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Corrosion puzzle: The Question

11 Dec 2013  | Michelle Woolley

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What is the case? Corrosion on printed circuit board assemblies (PCBAs). In this instalment, we provide all the information we obtained through conversations and visits to the customer's facilities (because the corrosion was happening at the customer's sites). With this information, you can formulate your own theories. In "Corrosion puzzle: The Answer," I will show you the resolution and explain it.

1. The PCBAs were from different facilities that were located in the southern part of China, Hong Kong, and Japan.

2. The corrosion was very extensive. In many cases, the PCB had been thermally degraded between two conductors. The conductors most affected carried voltage differentials of 180VAC. Figures 1- 3 show some of the most severe corrosion product that spread across the PCB surface.

3. Figures 4 and 5 show an optical image and a SEM image of a burned area clearly showing the damage to the PCB. In order to produce this damage, enough current must have passed between the two conductors to heat the wiring beyond its degradation temperature. The PCB had a degradation temperature greater than 260C. There were no anomalies associated with the printed wiring board (PWB).

4. The PCBAs are held vertically in a cabinet with forced air flow from the bottom front of the cabinet to the top of the back side where the fans are placed. This allows a diagonal air flow across the PCBAs.

5. Most of the corroded areas were not uniform around the centres but was biased towards one side as is shown in figures 1 and 2.

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

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