CAF failures
What exactly is CAF?
Conductive Anodic Filament
CAF is the Formation of Solid Cu Filament between different nets within PCB at different potentials.
Follows a glass weave pathway internally in the PCB.
Acts as dead short and then possibly the fire.
Multiple-step Process (PCB Internally)
- Pathway Formation between different PCB Nets internally
- Separation of glass/resin interface
- Incomplete wetting of initial coating of resin to glass
- PCB / PCBA Fabrication (Therm / Mech / Chem)
- Degradation of glass/resin bond – Temp/Humidity
- Can be Hollow Fiber
- Separation of glass/resin interface
- Electromigration-Ionic (EM) down a Pathway – Anode to Cathode
- Growth of Solid Cu Filament – Cathode to Anode
CAF Failure -can occur before solid Cu filament formation.
Electro-migration (EM) can lead to catastrophic drops in resistance.
What does CAF look like?
What exactly is Electro-migration (EM) for CAF?
- Electro-migration of e– and Ions under the influence of applied voltage (PCB Internally)
- Electro-migration of e– and Ions Between Vias (HW-HW)
- In 2 different electrical nets
- At different potentials (one hole relative to the other)
- Must be an electrical pathway between hole pairs
- not the pure resin
- but at the glass/resin interface or Hollow-Fiber (microscopic)
- resin holes between the glass filaments (macroscopic)
Elements Needed for CAF / EM Failure
Pathway
- Separation of Glass Fiber / Epoxy Interface
Material Fabrication – Incomplete wetting glass/resin PCB + PCBA Fabrication – Thermal / Mech / Chem Stresses Degradation of Glass/Resin Bond (Temp / Humidity)
- Hollow Glass Fiber
Moisture
PCB + PCBA Fabrication
Final Part Field Operation
Applied Bias Voltage
Operation of the Final Part (between Different Nets at Different Potentials)
Hollow Fiber Path Formation Example 1
Hollow Fiber Path Formation Example 2
Hollow Fiber Path Formation Example 3
Glass Cloth / Resin Perspective
Topographical View of Typical Glass Fabric
Vertical X-sect of Typical-Glass Fabric in MLB
4 Potential modes of CAF / EM failure (3 can be prevented by the use of correct Buttercoats)
Importance of Buttercoats
Drill Hole wall to Drill Hole wall with CAF filament growth.
DHW to Feature (Trace/Clearance) – Undesirable
DHW to Feature (Trace/Clearance) – desirable
Insufficient Resin Buttercoat
Acceptable Buttercoat
Factors influencing Electro-migration (EM) and subsequent CAF growth.
Design
- Proximity of Holes / Features – Critical
Materials
- Heavier Glass (7628) structures perform worse for EM / CAF
- Lighter Glass (106) structures can perform worse for EM / CAF
- Glass fiber distribution can cause filament nesting.
Type of Glass, Treatment, and Resin marriage
- Environment (End application)
Higher voltage/humidity/temperature -Accelerates.
- PCB Processes May Need to optimize for best CAF performance (Lamination / Drill / De-smear)
Low – Medium Layer Count Challenges to CAF Success
- Heavy Glass (7000 series): DSR – 4L, 6L, 8L, 10L
- Heavier Glass structures perform worst for CAF.
- Lower Resin Content vs Lower CTEz-axis Materials
- Low CTEz-axis – Ability to pass ATC Thermal Cycling
- Low Resin Content Not good for CAF – Not enough butter coat
- Compressed Stack-ups
- Glass touching copper – Stress Points + Shorter CAF Pathway
- Low Cost / Low Layer Count Solutions
- 7628 stack-ups – not good for CAF
- Dicy Materials preferred for cost – not good for CAF or LF
- Design Features shrinking – not good for Best CAF Perf
Best CAF Resistant Materials
- Proper Marriage of Glass / Glass Surface Finish to Resin Enhanced CAF Process @ Material Supplier
- Learn by CAF Testing TVs – Generally, 12,30 mil DHW to DHW testing coupons.
- Two Important Elements Needed for Best Resistance are the Best Glass match to Resin System and the Enhanced CAF Process at the Material Supplier
Best Glass Choices
- Not always obvious & Need to Test to find out.
- Some Best Choices can be more expensive such as Japanese Glasses
- Sometimes there are cost-effective solutions that depend on DHW to DHW spacings, Applied Voltages, and Environment.
CAF Enhanced Process at Material Supplier
- Better Surface Wetting or Glass Wet-out.
- Lower Viscosity Resin adjustment
- Glass Cloth prewetting
- Slower Treater Speed
Material Influencing Factors for Electro-migration (EM) and Subsequent CAF Growth.
- Right Glass/Finish
- Passing spacings can go from 17 mils to >30 mils.
- Enhanced CAF Process at the Material Supplier
- Passing spacings can worsen by 10 mils DHW to DHW
- Improper wetting of glass filaments – easy pathways
- Tripple Point voids – easy pathways
- All can lead to EM/CAF
CAF Materials -Pathways
Example of Non-wetted Glass in Hi-Tg Filled Phenolic of a 1501 core in Warp direction Material Supplier changing Treater Process and Spread Glass to both Warp + Fill.
Example of Non-wetted Core Glass or Classic Tripple Point in Mid-Tg Filled Phenolic Material Supplier Forgot to run Enhanced CAF Process at their Factory.
Best CAF Resistant Material Choices (~0-20%) of Total Material Cost
- Pre-selected Glass Type / Glass surface finish
- CAF Enhanced Process at Material Supplier
Some Material Suppliers include Enhanced CAF Processes automatically as part of their Standard Process and cost structure and some do not.
Special Glass is a Major Cost Adder of the above Two and not the Enhanced Process.
Other Perspectives
DHW-DHW
- 25 mil – Most good CAF Resistant Materials Should Pass (Glass/Resin?)
- 20 mil -Some Risk for good or best CAF Resistant Materials
- 15 mil – High Risk for best CAF Resistant Materials
- 12 mil – Best CAF Resistant Materials will most likely Fail.
Material Suppliers
- Better Understanding of variables needed: Glass, Process, Constructions, Better Marriages of Glass/Resin
Potential Applications Susceptible to Electro-migration Failure (Hard Leaks)
Automotive Industry
- Brake Controllers
- Other Controllers
Telecommunications Industry
- Any circuit with a high impedance – is radically affected.
- Phase Locked Loops (control of the system clocks)
- Analog and RF circuits
Other Industries
- A-D and D-A converters
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