Can SMD components be baked alongside microcontrollers that require baking?

[ivanmak]

So lets say I have an assembled PCB, but I forgot to bake components that are sensitive to moisture. Will it be safe for the entire PCB including SMD resistors, capacitors to be baked to avoid disassembling the PCB? Or can I least remove some components like capacitors? Also, how will the solder paste be affected (Solder paste: Chip Quik SMD291AX50T3)? The most sensitive components have an MSL level of 3, but some are different thicknesses which changes the baking times.

Any help would be appreciated

[ataradov]

Baking is needed to prevent issues during assembly. If the  boards are already assembled, why bake them at all?

[engineer_in_shorts]

Baking is needed to prevent issues during assembly. If the  boards are already assembled, why bake them at all?

This 👆

[ivanmak]

Oh, I meant assembled as in components placed on the PCB with solder paste applied, but the pcbs haven't been reflowed yet.

[2N3055]

Baking is needed to prevent issues during assembly. If the  boards are already assembled, why bake them at all?

Exactly!
Baking is to dry out packages so they don't crack (externally or internally) during soldering from trapped water vapor and water soaked material expanding differently.
 
If OP already soldered them, damage is already done, or if components were reasonably dry or OP was lucky nothing wrong happened.

MSL3 is one week at max 60% humidity at max 30°C.

It depends how long were the most sensitive components out of dry packaging, and what was air humidity at the time.
I have very dry air sometimes in my lab, less than 30% (my problem is static electricity, so need to humidify, and use ESD precautions).
I can keep components open for much longer periods, no problem..

Hard to know now. Worst part is that they might test OK and fail later.

[2N3055]

Oh, I meant assembled as in components placed on the PCB with solder paste applied, but the pcbs haven't been reflowed yet.

You have a different problem then. Your paste dried out and lost it's "tack", stickines. While handling boards components will fall off and move..
There is a defined process time from printing paste to component placement to reflow, and it is paste specific...

[tooki]

Oh, I meant assembled as in components placed on the PCB with solder paste applied, but the pcbs haven't been reflowed yet.

You have a different problem then. Your paste dried out and lost it's "tack", stickines. While handling boards components will fall off and move..
There is a defined process time from printing paste to component placement to reflow, and it is paste specific...

It’s a hypothetical question. They didn’t say anything about a delay between placement and reflow.

The question is: if they placed a board but realize they forgot to bake some components, can they bake the pasted, populated, but not reflowed board?

[jmelson]

It’s a hypothetical question. They didn’t say anything about a delay between placement and reflow.

The question is: if they placed a board but realize they forgot to bake some components, can they bake the pasted, populated, but not reflowed board?

Heat for one hour at 50C, then MAYBE heat for another hour at 70C.  I THINK the solder paste will still perform properly for reflow, but I've never tried this.  I generally have to bake boards using this prototcol with blank boards that have been in stock for over a couple months to prevent blistering of the boards.
Jon

[SiliconWizard]

Oh, I meant assembled as in components placed on the PCB with solder paste applied, but the pcbs haven't been reflowed yet.

I'm curious what others will say, but uh... baking a board with placed components over non-reflowed solder paste? Are you serious?

[ataradov]

Yeah, it is nuts. Depending on the value of the boards and ICs, I personally would probably just reflow it an hope for the best. Heating flux in the solder paste may not activate it, but surely it can't be good for it.

[bdunham7]

I'm curious what others will say, but uh... baking a board with placed components over non-reflowed solder paste? Are you serious?

Oh, and it's two sided! 

[bdunham7]

Oh, I meant assembled as in components placed on the PCB with solder paste applied, but the pcbs haven't been reflowed yet.

If this has actually happened, hopefully to one board, and you are just sitting there trying to figure out what to do, I'd just go ahead and reflow it right now.  Your situation is not improving with time.

[ejeffrey]

To answer the rest of the initial question: the rest of your components are fine.  They can all take the baking process.  The solder paste however cannot.  Best bet is to reflow anyway and hope for the best.  Your odds likely aren't that bad: depending on your local environment and the specific parts in question. 

[Doctorandus_P]

Once solder paste is applied to a PCB you only have a short time to do the actual reflow. Typically between half an hour and maybe a few hours at most. Any longer and you risc the solder paste drying out and the flux not working properly.

If you start baking an assembled (but not reflowed) PCB then the flux will almost certainly dry out during the baking process. So just reflowing the PCB and hoping for the best is probably the best option.

Alternatively, you could try baking the whole PCB, and then see if the flux is still tacky. Maybe you can spray liquid flux over the PCB. But such things are all experimental, and cost time. It's also a one off, because the next time you (hopefully) do it properly.

If reliability is important, then just clean the PCB, bake the parts that need to be baked and re-assemble and reflow the PCB. Big parts that are easy to handle or that are expensive are worth salvaging, but small SMT stuff is usually not worth your time to try to re-use.

[ogden]

If reliability is important, then just clean the PCB, bake the parts that need to be baked and re-assemble

This ^^^ definitely. As already said, baking solder paste is really bad idea.