Download Proposal for determining a better CAF acceleration equation

HDPUG Project Proposal
Determining a better CAF
acceleration equation
Joe Smetana (Nokia), Eric Lundeen and
Kevin Knadle (i3), Kim Morton (TTM)
October 2016
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.
Background
• “Classic” CAF (Conductive Anodic Filament) growth is a
two step process (see 1980’s Bell Labs papers)
1.
2.
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 100-150 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.
Example Acceleration Factor
(Sun Equation)
The Sun Model (Based on CALCE)
(L-2D )^2
t (f) = KP ----------------------------HVT
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).
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.
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.
Two Ways to Approach This
1) 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.
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 DOEParenthetical 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
Box Behnken
Option
Row # Voltage Temperature Humidity
1
35
50
50
2
35
50
85
3
35
85
50
Factor
4
35
85
85
Row # Voltage Temperature Humidity
5
100
50
50
1
6
100
50
85
7
100
85
50
8
100
85
9
67.5
10
A
B
C
35
50
67.5
2
35
85
67.5
3
100
50
67.5
85
4
100
85
67.5
67.5
67.5
5
35
67.5
50
67.5
67.5
67.5
6
35
67.5
85
11
35
67.5
67.5
7
100
67.5
50
12
100
67.5
67.5
8
100
67.5
85
13
67.5
50
67.5
9
67.5
50
50
14
67.5
85
67.5
10
67.5
50
85
15
67.5
67.5
50
11
67.5
85
50
16
67.5
67.5
85
12
67.5
85
85
13
67.5
67.5
67.5
14
67.5
67.5
67.5
15
67.5
67.5
67.5
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 15-16 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 250-300 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.
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.
Without Interactions – Temperature and Humidity
Temperature:
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.
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 nonlinear):
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
35
85
3
85
50
4
85
85
5
60
67.5
6
60
67.5
7
35
67.5
8
85
67.5
9
60
50
10
60
85
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…
Other
Need Automotive company participation
Bosch
Continental
Delphi
Others?
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