1. HDPUG Project Proposal Determining a better CAF acceleration equation
Joe Smetana (Nokia), Eric Lundeen and Kevin Knadle (i3), Kim Morton (TTM)
October 2016

2. Goals
Determine a better acceleration factor equation for CAF and quantify the effects of Voltage, Temperature and Humidity
Hopefully enable shorter testing time for CAF material qualification.
Evaluate higher voltage requirements of Automotive and hopefully prove that testing at higher voltages is not necessary.
Provide inputs to update the IPC specification on CAF testing.

3. Background
“Classic” CAF (Conductive Anodic Filament) growth is a two step process (see 1980’s Bell Labs papers)
Creation of a path by hydrolysis. (temperature and humidity effects resulting in “debonding” of the glass to resin interface creating a path for CAF)
Electrochemical growth
(CAF can also occur along other paths – hollow fibers, resin cracks, triple points, and similar, but these are typically existing paths – not what I’m calling “classic” CAF)
If there is No path, there is NO CAF!
When we have “pre-existing” paths, the data for 100V/65°C/85% RH suggests these fail in roughly hours or less at this test condition.
Current acceleration factor equations are clearly incorrect as they attempt to model this process as a single step.

4. Example Acceleration Factor (Sun Equation)
The Sun Model (Based on CALCE)
                   (L-2D )^2
t (f) = KP
                    H V T
t(f) time to failure
H relative humidity
V voltage (bias)
T board thickness (PTH height)
D extent of readily conductive region surrounding PTH
L initial interspace of the  electrodes
K laminate material constant at standard temperature
P PWB manufacturing process
This is a commonly used equation  single step. Includes Voltage in the equation from the beginning when clearly voltage has no effect on the path formation, but only has an effect after the path is formed and the electrochemical cell has been set up. Still useful – but could be overly restrictive (or not restrictive enough).

5. Additional Challenge
The Automotive industry is running some comparably very high voltages, and is requesting high voltage CAF testing.
CAF growth, once a path exists, “should” be directly proportional to the strength of the electric field
This is related to the Voltage (directly) and the distance between the “electrodes” (in this case – between PTH’s and their associated “drill-affected zone – the readily conductive region around the PTH).
To satisfy Automotive– some additional high voltages need to be considered to validate this in actual testing.

6. Separating the Two Steps
To have a good CAF acceleration equation, we need to be able to separate the formation of a path from the “onset of CAF” where
The “onset of CAF” is defined as the measurable onset of the electrochemical cell where a first failure would occur. This is where the resistance in the test net drops dramatically and the plating action/formation of a CAF filament starts.
Note that at this “onset” a full CAF filament of Cu does not exist, but the “plating cell” that would form it is set up. In a real system, this resistance drop would typically cause a failure, which in most cases would be “no-fault found” on analysis.

7. Two Ways to Approach This
Study each variable (voltage, temperature, humidity) as an independent variable
The assumption here is that the interactions are small or non-existent
Is this a real condition?
2) Study this as a Statistical Design of Experiments
Enables the evaluation of the interactions between the variables.

8. Understanding Voltage
First step
Given an existing path…. What is the time to failure?
Need materials with “existing” path
Critical issue to identify this to begin with.
Test at 100V (50V) 35V… (Central Composite or Box Behnken DOE- Parenthetical number is not the center)
Temperature: 85C, (65C), 50C (CCD?)
Humidity: 85%, (65%), 50% (CCD?)
Resulting DOE (Central Composite Design – 3 levels, 3 variables)
Statistical: 5 samples (each condition) of maybe 3 different materials with “expected” existing paths.
This is a lot of conditions (and doesn’t even account for high voltages)…
But it does allow evaluation of interactions
Can also do a “custom DOE” (non CCD, non BB) - where we ‘switch” the center ons to 50V, 65C, 65% RH – some software challenges on this… (at least with my DOE software)
Alternative (coming up)
Central Composite Option
Factor A B C
Row #
Voltage
Temperature
Humidity 1 35 50 2 85 3 4 5
100 6 7 8 9
67.5 10 11 12 13 14 15 16
Box Behnken Option
Factor A B C
Row #
Voltage
Temperature
Humidity 1 35 50
67.5 2 85 3
100 4 5 6 7 8 9 10 11 12 13 14 15

