Flat heater element design

[el salvadore]

Hi all,

I have question regarding to heater I want to build. It suppose to be a heater used for soldering a thin copper sheet (1mm thick) with other small copper components. Heater has to be flat as much as possible and reach the temp up to ~250C (solder melting point). The copper sheet surface is about 950x800 mm, so heater should have the equal size in contact surface area. Copper sheet needs to be heated up equally while soldering due to thermal expansion stress, so it needs to keep flatness as possible when is cooled back down, therefore this kind of flat design heater is needed. Also a heat radiation from the heater needs to be as much homogeneous as possible.

So, my question basically is, what would be the best way to build such a heater?

I was thinking about using some resist wire spiral poured into some kind of ceramic but before i start search for any suitable solution I'd like to know your opinion,

Thank you very much.

 

[Fish4Fun]

I would suggest using hot air. A cheap solution would be a standard heat gun (~$50 US) with a long thin "forcing cone" that created a 10cm * Xcm square opening, where Xcm is slightly larger than your solder joint width.

Fish

 

[el salvadore]

Fish4Fun: Thank you for your answer,unfortunately i don't think this one is going to solve my problem. Because hot air gun can provide heat only locally, what may cause a thermal expansion stress and lead to rippled surface of the copper sheet after is cooled. I'm not looking for the cheapest solution either so any other suggestions are very welcome.

Thank you.

 

[Fish4Fun]

el salvadore,

Sounds like you need a reflow oven:

http://en.wikipedia.org/wiki/Reflow_oven

https://www.manncorp.com/reflow-ovens/

Sorry, I assumed you were attempting to do this on a budget; if cost isn't a concern, there are pre-engineered solutions available. You would likely need to have a 800mm wide unit special built.

I mis-read your original post, I thought you were attempting to solder two sheets together, not components to a sheet. If a full blown commercial re-flow oven is more than you need, you might consider building a steel or aluminum support frame to support your copper sheet and heat it from below. This would likely be easier than casting a ceramic heater, and you could use gas, resistive heating elements or hot air as a heat source.

Regardless if you build a "heat gun" or simply buy "off the shelf" units, hot air is one of the most common methods employed in re-flow soldering applications, primarily because of the speed at which you can bring the copper to temperature.

The specific heat in your copper will require a minimum of ~600kJ (760cm^3 * 3.45 JCM^3/K * 225K) or ~.16kWh. If you want to bring the copper to temperature in 1min, this will require a minimum of 10kW input, and likely two to four times that depending on losses. Ideally you would like to reach soldering temperature in a 10-20 seconds, but this is a very large amount of copper.

If this is a production setting, and you have a lot of these sheets to solder, designing a hot air system that re-circulates the air and moves the sheets through the machine would save a lot in energy costs over time. If it is a limited run, I would seriously consider using propane or natural gas for your heat source. 9 * 100,000BTU burners placed below a steel/aluminum support rack should give you quick even heating.

I hope these thoughts give you some ideas.

Fish

 

[el salvadore]

Fish,

thank you for your reply. First of all, I will try to clarify more the use of heater I'd like to build. As I have already mentioned, it will be used for soldering copper sheet together with other components (also Cu, such as pipes, mounting brackets etc.) all together creating a mass about 17kg. The flat cooler will be result of soldering these components together.

As u correctly assumed, it will be only a limited run (~ 10), so reflow oven sounds like good idea but in my case it is bit more than I need actually. Thus the time is not really an issue, heat up material could easily take up to 10-15 minutes. Quality is more important though (accurate placed components and flatness of the sheet after it's being cooled down).

The support steel frame heated with gas is very good, simple and efficient idea and it was first thing came upon my mind when i started with design. But unfortunately, this one has one biggest disadvantage for conditions it's going to be used within -> control & safety.
I am an electrical engineer and building a gas device is not really my cup of coffee. Apart from this, I need to set a proper temperature point to heat up material, melt the solder and let it cooled down gradually.

So the best way how can I manage this, is to build a heater consist of resistant wires encapsulated in some ceramic mass. So far I decided for design of heater such as split it into 3 separate parts, each of them contains one heating coil encapsulated in separated ceramic mass and each of them connect to different phase (not sure if I'm going to use delta voltage (between phases) or just single phase voltage for each part of heater, here in EU is delta around 400V and single phase 230V). I can also easily build in some temp sensor in each of them and provide better troubleshooting, maintenance and control also in case if one of them gets broken. Connect all of them to logic control device allows me to have full control "in my hands".

