Reflow Soldering Oven with a Stencil and Soldering Paste

Reflow Soldering Oven with a Stencil and Soldering Paste

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On: 04 Agu, 2020

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Category: Circuit

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In this article I will talk about the transformation of a 35 € oven into a reflow soldering oven, i.e. the type of soldering that allows you to obtain quality assembled PCBs with a stencil and soldering paste.

The soldered paste is de...

In this article I will talk about the transformation of a 35 € oven into a reflow soldering oven, i.e. the type of soldering that allows you to obtain quality assembled PCBs with a stencil and soldering paste.

The soldered paste is deposited on the free pads of the PCB using a stencil. In order for the paste to properly weld the components, it is necessary to subject it to a heat treatment with a specific time profile, divided into several intervals, as shown below:
Thermal profile divided into intervals

thermal profile divided into intervals

thermal profile divided into intervals

Preheat

The solvents in the solder paste evaporate.

Soak

The solvents complete the evaporation, the flux activates and deoxidises the pads and pins of the components; the board is slowly heated so that the thermal gradient between its parts is minimized.

Reflow

The metal part of the alloy reaches the fusion, mechanically and electrically connecting the components to the board pads.

Cool down

Cooling solidifies the metal alloy.

Personally I have noticed that, assuming that the card is not extremely simple, the time required for the preparation for welding (spreading the paste by stencil) is amply rewarded by the speed and quality of the reflow process; this technique also allows you to weld no-leads components with precision and reliability, which is not at all obvious with the welder.

Furthermore, the finished card must not be cleaned of the flux and does not have burrs of any kind. Furthermore, if the volume of the oven permits, entire panels can be welded in a single step.

The construction of the oven was for me a reason for curiosity rather than necessity, since notions of digital control, thermodynamics, firmware and software programming are necessary.
Electric oven and insulation

In order for the PCB board to correctly follow the thermal profile, the oven must have sufficient power to heat it (by convection and radiation) at the required speed. It is also clear that, to follow the profile, an internal temperature sensor and an automatic control that acts on the power introduced into the chamber is required, modulating the power supplied by the heating elements.

So I chose a oven to heat the pizzas, I bought it in Austria by going to find a friend of mine who works there. It is a very inexpensive oven, to be precise it is called "MPO 3520 Multi pizza oven", brand Clatronic, with two quartz resistances that I paid around € 35. The declared power is 1000 W, and it is the largest I have found for such a small oven.

The power of the oven determines the maximum slope of the thermal profile: in particular, approximating the system as a closed-volume thermal power generator (resistances) (inside the oven) characterized by a fixed thermal capacity Ct and a fixed thermal resistance Rl ( walls of the oven towards the environment), we obtain the following equivalent circuit:

gambar 2

Image 2

Intuitively, little power and / or poor insulation (small RL) degrade both the maximum slope \ frac {\ delta T} {\ delta t} and the maximum achievable temperature. In particular, the latter is obtained when the thermal transient is finished and the power introduced by the resistors corresponds to that dissipated towards the environment. The quantification of the quantities of the equivalent circuit will be treated at the end of the article, while for now I will just say that:

More power is better
Greater isolation is better

This is intuitively sensible as the control will regulate the power fed into the volume: a greater available power will give a large margin to the control, keeping the system regulated away from saturation. Insufficient resistance power, on the other hand, will not be compensated by any control system.
The newly purchased oven looked like this (without pizza):

gambar 3

Image 3

Unfortunately I didn't take any pictures when I bought it.

The stock oven uses a reed temperature sensor as a thermostat: you can select which resistances to use (above / below / both) and the cooking time using the knobs. The heaters are controlled with hysteresis by the thermostat, which, when fully operational, maintains the temperature around its "trip" threshold. We do not care about this, since the only thing that will remain of this system is resistance.

