Toaster Oven reflow - Page 1
Forum section: electronics / projects

author: DaveW
author_rank: Frequent Contributor
timestamp: January 14, 2011, 12:00:34 pm
content: I've been looking at soldering at BGA and some higher density QFN packages at home recently and I started looking at using a toaster oven for solder reflowing. I couldn't find the oven used in the sparkfun writeup in the UKhttp://www.sparkfun.com/tutorials/60so I found this insteadhttp://www.amazon.co.uk/Andrew-James-White-Litre-Grill/dp/B003SE88Z4/ref=sr_1_1?ie=UTF8&qid=1295006018&sr=1-1-spellThe internal space is nice and roomy for any PCB I do at homeI took the side off to get to the controls. Very simple and the temperature control could be off as it's outside the oven but all the easier to modify!I simply changed the thermostat out for an SSR-didn't even need to cut any wires!A 3mm hole to get a thermocouple though and the modding is almost doneIt does heat up quickly enough, 15-100 deg. C in 110 seconds, 15-200 deg. C in 246 seconds. I'll put a higher temperature thermocouple in there and get it under PID control with a PIC. Looks like a quick simple way to get into reflow soldering, judging by what other people have done should be easily capable to doing BGA packages which I've had to leave before,

author: apex
author_rank: Regular Contributor
timestamp: January 14, 2011, 03:13:39 pm
content: Hey, cool!apex

author: tyblu
author_rank: Frequent Contributor
timestamp: January 14, 2011, 03:35:38 pm
content: Very cool -- will enjoy doing this myself in the future. Is it pretty even heat, or is there a convection fan inside? (Does it matter?)

author: Time
author_rank: Frequent Contributor
timestamp: January 14, 2011, 04:00:51 pm
content: hell, I have just done it on an unmodded toaster oven, thermo couple/meter lots with no problems

author: Chasm
author_rank: Regular Contributor
timestamp: January 14, 2011, 04:14:07 pm
content: The problems with such a conversion are usually the temperature probe and the distribution of the heat within the heater - and of course that you are playing with the mains.The temperature probe problem can be solved by using something designed for this task, usually a thermocouple. But those are not necessarily cheap, and have some pitfalls. (You have to keep the other end of it at a constant temperature or know the current temperature to compensate for it.) OTOH cheap part bin solutions like a diode are not too stable and introduce nonlinearity problems.Distribution of heat should be even. you don't want to burn one part of a board while another is not even melting. It can be improved with a fan -which of course has it's own pitfalls- or by using a oven that has top and bottom heating elements and thus heats up more evenly.And then there is this mains thing... Let's say it this way:Mains can kill you. Mains can burn down the house. Mains can kill your significant other.And usually most important: Insurance companies have the habit of not paying at all when tampered with gadgets are involved.So better know what you are doing before tampering with it.

author: DaveW
author_rank: Frequent Contributor
timestamp: January 14, 2011, 04:22:38 pm
content: Quote from: tyblu on January 14, 2011, 03:35:38 pmIt does have a fan inside, you can see the back of it herehttp://www.flickr.com/photos/21076036@N07/5354418734/#I've tried the thermocouple in a few places and there seems to be some variation of temperature with height, but not horizontally so it should be able to keep a consistent temperature across a large board.Chasm-this has top and bottom heating elements, four in total. Agreed on caution with mains-although this is one of the easiest mains modification I've ever done, the SSR just needs a 5V drive signal and provides full opto-isolation.Good point with the thermocouple, the solution I'll be using is a thermocouple, with a to-92 temperature sensor to give me the cold junction temperature at the PCB

