What is CAF (Conductive Anodic Filament) on PCB

What is CAF (Conductive Anodic Filament) on PCB

Many companies’products are plagued by CAF (conductive anodic filament), because they are unable to find the reason for CAF. The reason for the recent breakthrough is that we finally find the defective board with microshort-circuit phenomenon in the circuit board. Bad causes currently point to CAF.

Table of Contents
    Add a header to begin generating the table of contents

    What is CAF on earth

    CAF is actually the inside of the circuit board or welding green paint layer in the micro short circuit phenomenon. CAF means that DC voltage is applied to the printed circuit board and placed in a high humidity environment. In layer to layer, line to line, hold to hole or hole to line, CAF is applied to the printed circuit board.

    How to find out CAF on PCB

    In fact, it’s not easy to find this kind of CAF problem. First, you have to find out where the circuit board is short-circuiting, and then you have to cut all the lines that can be cut, and gradually narrow down the area where the short-circuiting is possible, preferably to the via to the VIA, or to the trace to the circuit.

    It is even necessary to measure the layer of copper foil short circuit, so that there is a greater chance to find evidence of micro short circuit under the cross section.

    Cut badly or inexperienced, not to cut the evidence disappeared, or when grinding the place is not short circuit imaging short circuit and cause misjudgment.

    Later, we took the board with a micro short circuit to the PCB board factory and asked for slice analysis on the spot. This time, CAF was really confirmed. However, the board factory thinks that the reason for the occurrence of CAF is that the PTH VS NPTH via and Blind Via design of our board are too close to each other.

    Now the board factory has changed the recommended distance of 0.4mm to at least 0.5mm. But now the hole edge to the edge of the hole (drill to drill) has been small to 0.1mm, but the requirement to have 0.5mm distance, this is actually very unrealistic.

    What may lead to CAF

    What-may-lead-to-CAF

    The copper metal in the high-potential anode will oxidize into Cu + or Cu ++ ions, and slowly migrate to the cathode along the existing bad channel of glass fiber yarn bundle, and the cathode electrons will also move to the anode.

    When the copper ions encounter electrons, they will be reduced to copper metal, and then form copper ions and then change back to copper metal. So repeated gradually from the anode to the cathode spread into copper trace, so it is also called copper migration.

    Many people who encounter CAF for the first time are continually perplexed by its repeated and intermittent behavior. Once the CAF has completed the channel conduction, it will be burnt off by joule heat with high resistance from time to time.

    Generally, when measuring CAF with three-purpose electric meter, it will be found that the short circuit is obtained at the beginning, but later the quantity is not enough, or the problem of copying the electric simulation is not always present. Even if the measured value will always drift, before the formation conditions of CAF completely disappear, CAF’s play will be repeated in the same place over and over again.

    To sum up, to form CAF defects, the following four failure conditions must be met at the same time, one of which is indispensable, that is, CAF can only be produced with absolute favorable conditions, geographical conditions and conditions. Therefore, accident is not accidental, but caused by a series of mistakes.

    Factors can influence CAF

    Water vapor :which seems impossible to avoid in the atmosphere or solvent is used to form electrolytes. For example, flux left on a circuit board or that is wet from a salty sea breeze are excellent electrolyte materials.

    Copper exposure (copper foil inside the circuit board when the base material, so it is unavoidable. In fact, as long as the material can form ions are counted, generally copper, silver is easier to form CAF.)

    Bias is a necessity of circuit design and therefore unavoidable. The so-called Javani effect causes potential differences between dissimilar metals.)

    Channel (seems to be the only way to improve this parameter). Should say the distance between the poles and obstacles! Shorter distances are more likely to occur because the insulation impedance is smaller.

    electric-field

    Under the action of electric field, the ElectroChemical Migration (ECM) reaction of metal ions occurs in non-metallic media, thus forming a conductive channel between the anode and cathode of the circuit, resulting in short circuit

    It is generally believed that electric ion migration is divided into two stages: the first stage, resin and reinforced material under the action of moisture, the chemical hydrolysis of silane coupling agent of the reinforced material, that is, on the epoxy resin/reinforced material along the leakage path of CAF, this stage is a reversible reaction. 

    In the second stage, under the action of voltage or bias, the electrochemical reaction of copper salt occurs, and the conductive channel is deposited between the lines to form a short circuit between the lines. This stage is an irreversible reaction.

    How to avoid and solve CAF

    If you want to solve or prevent the occurrence of CAF, in fact, you can start from the above four necessary conditions, as long as the elimination of any of the conditions can prevent its occurrence.

    Improve the Anti-CAF capability of circuit board materials

    The selection of circuit board is actually very important to protect the occurrence of CAF, but usually a penny a penny, generally have high CAF protection ability of the substrate need special requirements, the following is the selection of circuit board substrate anti CAF recommendations:

    ★ Reduce impurity ion content in materials.

    ★ The fiberglass cloth is well impregnated with resin and binds well.

    ★ When the substrate of circuit board is made, many bundles of glass fiber bundles are woven into cloth, and then resin tanks are introduced to impregnate, and then glass fiber bundles with resin are gradually pulled up or pulled out.

    The purpose is to make the resin fill into the gaps in the glass fiber bundles. If the parameters are not set well at this stage, it is easy to form gaps in the glass fiber bundles, allowing CAF gaps.

