Solder Reflow Oven in a Coffee Can

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As we do some prototyping for customers, we obviously need the proper tools to do that.  One customer's needs involved building a very tiny surface mount board for a prosthetic.  These are very laborious to hand solder under the microscope.  It's about like a 700ft tall giant trying to pound in a toothpick using a pickup truck for a hammer (and, as the pickup truck is too small for his fingers, he has to hold it in giant tweezers).  As this customer needed more than the usual number of prototypes, we decided to streamline this process somewhat and "do it right", rather than spending an engineer-day per board to hand build them (which the customer was willing to pay for, but try to find a truly willing engineer when it's just grunt work).

Vapor phase reflow theory:

Vapor phase reflow is a method of applying heat very quickly and uniformly to something using a vapor.  Usually what is being reflowed is solder.  Basically, a fluid is boiled at the bottom of a tank, and its vapors rise into the tank.  The top circumference of the tank is usually actively water-cooled so any vapors that make it that far are condensed and returned to the boiler via gravity.  A fluid is used that has a sharp boiling point, and a high heat of vaporization, which means it gives up a lot of heat per gram when it condenses on something.  Water would for instance be ideal if you wanted to heat something to exactly 100°C (or 97° C here in the mountains).  The industry uses specially engineered fluids for this that have all sorts of desireable properties such as inertness (won't corrode your stuff), dense vapor (won't come flying out of the oven), high heat transfer, and good boiling behavior (no bumping or foaming).

We use FluoroInert FC-70 ®, a 3M ®product  available from Acuity Sales (1-800-554-4905) for our reflow fluid/vapor.  The people at 3M specialty products (1-800-810-8513) will hook you up with a local supplier if you call.  This stuff is VERY expensive, but it isn't consumed in the process (when things are right), so it's not the most expensive part of the production.  For reference, we bought two 4 ounce bottles for a mere $106.67.  It looks like that may be nearly a lifetime supply for this oven.  It is an amazing substance that boils at 215° C, has an enourmous heat of vaporization, and has a vapor about 28 times as dense as air.  As it condenses on whatever you're trying to heat, it transfers heat very rapidly, but cannot heat the target material above the magic 215° C boiling point.  In fact, it works so fast that the internals of the SMD parts never come close to this temperature before the board is removed from the vapor.

Other techniques have been used for reflow soldering, including the basic wave soldering setup, hot oil, IR, and convection.  Vapor reflow is considered the best by most, as the other techniques all have problems.  Regular wave soldering pretty much only does one side at a time, and requires the components be glued down.  IR has the problem of uneven heating due to different IR characteristics of various parts.  The encapsulant used for IC's is IR transparent, but the silicon inside isn't, so you can have the problem of the heat going precisely where you don't want it.  Convection using air or inert gas heats more slowly as the gas doesn't have the heat capacity of a vapor.  This can lead to uneven heating of parts with varying thermal masses.  The attempt to overcome this by force circulation of the gas can blow the parts around.  Hot oil is messy.

This project:

For this size board (a segment of which is the background image here), a little under 2" x 2", a coffee can is a perfect fit and has some nice engineering properties for this application.   The thin steel is a lousy conductor of heat, perfect for a situation where you want solder temperatures at the bottom and vapor all condensed before escaping the can.  Heat will go through the thin steel easily in the thin dimension, but as far as heat transfer goes, the top of the can may as well be insulated from the bottom.  Steel is a fairly poor conductor for heat, as any welder knows.

First, obtain a coffee can.  This is easy here, as engineers tend to like coffee.  We used a Folgers can for mod 1.0 and a Maxwell House can for mod 2.0.  The Maxwell house cans have a lip around the top that we haven't decided to declare good or bad yet.

Strip all the paint off the can, or you're going to have a lot of smoke later.  Paint stripper makes it easier, but it still doesn't just come right off, you will need a wire brush in a drill or a lot of elbow grease and sandpaper time.

Be sure to see: It Almost Worked for this project, we had a ball with this one.  This page will soon be updated to remove most of the attempts that didn't work just so, or just wasted time and materiels.
Ugly but functional model 1.0, heating rope and water cooling. For the first model, we wrapped 5 turns of 3/16" copper tubing around the very top of the can for a cooling water flow.  This was soldered to the can, and flared to match our plastic water tubing size.  It works well, but anything that needs plumbing is a pain, so for the next model we will try a passive heatsink made from copper flashing.  In either case, these are soldered to the can using the basic plumbing tools for this.  Get "refrigeration grade" copper tubing if you can, as it is much easier to wind.  You will not need very much water flow, just a drip-drip does it fine, which is what inspired us to try a passive finned heatsink for the next model.   This model works, but it works ugly.  Heat is supplied by a 100 watt insulated heating "rope" we happened to buy cheaply as surplus.  They normally cost too much for Doug to design them into things.  The rope is wound in a spiral at the bottom of the can and glued with Silastic 650.  The insulation is to control the height of the resulting vapor column in the can, and to prevent burning the bench.  One of the ugly things about this is that it is now mechanically unstable, and should have feet that protrude through the insulation.

