Removing SMD chip with hot air gun - what is a safe temperature to use?

[djd_oz]

Hey there, I am suspecting this I have a faulty chip that I need to replace. It's an SMD chip with Lead solder. As far as I understand to remove it, we apply some flux onto the chip and then using a hot air gun the solder should start to melt and the chip can be lifted. What is a safe temperature to set the hot air gun at for this purpose?

[srb1954]

Before tackling the repair on your board it might be best to practice your technique on some scrap PCBs, preferably with similar size components to the one you want to replace.

Until you get your eye in you will probably put a few scorch marks on the PCB, damage some adjacent components or lift some pads or tracks off the PCB. Best to do that on boards you don't particularly care about.

[Fflint]

Before tackling the repair on your board it might be best to practice your technique on some scrap PCBs, preferably with similar size components to the one you want to replace.

Until you get your eye in you will probably put a few scorch marks on the PCB, damage some adjacent components or lift some pads or tracks off the PCB. Best to do that on boards you don't particularly care about.

I second the above. Also the temperature you use depends on the kind of pcb substrate. For example I recently decided to repair my pile of broken Chinese led floodlights. All of them use aluminium pcbs. No matter how long you cook them from above the solder will not melt. I learned preheating the board to 70-90degrees,then using biggest hot air nozzle with 390 degrees works with my setup to desolder individual leds. Also, your typical preheating device prooved to be completely unsuitable. I had to put the boards on a single burner electric hot plate (controlled by an external thermostat) to preheat it properly.

I heard it said multiple times in youtube vids etc that temperature settings are meaningless... I disagree if one uses the same equipment as the person recording and one can observe the technique used the temperature and air settings are helpful as a starting point.

[thinkfat]

I typically use 300°C to 330°C on leaded boards, but you might need more on boards that have large ground planes and can dissipate heat fast. A bottom heater might help, but I don't own one, and for small chips or sparsely populated boards you don't need one.

But note that it's effectively useless to give advice on exact temperature. Most hot air stations are not able to keep the temperature stable for every airflow setting and it depends a lot on where the temperature is measured. You might want to put a temperature probe on the board somewhere in the area you're working on to get an idea about the actual heat transfer, independent of what your hot air station is doing.

[Bicurico]

I have used a lot more than that! I have dessoldered with my hot air soldering station set at 450 degrees C.

I am not pro by any means and the most I did was in fact trying out how to do it on old PC motherboards.

What I have learned is this:

1) To much air flow will blow small components away (SMD resistors and capacitors)
2) To little heat and you will be heating forever without success
3) Too much heat will burn the PCB
4) Any plastic must be removed - it will burn/melt
5) large IC's need some practice and patience, because you must heat up all pins evenly by circular motion - best to use a bigger nozzle
6) There is a certain technique to actually remove the component. I just give it a periodic "kick" with some screwdriver. Once the temperature is right, the component will easily move and you know it is done

The best is to get some old PCB's like motherboards and start removing all the SMD components one by one, taking note how its done best.

Regards,
Vitor

[Microdoser]

Well, it depends.

If you use too little heat then you will spend a long time trying to remove the component. As a result, more heat will be put into the surrounding area and that can damage other components.

If you use too much heat, you can damage the board (specifically the glue that holds it together or holds on the pads and tracks) or scorch the surface. If you spend too long before removing the component, this can also damage nearby components.

If your airflow is too high, this can blow away components, will increase the area heated and might spread the heat to nearby components, this might not be good if nearby components are heat sensitive. Plastic shrouds on headers or processors particularly.

If your airflow is too little, you may not heat enough of the component you want to remove for all the solder to melt at once.

All this also depends on the board around the component. For example, if you have a large ground plane then that will suck heat away from the work area and you will have to adjust your heat and flow accordingly.

With practice, you should be able to mostly set your temperature high and only adjust your airflow. Then you adjust proximity and time spent heating to achieve the desired results.

I agree strongly with the idea of getting an unwanted board and practising. IMO this is your best way forward. Also, watch some youtube videos by people who do SMD rework and upload board repair videos.

You should watch for the indicators that the work gives you, like when the solder is melted on the component you are working on or when other components you are not working on also show signs of too much heat.

And always use flux. It vastly improves wetting and allows the solder to flow.

To be honest specific temperature advice might not even be helpful for you.

For some people that is number 5 heat and number 7 flow, for others it is 75% flow and 350C and so on.

You should learn your particular gun and the settings that work with the pieces you work on.

[djd_oz]

Thank you all for your comments, it's been really helpful.