    PCB layout design

    PCB layout design in bias and hole spacing avoidance

    PCB-layout-design-in-bias-and-hole-spacing-avoidance

    The board’s through-hole, line size, position, and stack design also have a definite impact on CAF because almost all requirements come from the design. As the product gets smaller and smaller, the circuit board density gets higher and higher, but the PCB process capability has its limit.

    The smaller the distance between adjacent lines with DC bias voltage, the higher the probability of CAF will occur. Basically, the higher the bias voltage or the smaller the distance, the higher the probability of CAF will occur.

    According to the information provided by circuit board manufacturers, the following is the PCB size design value recommended by most circuit board manufacturers for CAF protection:

    ● hole to hole distance (minimum) : 0.4 mm
    ● hole to line distance (minimum) (Drill to Metal) : 12 mil (0.3 mm)
    ● aperture advice: 0.3 mm

    Wicking control in PCB process

    Wicking-control-in-PCB-process
    When the mechanical drilling or laser firing of PCB produces high temperature, which exceeds the Tg point of resin, it will melt and form a gel slag, which will adhere to the inner copper edge and hole wall area, resulting in bad contact during subsequent copper plating. 

    Therefore, De-smear operation must be carried out before copper plating. The first station of gel removal must use sweller after soaking treatment for 1 to 10 minutes, so that all kinds of gel slag swelling and relaxation, to facilitate the subsequent Mn +7 penetration and bite. 

    However, slag removal operations can also cause certain chew on the through hole and there is the possibility of copper icking (wicking, core). Some circuit board operators, in order to speed up the fluffy operation, will adjust the temperature of the fluffy groove to a higher level, resulting in excessive loosening of the fluffy agent, increasing the length of the insertion, leading to subsequent copper migration.

    IPC-A-600 provides for the admissible standard of copper icking (Wicking) as follows:
    ●  Class 1, permeability of copper is more than 125 (including m (4.291 mil)

    ●  Class 2, the permeability of copper is more than 100 (including m (3.937 mil)

    ●   Class 3, permeability copper (including more than 80 m (3.15 mil)

    But with the progress of science and technology, 0.1mm (100µm) copper penetration seems to have not meet the actual demand, to 0.4mm hole to hole edge distance, subtract the size of copper penetration, the distance is only 0.4-0.1-0.1=0.2mm, in order to circuit board industry’s manufacturing capacity, It should be possible to control copper penetration below 50µm (2mil). 

    Comparatively, the hole to hole distance of the system factory in the Layout circuit board is also reduced to 100µm (4mil), which is really a big test for CAF prevention and control.
    In addition, in the mechanical drilling operation of PCB, if the feed speed is too fast, or the milling cutter exceeds the service life, it is easy to tear the glass fiber because of the external force of the milling cutter.

    Control of waterproof moisture in PCB processing

    solder-paste-printing

    In PCB Assembly factory, solder paste printing, parts attachment, high temperature back welding and so on May leave some pollutants on the circuit board. These pollutants may include solder, glue, dust, dew and other electrolytic substances, which may cause electrochemical migration phenomenon. A sealant can be used to close junctions that may create gaps to form CAF to prevent infiltration of moisture.

    Conclusion

    In summary,according to practical experience, CAF channel (gap) almost occurs along the same glass fiber bundle, so if the through hole or welding pad arrangement can be made a 45 degree Angle of cross wiring will help to reduce the incidence of CAF.

    FAQ

    CAF is actually the inside of the circuit board or welding green paint layer in the micro short circuit phenomenon. CAF means that DC voltage is applied to the printed circuit board and placed in a high humidity environment. In layer to layer, line to line, hold to hole or hole to line, CAF is applied to the printed circuit board.

    In fact, it's not easy to find this kind of CAF problem. First, you have to find out where the circuit board is short-circuiting, and then you have to cut all the lines that can be cut, and gradually narrow down the area where the short-circuiting is possible, preferably to the via to the VIA, or to the trace to the circuit. It is even necessary to measure the layer of copper foil short circuit, so that there is a greater chance to find evidence of micro short circuit under the cross section.

    1. Improve the Anti-CAF capability of circuit board materials
    2. PCB layout Design
    3. Wicking control in PCB process
    4.Control of waterproof moisture in PCB processing

    Related Posts

    Related Posts

    Introduction to DIP Package : Understanding the Basics

    Introduction to DIP Package : Understanding the Basics

    DIP package (Dual Inline Package) is a type of electronic component packaging used for integrated circuits (ICs), such as microcontrollers, memory chips, and operational amplifiers, ...
    Introduction to Through Hole Technology

    Introduction to Through Hole Technology – THT in Electronics Assembly

    Through-Hole Technology is another type of component assembly technique. Its name comes from its working principle: the leads of the components pass through holes drilled ...
    How to use PCB copper foil in electronic manufacturing?

    How to use PCB copper foil in electronic manufacturing?

    PCB copper foil stands as the backbone of modern electronics, quietly but indispensably enabling the functionality of myriad devices. Comprising thin, flat sheets of copper, ...
    Exploring PCB annular ring : function, composition, and process

    Exploring PCB annular ring : function, composition, and process

    PCB annular rings are crucial for ensuring reliable solder joints, stable component mounting, and proper signal transmission or power delivery on the PCB. In this ...
    PCB Manufacturing Process

    What is the PCB Manufacturing Process ?

    Printed Circuit Boards (PCBs) are the backbone of modern electronics, serving as the foundation upon which electronic components are mounted and interconnected. The PCB manufacturing ...
    Request a Quote

    Leave a Comment

    Your email address will not be published. Required fields are marked *

    en_USEnglish
    Scroll to Top