Resources

X-Toaster Reflow Oven Controller – Free Resources Downloads

Reflow Oven Controller - Resources Download

All downloadable resources for X-toaster are linked from this page. If you need other documents please contact us.

X-TOASTER REFLOW OVEN CONTROLLER - USER MANUAL

X-toaster-reflow-oven-controller-User-Manual
PDF files compressed with RAR.
Download it here

 

X-TOASTER REFLOW OVEN CONTROLLER - SKETCHUP MODEL

Reflow-Oven-Controller-Sketchup-Model-Download
You can use it to quickly make a project of your own chassis. Please note that the model has been designed with a tollerance of approx. +/- 0.5mm
Download it here

 

USER SUBMITTED REFLOW OVEN BUILD GUIDES

If you want to share your own X-toaster based Reflow Oven build guide send us an email, we'll be happy to post it here.

MHofmann-Reflow-Oven-Build-Guide

 M.Hoffmann Reflow Oven Build Guide

An elegant metal chassis build... with a touch of colour.

Download PDF

 

 

 

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PID Tuning for Toaster Reflow Ovens

A few Words about PID

PID controllers (Proportional-Integrative-Derivative) are widely used in many industrial processing systems.
To be able to control the heaters and closely follow the ideal reflow profile, a PID controller has been implemented. Since every PID controller must be tuned to work properly and the tuning changes for different environments (different ovens) you need to understand some basics on how PID actually works.

PID is based on “feedback”: at a fixed time frequency the temperature value is read from the sensor (thermocouple) and confronted with the setpoint (ideal targeted temperature).
The error is then used to calculate three variables P, I and D.
The result of the sum of those variable is then used to know how strongly the heaters should be turned on using a sort of “slow PWM” to drive them via the SSR.
It is to be noted that, while in a standard PID algorithms all three components are calculated based only on the error, to improve the response in our PID we decided to calculate D using the difference from the CURRENT OUTPUT value and the LAST OUTPUT value, following a technique also known as “Derivative on Measurement”.

Since PID variables may become very large, they are also limited to a maximum value of +/-999.
Keep in mind anyway that a PID value of 400 (sum of P,I,D) will turn on the oven constantly, so your values should remain much lower than maximum at any time.

Before proceeding to tuning X-toaster we will need to understand how PID variables change the behavior of the oven without using complex math and simplifying everything as much as possible: our goal is to be able to understand enough of the matter to be able to tune it.
For a more precise and complete analysis you can read some of the many articles around the web or you can refer to some academic literature on Controls System and Automation.

 

Proportional Term

This is the easier to understand. You simply have to multiply a Kp constant with the measured error at any given time.

Try imaging when you are driving a car and you want to reach a steady and fixed speed: at the start you would press your foot on the pedal with a large intensity (large error). While the speed increases over time (the error gets smaller) you surely will decrease the pressure on the pedal to have more control. You are simply using a P control. Kp tells you how strongly you have to react to errors by pressing the gas pedal more or less aggressively. When you start your car you never press your pedal all the way down: tuning P means searching a value for Kp large enough to let the speed rise quickly but not so much that you will reach a point where the car becomes uncontrollable.

In our oven the main problem with large Kp values are big overshoots because of thermal inertia. In the car example temperature would be more the road traveled than the velocity: if you want to reach a certain distance, even by releasing the gas pedal your car is still travelling because of its inertia: by reducing your Kp the car is more controllable because changes are slower.

Integral Term

The main problem in using only a P term is that, when you reach the target speed in the previous example the error become zero. By definition then, you will instantly and completely release the gas pedal (remember that with P you are simply multiplying the error by a constant Kp). As soon as you release the pedal the car starts to decelerate, the error changes and you will press the pedal again.
Of course this is not how people drive because by doing this you are introducing a bad oscillation on your car speed.
The integral term helps in removing steady state errors. Let’s see how it can help with our speed problem.
I is calculated by multiplying a Ki constant with the sum of all the errors measured during time (if you draw a graph for the "error function", I will be the area under the graph itself, hence “integrative”).
You can easily see that I will increase over time, while there is an error, and will become a fixed value when the error becomes zero.
Well, this is exactly what we needed with our car: when you reach targeted speed, the error becomes zero. You still have a P term equals to zero but an I term which is not zero so you are not going to release the pedal but, depending on how big is your Ki constant, you will keep on pressing the pedal at a bigger or smaller angle.
Tuning I means searching the Ki constant which allows you to have a pressure just right to maintain the speed.

