Conductive anodic filament, also called CAF, is a metallic filament that forms from an elector-chemical migration process and is known to cause printed circuit board (PCB) failures.
Conductive Anodic Filament (CAF) testing helps to determine the reliability of a printed circuit board (PCB) laminate material or a finished product. With conductor spacing and overall part sizes getting smaller and smaller, the necessity for this test is increasing.
Conductive anodic filament (CAF) failure is the growth or electro migration 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.
- Layer to layer.
The most common failure mode is hole to hole.
Testing Mechanism
CAF testing is performed under high temperature and high humidity environmental conditions, 65 or 85°C with 87% Relative Humidity (RH). The standard test method used is IPC-TM-650, Method 2.6.25A, utilizing standard coupon designs such as IPC-9253, IPC-9254, IPC-9255, and IPC 9256. The coupon designs should include different hole sizes; hole-to-hole, hole-to-plane, or Z-axis spacing; and glass fibre orientations to allow for all possible failure modes to determine if a laminate material will meet the necessary requirements.
IPC- TM-650 Method
For the IPC test method, the samples are placed under environmental testing for 596 hours, with a bias voltage being applied for the duration of the test. Periodic high resistance measurements are performed with the samples still under conditions. The data is reviewed for any large drops or high resistance shorting (≤1.00 x 107Ω). When CAF growth is suspected, we can perform a standard failure analysis within the failed area.
Conductive Anodic Filament (CAF) Formation is defined, in IPC-TM-650, Method 2.6.25A, as the growth of metallic conductive salt filaments by means of an electrochemical migration process involving the transport of conductive chemistries across a non-metallic substrate under the influence of an applied electric field, thus producing Conductive Anodic Filaments.
The main application of CAF is in PCB testing and Assembly.
Conductive anodic filament (CAF) formation is a significant failure mode inside multilayer PCBs. It results from an internal electrochemical process forming corrosion products between two opposite, and usually adjacent, charged copper conductors. It leads to lower resistance pathways forming within the laminate.
There has been a significant increase in concerns about the effect of CAF on board reliability due to:
01. the reduction of the inter-feature spacing caused by increased circuit density with finer PCB features and increased layer counts.
02. electronic circuits being subjected to increasingly harsh environments, especially in high reliability and safety critical applications.
03. higher soldering temperatures associated with lead-free solders which have the potential to affect laminate stability.
It should be applicable to boards used in all market segments, including high density interconnect, and automotive applications.