Transcript No Slide Title

The 21st annual IEEE SEMI-THERM Symposium
Fairmont Hotel, San Jose, 13 March 2005
Thermal measurements and
qualification using the
transient method
Principles and applications
One-day short course by András Poppe
Budapest University of Technology and Economics,
Department of Electron Devices
[email protected]
[email protected]
Thermal measurements and qualification using the transient method: principles and applications
1
MATHEMATICAL
DESCRIPTION OF
THERMAL SYSTEMES
(distributed linear RC
systems)
Thermal measurements and qualification using the transient method: principles and applications
2
Introduction
• Linearity is assumed
– later we shall check if this assumption was correct
• Thermal systems are
– infinite
– distributed systems
• The theoretical model is: distributed linear RC system
• Theory of linear systems and some circuit theory will be
used
For rigorous treatment of the topic see:
V.Székely: "On the representation of infinite-length distributed RC one-ports", IEEE
Trans. on Circuits and Systems, V.38, No.7, July 1991, pp. 711-719
Except subsequent 12 slides no more difficult maths will be used
Thermal measurements and qualification using the transient method: principles and applications
3
Introduction
• Theory of linear systems
d(t)
d(t)
W(t)
W(t)
t
t
Dirac-delta
weight function (Green’s function)
Response to any excitation:

T (t )   W ( y) P t  y  dy
0
or shortly:
T (t )  W t   Pt 
 = convolution
If the T response to the P excitation is known:
W (t )  T t  1 Pt 
 1 = deconvolution
(to be calculated numerically)
Thermal measurements and qualification using the transient method: principles and applications
4
Introduction
• Theory of linear systems
d(t)
d(t)
W(t)
W(t)
t
t
weight function (Green’s function)
Dirac-delta
• The h(t) unit-step function is more easy to realize than
the d(t) Dirac-delta
h(t)
h(t)
a(t)
a(t )  W t   ht 
a(t),
1
t
t
a(t) is the unit-step response function
If we know the a(t) step-response function, we know
everything about the system
the system is fully characterized.
Thermal measurements and qualification using the transient method: principles and applications
5
Step-response

a(t )  W (t )  h(t )   W ( y)  h(t  y) dy



a(t )   W ( y)  h(t  y) dy   W ( y)  1 dy

0
d
a (t )  W (t )
dt
a(t)
t
W(t)
t
• The a(t) unit-step response function is another
characteristic function of a linear system.
• The advantage of a(t) the unit-step response function
over W(t) weight function is that a(t) can be measured
(or simulated) since it is the response to h(t) which is
easy to realize.
Thermal measurements and qualification using the transient method: principles and applications
6
Thermal transient testing
h(t)
a(t)
The measured a(t) response function is characteristic to the
package. The features of the chip+package+environment structure
can be extracted from it.
Thermal measurements and qualification using the transient method: principles and applications
7
Step-response functions
• The form of the step-response function
– for a single RC stage:
R
C
a(t )  R  1  exp(t /  )
  R C
R
characteristic values: R magnitude and  time-constant
– for a chain of n RC stages:
C
C
C
n
1
2
n
R1
R2
Rn
a(t )   Ri  1  exp(t /  i )
i 1
t

 i  Ri  Ci
R1 R2
1
2
Rn
n
t
characteristic values: set of Ri magnitudes and i time-constants
If we know the Ri and i values, we know the system.
Thermal measurements and qualification using the transient method: principles and applications
8
Step-response functions
– for a distributed RC system:
n

i 1
0
  
n
n
a(t )   Ri  1  exp(t /  i )
i 1

a(t )   R( )1  exp(t /  )d
0
characteristic: R( time-constant spectrum:
R()
R1
R2
Rn

t
1
2
n
discrete set of Ri and i values
continuous R( spectrum
If we know the R() function, we know the
distributed RC system.
Thermal measurements and qualification using the transient method: principles and applications
9
Time-constant spectrum
Discrete RC stages
Distributed RC system
discrete set of Ri and i values
continuous R() function
R()

a(t )   R( )1  exp(t /  )d
0

If we know the R() function, we know the system.
R() is called the time-constant spectrum.
Thermal measurements and qualification using the transient method: principles and applications
10
Practical problem
• The range of possible time-constant values in thermal
systems spans over 5..6 decades of time
–
–
–
–
–
100ms ..10ms range: semiconductor chip / die attach
10ms ..50ms range: package structures beneath the chip
50ms ..1 s range: further structures of the package
1s ..10s range: package body
10s ..10000s range: cooling assemblies
• Wide time-constant range  data acquisition problem
during measurement/simulation: what is the optimal
sampling rate?
Thermal measurements and qualification using the transient method: principles and applications
11
Practical problem (cont.)
T3Ster Master: Smoothed response
70
a(t)
Measured unit-step
response of an MCM
shown in linear timescale
Temperature rise [°C]
60
50
40
30
20
10
t
0
0
200
400
600
800
1000
1200
Time [s]
Nothing can be seen below the 10s range
Solution: equidistant sampling on logarithmic time scale
Thermal measurements and qualification using the transient method: principles and applications
12
Using logarithmic time scale
T3Ster Master: Smoothed response
70
Measured unit-step
response of an MCM
shown in linear timescale
a(z)
Temperature rise [°C]
60
50
40
30
20
10
z = ln(t)
0
1e-6
1e-4
0.01
1
100
10000
Time [s]
Details in all time-constant ranges are seen
Instead of t time we use z = ln(t) logarithmic time
Thermal measurements and qualification using the transient method: principles and applications
13
Step-response in log. time
• Switch to logarithmic time scale: a(t)  a(z) where
z = ln(t)
a(z) is called*
– heating curve or
– thermal impedance curve
• Using the z = ln(t) transformation it can be proven that