 

Derivative Term

We have seen that when the speed changes too fast your car is difficult to control.
By using lower Kp values you can improve things a little bit, but everything is less reactive and travelling a certain distance would require too much time.
We need a way to keep reaction fast but to counteract changes too big to handle.

In our PID version, D is the difference between the output value and the previous output value, divided by the time between the two measurements and multiplied by a constant Kd.
If you can visualize it in a graph you can easily recognize it as the derivate of the output function over time (supposing a very small interval between measurements): it tells us how big is the slope of the tangent line to the "output function" for every single moment hence how fast the output is changing.
Well, this is exactly what we needed: when the output function is rising D is negative, when it is falling D is positive, when the output function rises or falls gently D becomes almost zero, when there are fast changes D becomes very large.

In our car example, at the start when the error is large the controller tells us to press heavily on the pedal. If the speed is not changing quickly we will keep on pressing the pedal but if the car starts moving at high speed we will instantly lower the pressure on the pedal because of the big negative D term now summing with P and the speed will increase more slowly. When the speed is reached, if for some reason your car slows down a little bit, D term will be small enough to allow for a quick speed recovery due to a still big enough P.

Tuning D term means searching a value for the Kd constant high enough to counteract for big reactions due to P, which usually brings big overshoots in temperature, but low enough that P is not affected too much that the heating becomes too slow.

 

In Short:

ERROR

is the difference between the SETPOINT (target temperature) and OUTPUT (value read by the thermocouple).

P - PROPORTIONAL TERM

tries to correct the error by applying an influence proportional to the error itself

I - INTEGRAL TERM

detect how error changes over time and tries to counteract steady state differences

D - DERIVATIVE TERM

tries to counteract abrupt changes imposed by the proportional term
P, I and D are then summed together to obtain a numerical value.
That value (let’s call it PID value) is then used to control the heaters via PWM: larger PID values impose the heaters to stay ON for longer

PID tuning is the process of finding the right values for the three terms Kp, Ki, Kd.

 

An Improved Controller

A lot of reflow ovens, even many industrial ones, can live with just a PID controller for a good reason: they have the ability to not only add heat but also to remove it.
Toaster ovens can only turn on the heaters and for that reason a PID controller may often become very difficult (if not impossible) to tune properly.
Thermal inertia and hysteresis are your worst enemies. Turning on the heaters make the temperature on the PCB rises only after a period of time and even when you turn off the heaters the temperature on your PCB usually continue to increase with the result of nasty overshoots and burnt components.

To solve the problem we added a few more controls:

 

Overshoot Compensation

Overshoots in reflow profiles are usually observed in the reflow phase, when your oven is asked to stop rising temperature and maintain it at a steady level for a period of time.
While small overshoots of 2/3°C are usually perfectly ok, if you observe more than 5°C with a standard PID controller you would have to start over the tuning process even if the other phases were perfect.
By using the overshoot compensation feature you can effectively change how the PID controller reacts only during the reflow phase by lowering the target temperature only where it is overshooting.

 

Previsioning

The practical result of hysteresis is that there is a lag from the ideal target profile and the real curve we can observe as read from the thermocouple.
When the heaters are turned on and off, the temperature on your board will take a few seconds to change proportionally.
This time-shift in the output temperature can be compensated by using the previsioning feature. By using only a standard PID controller there is no way to compensate for it.

