Further improvement of the T-962 soldering oven.


Description of a modernization of the Chinese soldering furnace.
Made on the basis of the project: T-962 reflow oven improvements.

Why this article appeared.

Our lab needs assembled miniature PCB for electronic prototypes. Previously, we had ordered PCB with assembly from Chinese factories, but the last few orders results was very disappointing. In fact, we lost half a year of time, thousands of dollars due to incorrect assembly and soldering. We tried to move the assembly in-house purchased the popular oven T962. But practically, in reality the only thing that more or less works for this PCB oven is the iron box. Hardware and software are practically unusable and destroys components by overheating. There are several attempts on the Internet to fix these problems with workarounds. Our modifications and software makes this oven suitable for practical use...

Calculation of uniform illumination of the PCB plane

First of all, we have to get around the many problems with unsoldered components. To do this, we need to calculate the optimal positioning of the heaters. So that there are no shaded areas and the heating energy is evenly distributed over the of the PCB circuit board surface. We must calculate the required number of infrared radiation sources and their location.

Let us take an example from the technique of illumination. Let there be a light source at a distance a above the plane. It will illuminate the surface. What is the energy of radiation falling on a unit area of the surface per unit time. See picture:

1
We assume that the source is spherically symmetric, and the light is radiated equally in all directions. Then the amount of radiated energy passing through a unit area perpendicular to the light flux changes inversely proportional to the square of the distance. Obviously, the intensity of light in the direction of the normal is given by the same formula as the electric field from a point source. If light rays fall on a surface at an angle θ to the normal, then I, the energy falling on a unit surface area decreases by a factor of cos θ, because the same energy falls on an area 1/cos θ times larger. If we call the strength of our source S, then In, the luminosity of the surface, is:

2
where er is the unit vector in the direction from the source, and n is the unit normal to the surface. The illuminance In corresponds to the normal component of the electric field from a point source with charge 4πε0S. Given this, we see that for any distribution of light sources we can find the answer by solving this equation. We calculate the vertical component of the field on the plane from the charge distribution in exactly the same way as for the light sources.

We need the PCB to illuminate evenly. We have long tubes of IR lamps radiating evenly along their entire length. By placing the tubes in regular rows on the ceiling, which is at height z above the PCB. What should be the optimal distance b from the tube to the tube if we want the surface illumination to be uniform to within one thousandth?

Answer:

1. Find the electric field from a set of uniformly charged wires with a gap between them equal to b.
2. Calculate the vertical component of the electric field.
3. Determine what b must be equal to so that the waviness of the field is not greater than one thousandth.

We saw that the electric field from a series of charged wires can be represented as a sum of terms, each of which gives a sinusoidal change in the field with a period b/n, where n is an integer. The amplitude of any of these terms is given by the equation below:

3
We only need to take the case for n=1, we want to get the field at points not too close to the lamps. To get a complete solution, we still need to determine the coefficients Ap, which we have not yet found (they are found by direct calculation). We only need to know A1, then we can estimate its magnitude by considering it equal to the mean value of the field. The exponential multiplier then gives us immediately the relative amplitude of changes. If we want this multiplier to be 10-3, then b is 0.91z.

If we make the spacing between the lamps equal to 3/4 of the distance to the ceiling, the exponential multiplier will then be 1/4000, and we have a reliability factor of 4, so that we can be quite sure that the lighting will be constant to within one thousandth. The exact calculation shows that A1 is actually twice the average field, so the exact answer would be b=0.8z.

According to these findings we find a simplified calculation formula. The distance between the centers of the heaters should be calculated by the short formula: x = h / 1.34 Where h is the distance from the center of the heater to the PCB board surface. The heating chamber have distance h = 40 mm, so the heater spacing should be 30 mm.

Adding additional heaters

As we calculated earlier, the heater pitch should be 30mm. In the original design of the oven this is not respected. Therefore, we disassemble the housing. Take out the soldering chamber and cut grooves to install additional heating tubes.

chamber fix
heater installed
The heating elements have been completely replaced with new ones. The old ones had to be thrown out - they had assembly defects, the сhinese had not wound the coil evenly.

