BGA soldering is perfect, did the chip get too hot? Did you destroy the chip by getting it too hot? Even on modern assembly lines this is an issue. But with the proper equipment you can adjust and measure the temperature profiles to at least get you in the right ballpark. On one board I did recently, the BGA's were being damaged. The solder balls looked great, but under a very nice X-Ray machine we could see that the gold bond wires melted from the heat.
I've been there. Not at the hobbiest level, but professionally as we were bringing up new boards while the assembly shop was learning to do BGA's. We had no JTag. No optical inspection. And the X-Rays were terrible. Our PCB had 11 BGA's on them. That was 2 years of hell I don't wish to repeat.
So, here's my recommendation:
Get someone who has the proper equipment, training, and experience to solder your BGA's. There are a lot of contract manufacturers that'll do a single BGA. It takes money, but that's way less than the time you'll spend trying to debug your own soldering.
If you must do it yourself, then you should get the proper equipment, training, and figure out how to get the experience required. For this to pay off in the end, you need to have a large enough company and need to justify the huge amount of time and money that you'll put into this.
But I would never try to just kludge something together. That's a recipe for, um, bad stuff.
If you are a manufacturer who assembles lots of PCBs for a paying customer, it is good economics to get the solder temperature profile exactly right, to reduce the incidence of tombstoning and other soldering defects.
On the other hand, if you are a hobbyist cooking boards one at a time in a toaster oven, then achieving the perfect solder profile is a waste of time. The best hobbyist profile is:
- Heat on high until the solder melts
- Shut off the oven and crack the door open until the solder freezes
- One or two passives will have tombstoned due to uneven heating. Use a soldering iron to rework them.
I realize that some hobbyists build elaborate temperature controllers, but this is because hobbyists enjoy building things, not because it is necessary for a reflow process.
Best Answer
If you don't follow the reflow profile properly, there are a few things that can happen
With those said, if your board is not the most dense board, like it doesn't have a bunch of decoupling capacitors underneath a processor, I wouldn't worry too much about it. You can easily observe whether something is soldered. It would still be a good idea to get a preheater since the copper pour issue is kind of annoying (You can literally watch it happen while using hot-air rework)
Since you do have a double sided board, then you might want to consider getting thermal adhesive. It is like a little red glue that you put on the components that you reflow first on to the board so that when you flip it over to reflow the other side, the parts don't fall off.