After ruining about 20 bucks worth of resistors and caps on the backside of the Hermes board I decided to plot my "reflow" oven temperature profile. I used a K thermocouple to measure the temp as close to the PCB as possible. The solder melted at 172° (10 degrees "below" the spec) which means the probe "lagged" behind the actual solder paste temperature. However plotting the temp over time probably gives a good indication if the process is at least within the ballpark of the correct reflow profile.
Per First Principles of Solder Reflow by EFD, Inc
I decided not to worry about exactly matching the solder profile and just get it "close enough for government work"
The reflow oven is a $20 1 KW toaster from walmart. It has a heating element at the top and bottom. In the "Toast" mode both heaters are on and the overall rate is too fast, resulting in solder paste splatter and SMD components jumping off the board! With 50% power (Bake) only the bottom element is on, providing 500 Watts of heat. Simply turning on the oven, without fancy controllers, provided a pretty good approximation of the recommended reflow profile:
- turn the heater on
- wait until peak temperature (or about 30-40 seconds after the solder paste becomes liquid)
- turn off the heater and open the oven door.
Here is the recommended starting profile for the 4300 Chip Quik w/lead Water based No-Clean solder Paste, from SMT Intl, LLC
And this is a generic Walmart 1KW oven at half power:
The toaster heats up a little slower than optimal in the preheat and reflow zones, but keeps a nice <1C° ramp up time during the "soak". I hope this is good enough for home brewing. :)