Homebrew PCBs

--- In Homebrew_PCBs@yahoogroups.com, "timbomcnuckle" <jaydag71@...>
wrote:
> Elektor did a great version of the
> taoster oven re-flow controlled with an AVR I think, lcd display,
> programmable curves, precision control (within reason),, I think the
> only complaint was lack of insulation, but you can always stuff with
> rockwool or something.
> 
>  I have used old style black&decker toaster-oven without any
> modification a few times but I got a better one now that I still need
> to finish. gluk.
I got a "GE" toaster oven from Wal-Mart for doing reflow.  After some
fooling around and toasting a couple boards but good, I figured out
that by putting the thermocouple into a PTH in the board, i'd be
monitoring ACTUAL board temperature, not the air temp.  The color of
the boards (or their IR absorbance, really) makes them get hotter than
the surrounding air.  This has worked out really well, and I know that
at least the couple aquare inches around the thermocouple are really
getting the exact temp profile I want.  The oven is not totally
uniform, but it really does quite well.  I'm using Kester SAC305
paste, and had been using this profile:
1.  ramp over 3 minutes to 185 C
2.  soak at 185 C for 1 minute
3.  ramp in 2 minutes to 235 C
4.  soak 1 minute at 235 C
5.  cool to 25 C in 5 minutes

I later upped the final temp to 238 C to help with the soldering in
the coolest spot of the oven.

The cooldown actually happens much slower, and at 100 C actual temp I
open the oven door.  The temperature control is quite good, usually
holding within 5 C of the setpoint, and getting closer as it get
hotter, where it really matters.

I bought an Omega 1/16th DIN ramp and soak temp controller on eBay,
and also got 1000 feet of #30 thermocouple wire with FEP insulation
from another eBay auction.  Aside from having to poke the thermocouple
into a hole in the board, this is working like a dream!

Jon