To be able to quantify the goodness of the oven insulation, it is interesting to measure the thermal profile that would be obtainable with the "factory" oven. As we will see, basic isolation can be vastly improved. To contextualize the measurement, we choose a slow profile for a lead-free high temperature alloy (Sn96.5 Ag3 Cu0.5, code SAC305). It is still possible to select alloys with a low melting temperature, but it remains a weak solution that limits the use of the oven.

gambar 4

Image 4

Looking at the image, we can decide to be "happy" by being able to replicate the heating ramps until the peak temperature is reached. Once this is achieved, the profiles for alloys with lower melting temperature will certainly be achievable.

To evaluate the performance of the stock oven, I measured the response of the system powered at maximum power (both resistances connected to the mains voltage). In practice, the system to be controlled has the power supplied by the resistors as input and the internal temperature as output. Below the response of the factory oven is compared with the profile we want to achieve.

gambar 6

image 6

The insulation present is already good: the initial heating speed is sufficient, while the peak temperature would be reached with a little patience. This curve is rather comforting, an automatic control could already allow the correct operation of the oven, however the efficiency of the oven is very poor and the margin is rather narrow, especially considering that the board, however small, will add a contribution to the thermal capacity of the oven. system worsening its performance.

In order for the power fed into the heating chamber to remain confined and to heat the PCB, it is necessary to improve the insulation already present. The interior of the oven is equipped with a slot for the recirculation of air placed on the bottom, moreover the walls are poorly reflective. I chose to insulate the oven chamber with aluminum tape: it is basically ordinary foil paper (perhaps a little thicker) which on one side was made adhesive with a high temperature glue.

power resistance trigger system

power resistance trigger system

I completely covered the inner chamber, also covering the glass lid and leaving only a thin slit to observe the inside. I also chose to direct both quartz resistances inwards (originally a metal grid was placed between the resistances and the chamber which limited their irradiation power). The thermocouple K is visible in the following photo on the right, in the future it may be useful to replace it with a clamp version to be fixed on the PCB being processed. I then added two "bubble" threaded bars to place the card.

gambar 11

gambar 11

first stage of feeding

first stage of feeding

Following the changes, the full power response of the oven has improved (even too much), as shown in the following graph.

integrated buck and reference voltage

integrated buck and reference voltage

Now that the oven is well insulated, we can be sure that we can robustly follow all the thermal profiles that interest us.
Control board

The control board that I made performs the following functions:

Internal temperature measurement
Partialisation of the power supplied by the resistors
Automatic profile check
LCD interface management for process status monitoring
Communication with software for setting the profile and parameters of the controller

The last point is more or less optional: if desired, the profile can be written in the firmware and remain unchanging, or it can be delegated to a simpler interface shown directly on the LCD. In my case I preferred to do things more completely and I wrote a management software that I will describe at the end of the article.

Below, in pieces, I report the schematic of the control board with a quick description of the various parts (I can't stand having to open the full-screen image when I read the articles).

compensated differential amplifier for measuring the temperature in the chamber

compensated differential amplifier for measuring the temperature in the chamber

Once hot, the quartz resistances reach a resistance of about 120 Ω. To control the power supplied, simply modulate the duty cycle through two TRIACs. Two TIC226Ms drive the two resistors, while their control and galvanic isolation from the logic is entrusted to the two zero-crossing TRIAC drivers MOC3041. These devices consist of an optoisolator, a mini-triac and a zero-crossing detection system of the mains voltage.

