Understanding LGA
The Land Grid Array (LGA) method is a cutting-edge way to connect electronic components, featuring a network of contact pads on one side of the PCB (Printed Circuit Board). This network doesn’t come with solder already applied, setting the stage for a unique connection process. By applying solder paste to the PCB and then heating it up (a process known as reflow soldering), a secure bond is formed between the LGA module and the PCB.
The Genesis of LGA
LGA technology, with its intricate grid of contacts, emerges as an exemplary solution for modern integrated circuits that demand a higher count of input/output connections. Unlike Ball Grid Array (BGA) packaging, which relies on solder balls, LGA utilizes a direct contact approach, ensuring a lower profile and a more stable connection.
LGA Design and Assembly Considerations
- LGAs, having less solder volume, undergo more rapid cooling, leading to a unique, intertwined grain structure, unlike the ‘Beach Ball’ structure seen in BGA joints.
- Addressing solder joint integrity further, the type of land pattern used plays a crucial role. Non-Solder Mask Defined (NSMD) pads, which feature larger solder mask openings than the PCB pads, are effective in minimizing voids. Conversely, Solder Mask Defined (SMD) pads, with smaller openings, are advisable when durability against physical impacts is critical, such as in handheld devices.
- The solder stencil’s design is crucial for LGA devices, where the solder mask openings must accurately mirror the device’s underside. This ensures the PCB lands align perfectly with the device’s lands.
LGA Solder Joint Inspection
- To ensure the best outcomes in soldering modules onto motherboards, it’s crucial to adhere to specific assembly practices. Utilizing x-ray imaging allows for a clear view of the LGA pad connections on the module and the motherboard, revealing the intricate solder links and any voids present within the solder pads. These voids, while unavoidable, typically occupy 5-10% of the pad area, though they can reach up to 40% in some instances.
- Before assembly, the modules are carefully dried and sealed in moisture-resistant bags to prevent outgassing during soldering, which could otherwise increase voiding in the solder joints. This step is especially critical if the modules have been exposed to a humid environment and necessitates following the JEDEC J-STD-033 standard for baking.
- Opting for a solder paste with high flux activity and excellent wetting properties is essential, as it quickly cleans pad surfaces and allows gases to escape before the solder solidifies.
- The design of the screen printing stencil should mirror the recommended setup for the component, with a typical thickness ranging from 4 to 8 mils (100-200 µm). Adjustments to stencil apertures may be necessary to prevent solder bridges or splatter, enhancing the quality of the solder joints.
- A ramp-soak-ramp reflow process is generally favored over a direct-ramp approach due to its effectiveness in minimizing solder joint voids by allowing more time for gases to escape. Careful control of the temperature ramp rate is necessary to avoid solder bridges or splatter. For SAC alloy soldering, a rapid cooling rate can enhance joint reliability by promoting a specific microstructure within the solder.
Deciding Between BGA and LGA for PCB
When selecting between BGA and LGA, consider factors such as mechanical stability, connection type, pin density, heat dissipation, and maintenance requirements. BGA is often preferred for high-density, high-performance applications such as smartphones, cameras, and high-speed communications equipment.
LGA sockets can allow for the CPU or other components to be upgraded or replaced without desoldering, offering flexibility in maintenance and upgrades. LGA finds its place in CPUs and camera modules, where ease of component replacement is paramount.
