Objective: This post presents the assembly of Surface Mount Components (SMT) on a 4-layer board using a cheap toaster oven. It aims to show that the toaster is sufficient to achieve remarkable results for DIY PCB boards.
Recommended Prerequisite articles to read
Some time back, I decided to explore the oven-based PCB assembly which had been something always wanted to do. Having read of its success in various DIY blog posts, I was convinced it was a near-perfect cheap solution for personal PCB projects.
However, most of the blog posts included adding some electronics to achieve the desired solder reflow thermal profile. Such a profile ensures that all the different components on the board are properly soldered without damaging them via overheating. Hence you need a way of controlling the temperature (heat) inside the oven by navigating through the four typical temperature zones of a thermal profile that include: preheat, soak, reflow and cooling.
I did not want to focus much on the temperature control part since my aim was to simply try out the oven to see if it could actually work on my boards. Hence my temperature control was purely manual.
I got a used oven from a recycle shop at less than $20. Since I was not going to use any additional thermal controllers, the oven is all I needed for my PCB assembly. It was pretty old, but adequate since it was solely for PCB work.
PCB and Components
The board I was working on was one that I needed for my personal projects.It is mainly for power supply and some light computing. Hence it has various DC buck and boost converters as well as an STM32 microcontroller. It is a 4-layer board and all the components, apart from connectors, are surface mount.
After designing the board, I sent the gerber files to JLCPCB for fabrication. They are extremely cheap, even for 4-layer boards. Most importantly, I had got positive reviews about their services and I was not disappointed either. For the components, I got from various distributors but mostly from their sister company, LCSC Electronics. At the moment I can not guarantee on the quality of the electronic components as they were extremely cheap. Nevertheless, all of those used on my board have been functioning as expected. Other components were ordered from Mouser and Digikey. Within two weeks, I had received the boards as well as the components without any major hiccups. A minimum of 5 boards was ordered.
PCB Preparation for ‘baking’
During the PCB fabrication, I had ordered a stencil as well. This is very important in SMT reflows.
The stencil enables easy application of the solder paste to the component pads without spreading all over the board. More so, it ensures that all the pads have solder paste since visual inspection is made easier.
The board needs to be fixed firmly on the table and then the solder paste is applied as shown here. A firm card such as expired credit card can be used to spread the paste evenly.
After the solder paste has been applied, the surface mount components can then be placed easily.
Testing the process
Since this was going to be the first time to use this oven for the soldering, I wasn’t certain of the heat it generated and how long the PCB needed to be heated. I didn’t have any thermal controller; hence it was bound to be a trial and error approach. I placed a few components on the board including the STM32 microcontroller to test out the heating process.
Within a few minutes (I didn’t record the time), the PCB was up in the legendary smoke. Prior to that, I had observed the solder paste melt in a glowing silver color. I should have switched off the oven at that point and opened the lid. Failure to do so in time, led to overheating and the board was burnt as shown here:
Getting it right
In the next trial, I was determined to get it right. Once the solder paste started melting, I switched off the oven and allowed the latent heat to finish off the heating process. I then opened the lid to allow (faster) cooling.
This was successful and all the SMT components on the top side were successfully soldered in one go.
Soldering Bottom Side
The bottom side was mainly inductors and some other large size components. I was not willing to risk putting back the board to the oven and decided to use a hot-air gun to finish on the few components on the bottom side.
The connectors were hand-soldered easily as they are through-hole. The final board is as shown here.
Testing the Board
The board was successfully tested, and all the functionalities were met including programming of the STM32.