(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


 

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[N8LP]

All that is undoubtedly true... but if you still want to try it, I would
think you would get better results with a convection kind of toaster oven.

Larry N8LP



Henry Gardiner wrote:

>   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
>
>
>  
>
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> You must be a subscriber to post to the list.
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>
>
>

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[EricJ-2]

 

-----Original Message-----

   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.

[Hams aren't switching to SMTs to avoid having to inspect every board. We
will still have to do that with SMTs no matter how we install them.]


   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.

[Technical hobbies don't advance by doing what "makes more sense." They
often advance when some wacky guy commandeers the family toaster oven to
solder parts and doesn't burn the house down...or poison them all with lead
exposure.]

Henry AC5LA

[Thanks for the background info, Henry. I never even realized such equipment
was used until one of the Elecrafters mentioned the toaster oven idea. There
was another article in Circuit Cellar magazine that said commercial ovens
also use infrared as well as air convection ovens. All in all, it is pretty
amazing technology. It's something I want to try just because it's...well,
just because.

Eric
KE6US


 

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