Purpose
Background
Interim Observations
Prior Research

Purpose:
This experiment documents and monitors tin whisker growth on one lot of pure tin-plated multilayer ceramic capacitors (MLCCs) mounted inside a hybrid using conductive (silver) epoxy and then subjected to extensive thermal cycling followed by long term ambient storage.  The experiment was initiated in 2001 by a non-NASA entity who then donated one of their test samples to NASA Goddard tin whisker researchers for continued monitoring.  Details of this study and interim observations were formally published in June 2002 at the American Electroplaters and Surface Finishers (AESF) SUR/FIN conference in Chicago, IL.  The paper also discusses similar but unrelated experiments by others attempting to induce whisker growth on tin-plated MLCCs.

1. J. Brusse, "Tin Whisker Observations on Pure Tin Plated Ceramic Chip Capacitors", AESF SUR/FIN Conference, June 2002, pp 45-61
2. Slide presentation is also available.

The NASA Goddard Whisker WWW Site will be used to provide updates as monitoring of these specimens continues.

Background:

In 2001 a hybrid microcircuit manufacturer informed NASA Goddard tin whisker researchers of their surprising observation of profuse whisker formation on pure tin-plated ceramic chip capacitors (MLCCs).  The hybrid manufacturer made this discovery during an evaluation intended to assess the electrical and mechanical integrity of joints made between pure tin-plated MLCC terminations and gold-plated substrate pads when using conductive (silver) epoxy instead of solder reflow for mounting.  Their experiments included the following basic approach:

In May 2001 the hybrid manufacturer donated one thermally cycled evaluation hybrid package (200+ cycles per Condition 1 above) to NASA Goddard for continued observation.  The package contained 6 pure tin-plated ceramic chip capacitors (size 0805) all reported to be from one manufacturing lot.  As received by NASA Goddard, the hybrid package had already been delidded.  The manufacturer supplied photos of the capacitor terminations to illustrate the extent of whisker growth they observed after temp cycle.  Upon receipt at NASA Goddard the sample was reinspected and the whisker formation was confirmed to be consistent with the extent of growth reported by the hybrid manufacturer with all 6 capacitors showing clear evidence of tin whisker formation.   NASA Goddard has since retained this sample in office ambient storage for ongoing observation and analysis.

NOTE:  The hybrid manufacturer reports they originally intended to use palladium/silver terminated capacitors in an application where the capacitors were to be mounted to a substrate using conductive epoxy.  Erroneously, the supplier of the capacitors provided pure tin plated capacitors (standard commercial offering) to fill the user's order.  CAVEAT EMPTOR.  The hybrid manufacturer noticed a visible difference in appearance of the termination finish at incoming inspection.  The subsequent evaluations were initiated to determine if the mistaken shipment of capacitors could be used for production anyway.

Interim (Summary) Findings (reverse chronological order):

August 2001 - NASA Goddard Inspection to Confirm Observations Reported by Hybrid Manufacturer

Q1.  What is the max length of whisker and typical length of whisker?  Quantify typical length of whiskers.
A1.  The max length of whisker was estimated to be 100 um and the typical length is estimated to be around 25um to 50 um.  This cap is populated with mainly two types of whiskers: toothpaste like (thick curly type) and needle like.  The density of the whiskers (including both types) is estimated to be 800 whiskers/mm2 (conservative estimate) which can be compared to GSFC experiment using pure tin plated brass substrates with ~50 whiskers/mm2 (max density sample).  Among entire whisker population, 5% is needle like whiskers that would grow potentially dangerously long.

Photos Courtesy of NASA Goddard Space Flight Center

Q2. EDAX on the cap termination to confirm if there's any lead (Pb) content?
A2. EDAX results showed Sn element exclusively.  No Pb was observed (Accuracy: Elapsed time was 500 sec.).

Q3.  Examine whisker growth at the conductive epoxy interface, and WITHIN the conductive epoxy?
A3.  The whiskers are present only on plated termination on both sides of the caps.  No whiskers were observable at the conductive epoxy interface nor within the conductive epoxy.

Previously Published Research on MLCCs and Tin Whiskers:

DISCLAIMER:
This website provides information about tin whiskers and related research. The independent research performed during the past 50+ years is so vast that it is impractical to cover all aspects of tin whiskers in this one resource. Therefore, the absence of information in this website about a particular aspect of tin whiskers should NOT be construed as evidence of absence.

COPYRIGHT NOTICE:
This material is presented to ensure timely dissemination of scholarly and technical work. For technical references posted herein, copyright and all rights therein are retained by authors or by other copyright holders. All persons copying this information are expected to adhere to the terms and constraints invoked by each author's copyright. In most cases, these works may not be reposted without the explicit permission of the copyright holder.

SECURITY BANNER