Question about PCB soldering

[thekingofmonks]

Is it a bad thing that the PCB I'm working on changes colors while I'm blowing hot air on it to remove/solder a component?

 

A typically safe temperature would within the range of 200-300 degrees Celsius, which is the temps my hot air gun is operating at that allows most things to come off, but is the change of color on the PCB a sign of damage?

 

fyi I do cover most of the board with aluminium on expensive boards, I'm not dumb

[Beerzerker]

16 hours ago, thekingofmonks said:

Is it a bad thing that the PCB I'm working on changes colors while I'm blowing hot air on it to remove/solder a component?

 

A typically safe temperature would within the range of 200-300 degrees Celsius, which is the temps my hot air gun is operating at that allows most things to come off, but is the change of color on the PCB a sign of damage?

 

fyi I do cover most of the board with aluminium on expensive boards, I'm not dumb

WRONG.
The very bottom of the temp range you stated is already hot enough to melt solder.

Going up to 300c is way too high and will burn a PCB in no time.
The average soldering temps used are 193 - 210c since solder melts around 193c/380f. Some solder types do need more heat but not alot more and as a standard this temp range is what to go with, even if it takes a little longer to get the solder to melt to get the work done.

300c is 572f and that's more than enough to cook a PCB, the max temp you've ever want to use period is around 218c/425f even though some setups can go higher in temp, mine can go up to 237c/460f but I don't do that.

It may seem to take a really long time to get it to melt but tryng to make it happen all at once is just asking for it. A PCB is NOT supposed to change color during the soldering/desoldering process because if it does you're cooking it.

[trag1c]

3 hours ago, Beerzerker said:

WRONG.
The very bottom of the temp range you stated is already hot enough to melt solder.

Going up to 300c is way too high and will burn a PCB in no time.
The average soldering temps used are 193 - 210c since solder melts around 193c/380f. Some solder types do need more heat but not alot more and as a standard this temp range is what to go with, even if it takes a little longer to get the solder to melt to get the work done.

300c is 572f and that's more than enough to cook a PCB, the max temp you've ever want to use period is around 218c/425f even though some setups can go higher in temp, mine can go up to 237c/460f but I don't do that.

It may seem to take a really long time to get it to melt but tryng to make it happen all at once is just asking for it. A PCB is NOT supposed to change color during the soldering/desoldering process because if it does you're cooking it.

I have to disagree and say those temps are way too low. Temperatures that are close to the melting point are fine when you're using a reflow oven since it's a precise temp/time curve you're following but doing any rework with an iron or hot air requires higher temps. Too low of a temperature can ironically burn the board or components around the component you're trying to rework. You need to have reserve temperature so that you can effectively heat the component leads. You ideally want to get in and get out with only a few seconds per joint with an iron. Hot air should be about 10-15s with no pre heat for a good size BGA package.

 

Even with leaded solder you want about 340C for either hot air or with an iron. 370-380 for lead free. Also preheating the board is a great way to shorten the time it takes to melt the solder while keeping the thermal stress and all of the components at a relatively safe levels. Flux is also never a bad idea.

 

OPs failing was most likely too low of temperature and not knowing when to take the component off leading to the FR4/soldermask burning.

 

Some sources for temps:

 

https://www.hakko.com/english/namari/pages/

https://www.ti.com/lit/pdf/slwa051

[Mojo-Jojo]

20 minutes ago, trag1c said:

I have to disagree and say those temps are way too low. Temperatures that are close to the melting point are fine when you're using a reflow oven since it's a precise temp/time curve you're following but doing any rework with an iron or hot air requires higher temps. Too low of a temperature can ironically burn the board or components around the component you're trying to rework. You need to have reserve temperature so that you can effectively heat the component leads. You ideally want to get in and get out with only a few seconds per joint with an iron. Hot air should be about 10-15s with no pre heat for a good size BGA package.

 

Even with leaded solder you want about 340C for either hot air or with an iron. 370-380 for lead free. Also preheating the board is a great way to shorten the time it takes to melt the solder while keeping the thermal stress and all of the components at a relatively safe levels. Flux is also never a bad idea.

 

OPs failing was most likely too low of temperature and not knowing when to take the component off leading to the FR4/soldermask burning.

 

Some sources for temps:

 

https://www.hakko.com/english/namari/pages/

https://www.ti.com/lit/pdf/slwa051

Agreed. 340c is my go-to temp for soldering ICs with leaded solder. 360 or even 380 if I'm soldering components to a large ground mass, such as FETs. Never had issues with the board discoloring.

