Video
Material Required
Little about Solder Paste
Basic Setup
Preparing the Temperature sensor Setup
Construction Part of Hotplate Element
Epoxy Potting Process
SMD Process and PCB Reflow Process
Conclusion
Limitation and Flaws, also Improvement

Published April 11, 2021 © GPL3+

DIY SMT Hotplate Project

Minimal SMT Hotplate from Cloth Iron's Element, OLED Displays the temperature of Iron via a 10K thermistor, pretty simple stuff yo

BeginnerFull instructions provided4 hours2,008

Things used in this project

Hardware components

JLCPCB Customized PCB

Microchip ATmega328

oled display SSD1306

Software apps and online services

Arduino IDE

Story

Hey Everyone what's up!

So this is my "DIY SMT Reflow Iron" made from Cloth Iron Element screwed onto a thick copper plate to transfer heat for achieving the desired heating which would be the correct melting temperature of Solder paste (which is in my case 270°C)

The goal for making this DIY SMT Reflow was simple, it's easy and cheaper to make a working SMT Reflow Hotplate instead of buying a proper hotplate.

Video-

Material Required

Iron's Element 700W-1000W
Copper plate 150mm x 250mm x 2.2mm
Wooden Board 460mm x 380mm x 20mm
M4 Screws 50mm Length
M4 Bolts
AC Cord
ESD Mat 145mm x 345mm

The rest of the parts are for Preparing the Temperature sensor.

PCB (Which was Provided by JLCPCB)
OLED Display SSD1306
Atmega328PU
16MHz Crystal
22pF Cap 0603 Package
10K Resistance 0805
NTC 10K
5V 1A Charger Circuit
3D Printed Enclosure/BOX
Potting Compound (Epoxy and Hardener)
Teflon wires RED, Black, White
PCB for Reflow
Solder paste
Arduino as ISP Programmer setup

Little about Solder Paste

The Melting Temperature of solder paste depends on the Flux percentage and the ratio of Sn-Pb which in my case is 63/37, Sn63-Pb37.

Sn-Pb solder has melting points from 140 to 270 °C higher.

Note- I bought this Solderpaste without checking the Melting temperature. Solder paste with a lower melting temperature should be used. (170-200°C)

Basic Setup

Here's the Block Diagram of this Hotplate Project!

The hotplate is powered Directly from the AC Supply, no Relay has been added to the hotplate line for power cutoff or Control. The reason for this is pretty simple, adding Relay to this setup means we need to monitor the temperature and cut the supply of Hotplate whenever the hotplate reaches a certain temperature and then reconnect when hotplate temperature goes down.

On the Iron's Element, there's a mechanical part in iron which is the thermostat.

The thermostat is an important component of an electric iron that regulates its temperature, when Iron reaches a certain Temperature, the Thermostat turns off the power of 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.

you should definitely check out this great article about the working process of iron from here.

So the bottom line here is that the iron already has a Power Cut Off circuit that controls the temperature of Iron, we only require a Temperature monitoring setup that displays the accurate temperature of the Copper surface.

For measuring Temperatures higher than 150°C, I Prepared an NTC Based Atmega328PU Setup which displays the Hotplate Temperature on an SSD1306 OLED Display.

Preparing the Temperature sensor Setup

In Order to Make this temperature Sensor Setup, I used a 10K NTC along with an Arduino Nano to set up a basic Temperature Sensor setup by following this Guide.

Setting up NTC is pretty easy, we just need to add NTC with the same Value resistance (in my case 10K) in series with the NTC according to the given schematic.

After Finalizing the Breadboard version, I prepared a Minimal Atmega328PU Setup by using a PCB from a different Project.

