Electronics > Manufacturing & Assembly

Reflowing lead free

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Simon:
I've been told that I should not use an IR oven with lead free due to the higher temperatures required as the black IC packages will absorb heat faster being IR and at the higher working temperature this is not good. I think leaded was around 180C and lead free is 230 so a 50C increase or an increase of nearly 30%.

What should I use then instead of the traditional cheap IR oven. Is this why the newer Chinese one have fans? what If I put a black item in there that will heat up and then the fan will blow that heat around?

Or do I just literally buy a small kitchen fan oven (mine goes up to just 230C) and perhaps put some aluminium in there, preheat that and then run the board reflow?

mikeselectricstuff:
With unleaded, there is a rather smaller margin between "reflow" and "incinerate", but it's probably doable with some experimentation with paste types, power and/or fan.
As regards  absorbtion, my gut feeling is to go for a lower element temperature (barely glowing) and longer time.
However if it's for prototypes, just use leaded.
 

Simon:
I would also need to do small volume manufacturing so it needs to be RoHS compliant. I see ovens that specify forced air convection but are IR. I think mine (ZB3530HL) has a fan on top of the IR elements.

So is it the adding of air flow that does it or are these IR ovens lying as I see IR ovens with air convection advertised as being for lead free What do I use for small scale prototype builds if not one of those cheap IR ovens. Or is there a combo of IR and the IR heating an object that the forced air then transfers the heat from.

Why was IR used in the first place? was it more efficient as it heated just the PCB and not the whole oven?

jmelson:
I've been doing lead-free reflow in a consumer-grade toaster oven for over 10 years.  I got a "ramp and soak" thermocouple controller and poke a micro-sized thermocouple into a plated through hole in the board to measure actual board temperature.  This works great, if the thermocouple is just in the air the boards get too hot and burn.  The ramp and soak feature allows me to program temperature ramps and holds.

Jon

Simon:
so what sort of power do you need to do this? I think my kitchen oven is 1kW. I take it then the thermocouple must be a tight fit in the hole.

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