The best heater for soldering are the ones that have the longest wavelength radiation. These are lamps with a carbon heater. They are very fast and the PID works great with them. In the picture below you can see the dimensions of the heating tubes for the T962. I couldn't find these so I bought the regular ones 220v 300Watt from this store: Infrared Quartz Tube Heater 25cm

heater spectral tab
The less heated the coil of the lamp is, the more long wavelength it radiates. This is very suitable for us. All the heaters are connected in parallel. The total power is 1.8 KW, but this is never used due to the partial heating mode.


MCP9600 Thermocouple Temperature Converter

Adding a quality measurement interface. The MCP9600 chip provides very high accuracy and stability: ±0.5°C typical Hot-Junction temperature measurement accuracy. There is no need to calibrate the sensor later on. For a full description see the Microchip website MCP9600


The PCB contains:

1. 4x Thermocouple Converters.
2. Bluetooth communication chip with PC.
3. External Bluetooth antenna connector.
4. Reset button for manual reset.
5. ISP button for manual programming.
6. Connector for I2С interface to oven.
7. Connector for ISP interface to oven.
8. Connector for SPI interface.
9. JTAG for BT chip.

Thermocouple PCB schematic

At first the MCP96L01 chip was used, but after tests it had to be replaced by the MCP9600. Because the MCP96L01 stopped the measurement when the thermocouple touched the ground. This is probably a design defect of the chip. Do not use the MCP96L01 - it It can cause catastrophic overheating of the oven.

Sensor check utility



Configuration utility
CP2112 board CDM USB driver
CP2112 debug board USB to I2C

Use the configurator to check or calibrate the sensors. The program quickly changes the settings and you can evaluate the quality of the sensors in real time. Use the CP2112 board USB to I2C adapter from Aliexpress for easy connection to your computer. The PC adapter is connected with 4 wires with the same name : VCC-3.3V, GND-GND, SDA-(I2C-SDA), SCL-(I2C-SCL).

Sensors calibration

We will calibrate the sensors to the middle of the measurement scale = 100°C. It is most convenient to do it by the value of the boiling point of water.

The MCP9600 supports 8 different types of sensors. I use the cheapest Type K thermocouples for the range: -40..+400°C. They are accurate enough for all soldering applications. They were purchased here: Aliexpress K-type


Connect thermocouples to the blue PCB connectors. Observe the correct polarity of the wires. Connect (+ Alumel wire) to the upper terminal, connect (- Chromel wire) to the lower terminal.

Heat a glass of water to boiling and lower the balloon junction of the thermosensors into the water to the center of the vessel. Run the configurator program and see the result of the temperature measurement.
Altitude, ft (m) Boiling point of water, °F (°C)
0 (0 m) 212°F (100°C)
164 (50 m) 211,6°F (99,8°C)
500 (150 m) 211.1°F (99.5°C)
1,000 (305 m) 210.2°F (99°C)
2,000 (610 m) 208.4°F (98°C)
5,000 (1524 m) 203°F (95°C)
6,000 (1829 m) 201.1°F (94°C)
8,000 (2438 m) 197.4°F (91.9°C)
Check the water boiling point table for your altitude. If the measured values differ from the reference water boiling point - enter the correction value in degrees Celsius in the sensor settings window "Calibration value". The resolution and noise filtering settings should be selected so that the MCP9600 chip internal measurement cycle is no longer than 245ms. The oven firmware makes 4 measurement cycles per second, and reads the data in 250ms time interval.


Installing the sensors PCB

You need to make two flat flex cables with connectors.
The first cable connects to the ISP connector on main controller board. Through this cable the oven processor will be able to data exchange to the computer via a wireless Bluetooth.

The second cable is the I2C interface and the +3.3v power supply. Here the oven processor reads the temperature values from the thermocouple chips.

