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For a longtime now I have been an advocate of the hotplate method that was first introduced to me by the folks at Sparkfun for simple and easy reflowing of solderpaste. Through the use of our stencils this makes for a very simple and easy to use method of preparing SMD products for the marketplace. Trhough some of the research work I am currently performing it had become necessary to prepare our own hot plates. I developed through the guidance of doctek and MightyOhm a very simple “P” Temperature control using an Amicus development board and a solid state relay. Using some simple thermal transfer equations and a matlab script I was able to calculate that for a 8″x8″x1/2″ aluminum plate that I sourced from Metal Express for $32 with local pickup.
l=8; % inches
w=8; % inches
t=.5; % inches
W=500; % Element Size in Watts
l=l*2.54; % cm
w=w*2.54; % cm
t=t*2.54; % cm
v=l*w*t; % cm^3
d=2.7; % density of Al in grams/cm^3
m=v*d; % grams
m=m/1000; % kilograms
Cs=0.9; % Specific heat of AL in Joules/(grams * K)
Cs=Cs*1000; % Specific heat of AL in Joules/(Kg * K)
Tset=200; % degC = 400 F
Tamb=20; % deg C = 68 F
Qn=Cs*m*(Tset-Tamb); % Heat Flux required
eff=0.5; % Heat Transfer Efficiency (Initial 0.8 - Real 0.5)
t=Q/W; % Time to heat in seconds
t=t/60; % Time to heat in mins
Q = 458732.9148
Time in Mins = 15.2911
I chose to use a 500 Watt Ring Heater from McMaster-Carr for my design due to pricing. However, in the future I would recommend a 1000 W heater. Heat time is approximately 10-15 minutes and nearly matches the calculations from the heat transfer analysis.
After getting everything working with the Amicus development board I began to work on the “optimization” and “safety” aspects of this project. Luckily for me I live near Midwest Electronic Surplus in Fairborn, Ohio. This local stomping ground often has everything you need for that prototype project. Luckily, I found 1″ ceramic standoffs here for $0.50. These bad boys cost >$3 from Digikey. I was also able to source >880F wire here for $0.10/ft versus the $3.15/ft Digikey wanted for this stuff. After much debate I finally gave in and moved from the Amicus board to the $35 TET612 – PID controller you see in the images above. The manual is not very good but will get you started. Operation is dead simple and the auto tune PID is a breeze with this configuration. Actual control of the ring heater is through a 25A SSR. Temperature control is through a K-Type Thermocouple that I had here in the office.
This project is by no means complete as I have still to add an inline fuse, proper grounding, and a protective metal enclosure. I plan to update once I get these pieces up and running. Stay tuned for more details in the near future!