Here we use a trick that may be useful to others, and in other situations that need water cooling.  We put a gallon jug or 5 gallon bucket up on a shelf, filled with water and maybe some ice cubes and antifreeze.  Set up a siphon down to whatever you're water cooling, and let the resultant coolant go into another bucket or jug.  We use a pinch valve we got from Small Parts Inc. to control the flow.  It takes suprisingly little water to handle most things, like this or a distillation column.  Often the water in the bottom jug can just be poured back into the top when the top starts to empty, as it has cooled to room temperature by then at these slow flow rates. 

At this point, you have a reflow oven.  The power (100 Watts) is just about right to simply plug the thing in.  We usually use a variac or a big dimmer for things like this around here.  Unlike model-1, model-2 seems to have a bit too much power, which I suppose is due to better thermal coupling from the new heater than from the glass-insulated wire rope of model 1, not to mention higher power with the McMaster-Carr heater.  We will need to control the power at less than full up. 

Air Cooled reflow oven Six 2" x 4" pieces of flashing were cut out and sweated to the sides of the can at the top.  They get fairly hot, which indicates that more would be good.  We believe that in this case, the plumbing may be worth the hassle.  Here's the completed reflow oven mod 2.0 in all its glory on our messy chemistry desk just after its first run.  The dryer hose coming in at the right is exausting at around 300 cfm to catch anything that gets out of the can, but it turns out to be precisely the wrong thing to do.  It disturbs the convection rotor in the can and sucks valuable Fluorinert out, while mixing the vapor in the can with water vapor from the fresh air.  The wire going into the can is a type K thermocoule, which the DVM at the bottom is reading.  White fumes, which aren't Fluorinert, start coming off at a rather low temperature, this is either a decomposition product or an impurity.  White fumes, which aren't? Fluoroinert, start coming off at a rather low temperature.  It seems these fumes are some decomposition product of Fluoroinert plus water vapor from the air.  In a relatively sealed system, there are no visible fumes.  The version pictured here used a heater from McMaster-Carr, please do check out It Almost Worked for more information on this model, and maybe a laugh or two.

Once the Fluorinert is bubbling merrily, lower your board into the vapor near the bottom of the can.  We use a cage made of baling wire to do this here.  The board will suddenly look wet, a sign you're into the vapor.  A few seconds later any solder on the board will melt and flow, and the board will appear to (mostly) dry.  You'll want to hold it up near the top of the can for awhile to drain every last drop of that expensive Fluorinert off before completely removing the board.  The actual Fluorinert vapor layer in the can is colorless and invisible.  You can only tell it is there by the condensation on the can sides.

Model 2.0 works a little less ugly than model 1.0, but the McMaster-Carr heater has too much thermal mass, and will create problems for the eventual computer automation we plan to apply to this.  Thus yet another heater was tried, and it's simply perfect.  It is a 100 watt quartz halogen bi-pin bulb plugged into a ceramic tube socket salvaged from some old surplus.  I'm sure you can just buy a socket for it, but there was no need here.  The tube socket was glued to the can bottom with the ubiquitous Silastic 650 and holds the bulb about 1/4" away from the can bottom.  We think it may work even better without the original heat spreader we put on the can bottom, and will try that next time.  However, "too fussy wastes man-hours", so for now we are using Model 2.1. (Quote is from Robert Heinlien's "Farmer in the Sky")  Here's what it looks like.

Model 2.1 with halogen heater, insulated base, and legs
A base was made with the bottom of another coffee can, cut right in the middle of a rib with the dremel tool and diamond wheel. This fits nicely inside the base of the original can, and will be lightly glued with hi-temp Silastic once everything is verified.   Some fiberglass was laid around the outer edge of the base, and an aluminum foil reflector fabricated.  Teflon coax (we had this laying around, you could use other high temperature wire, or baling wire wrapped with teflon tape for this) was crimped to the socket terminals with little pieces of copper flashing.  Can't use solder here!  The base has 3 4-40 standoffs used as feet, and they don't get hot due to the insulation and reflector.  We didn't mind holing the base, as it doesn't have to contain liquid.  This model runs fine with no insulation wrapped around the main coffee can.  The black paint on the can bottom and fins is high temperature paint from the auto parts store, which is meant for painting headers.  Theoretically, this should make it absorb IR and visible better, but who knows?  In other work we've been quite suprised to find that things that would seem quite black are actually IR transparent, including carbon black used in many paint pigments.

