Although LGA (land grid array) and BGA (ball grid array) packaging seem quite similar – LGA packaging is basically BGA packaging without the solder balls – there are some significant differences that can have an impact when used for power converters such as point of load modules.
BGA technology emerged as an alternative to packages with pins, offering much finer pitch so that packages could be shrunk. BGA also offers performance improvements based on shortening the distance between the mounted device and the PCB. Shorter connections typically mean less inductance, which further improves transient performance by reducing Ldi/dt losses.
Because of the finer pitch compared to pin modules, BGA technology also allows for multiple ground connections to be placed in between the power and data connections, rather than connecting them all to a single ground pad. This helps EMI immunity, but critically for power modules, it also allows for more connections to the host PCB ground plane, reducing the system’s thermal resistance further (the ground plane, a copper layer usually the whole area of the PCB, is one of the routes for heat to dissipate out of the system). These ground connections are often directly underneath the module, enabling heat flow from its hottest parts, in positions where it’s very difficult to get airflow.
AMP Group co-founder Ericsson Power Modules’ BMR466 uses the LGA package for thermal efficiency
LGA takes this idea a step further. Leaving off the solder balls, a much thinner layer of solder paste is instead applied to the host PCB and the module is then reflowed in much the same way as a BGA module would be. Since the solder paste layer is much thinner, we get the same advantages as the BGA package, but even more so; thermal resistance is even smaller, and inductance is even lower. For modern power-dense point of load modules, which can carry very high currents (Ericsson Power Modules’ BMR466 can supply up to 60A to MPUs/FPGAs/ASICs from 25.0×14.0mm, for example), thermal efficiency is more important than ever before; LGA’s thermal advantages can make a real difference. The technology’s lower mounting height, even though we are only talking about a fraction of a millimetre, can help lower the profile of the board so as not to obstruct air flow around the system.
Although it would seem to follow that reducing the amount of solder and making the connections much thinner would lead to less robust connections, in fact, the opposite is true for SAC (lead-free) solder. With SAC alloys, a smaller volume of solder in each connection means the connections cool and solidify quicker during reflow. This has a notable effect on the microstructure of the joints – an interlaced twinned dendrite microstructure is formed, which is much harder than the microstructure of SAC BGA connections, and it exhibits slower creep behaviour and it takes much longer for any re-crystallisation to occur. This adds up to much higher reliability of the joints post-manufacture.
Comparison of reliability for BGA and LGA packages using SAC (lead-free) solder
LGA also offers designers the ability to optimise the connections for either shock endurance or long-term reliability by choosing between two solder mask profiles. SMD (solder mask defined) means the solder mask hole is smaller than the LGA pad, so solder doesn’t cover the entire pad. This is used in applications like portable devices where the risk of impact forces is high, such as in portable devices which may be dropped. NSMD (non-solder mask defined) means the mask opening is larger than the LGA pad, so the solder overlaps slightly. This provides greater long-term reliability as it reduces the risk of solder fatigue, and it’s commonly used in industrial and avionics applications. These optimisations cannot be made using BGA technology as the solder balls are a standard size.
The effect of packaging choice on the performance and reliability of the end application is sometimes underestimated, but in fact, choosing the right technology can bring some distinct advantages to power module design. This is the very reason the AMP Group® has chosen to use LGA packages for its picoAMP and gigaAMP standards , so that customers can benefit from these advantages.