CALCE Researches Solutions for CAF Formation

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CALCE Research and Labs Offer Solutions to Conductive Anodic Filament Formation Problems

Conductive anodic filament (CAF) formation is a reliability concern that has dogged the electronic industry for decades and remains a concern even with board materials that suppliers claim to be CAF-resistant. The root causes of this class of failure are not well understood, and the types of tests being conducted by the laminate manufacturers and the OEMs to design and get board assemblies fabricated are long and expensive without being useful for determining CAF risks or preventing CAF in the field.

CALCE has been working in the area of understanding and preventing CAF over the last several decades and has made fundamental contributions that have helped many companies make engineering decisions to reduce risk of CAF, including:

  • Role of threshold humidity level in precipitation of CAF
  • Role played by hollow fibers in glass in increasing the risk of CAF
  • Development of a test method to detect hollow fibers
  • Development of an industry standard on “acceptable level” of hollow fibers

In addition, CALCE has worked on multiple failure analysis studies to identify CAF failures. Combinations of superconducting quantum interference devices (SQUID), in-situ electrical monitoring, and micro-level cross-sectioning have been used to determine the location and nature of CAFs.

CALCE research has also been incorporated into various industry standards, particularly those from the Institute for Printed Circuits (IPC):

  1. IPC 9691B, User Guide for the IPC-TM-650, Method 2.6.25, Conductive Anodic Filament (CAF) Resistance and Other Internal Electrochemical Migration Testing
  2. Amendment 2 to IPC-4412B, Specification for Finished Fabric Woven from "E" Glass for Printed Boards
  3. IPC-1601A, Printed Board Handling and Storage Guidelines

A recent publication by Dr. Bhanu Sood and Prof. Michael Pecht, “The effect of epoxy/glass interfaces on CAF failures in printed circuit boards,” (abstract below) was the focus of a CALCE talk regarding the concerns with CAF and methods to analyze low-resistance failures.

In an effort to aid the electronics industry, CALCE has been performing research and conducting tests to help prevent and reduce CAF formation of electronic components and parts. A list of recent publications on filament formation, failures, risks, and reliability issues can be found below.

The CALCE Test Services and Failure Analysis (TSFA) Lab is ready to provide the most efficient and cost-effective design reviews, reliability audits, material characterization, testing, reliability assessment, and failure analysis assistance to all interested parties.

Contact CALCE for your needs on technology selection and failure analysis relating to CAF.

Abstract: Reduction in printed circuit board line spacing and via diameters and the increased density of vias with higher aspect ratios (ratio between the thickness of the board and the size of the drilled hole before plating) are making electronic products increasingly more susceptible to material and manufacturing defects. One failure mechanism of particular concern is conductive anodic filament formation, which typically occurs in two steps: degradation of the resin/glass fiber bond followed by an electrochemical reaction. The glass-resin bond degradation provides a path along which electrodeposition occurs due to electrochemical reactions. Once a path is formed, an aqueous layer, which enables the electrochemical reactions to take place, can develop through the adsorption, absorption, and capillary action of moisture at the resin/fiber interphase. This study describes the experimental and analytical work undertaken to understand the glass-resin delamination and the methods used for analyzing this critical interphase.

Recent publications on filament formation, failures, risks, and reliability issues:

The Effect of Epoxy/Glass Interfaces on CAF Failures in Printed Circuit Boards , B. Sood and M. Pecht, Microelectronics Reliability, Vol. 82, pp. 235-243, March 2018.

Conductive Filament Formation in Printed Circuit Boards – Effects of Reflow Conditions and Flame Retardants, B. Sood and M. Pecht, 35th International Symposium for Testing and Failure Analysis, San Jose, CA, November 15-19, 2009.

A Variant of Conductive Filament Formation Failures in PWBs with 3 and 4 mil Spacings, K. Rogers and M. Pecht, Circuit World, Vol. 32, No. 3, pp. 11-18, 2006.

Hollow Fibers Can Accelerate Conductive Filament Formation, M. Pecht, K. Rogers, and C. Hillman, ASM International Practical Failure Analysis, Vol. 1, No. 4, pp. 57-60, August 2001.

Conductive Filament Formation: A Potential Reliability Issue in Laminated Printed Circuit Cards with Hollow Fibers, M. Pecht, C. Hillman, K. Rogers, and D. Jennings, IEEE/CPMT, Vol. 22. No. 1, pp. 60-67, January 1999.

Conductive Filament Formation Failure in a Printed Circuit Board, K. Rogers, C. Hillman, M. Pecht, and S. Nachbor, Circuit World, Vol. 25, No. 3, pp. 6-8, 1999.

The Physics of Conductive Filament Formation in MCM-L Substrates, M. Li, M. Pecht, and L. Wang, Proceedings of the INTERpack'95, Lahaina, Maui, HI, pp. 517-527, March 26-30, 1995.

Assessing Time-to-Failure Due to Conductive Filament Formation in Multi-Layer Organic Laminates, B. Rudra, M. Pecht, and D. Jennings, IEEE Transactions on Components, Packaging and Manufacturing Techniques - Part B, Vol. 17, No. 3, pp. 269-276, August 1994.

Tutorial: Failure-Mechanism Models for Conductive-Filament Formation, B. Rudra and D. Jennings, IEEE Transactions on Reliability, Vol. 43, No. 3, pp. 354-360, September 1994.

Conductive Filament Formation in Printed Wiring Boards, M. Pecht, B. Wu and D. Jennings, 13th IEEE International Electronics Manufacturing Technology Symposium, pp. 74-79, 1992.

Published December 20, 2018