Hi 

A long time ago I had a similar problem with a 22 layer mother board. The only
way to solder it was first preheat the board in an oven to approx 100C. The
board was then passed over a wave fluxer with a flux called Alpha 611SP this
flux had a high B.P. solvent. The track speed over the wave was set to 0.5
meters/min. This fixed the top-side soldering problem.
With new spray fluxers you should be able to use low BP fluxes but I think that
you require more than a 1.2% flux. Try Lonco 65.

regds Andrew

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