author: JohnS_AZ
author_rank: Frequent Contributor
timestamp: January 14, 2011, 04:49:03 pm
content: Very cool (hot?)It's not my intention to slam your project, but offer the following as FYI ...A lot of parameters that can be safely ignored when reflowing chip caps, resistors, and SOICs should be taken into account when trying to reflow BGAs and QFN/QFPs. Most importantly is the temperature profile. In a commercial reflow oven the temperature follows an exact and specific profile. In general terms, something like ....PRE-HEAT ZONE - The board is SLOWLY brought up to an idle temperature, just below the activation temperature of the flux. Most component guys recommend no more than 2 degrees C per second. The board will be brought up to about 170C in this zone.SOAK ZONE (flux activation) - The board is then heated to the point at which the dehydrated flux chemically activates and does it's thing, and time is allowed for the entire board and it's components to reach the same temperature. The board will go from about 170C to 220C in this zone.REFLOW ZONE - The board temp is then peaked into the reflow range of the specific solder paste being used. It was typically held there for another 30 seconds to a minute. The temperature depends entirely on your solder paste chemistry.OUT RAMP (cooling) - Finally the board is SLOWLY cooled down to a temperature of about 90C over the course of another 3-4 minutes.The importance of the gradual preheat is to avoid thermal stresses and fractures in the PCB and within the components.The importance of the soak is to avoid solder balls and voids.The importance of the controlled cool down is that you want it slow enough to avoid thermal stress, and yet fast enough that the solder can form the optimal granular structure to ensure a solid mechanical connection.Now, this CAN be accomplished with converted toaster ovens, but I've never seen it done. The oven conversion projects I've seen published that pretty much drive the temperature directly to reflow, and then kill the power, make me grimace.There are lots of references on the web, but here's a pretty decent one...http://www.rayprasad.com/home/rp1/page_70/smt_-_lead-free_reflow_profile_development_part_1.htmlGood luck, and keep us informed on how it all works out!

author: Chasm
author_rank: Regular Contributor
timestamp: January 14, 2011, 05:28:39 pm
content: Quote from: DaveW on January 14, 2011, 04:22:38 pmNice, with 4 elements and the fain there should be a pretty even heat distribution.Measure also with a (large) board inside to see if it changes the heat distribution.Fan... Maybe you have to mod the fan to lower the airflow. But you'll know that if it blows small parts from the boards during reflow.Quote from: DaveW on January 14, 2011, 04:22:38 pmYes, it is easy to do this in a safe way. After all this is a recurring problem. Still it is something to keep in mind.I had some of the toaster oven projects form Germany in mind. Several of them use thyristors to archive a more flexible heating profile conforming to the requirements JohnS_AZ laid out. (Using gulse packet modulation or whatever the exact English term is, letting a couple full sinus waves trough, and then pausing for a while.).Getting this right is a bit more complex than a relay.

author: DaveW
author_rank: Frequent Contributor
timestamp: January 14, 2011, 05:51:45 pm
content: John-this is a good point. I try and stay away from the PCB side at work (don't want to ever get dragged away from the design and get stuck doing PCB layout), but I'll have a chat with the guy that does that part. I can easily put ramps in and from the tests I've done this oven should be more than capable of keeping up with those rates. With PID running I will be able to control the cooling as well. I have to confess I have no experience with BGAs at all, but hopefully the ramping rates and profiles should be similar to qfn chips.Chasm-I did the measuring with a board (approx 300x120mm) inside on a wire rack and the temperature was even across it, although slightly different top and bottom. If this is a problem I should be able to fix that by adjusting the height inside the oven.If the fan proves too powerful (looks unlikely) then I shall probably add some baffles to diffuse the airflow rather than mod the fan itself as it seems to help keep the temperature very even.The solid state relay I'm using is quite capable of burst control (I think this is what you meant)-this isn't a mechanical relay, it uses mosfets and has a switching time in the microsecond range. I normally use triacs for this kind of thing, but I happened to have the SSR in the drawer and the faston terminals meant I didn't even have to cut the wires in the oven. I have used a similar system for the controller for an environmental chamber (415V, 15kW) before with excellent results.

author: Chasm
author_rank: Regular Contributor
timestamp: January 14, 2011, 06:26:33 pm
content: Burst control was what I was looking for, thank you.The problem with many of the projects is that they try to get as cheap as possible.Solid state relays like the one you used cost money, so do thermocouples and the AD595. Even space on PCB's costs money too. That can lead to unsafe designs.OTOH there is eBay with both it's surplus and cheap Chinese reflow systems. So you actually have to take a look at the market and keep the cost at least somewhat under control.Or you can make a business of it, take a look at auction: "reflow Oven TWS 800 SMT compact reflow and convection" no link because it stinks so much of a firetrap....

author: DaveW
author_rank: Frequent Contributor
timestamp: January 14, 2011, 10:52:43 pm
content: You aren't kidding about the AD595, nice chip though. I'll probably do a more discrete circuit as I've got some instrumentation amplifiers around, but useful chip to know. Looked up that reflow oven-looks terrifyingly similar to this homemade one! Thanks for everyone's input-some great links about the soldering profiles in particularHope it's been of interest,Dave

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 14, 2011, 11:05:14 pm
content: Quote from: DaveW on January 14, 2011, 10:52:43 pmFor a high-temp app like this, you can get away without cold-junction compensation - assuming your amplifier is at ambient temp, this isn't going to vary by anywhere near the other errors like evenness of heating. A simple instrumentation amp to boost the voltage, software lookup for linearisation and offset by a nominal CJ temp would be fine.BTW check that your SSR doesn't need heatsinking for the load you're running - most require a heatsink to run their full rated load.

