What’s Wrong with This Board?
The critical first step in any project at Foresite is performing a thorough visual inspection. This allows us to not only observe and document the state of incoming samples, but can also provide valuable information useful in determining root cause analysis. Below are selected examples of images captured during visual inspection and an accompanying description.
A client submitted field failure samples that had obvious signs of a fire event on a PCBA at high-current PTH connectors (battery and charge connections). Visual inspection of connectors removed from current production units showed obvious heavy residue entrapped in the connector body and deposited on the pins.
Figure 1 - Water Soluble Flux Residue Deposits on PTH Connector Pin
This led to analysis via ion chromatography which indicated that contamination typically associated with water-soluble flux residue was present on the surface of the board and in the connector body.
It was determined that liquid flux was being applied manually during the hand-soldering operation and the subsequent cleaning operation was not adequately removing the remaining active flux and flux vapor residues that had deposited on the bottom of the connector body. These active flux residues, in conjunction with humidity from the operating environment and the bias present on the connector pins, caused shorting of the high-current leads.
Figure 2 – Water Soluble Flux Residue Deposits on Bottom of PTH Connector Body
Figure 3 – Dendrites Originating from Plated Vias
Another client submitted NTF (no trouble found) PCBAs for failure analysis. Upon inspection of the samples, several areas were identified where dendrites were present with plated vias as the origination point. Ion chromatography analysis indicated high acetate sporadically across the samples. Two possible source of acetate were identified: 1) inconsistent cleaning of water-soluble flux residues or 2) conformal coating not being fully cured and allowing the acetate-based carrier to react with moisture in the environment. The origination of dendrites at plated vias indicates that residues from the board fabrication process were likely not properly cleaned from the boards prior to assembly.
Occasionally, after a localized extraction is performed on a PCBA using our C3, the solder mask will take on a hazy appearance at the extraction site. This is a tell-tale sign that the solder mask has not been properly cured. The heated extraction solution/steam will penetrate the solder mask that has been left porous by the improper cure and cause the hazy appearance. If we place the PCBA in a cross-flow oven, the normal appearance will return as the water is driven out of the material.
Figure 4 – Solder Mask Haze after C3 Localized Extraction.
Foresite has a suite of analytical equipment available to perform failure analysis of PCBAs, yet commonly simple visual examination of samples backed by experience and process knowledge can yield valuable insights to aid in root cause determination.