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Tests confirm resins systems have greater impact than glass finish on insulation resistance. But are CAF failures more a result of the fabrication process?
Conductive anodic filament (CAF) failure is the growth or electromigration of copper in a PCB. This growth typically bridges two oppositely biased copper conductors. This failure can be manifested in four main ways: through hole to through hole, line to line, through hole to line, and layer to layer. The most common failure mode is hole to hole (Figure 1).1,2
A combination of bias voltage (voltage applied during the test) and high relative humidity are known to cause a CAF failure during testing. The electrical failure is caused when a filament grows from a copper anode to a copper cathode.
Theoretically, CAF failure proceeds in two stages. The first stage involves the degradation of the resin glass interface followed by an electrochemical migration process, which allows the filament growth. The first stage is believed to be reversible, and the material's insulation resistance returns after baking and drying. However, the second stage of actual CAP growth is believed to be irreversible.
The mean-time-to-failure is a function of voltage bias, relative humidity, hole-to-hole and line-to-line spacings, temperature, and the resin system. The temperature relationship can be considered as an Arhennius relationship while power laws approximate the relationship with other variables.
CAF migration starts with a degradation of the resin glass interface by moisture. This primary step may be hastened by numerous factors including drilling, stress due to coefficient of thermal expansion (CTE) mismatch, thermal decomposition during processing, voids, and dryness in cores.
Once the resin glass interface degrades, the PCB behaves like a cell, and the following reactions may occur.
At the anode:
Cu [arrow right] Cu^sup n+^ + ne
H2O [arrow right] 1/2 O2 + 2 H+ + 2e^sup -^
At the cathode:
H2O + e^sup -^ [arrow right] 1/2 H^sub 2^ + 2 OH^sup -^
This generates the reaction products: copper ions and hydrogen ions at the anode and hydroxyl ions at the cathode.
The enthalpy of formation of copper ions is around 65 KJ/mole. Thus, the reaction needs an activation energy, and the reaction is temperature dependent.
Once the conductive path is set up, the ionic mobility and...