author: Jon Chandler
author_rank: Frequent Contributor
timestamp: January 15, 2011, 12:11:12 am
content: I've been developing a thermocouple circuit using the TI TMP513. It's designed for monitoring power supplies but it's perfect for this:I2C interfaceBuilt-in temperature measurement (for reference junction measurement) and 3 optional PN-junction measurements12-bit ADC with a range of 40 mV for measuring shunt voltage (which is the perfect range for thermocouples)It's less than US$4 and availableI read the thermocouple voltage, do a table lookup to the nearest degree and interpolate to get an accurate reading. The reference temperature is then added to the calculated thermocouple temperature to get the true temperature. It's working well and calibrates nicely.

author: Chasm
author_rank: Regular Contributor
timestamp: January 15, 2011, 12:57:38 am
content: Nice find!

author: Jon Chandler
author_rank: Frequent Contributor
timestamp: January 15, 2011, 04:48:49 am
content: I should also note about the TI TMP513....the shunt voltage ADC can handle positive or negative voltages so it's no problem if the reference junction is warmer than the thermocouple.I've limited the measurement range in my circuit to -269 degrees C to + 760 degrees or so. This keeps the table values in the range integer variables can handle. That's plenty HOT for me!

author: DaveW
author_rank: Frequent Contributor
timestamp: January 15, 2011, 01:33:27 pm
content: It would be good to put the electronics in the side of the oven, but that gets up to 65 degrees or so, so I will still add some cold side compensation-as you say though you could make this a lot simpler by keeping the electronics outside of the enclosure. This SSR is vastly overrated (about 4x) do I can get away without a heatsink-will probably bolt it to the side though.That's a nice chip, like you say should be perfect for this; I think 760 degrees should be sufficient!

author: Mechatrommer
author_rank: Super Contributor
timestamp: January 16, 2011, 01:47:50 am
content: how about usb controllable oven from PC? with display of current temperature reading/ramp graph? think i want to build this, when i settle down.

author: CafeLogic
author_rank: Regular Contributor
timestamp: January 17, 2011, 04:15:45 am
content: I just built a controller for griddle/toaster reflowing with graphing. It uses the TI ADS1147 which I highly recommend. I guess since there is a thread about it, I will finally get off my ass and shoot a video.

author: Jon Chandler
author_rank: Frequent Contributor
timestamp: January 17, 2011, 11:40:29 pm
content: Here's a PDF describing the TI TMP513 thermocouple circuit and the registers needed to read the temperature.I'll be posting an article soon on Digital-DIY.com with Swordfish Basic code for PIC18F-series parts.

author: CafeLogic
author_rank: Regular Contributor
timestamp: January 22, 2011, 02:45:47 am
content: OK, finally prepared something. I will post schematic details and such later. I don't know how Dave finds the time to do this every week.Videos:

author: Chasm
author_rank: Regular Contributor
timestamp: January 22, 2011, 03:10:36 am
content: Nice!One of the best conversions I've seen so far.

author: Mechatrommer
author_rank: Super Contributor
timestamp: January 22, 2011, 05:53:16 am
content: Quote from: CafeLogic on January 22, 2011, 02:45:47 amlol. do you use OS for the display? i can see the lcd interface is not the simple hibby jibby graph. is that menu-start-stop "soft buttons" functional?cant argue with chasm more. this is the best i've seen so far, esp the UI.

author: Zero999
author_rank: Super Contributor
timestamp: January 22, 2011, 11:35:41 am
content: What about safety?Have you connected the metal case to earth? I couldn't see any bonding on the photos and there's no way the case is double insulated from the mains; it's used as a heat sink and the thermal pad only provides basic insulation at best.

author: CafeLogic
author_rank: Regular Contributor
timestamp: January 22, 2011, 01:07:44 pm
content: Quote from: Hero999 on January 22, 2011, 11:35:41 amOf course it is connected to earth, along with toaster shell. There is no thermal pad, The TRIAC is the insulated tab type. I'm not all that concerned about the shock hazard. I am concerned about the fire hazard, I wouldn't leave it unattended because there is no back-up if the TRIAC fails short. I thought about using a relay.

author: CafeLogic
author_rank: Regular Contributor
timestamp: January 22, 2011, 01:11:03 pm
content: Quote from: shafri on January 22, 2011, 05:53:16 amThanks. NO OS. MPU is only 64K flash. I used the Stellaris graphics library for the buttons and some other stuff but the graph is a custom component. All buttons are functional. I will put up the source.

