What are CAF issues and how to address them?

Conductive Anodic Filament Issues

CAF Failures

CAF failures

Conductive Anodic Filament

CAF –> 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 –> then possibly the fire

CAF –> Multiple step Process (PCB Internally)

1)Pathway Formation between different PCB Nets internally

a)Separation of glass/resin interface

▪Incomplete wetting of initial coating of resin to glass

▪PCB / PCBA Fabrication (Therm / Mech / Chem)

b)Degradation of glass/resin bond – Temp/Humidity

c)Can be Hollow Fiber

2)Electromigration-Ionic (EM) down a Pathway –> Anode to Cathode

3)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

In the electronics sector, conductive anodic filament (CAF) failure is a prevalent and developing concern. A conductive salt containing copper can form within printed circuit boards, which has the potential to be a catastrophic failure scenario (PCBs). It’s a form of electrochemical migration that occurs from the anode to the cathode sub-surface along the epoxy or glass interface. The growth of conductive metal filaments across a dielectric medium is known as electrochemical migration.

What does CAF look like?

CAF

What is Electro-migration (EM) for CAF?

1)Electro-migration of e and Ions under the influence of applied voltage –> (PCB Internally)

2)Electro-migration of e and Ions Between Vias (HWHW)

a)In 2 different electrical nets

b)At different potentials (one hole relative to the other)

3)Must be a electrical pathway between hole pairs

a)not the pure resin

b)but at the glass/resin interface or Hollow-Fiber (microscopic)

c)resin holes between the glass filaments (macroscopic)

Incomplete Resin wetting –> Triple Point

Elements Needed for CAF / EM Failure

Pathway

I.Separation of Glass Fiber / Epoxy Interface

Material Fabrication –> Incomplete wetting glass/resin PCB + PCBA Fabrication –> Therm / Mech / Chem Stresses Degradation of Glass/Resin Bond (Temp / Humidity)

II. Hollow Glass Fiber? –> Today much better

Moisture

PCB + PCBA Fabrication

Final Part Field Operation

Applied Bias Voltage

Operation of Final Part (between Different Nets at Different Potentials)

Hollow Fiber Path Formation Example

Hollow Fiber Path formation

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 –> Key

Environment (End application)

Higher voltage / humidity / temperature –> Accelerates

PCB Processes

May Need to optimize for best CAF performance

(Lamination / Drill / Desmear)

More on Material Influencing Factors for Electro-migration (EM) and subsequent CAF growth

Do I really need the right Glass/Finish?

Yes !! –> Passing spacings can go from 17 mils to >30 mils

example = Supplier A in Country I vs same Supplier A in Country II

Do I really need the enhanced CAF Process

at the Material Supplier

Yes !! –> 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

CAF Materials Pathways
CAF Material Pathway

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

CAF Material Pathways

Same Example of Non-wetted core Glass but with Large Void in Mid-Tg Filled Phenolic

Material Supplier Forgot to run Enhanced CAF Process at their Factory

Best CAF Resistant Material Choices CAF Material Cost Adders

a)Preselected Glass Type / Glass surface finish

b)CAF Enhanced Process at Material Supplier

Some Material Suppliers include Enhanced CAF Process automatically as part of their Standard Process and costing structure…some do not

Special Glass –> Major Cost Adder of the Two

not the Enhanced Process

Other Perspectives

Industry Experts (HLC / Fine Glass / 5-10V Applications)

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

TTM GZ

Find CAF solutions for:

Alternate glass sources with similar performance –> Japan Crisis Potential alternate stack-ups for heavy glass

Materials that satisfy CAF, LF, ATC + Hi-volt CAF with LF Hvy Cu

Extensive CAF Testing Required

Potential Applications Susceptible To Electro-migration Failure (Hard Leaks)

Automotive Industry

a)Brake Controllers

b)Other Controllers

Telecommunications Industry

a)Any circuit with a high impedance –> radically affected

b)Phase Locked Loops (control of the system clocks)

c)Analog and RF circuits

Other Industries

A-D and D-A converters

Cerra Systems is one of the leading PCB manufacturing solutions providers with PCB manufacturing plants located in North America, Asia with certifications for MIL, AS, TS, ISO, & NADCAP. Our products are Rigid, Flex, Rigid-flex, HDI, RF, and hybrid PCBs addressing up to 64 layers and 2 mil technology. Cerra Systems  Major  PCB Market is Aerospace, defense, Medical, computing, cellular & Industrial.

We manufacture and supply a wide range of Printed Circuit Boards in India from very small, double-sided PCB to high layer count, high-density multi-layer boards to large tabletop size backplanes. We have dedicated and quick-turnaround operations capable of delivering PCB prototypes with a lead time as short as 24 hours. We also offer quick-turn prototyping with a lead time as short as 5 days, depending on the technology and layer counts.

For more details, visit us- http://cerrasystems.com/

Manufacturing Partners Profile includes: 
Argus Sytems (AESPL) –  PCBACable AssemblyBox Build
Emsxchange – PCBA, Cable Assembly, Box Build.