In summary, it is sufficient to turn on the led to the primary to power the corresponding power resistance from the next grid half-period. Conversely, in the absence of a signal, the optocoupler releases the gate of the TRIAC, making it switch off at the zero-crossing of the current.

first half of the microcontroller

first half of the microcontroller

In the diagram, referred to the first resistance, I wanted to highlight the heating phenomenon that increases its resistivity during the first few moments of power supply that I observed with a current probe.

second half of the microcontroller

second half of the microcontroller

The power supply for the logic is obtained through a small transformer and an integrated buck. The image shows the generation of the reference voltage that will be used by the microcontroller ADC for reading the temperature.

interface and connectors

interface and connectors

For the temperature measurement I used a K thermocouple, to simplify my life I used a fully integrated amplifier with cold junction compensation.

heatsink for triacs

heatsink for triacs

control board layout

control board layout

The microcontroller is a PIC18f26j50, a bit oversized for this application, but I had some in the laboratory.

interface on pvc

interface on pvc

complete card and interface

complete card and interface

As will be seen below, the board is connected to an alphanumeric LCD and two buttons to start and stop the welding process. The heat sink is advisable given the average power dissipated by the TRIACs.
PCB and wiring

I made the control board on 2 layers, the final dimensions are 100 mm x 100 mm.

Looking at the image, it is easy to recognize the horizontal line of electrical insulation, which divides the control section (bottom) from the power section (top). In the center, U7 and U8 are the opto-isolated gate-drivers, while on the left T1 is the power transformer. On the right, R21 (is a short) connects the reference of logic to earth. At the top there are the TRIACs Q2 and Q3, connected to the heat sink with a sheet of mica, and the F1 protection fuse.

In the lower section, the microcontroller manages the entire board, detecting the temperature through the thermocouple (connected to the TC connector) and driving the U7 and U8 drivers. On the left you can see the buck integrated with the inductance L1. All connections to LCD, serial and buttons are on the right, on the relative connectors.

I built the LCD interface, with the two start-and-stop buttons, using a white PCV panel and brutally removing the plastic facade that contained the original knobs. I inserted a LED that visually communicates the heating action, flashing with the same duty-cycle used for driving the resistances.

complete board and interface fixed to the case

complete board and interface fixed to the case

To fix the card inside the oven case I riveted two "L" supports to the sheet. I also purchased a fire retardant thermal blanket to improve the insulation of the room from the walls of the case. For the connection of the resistances to the control board I used the original cables for high voltage / high temperature, equipped with safety connectors that prevent their accidental release. All metal structures and the ground-plane of the board are connected to earth. The plug is German type, also in this case I left the original cable. The control board is kept at a distance and isolated from the closure case wall with an additional PCV panel.

Hi reported the blocks in the order in which they are found in the code (shown below), so as to facilitate their recognition.

The main loop performs low priority functions and is cyclically interrupted by three interrupt sources. The only case in which the low priority cycle influences the welding process is when the serial port receives a complete configuration message, otherwise it is transparent and the main functions remain delegated to the interrupt functions. The main cycle, in this particular case, disables the interrupts and cancels the heating process to write the configuration received via software to the program memory (which will be better described below). During normal operation, the cycle sends periodic status messages via serial and updates the LCD interface.

TMR2 generates a cyclic interrupt that monitors the pulses, implements a software debounce and determines when to start or cancel the heating process based on the user's selection.

TMR1 generates a cyclic interrupt exactly every 10 ms. Access to the interrupt increases a variable which is actually the control timer. During the execution of the interrupt, the compensated temperature of the probe K is sampled, the interval of the profile in which you are located is decided and the instantaneous reference temperature is interpolated. The temperature error is derived from the setpoint thus calculated and from the ADC sample. Once the PID status has been updated, the power to be supplied to the resistances in the next 200 ms is decided. In fact, the control regulates the duty cycle (equal for both resistances) of a rectangular wave of the period 200 ms. The control frequency is therefore 5 Hz.

Below is the section of code that performs the function described above. The tick variable is increased by one unit at each interrupt input, therefore every 10 ms, and is reset once 200 ms have been reached. At reset (every 200 ms) the error is then calculated and decided whether to turn the TRIACs on or off. The sysPower variable represents the average power delivered by the resistors during the 200 ms cycle and is used together with ticks to impose the corresponding duty cycle.

