Anti-CAF “Conductive Anodic Filament” CCL Material
Eric LIN 林坤正
Nan Ya plastics corporation, Electronics material Div.
Technical Department of Copper Clad Laminate
 
Introduction
Conductive Anodic Filament (CAF) has been increasingly concerns on printed circuit boards in the last few years. The major factors are current trend toward smaller size, lighter weights, thinner and higher performances that the density of PCB shall be designed with more closely spacing conductors, much smaller pitches and single-ply dielectrics such as mobile telephones, notebook computer and camcorder. In the other hand, some electrical applications must be exposed in extreme environment. For example, the safe necessity of automotive like engine control and brake control system. There cannot occur electrical shorts phenomenon, so the insulation resistance of material becomes very importance. These factors are pushing the demand for anti-CAF materials.

CAF is an electrochemical corrosion process that causes electrical shorts, when copper metal is dissolved at the anode and migrates to the cathode through the resin matrix/ glass fibers interface. CAF can occur between any two oppositely adjacent conductors when the conductors are in contact with the glass fibers. There are four ways that include hole-to-hole, line-to-line, hole-to-line and layer- to-layer. (See figure1)
 
 
 
Mechanism of CAF
CAF failure has two major step .The first step is degradation at resin/glass fibers interface that result in path formation. This may be poor wetting during treating or lamination, chemical hydrolysis of the coupling agent and impure raw materials. The second step is electrochemical corrosion reaction that result in loss of insulation resistance. After degradation and path formation with a continuous moisture medium between conductors, an applied voltage produces an electrochemical process that copper ion dissolved from anode and migrated to cathode through the resin/glass fibers interface. The insulation resistance will be drop and final to occur CAF phenomenon.
The reaction process as follows.
1. Cu ® Cu2+ +2e- (Copper dissolved at anode)
  H2O ® H++OH-  
  2H+2e- ® H2  
2. Cu2++2OH-®Cu (OH) 2 (Copper move from anode to cathode)
  Cu (OH) 2 ®CuO+H2O  
3. CuO+H2O®Cu (OH) 2®Cu2++2OH- (Copper deposited at cathode)
  Cu2++2e-®Cu  

 
 
 
Influence Factor of CAF
CAF is an electrolytic corrosion process. The essential elements of electrolytic corrosion are combination moisture, potential difference and poor material quality. (See figur2) If we remove any element of triangle and the electrolytic corrosion stops.
The formation of CAF should be dependent on operating condition, test pattern, laminate quality and manufacturing process. All these factors are discussed as follows:

The operating condition includes temperature, humidity, and voltage. Electrochemical corrosion will be accelerated by critical operating condition. The reaction process of CAF has explained that moisture is an electrochemical corrosion medium because mechanism of CAF requires moisture to operate. Most laminate materials will absorb moisture by surface absorption and diffusion in the interior when exposed in high temperature and high humidity environment. Higher temperature and humidity will accelerate the speed of moisture absorption that can result in quicker degradation and path formation. The difference moisture absorption of resin and glass fibers also can lead to interface stress. Resin will swell due to absorption of moisture. The swelling stress can lead to de-bonding at resin/glass fibers interface. Voltage bias is a driving force for CAF formation. Higher voltage can lead to quicker failure.

The test patterns have four configurations that include hole-to-hole, line-to-line, hole-to-line and layer- to-layer. Experiment shows that test pattern with smaller spaces between adjacent conductors are easier to occur CAF. The hole-to-hole failure is the most common CAF failure. (See figure 3) This reason is that through hole has higher area of glass fibers contact with copper metal. There can detect copper on the resin/glass fibers interface by EDS analysis.(see figure 4) In the others , hole-to-line or line-to-line failure are not absolute relationship with CAF. Because of the probability of glass fibers contact with copper metal are very small.
Laminate qualities have several items to prevent CAF formation. The major factors have glass cloth, resin, impregnation and lamination. Glass cloth must be make proper treatment. Silane is a bridge between glass fibers and resin, so silane type and treatment are a significant process. Chemical hydrolysis of silane can lead to de-bonding due to absorption of moisture. In addition to finish, a special process will let glass fibers splay more uniform and puffy to help resin complete wetting each glass fiber. To insure the bonding of glass fibers/resin is absolute. Resin must be pure which mean low content of ion impurities. Because of ion impurities can occur corrosion phenomenon when matrix absorb moisture. There will accelerate copper sale production. Different resin type also has different CAF performance. Much paper show when resin has higher glass transition temperature and low moisture content that will has excellent anti-CAF performance. For example, Bismaleimide triazine (BT). Cyanate esters (CE) and FR5 can be higher anti-CAF properties than FR4 laminates. Path formation of CAF may result from incomplete bonding between resin and glass fibers interface. To enhance wetting ability in impregnation process that will increase bonding strength at the resin/fiber interface. Foreign materials are conductive medium that can decrease insulation resistance, so cleaning environment is also important in impregnation and lay up process. The voids must be removed completely during lamination. Because of voids will promote the accumulation of moisture in laminate.

