I've left wet paste on a board overnight in cool weather without problems, but I'd be worried it would dry out over a few days - especially if left uncovered in a refrigerator (although I'm not sure if this would adversely effect the reflowing).
If there are pads under the parts, then soldering with an iron is out of the question, but a hot air rework gun should work fine. Depending on the complexity and the number of missing parts, I would either:
- Flow now and solder the additional parts later (you can place a cube of metal under specific parts of the board to reflow individual parts on the hotplate).
- Clean the board and restart when you've got the new parts
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.
Best Answer
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The biggest issue with the r***R, as with all surface reflow tools, is that double-sided loads are essentially impossible to create. Flipping the board over means that it will be held away from the heating surface, resulting in at best uneven heating of the board.
A reflow oven uses either infrared radiation or air convection to heat the board instead, which means that suspending the board by its edges does not impede the reflow process (and in some cases may be required), making double-sided loads possible. It also allows for a mixture of SMT and TH devices to be reflowed (insomuch as TH devices can be).
The flip side is that with a surface reflow tool heat can be transferred to and from the board much more quickly than via air convection, which allows tighter adherence to JEDEC reflow profiles.
In addition, the exposed nature of a surface reflow tool means that not only can parts be manipulated during reflow as you note, but also during rework after the board may have been in service for some time. This of course requires tighter thermal control than has previously been available using more... primitive tools.
As for storage, this refers to long-term thermal profiles, a month or longer. The few minutes during reflow or even rework have been factored into the construction of most devices from reputable suppliers.