elson at pico-systems.com
Fri Aug 28 12:41:23 CDT 2015
On 08/28/2015 11:52 AM, Paul Koning wrote:
>> On Aug 28, 2015, at 11:44 AM, Mark J. Blair <nf6x at nf6x.net> wrote:
>> Well, I guess I'll need to build a little convection tape baker, then. Maybe I'll use something like an Arduino to control the temperature. I want to build a toaster oven solder reflow over, too, which might use the same controller. But operating at a much higher temperature, of course.
> If you can puzzle out German, you might find this article worth reading. It describes SMD reflow in a toaster oven, and some of the pitfalls. The main one is that the temperature sensor needs to be attached to the board, or to a (blank) copy of that board. If it's attached to something with a significantly different copper to etch ratio, it will be heated differently and your board will either not heat enough, or be overheated -- possibly enough to scorch it.
I have made thousands of boards in a large GE toaster oven.
I got a ramp-and-soak thermocouple controller on eBay. They
are usually $600-800, but I got it for a song. It allows
you to set temperature ramp rates and hold times. I ramp to
180 C, hold for one minute, than ramp to 230 C (for leaded
solder) or 245 C for lead free, hold for one minute and then
cool rapidly. The trick is to poke a tiny thermocouple into
a plated through hole in one of the boards. Then, the
controller is measuring actual temperature. I tried having
the thermocouple in the air, and the boards got seriously fried.
With good solder paste applied in the right amounts, the
process is totally amazing. I make my own solder stencils
using essentially PC board etching techniques. I make litho
film photomasters with a home-made laser photoplotter, then
use those to expose PC board dry film photoresist to 0.003"
brass shim stock. Develop the resist and etch in ferric
chloride. it is important to shrink the apertures so as not
to get too much solder paste on the chip's pads. The finer
the lead pitch, the more you have to shrink the apertures.
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