Reflowing lead free

[Simon]

I've been told that I should not use an IR oven with lead free due to the higher temperatures required as the black IC packages will absorb heat faster being IR and at the higher working temperature this is not good. I think leaded was around 180C and lead free is 230 so a 50C increase or an increase of nearly 30%.

What should I use then instead of the traditional cheap IR oven. Is this why the newer Chinese one have fans? what If I put a black item in there that will heat up and then the fan will blow that heat around?

Or do I just literally buy a small kitchen fan oven (mine goes up to just 230C) and perhaps put some aluminium in there, preheat that and then run the board reflow?

[mikeselectricstuff]

With unleaded, there is a rather smaller margin between "reflow" and "incinerate", but it's probably doable with some experimentation with paste types, power and/or fan.
As regards  absorbtion, my gut feeling is to go for a lower element temperature (barely glowing) and longer time.
However if it's for prototypes, just use leaded.
 

[Simon]

I would also need to do small volume manufacturing so it needs to be RoHS compliant. I see ovens that specify forced air convection but are IR. I think mine (ZB3530HL) has a fan on top of the IR elements.

So is it the adding of air flow that does it or are these IR ovens lying as I see IR ovens with air convection advertised as being for lead free What do I use for small scale prototype builds if not one of those cheap IR ovens. Or is there a combo of IR and the IR heating an object that the forced air then transfers the heat from.

Why was IR used in the first place? was it more efficient as it heated just the PCB and not the whole oven?

[jmelson]

I've been doing lead-free reflow in a consumer-grade toaster oven for over 10 years.  I got a "ramp and soak" thermocouple controller and poke a micro-sized thermocouple into a plated through hole in the board to measure actual board temperature.  This works great, if the thermocouple is just in the air the boards get too hot and burn.  The ramp and soak feature allows me to program temperature ramps and holds.

Jon

[Simon]

so what sort of power do you need to do this? I think my kitchen oven is 1kW. I take it then the thermocouple must be a tight fit in the hole.

[48X24X48X]

In my case, for LF SAC305 paste, I still uses my hacked oven. 900W with IR heater at 12 liter of volume space inside the oven. The IR heater however has partially exposed metal cage to reduced the direct heat. The key is the volume (not so big) so the temperature can go up faster. Up to now, this setup still better than my T-937 for high temperature lead-free as it doesn't melt my plastic connectors. And as always, never ever put the thermocouple tip hanging in the air, put it inside a through hole pad, best is the ground pin. The only drawback is the maximum size of PCB you can put in...

[Simon]

Yes the power to volume (and weight) ratio is key for responsiveness which is why I would consider putting a block of aluminium in the oven and preheating it so that it can release heat. I don't know how important the cool down time is but if I just open the door and leave it it will still take time to cool.

[IconicPCB]

The purpose of the fan is to increase the thermal mass of the oven.
Putting additional aluminium block may seem like a good idea but it is not. It limits the oven dynamic.
Stick with the minimum actual thermal mass and use a fan to minimise temperature gradient between components.

Use a temperature probe to measure PCB temperature as directly as You can with as little lag as practicable to control thermal profile as tightly as you can.

[peter-h]

See this thread
https://www.eevblog.com/forum/manufacture/t962-reflow-oven-233972/msg3614282/#msg3614282

One can do this job very well using these 150 quid ovens. Well, I changed the controller for the 150 quid ES one so my oven became a "300 quid oven". The key thing is to extend the sensor so it measures the actual board temperature. All the time one is measuring something "in the air" it is going to be very hit and miss especially if you have different size components. People have done all kinds of weird mods, with fans, but the effective fan mods are difficult.

However I use leaded solder paste for prototyping. Volume is contracted out and is lead-free but the contractor we use has vapour phase ovens which are excellent for difficult boards. OTOH I am sure nobody will notice if you use leaded solder even in production On a specialised item, low volume, who will be checking? The "control and monitoring" exemption made a complete mockery of lead-free soldering; most small companies were using that exemption. I don't remember when it ended, but by the time it did, everybody lost interest in ROHS.