author: Zero999
author_rank: Super Contributor
timestamp: January 22, 2011, 03:06:52 pm
content: Quote from: CafeLogic on January 22, 2011, 01:07:44 pmGood point. You could use a thermal fuse or a bimetal switch which is resettable or better still use both for piece of mind.

author: Chasm
author_rank: Regular Contributor
timestamp: January 22, 2011, 03:29:25 pm
content: Quote from: Hero999 on January 22, 2011, 03:06:52 pmSince this is a failsafe I'd opt for a thermal fuse.It should never trigger in normal operation, if it does trigger I really don't want the device to restart after a cool down period.

author: Zero999
author_rank: Super Contributor
timestamp: January 22, 2011, 04:29:36 pm
content: Come to think of it, I agree, although having a bimetal trip at a lower temperature as well as the fuse, would save money when you're experimenting.It's also good to have two layers of protection, the bimetal strip, just in case there's a mistake in the code and a fuse in case the switch contacts weld closed - such a system should easily pass both industrial and domestic safety standards.

author: CafeLogic
author_rank: Regular Contributor
timestamp: January 22, 2011, 04:29:49 pm
content: I thought about a thermal cutoff and even noted that in the source code. The problem is that it requires all sorts of experimentation. You have to find the temperature of the adjacent cavity when the toaster reaches a dangerous point and then get the thermal cutoff for that temp. Since I have zero intention of making a commercial product, I'm not going to bother. I overspeced the TRIAC but really the solution for my use is common sense. Make sure the area is clear of flammable objects, and don't go to lunch while it is reflowing.I was actually surprised at the lack of protection in the toaster. Microwaves always have 1 or 2 thermal cutoffs plus current protection but this had nothing. Just the mains running straight to the element through a bi-metal switch. So anything I do is an improvement over the safety I experience while making toast.

author: CafeLogic
author_rank: Regular Contributor
timestamp: January 22, 2011, 04:39:11 pm
content: Quote from: Hero999 on January 22, 2011, 04:29:36 pmOn one occasion while I was doing the software it locked up while the element was on (stack overflow). Of course I have since enabled the watchdog but that is a very legitimate concern, especially when it is complicated. My opinion is that a single engineer simply does not have the resources to make a commercial product like this, you need multiple people to review it and you need adequate resources for product testing and refinement.

author: CafeLogic
author_rank: Regular Contributor
timestamp: January 23, 2011, 05:32:36 am
content: Full details have been postedhttp://www.cafelogic.com/articles-2/a-reflow-controller-for-soldering-with-a-griddle-or-toaster/

author: Zero999
author_rank: Super Contributor
timestamp: January 23, 2011, 11:47:41 am
content: Quote from: CafeLogic on January 22, 2011, 04:29:49 pmThere are different categories of safety protection according to the risk and consequences of a failure.I think here it comes down to how long the appliance is likely to be left on unattended. It's unlikely you'll leave the toaster switched on while you're out of the room for a long length of time and there's normally a timer with a maximum delay of 10 minutes or so. In other words you'll probably smell burning plastic before the toaster catches fire and if it does you'll be there to turn off the power and throw a fire blanket over it. On the other hand a microwave could be left on for more than an hour unattended, which is long enough to burn the house down.Quote from: CafeLogic on January 22, 2011, 04:39:11 pmYes software can never be relied on its own for safety critical applications. There need to be other layers of protection, unless the design consists of three MCUs running different firmware, monitoring each other to ensure it fails safe. The whole design needs to be peer reviewed by several safety engineers.You also need to be careful with devices such as switches, relays and semiconductors which often fail dangerously i.e. closed circuit. Often the best option is to buy a specially approved safety relay.I like your write-up by the way and even though it's obviously potentially dangerous the disclaimer should stop you being sued lol.

author: Mechatrommer
author_rank: Super Contributor
timestamp: January 23, 2011, 02:59:34 pm
content: Quote from: Hero999 on January 23, 2011, 11:47:41 amsorry i'm not familiar with the design, but the way i see it, they need to be separated power supply circuit too. even if many mcus, but if one supply is shared, if that supply fail, all the mcu will fail too.

author: Zero999
author_rank: Super Contributor
timestamp: January 23, 2011, 03:51:33 pm
content: Quote from: shafri on January 23, 2011, 02:59:34 pmI don't see how that's practical, normally there's only one power source available and the device should be designed to fail safe even if the PSU fails.

