BGA/area array stencil aperture : Round or Square?
What is the best aperture geometry for depositing paste on round ball grid array pads on PCB’s. To find the answer we need to rephrase the question. For the same volume of paste deposit what is the best shape which gives the largest clearance between the deposits. For stencil of thickness “s” a round aperture with diameter of “d” will deposit (3.14*(d/2)^2*s )=0.78*d^2*s paste by volume. To have same volume of paste from a square aperture the size of the sides of the square need to be 0.88*d. So choosing a square aperture will give us better clearance between deposits as with square aperture the clearance will be increased by 0.12*d