However,  it is at this point that the real fun begins.  For the process to work well, you will need to preheat your board at a particular time vs temperature profile.  Your toaster oven isn't slick enough for this job, even with a thermocouple and a variac (and that's getting into grunt work again, shame on you).  The profile you want is from 1-2 degrees C per minute, heating to a temperature of about 150 degrees C.  This activates the rosin and gives it time to work, effectively glueing the parts down to the board in the process.

Pre-heat oven:

What is needed is an integral preheat oven that doesn't expose the board to cold air in between the steps.  Our baling-wire board cage will extend through some small holes in the top, so we can raise and lower the board without opening the top at all.  The wires will have 4/40 nuts soldered on to prevent shoving the board all the way down into the liquid.  What is done is the board is preheated, then shoved down into the Fluorinert vapor, then, when the solder has melted, the whole top is simply lifted off so the board can cool.

Preheater model 1.0 showing heater resistors, insulation, and (flawed) window.
This was made with the rest of the can left over after cutting the base off.   Like the base, the fact that it was cut off in the middle of the rib means it fits into the main can nicely.  A 1/4" thick piece of Lexan (Lowes) serves as the top and as a window.  Near this window, you can see the thermocouple entering at the top left.  This is just plain old type K thermocouple wire, obtained at DigiKey, with the end twisted and then welded with an oxyacetylene torch.  We tried Mapp gas, but without the oxygen, it just wasn't hot enough to melt the wires.  You can just barely make that work if you hold the wires just above an anvil while heating with the Mapp, and hit them with a hammer while orange hot.  It might take more than one try.   Ringing the bottom of the can are 12 1.0 Ohm 3 watt  Ohmite "vitreous enamel" resistors (DigiKey again) hooked in series.  These can indeed be run well above their ratings without harm.  Fiber shoulder washers and just plain washers insulate the resistor leads and the 4-40 screws from the can.  As the heating power is marginal with a 24 volt supply (at least while sitting on those heatsink fins), a thin layer of insulation was wrapped around the can and secured with fiberglass cloth (boat store), rubber bands, and the ever handy Silastic 650.  We plan to add a pair of 20w, 12 volt halogen bulbs to this for more heat and useful light, after brazing some wire to the bases of them.  No solder used here either.  The white arc on the Lexan is a bruise from an earlier use of it for something else.  Why waste the good stuff on prototypes?  Small holes will be drilled into this to admit the baling wire board cage handle.  For this part of the job, the Maxwell House can lip is definately good.

A run with this showed us something new.  No white fumes appeared at all.  It seems the Fluorinert doesn't like being mixed with fresh air (water vapor?), and with this on top of the main can, there is pretty much no circulation.  For whatever reason, this solves that problem anyway, and losses of Fluorinert are very tiny. 

We designed a PID controller for this project based on a MicroChip 18LF452.  Digikey sells type K thermocouple wire, and LTC1152 no-drift opamps (and most of the other parts).  Cold junction compensation will be done digitally using data from a  TI TMP 100 I2C temperature sensor placed near the terminal block used for external thermocouple connections.   We use the ICD-2 and the Hitech PIC18 C compiler for things like this, reusing software we've written over the years to speed things up.  It is always easy to go "up" in PICs, and often easy to go down, if the more advanced capabilities of the later family aren't used by the code.  This controller has 2 thermocouple inputs, a solid state relay to control the main heater, and a FET driven by a PWM source on the PIC chip for the pre heater.  We added a communications link to talk to a PC or other host, and ran the various spare PIC pins out to terminals for later use (you never know).  A 16x2 LCD display and a few pushbuttons for settings round things out.  The whole thing, including a massive power transformer, fits easily in the large project box from Radio Shack.  Here is a picture of the partially stuffed board.  The unstuffed front panel board is behind it.  The wiring terminals are stuffed on the bottom of the board to make drilling the clearance holes in the project box easy.

PID controller board photo 

One of the thermocouples is used to monitor the preheat temperature, the other is used to know when the vapors have risen in the can to working height.  There's no point in boiling the Fluorinert the whole time, so the main heat will only be turned on when the preheat cycle is nearly complete.     The solid state relay in the controller will allow for proportioning the main heater.  The big transformer is for a large 24 volt DC supply that is PWM-switched by an onboard FET into the preheater resistors.  Frank did the layout work here, ain't it purty?

We plan to make this design available to all under the GPL.  "Send a self-addressed, stamped email" for the files.
Here's the address as a jpeg for the nasty spambots to miss:

(Note to self, more on this when the thing is finished and tested!)

All trademarks are property of their respective owners.  Sorry if I got too lazy to put in the little ® thing everywhere I could have.  These names are all well known by everyone in our business, and we obviously have no interest in "stealing" them.

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