 

Ki Limit Factor

Since the only steady state to keep in a reflow profile is the short one in the reflow phase, integral term is usually kept at minimum to improve the system response and to make tuning easier.
Sometimes though, for certain application which requires both longer phases with a constant temperature AND steep temperature gradients, tuning may become difficult because you need higher ki values for the constant phases but those higher values tends to create overshoots when there are changes.
Moreover, since it is not possible to actively take the heat out of the oven “I” term tends to grows over time to very large values which results in the oven heating even during overshoots.
Ki limit factor is a constant which come to play when an overshoot occurs: the bigger you set this parameter the faster the value of “I” returns to zero during any overshoot phase.
Please note that this parameter is usually not required for reflow soldering and it should be kept to zero (disabled).

 

Smooth Radius

This parameter changes the ideal profile by smoothing the knees around the nodes. It helps preventing lags and overshoots by “anticipating” what will be required from the next phase.

 

Preheat Until

This parameter acts at the very beginning of the reflow process. It simply bypass the PID controller and turn the heaters to a fixed ON until the chosen temperature, effectively converting the controller in an “open loop” mode. It is useful to prevent big integral summing and to quicken the heating at the start.
You should use a value big enough to just reach the same slope of the first segment before giving control back to the PID.

 

Fixed Ramp Up

This parameter acts at the beginning of the ramp up in the reflow phase. PID controller is bypassed and the heaters are turned to a fixed ON for the specified amount of seconds. This is useful to improve react time of the oven during the most difficult part of the reflow profile.
Please note that this parameter overrides completely both your profile and any overshoot compensation values: if you set it to a value too large your temperature will rise without any control! Before using it with a real reflow process be sure to test it accurately.

 

PID Tuning your X-toaster Reflow Controller

PID tuning may be a little bit tricky. Before going any further be sure to know and understand:
-    Your oven capabilities, the rate at which it can ramp up
-    What is the profile that you are going to use: temperatures and time intervals between each node.
-    How a PID controller works and how the constant Kp, Ki and Kd will affect the behavior of the oven

Be prepared for a few test runs and always let the oven cool down to at least 35/40°C before proceeding with another test.

The process of tuning may take from 30 minutes to a few hours, mostly because you have to let the oven cool down before starting a new test.
Keep a cellphone or a camera at hand and shoot a photo of the display at the end of every test to keep tracks of your progress.
Always check the calculated values on the display, they can tell you what to change.

Reference values for the PID parameters are:
Kp:    15~90, it depends on how much your oven is powerful and on how much inertia it has.
Ki:    0.01~1, it can be increased for different purposes apart from reflowing.
Kd:    1/5*Kp ~ 2*Kp, usually higher with powerful heaters and lower for weak ovens.
Your specific oven may require larger or lower values than those specified above.

Remember that changing the value of a constant also change the behavior of the others: the entire controller is dependent upon the values and the correlation between the three.

For reference these are the typical effects of increasing each variable:

Make sure that the profile you are using is compatible with your oven maximum capabilities.
Program the profile in your oven starting from the last node, you won’t be able to set the time one node later in time than the following one.
Once the profile has been programmed save it and enter the advanced options screen.

 

Let's start...

Let’s start with the “Preheat Until” parameter.
We are aiming for a value high enough to quickly bring the oven to temperature and to “move” the heaters from their steady state ambient temperature giving them some inertia.
Set this value to 60°C.

The “Fixed RampUp” parameter will be useful during the most important ramp of the entire reflow process, start with a value of 5s, we will tweak it later.

Ki limit factor is not usually needed for reflowing, set it to zero.

Set the “Smooth Radius” to 5s, again we can increase it later if needed.

The “Previsioning” parameter is strongly dependent on the size and internal mass (inertia) of your specific oven. Set it to 7s as a starting point, we will probably increase it later.

Leave “Overshoot Compensation” to zero, before changing it we need to know if and how much we are overshooting during the reflow phase.

Set Kp=15, Ki=0 and Kd=0

Save and start a reflow process with the programmed profile.
The first thing to check is the “Fixed RampUp “ parameter. You can recognize its influence because the curve is drawn in pink instead of red and, while it is acting, all PID values are MAN (manual).
This parameter should act until the slope of the output is more or less equal to the slope of the first segment of the profile. If it is too high you will get overshot.
Observe its behavior, abort the session, change the parameter and start again until you obtain what we are looking for.