The PCB processes include line manufacture, lamination, drilling and surface coating that can result in CAF failure. In lamination process include temperate, pressure must be correspond with performance of prepreg to insure adhesive strength of resin/core and none any lamination voids in PCB. These defects can result in delamination under environmental stresses, which provide CAF path. The roughness of hole must be minimized that means straight and smooth. If drilling qualities are poor that include delaminations, voids and cracks on the wall of hole. Chemicals can be trapped inside the voids and cracks during PTH process. Moisture may also be transferred by capillary action a long cracks of glass fibers / resin interface during high temperature and humidity environment.

 
 
 
Test Specification
There are not any public specifications for CAF test. The test condition, pattern, method and judgment standard are dependent on requirements of PCB markers. Generally, the test condition include that humidity set from 85%RH to 95%RH, temperature set from 60℃ to 90℃, applied voltage set from 10VDC to 100VDC, test voltage set from 10VDC to 500VDC and total time are from 240hrs to 1000hrs. Test pattern include that structure is from 2 layers to 10 layers, hole to hole distance is from 0.35mm to 1.2mm, hole to line distance is from 0.4mm to 0.85mm and line to line distance is from 0.100mm to 0.8mm. The test methods usually have two ways. First way, sample can be measured in chamber by automatic resistance machine every time that call the on line test. The machine can automatic stop when occur CAF migration (see figure 5). Second way, sample must be removed from the chamber to measure resistance when pass at regular intervals that call the off line test. (see figure6). The judgment standard is insulation resistance that must be large 106 ohms in the on line test and large 108 ohms in the off line test.
 
 
 
 
 
Promotion and Evaluation System
There have been promoted from raw materials, manufacturing processes and qualities of laminate to develop anti-CAF FR4.
There are three major directions.
1. To prevent pathway formation of CAF.
2. To reduce disruption of epoxy resin and minimize water absorption.
3. To enhance bonding strength between the interface of epoxy resin and glass fibers interface.

In order to evaluate CAF performance of new products, there have design various test pattern and test condition to evaluate anti-CAF FR4 materials. The details are described in tableⅠ. Figure 9,11 show evaluation results regarding standard FR-4 materials. Figure 10,12 show evaluation results regarding Anti-CAF FR-4 materials. Standard FR-4 materials failed at 1250 –1750hours in test1 and failed at 120-168hours in test2. Anti-CAF materials have been passed each test that keep high insulation resistance.

Pattern   Test1 Test2
Layers Double side 8 layers
Board thickness 1.6mm 1.6mm
Hole diameter 0.7mm 0.35mm
Hole wall to Hole wall distance 0.4mm, 0.5mm
0.6mm		 0.35mm, 0.45mm
Test condition Temperature 85℃ 85℃
Relative humidity 85%RH 85%RH
Applied voltage 50VDC 50VDC
Measure voltage 500VDC 100VDC
Measure frequency Samples are removed from the chamber and measurement IR at every 250 hours (off line test) Samples are measured IR in chamber at every 24 hours (on line test)
Total time 2000hours 240hours
Judgment standard Bellow 108ohms NG Bellow 106ohms NG
TableⅠ: Various test pattern and condition

 
 
 
 
 
 
Conclusions
1. How to reduce the degree of degradation at the resin/glass fibers interface, it is a key point to promote CAF performance.
2. CAF failure is also highly dependent on the moisture content that relationship with resin type and hardener.
3. Besides quality of laminate, the PCB manufacturing process will can contribution to CAF performance.
4. Although there are many different test pattern and test condition for CAF test, the purpose is evaluation CAF performance. Generally, there will be dependent on requirement of PCB makers.


 
 
 

References
1. Balu Rudra and Michael Pecht “Assessing Time-to-Failure Due to Conduction Filament Formation in Multi-Layer Organic Laminates”
2. Balu Rudra and David Jennings “Failure Mechanism Models for Conductive Filament Formation”
3. J.P.Mitchell and T.L.Weisher “Conductive Anodic Filament Growth in Printed Circuit Materials”
4. Anand A. Shukla, Terrance J. Dishongh, Michael Pecht and David Jenning ”Hollow Fibers in Woven Laminates”
5. Simeno J. Krumbein “Electrolytic Models for Metallic Electro migration Failure Mechanisms”