A customer contacted the EMPF and requested a root cause analysis of a sensor failure...

Recently, a customer contacted the EMPF and requested a root cause analysis of an automotive sensor failure. After several conversations with the customer to discuss the failure modes and to determine the customer’s requirements, the EMPF used the following test methods: electrical testing, optical microscopy imaging, Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM/EDS) and Fourier Transform Infra-Red analysis (FTIR) for failure analysis. Several samples of the sensors were supplied by the customer to the EMPF, and analytical results were obtained.

Test Methods:

Optical Microscopy Imaging - The sensors provided by the customer were examined and several images were generated which facilitated the electrical measurements. These images were taken with an Olympus Stereo Microscope (Figure 2-1).

SEM/EDS Analysis - Scanning Electron Microscopy was performed using an Amray 1830 Scanning Electron Microscope to image the sensor for the determination of physical size, layout, and composition of the patterned resistor elements on the sensor element.

FTIR Analysis - The hot film sensor die was removed from the sensor base and the infrared spectrum of the residue was determined through a Thermo-Electron Avatar 360 FTIR with the Inspect IR plus microscope.

Results:

▲ The SEM/EDS analysis of the sensor indicated the sensor was predominately platinum (Pt). Silicon (Si) was also observed, but may have been the result of a thin film of Pt.

• The amount of residue on the sensors varied
  
• The characteristic Infrared signature of the conformal coating following wave-number (cm-1) assignments:
  
• 3284 (most likely moisture in the air or on the sample)
  
• 2361 (CO2 found in all spectra due to atmosphere)
  
• 2341 (CO2 found in all spectra due to atmosphere)
  
• 1264 (due to Si-CH3 )
  
• 1106 (due to Si-O-CH3 asymmetrical stretch)
• 950 (due to Si-O-CH3 symmetrical stretch)
  
• 822 (due to Si-C)
  

▲ The spectrum of the residue on each sensor was the same based finger print region (1500cm-1 to 500 cm-1).

• The characteristic Infrared signature of the residue included the following wave-number (cm-1) assignments:
  
• 2360 (CO2, found in all spectra due to atmosphere)
• 2340 (CO2, found in all spectra due to atmosphere)
  
• 1259 (due to Si-CH3 )
  
• 1083 (due to Si-O-CH3)
  
• 1015 (due to Si-O-CH3 asymmetrical stretch)
  
• 864 (due to Si-O-CH3 symmetrical stretch)
  
• 793 (due to Si-C)

▲A comparison of the finger print region (1500cm-1 to 500 cm-1) of the samples and the conformal coating (Figure 2-2), indicated slight differences in the spectra between the coating and the residue. The characteristic absorption bands can shift depending upon the number and type of groups surrounding the functional group. For example, the spectrum for a polymer will have slightly shifted bands versus the monomer (starting material).

Conclusions:

The residue collecting on the sensor consisted of silicone and is most likely due to outgassing from the silicone conformal coating on the hybrid sensor. This residue is a distribution of molecular weight fragments consisting of monomer and oligomers (di, tri, and molecules consisting of multiple repeating groups).

A polymer can revert back to its raw materials, or a shift in the distribution can result from the volatilizing of lower molecular weight components depending upon the polymer. This can be affected by temperature and humidity and is dependant upon the type of polymer, degree cross-linking, and the degree with which the polymer is cured. In this application, the environment is such that a breakdown of the conformal coating is reasonable.

Recommendations:

This particular conformal coating may not be the best choice for this application. There are numerous low volatile organic compound conformal coatings available. A silicon conformal coating is a good choice because of its low mechanical stress properties, using very low VOC materials such as those used by the aerospace industry would eliminate outgassing residues.
Viscosity, operating temperature range, and optical clarity requirement will also guide material selection.