On 6/20/2014 1:32 PM, Brian Reay wrote:
Percy Picacity wrote:
On 2014-06-20 11:02:44 +0000, Jerry Stuckle said:

On 6/20/2014 3:54 AM, Brian Reay wrote:
Michael Black wrote:
I seem to recall from early articles about surface mount the suggestion
of using a dab of flux to hold the component in place. It was only to
hold it until you actually had something soldered.
That said, I really think a good magnifiying glass or microscope makes
sense. I was trying to figure out something, and got out the jeweller's
loupe, and it's amazing how much bigger everything is. That's not going
to work for soldering, but getting good magnification that will stand on
its own is bound to help a lot. And good lighting. Looking at that board
with the loupe almost makes the idea of soldering surface mount within
my capability.
Michael,
For SMD projects, rather than repairs, you can use a domestic oven to
solder the boards. Ideally, get a small table top model (the type of thing
student or caravaners use) but I did a test run in a full sized kitchen
one. The key thing is to be able to get to 230C or so at least.
You need to plot a graph of the way the oven temp rises using a
thermocouple (I borrowed one but they are cheap enough to buy). Basically
you put the board it with all the components held in place by solder paste.
Set the temp to about 200C and watch as it rises. When it gets to 100, turn
oven off for 3 min, this is known as drying time and is to drive the
solvent from the solder paste, them set temp to 250 or as high as you can
if lower. Now watch carefully, as soon as the solder paste melts and flows
to make the joints, turn off the oven and open the door.
I've tried this a couple of times and the results are surprisingly good. No
dry joints, no moved components.
The graph will help you predict when things are about to 'happen' and the
thermocouple is essential if your oven temp. gauge isn't accurate.
Brian,
I hadn't heard of using a domestic oven like this. It sounds like an
interesting idea. But wouldn't it be a bit hard on the components? I
wouldn't think many would like being heated his hot.
The commercial assembly lines I've seen use robot soldering irons
instead of heating the entire board.

I believe stencilling on solder paste, robot placement of components, and
melting the whole board in a reflow-like process with an accurately
controlled temperature vs. time profile is pretty standard for
complicated multilayer boards nowadays.


It is, and has been for some years, in fact decades, even in relatively
small batch production. The quality achievable is far higher that you can
get with manual assembly- once the process is right. Sometimes things like
edge connectors are pressed into holes which have been left solder free on
purpose. The plated through holes are designed to 'cold weld' (like a
crimp) when the connector is pressed home. It isn't soldered. This is
common on COTS boards used in military kit and custom boards in the same
LRUs. It is a very reliable technique.


Robotic soldering irons are NOT "manual assembly".

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