Custom low air volume reflow oven based on flat-iron 1000W 230VAC 50 Ohm heater - Page 1
Forum section: electronics / projects

author: eneuro
author_rank: Super Contributor
timestamp: May 11, 2015, 10:25:30 pm
content: Hello,I've foundmade not in Chinaflat-iron heater 1000W (leads to checked with multimeter 50 Ohm resistance while rated @ old grid 220VAC) and by adding aluminium plate and filling this ceramic heater with CNC aluminium to form nice aluminium plate with shown ceramic flat-iron hetaer inside aluminium block, while active PCB heating area ~7.8*10^-3  and air volume in version shown below ~600*10^-6  , so very small and even lower volume in final design while reused glass light bulb cover could be changed with glass plate, to allow better monitoring using visual camera, upgrading in the future to combined with thermal imaging camera (Seek Thermal probably with circular motion implemented and enahenced lenses), maybe it could be quite usefull reflow oven for small PCBs which are in my interest now 5cm x 5cm max sizeI can put two 5cmx5cm PCBs easy and integration with custom pick & place machine is in my interest which will be another interesting project to try automate production of short series of PCBs on customer demand without overhead and messing with sending and waiting for assembly in external remote places like we knowAnyway, for the moment wondering if used 1000W flat-iron heater will allow to reach inside this glass cover temperatures needed for solderingAlso, considering apply some physics based on known low air mas, heating area and cooling area to monitored ambient temperature as well as small PCB with temp sensor sitting on aluminium plate or with thin ceramic insulator to allow hot air do the job.What kind of control could be the best, assuming I'll have three temperature sensors and later visual camera?Lets forgot now about thermal imagining, while special plate probably needed to allow IR go througth cover.How do you think, visual imagining software capable to detect solder changes could be helpfull there or simply overkill?I have idea to drill a few holes perpendicular to wooden prototype case walls insulating glass cover, which could allow mix inside external air at ambient temperature to help keep given thermal profile as close as possible to needed parameters, while turning off heater with quite huge additional aluminium thermal capacity could be not enougth to be able cool air inside without opening glass cover before reflow cycle is over

author: eneuro
author_rank: Super Contributor
timestamp: May 12, 2015, 08:41:39 pm
content: After quick research at local market and comparing power of available electric ovens I'm prety confident that 1kW flat-iron should be fine, while found even electric oven rated at 800W, another bigger with chicken inside like thishttp://www.diytrade.com/china/pd/1994626/Electric_oven.html1.3kW, but well I have so small amount of air to heat and no plans to cook meal, but small PCBs in a automated wayIt is time to add AC 230VAC mains current monitoring using my Hall sensor module integrated with MPU, so with I2C network for other needed sensors and additional GPIOs it should do the job and be easy to add optoisolated power stage module (moc3041+triac) on another 5cm x 5cm PCB and another PCB for needed sensors and optoisolated datalink to PC to upload thermal profile and monitor "cooking" PCBs on laptop screen if neededBottom aluminium plate will be thermaly insulated using this 1.5cm-2cm mineral wool with aluminium glued which I've succesfully tested during winter fith fireplace. This should easy avoid transfer heat from aluminum plate and let it go only througth not insulated top plate with glass window3D aluminium plate model is under design to be ready send to CNC factory to make needed cuts in a few mm aluminium plate to hide flat-iron heater inside and make safe connections to its 50 Ohm 230VAC internal wiresTime to do some math.Hyperphysics: Heat Conductionbased onHyperphysics: Thermal Conductivitytables.Code:Silver: 406 [W/mK]Copper: 385 [W/mK]Aluminium: 205 [W/mK]Brass: 109 [W/mK]Steel: 50.2 [W/mK]Wood: 0.12 [W/mK]Glass: 0.8 [W/mK]Concrete: 0.8 [W/mK]Fiberglass: 0.04 [W/mK]Air: 0.024 [W/mK]Air properties and this what we might need to beter predict heat capacity:http://www.engineeringtoolbox.com/air-properties-d_156.htmlThat is interesting at ~300*C air density is ~2 times lower than at room temperature

author: eneuro
author_rank: Super Contributor
timestamp: May 13, 2015, 04:12:05 pm
content: Managed flat-iron to be installed safely between two aluminium plates, while will have to wait some time for machine cut CNC parts.Anyway, it is interesting if with this simple setup I will be able heat aluminium plates with flat-iron inside to achieve those temperatures to be compilant with JEDEC JSTD-020 temperature profile.They recomendnitrogen atmospherefor lead-free reflow soldering-it comes from OSRAM SMD leds datasheet.Do you replace air with nitrogen in your reflow ovens in small production PCB soldering?Makes a lot of sense, so thinking how to add silicone pipes to be able replace air under glass cover at the begining of soldering-probably 2 pipes for regular air and additional 2 for optional nitrogen, but need to first find optimum thermal capacity to be able implement required 3*C/s (maximum 6*C/s) ramp down-maybe external fans to cool down glass cover, while I'd like to test nitrogen atmosphere soldering too, so maybe something else-closed circulation of nitrogen/air with internal cooling system without need to interfere with room air.Additionally I've played for a while with heat conduction calculator and when entered expected area of heat loss in my custom oven througth a few mm glass cover, than I've got for 250*C inside oven and worst case 15*C room ambient temperature ~300W losses, so assuming quite good insulation of rest of the oven from room temperature, this needed power to keep 250*C temperature is not too much and at 175*C inside I;ve got ~200W power needed, so probably will limit PWM to 33% for first runs to collect some dataWe'll see what happends in real experiment in attempt to recreate this temperature profile, which will be default and scalable using potentiometer to easy change this profile maximum temperature 245*C in the case if some manual adjustments were needed.Another set of theory review with heat capacity tablesHeat Storage in Materialswith heat or energy storage calculation and I think, it is time write some MPU code to heat something using sophisticated software with this flat-iron based oven dimensions and used materials to better predict temperature change inside oven

author: eneuro
author_rank: Super Contributor
timestamp: August 29, 2015, 07:57:17 pm
content: There is some progress and now I can have easy 4kW input heater power and bigger heating plateI had in my hands steel St3 plates (looking for aluminium in final design, but CNC G-Code is written to drill those holes in 0.200m x 0.200m plates, so it will be easy manufacture the same but in aluminium) used in another project, so desided make a try with those steel plates:Its size will allow me put at the bottom of my oven easy up to four such iron heaters or maybe try inductive heating, since there is a lot of space for heating element and everything will be covered on back with shown a few posts earlier thermal resistant up to 800*C degrees insulation, so the only way heat can go easy outside my oven will throuhth glass cover on the top (not shown there it will be flat)There will be no crappy plastics, but steel, ceramic base and nice to touch wood... to be lucky to reflow PCB in a way not to damage elementsThis thing could be also usable to... heat up cup of tea, so of course I need add separate software code to keep something put inside at given temperature, regardless air room temperature isBTW: If someone knows tricky ways to make this ST3 steel as black as possible and maybe protect at the same time against corrosion let me know