Reflow oven first tests with questions... - Page 1
Forum section: electronics / projects

author: alank2
author_rank: Super Contributor
timestamp: May 25, 2014, 01:59:04 am
content: Hi Everyone,So I'm ready to make the jump to smd and try my hand at some stenciling and baking. I worked on an avr based PID controller last year with a converted toaster oven that has 2 elements on top and 2 elements on bottom. My first test shows that I tweaked the PID algorithm pretty well for the sensor in air (not attached to a PCB), but then I thought I'd attach it to a small test pcb with a paperclip and see how that would go. Here are some pictures and I've got some questions at the end for anyone who has experience here!I am using a profile that goes like this:warmup ramps to 150 deg C @ 3C/s and the stage does not end until 90s after the the temp reaches 100Creflow ramps to 200 deg C @ 3C/s and the stage does not end until 90s after the the temp reaches 185Ccooldown ramps down to 25 deg C @ 6C/sOven does have a convection fan I run all the time during the cycle.My oven has a metal pan that sits on top of the metal bars so I left it in, but I wonder if I'd get better results without it (allowing the bottom two elements more direct access to the pcb's) - So this is question #1.series 1 is PV (present value, temp), series 2 is SV (set value), series 3 is CO (controller output). The CO is 0 for completely off to 120 for completely on (120 half cycles @ 60 Hz):Sensor placed about 1" above the metal tray. I suspect when I was playing with this last year, I must have tweaked the PID parameters by putting the sensor in air:Sensor paper clipped to a small pcb:Question #2 is - should I be taking the temperature of the pcb surface? Is this the best place to sense? Assuming this is true, I'm going to need to go back and tweak my PID parameters for this condition to improve performance.Question #3 is - I am a bit surprised as the temp spikes with it was attached to the pcb. I expected the pcb to dampen the temperature changes much moreso than the air but yet between 400 and 500 seconds it was a spiky up and down deal. Does this make sense? Why would this be? My sample time is 5s between pid operations so each temperature is an average of all the readings the unit took for that 5s (4 per second so 20 measurements from a max6675).Any other thoughts?Thanks,Alan

author: Paul Price
author_rank: Super Contributor
timestamp: May 25, 2014, 05:02:53 pm
content: Looks like you might have some problem with the temp sensor or the wiring/interface to it.

author: alank2
author_rank: Super Contributor
timestamp: May 25, 2014, 05:39:47 pm
content: Quote from: Paul Price on May 25, 2014, 05:02:53 pmI'm using a max6675 on a breadboard. The above is a 5s average of 20 measurements from it. Maybe I need to get a list of 250ms measurements and see what they look like to see if there is error or bad readings coming through...Thanks,Alan

author: Paul Price
author_rank: Super Contributor
timestamp: May 25, 2014, 05:55:51 pm
content: Just how does this oven work for you? Does it do the heating/cooling/soldering correct job if you feed it a PCB?

author: alank2
author_rank: Super Contributor
timestamp: May 25, 2014, 05:58:19 pm
content: I haven't tried yet, I want to get the profile right before trying...

author: Paul Price
author_rank: Super Contributor
timestamp: May 25, 2014, 06:03:03 pm
content: Your first graph shows almost an ideal response, that is why I cannot understand the erratic readings from the second picture of operation.

author: alank2
author_rank: Super Contributor
timestamp: May 25, 2014, 06:05:20 pm
content: Quote from: Paul Price on May 25, 2014, 06:03:03 pmI know, I'm going to run another test with the probe pressed against a pcb again and see if it turns out the same or similar. I know I need to tweak the PID parameters for the sensor being on the pcb vs. measuring open air.

author: Paul Price
author_rank: Super Contributor
timestamp: May 25, 2014, 06:29:30 pm
content: You got it right in the first graph shown..revert to that software and probe configuration. You second graph shows rapid oscillations of control output. This means you've either made a big tweak in your software or your temperature probe is gone sour.

author: alank2
author_rank: Super Contributor
timestamp: May 25, 2014, 08:46:47 pm
content: The difference between the first and second is that the first one has the probe held in air, touching nothing. The second has the probe bead pressed against the board with a paperclip.

author: Paul Price
author_rank: Super Contributor
timestamp: May 25, 2014, 10:54:45 pm
content: This does not seem to explain the entirely different control output behavior, I say try it again with the probe off the board. If that works the same as your first diagram you are ready to cook PCB pie.Could the probe tip be possibly grounding or shorting out to something on the PCB on the hot seat?

