CAF was first identified in 1976 by researchers at Bell Lab. The first studies involved tests of different coatings of UV-cured resins using FR-4 fine-line flexible printed circuits. Further failure-preventing variables were also studied, including the number of glass-reinforcement layers, cover coat and the thickness of the epoxy butter coat layer. Researchers found they could model time to failure on a log-normal plot and identified temperature, humidity and bias as factors that sped up the process. They also described four substrate-related failures.
In 1979, researchers first used the term CAF to refer to these failures. The paper that first used it focused on how materials and conductor orientation related to CAF formation. That same year also saw the introduction of the two-stage model. Investigations into CAF has continued to this day, with researchers testing how humidity, temperature, bias, materials, conductor orientation and other factors influence its formation.
Most of this research has focused on traditional laminates, such as FR-4, G-10, BT and MC-2. More recently, though, some have applied newer materials, namely CAF-resistant or halogen-free laminates, which typically have improved thermal properties.
How to identify PCB CAF?
Identifying CAF once it occurs is challenging, making it difficult to inspect and study. CAF often occurs in layers buried within PCBs. It also can appear alongside other contributing failure factors, making it hard to realize when CAF is a solely-responsible failure mode.
IPC Electrochemical Migration Testing:An IPC standard test to measure the resistance to the flow of current across the surface of a PCB substrate.
Temperature-Humidity-Bias (T-H-B) Testing: A SIR test that takes into account the processing temperature, relative and aging humidity and voltage bias.
You can also use various methods to image CAF formation on a PCB. These techniques include:
Scanning Electron Microscopy (SEM): This method involves the use of a primary electron beam gun that sends electrons toward the positively charged anode in a vacuum through electromagnetic lenses. You can operate this device in either secondary electron (SE) mode — which is ideal for surface-topography imaging — or backscattered electron (BSE) mode, which allows for atomic number contrast.
What are the factors of PCB CAF?
- Electrolyte – Water
- Voltage bias – Force that drives the reaction
- “Pathway” – A way for the ions to move from the anode to the cathode
- The following factors accelerate CAF formation
- Higher water content
- Higher Voltage levels
- Higher temperature (increases reaction rate of corrosion