[Elecraft] (OT) Baking PC boards in the toaster oven...

[Henry Gardiner]

   SMT soldering was designed to obtain high volume and high quality solder 
joints with great uniformity so that detailed inspection and rework are not 
necessary.  It does this at considerable investment expense up front.
   The paste solder must be applied in a precise amount.  This means a 
stencil printer (most use optical alignment) and multi-hundred dollar 
stencils, or a robot that applies dabs in precisely metered amounts.  Hand 
application will not work adequately.  Smear the solder on those narrow smt 
ics even slightly and a solder short results.  Not enough solder, and the 
lead won't wet.  Too much, and the joints short or are hard to 
inspect.  You end up with the extra step of inspecting every joint (which 
is not done often in the industry) and doing a lot of touch-up by hand.
   Then the component must be placed precisely on the pads of solder 
without lateral motion that would smear the solder and cause solder joint 
problems.  In industry, robots do this at high speed and precision using 
software that determines from coordinates and optics where to put each 
component.  Just about impossible to do by hand unaided at any speed.  But 
a person could rig up some very slow manual pick and place device.
   Circuit boards that have been sitting around for months start to lose 
their wettability.  This adversely affects the uniformity of the solder 
joints.  The high volume of a factory provides considerable protection 
against this problem.
   Most smt ovens heat the air and blow the air around the boards at high 
volume.  Compared to radiant heating this provides much better regulation 
of component temperature-time profiles despite differences in component 
colors and geometries, and the biggest factor, the thermal mass of the 
circuit board itself.  The circuit board forms half the joint.
   To obtain the temperature vs time profile needed, most factory smt ovens 
are actually a chain of several ovens connected together linearly over a 
moving belt.  Each oven operates stabilized and uniform at a different 
point in the temperature profile.
   Unless you want to buy good equipment, you will be inspecting every 
joint, doing a lot of touch up and replacing fried components.  You'll have 
a lot of field failures from intermittent joints.  Toaster ovens don't make 
sense.  Single cavity ovens could be made to work well if there's a lot of 
moving air and if the walls of the oven and the air can be kept very close 
to the temperature profile, including the cool-down phase.
   For experimenters, it makes much more sense to stick with the larger smt 
sizes and solder them in by hand with a soldering iron and hand-held solder.

Henry AC5LA