Homebrew PCBs

On Jan 31, 2008 11:27 PM, eeks123 <eeks123@...> wrote:
> yes, air flow and small components are apparently a bad combination. i
> suppose it's a try-and-try-again until you learn which distance is
> best, taking care where you point with the gun at, how much time a
> component can be exposed to that temperature, etc. you can solder a
> lot faster than with an oven, but you need to learn how to work with
> it. plus chances to damage something seem to be higher, when
> learning/starting

Actually, once the solder is reflowed there is no problem. The surface
tension of solder is huge. It is no problem to use hot air on
components that are already soldered, unless you select a really much
too high air setting. The problem is when reflowing fresh paste. Some
types have a flux that "melts", resulting in puddles of flux/solder
mixture. Other types don't seem to run about so much.

Also, there is not that much chance of damage. It appears the heat is
"softer" than the heat shock from an iron. Even if component
manufacturers only guarantee two solder cycles or so, you can usually
reflow a component many times without damage. The same is true for
damage to the PCB. I have swapped components with hot air 20, 30 times
for sure, without any harm to the PCB pads. As long as the temperature
is correct and you do not apply force all is well.

>
> i didn't knew there are two types, i supposed all components would
> swim, and that was also a reason to use soldermask -to avoid
> solderpaste flowing away through tracks-. so the first one is better
> but has a shorter shelf-life :( how do you know which type of
> solderpaste you are buying? through composition?

There are definitely large differences in paste viscosity during
heating. The one i use now is Edsyn CR44. This stuff could never be
used for mass production, it melts like chocolate when you get near it
with the heat. But it dries very slowly, making it practical for lab
use. I have tried other types but the syringes either came half dry or
dried before i could use them up. Some pastes dry up instead of
melting, with those there is virtually none of that puddle making.
Alloy composition seems to be of little consequence in this regard,
the flux and other additives seem more important.
Traces don't carry away much solder, and you can always apply a little
extra if broad traces connect. Remember surface tension, solder
_likes_ to make a nice fillet between component and PCB. It does not
like to spread out in a flat sheet.


> i've seen this explained on a webpage, it's important the way tracks
> go to the component, and also if there any very near to it.

You've done your homework then. Don't think to much about it though,
you'll quickly learn what to avoid by experience and the faults are
easy to rework even if something is pulled where it shouldn't go.

ST