Effect of cooling rate on the isothermal fatigue behavior of CBGA solder joints in shear

This paper investigates the distribution characteristics of the isothermal fatigue lifetime of ceramic ball grid array (CBGA) solder joints in shear, Placement direction of the board-level assembly on the oven conveyor during reflow critically influences the fatigue lifetime of solder joints in shea r: the front or outer solder joints have a longer shear lifetime than the r ear or inner ones, The solder joints that moved diagonally during reflow ha ve a longer fatigue lifetime and a tighter distribution. Cracks initiated i n the eutectic solder region on the card and package side and tend to propa gated in that region, while final failure occurred mainly on the card-said eutectic solder region. This phenomenon can be explained that the front or outer solder bumps have a resistant effect to the gas fluid which passes th rough the rear or inner solder bumps, and lower these solder joints' coolin g rate during solidification. Fast cooling rate can cause a more fine-grain ed and homogeneous microstructure in eutectic solder alloy, which can delay crack initiation and slow crack growth. When the board-level assembly move s diagonally during reflow, the resistant effect of front solder bumps to t he gas fluid reduces markedly. So the fatigue lifetime of solder joints and its distribution characteristic enhance substantially. The theories of flu id dynamics and heat transmission are used to calculate the decrease of gas fluid velocity and the corresponding reduction of mean coefficient of heat transfer (h(m)).