BGA Socket Considerations: Prototype to Reality
December 31, 1969 |Estimated reading time: 6 minutes
BGA socketing systems are essential during the design, testing and production phases of products containing high-performance ICs. This article discusses the tradeoffs associated with the interface between the socketing system and the PCB. It also focuses on the impact these requirements have on socket form factors and available mounting options.
By Ila Pal
BGA socketing systems are an essential option during the design, testing and production phases of new product development processes. In response to these demands, sockets featuring a variety of contacting methods, form factors and mounting techniques have been developed. Socketing systems that perform at bandwidths exceeding 10 GHz and with ball-pitch spacing down to 0.5 mm now exist. As BGA devices with finer-pitch, larger I/O counts and increased gate densities are used, the requirements for sockets continue to increase accordingly. These performance expectations place critical demands on the mechanical and electrical requirements of socketing systems. The challenges for shortened contact lengths and controlled impedance geometries are being met by various approaches. These approaches may include contactors consisting of embedded wire arrays, spring pins or precision pin and socket interfaces. Table 1 shows a comparison of electrical data for these contactors. For many applications, a universal need for any contactor approach is to provide the most reliable connection in the smallest possible footprint.
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Product Prototyping and Design Phase
In the prototype and design phase of a new product introduction (NPI) process, it often is important that the prototype configuration match the final product configuration closely, both mechanically and electrically. This close matching will have the benefits of shortening the overall development cycle and reducing the potential for future design changes due to unanticipated performance shortfalls. From a socketing system perspective, both the overall size of the socket and the method of attachment become important considerations, often resulting in the need for a socketing system with mounting-footprint dimensions minimally larger than overall BGA-package dimensions.
Small Footprint - High-performance BGA applications require the smallest possible socket form factor due to the need to place related components in close proximity to BGA pads. Socket systems create keep-out areas, zones or areas unavailable for other components, such as decoupling capacitors and resistors, due to mechanical interface with the socket housing and attachment points. A small footprint allows the design engineer to match final product performance and end-use conditions, whether or not a socket is planned for use in the final product. It provides for the shortest possible traces to minimize loading and allows for close proximity of any necessary decoupling components.
Mounting Methods - Various mounting methods can be considered for GHz-bandwidth BGA sockets. These include fastener mounting, epoxy mounting and direct-solder mounting.
Figure 1. Fastener mount socket with backing plate.
Fastener Mount - Using fasteners to attach the socket to the PCB eliminates the need to attach the development IC permanently to the PCB. This provides flexibility to replace components without need to unsolder and rework (Figure 1). In this configuration, mounting of the socket is accomplished using corner fasteners. The use of a backing plate to provide the necessary overall socketing-system stiffness and rigidity is provided depending on the size of the package. The necessary keep-out area, while small when properly engineered, still is a significant percentage of the overall package dimension. Fasteners also have the potential drawback of requiring thru-holes in the PCB, which may or may not be problematic depending on routability and final-product requirements. Small hole size for fasteners, as well as a small (≤2.5 mm) border area, help minimize the socket footprint.
Epoxy Mount - If thru-holes are unacceptable, or if a smaller footprint is required, an epoxy-mounting option may be considered. While this creates somewhat permanent bonding of the socket to the PCB, the socket may be designed such that contacting elements are replaceable should damage or excessive wear occur. The socket can be placed easily using a precise placement guide. A ring of epoxy around the socket holds it in place.
Figure 2. Solder-mount socket.
Solder Mount - A third option to either the fastener or an epoxy-mount approach is to consider a direct solder-mount approach (Figure 2). This socket solders directly (requires no tooling holes) to BGA pads, but typically requires a larger keep-out area due to stiffness requirements to compensate for co-planarity considerations. Standard surface mount methods can be used to attach the surface mount base of the socket, using low-temperature eutectic solder balls, to the target PCB. The IC then is dropped into the socket base and the lid, which contains a screw mechanism, is added and tightened.
A measure of comparison between the three mounting methods is shown in Figure 3. Direct mounting of the socket by epoxy provides the highest potential density, followed by fastener- and direct solder-mounting methods.
Figure 3. Fastener-mount, solder-mount and epoxy-mount keep-out area to device area.
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Product Testing and Production Verification Phase
Depending on the number of insertion or withdrawal cycles needed during the testing and production phase of a project, the contacting method used in the socketing system may change. Typically, spring pins (pogo) offer the most suitable performance over the highest number of cycles relative to other contactor approaches. Depending on device speed and system requirements, a shortened pin length may be important for electrical performance matching to the final application.
If the PCB footprint required for testing and production remains consistent, potential advantages in time and cost may be realized in the overall migration path from prototype to a fully tested product. However, other considerations, such as ease of placement of the device to be tested in the socket, may require form factors that impact overall socket size. These do not impact overall footprint dimensions necessarily, depending on socket design. Due to the high-contact forces, a fastener mount with backing plate normally is recommended.
End-use Product Applications
In high-performance, end-use product applications, the requirement for a small footprint remains, often dictated by the overall available space and the option to attach the device to the board directly (Figure 4). The consideration of pluggable, compact sockets often is made as an option to facilitate product replacements, upgrades and repairs in the field. Direct component replacement requirements result in the need to solder the socketing system directly to the target board. Solderability, in terms of meeting coplanarity requirements, and in the prevention of solder wicking into the contact interface, is important. Because of high pin counts, a low-insertion force is important for usability. A key to success is the ability to withstand multiple reflow cycles without a loss of reliable contact due to substrate warping and solder wicking into the contacts. These sockets require the same space as the target IC device.
Figure 4. Small form factor, low-profile BGA SMT socketing system.
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Conclusion
BGA-socketing systems perform important roles in the prototype, production and end-use phases of many new product development programs. A range of contacting methods and mounting methods are available to meet the needs of product designers. The ability to use sockets featuring compatible form factors and small footprints provide significant potential performance and time-to-market advantages as systems migrate from prototype to reality.
ACKNOWLEDGMENTS
The author would like to thank Gary Forsberg for his assistance with this article.
Ila Pal, director of research and development, Ironwood Electronics, may be contacted via e-mail: mannan@ironwoodelectronics.com.