Electronics > Projects, Designs, and Technical Stuff
Vapour phase Soldering
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IanB:
Why wouldn't one just use a hot air convection oven?
jeremy:

--- Quote from: IanB on January 20, 2015, 10:47:21 pm ---Why wouldn't one just use a hot air convection oven?

--- End quote ---

When one BGA is worth $50, you don't have to make many toaster oven mistakes to cover the cost of the fluid :P
IanB:
I'm not necessarily talking about home brew solutions, though it might apply here too.

If we are trying to solve the problem of uneven heating due to radiant paths, then a circulating hot fluid can help. But why does the hot fluid need to be some expensive chemical? If you circulate hot air around the board it seems you should get even heating in a similar manner at lower cost.
wraper:

--- Quote from: IanB on January 20, 2015, 10:55:43 pm ---I'm not necessarily talking about home brew solutions, though it might apply here too.

If we are trying to solve the problem of uneven heating due to radiant paths, then a circulating hot fluid can help. But why does the hot fluid need to be some expensive chemical? If you circulate hot air around the board it seems you should get even heating in a similar manner at lower cost.

--- End quote ---
You won't get even heating with hot air. The main "thing" in this fluid is boiling temperature, it will condense at coldest parts (read heat them most), therefore even heating is guaranteed. Also you cannot overheat the parts.
mikeselectricstuff:

--- Quote from: IanB on January 20, 2015, 10:55:43 pm ---I'm not necessarily talking about home brew solutions, though it might apply here too.

If we are trying to solve the problem of uneven heating due to radiant paths, then a circulating hot fluid can help. But why does the hot fluid need to be some expensive chemical? If you circulate hot air around the board it seems you should get even heating in a similar manner at lower cost.

--- End quote ---
The difference is latent heat of vaporisation.
The heater vaporises the liquid at its boiling point. Additional energy just turns more liquid to vapour without significantly increasing the temperature of the vapour. When vapour condenses on the PCB, it transfers heat efficiently, and will keep condensing as long as the PCB temp is below the vapour temp.
 
With air, there is no inherent control of temperature, and this will vary significantly due to convection etc. And you will need to start with air at a higher temp to compensate for this to get the temp you want at the surface, and this will vary with different PCBs

It also helps to be using an atmosphere that's chemically inert to avoid oxidation - this is why some conventional reflow setups use nitrogen, especially for higher temp leadfree prcesses.

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