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Via-In Pad Plated Over (VIPPO) Design Considerations
Via-In Pad Plated Over (VIPPO) Design Considerations
This work has evaluates BGA packaging and PCB design parameters to characterize their influence on double-reflow solder separation failure mode for mixed VIPPO BGA pad footprints.
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Authored By:
S.Y. Teng, P. Peretta and P. Ton
Cisco Systems, Inc.
San Jose, CA, USA

V. Kome-ong and W. Kamanee
Celestica Thailand
Laemchabang Sriracha Chonburi, Thailand

Summary
Increasing signal speeds (up to 28Gbps), board functional density and PCB layer count/thickness (up to 140 mils) are increasing the challenges associated with PCB design, especially with respect to the signal integrity. These constraints have forced CAD design engineers to use via-inpad plated over (VIPPO) structures in conjunction with traditional designs, such as dog bone traces, microvias, skipvias and pad-with-trace along with incorporating VIPPO back-drilling techniques in order to achieve routability and signal integrity requirements.

Under certain conditions, the use of VIPPO with other pad structures within a BGA footprint can result in a unique failure mode in which the BGA solder joint separates between the bulk solder and the intermetallic compound (IMC) either at the package pad or PCB pad interface, depending on whichever is the weaker interface. It can be either a complete or partial separation and hence, may or may not be detected at ICT/functional test. It is also extremely difficult to detect with inspection methods and is therefore a high risk for potential escapes to the field.

This solder separation occurs when the BGA component is subjected to a second reflow, i.e. during top-side SMT if the component is on the bottom-side of the board or during rework of an adjacent component. Details of the failure mode, including root cause hypothesis and specific case studies, will be discussed in this paper.

Test vehicles have also been designed to evaluate the influence of various package and PCB design parameters on this failure mode. These include BGA package body size, BGA pitch, VIPPO drill hole size, pad design (NSMD/SMD), microvia/skip via structures, backdrill depth, etc. Based on this work, PCB design guidelines have been established in order to characterize the limits and conditions for acceptable usage of VIPPO structures in order to prevent this type of failure mode from occurring in new product designs.

Conclusions
This work has evaluated various BGA packaging and PCB design parameters in order to characterize their influence on this double-reflow solder separation failure mode for mixed VIPPO BGA pad footprints. It was observed that this failure mode is not dependent on the BGA pitch and can occur for ≤ 1.0 mm pitch components. It was also found to be independent of the VIPPO drill hole size, occurring with both 9.8 and 7.9 mil drill hole sizes. However, this failure mode has been shown to be sensitive to the BGA package body size, with the risk of occurrence decreasing for large package body sizes. It is also suspected that the package weight, BGA density and ratio and location of VIPPO-tonon-VIPPO pads within the BGA array may also play a role along with the package body size.

As previously noted, the current guidance recommends not to mix the VIPPO pad structures with non-VIPPO pads or deep-backdrill VIPPO structures within a single BGA footprint. Further investigations are still needed to establish more specific guidance regarding the usage of these VIPPO structures with non-VIPPO pad designs or deep-backdrill VIPPO structures. As performance requirements continue to advance, these types of PCB structures and designs will become a necessity for future generations of products. Hence, these design guidelines provide only a short-term solution to address this failure mode. More detailed understanding of the mechanisms driving this failure mode and how to control them are needed to develop a long-term solution that can allow implementation of these mixed VIPPO designs.

Initially Published in the SMTA Proceedings

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