author: AlfBaz
author_rank: Super Contributor
timestamp: May 25, 2014, 11:03:16 pm
content: Quote from: Paul Price on May 25, 2014, 10:54:45 pmWouldn't attaching the probe to the pcb add thermal inertia, thereby screwing the PID params that worked in free air?

author: alank2
author_rank: Super Contributor
timestamp: May 25, 2014, 11:04:35 pm
content: Quote from: Paul Price on May 25, 2014, 10:54:45 pmI wonder about that, I'm going to repeat the test, maybe it was grounding.I did a CO change test today with the sensor on the PCB.My controller output CO can be from 0 (fully off) to 120 (fully on, 120 half cycles, 60 cycles). I first set the CO to 5 and let it stabilize, and then changed it to 15 and let it stabilize. I'm going to try to re-tune the PID parameters using this and see how it goes.

author: alank2
author_rank: Super Contributor
timestamp: May 25, 2014, 11:05:42 pm
content: Quote from: AlfBaz on May 25, 2014, 11:03:16 pmI think you are right, absolutely, working on returning now.

author: hamster_nz
author_rank: Super Contributor
timestamp: May 26, 2014, 12:01:33 am
content: Does anybody have a rough analogy for what is going on?I always assumed that it worked like thisBecause most of the heat is IR and can go through air without heating it, at a first approximation it looks like this:With the air temperature behind a low pass filter caused by the metalwork (and your PCB). I think this explains the graphs quite nicely (with free air being smoother, and the PCB surface being spikey.

author: Paul Price
author_rank: Super Contributor
timestamp: May 26, 2014, 12:25:15 am
content: Some Ideas to ConsiderThese thermal diagrams makes some sense but how does this help establish real-world values to the variables expressed so much more qualitatively than quantitatively.A probe thermally interfaced to a PCB with paper-clip would not induce the wild CO oscillations you show. Show me how this could be possible. Could just turbulent air cause this? The temperature on/off CO hysteresis must be very much too low.Look at it this way. The internal temperature measured by in free air is actually feeding a integrator (the inertia of the PCB's thermal mass). What this means is that if the target temperature is achieved in air, it is only a matter of seconds or minutes before the PCB warms/cools itself to the same temperature. That's all there is to it.What this means, if you give the PCB enough time to rise to the hot air temperature, you can then use this information to derive an algorithm to determine how long the new plateau of temperature must be sustained to raise the board to very close to the same temperature of the internal air (at the critical solder paste melt temperature stage of this thermal joyride).The temperature will rise from it's its present temperature to 63.7% of the air temperature in one thermal time-constant and the thermal time-constant is directly proportional to the thermal mass of the PCB. in 5 time-constants the board will reach 99.9% of the air temperature.But this may all boil down to that you could be able to attach and shield(from moving air. it is the board temperature you interested in, not the turbulent air) one or more temperature probes that are thermally intimate with tiny spots on the PCB(using thermal heatsink compound) surface and then use these temperature readings to tell if the board, in fact the whole board, is at the right temperature..

author: hamster_nz
author_rank: Super Contributor
timestamp: May 26, 2014, 12:45:14 am
content: Quote from: Paul Price on May 26, 2014, 12:25:15 amAir isn't what is doing the warming in these small ovens, it is the IR radiation from the heater than does most of the heating, and air is only really involved in taking heat away from the the board. And even blowing air in at room temperature you cant can't cool a PCB off as quick as it heats up.You can prove this if you don't use a tray under your PCB when doing reflow - the area of the PCB that are shadowed from the elements by the wires of a metal shelf will be much cooler, leading to uneven soldering - likewises you get a pattern of lines on the bottom of any toast you make (but do this before you use the oven for its true purpose in life...).

author: Paul Price
author_rank: Super Contributor
timestamp: May 26, 2014, 01:07:31 am
content: Fascinating, but look at the edit I made to what you've already replied to. Will multiple probes upon spots on the board exact the temperature of the board? Would it be necessary to engage in dithering the PCB's position to ensure even heating?I am soon ready to attempt to build one of these thermal joyrides myself.

