also frees the operator to do other things while the cycle
runs, such as prepare the next batch of parts or start
checking the previous batch.
One of the neat things about the Beta Layout kit
I used is the included stainless steel solder stencil
(Figure 10). A solder stencil is usually a sheet of thin
stainless steel or mylar which has cutouts for all of the
solder pads on the PCB. By controlling the thickness of
the solder stencil, you can place a known quantity of
solder paste on each pad. This minimizes the chance of
solder bridging or tombstoning parts since uniform solder
paste distribution is almost guaranteed. The PCB alignment
fixture consists of several L-shaped pieces of circuit board
in two standard thicknesses. I chose the pieces that most
closely matched the thickness of the PCB in the kit I was
working with. Refer to Figure 11.
The larger L-shaped PCB is attached to a piece of
hardboard or Masonite type material with the provided
masking tape. The kit PCB is then placed in the inside
corner of the L. A smaller L-shaped PCB is used to hold
the opposite corner of the kit PCB in place and is affixed
with tape. The stencil is then manually aligned to match
the solder pads on the kit PCB. Once the stencil is
aligned, it is taped to the fixture along one edge with the
provided masking tape. Taping along only one edge allows
the solder stencil to be lifted safely away from the PCB
once the solder paste is applied.
Let’s look at how we get solder paste onto the PCB.
The solder paste comes in a 100 gram container that
should be kept refrigerated to prolong its shelf life as
mentioned previously. This particular paste is lead free.
The solder paste should be taken out of cold storage and
brought to room temperature before use. The paste
should also be stirred for 15 seconds or so before being
applied. I used a disposable plastic knife for stirring and
applying the paste to the stencil.
The solder paste is applied by putting a generous
bead along the edge of the stencil near the masking tape
(Figure 12). Since the solder paste is thick and sticky, the
“bead” of paste looks more like a small mountain range.
The included stainless steel squeegee/spatula is then used
to scrape or spread the solder paste across the stencil,
thus depositing a layer of paste into each opening of the
stencil (Figure 13). Once the solder paste is spread over
each stencil opening, the stencil can be flipped up to
January 2015 49
Figure 8. Status shown during the learn cycle.
Figure 9. Staus display of a solder cycle in process.
Figure 10. Stainless steel solder mask.
Figure 11. Board held in alignment fixture, between
two L-shaped green PCBs.