Why do you think that assembling the BGA, QFP, and fine pitch parts will solve your bridging problem? I would think doing this may cause you more grief by having these more massive components being reflowed upside down fall off during the second pass assembly. The passives are better suited for multiple reflow passes due to their smaller mass and better tolerance to withstand multiple reflow cycles compared to the BGAs, QFP, and fine pitch parts.
What is the earliest stage of the assembly process that are you seeing these shorting problems?
Scenario 1: After printing? In this case some questions to ask yourself are: a. Is the stencil designed correctly? b. Is the stencil damaged? c. Are the squeegees damaged? d. Is the solder paste rheology and formulation appropriate? e. Is the machine calibrated and operating properly? f. Are suitable printing process parameters being used? g. Is the board being fully supported with proper tooling? h. Is the under stencil wiping system working? i. When the board is printed the second time, is it still warm from the first reflow?
Scenario 2: After placement? In this case the placement pressure is probably too high, causing paste to squeeze out under the balls or leads and bridge.
Scenario 3: After reflow? Some things to check: a. Are you using nitrogen? b. Is the temperature ramp too aggressive? c. Is the solder paste you are using sensitive to hot slump?
-Jeff
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