author: CafeLogic
author_rank: Regular Contributor
timestamp: January 23, 2011, 05:55:13 pm
content: Quote from: Hero999 on January 23, 2011, 11:47:41 amUnfortunately, I know exactly what the reason is. Some time ago, my then girlfriend decided to microwave her hot-pocket on top of a plate covered in strips of conductive foil (the bottom was labeled "do not place in microwave"). I come downstairs after she is yelling for me and there is pile of flaming smoldering ash inside the microwave. I could just imagine the massive arcs that were flowing in between the strips of foil. The microwave was dead, I later found the thermal cutoff was blown which probably saved my ass. So I am pretty sure all the circuit protection in a microwave is because of the potential for the high power electromagnetic waves to create electrical arcs and thus cause fires.Add to that the electrocution risk. Here in the US, we have 120V for our normal appliances and 240V for big appliances. I am not going to say that 120V is not dangerous, because it is, but it takes an unusual set of circumstances to seriously hurt yourself with it, mostly involving water. The microwave on the other hand, creates many thousands of volts, enough to seriously injure someone in an instant on a dry day. The microwave is one of the most dangerous appliances in the home.

author: Zero999
author_rank: Super Contributor
timestamp: January 23, 2011, 06:03:31 pm
content: You're right the microwave is the most dangerous appliance in the house, what's more using an isolation transformer to power it doesn't help because the high voltage neutral is bonded to the metal case.Did you try replacing the thermal fuse? It may have been all right.

author: CafeLogic
author_rank: Regular Contributor
timestamp: January 23, 2011, 06:22:19 pm
content: Some stats to back me up. I found these a while back and thought they were interesting. This is from the US Consumer Product Safety Commission, covering electrocutions in the US in the year 2000. The most common is work place stuff involving arc flashes and construction workers from HV power lines, but this only covers the smaller percentage of consumer-product related deaths. Here is a partial breakdown.Total deaths 150:CauseNumberMy thoughtMetal ladders hitting overhead powerlines22Ouch.Wiring25Willing to bet this mostly involves missing or defective GFCI outlets in the bathroomAir conditioners8This is 240V here (high humidity/rain could make trouble)Pumps7Pool pumps, sump pumps and the likeWater heaters3240VRefrigerators2I have no idea how this happensPower tools15These are often 240VLighting12They specifically mention underwater lighting but could also be HV fluorescent ballastMicrowaves7These make up 50% of small appliance incidents, there are no toaster incedents listedI attached the report in case you want to see the whole thing.

author: CafeLogic
author_rank: Regular Contributor
timestamp: January 23, 2011, 06:23:50 pm
content: Quote from: Hero999 on January 23, 2011, 06:03:31 pmYes, I found the fuse on eBay, worked like new.

author: CafeLogic
author_rank: Regular Contributor
timestamp: January 23, 2011, 06:53:56 pm
content: I just checked and the CPSC came out with a new report in June. Dramatic reduction, only 46 deaths in 2006. Prob more widespread use of GFCI in homes and products.http://www.cpsc.gov/library/foia/foia10/os/2006Electrocutions.pdfThis caught my attention:However, apartial explanation for the increase of recreational equipment-related electrocutions in 2005 is asingle incident where four individuals were electrocuted while moving a tent, which happened tocome in contact with a power line. This one incident accounted for more than half (4 of 7) of therecreational equipment-related electrocutions for the year.That sucks

author: Zero999
author_rank: Super Contributor
timestamp: January 23, 2011, 07:12:35 pm
content: 240V in the US isn't really much more dangerous than 120V as it consists of two 120V live wires, 180oout of phase with each other so the voltage relative to earth is still 120V. The person would have to be pretty stupid to make contact with both live wires simultaneously.We also have 110V appliances used on construction sites in the UK but it's safer than the US because it's two 55V phases 180oapart (single/split phase) or two 63.5V phases 120oapart (three phase).In the UK most underwater lighting has to be 12V to prevent electrocution.

author: CafeLogic
author_rank: Regular Contributor
timestamp: January 23, 2011, 07:17:35 pm
content: Quote from: Hero999 on January 23, 2011, 07:12:35 pmlol, I never even considered that but you are right.

author: RayJones
author_rank: Frequent Contributor
timestamp: January 24, 2011, 04:21:30 am
content: But your eyes roll an extra 10 times per second despite our 240V over ground in Australia

author: Chasm
author_rank: Regular Contributor
timestamp: January 24, 2011, 01:40:09 pm
content: On a second and closer look at the pictures the ARM board seems to be awfully close to the HV parts.