Bob Cantrell, Ericsson Power Modules
LGA package technology can deliver important advantages for the high-performance Point-Of-Load converters needed to feed power-hungry, fast-reacting data centers and network infrastructure. As the power demands of high-performance Cloud and telecom computing systems continue to increase, converter modules that offer the greatest current density deliver advantages for board designers.
Even a small saving in module size can have a significant impact in systems that may have 10, 15 or more Point-Of-Load (POL) converters supplying multiple power domains and multi-rail devices like processors, FPGAs or ASICs. On the other hand, interoperability between modules from different manufacturers helps in dealing with the complexities of digital or software-defined power architectures. The Architects of Modern Power (AMP Group) consortium has established a platform for delivering the best of both worlds.
Recognizing the role of package technology in the drive to maximize module current density, AMP Group co-founder Ericsson Power Modules is sharing its Land Grid Array (LGA) know-how to enable a new generation of interoperable non-isolated point-of-load converters that deliver industry-leading current density and thermal performance and reliability.
Evolving Area-Array for Power Conversion
Area-array IC packages such as the Ball Grid Array (BGA), which make use of the package underside to site I/Os, have become extremely popular for devices such as microprocessors, memories and FPGAs that have a high I/O count and must occupy only a small PCB area.
In addition to its high I/O density, the BGA package also benefits from low overall thermal resistance.
The large number of small-diameter spherical solder balls that constitute the I/O array are able to conduct heat efficiently into the PCB and subsequently dissipate it into the ambient environment. In addition, since BGA interconnects are shorter than those of leaded devices, package inductance is also low. This ensures superior electrical performance, particularly in the presence of large, rapidly changing currents.
As area-array packages have evolved, the LGA has been developed featuring large flat I/O pads instead of BGA solder balls. These pads have outstanding low inductance and efficient heat-flow characteristics. The improved heat conduction through the pads into the motherboard benefits power converters as it allows more efficient cooling, particularly if the module design itself is optimized for this method of heat removal.
LGA Assembly Advantages
The soldered connections of LGA packages provide additional benefits to the systems manufacturer. When LGA packages first entered widespread use, some manufacturers assumed that the low height of the soldered joints would result in lower reliability compared to BGA joints. In fact, it is the choice of soldering materials, rather than solder-joint height, that has the major bearing on reliability.
Tests have shown that where tin-silver-copper (SAC) solder alloys are employed, the results for LGAs are often superior to others. The improvement can be explained through an examination of the microstructure of the LGA solder joints.
An LGA joint contains a relatively low volume of solder in relation to its surface area, which allows very fast solidification after reflow. As a result of this, the solder joint has an interlaced grain structure that is quite different to the large-grain structures typical of solder joints that are slower to reform, such as the solder balls of BGA joints. The interlaced microstructure ensures a hard solder joint that exhibits slow creep behavior and delayed re-crystallization. These factors help maximize reliability in the field.
The LGA is fully compatible with the reflow processes used for BGA-based assembly, as the same core solder reflow processes are employed. It can also be used with PCB pads that are either solder mask defined (SMD) or non-solder mask defined (NMSD). An SMD-type pad may be recommended if the application has a high risk of suffering impact forces. The NSMD pad, on the other hand, helps enhance reliability and is often chosen for long-life applications where solder fatigue over time may be a greater concern.
Design to Maximize Package Benefits
Ericsson Power Modules has taken advantage of the high area efficiency, low inductance and superior thermal performance made possible by LGA technology in its BMR466 PMBus-compatible, 60A, non-isolated dc-dc POL converter for high-performance Cloud, telecom and industrial power applications. Up to eight units can be connected in parallel to power loads drawing up to 480A. The design is licensed to all AMP Group members thereby making advanced and interoperable modules, which provide the assurance of consistent behavior in digital and software-defined power architectures, readily available in the marketplace.
The internal layout of the module is engineered to achieve a low package profile. At just 7mm high, the BMR466 minimizes interference with cooling airflow across the board. Moreover, Ericsson has optimized the arrangement of the LGA solder pads to ensure enhanced thermal performance and enable the module to dissipate heat very efficiently while at the same time benefiting from an extremely compact footprint of 14mm x 25mm. Cooling is also aided by convection over the top of the module.
The LGA contacts are positioned symmetrically to maximize mechanical contact and boost reliability after soldering. To help further enhance the quality of the soldered joints at the connection to the PCB, Ericsson applies gold plating to the LGA pads. This minimizes any risk of contamination between solder chemistries, thereby ensuring optimum reliability. Figure 1 illustrates the module’s low profile, symmetrical LGA pad array and large upper surface area for convection cooling.
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Figure 1. LGA package technology helps boost the current density of POL converters.
The LGA technology has also enabled all internal connecting leads to be eliminated, thereby significantly reducing package inductance. In addition, a high number of the external contacts are ground pins. Together, these features enhance noise immunity and minimize electromagnetic emissions.
The superior thermal performance achieved through the use of LGA technology has enabled temperature-related derating of load current to be reduced, thereby allowing higher output current to be sustained without sacrificing reliability. After derating is applied, the current capability of the BMR466 is comparable to that of competing POLs that have higher maximum-current rating but occupy more than twice the surface area and nearly four times the volume.
Proof in the package
LGA package technology has delivered multiple advantages for point-of-load converters tasked with powering the next generation of high-performance servers in industrial, telecom and Cloud computing. Lower package inductance, extra ground pins, enhanced heat dissipation through conduction and convection, and a compact footprint with low mounted height ensure the high current density, high reliability and improved noise performance that tomorrow’s systems will need to meet the high expectations of the most demanding end users.
Ericsson’s BMR466 module is the first product to meet the ‘gigaAMP’ standard recently introduced by the AMP Group, which defines a 60 A dc-dc POL converter in this compact 25.1 x 14.1 mm LGA package. Footprint- and software-compatible modules leveraging this technology support the Group’s strategy to make life easier for system developers seeking a competitive edge through advanced power supply design and exemplify the benefits that this alliance between CUI, Ericsson Power Modules and Murata is bringing to the industry in its continued efforts to establish standards that cover every aspect of board-level digital power technology.