Actions
Forum Thread Details
  • Replies 8 replies
  • Subscribers 155 subscribers
  • Views 398 views
  • Users 0 members are here
Related

Thermal profile methodology for DIY reflow — what I know, what I'm missing

pvit

Hi everyone. I built a compact reflow micro-table for hassle-free PCB assembly at home:  Reflow Micro Table: Compact USB PD Reflow Table with Browser Control 

I'd like to write a general guide on setting up thermal profiles, but I lack the practical experience to make something solid. I'm not talking about production-line requirements — more about "reasonably good" for advanced DIY.

What I know so far as hard constraints:

  • For RoHS/leaded: TAL no more than 60 seconds. For LTS — I'm not sure.
  • Ramp rate: 1–2°C/sec (except the start)
  • Cooling rate: ~4°C/sec; passive air cooling is acceptable (not sure about LTS).

Everything else seems to depend on the specific paste and equipment — recommendations vary widely. The bigger frustration is that datasheets give you a profile to follow but don't explain how much the parameters can actually vary. It's like the difference between a datasheet and a reference manual :)

A colleague with hands-on production experience described this simplified approach to me — I'd be curious whether it holds up in practice:

  • Ramp linearly at 1–2°C/sec
  • Wait until paste visibly melts across all components
  • Turn off immediately and let it cool

I'm keeping the scope to LTS and leaded — RoHS/SAC peak temperatures and tighter process windows are an unnecessary complication for beginners.

Questions

  • How important is a soaking plateau, and how does it depend on the paste type?
  • How long can components and the board stay above a given temperature before damage becomes a concern?
  • Is there a minimum time above liquidus required for proper intermetallic formation?
  • How critical is cooling rate — does the solidification speed affect joint quality, and what is generally preferable?
  • and so on...

I'd really appreciate input from people with real PCB assembly experience to help systematize the practical nuances — the goal is to put together beginner-friendly guidance on building reflow profiles from scratch.

I've also tried to document my limited experience with LTS pastes here: github.com/.../soldering_paste.md — feedback on what could be improved there would be much appreciated as well.

Parents

  • The colleague's method work well for me with an extra step to carefully push the soldered board off the plate after turning off. My plate has enough heat capacitance that the cool down rate is way too slow.

    On your plate (very nice and clean design btw), I would be interested in the temperature difference between your calculated temperature and the temperature on the top side of the board to be soldered. My experience is that the top temp always lags a few degrees behind the plate temp itself. If you want/need to be profile correct, this difference matters and needs to be taken into account. 

    To your questions:

    The soaking plateau gets more important the more copper you have in your board. A 2-layer board is very forgiving, but 4 or 6 layers are happier with the soak to get a more even temperature when ramping up into the liquid phase. Also prevents tombstoning. 

    Max temperature is mainly a concern of the FR4 material used, components are quite robust and have a little convection cooling working for them. The standard FR4 with Tg = 130degC of the cheapest offerings of your favourite Three-Letter board house works OK for one cycle of leaded solder profile. A lead-free profile with temperature up to 260 degC requires Tg of 170degC or better. The substrate gets brittle with compromised structural integrity.

    Cooling rate is again a concern of tombstoning as a slower rate gets more uneven cooling. 5 or 6 degC/sec is preferred over slower rate. 

    And don't be afraid of lead-free solder process, it is no different when you use quality ingredients (as long as we are not talking low temperature solder). 

Reply

  • The colleague's method work well for me with an extra step to carefully push the soldered board off the plate after turning off. My plate has enough heat capacitance that the cool down rate is way too slow.

    On your plate (very nice and clean design btw), I would be interested in the temperature difference between your calculated temperature and the temperature on the top side of the board to be soldered. My experience is that the top temp always lags a few degrees behind the plate temp itself. If you want/need to be profile correct, this difference matters and needs to be taken into account. 

    To your questions:

    The soaking plateau gets more important the more copper you have in your board. A 2-layer board is very forgiving, but 4 or 6 layers are happier with the soak to get a more even temperature when ramping up into the liquid phase. Also prevents tombstoning. 

    Max temperature is mainly a concern of the FR4 material used, components are quite robust and have a little convection cooling working for them. The standard FR4 with Tg = 130degC of the cheapest offerings of your favourite Three-Letter board house works OK for one cycle of leaded solder profile. A lead-free profile with temperature up to 260 degC requires Tg of 170degC or better. The substrate gets brittle with compromised structural integrity.

    Cooling rate is again a concern of tombstoning as a slower rate gets more uneven cooling. 5 or 6 degC/sec is preferred over slower rate. 

    And don't be afraid of lead-free solder process, it is no different when you use quality ingredients (as long as we are not talking low temperature solder). 

Children
  • in reply to wolfgangfriedrich

    Personally, I have nothing against lead-free processes. But there are technical limits - existing power does not allow to reach desired ramp up speed for lead free. It's possible to reduce working area, but then comparison with MHP50 will be less impressive :). And as you noted, after additional temperature correction, to compensate losses, things may become too risky.

    wolfgangfriedrich said:
    I would be interested in the temperature difference between your calculated temperature and the temperature on the top side of the board to be soldered

    According to cheap chinese thermal imager, and visual inspection of paste melting, in static state and 4-layer board with power polygons inside

    • about 10-15°C with LTS profile
    • about 20°C with leaded profile
    And with notable delays on ramp up. As you can see on video, bottom right corner - one pin of buzzer is melted even after cooling phase started (probably it was located on corner screw hole). But this is very heavy scenario for "home use". Self-assembly is more acrobatics than normal use :)
    Profiles are already adjusted - the leaded one has 230°C top instead 205°C. Probably, this can be more optimized, but this require more experience than I have in this area.
  • in reply to pvit

    When I did have my DIY hot plate, I recorded some temperature profiles. With just turning of and convection cooling, the down ramp was never as steep a decline as the required profile. And the up-ramp was basically linear if there is not enough power or too much mass to heat. 

    (+) Arduino Hot Plate (2) - What's for dessert? - element14 Community

  • in reply to wolfgangfriedrich

    Yeah. Thin plate => bad heat distribution. Fat plate => slow cooling. I use Pi5 fan to blow from the bottom. It improves cooling speed 2x-3x until melting point reached. But that's still 1°C/sec. Something more serious will not fit device size.

    The cheat is to end profile graph on melting point instead of room temp, lol :).

    image