Now let’s focus on the PID controller itself.
By now we only enabled P term with a small Kp constant which will probably be too weak.
Increase its value until the red output curve is able to follow the green ideal profile. A few overshoots over the nodes are ok and will be addressed later. If Kp is too low you will observe slow rises and output will mostly be under setpoint. If it is too high there will be oscillations in the output.

If you notice a “shift” in time between the ideal profile and the output you can adjust it by changing (increasing or decreasing) the “Previsioning” parameter.

Once you have a value for Kp we can try to deal with the overshoots that you probably have.
Let’s change the value of Kd to 1/5*Kp. And start a test session. Increase Kd until the overshoots are gone.
The effect of increasing Kd is to counteract Kp action when the output is changing very fast. As a result you will probably get a slower response during the preheat phase and the ramp up of the reflow phase. To compensate for it you can increase the “Preheat Until” and the “Fixed RampUp” values.
If higher values of Kd are introducing oscillations try increasing Kp again. As always proceed in small steps.

If your output is almost perfect but you are still observing some small overshoots over the nodes and the ramps are too late in respect to the ideal profiles try increasing the value of “Smooth Radius”.

At this stage you should have a nice output until the reflow zone, where maybe you still have to work with the steep rampup and the overshoot.

If you notice that the ramp is not heating fast enough to reach targeted reflow temperature in time try increasing the “Fixed RampUp” value. If the temperature is overshooting take note of how much you have to compensate for it and program the “overshoot compensation” parameter with that value.

We still have to deal with Ki.
As we saw, the integral term of our PID is used for steady state error compensation: since the only constant temperature in our profile is during the very short peak of the reflow phase you can probably leave Ki to zero and still have a perfectly nice output.
If you observe some kind of oscillation during the constant part of the reflow phase you can improve it by increasing Ki.
Start from a very low value, something around 0.01 and increase it in small steps until the oscillations are gone and the constant temperature is kept nicely.
By increasing Ki you may start to have some new overshoots in your output. If that happens increase again Kd a little bit until they are gone. If the output oscillates tweak again Kp increasing it again in small steps.
When the output satisfy your requirements you are finally ready for your first reflow.

Congratulations!

 

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Some Basics on Reflow Soldering

PCB-solder-paste

Why Reflow?

Because it is easy, fast, reliable and gives professional results with very little skills to master.
While many SMD components can be soldered with just a standard iron this is generally a very slow process. It also requires some skills to be able to solder smaller components and high-density pins.
Some footprints like QFN are very difficult to be soldered with an iron even for a trained technician and often those joints are not good enough even for prototyping purposes. BGA components are of course impossible to deal with and for them a reflow oven is a must.
Reflow ovens are the first choice in the industry for production and prototyping, the only downside are their cost and the costs related to reflow technique: you are going to need a stencil for every board and you need to use solder paste which is generally quite expansive.
Lately these costs, thanks to some Chinese production facilities and resellers (like Elecrow, SeeedStudio, ITead, etc…), have been lowered from several hundreds of dollars for a steal stencil to less than 20 USD, allowing even hobbyists to build devices with modern and difficult footprints.
The only high cost device to buy is a reflow oven.
Industrial ovens usually costs more than 1.000 USD and the cheaper Chinese alternatives on the market (which still are quite pricey, around 400/500 USD) are simply too buggy and do not have a build quality good enough for a reliable (and secure) use.

A toaster oven conversion is usually a much cheaper (and often better) alternative, but to be able to control the temperature to follow the suggested thermal profiles you are going to need a controller capable of compensating for the high thermal inertia usually related to cheap mass produced food ovens.
These toaster ovens also transmit heat mostly by radiation than convection and this is another factor to take into account when building a controller.
X-toaster has been designed to follow any reflow profile, to be adaptable to any oven and to compensate for inertia and hysteresis.

