Authored By:
Tobias Bernhard, Roger Massey, Senguel Karasahin, Frank Brüning
Atotech GmbH, Berlin, Germany
Summary
Micro vias, be they mechanically, or more typically laser drilled have revolutionized the technical capabilities of today PCBs. Their high hit rate, and low-cost during manufacture, in combination with their small real estate requirements enable the High-Density Interconnects that have allowed PCB and product designers to push their applications to capabilities beyond anything thought possible with more traditional drilling techniques. That being said, with their number easily running into the many 1000s in a single PCB assembly, there is increasing concern about the long-term reliability of micro vias, especially when used in a stacked configuration.
Over the last few years, there has been a slow, but increasing number of reports concluding that stacked micro vias are failing preferentially when compared to an alternative staggered via design. However, while a staggered interconnect arrangement could be seen as a solution to satisfy the reliability demands, they are usually undesirable as they consume larger amounts of real estate which can’t be tolerated in many applications.
With strong evidence available, the preferential failure of stacked micro vias can’t be denied, and as such, there is now a growing number of investigations ongoing to examine microvia reliability and determine if and why staggered micro vias fail preferentially, and also trying to identify what is the ideal microvia structure for best reliability performance.
As a result of the ongoing need to understand the nature of any failure, there is an ever-increasing array of analysis tools being drawn on in order to inspect microvia structures. Optical microscopy has in many cases been superseded by the Scanning Electron Microscope, and the SEM is now being supplemented with other tools such as the Focused Ion Beam Microscope and Tunneling Electron Microscope to name a few. However, with each new analysis tool there comes a wealth of new information, and this needs to be understood and interpreted before that information can be valuable.
As part of investigations into microvia failures, we review the published data and find that while these new analysis tools are being readily used, there is what we consider to be some misinterpretation of the data, leading to inaccurate route cause diagnosis, and conclusions that are questionable at best, or wrong and misleading at the worst.
This paper summarizes these initial investigations and discusses the misinterpretation of data as well as offering some insight into other microstructural characteristics which will likely impact the physical properties and reliability of plated blind micro vias.
Conclusions
There is an increasing awareness and concern regarding the reliability performance of stacked microvia in advanced PCBs leading to extensive works investigating BMV failure. Through a number of associated investigations, it has become apparent that the use of SEM-FIB has become popular as it offers an enhanced understanding of the complex crystallography present within the microstructures of a plated and filled BMV. Care should be taken however when utilizing SEM-FIB as there is a risk of preparation artifacts which can impact the conclusions drawn, as such, and based on our own experiences, a recommended best practice for the investigations of failed BMV is offered.
Initially Published in the IPC Proceedings
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