Lead To hole Ratios are fairly important but not normally a cause for Blow Holes.
If you are using Single Sided PCBs then many things are blamed on lead to hole ratios, when in fact the problem is incorrect drilling leaving a laminate burr inside the hole wall which then naturally de wets solder.
If the problem is with PTH boards then this can be caused by HASL fluids attracting moisture into the holes, sometimes baking works, but sometimes there is too much high boiling solvents left to bake out as they are there to withstands high temperature so stands to reason they cannot be baked out.
This can be tested for as if you leave an Iron tip for 4-5 seconds on the blown joint you will see an acrid smoke come off with the solder fizzing and popping and a black spec appear on the molten solder normally.
If it is only in a certain area of the PCB then it could more likely be thin plating so you get hole wall break out, get that area micro sectioned and wall plating including Copper thickness tested.
If the PCB is Silver Finish then this too can produce blow holes and the only thing that really helps is to flow the PCB twice, we believe there is resist sometimes improperly removed or leached/bled over the holes and by soldering it twice this actually removes the residue and gives an improved solder joint.
Would be helpful to know what solder finish PCB is and what type i.e. single or double sided.
Hope it helps