PCB that I'm using here was from a Different project which was this-

https://www.hackster.io/Arnov_Sharma_makes/arduino-synth-v2-8b9dd3

I prepared the PCB in a PCB designing software and then gave this PCB to JLCPCB for samples

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I first added the necessary SMD components to the PCB which were mentioned in the schematic above.
Now, the PCB that I'm using doesn't have a breakout point for OLED Display, so I modified the PCB a little bit by cutting the copper traces of Four connectors and then I added VCC, GND, A4, and A5 Pins separately to that connector by jumper wire.
after that, I added THT Components like the IC and burge strip.
Next, I removed the OLED from the previously made breadboard setup and connected it to the current setup along with the 10K NTC.
After doing that, the Last Step is to Flash Bootloader in the Atmega328PU and upload the code to it and for that, I used my "Arduino as ISP Programmer Board" which is an Arduino Nano flashed with Arduino as ISP sketch. by connecting the SPI Pins of the ISP Programmer with any target Microcontroller's SPI Pins, we can flash the Microcontroller (you can learn more about that from here in detail), I first burned the bootloader of Atmega328PU and the Uploaded the Main sketch to it.
The end result was this DIY Temperature sensor that displays the Real-time Temperature measured by the NTC.

Construction Part of Hotplate Element

The Main Part of this Project is the Element Setup. We can put PCB Directly onto the Iron's Surface and it will reflow the PCB without any issue but the iron surface area is not enough to reflow large PCBs, to increase the Reflow Surface, I bought a Copperplate of size 150mm x 250mm x 2.2mm.

My plan here is to add the copper plate onto the Iron's Element. The copper heat transfer rate is pretty great and it will be perfect for this project.

I Drilled Mounting holes in Iron's Element and copper plate, with M3 Screws I secure both of them and made a huge Heating Plate setup.

Three Holes were for Securing Iron's Element with Copperplate, and Four holes were for adding the copper plate onto the wooden base.

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Preparingthe Base

As for the Base, I've used a wooden board of size 460mm x 380mm x 20mm.

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I've Drilled Mounting holes in the wooden base according to the Hotplate holes and then used M4 Screws, Washers, and Bolt to hold the Copperplate in Mid Air away from the wooden surface.

But Before Screwing them all together, I painted the wooden board and added an ESD Mat on it.

Adding ESD Mat here is not necessary, I added this because it's heat resistive and after Reflowing any PCB, we can place that PCB (which is still HOT) on the Mat, and the mat won't melt.

I used Rubber Adhesive to attach ESD Mat with the wooden base.
At Last, I Added an AC Connector for connecting the Hotplate AC Supply with AC Cord. to keep AC Cord in place, I added a plastic part that holds the cord in place.
After setting the Hotplate on Base, I added the Temperature sensor Setup on Base with Screws.
Now, to power the NTC Setup, we required 5V and this setup runs on AC Supply.

To get DC from the AC Supply, I took a 5V 1 A Charger Circuit and designed a Custom Body for it.

I 3D printed the Body with White PLA and then prepared an Epoxy Mixture for potting the whole box in order to make a DIY Isolated 5V Power Supply!

Epoxy Potting Process

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The potting process begins when the substrate is placed inside the pot. A liquid compound is then poured into the pot, filling it and covering the device completely. The liquid hardens, encasing the device inside it (in our case, an AC-DC Charger Circuit)

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I Pour Epoxy and Hardener into Two different paper cups (each 30ml) and then mixed both of them together.

Epoxy Resign stays liquid but when the hardener is added, its composition starts to change and it solidifies.

After mixing Epoxy and Hardener together, I placed the charger circuit inside its 3D Printed Body and pour the mixture into it.

It solidifies completely after few hours and the end result was this HARD and ISOLATED 5V Power Supply.

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I connected this potted power supply with the Temperature sensor setup and with AC Supply according to this schematic and our setup was basically completed!

SMD Process and PCB Reflow Process

Reflow soldering is the most widely used method of attaching surface mount components to printed circuit boards (PCBs). The aim of the process is to form acceptable solder joints by first pre-heating the components/PCB/solder paste and then melting the solder without causing damage by overheating.

For the final checking of this process which is to Reflow some PCBs on it, I will be using these two PCBs which are from my two previous projects!

