PCB CAF Troubleshooting Guide
PCB failure modes can be classified into two categories:
- Mechanical Failure
- Electrical Failure
But there is also a third class of failure which is like a hidden treasure and that is “Electrochemical Failure Mode”. In other words, this failure is known as Conductive Anodic Filament (CAF), which is the result of the electrochemical reaction that takes place in the presence of water contents between Copper in PCBA and resin system in PCB substrate. In the following content, FS Tech will introduce the causes, detection methods and solutions of CAF failures in detail.
What is CAF
CAF is considered a type of PCBA failure and is a result of electrochemical migration and this process is termed CAF Formation. It is carried forward by applied voltages which in some conditions form copper oxide and cuprous salts. These oxides and salts are conductive in nature and grow in PCB substrate. In this process copper salt conductive filament formed within PCB that develops along the epoxy within dielectric material i-e from anode to cathode. One of the most favorable conditions for CAF to grow is when there is void present in PCB and low resin in tight weaves. This failure mode is gaining concern among experts as it has catastrophic effects and can result in PCB failure.
Three substances in printed circuit boards that cause electrochemical reactions:
- Water or other volatile chemicals, used while PCB fabrication, absorbed by PCB substrate
- Decomposition of the resin system, and due to voids that are closer to the anode.
The formation of CAF can be described in to following two points:
- Degradation of resin glass
- Electrochemical migration
Following are some of the conditions necessary for CAF to develop:
- For an electrochemical cell to be formed, an electrical charge carrier must be present.
- Water, may be due to humidity or other environmental conditions.
- Voltages that may drive the electrochemical reaction
- For the movement of CAF growth from anode to cathode, a specific pathway must be there for the formation of CAF
- Other conditions like High humidity, high temperature, soldering flux contents, and high voltages gradient between cathode and anode.
Detection of CAF
Once CAF occurs, then it becomes a challenging problem and is difficult to study and PCBA test because sometimes it may grow in the middle layers of PCB. With the advancement in technology, there are now different testing methods to check if there is CAF growth in PCB substrate or not. Above all known testing methods, the “Surface Insulation Resistance (SIR)” tests are electrical methods that are at IPC standards. Generally, SIR includes the following two types of tests:
- IPC Electrochemical Migration Test: This test includes the measure of the resistance offered by the PCB surface substrate to the flow of current passing through it.
- Temperature-Humidity-Bias Test: In this simple test, only processing temperature, voltage biasing and relative humidity are taken into account.
There are further testing methods to test CAF and to image CAF formation in PCB. These tests include
- Energy Dispersive Spectroscopy (EDS): Electronic beam gun is used to send an electronic beam to identify some elements like copper, bromine, and chlorine. ∙ Focused Ion Beam (FIB): This method is used to magnify surfaces at a very high resolution.
- Transmission Electron Microscopy (TEM): This is used to detect crystallographic structure and to identify other phases of materials in PCBA. This system works on the principles of a light microscope.
- X-Ray Photoelectron Spectroscopy (XPS): This technique is also known as Electron Spectroscopy for Chemical Analysis (ESCA), this technique is usually used to detect chemical compounds on PCBA surfaces. Generally, this method is used to analyze the surface of PCBA.
- Fourier Transform Infrared Spectroscopy (FTIR): This method examines organic components in PCBA substrate and generates wavelength and intensity spectrum. Ion Chromatography: This method separates ions and can be termed ion-exchange chromatography
How to Avoid CAF Failures
There are several actions to take to reduce the chance of CAF failure. The hunt for solutions to this CAF failure is ongoing but avoiding the circumstances that allow CAF creation will assist. Here are some things to think about:
Moisture and Humidity
CAF requires moisture and water contents to grow, which means more water contents results in more chances of CAF failure in PCBA to happen. So, it is important to have a keen eye on moisture content and get rid of water content as efficiently as possible.
Some processes during PCB fabrication may cause acid to remain on the PCB substrate, if not cleansed well, then this leftover acid usually leads to the growth of CAF. So, it is important to keep PCBA well-cleaned and remove any type of acid from it.
Bias and Voltage:
As discussed earlier, there is the force required to drive the electrochemical reaction and here biasing and voltage is the force that aids the formation of CAF in a favorable environment.
The materials utilized to construct the board are another important CAF-failure cause. One of the most effective strategies to reduce CAF production and failure is to use CAF-resistant materials. Various studies conducted on laminate materials have measured the vulnerability to CAF failure. According to the findings, the material which is most resistant to CAF is “Bismaleimide Thiazine (BT)”, whereas MC-2 is the most vulnerable. In crux, the laminates which are highly thermally resistant, are the most resistant to CAF development.
The design of a PCB and its fabrication are both also important factors in influencing its CAF resistance. PCBs with greater gaps between voltage-biased features fail lesser than the ones with fewer gaps, while this is thought to affect just the second phase of the CAF-formation. CAF resistance is influenced by the size of the drilled holes and the thickness of copper in plated through-holes, in addition to hole-to-hole and line-to line spacing. Having more biased voltage characteristics increases the probability of CAF formation.
More Interesting PCBA Contents
CAF failure is only one of the common problems of circuit boards. The reasons for the failure also include irregular use of flux, uncleaned PCB, assembly problems, etc. Read the following to help solve your PCB manufacturing or assembly problems.
Printed circuit board cleaning steps