tomb-stoning of two-legged parts (they
stand vertically due to uneven surface
tension of molten solder on the legs).
Too much solder results in solder bridges
between leads on integrated circuits of various packages.
Too little solder results in pins or pads that are not
connected between the traces on the PCB and the pads
on the device.
These are especially hard to rework — if not impossible
— on QFN and BGA parts. So, the stencil makes it much
easier to apply the desired quantity of solder correctly in
the first place, and will result in less rework after reflow
soldering. Considering that the stencil is free with a PCB
order, why not use it to your advantage. It is much more
satisfying to do a quick visual inspection on a properly
soldered PCB and then start testing assembled boards, than
to have to fix tombstones and bridges. (Hey, that sounds
like a new version of the snakes and ladders game!)
Opening Up the Box
A well packaged toaster oven with the reflow kit arrived
in a single large box. Packed inside was a complete kit of
everything I needed to get started (Figure 4). Contents
included lead-free solder paste (which should be refrigerated
to extend its shelf life), masking tape for stencil alignment, a
solder squeegee/applicator, tweezers for component
placement, a digital multimeter (with 9V battery, leads, and
temperature probe), a PCB alignment kit, sample PCBs with
an SMD parts kit, and solder paste stencil.
These items are part of the training opportunity which
allows you to build a non-working PCB to test your ability
to apply the solder paste, to identify and place the parts
on the PCB, and finally to run the PCB through the reflow
January 2015 47
Figure 2. Beta Layout's reflow controller —
Figure 3. Solder paste — this
one contains no lead.
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The solder paste needs to be kept cold to prolong its
shelf life. Putting it in the household fridge seems easy
enough, but it is not a good idea. A small dormitory type fridge
would be better, and make sure to label it accordingly.
My former workplace had a small fridge that was used
to store solder paste and isopropyl alcohol. It was marked
“solder and alcohol only.” (Of course, it only took a couple
days for the sign to be modified to read “lead and vodka
The reflow oven should never be used for food once it
has been used for soldering. It is common sense that we don’t
want chemicals, metals (heavy or otherwise), or surface-mount parts in our food. So, be safe and do not be tempted to
use the reflow oven for food.
Others may see this nice looking toaster/counter-top
oven and think it is just the thing for heating up their lunch.
Without knowing its history, they will be taking a risk. I would
suggest permanently marking the oven in some manner,
stating that it is not
safe for food.
Figure 4. Reflow kit
and SMD parts.