So far I can not move further within calculations because I've got stuck on develop of processing such a heater. The ceramic mass is unknown for me yet, i can not find any sufficient material thus I don't know the requirement parameters.

Thank you.

 

[Fish4Fun]

ElSalvador,

Fish,
all together creating a mass about 17kg.

Resistive heating, while plausible, is silly with this mass/volume.

As u correctly assumed, it will be only a limited run (~ 10), ...heat up material could easily take up to 10-15 minutes. Quality is more important though...

USE GAS

The support steel frame heated with gas is very good, simple and efficient idea and it was first thing came upon my mind when i started with design.

Resolve those issues, do not make this more complicated than it needs to be.

So the best way how can I manage this, is to build a heater consist of resistant wires encapsulated in some ceramic mass. So far I decided for design of heater such as split it into 3 separate parts, each of them contains one heating coil encapsulated in separated ceramic mass and each of them connect to different phase (not sure if I'm going to use delta voltage (between phases) or just single phase voltage for each part of heater, here in EU is delta around 400V and single phase 230V). I can also easily build in some temp sensor in each of them and provide better troubleshooting, maintenance and control also in case if one of them gets broken. Connect all of them to logic control device allows me to have full control "in my hands".

Quite honestly the control system is NOT dependent on the method of heat input chosen.

So far I can not move further within calculations because I've got stuck on develop of processing such a heater. The ceramic mass is unknown for me yet, i can not find any sufficient material thus I don't know the requirement parameters.

You are making this FAR MORE DIFFICULT than it needs to be. Heating copper homogeneously is almost a given, you would have a hard time messing that up in any reasonable time period given a 17kg mass.

I am an electrical engineer and building a gas device is not really my cup of coffee.

As luck would have it I am NOT an engineer @ all, but I have taken all of the core EE classes, and have 30+ years experience in welding/soldering, and I am here to tell you that you are making this a lot harder than it really is. You could make this as easy as heating a steel plate with oxy/act, or as complicated as a temp controlled ceramic heater, but @ the end of the day, the copper is going to heat fairly homogeneously w/o regard to method if time > a minute or two. I know you want to define everything, fine, but in the real world it is likely your "shop guys" could do this with a torch w/o any specs. Trust your journeymen, they will make this work.

Honestly, if you were doing even 1k+ units it would pay to design/order a heater, but @ ~10 units, let the journeymen mess up 10 units and deliver you 10 units, it is the best benefit/cost process.

If you are bound and determined to build a ceramic heater, and you want advice on resistive heating using a 400V 3phase source, of course you use three, single phase circuits with either SCR or TRIAC control elements, you could build in a feed-back loop to sense the temp of the mass, or the copper, but really this is secondary information. The heat mass/specific heat of the ceramic is unimportant, the only parameter you are concerned with is the temperature of the copper and components. I would suggest a shop tech & a laser temp sensor (or they could just look to see if the solder were flowing) to achieve this.

I do not mean to be ugly, but EXACTLY this type of engineering is what cripples low yield design projects. Engineers frequently want to use everything they learned in school to boil water, when boiling water was solved quite some time ago, by idiots. You obviously take your job seriously, and trust me when I say that is a GOOD THING, but you are making this project far more complicated than it needs to be. Heating copper homogeneously is pretty simple, apply heat over time. Re-Flow solder is pretty simple: achieve flow temp. Anything beyond these two concepts simply complicates things. If you were building 10k+ units and the chore was to minimize energy costs, sure, that is a good reason to refine the process, but @ ~10 units? Apply heat and go forward. Screw up 75% of yield and you are GOLDEN. Building a ceramic heater using company resources to prevent a loss of <$100 worth of copper is silly. Remember, at the end of the day, the same journeymen will use a torch, or watch your ceramic get hot; trust their skills or don't, but their labor is a fixed expense, we are only haggling over $1k in materials.

I hope I have not offended you, but I own my own business, and I hate waste in ALL of it's manifestations. While I am NOT an engineer, I make a lot of things work every day, despite engineering specifications. This in no way implies or presumes that I do not appreciate engineering, or that I hold it in contempt, i just hate to see it misapplied.

I hope you take this in the good spirit it is meant in,

Fish