CAF Failures
CAF –> First the Formation of Solid Cu Filament between different nets within PCB at different potentials –>follows a glass weave pathway internally in the PCB –>soon acts as dead short –> then possibly it fires up.
Multiple step Process (PCB Internally)
1)Pathway Formation between different PCB Nets internally
a)Separation of glass and resin interface too.
▪Incomplete wetting of initial coating of resin to glass
▪PCB / PCBA Fabrication (Therm / Mech / Chem)
b) Degradation of glass/resin bond – Temp/Humidity
c) As a result, Can be Hollow Fiber
2) Due to Electro-migration-Ionic (EM) down a Pathway –>Subsequently changes from Anode to Cathode
3) Consequently, Growth of Solid Cu Filament –> Cathode to Anode
CAF Failure –> can occur before solid Cu filament formation
Electro-migration (EM) –> Meanwhile, can lead to catastrophic drops in resistance
In the electronics sector, conductive anodic filament (CAF) failure is a prevalent and also developing concern. Although conductive salt containing copper can form within printed circuit boards, which has the potential to be a catastrophic failure scenario (PCBs). Nevertheless, It’s a form of electrochemical migration that occurs from the anode to the cathode sub-surface along the epoxy or glass interface. In addition, The growth of conductive metal filaments across a dielectric medium is further known as electrochemical.
How does CAF look like?
What is Electro-migration (EM) for CAF?
1)The Electro-migration of e– and Ions under the influence of applied voltage –> (PCB Internally)
2)Electro-migration of e– and Ions Between Via (HW–HW)
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 in Addition Needed for CAF / EM Failure
Firstly, Pathway
I.– Separation of Glass Fiber / Epoxy Interface
thereafter, Material Fabrication –> furthermore, Incomplete wetting glass/resin PCB + PCBA Fabrication –> Therm / Mech / Chem Stresses Degradation of Glass/Resin Bond (Temp / Humidity)
II.– Hollow Glass Fiber? –> Certainly, much better
Later, Moisture
PCB + PCBA Fabrication
Final Part Field Operation, although
Applied Bias Voltage and then
Lastly, Operation of Final Part (between Different Nets at Different Potentials)
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.
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.
Since bias voltage is the force that drives the reaction, a low-voltage bias will significantly decrease the chances of CAF formation. Lower voltages decrease CAF performance as well.
Hollow Fiber Path Formation Example
Factors influencing electro-migration (EM) and subsequent CAF growth
Design
Proximity of Holes / Features –> Critical
Materials
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.
Similarly, Heavier Glass (7628) structures perform worse for EM / CAF Lighter Glass (106) structures can perform worse for EM / CAF . In addition, Glass fiber distribution can cause filament nesting
Type of Glass, Treatment and Resin marriage –> Key
Finally, Environment (End application)
Higher voltage / humidity / temperature –> Extremely Accelerates PCB Processes
Likewise, May Need to optimize for best CAF performance
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.
(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 !! –> Meanwhile, Passing spacings can go from Especially17 mils to >30 mils
for example = likewise, Supplier A in Country I vs same Supplier A in Country II
Do I really need the enhanced CAF Process at the Material Supplier
Indeed !! –> Whereas, Passing spacings can worsen by 10 mils DHW to DHW
–>On the contrary, Improper wetting of glass filaments –> easy pathways
–>As a result, Triple Point voids –> easy pathways
–>Summing up, All can lead to EM/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 Triple Point in Mid-Tg Filled Phenolic
Sometimes, Material Supplier Forgot to run Enhanced CAF Process at their Factory
Same furthermore Example of Non-wetted core Glass but with Large Void in Mid-Tg Filled Phenolic
~Sometimes, Material Supplier Forgot to run obviously Enhanced CAF Process at their Factory
Best Resistant Material Choices CAF Material Cost Adders
a)Pre–selected Glass Type / Glass surface finish
b)CAF Enhanced Process at Material Supplier
Thus, Some Material Suppliers include Enhanced CAF Process automatically as part of their Standard Process, and then costing structure…some do not
Moreover, Special Glass –> Major Cost Adder of the Two
yet 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 :
Certainly, In any case 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 –> In contrast, Japan Crisis Potential alternate stack-ups for heavy glass
However, Materials that satisfy to demonstrate CAF, LF, ATC + Hi-volt CAF with LF Hvy Cu
Lastly, Extensive CAF Testing Required
Potential Applications Susceptible To
Electro-migration Failure (Hard Leaks)
Summing up,
- Automotive Industry
a) Brake Controllers
b) Other Controllers
2.Telecommunications Industry
a) Since, Any circuit with a high impedance therefore it radically affected
b) Phase Locked Loops (control of the system clocks)
c)Analog and RF circuits
and Other Industries
A-D and D-A converters
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Emsxchange – PCBA, Cable Assembly, Box Build.
