[hpsdr] SMT Soldering and Circuit Board Heat Question

[Bob Fish K6GGO]

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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