Rockwell Collins, Cedar Rapids, IA CHARACTERIZATION OF THE CAPABILITY OF CONFORMAL COATING TO INHIBIT TIN WHISKER GROWTH Dave Hillman Rockwell Collins, Cedar Rapids, IA david.hillman@rockwellcollins.com

Agenda Background Objective Test Procedures Test Results Discussion Conclusions Questions Photo Courtesy of Bob Ogden

Background Graph Data Source: Issues and Outlook for Lead-Free Electronics in Military & Aerospace Applications (April 2015)

Background Photo Credits: Left – Rockwell Collins, Right – NASA Card Guide tin plated Beryllium

Background Cell Source: S. Meschter, “ReMAP Materials Project M2: High Temperature High Humidity Corrosion and Tin Whisker Evaluation of Bi containing Leadfree Alloys, SMTA ICSR Conference, 2016.

Background Cell Source: S. Meschter, “Strategic Environmental Research and Development Program (SERDP) Layered Nanoparticle Enhanced Conformal Coating for Whisker Mitigation, SMTA ICSR Conference, 2016.

Background: Woodrow Data T. Woodrow Conformal Coating Data [7]

Background: NPL Testing Daisy chained SOIC M. Wickham, National Physical Laboratory (NPL), [8]

Background: IPC Committee Test Conformal Coating Types: AR = acrylic ER = epoxy UR = urethane SR = silicone XY = parylene Application Method: Machine or Manual Spray Dip Brush Other Application Volunteers to collect and retain detailed documentation of application method –NO IP will be disclosed. Sufficient baseline application process details have been disclosed for every application method. Test Vehicle

Background: IPC Committee Test

Objective: Conformal Coatings: The objective of the investigation was to characterize the ability of different conformal coatings to trap and/or contain whiskers within the coating. Conformal Coatings:

Test Component:

Testing: Test Parameters: A representative plating lot was tested A total of 24 test samples were tested The inspection intervals were 3000 hours, 6000 hours and 9000 hours No ‘preconditioning’ was conducted on the test samples Conditioning parameters: High Temperature/Humidity: 85ºC and 85%RH, non-condensing environment

Inspection: Scanning electron microscope (SEM) inspection was conducted on the test samples per the prescribed intervals. SEM inspection was conducted using 100X to 5000X magnifications. Once a test vehicle was removed from temperature/humidity conditioning and inspected, it was not returned to the chamber. Every lead of each component Tinman was assessed at each inspection interval. Inspection at Zero hours: No tin whiskers on any samples

3000 Hours/Control:

3000 Hours/Acrylic A:

3000 Hours/Acrylic A:

3000 Hours/Acrylic B:

3000 Hours/Acrylic B:

3000 Hours/Parylene:

6000 Hours/Control:

6000 Hours/Acrylic A:

6000 Hours/Acrylic B:

6000 Hours/Acrylic B:

6000 Hours/Parylene:

9000 Hours/Control:

9000 Hours/Acrylic A:

9000 Hours/Acrylic A:

9000 Hours/Acrylic B:

9000 Hours/Acrylic B:

9000 Hours/Parylene:

Discussion/Conclusions: The investigation results show that the Acrylic A and the Parylene conformal coating provided acceptable tin whisker risk mitigation as no instances of whiskers escaping or rupturing the coatings were observed after 9000 hours (375 days) of conditioning. The Acrylic B conformal coating did not provide acceptable tin whisker risk mitigation as multiple instances of whiskers escaping or rupturing the coatings were documented. A portion of the whisker escapes/ruptures are attributable to inconsistent Acrylic B coating application, but significant numbers of whiskers were able to escape/rupture the coating. The reader should not conclude that the Acrylic B conformal coating provides no tin whisker risk mitigation; this result indicates that the coating should be used in conjunction with additional risk mitigation methods such as solder poisoning [10, 11] and design attributes such component spacing. The reliance on a single tin whisker risk mitigation methodology is not an engineering best practice and the use of multiple concurrent risk mitigation strategies is preferred. The investigation results demonstrate that conformal coating can play a critical role in an overall tin whisker risk mitigation strategy.

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