 

Using enough flux is also important, especially also take care not to cook the flux too much as it will carbonize and conduct electricity. Always clean that stuff off.

[LogicalDrm]

-> Moved to Hobby Electronics

[Guest]

8 hours ago, Beerzerker said:

WRONG.
The very bottom of the temp range you stated is already hot enough to melt solder.

Going up to 300c is way too high and will burn a PCB in no time.
The average soldering temps used are 193 - 210c since solder melts around 193c/380f. Some solder types do need more heat but not alot more and as a standard this temp range is what to go with, even if it takes a little longer to get the solder to melt to get the work done.

300c is 572f and that's more than enough to cook a PCB, the max temp you've ever want to use period is around 218c/425f even though some setups can go higher in temp, mine can go up to 237c/460f but I don't do that.

It may seem to take a really long time to get it to melt but tryng to make it happen all at once is just asking for it. A PCB is NOT supposed to change color during the soldering/desoldering process because if it does you're cooking it.

 When I de-lid soldered chips, I'm not exceeding 230c. Cause if I did, the pins would fall off the bottom of the chip. That and the PCB changes colors. 300c bakes the PCB pretty quickly. 

[Beerzerker]

7 hours ago, trag1c said:

I have to disagree and say those temps are way too low. Temperatures that are close to the melting point are fine when you're using a reflow oven since it's a precise temp/time curve you're following but doing any rework with an iron or hot air requires higher temps. Too low of a temperature can ironically burn the board or components around the component you're trying to rework. You need to have reserve temperature so that you can effectively heat the component leads. You ideally want to get in and get out with only a few seconds per joint with an iron. Hot air should be about 10-15s with no pre heat for a good size BGA package.

 

Even with leaded solder you want about 340C for either hot air or with an iron. 370-380 for lead free. Also preheating the board is a great way to shorten the time it takes to melt the solder while keeping the thermal stress and all of the components at a relatively safe levels. Flux is also never a bad idea.

 

OPs failing was most likely too low of temperature and not knowing when to take the component off leading to the FR4/soldermask burning.

 

Some sources for temps:

 

https://www.hakko.com/english/namari/pages/

https://www.ti.com/lit/pdf/slwa051

I still say that's just too hot because the point is to melt the solder and that's it. No need to go overboard with heat if it's already hot enough to make the solder liquify because that's what you are doing when you solder something.

Preheating a board is a good thing and you are correct about it being useful, it does help things and of course flux is a necessity to get a good bond to the solder and part you're working on.

BTW I"m basing what I'm saying with my work done with a standard soldering station that has an iron and airwand, no ovens or anything of that sort in use here. I've used the temp settings I stated without issue for as long as I've been doing this.
What you are saying is it takes too long to get everything up to temp and because it takes so long, it's letting everything there and around it heat up to frying point - That I CAN agree with. 
So far I've used the temps I've stated and haven't burned a PCB that way, it was when I jacked the heat up to the upper range is when the PCB burn occured for me when I had it that high so I don't go that high now.
I will preheat the area with the airwand on a low airflow setting around the component first with a lower temp, usually around 350f / 176c and do that for like a minute to be sure the area is ready - I then use what's required to get the solder to melt and that's it.
Works great.

I do know if using a higher airflow speed, that in itself can thermally oversaturate the area and it will get hotter than the actual temp you set leading to excessive heat and well... You know the rest like I do about that.

BTW I don't use silver solder, I use 60/40 and maybe that's why we are stating different solder temps to use because I already know the silver stuff has a higher melting point than 60/40 does. In that case I can see the need for higher temps but since I use 60/40 it's not required for my work.
I also noted one of the links ( TRF3761 QFN Installation Using a Hakko Hot Air Rework Station - slwa051.pdf ) you gave refers to a hot air rework station, not a typical soldering situation most of us would be in.
 

[mariushm]

Lead free solder starts melting at 217 degrees Celsius. Your hot air gun temperature should start somewhere around 280-300c, unless that component is particularly sensitive to heat (like some leds or some optical stuff, ccd/cmos sensors etc). For most surface mount stuff (resistors and capacitors) even 350 degrees Celsius will be fine.

 

Components are soldered onto traces which absorb some heat. The component itself also has some thermal mass, it takes some time to "absorb" heat and become hot, and during that time it takes heat away from the terminals of the component.

Motherboards are especially hard because they often have thick traces, 1-2oz thick... they suck more heat than normal.  The ground and voltage traces on a motherboard are particularly thick and big (to keep voltage drop on the motherboard as low as possible) so that whole layer acts like a heatsink.

