CUSTOMER ISSUE: The EMPF Helpline received a call from an EMS (electronic manufacturing services)
provider who was experiencing failures on PWAs (printed wiring assemblies) that were in a large number of automobiles
after only one year in the field.

The suspected source of the open circuits was 0402 ceramic chip resistors that had small black dots near the electrode terminations. They were failing in an electrically open condition. Surprisingly, electronics manufacturing processes had not changed. Only the environment had changed as the board was relocated from within the interior of the vehicle to under the hood.

Silver is quite reactive and tarnishes easily in the presence of sulfur. A household example is unused silverware on display, the dark tarnish is primarily composed of sulfur. Energy Dispersive Spectroscopy (EDS) determined that the chip resistors with small visible corrosion spots on the exterior of the component contained both silver (Ag) and sulfur (S). Through examination of their relative atomic percentages, it was suspected to be a non-conductive Ag2S phase.

The failed resistors were then crosssectioned from the assembly, and subjected, along with a series of new resistors from commercial vendors, to rootcause analysis tests:

Cross-sectioning/SEM (Scanning Electron Microscope) analysis was used to look for cracks in the solder joints.

EDS Energy Dispersive X-Ray analysis and elemental mapping was used to determine the presence of unwanted chemical species at the resistor contacts.

No indication of cracked solder joints or poor workmanship was found. However, sulfur contamination and the presence of Ag2S, accounting for the electrical opens, were identified.

Upon confirmation of sulfur contamination as the root cause for the opens, samples of the product packaging plastic components and gaskets were submitted for Fourier Transform Infrared Spectroscopy (FTIR) to determine candidate sources for the sulfur contamination.

Results
Figure 1 reveals a plan view of nodules coinciding with the electrical open failure of a chip resistor in the product.

Figure 2 is an elemental analysis created at ACI using the EDAX capability of the SEM (Scanning Electron Microscope). The predominantly silver layer beneath the electrode contact of the chip resistor on a failed open resistor is shown. There is substantial S (sulfur) contamination in the silver (Ag) layer. The material structure is identical to the material of the nodules noted near the electrode contacts of the failed resistor (see Figure 1). This material is reasoned to be Ag2S, and is non-conductive. This is the apparent mechanism causing the electrically open failures of the resistors.

The next step was to determine likely candidates for a source of the contaminating sulfur. Several samples of plastic materials of the case, cover, and various gaskets used in the final product assembly in close proximity to the failed PWA were submitted by the customer to ACI for analysis. FTIR (Fourier Transform InfraRed) scans of the plastic components and rubber gasket material were run at ACI. There was a definite concentration of sulfur present in the gasket material that potentially was the source of the sulfur causing the failure. The plastics used in the container were confirmed by FTIR to be free of sulfur.

Recommendations
Electrode terminations for ceramic chip resistors contain both silver and a minor amount of palladium (Pd) the latter serves to protect against corrosion mechanisms involving sulfur. The Pd amount present in the resistors was questioned, as well as the ultimate source of the sulfur. Use of resistors having higher Pd (3% or above) concentrations in the Ag layer under the terminal metallization on the resistors was recommended as a general precautionary measure. Furthermore, ACI recommended minimizing the use of rubber gaskets containing sulfur for this situation.

Consultation with ACI partner Cookson Electronics revealed that Parylene conformal coating although expensive, is expected to be very resistant to any sulfur contamination ingress. The natural tendency of sulfur is to be in a cluster of eight sulfur atoms per cluster, and the Parylene is known to have much smaller porosity structure.

Satisfied with these results and recommendations, the customer elected to change the resistors to those showing a more sulfur impervious boundary layer, and to change to sulfur free rubber gaskets for the products wherever possible. For this application, the cost for Parylene conformal coating, although expected to add additional protection, is prohibitively expensive.