Re: gEDA-user: Cheap 4 layer manufacturer?

[marvindickens@xxxxxxxxxxxxx]

On Tuesday 11 October 2005 10:39, Randall Nortman wrote:
> On Tue, Oct 11, 2005 at 10:12:02AM -0400, DJ Delorie wrote:
> > > The oven a 1500-watt Oster, and was $60 at Target.  It's got four
> > > heating elements (two top, two bottom) and, as I mentioned, a
> >
> > /me wonders if it's better to only heat the board from the bottom
> > (i.e. move all the heating elements to the bottom).  For BGAs that
> > would mean that the heat didn't have to go through the heat-sensitive
> > BGA to get to the solder balls.
>
> Not a good idea if you want to be able to do boards with components on
> both sides.  When doing the second side, there will be components on
> the bottom, and I think you would get them too hot by the time the top
> got hot enough to melt the solder there.
>
> I wonder -- do the real reflow ovens use bottom heat on the first side
> and then switch to top heat when doing the second side?

All of the ovens that I've worked with heat the top and the bottom.  In fact, 
the initial stage involves pre-heating the bottom of the board for 5 or so 
minutes. During the pre-heat, the upper element is off. The heat irradiated 
by the lower element is below that of the temperature required for solder 
reflow. Once the board is pre-heated, the upper element is turned on to 
initiate the reflow of solder on the componets located on the top of the 
board. 

The board is then allowed to cool. The process is repeated to attach devices 
to the the other side of the board. 


Regards

Marvin Dickens