My soldering iron is dirt cheap, low power and pretty poor for soldering SMT components. After looking at the price of a more suitable replacement I decided that it was worth looking at the option of making an SMT Reflow oven from a mini/toaster oven, it would cost less than a decent temperature controlled iron and be fun to create.
I seem to be suffering from several of the symptoms of lead poisoning already, so use lead-free solder and want to continue, which means a higher temperature is required.
Googling for lead-free solder reflow temperature profiles gives lots of results from both solder and parts manufacturers (Altera, Wolfson Micro, Lattice Semiconductor, and Kester), and the 'standard': IPC/JEDEC J-STD-020D.1. These suggest that a max temperature of between 235-255Â°C is required and the ability to ramp up the temperature towards the max to reduce the time at the high temperatures (30 seconds within 5% of the max).
This is tough for a mini oven, many are advertised with a maximum of 220Â°C. However, some experiments recording the temperature of my parents cheap Hinari Tiny Top with a thermocouple attached to a multimeter, showed that it could get up to over 255Â°C (the ovens thermostat seems to cut out at around 260Â°C - less than the rated 280Â°C) despite only using 630 Watts. The only caveat is that as it approached the maximum it could only raise the temperatures by about 0.5-1.0Â°C/Second. However, the oven was also loosing a lot of heat, particularly from the top, so some insulation may help things.
I considered some of the more powerful ovens (1300-2500W), in particular one with a fan would have been good to regulate the temperature around the oven, but they have correspondingly larger spaces to heat - 10-24 litres - compared with the smaller 6.5 litres. I decided to go with a Hinari HTP033:
- It has two heating elements which allows for more slightly more fine grained temperature control (rather than on/off of a single element)
- Has a transparent door allowing the reflow process to be monitored
- The housing looks easily to insulate and modifiable
- Â£20 delivered from ebay (Amazon were out of stock)
- Insulate Mini Oven.
- Thermocouple to monitor temperature.
- Microcontroller monitoring the temperature and controlling the heater elements.
- USB interface for control/uploading and downloading temperature profiles.
As stated above - if the ovens heating elements aren't quick enough, then using a microcontroller to control the temperature is redundant overkill. Its just enough to turn on the oven, put the board in when the temperature gets high enough, then turn off and open the door when the maximum temperature is achieved. If this was the case the backup plan was to just add an LCD screen showing the current temperature and use the oven manually.
I thought that measuring the temperature would be simply a matter of connecting a thermocouple (which creates a voltage difference based on the temperature difference between the two metals in its probe) to an Analogue to Digital Converter in some configuration and reading off the temperature value, but it appears to be a fairly non trivial task. Luckily there are a number of off the shelf integrated circuits that the thermocouple can be connected to directly and produce a digital temperature reading. I choose the MAX6675, which provides 0.25Â°C resolution from 0-1023Â°C on a simple to use SPI output in an 8 pin SOIC.
To get temperatures from the oven a Teensy at90usb162 board was used to poll the temperature every second and send it to the USB, where it was copied into a spreadsheet.
Chart below shows two test runs of the oven with both elements on (I turned off the oven earlier on the second run).
Surprisingly it got up to to well over 300Â°C, before I cut the power, with no sign of a thermostat limiter. The temperature rises at roughly 0.9Â°C a second to 200Â°C then slows down to about 0.5Â°C/Sec. to 250Â°C. Although the slow down towards the peak is disappointing - this is probably just about usable although it would be better to reduce the time spent near the maximum. The amount of heat escaping from the oven is fairly excessive - the temperature of the top of the oven was approaching 100Â°C so some insulation will help to increase the gradient.
After some research I chose to use Reflect-A-Gold sheet for insulation, this aerospace/automotive heat insulation isn't the cheapest available but was fairly easy to use and claims to reflect 80% of radiant heat up to 850Â°F.
The HTP033 is made of riveted sheet metal with a slightly thicker back plate to keep it rigid, after drilling out 4 rivets the oven basically falls apart. The side and top comes away from the internals of the oven allowing it to be just about completely wrapped with the Reflect-A-Gold sheet. Most of the oven had two layers of sheet, with several layers around the edges of the door to increase the closeness of the fit (without there was a 5mm gap with the door closed).
Results after insulation are are shown below, the oven was turned off as close as possible to 245 Â°Centigrade.
After insulation the oven temperature increases by about 1Â°C/S. to 200Â°C and 0.75Â°C/S. after that (Compared with 0.9Â°C/S. and 0.5Â°C/S. respectively for the uninsulated oven). Temperature inside the case behind the ovens controls went up to about 50-60 Â°C - perhaps still a little too hot to house the electronics.
Conclusions and Future Work
I'm fairly happy with the performance of the oven, considering its cost and size. Next steps are:
- More temperature tests to see if the performance can be increased.
- Do some test 'reflows' - just turning on the oven and turning off when it reaches the maximum temperature to see what temperature is required by the solder and how good the results are.
- Develop a controller/relay board and investigate controlling the heating elements under processor control.
- Other projects: "Toaster oven SMD", "SMD ReflowToaster Oven", "Easy Reflow", "Have you seen my new soldering Iron?", "Reflow Skillet". And products: "techFX reflow 2.0 controller with PID control!", "Artic Reflow Oven", "GF C2 Reflow Oven".
- Various reflow temperature profiles: Wolfson Micro, Lattice Semiconductor, and Kester R206 Solder Paste.
- IPC/JEDEC J-STD-020D.1 Moisture/Reflow Sensitivity Classification for Nonhermetic Solid State Surface Mount Devices
- Taking the Pain Out of Pb-free Reflow, Lead Free Magazine
- MAX6675 K-type Thermocouple-to-Digital Converter
- Reflect-A-Gold Heat insulation sheet and tape