 

Apply liquid/gel FLUX all over the pads/terminals so that the oxides and crap on the surface of the terminals / pads breaks down as the flux is heated by the hot air gun, and no longer act as heat shield, and now the heat can actually heat up the solder and pins/terminals of the component.

 

Get your hot air gun maybe 5-10 inches away from the surface so that the component (and area) slowly gets warm up to 80c or so over the course of a few seconds and you give time for the flux to do its magic, then bring the hot air gun closer to area you want to desolder - bringing it closer will increase the actual temperature the board is subjected to.

Move the hot air gun around a big chip, if that's what you want to desolder, so that all the sides of the chip heat up evenly.

 

Keep in mind that hot air guns have the air temperature measured inside the nozzle somewhere, and sensors are often poorly calibrated, giving you +/- 10 degrees.  With even cm / inch of space between the nozzle and the work area, the temperature will decrease.

 

60/40 melts at around 180c and has a wider region where it's between states, not melted fully, not solid fully ... it's a few degrees Celsius.

63/37 is euctetic, it has a very narrow transition point, at 183c, basically snaps from solid to liquid at that temperature.

 

Silver solders can have lower melting temperatures, not higher. For example 62/36/2 is also euctetic like 63/37, and transitions at 179c

The lead free solders with silver are indeed melting at 217-220 degrees Celsius just like regular lead free solder.

 

[mr moose]

I have been soldering at 450c for the better part of a decade, never had an issue with killing parts,  Caveat with this is the digital readout on my station could easily be inaccurate making it more like 400c and the I don't typically do highly temp sensitive stuff.  Mostly audio chips.   I have done the odd phone and motherboard and have yet to discolor the PCB.

 

 

 

[thekingofmonks]

I'm getting replies with completely different opinions and experiences...

 

Some say it's sub-300, some say 340, one says it's past 400.

 

Who am I supposed to trust??

[mariushm]

The answer is that it really depends on lots of factors. 

For hot air guns, it depends on the nozzle (how wide the air exit is) , the air pressure (how fast the air comes out) and how far away you are from the piece you want to heat up.

 

For soldering irons, it depends on how big the tip is, how much "heat reservoir" the tip has (because a bigger tip has a bigger reservoir, so it cools slower, hence station doesn't need to pump a lot of energy back in the tip to maintain that temperature), and how fast the station reacts to tip cooldown.

 

Basically, say you have the iron configured to 250 C ... the moment you put the tip on some pads connected to a thick ground plane for example, the ground copper plan will start sucking heat away from the tip, and the tip temperature may drop down to 200c .. and now the soldering station has to react and pump a lot of energy in the heating element in the tip to raise the tip back to 250c while the heat is still sucked away from the ground plane.

 

You just can't use a soldering iron close to the melting temperature of solder (180c-183 for leaded solders, 217c for lead free solders)  - usually you set it around 50-100c, 300c is perfectly reasonable.

 

I have a Hakko 936 (original), and I'm not even sure at what temperature is set... yeah, just looked and it's set at 275c because I soldered some thin wires to a lithium battery pack and didn't want to overheat the pads on the battery. Usually, I keep it in the 300-350c range.

 

I also have a Pace HW 50 soldering station that you can only adjust temperature by plugging a "module" in it... doesn't have knob to adjust temperature. They're basically stereo jacks with a resistor hidden inside that sets the temperature threshold.

The "modules" are hardcoded from 5 to 8.5 , where 5 is 500F (260c) and goes up in 50F increments to 8.5 which is 850F or 454 Celsius  The module 6 is the most common I use, so around 320 Celsius.

 

As you solder lots of things, you'll start to get a feel about how much heat is right and don't worry that much about it.

As the Pace tutorial say, the temperature is just one part of the whole, and whatever temperature allows you to finish a joint in reasonable time is the right temperature

 

See the explanation at 12:15  but really, if you have time, watch the whole video  - the tools in it ,may look outdated, but everything he says is still valid (with the exception that you won't damage the board so easily, they make circuit boards with better glues and thicker copper pads so it's harder to damage them):

 

 

Dave from EEVBlog also has a very good soldering tutorial (worth watching parts 1 and 2 fully) and he gives some details about temperature at 23:00 :

 

 

 

[mr moose]

20 hours ago, thekingofmonks said:

 

 

Some say it's sub-300, some say 340, one says it's past 400.

 

 

I say that as it relates to my iron with a caveat. 

 

Really what it comes down to is if the solder is melting too quick and the pcb is burning then turn your temp down (regardless what it says it is) and see what happens.  After a while you won't bother with the numbers and just find the setting that works.    If I let the numbers take priority over my experience I would have a shit ton of cold joints.