// PID update
if(tick == 0)
{
    t_error = sysState.Tsetpoint - sysState.Tmeas;
    
    if(first_lap)
    {
        error_prev = t_error;
        first_lap = 0;
    }
    
    integrator += t_error*(pidCoeffs.KI/Tctrl);
    
    // Saturation
    if(integrator > 5)
        integrator = 5;
    if(integrator < -5)
        integrator = -5;
        
    sysState.power = pidCoeffs.KP*t_error + (pidCoeffs.KD*Tctrl)*(t_error - error_prev);
    sysState.power += integrator;
    error_prev = t_error;
}

if(sysState.power > 20)
    sysState.power = 20;
else if(sysState.power < 0)
    sysState.power = 0;
if(tick < sysState.power || sysState.power == 20)
{
    TRIAC1 = 1;
    TRIAC2 = 1;
}
else
{
    TRIAC1 = 0;
    TRIAC2 = 0;
}

complete interior of the oven

complete interior of the oven

complete interior of the oven 2

complete interior of the oven 2

block description of the control board firmware

block description of the control board firmware


gambar 12

image 12


display screen of the thermal profile set and executed

display screen of the thermal profile set and executed


profile and pid setup screen

profile and pid setup screen


gambar 13

image13


dynamic system for calibration of thermal parameters

dynamic system for calibration of thermal parameters


measured and simulated response with calibrated parameters

measured and simulated response with calibrated parameters


dynamic controlled system

dynamic controlled system


simulated profile and pid controller output (z)

simulated profile and pid controller output (z)

UART, which is the peripheral that implements the serial port, generates an interrupt upon receipt of each character sent by any software on the PC. Upon receipt of a complete configuration string, it communicates to the low priority loop that the program memory must be updated with the new settings. It is possible to configure the thermal profile or the parameters of the PID.

The firmware was written for the XC8 Microchip compiler. The entire MPlabX project (containing the .c sources, for those wishing to view them), is available here.
C # management software

For the visualization and management of the temperature profile and, while I was there, to calibrate the parameters of the PID, I created a C # software that communicates with the oven via a serial port.

The software also allows you to view the temperature curve and setpoint in real time during the welding process. Curves can be saved and loaded in .csv. The following image shows the required profile (in blue) and the actual measured temperature (orange). At the end of the reflow phase (temperature peak) the LCD notifies the user to open the door to ensure rapid cooling of the card being processed.

When the oven is switched on, the software receives the parameters currently stored. In the setting screen you can view the profile just received or set a new one. In the same screen you can edit the parameters of the automatic controller.

Equivalent controlled dynamic system

It may be interesting to return to the simplified circuit introduced at the beginning of the article and to deepen the question, analyzing the dynamic system that describes the whole system. This analysis can help to select the parameters of the automatic controller.

We consider the power supplied by the resistors as a system input and the chamber temperature as an output. Implemented the dynamic system that describes the thermal circuit, we can calibrate its parameters using the step response previously measured. The following system is used to calibrate the thermal capacity and the loss resistance by comparing the measured response stepresp to the simulated matched temperature. A delay has been added to the dynamic system which approximates the propagation of heat from the resistances to the center of the chamber, where the thermocouple is positioned. The best approximation would probably be a weighted combination of single poles that describe the thermal inertia of the resistance, the radiation and the convection. I estimated that it would not be worth it.

At this point, with a dynamic system that approximates the real one sufficiently well, we can choose the parameters of the automatic control. This procedure, of course, can also be done experimentally.

The dynamic controlled system is shown below, where the automatic controller was implemented in the PID sampled version (z) with the corresponding period of 200 ms. The diagram also shows the saturation and equivalent gain seen by the firmware (see the code above). The controller is provided with the profile to follow tspr. temp is the simulated temperature, while ctrl is the output of the PID, corresponding to the firmware sysPower.

Note: a second saturation, in the firmware, is applied directly to the integrator. In the following system this is not present. The difference in this particular case doesn't matter when the supplement never saturates.

 

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