author: alank2
author_rank: Super Contributor
timestamp: May 26, 2014, 01:36:30 am
content: New PID parameters improved things quite a bit so far before any tweaking:

author: AlfBaz
author_rank: Super Contributor
timestamp: May 26, 2014, 01:46:44 am
content: Quote from: alank2 on May 26, 2014, 01:36:30 amNice...Doesn't look like you'll get any faster heating based on you controller output. I'd be inclined to increase your "soak" time to ensure "heavier" components are up to temp before going for reflow.If you have any parts that are borderline crispifying from prolonged reflow temperature, perhaps an alarm at the end of reflow to remind you to open the door for a quicker cooling ramp

author: Paul Price
author_rank: Super Contributor
timestamp: May 26, 2014, 01:48:28 am
content: The story looks good. Howja doit?Where did you place the thermocouple?

author: alank2
author_rank: Super Contributor
timestamp: May 26, 2014, 02:05:39 am
content: Thanks AlfBaz - thanks for the tips!Paul - thermocouple is clipped to a pcb and setup with a little tension to push the bead of the couple into some soldermask. I changed the PID parameters to PGain from 5 to 3.25, IGain from 0.002 to 0.005, DGain from 100 to 50.I then repeated the test with DGain double to 100. It looks like the CO was a little jumpier, but it didn't overshoot as bad:

author: hamster_nz
author_rank: Super Contributor
timestamp: May 26, 2014, 02:31:36 am
content: I'm a little confused at the lag between the green and blue lines - are they all aligned in time? I have only a few seconds between power on and temp change (basically the time for the heating element to warm from cold) but yours looks like a bit more.My oven was just the smallest, least expensive domestic bench-top oven I could find. Maybe yours is different...How I avoided overshoot in my design was to have a set point about 5 degrees below the target temperature, active for 10 seconds, that held up the profile until it's target temperature was hit. This is then followed with the desired temperature / hold time.So if aiming for 125, it hits 120, the heater goes off and it 'coasts' for 10 seconds up to the target temperature before the 125 setpoint becomes active.I managed to get an OK profile

author: StrangeloveMD
author_rank: Contributor
timestamp: May 26, 2014, 04:04:31 am
content: Nice work thus far! Luckily I've only just been looking in on this whole "temperatures change during the reflow process" thing, so I can actually find some of my bookmarks...Attaching Thermocouples:Tis from '98, but good.LEDs are fun.TCs in General:According to OSRAM, thermocouples do exist. We are on the right track.And of course, the real opus of concern; IPC-7530: Guidelines for Temperature Profiling for Mass Soldering Processes (Reflow & Wave)The document shows its age, but mostly talks about temperatures in terms of air temp as measured 1~4cm above board topside (this will depend on the oven) as I recall.Hope something in there is useful to you, as I believe a few of the concerns brought up in this thread are addressed. I also think it may be prudent to preheat the oven as much as possible, and consider a board heater (or preheat mode for the oven) that lets the board slowly ramp up to an easier "starting" temp.hamster_nz: Thank you for posting your curve with the paste limits, it's rather my preferred format. Now, if only we could all move on to plotting them as bounded byboardlimits. We'd really be onto something!

author: Paul Price
author_rank: Super Contributor
timestamp: May 26, 2014, 08:26:44 am
content: How big was your board? What was on it..just resistors and caps..any MCU in 100+ leaded packages?

author: hamster_nz
author_rank: Super Contributor
timestamp: May 26, 2014, 08:40:39 am
content: Quote from: Paul Price on May 26, 2014, 08:26:44 amI've been building batches of half a dozen daughter boards at a time, each with about a dozen or basic bits on it - nothing harder than a SOT23-3..

