I thought this is all I need. I have used hot plates in the past (coffee cup warmer). I can attest that it is easy to overheat a circuit board sitting on the plate. I carefully measured with my heat probe while setting the control. I set it on a temperature just under 200 degrees C and did a test board. This is the LOW setting on the vernier knob. That went ok. Later I decided to do a couple other test boards. I put the temperature control back to the same setting and placed the boards on the plate. My failure was assuming that the temperature would return to the same setting. Wrong! Either the hot plate failed or something else happened. The boards got very hot very quickly and scorched the backside, ruining them. Maybe the hot plate failed, I will check it out.
I decided to make something that I can precisely control to assemble the surface mount components on circuit boards. I purchased from Ebay several items including this 6 x 6 x 1–1/2 inch aluminum plate for a thermal mass to sit on top of the hot plate.
A type K thermocouple temperature probe screws into the plate to sense the temperature.
A ramp and soak PID controller allows me to ramp up and down the temperature at a controlled rate and then soak for awhile before going to another set point. Up to 30 set points can be programmed. The whole idea is match the required temperature profile for SMD assembly.
The PID drives an external SSR or solid-state relay to switch the hot plate current on/off.
After I get all the components and test it out, I plan to devise a housing with internal circulating fan. The circuit board to be assembled will probably sit on a wire rack just above the thermal block.
I have concluded that the lowest cost solution to board assembly is to do it in-house. I plan to use metal solder paste templates with screening jigs to do the screening and assembly work for the project. I will also need a stereo microscope and manual x-y table to aid in placement of the fine pitch parts.
The ordered items should all arrive before the end of the year.
73 K5OOR – Virgil, HF Projects