Temperature curve for lead-free soldering in an oven.

[luiHS]

 
Hi
I weld my boards in the oven and so far I have used tin paste with lead, specifically Loctite MP218, the one that has worked best for me by far. Now it seems that all the solder is lead-free, and although I have still got the MP218, I would like to try lead-free soldering.

The question is what temperature curve should I use in the oven, I have two, a T962A and a T962C. Both have some pre-programmed temperature curves, and others so that the user can set their own.

I started out using curve 2, but have been using curve 1 for a while now even if it's with leaded tin. Which of the 4 curves that I attach would be the ideal one for lead-free soldering, or better define my own curve and which one should it be?.

It appears that curves 3 and 4 are made for lead-free solder, with a peak temperature of 257 degrees. I don't know if this could be excessive and could burn some sensitive components like microcontrollers and others.

Regards

[abquke]

Waveforms 3 and 4 look to be just about identical. Normally I'd say "just try it" but I can sympathize with not wanting to risk microcontrollers these days. Getting components is one thing, keeping them hot for too long is another. Those temperature profiles are intended to not keep things hot for more than is necessary.

If you really don't want to go to those temperatures or have components that really can't handle it AND want to be lead-free you could look into bismuth-tin solders that have a reasonably low melting point (SnPb-ish?) but are lead free.

[Psi]

Seems fine to me, you do need a higher temp for lead free.

257C shouldn't damage anything.

[48X24X48X]

Puhui's temperature curve shown on the LCD display is compensated and is not the real theoretical curve you would expect from your paste datasheet. Basically the curve will be higher temperature as their thermocouple measure the air in the oven rather than the board temperature. This is just their "assumption" on how much the difference between the air and your board temperature will be. Best solution would be loading your own profile and measure the board temperature manually and tweak the numbers until it's close to your intended value. This is what I did with my T937 oven.

[tooki]

It appears that curves 3 and 4 are made for lead-free solder, with a peak temperature of 257 degrees. I don't know if this could be excessive and could burn some sensitive components like microcontrollers and others.

What gives you that idea?? Lead-free reflow soldering is the norm for mass-produced items these days, so billions of chips of all levels of complexity are reflowed at those temperatures every year. See the attached JEDEC profiles.

[luiHS]

 
I tried today with curve 3, which raises the temperature up to 257 degrees and it did not work for me. An SMD connector came out with melted plastic. On the other hand, the welds have been quite ugly, seen under the microscope they are seen with many small balls, they do not seem reliable to me. I don't know if it's a matter of adjusting the curve times, maybe increasing the preheating times.

As for the maximum temperature, I have read that 240 degrees could be enough. What I am clear about is that 257 degrees burns the plastics of the SMD connectors.

The paste tin I tried today was this:
https://www.tme.eu/en/details/esac305_250/other-lead-free-binders/ag-termopasty/art-agt-026-easy-print/

I have ordered another one from Mouser, which will arrive tomorrow, it is this:
https://www.mouser.es/ProductDetail/910-TS391AX250

And as a last option is to go back to using the leaded MP218, I have ordered it from RS Amidata, but it will take a few days to arrive, and that is why I bought the two unleaded ones from Mouser and TME, but I think it was wasted time and money .

Is there anyone who has a soldering oven of this type or similar, solders with lead-free tin paste, and can share the characteristics of the curve used (times and temperatures)?

Could it be that with the curves that I have selected the solders go wrong because the maximum temperature peak only lasts 10 seconds, and it should last 20 seconds for lead-free solder?

[Psi]

I'm surprised you melted a connector at 257C.

Maybe there are hot spots in the oven over 257C

[tooki]

That was my thinking too.

[Psi]

That was my thinking too.

You could test by setting hot air around 260C and see if it melts the connector.
Then keep going up 10C at a time until it melts. That should tell you how bad the hot spots are

I seem to recall people on here adding a metal plate to the top of their T962A/T962C to spread the heat out more evenly.