Makeshift Reflow Hotplate

Hello and welcome back.

Here's something HOT: the Makeshift SMT Reflow Hotplate, made completely from scratch.

This project's centerpiece is a heating element that was salvaged from a cloth iron. It is powered by AC power and regulated by a circuit comprised of an XIAO microcontroller and a MAX6675 TEMP sensor.

The goal was to create a low-cost, functional hotplate for reflowing SMD circuits; this is actually version 2 of the hotplate, which I created nearly four years ago.

Previous hotplate project:

https://www.instructables.com/DIY-SMT-Hotplate-Project/

This configuration works like this: Between the heating element and the AC supply, a relay in the circuit operates as a switch.

The microcontroller is connected to the relay. The heating element's temperature is measured via MAX6675. The microcontroller disrupts the AC power to the heating element and turns OFF the relay when the reading goes above the set threshold.

The heating element turns ON when the temperature drops below the set threshold and the power reconnects. The hotplate surface remains at a steady temperature by continuing this operation.

This Instructables is about the whole build process of this hotplate project, so let's get started with the build.

These were the materials used in this build:

• Custom PCB
• XIAO SAMD21
• Hotplate's Heating Element Salvaged from OLD Project
• M5 Nuts and bolts
• M3 Screws
• MAX6675 TEMP SENSOR IC
• K-Type Thermistor
• SSD1306 Display
• Wooden board
• AC Cord
• A03400 N-channel Mosfet
• 10K Resistor
• Relay 5V
• Isolated Power Supply Module

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.

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.

Let us now analyze the main control circuit.

Three sections make up this circuit: the XIAO SAMD21 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 SAMD21 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 SAMD21 DEV Board, to control the relay. Mosfet is the relay's driver.

Our MAX6675 IC gathers temperature measurements and sends them to the XIAO SAMD21 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.

After finalizing the PCB and generating its Gerber data, it was sent it to SEEED Studio for samples.

The PCB was ordered in a white solder mask with black silkscreen.

PCBs were received in a week, and their quality was super good considering the rate, which was also pretty low.

Seeed Fusion PCB Service offers one-stop prototyping for PCB manufacture and PCB assembly, and as a result, they produce superior-quality PCBs and fast turnkey PCBAs within 7 working days.

Seeed Studio Fusion PCB Assembly Service takes care of the entire fabrication process, from Seeed Studio Fusion Agile manufacturing and hardware customization to parts sourcing, assembly, and testing services, so you can be sure that they are getting a quality product.

After gauging market interest and verifying a working prototype, Seeed Propagate Service can help you bring the product to market with professional guidance and a strong network of connections.

Next is the PCB assembly process.

• Using a solder paste dispensing needle, we first add solder paste to each component pad individually to begin the PCB assembly process. In this instance, we are using standard 37/63 solder paste.
• Next, we pick and place all the SMD components in their places on the PCB using an ESD tweezer.
• With extreme caution, we lifted the complete circuit board and placed it on the SMT hotplate, which increases the PCB's temperature to the point at which the solder paste melts and all of the components are connected to their pads.
• Next, we add all the THT components, which include the relay, header pin, CON2 screw terminals, and isolated power supply, to their locations and then solder their pads using a soldering iron.

This is the finished circuit with all of the THT and SMD parts mounted to the PCB.

Next, we install the SSD1306 display after positioning the XIAO M0 DEV board into the header pins.

Following circuit completion, we connected the K-type Thermistor supplied by the MAX6675 module to the MAX6675 IC's CON2 screw connection.

In order to test the circuit, we heat the temperature sensor with a lighter and set the threshold to 40°C. The LED turns off when the temperature rises beyond 40°C and reconnects when the temperature falls below 40°C.

Here's the code used in this build-

int thermoDO = 9;
int thermoCS = 7;
int thermoCLK = 8;

const int RelayPin = 1; 
const int LED = 0;
const int threshold = 150;//cuttoff temp

MAX6675 thermocouple(thermoCLK, thermoCS, thermoDO);

void setup() {
  display.begin(SSD1306_SWITCHCAPVCC, OLED_ADDR);
  pinMode(RelayPin, OUTPUT);
  pinMode(LED, OUTPUT);
  Serial.println("MAX6675 test");
  // wait for MAX chip to stabilize
  delay(500);
  display.clearDisplay();
  Serial.begin(9600);
}

void loop() {
  // basic readout test, just print the current temp


  display.setTextSize(2);
  display.setTextColor(WHITE);
  display.setCursor(10, 20);
  display.println("C= ");

 
  display.setTextSize(2);
  display.setTextColor(WHITE);
  display.setCursor(60, 20);
  display.println(thermocouple.readCelsius());

  display.display();  
  display.clearDisplay();
  delay(300);

  if (thermocouple.readCelsius() > threshold) {
    digitalWrite(RelayPin, LOW);
    digitalWrite(LED, LOW);
  } else {
    digitalWrite(RelayPin, HIGH);
    digitalWrite(LED, HIGH);
  }

   // For the MAX6675 to update, you must delay AT LEAST 250ms between reads!
   delay(500);
}

CUTOFF FEATURE

void loop() {
  // Code to display temperature on the OLED screen (not shown in provided snippet)

  // Read temperature from MAX6675
  float temperature = thermocouple.readCelsius();

  // Code to display temperature on the OLED screen (not shown in provided snippet)

  // Turn on/off relay and LED based on temperature threshold
  if (temperature > threshold) {
    digitalWrite(RelayPin, LOW);
    digitalWrite(LED, LOW);
  } else {
    digitalWrite(RelayPin, HIGH);
    digitalWrite(LED, HIGH);
  }

  delay(500);  // Wait before the next temperature reading
}

• The loop function continuously reads the temperature from the MAX6675 sensor and displays it on the OLED screen.
• If the temperature exceeds a defined threshold (threshold), it turns off the relay and LED; otherwise, it turns them on.
• There is a delay of 500 milliseconds between temperature readings.