1. Attiny85 Game Boy

2. Nanoleaf Project

I designed the PCBs in OrCad cadence and send them to JLCPCB for samples.

you can check them out from here if you want to get great PCB service for a low price. https://jlcpcb.com/PCH

The first step here is to add solder paste on each pad one by one with a proper solder paste dispensing method.
I used a Broken tweezer to scrape little solder paste from the solder paste box, I have to add solder paste to each component pad one by one, this time it's easy as this PCB required few SMD components.

An ideal method for this is to use Stencil which you could also get from JLCPCB, another method is to use a decent solder paste dispenser which required a different type of solder paste that has a lot of flux in it. flux makes the solder paste less hard. (In my case, solder paste is pretty hard)

Next, we have to place components in their assigned place one by one with help of a sharp tweezer. I'm using 10K ohms resistance of Package 0805 here along with a 1K Resistance of package 0603 and an indicator LED with the same package, also an A7 Diode.
Now, we need to lift this PCB and Place it onto the hotplate. Turn on the Hotplate and wait for few mins.
After Few minutes, the PCB starts heating up as the temperature increases and when the temperature hits that peak of 270°C, solder paste melts completely.

Here's a Closeup shot-

I Did the same process with Nanoleaf PCB,

I Added solder paste on LED Pads one by one which really took time and effort but after the hellish work of adding solder paste to each pad, I place SMD WS2812B LEDs on their assigned location and put the whole PCB on the Hotplate for the reflow process and Result was pretty nice.

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Conclusion

Hotplate Did actually work, and it's pretty usable for reflowing PCBs for Projects.

It can even reach up to 300°C which can be used to reflow even Metal PCBs, overall I'm happy with the result.

Limitation and Flaws, also Improvement

This Hotplate is working Properly however, its temperature sensor setup didn't seem to work after when Hotplate reaches 270°C.

This might be because I'm using a 10K NTC. 100K NTC should be used here which I already have ordered.

This Hotplate currently requires a reliable Temperature sensor setup, if the DIY Temperature sensor won't work, a proper Panel Temperature sensor module can be used here which costs around 15-20$ but it can display accurate temperature readings even at higher temperatures.

Anyways, for Part 1 this is it.

I will make the necessary changes in this Hotplate project and make a part 2 of this project so stay tuned for that!

Leave a comment if you need any help, peace.

Schematics

Code

OLED Temperature display

#include <Wire.h>
#include <Adafruit_GFX.h>
#include <Adafruit_SSD1306.h>

#define SCREEN_WIDTH 128 // OLED display width, in pixels
#define SCREEN_HEIGHT 64 // OLED display height, in pixels


Adafruit_SSD1306 display(SCREEN_WIDTH, SCREEN_HEIGHT, &Wire, -1);

int ThermistorPin = A0;
int Vo;
float R1 = 18000;
float logR2, R2, T, Tc;
float c1 = 1.009249522e-03, c2 = 2.378405444e-04, c3 = 2.019202697e-07;


void setup(){
  display.begin(SSD1306_SWITCHCAPVCC,0x3C);
  display.clearDisplay();
  Serial.begin(9600);
}

void loop(){
  Vo = analogRead(ThermistorPin);
  R2 = R1 * (1023.0 / (float)Vo - 1.0);
  logR2 = log(R2);
  T = (1.0 / (c1 + c2*logR2 + c3*logR2*logR2*logR2));
  Tc = T - 273.15;
 Serial.println(Tc);
  display.clearDisplay();
  display.setTextSize(3);
  display.setTextColor(WHITE);
  display.setCursor(0, 40);
  display.println("C");
  display.display();
  display.setTextSize(4);
  display.setTextColor(WHITE);
  display.setCursor(3, 0);
  display.println(Tc);
  display.display();
  delay(1);



}

Credits

Arnov Sharma

267 projects • 273 followers

Just your average MAKER

DIY SMT Hotplate Project