Novel Whisker Mitigating Composite Conformal Coat Assessment
WP-2213
Objective
An electronic assembly is created by integrating thousands of parts from multiple suppliers utilizing a host of circuit card manufacturing processes. With the lead elimination from electronics resulting from the European Union Restriction of Hazardous Substances (RoHS) legislation, many of the heritage aerospace and defense commercial off-the-shelf tin-lead materials and processes have become obsolete. Most notably there has been a significant increase in pure tin and lead-free-tin part finishes and solders in aerospace and defense products resulting in an increased risk due to tin whisker shorting. As part of their adaptation strategy, original equipment manufacturers are relying on multiple mitigation methods to manage the tin whisker risk. One of the main mitigations is the use of conformal coating to prevent tin whisker electrical shorting. Current low-cost coating spray processes generally provide incomplete coverage and insufficient coating thickness, especially for area array devices and electronic devices with low stand-off heights. There are vapor-deposited coatings that provide complete coverage but have yet to demonstrate complete whisker mitigation, are difficult to repair, and have a high manufacturing cost.
The objective of this project is to develop a coating that provides the inclusion of nanoparticles to improve strength and toughness while maintaining a uniform film and retarding or preventing whisker penetration. Small nanoparticles provide improved conformal coating mechanical properties while having a low impact on the coating's transparency and viscosity characteristics.
Technical Approach
Desired qualities of a coating that will exhibit superior whisker mitigation include complete coverage, high strength to buckle whiskers, and high elongation prior to breaking to capture whiskers within the coatings. Furthermore, for the coating to be effective, it must retain these material properties over a range of humidity, temperature, and fluid exposure. The key elements being evaluated are (1) coating and particle chemistry (2) mechanical properties enhancement (3) coverage quality, (4) layered coating application, (5) fluid resistance, and (6) rupture resistance due to whiskers. Nanoindentation and nanoscratch testing will be used to characterize the deposited coating properties and flexure beam samples will be used to determine the surface stress between the coating and the tin surfaces over a range of conditions. Through the use of structured experiments and finite element analysis, the project will seek to correlate measured coating properties to the whisker penetration resistance.
The nanoparticle filled coating development will begin with the functionalization of silica nanoparticles, which are added to a suspension media that will then be mixed into the base coating material. The base coating will be a urethane-based chemistry that meets the requirements of IPC-CC-830 and MIL-I-46058. The nanoparticle surface modification will be designed with bonding sites that will tightly bind the particles to the resin matrix and assure uniform dispersion without agglomeration. The initial evaluation of particle size and loading will be based on viscosity, transparency, dynamic modulus, nanoindentation, and nanoscratch adhesion testing.
The dip coating process will be used as a baseline coating application method since it provides good coverage and manufacturability. The experimental and analytical assessment will evaluate various thicknesses and layer combinations of filled and unfilled coatings on idealized and real electronic assemblies. The coverage and thickness on various metal features will be assessed using scanning electron microscopy and cross-sectioning. In addition, whisker penetration resistance will be assessed using a novel tin-silver-copper alloy that has rare earth elements added to obtain a rapid whisker growth. In addition, more traditional high temperature/high humidity and thermal cycling will also be used in the assessment.
Benefits
The improved whisker mitigation of the nanoparticle coating will result in increased aerospace and defense electronic assembly reliability. The results of this work will be submitted to both the IPC and the Government Electronics and Information Technology Association (GEIA) committees responsible for conformal coating of aerospace and defense electronics. Recommendations will be made for inclusion in documents such as GEIA-STD-0005-2, IPC-HDBK-830, IPC-CC-830 and IPC - J-STD-001 when conformal coatings are used for whisker mitigation. (Anticipated Project Completion - 2016)
Points of Contact
Principal Investigator
Dr. Stephan Meschter
BAE Systems
Phone: 607-429-8828
Fax: 607-755-6615
Document Types
- Fact Sheet - Brief project summary with links to related documents and points of contact.
- Final Report - Comprehensive report for every completed SERDP and ESTCP project that contains all technical results.
- Cost & Performance Report - Overview of ESTCP demonstration activities, results, and conclusions, standardized to facilitate implementation decisions.
- Technical Report - Additional interim reports, laboratory reports, demonstration reports, and technology survey reports.
- Guidance - Instructional information on technical topics such as protocols and user’s guides.
- Workshop Report - Summary of workshop discussion and findings.
- Multimedia - On demand videos, animations, and webcasts highlighting featured initiatives or technologies.
- Model/Software - Computer programs and applications available for download.
- Database - Digitally organized collection of data available to search and access.