[hpsdr] SMT Soldering and Circuit Board Heat Question

[David Toepfer]

QUESTION:

Since you say the point is to head the board up to 100C (212 deg F), can't one
simply stick the board in a sealed bag (suck out most air you can) and drop it
in a pot of boioling water?  Boiling water never get's hotter than 100C.  Some
simple trial/error should be able to determine how long of a boil boards of
specific material and size.  Seems cheaper.  No hot plates, ovens, or
thermometers to buy either.

dt
.

--- Bob Fish K6GGO <rwfish at comcast.net> wrote:

> ***** High Performance Software Defined Radio Discussion List *****
> 
> Hi Bruce,
> The hot plate is only to bring the board and the parts on it up to an 
> idle temperature of about 100deg C. (212 Deg F). No hotter than that. 
> The board and parts can stay at that temp all day without damage. The 
> temperature profile that the manufacteurs use in the big reflow ovens 
> goes like this; the temperature ramps up to 100 degrees C (your hot 
> plate temperature) and stays there for 3 or 4 minutes. This is to 
> achieve thermal stability and to let the solder paste outgas a little 
> bit, then it continues to ramp up to around 200 degrees C (this is where 
> the solder melts) but does not stay there, it imediately cools down to 
> room temperature.
> To simulate this profile with the tools you have, all you have to do is 
> put the solder paste on the pads where the parts go and this is the big 
> secret to success: use way less paste than you think you will need. On 
> the fine pitch parts all you really want is a sweat joint, other wise 
> they will bulge over and short to the adjacent pin. After you place the 
> parts in the paste, place the board on the hot plate and bring it up to 
> 100C (220F) let it soak there for 3 or 4 minutes, then with the board 
> still on the hot plate at 100C use the heat gun on the board with a 
> waving motion until the solder paste turns from dull gray to a shiny 
> silver in color. It may not happen to the whole board at once, 
> especially if it is a large board, but once it does your done. Turn off 
> the hot plate and heat gun and let the whole works cool down to room 
> temperature. The higher teperature is where damage could occur if left 
> there for too long. So, once the paste turns shiny silver take the heat 
> gun away. By the way, you can do two sided boards if you put small metal 
> stand offs on the board and let the previously soldered parts hang 
> upside down while you do the other side. They just reflow again.
> I was a little worried when I first tried it also, But it works great 
> with a little practice you will be a Pro. Just make sure you keep the 
> hot plate at 100C and no hotter. The parts can stay at that temp all 
> day. Just use the heat gun to briefly heat the paste to melting temp.    
> Good Luck! Let me know how it went!
> 
> 
> Bob   K6GGO
> 
> 
> Bruce K3CMZ wrote:
> 
> >***** High Performance Software Defined Radio Discussion List *****
> >
> >
> > I have been trying to learn how to solder these smt parts.
> >
> >What I have is some solder paste with a melting temp of
> >361 degrees F, and a cheap six inch hot plate. The hot
> >plate is made out of cast iron and I an able to measure
> >the temp with fair accuracy.
> >
> > Now my concern is this; how much heat can the circuit
> >board stand before it turns to junk? My plan is to heat
> >the circuit board with the smt parts and solder paste
> >and finish this off with the embossing tool that Cash
> >Olsen uses. But, I am worried about the heat on the
> >board being too much, prehaps some of you have gone
> >throuh this and know if the boards can stand the heat.
> >
> > Bruce K3CMZ
> >
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