9. Classic CAF (Wear-out)
If we want to study Classic CAF AND understand the interactions, the SAME CCD and/or BB DOE (or custom DOE) can be run on materials that don’t have existing paths. So it is still runs
This “should” give us 3 parameter Weibull results (based on history).
Can enter as DOE data points either failure free time or characteristic life (or both) for developing acceleration factor equations.
Need to identify materials that fail by Classic CAF
Don’t want them to last “too long” – because the test could go way to long. A good material for this testing (based on 100V/65C/85% RH would be one that fails in the hour range). Hard to identify material options here (need at least 3) since fabricators are always trying to improve CAF performance… previous failures don’t necessary represent what we’d see on a new evaluation.

10. Alternative option – Don’t study Interactions
For “existing paths” this makes sense – no-one wants existing paths. Just need to know “how fast” they “weed out”.
Use a standard test condition (65C/85%RH) and test at 100V, 50V, 35V until failure.
Can also put some very high voltage test conditions in this mix (300V?) to confirm for Automotive.
From this data, we now know our voltage acceleration factor for an existing path (if we assume there are no interactions with temperature and humidity – probably not a valid assumption)
BUT – the current data suggests that voltage is not critical to long term classic CAF…. Once the path exists, then the failure occurs “quickly” AND (more importantly) Voltage has no influence on creating a path. If we make this assumption, then we can look (beyond this testing) as if Voltage is a constant and test only for temperature and humidity.

11. Without Interactions – Temperature and Humidity
Set Voltage to 100V, Humidity to 85% and vary temperature:
35°C, 65°C, 85°C
We would expect this to be an Arrhenius function and we could create a temperature acceleration factor based on this.
Humidity:
Set Voltage to 100V, Temperature to 85°C and vary humidity:
50%, 65%, 85%
Statistically derive a main effect humidity acceleration factor.
At this point – you would have 3 acceleration factors – one for temperature, one for humidity, and one for voltage – but the assumption would be that there is no interaction. And, more importantly, the voltage has NO effect until the path exists.

12. Studying the Interaction between Temperature and Humidity
If we keep voltage constant (100V), then we can do a DOE of just Temperature and Humidity (3 levels needed since this is non- linear):
Central Composite Design (CCD) – or custom. Still have 10 plus 3 (Voltage) tests (plus High Voltage Test).
This option is way simpler for looking at high voltages.
Row #
Temperature
Humidity 1 35 50 2 85 3 4 5 60
67.5 6 7 8 9 10

13. Identifying the materials
Challenges
Identifying the materials
Materials with “existing paths” (ideally like 3)
Materials that fail by Classic CAF wear-out “fast” (or the test will go too long) (ideally like 3)
Getting “buy-in” from the Automotive industry
Determining the TV (this isn’t hard – but need to ensure we have consensus)
Determining the stackup and getting the boards built.
Number of test/chambers/nets
Time requirement for the lower temperature and lower humidity testing could be very long…

14. Other Need Automotive company participation Bosch Continental Delphi
Others?

15. Basic Steps Create the team to work on this
Need to identify Automotive participants
Identify or intentionally make materials
Existing path materials (small paths important)
Classic CAF fail materials short time fails
Decide on a test approach
Based on the proposals in this presentation
Agree on the test vehicle/stackup.
Hopefully we don’t have to design it! The Automotive High Voltages challenge our most common TV designs.
Build the TV’s
Test the TV’s per the DOE
Complete Statistical analysis of results
Summary Paper
Submit inputs to IPC committee