Issue link: https://iconnect007.uberflip.com/i/969348
APRIL 2018 I DESIGN007 MAGAZINE 35 are producing reliability challenges from the opposite end of the IC size scale. These ICs will have a longer displacement to their neu- tral point and they can have higher power dissipation self-heating temperatures which increases the sheering strains in their outer sol- der balls. As previously noted, current automo- tive BGAs are capable of a mean life of 3,000 to 8,000 thermal cycles from -40˚C to 125˚C. These BGAs are in the 11-15 mm square pack- ages with 144 to 225 solder balls. But advance automotive application will be requiring much larger ICs such as 17x17 mm BGA-400, 23x23 mm BGA-760 and 29x29 mm BGA1313. The latter of which is projected to be capable of a mean life of only 820 thermal cycles under the reduced temperature range of -40˚C to 85˚C (Figure 2). 3. Warpage Issues The previous thermal cycling solder fatigue examples dealt with thermal cycling conditions that produced stresses in x-y plane of a PCB. When thermo-mechanical cycling stresses are limited to the x-y plane of the PCB, solder attachment are under compressive loads. Sol- der is similar to concrete in that it is stronger and more durable under compressive loads. However, due to the larger CTE of PCBs com- pared with the effective expansion of the com- ponents or inherent warpage of either the PCB or the components, warping curvature can also be encountered that produce z-axis stresses. Such warpage of either a PCB, components, or both can also be a life limiting factor especially as components get larger and PCBs get more complicated. Warpage conditions apply z-axis stress to solder attachments. During portions of the thermal cycle, the solder can experience tensile stresses which produce a tearing strain that can tear solder apart in a tenth of the num- ber of cycles needed to produce fatigue failures under compressive thermal cycles. Cooling to extreme temperature can induce plastic strains, Figure 2: Comparative IC package thermo-mechanical cycling solder fatigue durability. Figure 3: Examples of thermal cycling-driven PCB and component warpage. Direction of board and package expansion around the corner joint in QFN at (a) low and (b) high temperatures.