Electrovert Bravo 8105, Руководство по программированию и эксплуатации

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A C C O M P L I S H I N G T H E R E F L O W P R O C E S S
Understanding Process
11-114 Bravo™ 8105 Programming and Operations Guide Rev. 2
1 1
Board Design, Stencils
and Solder Paste
Experience shows that evenly–populated boards are easier to profile and reflow
than complex boards with diverse components because the thermal mass is more
uniform on a board that’s evenly populated. Additionally, as a component’s size is
reduced, the forces of surface tension become increasingly important. In general,
it appears that the lesser the wetting angle of the solder, the lesser the risk of
defects, particularly tombstoning. The wetting angle is particularly affected by
the physics involved in wetting, the volume of the solder and the pad geometry.
Reducing the SMT footprint extension decreases the solder contact angle and
aids in preventing defects with smaller components.
The stencil printing of solder paste is the most important step in PCB assembly. If
the process is not set up or operated correctly, then the final assembly yield
suffers. The reflow process cannot correct inadequate or incorrect solder paste
placement, stencil design, or paste materials.
Surface mount devices are referenced by the pitch of the component leads. The
pitch partly determines the type of powder required in a solder paste, i.e., the
tighter the pitch the finer the required powder. In general, estimate that space
between two leads should be about three (3) solder spheres.