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Guide to PCB CAF Issues

PCB-CAF-Issues

Conductive Anodic Filament (CAF) failure is a common and growing concern in the electronics industry. It has the potential to be a catastrophic failure mode, where a conductive salt containing copper can form within printed circuit boards (PCBs). It’s a type of electrochemical migration that grows along the epoxy or glass interface from the anode to the cathode sub-surface. Electrochemical migration is a process in which conductive metal filaments grow across a dielectric material.

What Is Conductive Anodic Filament Formation?

CAF formation is the term for the process by which CAF grows. CAF formation is described as a two-step process:

  1. First, the resin glass interface degrades, which is believed to be reversible.
  2. The second stage, the electrochemical migration, is not reversible.

CAF failure refers to the electrical failure that results from CAF formation. The failure occurs when the CAF grows from the anode to the cathode.

For CAF to form, you need several things present:

  • Electrical-charge carriers, which enable the formation of an electrochemical cell.
  • Water, which occurs due to humidity and moisture build-up and dissolves the ionic material, sustaining them in their mobile ionic state.
  • An acid environment near conductors to enable corrosion at the anode.
  • A voltage bias, which is the force that drives the reaction.
  • A pathway for the ions to take as they move from the anode to the cathode.

Many other factors are believed to accelerate the formation process including high temperatures, high humidity, repeated thermal cycles, high voltage gradient between anode and cathode, some soldering flux ingredients and more. Other problems, such as component failures and exceeding maximum operating temperatures might also contribute to a CAF-related failure.

Historical Perspective of CAF

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, covercoat and the thickness of the epoxy buttercoat 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.

In recent years, learning how to prevent CAF failure has become a more pressing concern as manufacturers produce smaller PCBs with higher circuit density. Boards now also see more use in harsh environments and conditions where reliability is critical. Additionally, the use of lead-free soldering and the increase in material choices have bolstered interest in the topic.

Although we now know much more about CAF than we did before 1976, research continues. As technology improves, we learn more about why CAF occurs, how to identify it and how to prevent it.

How to Avoid CAF Failure

There are many different measures you can take to minimize the risk of CAF failure. Research into ways to avoid this problem are ongoing, but avoiding the conditions enabling CAF formation will help prevent it. Here are some of the factors to consider:

  1. Moisture and Humidity

Because it requires an electrolyte, a higher water content increases the chance of CAF failure. Increased humidity leads to higher moisture content, which decreases CAF performance.

Increased humidity leads to higher moisture content, which decreases CAF performance

  1. Processes That Lead to Acid Contamination 

Processes used during fabrication can introduce acid contaminations, which increases the likelihood of CAF formation. The use of some soldering fluxes and the introduction of acid residues during the plating process are examples of this.

  1. Bias and Voltage 

Since bias voltage is the force that drives the reaction, a high-voltage bias will significantly lower the chances of CAF formation. Higher voltages decrease CAF performance as well.

  1. Pre-Existing Defects

Pre-existing defects such as fracturing, voids, wicking, contamination and misregistration can also create pathways for problematic filaments. You need to be careful when drilling holes so as not to cause damage to the board. Such damage can create these pathways by causing cracks, wicking and other defects. Drill speed, feed rate and other factors influence how likely these issues are to occur. Partial defects such as incomplete bridging between features can contribute as well.

High temperatures like those from environmental temperature, repetitive thermal cycling and reflow with a high peak temperature create more stress on a board and increase the likelihood of damage as well CAF formation.

High Temps and PCB CAF Issues

  1. Materials

Another influential CAF-failure factor is the materials used to make the board. Using CAF-resistant materials, as Millennium Circuits Limited does, is one of the most effective ways to prevent CAF formation and failure.

When it comes to laminate materials, various studies over the last 30 years have measured the susceptibility to CAF of various laminates. The research suggests that bismaleimide triazine (BT) is the laminate that is most resistant to CAF, while MC-2 is the most susceptible. Laminates with high-thermal resistance tend to resist CAF formation better.

It’s important to note, though, that even identically specified laminate materials from different suppliers can differ in CAF resistance. In general, however, the susceptibility of various laminates from least to most susceptible is as follows:

  • MC-2
  • Epoxy/Kevlar
  • FR-4 and PI (approximately equal)
  • CE
  • BT

PCB manufacturers frequently use glass finishes and resin systems to increase insulation resistance and prevent CAF formation. Both of these materials are considered useful for this purpose. Tests of the two substances, however, have shown that resin systems have a more significant impact than glass coatings. Using the two together can be an ideal solution.

Further research has revealed that DICY-cured resin may be less likely to promote CAF growth than phenolic-cured resin. Finished fibers are less likely than heat-cleaned and loom-state fibers to form CAF with loom-state fibers having the highest susceptibility to CAF growth. Fiber cleanliness, distribution and hydrolysis resistance — which helps preserve glass-resin bonds — influence the effectiveness of glass cloth or silane coatings. Resins with low-moisture absorption, enhanced pure-resin components and bolstered chemical stability — including hydrolysis resistance — have better CAF performance.

Other material-related factors include finish type, such as HASL, ENIG, immersion silver or immersion tin, and solder-mask type.

  1. Design 

The design and fabrication of a PCB also plays a crucial role in determining its CAF resistance.

