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    The Concept of Reflow Soldering

    Reflow soldering is a widely used process in electronics manufacturing for soldering surface-mount components to PCBs.

    Reflow soldering involves melting solder paste to create a permanent connection between the electrical components and the PCB.

    The solder paste is a mixture of small solder particles and flux. The flux serves multiple purposes, including removing any oxide layers on the metal surfaces, promoting wetting, and preventing oxidation during the soldering process.

    A precise amount of solder paste is applied to the pads of the components first, and then surface-mount components are placed on the solder paste-covered PCB. The components are positioned based on the design layout

    The PCB with components and solder paste is then passed through a reflow oven.

    We will be using a reflow hotplate for our case. A reflow oven and a reflow hotplate are fundamentally different from one another; an oven is enclosed, while a hotplate is open. Hotplates are used for circuit rework, whereas reflow ovens are used for professional work, such as in SMD circuit production lines.

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    Existing Hotplate

    There are already hotplates on the market with more useful features like cutoff features, manual temperature control, strong bodies, etc.

    Every size of these hotplates is available. A common example of a small hotplate is the Miniware MHP30, whose reflow surface measures only 30 by 30 mm, making it ideal for reflowing smaller PCBs.

    The only drawback of these hotplates is their price, which can be expensive for a beginner. However, one solution to this problem is to create a do-it-yourself hotplate that functions similarly to the original.

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    Let us now analyze the main control circuit.

    Three sections make up this circuit: the XIAO M0 Microcontroller section, the MAX6675 Setup, and the AC section, which is connected to an isolated SMPS and relay.

    Let us begin with the primary part of this circuit, the XIAO M0 DEV Board, which is linked to the SSD1306 display via I2C. Furthermore, the MAX6675 Sensor setup and the relay are linked to the XIAO.

    We used a straightforward Mosfet switch configuration, whose gate is controlled by the XIAO M0 DEV Board, to control the relay. Mosfet is the relay's driver.

    Our MAX6675 IC gathers temperature measurements and sends them to the XIAO M0 DEV Board. The XIAO Microcontroller then uses the temperature readings to regulate the mosfet's gate, which ON and OFF switches the relay.

    One end of the relay is linked to live AC on the AC side, and the other end is attached to a CON2 screw terminal, whose pin 2 is connected to AC neutral. The relay in this configuration acts as a switch between the heating coil and the AC power source.

    In order to power the microcontroller and sensor from direct AC, we utilized an isolated power supply module that converts 240V of AC power into 5V and 3.3V.

    Additionally, we have added an LED with the I/O pin of the XIAO MCU. This LED will show if the relay is turned on or off by copying the relay state.

    After finalizing the PCB schematic, we generated a netlist and prepared the PCB design. For further isolation, we have added a slot between the AC and DC sides.