 

Things to Know

When dealing with reflow soldering there are some things to know which usually are not needed when hand soldering with an iron.

Moisture Content

The first is that some components are sensible to thermal chock and the moisture content of their packages may be an issue.
When you order some big microcontroller from Digi-Key or Mouser they usually come in a sealed bag with a humidity indicator and a desiccant pack inside. That’s because the moisture in the package when exposed to high temperatures vaporizes and, if you do not give this vapors enough time to exit the package, the result could be a cracked microcontroller.
This lead to the first rule: only open these packing when you are going to solder them on the PCB and check the indicator that the moisture has not gone over the maximum allowed percentage.
If you have a lot of components to be soldered in many PCBs during more than one day, always use an ESD safe sealed bag and keep them in it, with the indicator and the desiccant. If you have some sensible components sitting in a box for more than a few days you should try to let the moisture in it evaporate gently before attempting any reflow.
X-toaster comes with a preprogrammed desiccant profile to keep the components at 125°C for 60/90 minutes.
Even so keep in mind that this is not an ideal process and that micro fractures could be forming.

Hot and Cold Spot

It may be quite difficult, even with big professional ovens, to keep a uniform temperature across the entire PCB.
When using a toaster oven this problem can cause some serious issues like pads where the solder paste do not reflow correctly or, even worst, components which overshoots the target temperature.
Moreover, since in the toaster ovens the heat is transferred mostly by radiation, remember that bigger black components heats faster than smaller ones. Ideally you should measure temperature over the entire board and across every component to be sure to have a profile with a peak temperature high enough for reflowing but not too high to cause thermal shock on the bigger components.
Of course this is not very practical and you should be ok by following a couple of simple rules:
If you get unsoldered pads but you are already hitting peak temperature try to make your profile last a little longer for every phase (while keeping it inside the maximum limits). This is usually enough to have the entire board heat mostly at the same temperature.
A convection fan may help in spreading the heat with more uniformity: if your oven have one make some experiments turning it on for one or more phases of your profile.
Check the tray you are using to sit the PCB inside the oven. The standard toaster oven tray (simply made with steel bars) sometimes absorbs too much heat and you get cold strips where the PCB come in contact with the tray bars. If this is your case build another tray with some steel mesh or try to raise the PCB from the tray with some metal spacers.
When designing your board keep in mind that large ground planes absorb much more heat and inertia may cause the pins sitting directly on these planes to overshoot. If you have such problems do not put your pads directly over these planes. Instead connect them with short tracks and eventually try to use thermal reliefs.

 

Leaded and Lead-Free

Until a few years ago, every solder joint were made with Sn-Pb alloys and everyone where quite happy about it.
Then someone started to think about all the electronics discarded in the bin every day by billions of people and the consequent lead pollution.
This (and a few other reasons) brought to the concept of RoHS and lead-free electronic and the Sn-Pb was substituted with other alloys like Sn-Ag-Cu.

Today leaded solder is not allowed by laws of many countries, particularly in the European Union and, even if you live in a country where leaded solder medium are allowed, you should consider using lead-free solder paste.
Lead poisoning is something you should be concerned about, especially if you work in a home-lab.
When absorbed by your body, lead is a cumulative poison affecting your health in several nasty ways.
Lead poisoning is not easy to recognize and children are particularly vulnerable to lead, which impede their brain development.

The main problem in using lead-free solder paste is the higher temperature needed for reflowing.
When using lead-free pastes you need to use profiles with high peak temperatures (around 250°C) and you have to take special care to make sure that overshoots are kept to a minimum and timings are carefully respected to avoid damages to your PCB and components.
When programming your reflow profiles, you should refer to the suggested profiles published by the producers of your solder paste and by the manufacturer of the components of your board.
If you have no access to those documents and for general reflow applications you can always use the standard profiles published by JEDEC.

