A customer called in to the Helpline with a signal quality issue on input and output pins/leads of a component on their board assembly. The customer mentioned that the board assembly had been serviced a few times and signal quality had not improved. Residue was also found at the pin/lead tips of the component.

The Helpline team member’s initial response was possible oxidation on the pins. An investigative plan was formulated, consisting of the following:

• Visual inspection – optical microscope and x-ray
• Surface finish analysis – Auger analysis
• Cross-sectional optical analysis – Scanning Electron
Microscopy (SEM) with Energy Dispersive
Spectroscopy (EDS) analysis

The visual inspection revealed areas of concern on the pins/leads of the component. It was noted that the plating appeared to be scratched, possibly exposing the inner layer of the pins/leads (Figure 4-1). The residue on the tips was found to be discoloration on the surface of the pins/leads, which could have been due to oxidation as initially suspected. X-ray inspection showed no problem with the die of the component and the mating socket.
The Auger analysis indicated the presence of Lead, Tin, and high levels of Carbon on the gold plating of the socket. This suggested that the Lead and Tin were transferred from the pin/lead to the socket. The area referred to can be seen in Figure 4-2.

The SEM images showed the area of the scratches and EDS reading of the socket further supported the Lead and Tin transfer to the gold plating of the socket (Figures 4-3 and 4-4).

The high levels of Carbon present piqued some interest. A Fourier Transform Infrared spectroscopy (FTIR) analysis was conducted to determine the origin of the organic compound. The result implied that the component was handled with bare hands.

Conclusions
1) The contamination from handling the component pins/leads with bare hands increased the chances of added resistance to the pin/lead and socket.
2) The results from the Auger and SEM/EDS analysis pointed to the phenomenon of fretting corrosion.

Fretting corrosion is a process caused by the motions of mating contact surfaces and results in contact failure. Fretting is also influenced by vibration and thermal excursion. In conditions where the resonating and operating frequency match, vibration will have an impact on the degree of fretting. Aircraft and shipboard applications commonly have these conditions which can affect the connections of a component to its socket. Extreme temperature change can cause expansion of materials with different thermal expansion coefficients.

Increasing the holding force of the socket to the pin/lead is not a viable solution. Over time, those forces will also degrade and create the same fretting conditions. Repeated replacement of the component also degrades the holding forces.

Recommendation
With the possibility of vibration and thermal exposure to the board assembly, one solution is to match the plating surfaces of both the pin/lead and the socket. This will prevent any mismatched material transfer. A lubrication to reduce friction between the mating contacts can minimize wear on the contact surfaces.

If you have any questions regarding the diagnosis of connector issues with board assembly components, please contact the EMPF Helpline at (610) 362-1320. A manufacturing expert will be able to offer technical insight and appropriate advice regarding your concerns.