Land Grid Array or LGA Manufacturing: Packaging and Substrate Concerns

November 7, 2019 Cadence PCB Solutions

Drones assisting in the farming of crops

 

Technology has changed farming practices more than anyone could have imagined even 20 years ago. Precision farming involves the use of grid sampling, remote sensing technologies, and analytics to check soil conditions for available nutrients and to efficiently use lime, gypsum, and fertilizers. Soil Grid Tests performed in laboratories analyze samples taken from different locations in a field to monitor the impact of fertilizer applications on the entire field. With the data in hand, a farmer can learn how to mitigate the over- or under-application of those chemicals.

Precision grid sampling divides a field into smaller regions and limits the number of soil samples used for analysis. With the field sub-divided into smaller parcels, farmers can use drones and imaging technologies to create georeferenced color soil maps. Then, the farmer—with the assistance of smart glasses that use optical see-through technologies to show virtual content can find the ideal sample points. With those points in hand, a farmer can collect only the soil samples that represent specific management zones. By minimizing the number of soil samples, farmers save the cost per sample charged by laboratories.

Precision farming practices have interesting parallels with the precision that we constantly pursue in our world of electronics and PCB design. In farming, practicing precision reduces costs and prevents problems that could harm production over a lengthy period of time. As we work with high speed application, observing precision prevents errors and allows us to meet product design criteria. 

Land Grid Arrays Do Not Test Soil. ….Or Do They?

While not directly testing soil, land grid array (LGA) technologies power the drone remote sensing and wearable augmented reality technologies that allow farmers to discover the optimal soil test points. PCB designs that rely on Very Large Scale Integration (VLSI) and Very Very Large Scale Integration (VVLSI) require component packages that can accommodate the circuitry. Those designs incorporate LGA technologies—also listed as Quad Flat No Lead (QFN) and Micro Lead Frame Plastic (MLFP) packages--into RF and microwave applications.

Because land grid arrays have a very small footprint, the technology has become especially useful for handheld devices. For example, a digital power module LGA package includes the controller die and FET die, ceramic chip capacitors, thick-film capacitors, and an inductor. Molding compound encapsulates the components so that a flat surface forms at the top of the package.

Manufacturers developed the land grid array packaging technology as a method for either fitting components into a socket or soldering the components using surface mount technologies. Rather than redefining component layouts, the LGA design functions as a standard flip-chip Ball Grid Array (BGA) but without spheres. Both an LGA and a BGA use the same substrate, the same electroplate bumps, and the same die attach procedures. The key difference between an LGA and a BGA is the use of flat contacts—rather than ball contacts—for the LGA.

The square grid of flat contacts found on an LGA interconnect with a grid of contacts on a PCB. The non-socketed configurations of an LGA use a flat contact that solders directly to the PCB without any pre-deposit of solder. Because LGAs do not have spheres, the solder connection occurs through solder paste applied at the PCB. The solder paste melts during the reflow process to create the solder connection.

 

LGA on a CPU

Utilizing land grid arrays in your designs requires clear manufacturer communication.

 

Although LGAs have many of the same characteristics seen with BGAs, the lack of spheres for the LGA reduces the mounting height of the LGA and creates more space for small form-factor designs along with the advantage of less thermal resistance. In addition, LGAs increase the reliability of board level designs, reduce risks associated with damaged spheres.

Solder Joints for LGAs: I’ve Lost the Connection

Land grid arrays have thin solder joints when compared to other package solder joints. In most—if not all—instances, a thinner solder joint offers less reliability. As an example, a thinner solder joint results in increased failures in thermal cycling and the inability to meet the 3,000 failure-free 0 to 100o thermal cycle benchmark that product manufacturers desire.

Research has shown that the use of lead-free tin-silver-copper (Sn/Ag/Cu or SAC) solder alloys—rather than tin-lead (Sn/Pb) solder alloys--compensates for the thinner solder joint and establishes a dramatic increase in reliability for the LGA solder joints. The use of a different solder alloy creates a different microstructure for the solder. Reliability increases because the different microstructure features a lower volume of solder that requires more undercooling before the solder becomes solid. In turn, more undercooling allows the solder joint to have an interlaced structure that produces a better joint.

Along with the selection of the correct solder paste, LGA reliability also depends on having a solder reflow profile that matches the recommendations given by the solder paste manufacturer. The profile includes the temperature range that gives the optimal dwell time and the best solderability for all components. Solder joints require a peak temperature minimum for full reflow. Maximum peak temperatures specify the highest temperature that a device can handle. Higher temperatures during reflow ensure the full drying of no-clean paste fluxes.

There’s a Great Void Somewhere Out There 

Scientists refer to the Bootes void as “The Great Nothing” where very few galaxies exist. Once a void occurs, nothing exists that can attract matter and the void increases. In a much smaller scale, voids happen when soldering a BGA or LGA module. As gases escape during the reflow and soldering process, voids become apparent in the solder joints. A void can consume 5-to-10 percent of the pad area. 

 

LGA CPUs that may utilize SMD or proper solder joints

LGA CPUs are best utilized when effective packaging is considered.

 

The method for avoiding the air pockets depends on the product application. If the product requires additional mechanical strength, the PCB design process can designate the use of Solder Mask Defined (SMD) pads. An SMD pad has a small solder mask hole and creates a solid bond between the pad and PCB that withstands dropping, bending, and vibration. Other systems—such as industrial hardware and next generation motor control avionics—have a higher risk of solder fatigue and may require the use of (Non-Solder Mask Defined) NSMD solder pads that have a mask opening larger than the pad provide a very consistent solderable surface.

Other methods for minimizing voids include maintaining clean soldering surfaces also reduces the opportunity for outgassing. In addition to having clean soldering surfaces, the use of high flux activity solder paste that has excellent wetting abilities decreases the volume of gases as the flux interacts and decreases the amount of metal oxides. Observing best practices and keeping the solder paste away from any contaminants also reduces the opportunity for voids.

LGA Packages Support New Technologies

Drones, wearable devices, and interplanetary landers rely on small form-factor system-in-package modules that reduce the volume, mass, and power of the electronics used for power management, processing, and actuator control. The reduced thermal resistance available through the use of land grid arrays allows new technologies to work in optimal conditions.

Utilizing Cadence’s suite of design and analysis tools will keep your packages manageable at all stages of your design cycle. Allegro is host to many potential customization options you need for the designs you’re working on, whether simply one or two layer boards or finite and particular LGA options. Allegro will certainly be able to equip you with the analysis and production means you need for all of your packaging needs. 

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts

About the Author

Cadence PCB solutions is a complete front to back design tool to enable fast and efficient product creation. Cadence enables users accurately shorten design cycles to hand off to manufacturing through modern, IPC-2581 industry standard.

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