Unfavorable environmental conditions such as temperature fluctuations or high humidity do not only affect the surface of printed circuit boards. Conductive paths can also form inside the PCB between conductor structures with a potential difference, which can lead to a short circuit and cause a failure. In addition to the surface resistance, it is therefore also necessary to evaluate the volume resistance of the PCB with regard to its long-term resistance. The CAF test is used here as a suitable method.
What does CAF mean?
CAF stands for Conductive Anodic Filament and describes the physical-chemical effect of ion migration - usually copper compounds - along glass fibers within the PCB. As a result of the migration, conductive paths are created which lead to a reduction in the insulation resistance between live conductor paths. In extreme cases, this causes a short circuit or failure of the module.
What characterizes the failure process inside printed circuit boards?
In comparison to superficial change processes, which are achieved by means of SIR test failure processes inside the PCB are generally slower. In addition, they are much more strongly influenced by the choice of material, layout and processing conditions than by incorrect handling or insufficient cleaning. The CAF test is becoming increasingly important, especially with ever smaller conductor spacing and overall component sizes in the course of miniaturization.
How does the CAF test work?
The test specimens with specific test structures are connected in the climate chamber with cables that are routed to the resistance measurement system on the outside. The system measures the absolute resistance and the leakage current at regular intervals, for example between rows of holes with opposite polarity or between holes and internal conductor runs.
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Typical test conditions:
- Pre-aging: 3x reflow simulation lead-free soldering
- Temperature: 85° C
- Relative humidity 85 %
- Bias voltage: 100 - 1,000 V
- Test duration: 1,000 to 3,000 h
Either the lower resistance limit (10 MOhm), the maximum resistance drop in relation to the initial resistance (3 tenths of a percent) or the upper leakage current limit are defined as failure criteria. The failure times are evaluated using Weibull statistics and the desired service life parameters for this type of fault are supplied.
How can failure processes inside the printed circuit board be avoided?
The aim is to achieve a high resistance to degradation of the internal insulation properties. The following factors play a role here:
- The selection of CAF-resistant base materials
- The use of layer structures that are optimized in terms of z-axis expansion in order to avoid delamination during processing
- A layout check for minimum distances
- The reduction of layout offset, which leads to shorter insulation distances
- The avoidance of process residues in the form of particles that shorten distances
- Clean handling
- High cleaning quality in all intermediate processes
Where can I find more information about the CAF test?
Detailed information on the CAF test procedure and valuable expert tips directly from the KSG Group's quality laboratory are available in the XPERTS online seminar "From gigaohms to short circuits: PCBs in the CAF climate test" by Dr. Swantje Frühauf and Holger Bönitz. Here to go to the registration.
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