assembly of printed circuit boards by reflow.
In this article, another homemade master proposes to consider the installation of printed circuit boards using iron parts.
Tools and materials:
-Sole of the iron with a heating element 700 W-1000 W;
-Copper plate 150 mm x 250 mm x 2.2 mm;
-Plywood 460mm x 380mm x 20mm;
-M4 screws, length 50 mm; -M4 nuts;
-Anti-static mat 145 mm x 345 mm; -Printing board; -OLED display SSD1306; -Atmega328PU; -Crystal 16 MHz; – Ceramic capacitor 22pF 0603;
-Resistor 10K 0805;
-Temperature resistor 10K; -3D-printer; -Component for pouring (epoxy resin and hardener); – Wires;
-Printing board for reflow;
Step one: about paste and thermostat
The melting point of solder paste depends on the percentage of flux and the ratio of Sn-Pb (tin/lead). The wizard uses solder paste, in which the ratio of tin to lead is 63/37.
The heating element is powered directly from an AC power source. The master does not put a relay in the circuit to turn off the power or control. The fact is that the iron already has a thermostat.
The thermostat is an essential component of an electric iron. With the help of a thermostat, the temperature of its heating is controlled. When the soleplate of the iron reaches a certain temperature, it cuts off the power to the iron. A mechanical thermostat was installed on old irons. The mechanism consists of a bimetallic plate, which is made of two metals of different thermal conductivity. When the temperature of one of the metals exceeds a certain limit, the plate begins to bend towards the metal with a lower coefficient of expansion. As a result, the strip ceases to physically connect to the contact point and the circuit is opened.
The point is that the iron already has a power cut circuit that controls the heating temperature, you only need a temperature control circuit that displays the exact temperature of the copper plate (installed on top of the sole iron).
The master will use an Atmega328PU and a thermistor. The temperature will be displayed on the SSD1306 OLED display.
Step two: temperature sensor The printed circuit board, which the master uses, was from another project.
The master ordered the printed circuit board from the corresponding service. After its manufacture, he proceeded to the installation according to the diagram. I installed the necessary SMD components.
There is no room for an OLED display on this PCB, so it needs some work. The wizard adds pins VCC, GND, A4 and A5 to the board.
Then he installed the rest of the components.
Next, you need to flash the Atmega328PU and load the code into it. The code can be downloaded below.
The end result is a homemade temperature sensor that displays temperature in real time.
Step three: heating element
You can place the PCB directly on the soleplate of the iron and melt the PCB without any problems. The problem is that the surface area of the iron is small for reflowing large PCBs. To increase the area, the master bought a copper plate measuring 150 mm x 250 mm x 2.2 mm.
The heat transfer rate of copper is quite high. He drilled the mounting holes and screwed the plate to the sole of the iron.
There are seven holes in total, three for attaching the plate to the iron and four for attaching the entire structure to the board.
Step four: base
For the base, the master uses 460 mm x 380 mm x 20 plywood mm.
The master drilled the mounting holes in the plywood. Then I painted it and attached a heat-resistant mat to it.
After the circuit board has melted, the board can still be hot on the mat.
He used rubber glue to fix the mat.
The holder for the electrical cord, the temperature control board and the power supply for the board are also fixed on the base.
Step Five: Power Adapter
The board needs 5V DC to power. The master uses a ready-made module to convert 220 V AC to 5 V DC. To install it on a 3D printer, I printed the case. Then I placed the module in the case and covered it with epoxy.
The file for printing the case can be downloaded below.
Step Six: Reflow Process
Reflow soldering is the most widely used method of attaching SMD components to printed circuit boards. The goal of the process is to form solder joints by preheating the components/PCB/solder paste and then melting the solder.
The first step here is to apply solder paste to each pad.
The ideal method for this is to use a stencil. If you order the production of a board on one of the services, you can also order a stencil on it.
It is also convenient to apply the paste using a dispenser. But here you need to use a paste that has a liquid consistency.
Then you need to place the components on the intended places. It is convenient to do this with tweezers.
After all the components are in their places, you need to place the board on the heating plate. Then turn on the device and wait a few minutes.
After a few minutes, as the temperature rises, the PCB starts to heat up, and when the temperature reaches a peak of 270 ° C, the solder paste will completely melt (different pastes have different melting points).
According to the master, the device works well overall. The temperature of the heating element reaches 300 ° C, which makes it possible to use it for reflowing even metal printed circuit boards.
But there are also disadvantages. Temperature control only works up to 270 ° C. The master believes that this is due to the use of a 10K thermistor. In his opinion, it is necessary to replace the element with another one with a value of 100K.
You can watch the video for more details about this item.
assembly of printed circuit boards by reflow.