The last cable, this is the RF cable. It is used to connect the external Bluetooth antenna. The external antenna provides a reliable connection to the oven at a distance of about 15 meters. In the photo below you can see the device already installed.

pcb1
pcb2


Complex strategy for a simple oven

In a simple device design, we have no bottom board heating, no multi-zone heating, no conveyor belt. Therefore, using the standard method and soldering profiles from a professional furnace will not lead to success.

If your PCB board has components of different sizes and weights and different heat capacities. Different waiting times for warm-up will be required, so that the contact pads under the larger components have time to reach the melting point temperature of the solder paste.

If the temperature profile moves too fast, the massive components will not have enough time to warm up and the soldering event will not occur. Small components will solder but large components not.

For this obvious reason, the soldering algorithm will not be simple. It must be adaptive and smart enough. As the process progresses, it has to analyze sensor data and dynamically change the speed of the profile until the set temperature is reached.

We should use several temperature sensors on the PCB at the same time. The first one should be mounted next to the most thermally sensitive component, e.g. the plastic LED housing. The next sensor is next to the most massive component like a transformer. Add the sensor in the center of the board, where the temperature will probably be maximal. In the settings of the control software, should specify in which role sensor is used and its coordinates on the board.

We will use two upper temperature sensors to limit overheating. These sensors are located directly near to the heater tubes and have a low moment of inertia. In the areas of rapid temperature increase, due to the significant transport delay of the main control loop, The PID will ramp up the heating power to maximum and this in a few seconds can melt the plastic components with shortwave IR-light radiation. To prevent this from happening, let's add to the system a protective algorithm for dynamic limit the heating power.

At the same time, it is necessary to calculate the permissible electrical power, to be supplied to the heaters. If the heating energy is less than the optimum - the process time will be delayed, the solder flux in the paste will boil out and the soldering will not comlete well. If the heating is too fast - the radiation will damage the delicate plastic components. Because plastic have poor thermal conductivity, and t-sensor on the PCB have the delayed response. The overheating point is difficult to detect in right time.

If we need a guaranteed quality soldering, the pro version of the of the oven control program, we can load a pcb board dsn file. This will be analyzed and a mathematical model of the temperature distribution over the surface area. The soldering algorithm will then use this model to calculate uniform heating.

These measures and the complication of the softwares will give us the ability to to assemble prototypes with high quality, from the first soldering cycle, without additional rework and damage to components.


Install the Bluetooth driver

Communication with the oven is supported only via bluetooth connection. The program on the computer side will need use external usb bluetooth dongle. This can be:

Broadcom
Any model based on the BCM20702 A0 chipset
GBU521 GBU522 ORICO GMYLE

Qualcomm
Any model based on the CSR8510 A10 chipset
There are a lot of them on Aliexpress for a small price.
TEROW СSR8510 A10 BT USB Adapter

Install driver

The program use own built-in bluetooth stack and it needs to install the standard WinUSB driver for BT dongle. This section describes how to do this.

Download the Zadig driver installer
In the menu, turn on the "Show All Devices" option.


Select your bluetooth dongle from the list of devices by the name of the chipset.



Choose the type of driver to install: WinUSB, press the Replace Driver. After some time the driver will be installed and you can run the program and try connect to the oven.



Software for soldering

Now should open the soldering control program 'T962.exe' and connect to the oven. You will be able to to quickly change many important settings and operation modes.

Calibrating the heater sensors.
Open the tab 1 "ADC" and calibrate the temperature sensors of the heater.


The calibration can be done at two points 0 and 100 degrees. Before, set the parameters 'Offset' = 0 and 'Gain' = 1.

Immerse the thermocouples TC1 and TC2 in boiling water. Use the trimmer resistor on the controller PCB board, set the sensor output to 100 degrees.

Now short the thermocouple with a short wire or tweezers. The reading on the display should be the same as the value from the CJ temperature sensor. If it does not match, enter a correction constant in the 'Offset' edit box. The 'Offset' parameter corrects the parasitic shift of the zero level of the analog thermocouple amplifier.

