A.Caputo, W.C. Roth, B. Grossman, B. Aspnes, X. Ye, W. Acevedo, J. Landeros, and S.A. Aravamudhan
5200 NE Elam Young Parkway
Hillsboro, OR 97124
Industry demand for high-speed server product performance such as PCI express requires higher pin counts in order to support memory channels which in turn is driving pitch reduction in the printed circuit board (PCB). To provide better signal integrity for high-speed signals, ultra-low loss (ULL) PCB materials may need to be used, and back drilling is recommended to remove via stubs and minimize signal loss. Back drilling is the process whereby plated through holes (PTH) are drilled from the stub-side of the PCB with a larger drill diameter (i.e. back drill) to a specified depth in order to reduce the stub length. This improves signal integrity by minimizing interference or signal loss due to excess stub length. Current industry back drilling capabilities have supported greater than 1mm pitch with a minimum back drill-to-metal gap of greater than 0.15 mm. For pitches < 1mm, the drill-to-metal gap will need to be reduced to less than 0.15 mm. In addition, primary drill (PD) diameters will need to scale down.
These changes pose manufacturability challenges with primary drill registration and higher aspect ratios (i.e. PCB thickness/PD). Reduced spacing compounded with drill registration issues can result in exposed copper, slivers//clipped traces, and layer-to-layer misregistration. Industry PCB manufacturing capability and experience with these finer pitches is immature. Next-generation server platforms will push the limits of current PCB industry capabilities, creating a need to identify and provide solutions to enable future manufacturing technologies for server PCBs requiring < 1 mm (0.94 mm) pitch designs. This paper will assess PCB vendor drill registration capability and will also evaluate PCB reliability using electrochemical migration (i.e. conductive anodic filament or CAF) and via reliability (i.e. interconnect stress testing or IST) testing. PCB manufacturing capability will be characterized as a function of back drill-to-metal gap capability and provide potential solution paths to enable PCB suppliers to fabricate reliable 0.94 mm pitch server boards.
The movement to high-speed server designs in order to meet customer demands for better electrical performance may result in higher demand in the future for the adoption of back drilling in the PCB manufacturing process. Ultra-low loss (ULL) PCB materials may also be needed to satisfy the electrical performance requirements. Limited PCB manufacturing and reliability data exist for future fine pitch designs using BD-metal gaps < 0.175 mm and ULL materials. This work found that PCB fabricators tested in this study are capable of manufacturing server designs with PD registration ranging from ~ 0.050 mm to 0.125 mm, but there is variability from supplier to supplier. The data suggests that designs that have BD-metal gaps ≤ 0.175 mm could result in exposed copper or a reduced BD-metal gap due to drill misregistration. If the copper is exposed or mechanical damage with a reduced BD-metal gap occurs due to drill misregistration, this could pose CAF or ECM risks. This work found that plugging is a solution to mitigate CAF risks for BD-metal gaps at ≤ 0.175 mm.
The IST data shows that there may be some via reliability risks for higher aspect ratio PCB designs, and a better understanding of the PCB suppliers drilling, and the plating process is required. Failure analysis and more testing is required for the current suppliers, and there is a need to test a wider range of ULL material to get a better understanding of the influence of PCB material on the via reliability. It is important for customers to work with there PCB suppliers to better understand the PCB manufacturing process flow and run scout lots during the PCB manufacturing process so that the drilling, plating, scaling of the inner layer registration, and via plugging can be optimized. Further work is also needed to better understand the impact of ULL materials on PCB reliability.
Initially Published in the SMTA Proceedings