Boards that have a smaller gap between voltage-biased features fail faster than those with larger gaps, although this is believed only to impact the second step of the CAF-formation process.

Hole to Hole PCB

In addition to hole-to-hole and line-to-line spacing, the size of the drilled holes and the thickness of copper in plated through-holes impact CAF resistance. Having more biased features also increases the number of opportunities CAF has to form. Anodic vias also fail faster than cathodic vias. Their warp and weft direction also play a role. Vias staggered at 45-degree angles have higher resistance to CAF. If fill voids, glass stops, wicking or other issues are present after fabrication, they can act as pre-existing pathways for CAF growth.

Other processes that can increase the likelihood of CAF formation include the reflow and desmear processes.

Many different aspects of a PCB, including aspects of its design and fabrication, impact its resistance to CAF formation. These factors include:

  • The materials used, including laminates, resign systems and glass coatings.
  • Design factors such as hole-to-hole distance and layout.
  • Processes such as soldering and reflow.
  • Voltage levels.
  • Bias voltage.
  • Pre-existing defects and other fabrication quality issues.

Since so many factors influence CAF performance, you should consider CAF during every stage of the PCB-production process. Optimizing your board for CAF resistance will result in a more reliable end product.

Identifying 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.

You can, however, use various advanced testing methods to check for and characterize CAF formation and failure. These tests include IPC-standard electrical methods known as Surface Insulation Resistance (SIR) tests, which include:

  • 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.
  • Energy Dispersive Spectroscopy (EDS): This incident electron beam can be used to identify elements such as copper, chlorine and bromine.
  • Focused Ion Beam (FIB): With this technique, you can magnify a surface at high resolution and then make a thin cross section to obtain a 3-D image.
  • Transmission Electron Microscopy (TEM): This system, which works similar to a light microscope, can identify material phases and determine the crystallographic structure.
  • X-Ray Photoelectron Spectroscopy (XPS): Sometimes referred to as electron spectroscopy for chemical analysis (ESCA), this method is a surface-analysis technique that can identify chemical compounds.
  • Fourier Transform Infrared Spectroscopy (FTIR): This technique analyzes organic constituents and creates a spectrum of intensity and wavelength readings.
  • Ion Chromatography: This process, which may be either anion or cation-exchange chromatography, separates ions and polar molecules.

Conductive Anodic Filament-Resistance Test

Conducting a test of a board’s CAF resistance helps to ensure its overall quality and reliability. IPC, the trade association for the circuit board and electronics assembly industries, has a standard test procedure for CAF resistance titled IPC-TM-650 Method 2.6.25A. The use of a standardized-test method allows for easily understandable and comparable CAF-resistance ratings.

For this test, you can use IPC-standard coupon designs such as IPC-9253 and IPC-9254, which have 10 layers and dimensions of approximately 125×175 millimeters, as well as IPC-9255 and IPC 9256. Some original equipment manufacturers also use their own coupon designs. When testing a laminate for CAF performance, you should use several designs that have a range of feature spacings, layouts and hole sizes.

The IPC test method recommends using at least 25 test boards per sample lot per bias level, which provides 4,200 potential in-line, hole-to-hole CAF failure points and 7,800 potential diagonal hole-to-hole CAF failure sites for every sample and condition set.

The IPC test method involves testing these boards under high temperatures of around 65 or 85 degrees Celsius (149 to 185 degrees Fahrenheit) and high humidity conditions of 87-percent relative humidity (RH). After you apply 100 volts of direct current to obtain initial insulation-resistance measurements, the samples undergo 96 hours of temperature and humidity conditioning, followed by another round of insulation-resistance measurements.

Then, the samples undergo a minimum of 500 hours of testing with a continuously applied bias voltage. Take high-resistance measurements every 24 to 100 hours during this time. After 500 hours, take another set of insulation-resistance measurements.

After the test, you review the data for any significant drops or high-resistance shorting. If the board fails and you suspect it is a CAF failure, you can perform a failure analysis using test methods such as those listed above to confirm the cause.

CAF Failure Test

The test will give you a percent failure rate at 500 hours for each spacing. You can describe test results in terms of CAF resistance or CAF quality. CAF resistance focuses on determining whether or not a material system is susceptible to CAF formation and does not emphasize defects. CAF quality is a material’s failure-rate model and may include defect-based failures.

CAF-Resistant PCB Materials From Millennium Circuits Limited

Resistance to CAF should be a primary concern for any printed circuit board project.

As PCBs become smaller and features must be placed closer together, the risk of CAF formation increases. For boards used in certain conditions and in applications where reliability is critical, CAF performance is especially vital.

That’s why all of the materials that Millennium Circuits Limited provides are resistant to CAF. We offer a range of material varieties, flex and rigid-flex circuits and surface finishes as well as high power and thermal boards. We also provide engineering services, including panel creation, design rule check and controlled-impedance modeling. We handle orders of any size, from prototyping to high-volume orders with quick turnaround.

For high-quality products, excellent customer service, reliable on-time delivery and competitive pricing, trust Millennium Circuits Limited for all of your PCB needs. Browse our website to learn more about our capabilities. You can also contact us with any questions you may have and request a quick quote by filling out this online form.

PCB Glossary

  • Desmear

    The removal of friction-melted resin and drilling debris from a hole wall.