Best temperature for working with hot air stations?

[belasajgo]

I would need a little bit of help and advice in choosing the right temperature and airflow for microsoldering with a hot air station. I could not find any similar topic on the forum. I have about 15 years of experience in soldering/desoldering with classical soldering stations with soldering irons (I mainly used Weller, but also worked with Ersa and JBC) and I also know when which tip and temperature is the best to use, but I almost never used a hot air station until now, so that's why I am asking for advice.

I purchased a few months ago a Quick 861DW hot air rework station and I already used it for many hours to desolder components from dead circuit boards (to get used to it), but I would like to know what temperatures would be recommended when soldering and desoldering small and medium size SMD components and what airflow would be the best to use, to not blow away the smaller components from the board. I also don't want to destroy the components with too high temperatures, that's why I prefer lower temperatures, but then I have to apply the hot air for a longer period of time, what also is not the best thing to do. I am mainly interested in the recommended temperature/airflow for (de)soldering small passive components like 0805 or 1206 resistors/capacitors, and for (de)soldering smaller SOIC and QFP ICs. I am not interested yet in working with BGAs.

Furthermore I would like to know if you apply the hot air directly just to the specific area where you want to (de)solder some components, or if you try beforehand to preheat the whole board a little bit with the hot air station, to prevent mechanical stress in the board due to the large temperature difference between the hot and the cold areas of the board? I don't own a board preheater yet but I also don't plan to buy one. Thank you in advance for the help and for the recommendations!

[Siwastaja]

Should try to preheat as much as possible, over a large area.

If you need to go much over 300degC (for leaded; lead-free requires a bit more), that's a telltale sign you are heating a spot too small, and the heat doesn't get into the PCB. Heat the board. If you only have the single hot air station available, the only way is to preheat a large area, using a higher flow rate, and moving the nozzle around. Note that you can use fairly high temperature setting: the fact you are moving it all the time means the components and board "see" a much lower temperature, they don't have time to heat up fully.

When I did not have any kind of hotplate available, I sometimes quickly taped a soldering iron on the edge of the table, so that the board comes on top (about an inch away) from the iron. It's an ugly hack, but provides quite good board heating. Doesn't work too well if the component to be removed is exactly in the middle... The board tips over unless you tape the PCB down as well...

Now I use a 3D printer heatbed PCB, I run it at about 140-150 degC. That's great, such temperatures are well acceptable even for extended times. Reduces the temperature requirement of the hot air station, often just some 270 degC does the job quickly enough because the board is already hot and less temperature differential is required to move the small amount of heat energy.

[D3f1ant]

Since your replicating a reflow process, the PCB and component typically shouldn't go over 260  degC or you risk damage (check datasheet for particular component). To achieve that will depend on the PCB and part you are reflowing, there is no one value that will work for everything.

Yes preheat the board  (but it's tedious without a dedicated preheater), several minutes at 150-160 is a good start.

[janoc]

Now I use a 3D printer heatbed PCB, I run it at about 140-150 degC.

Uh, do yourself a favor and buy one of the cheap preheater hotplates instead. Or use one of the cheap PTC heaters - buy one for the temperature you need and it self-regulates.

The PCB heated beds for 3D printers are not designed to run this hot (most 3D printing never runs the bed above 80-100 C) and it will likely warp and delaminate over time (it is normal FR4!).

In addition the connections tend to be pretty flimsy (usually just a wire soldered directly to the board) and the 3D printers using these have to go to a lot of length to provide strain relief to avoid damaging the high current carrying wires (I have seen fires because of broken wires!). Also scratching the surface could expose the tracks (they are only covered by soldermask) and cause a short through the board you are heating, potentially sending 12-24V (depending on type) at high currents through a part of your board.

This isn't really a good way to build a preheater for soldering, IMO.

[Siwastaja]

Now I use a 3D printer heatbed PCB, I run it at about 140-150 degC.

Uh, do yourself a favor and buy one of the cheap preheater hotplates instead. Or use one of the cheap PTC heaters - buy one for the temperature you need and it self-regulates.

You'd be surprised to see how self-regulating it is. I turn the lab power supply voltage knob so that the current initially shows about 12A. When about +130-140 degC, the current is down at about 9-10A, and the temperature is fairly stable. I may turn the current limit knob down a bit at this point.

It's an equilibrium between the power flow in, heat flow out.

It's working great for me.

It's not perfectly exact, but I can control it within about +/- 10 degC with minimal manual intervention.

The PCB heated beds for 3D printers are not designed to run this hot (most 3D printing never runs the bed above 80-100 C) and it will likely warp and delaminate over time (it is normal FR4!).

Normal FR4 is rated somewhere around 130, maybe 140 degC, for continuous use.

Hitting 150 degC for a few minutes, no problem whatsoever. Besides, there's no need to go for 140-150. 120 already does great job for a preheat, and is well within FR4 spec. It's just, I have tried it up to 150, for fun.

Also scratching the surface could expose the tracks (they are only covered by soldermask) and cause a short through the board you are heating, potentially sending 12-24V (depending on type) at high currents through a part of your board.

This is a real risk. I use a few pieces of 3mm heat-resistant fiberglass tubing to separate the boards, leaving air pockets between.

This isn't really a good way to build a preheater for soldering, IMO.

Yeah, at least in theory, I fully agree. A crappy hack. Wouldn't have recommended to anyone. Got it for free. Took 2 minutes to solder in connectors and connect to a supply. I have just used it no problems, with zero investment in cost or time. Surprised myself as well how well it works. It has worked so well I haven't had to look for anything else. Will do when I see any actual problem arising. YMMV.

The best thing: low thermal mass, so can heat up or cool quickly. For reflow cooldown, I tend to turn the current limit knob to 4-5A first to limit the rate.

I also use it for curing underfill. Just control the current and time. Inject underfill, heat at 12A for a minute, then set at 6A for 15 minutes... Or something like that, forgot the exact times already. Verify with an infrared thermometer, the process is always within +/- 5 degC.