Toaster Oven to Reflow Oven – Part 2
Last time I left you with a video showing the control board running a toaster oven. Today, I’m going to start with a video showing a PCB getting reflowed in the oven. I will cover the changes I made after the video.
The first and most important of the changes, a new temperature sensor. The diodes used in the original design are only rated to 175c and wouldn’t work at the temperatures needed to reflow solder. After doing some searching online, I found what looked like a good and cheap replacement. It’s a 50K Ohm NTC thermistor part number 135-503LAF-J01. It’s in a glass package like a small through hole diode which worked well because it made mounting the thermistor much easier.
Speaking of mounting thermistor, it isn’t as easy as running some wires and soldering it in place. Wire that will work up to 300c is expensive and the solder will melt every time a board is reflowed. The solution I settled on, with some ideas from my roommate, was to drill a hole in the side of the oven, cover the hole with epoxy, and run some thin copper tubing through 2 holes drilled in the epoxy. The thermistor leads are then crimped into the copper tubing to make the final electrical connection.
The first step in mounting the thermistor was to drill a hole in the side of the toaster oven. I used a 1/2 inch drill bit but anything close will work. The only requirement is allowing enough room for the 2 small copper tubes to pass through without shorting. I left plenty of room on all sides and drilled a little above the top oven rack slot to allow the sensor to rest on top of the board that was being reflowed. Here is a photo of the hole I drilled.
I placed a bit of duct tape on the inside of the oven to help keep the epoxy in place. I tried to put the tape on weakly and used the blunt end of a drill bit through the hole to push the tape away a little so the epoxy would be able to flow under the tape a little. That would help keep the epoxy from coming loose later. Here is what the tape looks like inside the oven. As you can see it doesn’t have to be pretty.
Next, I placed the oven on its side so gravity would help pull the epoxy through the hole onto the tape. After looking up several epoxies I settled on JB Weld. It’s rated to work up to 300C and it’s easy to find. I wasn’t sparing and made sure to work the epoxy under the tape and completely cover the hole. Here is what the wet epoxy looks like.
Now, let the epoxy cure for 24 hours. Once the epoxy is cured remove the tape from the inside and drilled 2 small holes. The holes should be just big enough to allow the small copper tubing to fit through. I got the tubing at Ace Hardware, but other hardware store should have it. It came in a pack of 3 – one foot long pieces. Here is a photo of the tubing with a small diode for size reference.
And a photo of the holes in the epoxy.
I then pushed the tubing in until it was about halfway into the oven, placed the leads of the thermistor into the tubing and crimped it down with pliers. I cut the excess tubing off with cutters and soldered wires from the tubing to the control board. Polarity doesn’t matter with a thermistor. That took care of the hardware mods, you can see the thermistor resting on a PCB in the following photo.
It’s nothing fancy, but it works quite well. The tubing has a bit of spring and gently holds the thermsitor on the board for better thermal contact.
With the hardware modified, I needed to update the software to compute the temperature from the thermistor reading. I settled on using the Steinhart–Hart equation to compute the temperature. This meant the first step was to convert the reading from the A/D converter into ohms. This was actually very easy since it was just ohms law, R = E/I. E in this case is the A/D result times VCC (3.3 Volts) divided by the total counts of the A/D (1024 for the 10bit A/D). The current “I” in the equation is set at 55uA in the CTMU module. I had the PIC do the math using floating point since I was going to need floating point later for the Steinhart-Hart equation.
Now that the resistance of the thermistor is known, all I need to know was the A,B, and C constants to plug into the Steinhart-Hart equation. This is where I ran into a bit of a issue because there isn’t really a good datasheet for the thermistor I bought. If I was to do this again, I would make sure to buy a part with a good datasheet. Thankfully, the internet came to the rescue and I found an excel spreadsheet that calculates the three constants given temperature and resistance values at 3 points. If you do a search for Stein1.exe you will find the site that has the spreadsheet. I had to heat the oven up and measure the temperature and resistance at a couple different temperatures. Once I plugged the data into the spreadsheet I had the constants I needed.
With the constants and thermistor resistance known, it was simple to have the PIC calculate the temperature. The PIC does all the math, including calculating the natural log of the resistance. The Steinhart-Hart equation provides temperature in Kelvin, so the program converts to Celsius and, if a flag is set, converts to Fahrenheit. This number is fed into the PID control loop and controls the Triac output level. The floating point math uses about 10% of the code space in the pic, but there is still room left over for additional features.
While I was working on the code, I also added a new feature. The controller on power up checks to see what the line frequency is (50Hz or 60Hz) and automatically adjusts timings for Triac control. For use outside of the USA, the only change needed will be a transformer rated at the appropriate input voltage that provides around 9V AC out.
With the code updated, I had to give the oven a try and solder a part. I settled on soldering a 44 pin TQFP part, a CPLD in this case. I will put pictures below, but it went very well. There were a couple solder bridges but that was caused by to much solder paste. This was my first time using this paste and the oven but it was nothing a little solder wick couldn’t fix. A little less paste next time and it would have been good right out of the oven. I used a ROHS solder paste from Digikey.
Solder paste on the board.
Board cooking in the oven.
Fresh out of the oven, you can see a couple of solder bridges.
After cleaning up with solder wick. It was a success!
Need to clean up the residual flux with some IPA, but the solder looks good.
Next, I want to add more options for setting up the PID control loop to the user interface. Stay tuned for that update.