Brad -
While excessive air velocity at PCB surface level may move parts, I have yet
to see it cause bridging (of course, there is always a first time).  Most
likely, what you are experiencing is "hot slump" due to solder paste
deficiencies and/or stencil deficiencies.
A print may look good after printing - well defined, brick shaped.  However,
during the reflow profile, particularly during the Preflow Soak, as well as
reflow, with the solvents containing binding agents and rheology modifiers
being vaporized off, the metal is left more "on its own" (depending upon the
solder paste).  Thus the metal may slump if it is deficient in binding
agents, the aspect ratio is incorrect, the solder paste particle size is too
large and/or the stencil is badly worn or damaged.
Try testing by printing on a blank laminate and observe the pattern right
after printing (cold slump).  Wait an hour at ambient and re-observe.  Then
place the board in a box oven at 150 C for two minutes (simulating the
Preflow Soak ).  Remove the board and observe - is there slump?   I use this
test (and variations) for both such diagnostics as well as evaluating solder
paste.  Repeat the test with either a different solder paste or stencil.
Keep us posted!

Best regards,
Phil Zarrow
ITM Consulting
Durham, NH  USA
www.ITM-SMT.com

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