author: mjkuwp
author_rank: Unknown
timestamp: May 26, 2014, 02:47:57 pm
content: thanks for sharing your project in so much detail! : )i recommend having a look at the recommended maximum TAL (time above liquid) for the solder and also heat tolerance of some of your parts. I feel like you should be getting the boards cooled down faster. What I typically do is open the oven door at 215C to 225C and aim a fan into the oven to force the temp down. Even just opening the door does not bring the temp down fast enough.so far as the controller goes...If you have a look at the first plot in my write-up you can see that sometimes this PID loop boils down to... full on - light duty - full on. OFF. however, i still like having the PID loop. The controller is helping to compensate for the lag in response of the elements.http://mjkuwp94.tumblr.com/ReflowOvenI have a moderate sized Black & Decker convection oven purchased at Amazon or Wal-Mart in USA. It is 1200W out of the box but I shortened the heating elements and boosted this to about 1700W. I also insulated the oven and even with these two efforts it will barely do 2 deg C/ second. I wonder what rate you are achieving when you target 3C per second?I get bad readings from my thermocouple and board as well so I just made sure to write code to help ignore them. I think this was faster than trying to chase down the hardware problem - at least in my case. I have done more than a dozen boards and they have all gone well. Most recently I have been holding the thermocouple to a scrap pcb using a combination of aluminum tape and kapton tape. One time I used wrapping wire to tie one in place. I definitely got the best results in my first few boards on which I mounted the thermocouple directly using extra-high temperature solder. however it is far too tedious to do that every time.I would be curious to know your oven wattage and interior size. It would be interesting as a comparison to my own oven.

author: alank2
author_rank: Super Contributor
timestamp: May 26, 2014, 03:26:21 pm
content: mjkuwp - I have the same black and decker oven you do. How hard was it to shorten the elements? What did you use to insulate it?

author: mjkuwp
author_rank: Unknown
timestamp: May 26, 2014, 07:14:56 pm
content: 9379K93 from McMaster-CarrUltra-High Temperature Foil-Faced Insulation Stripand also a chunk of stovepipe wire from the local hardware store to knit it down and into place.I should have mentioned that if I had to do this again I would try to make the oven work without doing the insulation and I would test it also without boosting the elements. It was a lot of labor to do those things and I'm not sure it is worth the time. this of course depends how much time you have.I got the idea for shortening the heater wires from here:http://www.buildlog.net/forum/viewtopic.php?f=2&t=1013It was fairly easy to do the shortening. Just involves disassembling things carefully. The wires have to be terminated by crimping so you have to have some good crimp connectors and tools.

author: alank2
author_rank: Super Contributor
timestamp: May 26, 2014, 07:58:09 pm
content: How important is it to measure the actual boards being baked? Is it workable to just pick an old board roughly the same size and attach the thermocouple to it permanently so you don't have to clip it on and off other boards?

author: mjkuwp
author_rank: Unknown
timestamp: May 26, 2014, 09:48:19 pm
content: Quote from: alank2 on May 26, 2014, 07:58:09 pmyes, this can work. It is what I do now. Since I started using a 'donor' or 'sacrificial' board I noticed that the temperature would be off by about 10C. I keep on eye on my temperature reading and note when the solder actually melts. In my head I do the math and make an adjustment to when I open the door on the oven. I know this is not perfect but it has worked well so far.

author: alank2
author_rank: Super Contributor
timestamp: May 26, 2014, 10:03:10 pm
content: Well, I baked my first board, and it works perfectly!I only had some really old solder paste to use (probably >2yrs old), but I tried to rejuvenate it a few months ago with some liquid flux when I got my pneumatic dispenser. My only complaint is that it puddled out even before I could place components on it - I couldn't see the soic pads any more, but it still lined up.I'm going to try my hand at some stenciling with some newer paste soon.

author: StrangeloveMD
author_rank: Contributor
timestamp: May 27, 2014, 12:19:03 am
content: Awesome, alank2. You can probably dry your current paste out a bit, and carry on using it. Still, it would be nice to have a fresh tube around. (You know, just to eat up its shelf life)mjkuwp, your efforts were probably worthwhile. If nothing else the insulation will effectively help to even the heat distribution in the oven body.It is nice to be able to jump to your peak (air) temp right when you hit liquidus, but heating is generally less of a concern in hobby ovens, compared to cooling. Cooling rate, meaning everything after the "peak dwell" period down to liquidus point, is going to determine the internal structure of the solder joint. Cooling too quickly/slowly may lead to cracked joints down the road, especially on devices "gettin through dem thermal cycles". (rapid and/or extensive thermal cycling.) Of course, this is all assuming you've met a profile that doesn't cook anything to begin with.Also, I made a post a bit further up with some links that may warrant a second glance. I've had to go back over them a few times in working on my oven (in slow progress), but there have been a few questions asked here since (board vs air temp, &c.) that are at least partially covered.P.S.: I am a bit jealous of your controller UI, mjkuwp.

