Introduction
QDR SRAMs are a family of products defined and developed by the QDR consortium comprised of Cypress, IDT, NEC, Micron and Samsung. QDR SRAMs are offered in various configurations starting at the 9M density. The QDR SRAM is offered in a 165 ball Fine Pitch BGA (FBGA) package. The 9M and the 18M QDR SRAMs are offered in a 13mm x 15mm
package outline, while the 36M and the 72M QDR SRAMs will be offered in a 15mm x 17mm package outline. Appendix 1 and Appendix 2 shows both the package drawings.

The QDR SRAM pinout is a mirror image pinout. This has been defined to simplify board layout and reduce the overall board area. The pinout is defined to allow two QDR SRAMs
to be placed on opposing sides of a system board. For more information on this application, refer to the Appnote “QDR: Layout considerations”. This allows depth and/or width expansion to suit the system needs. This also allows double the density in the same board area. This particular configuration is popularly called a Clamshell. Figure 1 shows a simplified drawing of a clamshell.

This configuration allows simpler system design and shortens the trace length on most traces from the controller to the QDR SRAMs. The Clamshell configuration also allows traces to be routed through the internal layers of the system board. This configuration also needs careful considerations from a manufacturing standpoint. This application note discusses some of the concerns which every designer has to be aware of during mounting and reworking of the devices.

Clamshell Configuration
The pinout on the QDR is defined such that if one side of the package has a data pin, the other side would be a No Connect pin so that the trace running from the top to the bottom and vice versa (data to the No Connect pin) could actually come off on the other side of the PCB and get routed instead of drawing this trace from the inner layers. Since all these pins
are closer, a common signal driving both the SRAMs must travel about the same distance, reducing reflections and increasing signal integrity.

Manufacturability
Care has to be taken during assembly of a clamshell configuration. Improper processing during assembly can cause yield issues during manufacturing.

1)Vias used for clamshelled devices should be filled with non-conductive epoxy and should be tented (covered with LPI soldermask). If this is not done molten solder could pass
through the open vias during reflow and affect the QDR connections on the opposing side of the PCB.

2)Inspection of devices using X-ray equipment is standard practice during the assembly of FBGA devices. X-ray inspection of clamshelled devices becomes more difficult. When
testing for physical inter-connectivity on one side of the board, the solderball of the FBGA on the opposing side of the board overlaps the side at focus and might provide a misleading image. This is easily overcome by adjusting the intensity of the X-ray beam and looking at only one side of the board. In this way, the solderball on the other side of the board does not overlap the solderball at focus.

Board Repair/Rework
When removal of an FBGA in a clamshell configuration is required the bottom side of the PCB is first pre-heated to approximately 100C. A suitable rework nozzle is then positioned
over the FBGA and the temperature is elevated to approximately 200C by virtue of forced hot air. When the solder achieves liquidus, a vacuum tip extends from the nozzle and
grasps the component. Utilizing z-axis control, the component is removed from the PCB. Care must be taken to ensure that the PCB is not overheated as this may affect the connections on the opposing side of the PCB.

X-ray inspection and electrical testing should be performed after a rework cycle to ensure proper connectivity.

Conclusion
Clamshell assembly must be handled carefully to minimize yield loss. PCB design techniques can alleviate many assembly issues associated with clamshelled devices. Reflow and rework profiles must be tightly controlled to avoid impacting components on the opposing side of the PCB. This application will be updated with more inputs from customers and contract manufacturers.