 

JEDEC Standard Profiles

The JEDEC Solid State Technology Association, formerly known as the Joint Electron Device Engineering Council (JEDEC), is an independent semiconductor engineering trade organization and standardization body.
They published a few documents on reflow soldering with a couple of leaded and lead-free standard profiles you can refer to when programming X-toaster.

By looking at the graph (JEDEC/IPC J-STD-020) you can easily understand that there is not a strict curve to follow, instead there are areas where to fit you profile.
Even peak temperature (Tp) is not fixed for every situation, but changes depending on the packages thickness and volume.

When programming the nodes on X-toaster these will be the corresponding phases:

PREHEAT

Flux solvent are evaporated during this phase and your board is quickly brought to a little bit more than half reflow temperature

SOAK

Solvents evaporation completes during this phase. Flux starts its deoxidizing action on parts to be joined and board is kept to an almost stable temperature for a certain amount of time to minimize thermal gradient

REFLOW

In the blue zone there is the actual melting of the alloy which forms the mechanical/electrical joint

COOL-DOWN

A quick cooling would be desirable to create finer grain structure and stronger joint. This is actually not always possible with toaster ovens where there are no exhaust/cooling fans. All you can do is simply open the oven’s door and wait until the temperature is low enough.

 

LEAD-FREE Parameters

On the table we can identify the maximum ramp up and ramp down rate (3°C/s and 6°C/s respectively) which, in our case, are not a big problem because you will hardly be able to obtain steeper rates with toaster ovens.
What may be a problem instead are maximum time above liquidus (217°C, 60-150s max) and maximum time within 5°C of peak (Tp).
If your oven is not powerful you could be forced to increase those values to allow temperature to raise enough. That is why choosing a powerful and small (quick) toaster oven is so important for reflowing. You should be able to reach more than 1°C/s when ramping up (ideally at least 1.5°C/s) with your oven.
If you can’t you should consider another oven or adding more heating elements.

 

Sn-Pb Parameters

As expected Sn-Pb parameters are much more “relaxed”.
The most important factor is the peak temperature of only 225/240°C compared to the much higher 245/260°C of the lead-free profile.
JEDEC recommends shorter time within 5°C of Tp and also shorter time above liquidus compared to the lead-free parameters, but if your application is not critical you should be ok with longer profiles.
If your oven is not powerful enough for lead-free alloys and you are not willing to change or mod the oven, Sn-Pb may be your only choice for reflowing.

 

Draw your Reflow Profile

We have seen that you have some kind of freedom to draw a profile suittable for your applications and your particular oven.
At this point you really need to know what is your oven capable of, what kind of solder paste you will be using and which component sizes will be mounted on your board.

Knowing the rate at which your oven can ramp up and cool down is easy enough using X-toaster controller.
After setting up your oven, controller and thermocouple like suggested in the article "X-toaster Installation & Wiring", program a profile with a high Kp and a steep temperature rise (5s) to around 150°C and let it run for at least a couple of minutes or simply set the "Preheat until" parameter to 150°C. Your oven will be fired on constantly and you will be able to observe how much your oven is ramping.
For the cooldown you can do the same in reverse: at the end of the previous profile set a steep cooldown to 25°C and make it at least a couple of minutes long. When the last phase start, open your oven’s door fully and you can observe at which rate your oven is cooling.

You will probably find that your ramp up is around 1.5°C/s and you are cooling down at no more than 2 or maybe 3 °C/s.

It would be pointless to program a profile which your oven will never be able to follow. Make sure to use the previous limits as a reference.
If you find that your oven is a little bit slow in the pre-reflow ramp-up you can tweak your profile to make that ramp a little bit easier, for example by using the whole allowed limits for the SOAK phase (150°C-200°C for lead-free and 100°C-150°C for leaded).

Special care should be dedicated to the peak temperature.
Once you know which peak temperature you a targeting based on your components size, draw a profile which respect the maximum time allowed for it and run a test.
If you can, diminish the time within Tp as much as possible. If you observe overshoots try to use the Overshoot Compensation feature in the ADVANCED options.

Of course you should also set the PID parameters. These are addressed in the "PID Tuning" article.

 

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