Building a Surface Mount Oven
Supplies Needed and Approximate Cost
1Any convection oven should work. The oven we chose was the Sanyo SK-VF7S Digital Convection Oven, Stainless Steel Finish.
2The temperature controller we chose was the CN1501-K.
Remember from basic physics that there are three main types of heating: Conductive, convective, and radiative. We need to change the properties of the oven you bought to balance these three types of heating.
Maximizing Convective Heating
Minimizing Radiative Heating
Most ovens are stainless steel or aluminum inside the cavity. This is an almost perfect reflector for heat. Since the reflectivity of solder and the copper traces on the board is very high, and the reflectivity of the components (particularly black encapsulated IC's) is very low, the components will get hot much more quickly than the board which is bad. To fix this we make sure there is not direct line of sight from a heating element to the PC board.
One disadvantage of the black paint is that the sides of your oven are going to get a lot hotter than they normally would. Make sure when you operating you don't touch the oven!
To accomplish conductive heating, an aluminum plate 1/16'' thick replaced the oven rack. This surface is where the PCB will be placed. The aluminum plate was painted black in order to reduce the reflection of heat and make sure it got hot as quickly as possible.
Putting it all together
Rewiring the heating elements
The side panel of the oven will no longer be used. Instead, the heat elements will be controlled by the temperature controller.With a soldering iron, connect one wire from the heating elements to the hot wire (black) of the power cord and the other to the neutral (white) wire.. The elements should be wired in parallel, not series.
Connect the ground wire from the power cord to the case for safety.
This power cord will then plug into the temperature controller which is connected according to the diagram below.
There will be two cords coming from the oven. One is to control the heating elements, the other operates the fan. You want the fan to run continuously, even when no power is applied to the heating elements.
Use the diagram above to wire the temperature controller and thermocouple.
It is important to mount the thermocouple DIRECTLY onto the surface on which the PCBs will rest. Not doing so will result in the thermocouple's measurement of ambient temperature and not the surface temperature of the PCB. Run the wire out through a hole in the oven and provide some strain relief.
Programming the Temperature ControllerWe use the manual from the temperature controller to program the ramp and soak profile. The specific values for the ramp and soak profile we developed can be seen in the table below.
Does it Work?
Yes, very well. The photos below show boards we milled using our T-Tech PCB mill and ran through our home-made oven
Other SMT Oven Projects
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