[mazurov]

I reflow SAC305 in a toaster oven set up similar to @jmelson ; I also equipped mine with nitrogen port and valve which opens at the end of soak step. More than 10 years in operation, no issues so far.

[Simon]

So what do you do with the nitrogen?

Also found this: https://www.newburyelectronics.co.uk/news/low-cost-smd-lead-free-solder-paste-reflow/ I think their conclusion was you need really good profiles. I'm going to guess that put a thermocouple in the PCB and set the temperature, later build a controller to automate the profile. As it is with my IR ovens I took to manually processing the boards. I had the use of one of those large ones and found that wafting the fans occasionally would boost the temperature, once I manually controlled the process things were better. I then put some anodized aluminium scrap bits in the back to absorb some IR light and used that as a boost.

[mazurov]

So what do you do with the nitrogen?

I blow it (gently) into the oven. Helps with the edges of large boards when you need to stay on the top of the curve longer.

Smaller boards are easier and don't require nitrogen. 1 sq.inch small ones reflow on a preheater just fine, I don't even bother with an oven.

One other thing that I think is beneficial. I covered the door with aluminum foil from the inside.  Helps with larger boards.

[Simon]

So is this about air distribution or the nitrogen being better at distributing heat? Looking around nitrogen is used in reflow of lead free as it is an inert gas and once it has replaced the air containing oxygen prevents oxidization and improves reliability.

[mikeselectricstuff]

So is this about air distribution or the nitrogen being better at distributing heat? Looking around nitrogen is used in reflow of lead free as it is an inert gas and once it has replaced the air containing oxygen prevents oxidization and improves reliability.

Yes - with N2 it can stay hotter for longer before things oxidise, giving the whole board time to get hot enough

[jmelson]

so what sort of power do you need to do this? I think my kitchen oven is 1kW. I take it then the thermocouple must be a tight fit in the hole.

I was surprised to see this is a 1500W toaster oven.  The thermocouple is a micro-size #30 AWG) teflon  thermocouple that I got a 1000 foot spool of for a song on eBay because it is an unusual type (Type E).  I just set the ramp and soak controller for type E thermocouples.  The thermocouple is not a tight fit in the plated through holes, it makes good enough thermal contact due to the tiny wire size.  This oven has two linear heating elements that run left-right above the rack and two more below.

[Simon]

Well I ran my kitchen "toaster" oven yesterday with a thermocouple in it wrapped around a bar on the middle shelf. I could not get too hot as the sensor an my meter did not look like the wire would take too much heat (Fluke 179) so I aimed around 100C and ended up running tests at 150C without damage. I've found my other thermocouple so will do more testing when I get a battery for the thermometer I have that being a bit of junk off ebay just self discharges the battery.

Anyway I tried measuring the ramp rate up to 100C. It is about 0.7C/s so not the fastest. I guess I need to find the most powerful oven for it's size that I can. Obviously the more element the more thermal inertia. even this thing continues to heat once the elements are off to up to 20C beyond the cut off temperature. Once the oven is stabilized the thermostat keeps it at around +/-3C of the target which is not the same temperature as what the dial is set at. I sort of see now why cooking with it sometimes is a challenge.

[peter-h]

I am getting great results by placing the measurement thermocouple (see thread I linked) onto a piece of PCB material, lying next to the board being soldered. I'd say within 5C.

Measuring the actual PCB is even better but you might touch some component with the TC and move it.

Still, edges of the PCB are cooler than the centre. I think that is due to the largely radiant heat source.

Re the nitrogen, I am surprised that does anything. The issue is not oxidation AFAIK; it is getting the temperature high enough (for lead free) without damage to components. But maybe it works; a cunning solution for sure.

[Simon]

Yes some representative piece of PCB is probably a good idea, that way the reflow sensing does not interfere with the design.

The nitrogen is inert, air has oxygen that will oxidise and the hotter the more easily so replacing the air with the inert nitrogen helps stop bad joints.