Hot air rework temperature help

[caphair]

When removing/replacing chips. Is it safer to use little to no air flow but higher temp heat or lower heat with more airflow?

My hot air station is a model 852d+ with a max temp setting of 500c and max airflow temp of 8.

I was testing melting a ball of lead free solder with a 4mm tip and at 365c 1 air flow speed it melts in 20 seconds. At 340c 4 air speed it melts same amount of time but not sure if more air speed is dangerous for near by components or components on the underside of the board.

Is the goal to remove chip as quickly as possible or at a slower time? I'm working on small mobile device boards that don't require a preheater.

I've ran into issues before while practicing where components underneath on opposite end of the board suffered from the heat causing shorts under the bgas.

[diif]

Re: Hot air rework temperature help

Using a preheater means you can use your hot air for less time and have less of an effect of components at the opposite end of the board.
I have mine set to about 380-400c.
Air speed depends on what I'm removing.

[stj]

Re: Hot air rework temperature help

too much airspeed will cause the heat to effect a larger area.

you need to set it for the job - desoldering a chip needs more air than a transistor because your moving the jet around the package.

also, temp needs to be set based on both the solder and the distance beween the part and the nozzle.

[caphair]

Re: Hot air rework temperature help

too much airspeed will cause the heat to effect a larger area.

you need to set it for the job - desoldering a chip needs more air than a transistor because your moving the jet around the package.

also, temp needs to be set based on both the solder and the distance beween the part and the nozzle.

Hmm so for a bigger chip, more airflow but my concern is preventing opposite side of PCB chips getting too hot that solder melts. Would closer high temps less airflow have more or less chance of that happening?

[stj]

Re: Hot air rework temperature help

i keep the airflow pretty low, and the nozzle maybe 5-10mm from the work

[caphair]

Re: Hot air rework temperature help

i keep the airflow pretty low, and the nozzle maybe 5-10mm from the work

What would your "pretty low" airflow be compared to mine having a max setting of 8?

[stj]

Re: Hot air rework temperature help

couldnt say, my gear is different.
probably about 20-25%

[petehall347]

Re: Hot air rework temperature help

do you not use masking ?