author: alank2
author_rank: Super Contributor
timestamp: May 27, 2014, 12:28:30 am
content: My measurements so far show it will heat around 1 deg C per second and it cools with the door open at 0.35 deg C per second. Do I need to consider doing some things to improve these?

author: StrangeloveMD
author_rank: Contributor
timestamp: May 27, 2014, 12:42:50 am
content: I'm not sure if you need to, but my understanding is it will give you a much better joint, especially on the long term. (Ahhhh! Thermal cycling!) Of course, the more control you have over either of these rates, the more capable your oven will be, especially in the future. I haven't looked into the window you have to hit on some of the "high pin count" bump packages, but I can't imagine they enjoy being thrown in a blast furnace, as it were.Cooling rates are usually recommended in low single digit Celsius. 2*C/s to 4*C/s, with "more than 6ish" seeming to get fairly dangerous.But what is cooling down at that rate? Air, PCB, joints? Is that a window you must get through or a range in which the value you must "hit" lies? I can probably clarify if you have some questions on the above links, as I am just barely moving on from them to some slightly more in depth stuff.

author: hamster_nz
author_rank: Super Contributor
timestamp: May 27, 2014, 01:41:33 am
content: High cooling rates cause issues with the grain in the solder, making the joint brittle, and more likely to fail due to mechanical shock.http://www.ami.ac.uk/courses/topics/0244_tsm/has quite a lot of interesting info, and on a side notehttp://www.ami.ac.uk/courses/topics/0132_rs/is a quite interesting read about the reflow process in general..

author: alank2
author_rank: Super Contributor
timestamp: May 27, 2014, 01:45:43 am
content: What about low cooling rates - mine has been averaging 0.35 deg C per second, is that too slow?

author: StrangeloveMD
author_rank: Contributor
timestamp: May 27, 2014, 03:56:41 am
content: Quote from: alank2 on May 27, 2014, 01:45:43 amhamster_nz has provided some great direct links to information on over/under cooling. I specifically recommend the "Self Assessment Questions".Also, the welders have been going on about this sort of thing for ages:http://www.aws.org/wj/supplement/WJ_1972_11_s536.pdfFor anyone looking for a simple answer:Undercooling (providing nothing gets cooked) is probably not as dangerous as cooling too quickly (>6C/s seems an absolute max, but I'm still reading) in terms of likelihood of damaging a board. I would say leave it in the oven a lot longer, though. The "slowly cooled" joints will want to be nice and stationary, asforto grow whatever structure they can, prior to encountering loads.At the end of the day, one simply reflows. If the board works, one has succeeded.

author: alank2
author_rank: Super Contributor
timestamp: May 27, 2014, 10:12:35 pm
content: Hi Everyone,How does this look?I'm trying to get it to match this type of solder / profile A:http://amtechsolder.com/pdf/4300_5Fspec.pdf135s in <140140s in 140-180 range70s in >183What do you guys think? Any recommended changes?

author: hamster_nz
author_rank: Super Contributor
timestamp: May 27, 2014, 10:55:01 pm
content: I think that is looking pretty good - did you reflow any components?The one thing I am not sure about is the climb to the peak temp. IIRC you want to take it to peak temp pretty quickly (at up to 3 degrees per sec IIRC), and then drop off once everything should have reflowed. Looking at your controller output it is aiming for a soft arrival at the target peak temperature.A soft arrival at the initial heat/ramp phase is what you want, but for the peak it keeps your components hotter for longer than they need to be.Maybe when you jump your set point to 210, instead jump it to 250 for 50 seconds, then back to 210. That should steepen up the climb, rather than it backing off after 25 seconds (between 275 and 300).Also, your peak temperature is looking a little low. Shouldn't it be around 220? (as per the PDF).

author: alank2
author_rank: Super Contributor
timestamp: May 27, 2014, 11:03:46 pm
content: Quote from: hamster_nz on May 27, 2014, 10:55:01 pmNot yet with this profile, but I did do a board the other day and was very pleased that is worked!Quote from: hamster_nz on May 27, 2014, 10:55:01 pmI'm using my own controller code. The final stage "reflow" changes the SP to 205 and the stage ends when it hits 205. I thought about the quick jump, but the CO does bump to maximum so I figured I was pushing it hard enough to try to get there and then let it cool.Quote from: hamster_nz on May 27, 2014, 10:55:01 pmI'm being a bit conservative, don't want to burn or pop anything but still get the job done. Do you think I really should get it up to the peak temp?Thanks,Alan

