DIY SMD Reflow Oven (cont.)
Before I had even started on this project I had decided to use the open source Reflow Master temperature profile controller designed by Seon Rozenblum. If you look for available controllers you will find a few that have come and gone in relatively short production runs. The above unit is (or was) still available and has a nice color display that shows both the target profile temperature curve and the current temperature progress against it. It has four built-in temperature profiles for some common leaded and lead-free pastes, and more can be added if you get into the code. It also has provision for an exhaust fan that can be switched on as a cooling aid after the reflow peak, helpful because the oven cool time is much longer than the temperature profile demands. I chose not to use this, however, because an exhaust fan does nothing if there is no air intake, and if I have to be present to open the door for that intake, I don't really need the fan.
One of the other available temperature controllers out there can trigger a standard RC servo to prop the door open at the appropriate time, but it's still best to monitor things in person for the whole five minutes or so that it takes to reflow a board.
The space on the control end of the oven was limited, so I knew better than to try to fit this controller inside the oven body. The solid state relays (SSRs), their heat sink, and a 5v power supply would be located inside the oven and the controller and display would be housed in a separate box (I haven't started on this yet). A multi-pin connector on the back of the oven provides the connection port for the controller.
Even though this particular controller does not have separate outputs for the top and bottom heating elements, I decided to use separate SSRs for the top and bottom with separate toggle switches on the front panel so that I could manually preheat the oven or override the controller if the temperature for any reason lagged the desired profile. The SSRs require heatsinking, so I machined a plate from 3/8" thick aluminum that would fit the space and also provide a mounting surface for terminal blocks and the power supply. This plate had to fit around some obstacles (note the cutouts for the convection motor and the bell).
The only original control that I used was the timer switch. A new front panel was made from stainless steel and a small fan was put in the bottom to push air over the heat sink and out the bottom and rear vents.
The choice of solid state relays wasn't trivial. The most commonly available brand on eBay is Fotek, which is a real manufacturer of quality relays, but as far as I can tell nearly all on eBay are Chinese clones that have a habit of failing, often in the shorted state. As it turns out, post-mortem analysis by a few inquisitive users has shown that some of the clones use severely underrated triacs - in one case someone found the clone SSR-40 amp relay board populated with a 6 amp triac. Fortunately I found these Panasonic relays from Mouser for about $14 each.
I was now able to fire up the oven for the first time to get some idea of the heating and cooling rates and to see if it could keep up with the controller profile. With all of the elements turned on, it could generally reach the peak temperature within the required number of seconds, but keeping up with those sections demanding a higher-slope temperature rise would be difficult. Plus there was a lot of radiant heat escaping through the front glass, so I applied a sheet of polished aluminum with a viewing window to the inside of the glass to help reflect most of the energy before it even reached the glass. Here is the inside of the oven showing the clip-on thermocouple and the reflective panel on the door:
This was a huge help for increasing the rate of the temperature rise, but it presented a new problem. The front glass was much cooler, except in the area of the window, where it was still very hot. This could have exploded the glass in my face, but fortunately it held together and I was able to make a couple of sliding shutters (also polished aluminum) that can be left closed until the crucial moment where you want to view the reflow action, and then closed again after that.
As a final test before buttoning up the oven I timed the heating rates and found that I could go from room temperature to 185 Celsius in 145 seconds, which should be able to keep up with the profiles. Some people have lined the inside of their ovens with a very expensive adhesive gold film (gold is highly reflective at infrared wavelengths) which improves both the heating and cooling rates. And it looks impressive as well. But I thought I would try going without at first because it does add probably over $100 just for the material.
Now I have to finish an enclosure for the Reflow Master controller. I'm thinking it would be a good idea to put a standard PID temperature controller in the same box so I could use the oven for other purposes as well (powder coat baking, drying). I also really need to go back and put some type of hardware temperature limit switch inside the oven too. With the manual override switches on the front panel I don't need to have a China syndrome in my basement if I should forget and walk away with the heating elements left on...