This was the previous Hotplate project, which expanded the heating area by connecting the cloth iron's element to a copper plate.

A mechanical mechanism called a thermostat was used to control the heating element of the original version of the hotplate.

The thermostat is an important component of an electric iron that regulates its temperature. When the iron reaches a certain temperature, the thermostat turns off the power of the iron. This mechanism consists of a bimetallic strip that is made from brass and iron. When the temperature of the iron exceeds a certain limit, the strip begins to bend towards the metal with a lower coefficient of expansion. As a result, the strip ceases to be physically connected to the contact point, the circuit opens, and the current ceases to flow.

We're going to use the heating element of Version 1 hotplate in Version 2, but with some major changes.

• First, the bolts holding the heating element to the wooden base are unscrewed.
• Next, we unplug the AC supply that was passing through the live and neutral pins of the thermostat and heating element, as well as the thermostat from the heating element.

To take temperature readings after removing the mechanical thermostat from the heating element, attach the probe of a K-type thermistor to the surface of the heating element. The closer the thermistor probe is placed to the heating element, the more accurate the measurement will be.

We create a mounting plate out of aluminum sheet that fits into the mounting holes on the existing thermostat. We then attach an aluminum block to the aluminum sheet to hold the thermocouple probe in place.

The thermistor probe can easily be tightened onto the aluminum block because the threads on this block are the same size, M6.

As for the base, we use a 340x280mm plywood board.

• Using a sharpie, we mark the location of the heating element plate's hole on the plywood after positioning it on one side.
• We follow the same procedure for the circuit and then use a 6mm drill bit to drill holes for mounting the heating element plate.
• For circuits, we use a 2.5mm drill bit.
• Using a 10mm spade bit, we drill countersink holes in the 6mm hole on the bottom side of the wooden board. To install the heating element plate, we will be adding four M5 bolts from the back; the bolt head will go into the 10mm drilled hole.

We use four M5 bolts and nuts to mount the heating element.

• To hold the four bolts in place, we first set them in the correct spots and tightened them with an M5 nut on each of them.
• Next, we place a nut on each bolt.
• The heating element plate was then installed on four M5 bolts; the nut that was placed earlier will keep the plate off the ground.
• We tighten the heating element plate in its position away from the wooden surface by adding four more M5 nuts, which will fully secure the assembly.
• Next, we use M3 screws to install the circuit onto the hardwood base. Four threaded inserts are used as spacers to maintain the circuit above ground.

We model and 3D print two wire holders in order to keep the wires securely fastened to the wooden base.

These were made from yellow glass PLA with a 0.4-mm nozzle and 20% infill.

• We tighten the wire holder with a base using two M3 screws after first securing the heating element's AC terminals.
• The AC main power cord is then secured, and the wire holder and base are fastened together using the same M3 screws.

The wiring followed next, and it was really simple. We started by connecting the K-type thermistor terminals to the CON2 screw terminal port that was supplied on the circuit.

The AC Heating Element Terminals are next connected to the CON2 screw terminal, and finally, the live and neutral AC Power Cord connections are made to the CON2 screw terminal of the circuit.

Additionally, the hotplate surface is connected to the ground port of the AC power cord, preventing electrocution in the event of a short circuit.

Please note that working with AC is no joke. Remember, prioritizing safety is crucial when working with AC supply systems. Taking precautions helps create a secure working environment and reduces the risk of accidents or injuries.

After the wiring, the hotplate is now complete.

For testing the hotplate, we have to prepare a SMD circuit for reflow.

• After applying solder paste to each component pad, we arranged each SMD component in its designated spot one by one.
• Next, we place the circuit on the newly finished SMT reflow hotplate surface.
• We put up an overhead PCB microscope to watch the solder paste melting process in order to get a close-up look at the solder reflow process.
• Solder paste melts, and components are soldered onto their pads as soon as the hotplate reaches the melting temperature.

After operating for a while, the hotplate switches off when it reaches its threshold temperature. A relay then disconnects the power to the heating element, and the hotplate turns off again when the temperature falls below the threshold voltage. This process repeats in a loop.

By doing this, you may avoid the overheating and burst of the heating element that was a problem with the previous version. The reconnect feature in the new version also helps to eliminate this issue, so you can expect this hotplate to live much longer.

The Hotplate Project is operating smoothly; the final touch is the addition of an ESD sheet to the side of the heating plate, which allows the user to place the freshly reflowed PCB there while it cools.

The back side of the ESD sheet is coated with wood adhesive, and the sheet is then attached to the wooden board.

The REFLOW HOTPLATE PROJECT is now complete.

As verified in the previous step, the hotplate is functioning as expected.

The cutoff feature can be adjusted in the code because, after a few uses, it is evident that the temperature of the TOP surface and the temperature probe aluminum block differ; that is, if the aluminum block's temperature is 150, the actual temperature may be 200. This significant difference in readings can be fixed by positioning the thermometer probe as close to the top surface as possible.

It is therefore necessary for us to machine a new top surface that will be sufficiently thick to allow us to mount the element from below and attach the thermistor from one side.

Although there will soon be a new concept, the project is now functioning and will continue to do so for the next three to four years.

Leave a comment if you need any help regarding this project. This is it for today, folks.

Thanks to Seeed Studio for supporting this project.

You guys can check them out if you need great PCB and stencil service for less cost and great quality.

And I'll be back with a new project pretty soon!