author: mjkuwp
author_rank: Unknown
timestamp: May 28, 2014, 12:32:08 am
content: yes, you can go ahead and let your board get hotter. Caveat being that you make sure to consider that the feedback is not directly on your pcb.Many or most components have been designed to handle lead-free solder so you have a pretty big window.I think your profile is looking really good. : )

author: alank2
author_rank: Super Contributor
timestamp: May 28, 2014, 12:20:02 pm
content: Last night went pretty well. My first stenciling was great, a friend of mine gave me some fresher paste. Wipe the paste across, scrape the excess, next pcb. Surprising how fast that is to paste a board. The tough part for me was placing parts without smearing the paste, hopefully practice will improve that. I baked 10 boards and tested one so far and it works great. All in less than 2 hours.

author: alank2
author_rank: Super Contributor
timestamp: June 04, 2014, 03:35:15 pm
content: Here is the latest profile I have been testing. I finally added graphing to my pid control application:My oven can only heat at a maximum of 1.1 C/s - will this be a problem and do I need do think about tweaking it for more heating power?I'm trying to follow profile A fro this:http://amtechsolder.com/pdf/4300_5Fspec.pdfMy oven spends 160s in 0-140 deg, 105s in 140-180 deg, and 95s in 183-218 deg...My profile:Code://{SET_AUTO, "Optional", SP, MaxChange, Fan },//{DELAY_THRESHOLD, "Optional", 0(<=) 1(>=), PV , Seconds },{{SET_AUTO, "Stage 1 - Preheat", 140, 0, 1 }, //sp=140{DELAY_THRESHOLD, "", 1, 140, }, //wait for pv>=140{SET_AUTO, "Stage 2 - Soak", 170, 0, 1 }, //sp=170{DELAY_THRESHOLD, "", 1, 140, 90 }, //wait for pv>=140 for 90s{SET_AUTO, "Stage 3 - Reflow", 230, 0, 1 }, //sp=210{DELAY_THRESHOLD, "", 1, 210, 10 }, //wait for pv>=210 for 10s{SET_AUTO, "Stage 4 - Cool Down (Open Door)", 0, 0, 0 }, //sp=0{DELAY_THRESHOLD, "", 0, 70 }, //wait for pv<=70{END, "Completed", }, //boards can be touched};

author: mjkuwp
author_rank: Unknown
timestamp: June 04, 2014, 05:22:21 pm
content: In declaring that your oven can do 1.1 C/s do you mean that the board heats up at this rate? Not a thermocouple inside the air of the oven?I will assume you have attached to the board. My oven has been boosted and has insulation inside it and in the walls. It can heat up a board at about 1.5C per second in the first stage (as it approaches the soak). I put foil-backed insulation inside the oven in an attempt to reduce the volume. The foil on the insulation hopefully reflects energy back to the board. I also think it is important that the foil has less mass than the oven walls so it tends to have the affect of reducing the thermal mass of the oven. I also have insulation between the walls of the oven and the decorative outside sheet metal but I am not sure what that really does.All that work... and I only got 1.5C versus 1.1C.My suggestion is that you change your profile to support 1.1C rise and see where that gets you.or.. Maybe go full on power until 80C and then try to have a gradual rise from 100C to 150C by the end of your soak period. At the end of soak or at 150C go full-on power again. {Between 80C and 100C there is thermal inertia of the elements as I am sure you have seen}I think the above is about what my PI controller ends up doing.

author: alank2
author_rank: Super Contributor
timestamp: June 04, 2014, 05:34:55 pm
content: Quote from: mjkuwp on June 04, 2014, 05:22:21 pmYes, it is a thermocouple attached to a board.Quote from: mjkuwp on June 04, 2014, 05:22:21 pmWere you the one who shortened the elements so they would run hotter too? That did look like a lot of work.Quote from: mjkuwp on June 04, 2014, 05:22:21 pmI may play around with it some more, but it somewhat adapts itself to what the oven is capable of. I set it up so that it will stay in a step until the step is satisfied (for example the soak from 140-180 is 60-90s and I set it to stay until 90s above 140 is obtains, so while I can't get the oven hot enough fast enough to do it as quickly as they do, I do make sure the step has been satisfied with a delay that says we need to be above 140 for 90 seconds consistently before it moves on.Thanks,Alan