Disconnect the wire and repeat the calibration process several times. The 'DGain' parameter is needed to fine tune the gain without disassembling the oven. Do not need make this settings absolutely exact. Because of the poor quality of the Chinese circuit design, this sensor will still drift. But this is enough for us to function properly.

Sensors configuration.
Go to tab 5 'T-Sensors' and configure which ones you want to use. In the example below, you see. Two upper thermocouples are used to limit the overheating of the heaters. Their measurements are averaged.


PCB TSx sensors are used for precise soldering control. In this case I installed the first one next to the miniature plastic component. The second next to the chip and the third sensor in the center of the board. I chose a temperature calculation algorithm with priority to protect the thermo-sensitive components. The TS buttons show the dialog box for the individual setting of the PCB sensors.

Heater configuration.
Open the tab 3 "Heater". Here you should set the control parameters for your hardware: 'Heater Total Power' - total power of heaters installed in the soldering chamber. 'Heater Power Limit' - at this level the maximum energy supply is limited by the rapid increase in heating. 'Heater Temperature Limit' - parameter dynamically limits the heating to this temperature level. 'Fan Speed' - determines the speed of ventilation in the chamber when soldering in heating mode.

The 'Heater/Fan manual control' handles provide direct access to control of the heater and fan. Be careful, in this mode the automatic control is disabled and you can easily "fry" the contents of the soldering chamber.



PID configuration.
Open the Tab 2 "PID". Run tests and see the temperature of the pcb matches witch profile. The difference should not be more than 2..5 degrees. I easily got a deviation of no more than +/-1 degrees. If the error is larger, you need to adjust the parameters of the PID regulator.


For this purpose set the temperature in the chamber, for example 'Set Point' = 100 degrees. Press the 'Run Manual' button, the oven will start heating. By adjusting the P coefficient, get close to 100x0.9 = 90 degrees. Then adjust the minimum coefficient I so that the temperature is close to to the set point of 100 degrees. Then, by turning the heat on and off, adjust the minimum D factor. At the best setting, the temperature should be set as quickly as possible with minimum overshoot. Below you can see the sequence of optimum manual adjustment of the PID coefficients.



System efficiency test.
Now you can go to the 'Reflow' tab and select the solder paste temperature profiles to check. If the PID has been set correctly, you will be pleased with the result.


Ramp Speed Test shows tremendous power and excellent accuracy of furnace. Made a quick transition to almost 200 degrees step in 15 seconds. Temperature drift no more than 2 degrees. This potential will be enough to perform any standard solder profile.



These graphs show that the thermal profiles are accurate to at least +/- 2 degrees in the heating section. On the cooling section, the board cools down more slowly. This is because there is not enough cooling fan power. It should be three times more powerful. But it's not critical for me. I don't want to change the fan, I'll wait until it cools down a few minutes longer.

Note the graph of the power control (yellow line). The energy is supplied smoothly, without sharp peaks. This protects the electronic components on the circuit board from damage by shortwave IR radiation.

Soldering profile management.
Open the tab 4 "Reflow". Here you can load any of your own profile. From a file or draw it and change it in the editor. This is convenient because there are only 3 soldering profiles in the memory of the oven. Press 'Settings' to select the desired soldering options. If necessary, you can enable a process execution log and save it as a picture or file. The 'Launch Reflow' button starts or stops the soldering.



To be continued...

As the work progresses, I adding the text and post a firmware update. Check in from time to time if the topic is of interest to you.

The article is in the process of being written.


Download software and driver

Application download:
Configuration utility (exe)
T962 (exe)

Download CDM USB-I2C driver:
USB PC connection driver
USB driver installation help

Download latest update:
T962 firmware (fixed version 0.5.2)
T962 firmware source code (IAR)
How To flashing firmware

Circuits and chips:
Thermocouple pcb sch
Chip MCP9600