The things you can do with a Toaster Oven
After finally being able to source the Inductors from RS components in the UK I had thought that I was ready to go, as I had already placed an order for the solder paste. But I received a call from the rep and the particular brand and type I had selected was not available or recommended so I had to wait another 10 days until a better alternative was back in stock. So after having to wait another two weeks, yesterday evening UPS made the final delivery.
It is usually not a good idea to start new endeavors such as populating an SMD board with almost 50 components for the first time and reflow soldering in a hacked toaster oven for the first time at 6:00PM in the evening, after a full day of work with another one coming up the next day, but I had been almost giddy with anticipation and just could not help myself!
I re-stenciled the solder paste twice as I was not happy with my first two attempts. However, it turned out to be easier than I had thought. Then I started placing the first components on the board. Reflowed on the board the smallest resistors and capacitors look at least 50% bigger than they actually are. And while placing these was not easy and somewhat time consuming, the surface tension of the melted solder pulled these into the correct position. One of the big ICs fell onto its spot less than ideally positioned and the nicely stenciled solder paste spots were very smudged after repositioning the component. This did not look so good! But I was to curious about the reflow solder process, so I continued to populate the board.
When I finally was ready to place the thing in the modified toaster oven I fully expected it not to work. The reflow process is actually quite fast – somewhere around 4-6 minutes – for lead free ROHS compliant solder and I watched the process with eagle eyes.
For the first 3-4 minutes of pre-heating and soaking nothing happens visually, but when the systems goes into the reflow temperature zone one can see the solder getting shiny and flowing and you can see the small components reposition themselves. This looked surprisingly good and my outlook changed from No-way-Jose to “hey, this may actually work”
Reflowing this not so simple board with almost 50 components and several several fine pitch ICs worked great right the first time. I attribute that to the following three things:
- I did not get the cheapest available equipment but bought things that required little hacking and had proven to be successful. After all, I was not looking for yet another side project but an inexpensive but functioning solution. So I bought the reflow Oven Controller Shield and a Panasonic IR toaster oven that someone on the RocketScream forum had reported successful results with. I also made sure I bought the recommended K-Type thermocouple.
- I followed the set-up instructions for the Arduino Reflow Oven Controller Shield and attached to and placed the tip of the thermocouple on a PC board that I placed in the reflow oven with the board to be reflowed.
- I paid attention to the Reflow profile of the solder paste I chose (Amtech NC-560-LF). The the heating part of the profile did not need much attention. If the oven has enough wattage it will heat up quickly enough and the PID control characteristics of the reflow Shield followed it precisely. I had copied and pasted the serial output into a spreadsheed and saw that the cooling, however, was much slower than recommended. Anyone who has ever had material sciences in school/college knows that the cooling of molten metal is as important to the proper crystallization of the solder joint as the heating is for activating the flux (etc.) so I made sure that I opened the oven door to get to the correct cool down speed -2-4 degrees celsius.
But of course just because the board was nicely soldered still does not mean that it’ll work. And it did not in the first attempt as the power LED remained unlit. When placing the components I had switched the polarity of the one diode in the switched on-board power supply but this was an easy to detect mistake and because the diode was one of the bigger components it was easy to fix with a soldering iron.
So here it is, the first functioning prototype: