Homebrew PCBs

Yes, sufficiently high temperature and dwell time are necessary. Lacking a 
controlled process, the simplest thing to do is cook and then overcook a few 
junk boards on a skillet and see what happens. (I have a few I can spare if 
you have not enough of your own. :)


----- Original Message ----- 
From: "Alan King" <alan@...>
To: <Homebrew_PCBs@yahoogroups.com>
Sent: Monday, November 21, 2005 2:46 PM
Subject: Re: [Homebrew_PCBs] Re: Tinning the board? - reflow


> Stefan Trethan wrote:
>
>>Anyway, i bought the more expensive silver paste because it must surely be
>>better ;-)
>>
>>BUT, it seems to reflow in the same strange manner as the leaded paste.
>>
>>When you heat it until it has a dry, grayish appearance, and then wipe it
>>off, it will come off as sort of dried paste/powder, BUT the board will be
>>tinned in a thin, shiny layer.
>>
>>When you heat it further, until it reflows, it will get a dull, rough
>>appearance from where the particles melted.
>>
>>
>>I don't understand the first thing. How can it tin the board without
>>melting, at all?
>>What happens?
>>
>>
>>
>>
>  Think of having your board right at the melt point, and taking a solid
> lead pencil and writing on the traces.  Pencil stays solid, but where
> you're touching the traces is melting.  Traces are above the melt temp,
> but you're only getting melt when you make it touch.  For sure it is
> melting some, just not a lot.  Remember it takes a lot more energy for
> the phase change, so right near the melt temp you'll melt the surface
> but not have near enough extra heat to melt the rest of the solder
> balls.  Might also be doing something with the flux at a higher temp, as
> someone else noted recently it stays on much longer just at the melt 
> temp..
>
>  Think of how you draw on your iron tip with solder, as the iron is
> just heating up, leaves a nice shiny path.  And won't melt the rest of
> the solder very fast yet, because it doesn't have enough extra heat.
>
>  Things well above the melt temp uaually oxidize much faster than
> things right at it, part of the reason a temp controlled soldering iron
> is a good thing too..
>
> Alan
>
>
>
>
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