d
a( z )   R( )exp(z    exp(z   ))d
dz
0
*Sometimes Pa(z) is called heating curve in the literature.
Thermal measurements and qualification using the transient method: principles and applications
14
Step-response in log. time
• Note, that da(z)/dz is in a form of a convolution integral:

d
a( z )   R( )exp(z    exp(z   ))d
dz
0
Introducing the wz ( z)  exp(z  exp(z)) function:

d
a( z )   R( )  wz ( z   )d
dz
0
d
a ( z )  R ( z )  wz ( z )
dz
• From a(z)
R(z) is obtained as:
d

R( z )   a( z )  1 wz ( z )
 dz

Thermal measurements and qualification using the transient method: principles and applications
15
Extracting the time-constant spectrum in
practice 1
T3Ster Master: Smoothed response
60
VIPER1-2 - Ch. 0
d
dz
Temperature rise [°C]
50
40
a(z )
Numerical
derivation
1
 wz ( z)
T3Ster Master: Derivative
30
18
VIPER1-2 - Ch. 0
16
Numerical
deconvolution
20
0
1e-6
1e-4
0.01
1
Time [s]
Measured thermal
impedance curve
100
12
T3Ster Master: Tau intensity
18
10
10000
d
a (z )
dz
8
6
4
2
0
1e-6
1e-4
0.01
1
Time [s]
VIPER1-2 - 0
16
14
Time constant intensity [K/W/-]
10
Derivative of temp. rise [K/-]
14
100
12
10
8
6
10000
R(z )
4
Derivative of the
thermal impedance
curve
2
0
1e-6
1e-4
0.01
1
100
10000
Time [s]
Time-constant spectrum
Thermal measurements and qualification using the transient method: principles and applications
16
Extracting the time-constant spectrum in
practice 2
a(z )
Must be noise free, must have high time resolution
(e.g. 200 points/decade)
d
dz
Numerical derivation should be accurate: high order
techniques yield better results.
1 wz ( z)
Numerical deconvolution: Bayes-iteration (for driving point
impedance only), frequency-domain inverse filtering (both for
driving point and transfer impedances)
R(z )
False values with small magnitude can be present due to
noise enhancement in the procedure. Negative values
represent a transfer impedance.
Danger of noise enhancement  filtering  loss of
ultimate resolution in the time-constant spectrum
Thermal measurements and qualification using the transient method: principles and applications
17
Using time-constant spectra
• The time-constant spectrum gives hint for the
time-domain behavior of the system for experts
• Time-constant spectra can be further processed
and turned into other characteristic functions
• These functions are called structure functions
Thermal measurements and qualification using the transient method: principles and applications
18
Break!
Thermal measurements and qualification using the transient method: principles and applications
19
INTRODUCTION TO
STRUCTURE FUNCTIONS
Thermal measurements and qualification using the transient method: principles and applications
20
Example: Thermal transient measurements
heating or cooling
curves
Normalized to 1W
dissipation: thermal
impedance curve
Evaluation:
Network model of a thermal impedance:
Thermal measurements and qualification using the transient method: principles and applications
Interpretation of the
impedance model:
STRUCTURE
FUNCTIONS
21
How do we obtain them?
Thermal measurements and qualification using the transient method: principles and applications
22
Structure functions 1
• Discretization of R(z)  RC network model in Foster
canonic form
(instead of  spectrum lines, 100..200 RC stages)
Ri=R(i)
i=exp(zi)
Ri
Ci=i/Ri
• A discrete RC network model is extracted  name of the
method: NID - network identification by deconvolution
Thermal measurements and qualification using the transient method: principles and applications
23
Structure functions 2
• The Foster model network is just a theoretical one,
does not correspond to the physical structure of the
thermal system:
thermal capacitance exists towards the ambient (thermal
“ground”) only
• The model network has to be converted into the
Cauer canonic form:
Thermal measurements and qualification using the transient method: principles and applications
24
Structure functions 3
• The identified RC model network in the Cauer canonic form
now corresponds to the physical structure, but
• it is very hard to interpret its “meaning”
• Its graphical
representation
helps:
n
C    Ci
i 1
• This is called
cumulative structure
function
n
R   Ri
i 1
Thermal measurements and qualification using the transient method: principles and applications
25
Structure functions 4
The cumulative structure function is the map of
the heat-conduction path:
logC
n
Cn   Ci
Rthja
ambient
i 1
Cj
C i
n
Rn   Ri
i 1
Ri Rj
junction
Ri
Ci
Thermal measurements and qualification using the transient method: principles and applications
ambient
26
Structure functions 6
Cumulative (integral) structure
function
Calculate dC/dR:

differential structure function
air
Thermal measurements and qualification using the transient method: principles and applications
27
What do structure
functions tell us and how?
Thermal measurements and qualification using the transient method: principles and applications
28
A hypothetic example for the explanation
of the concept of structure functions 1
T(z)
An ideal homogeneous rod
z = ln t
1D heat-flow
P(t)
Rth_tot= L/(A·l)
1W
t
Ideal heat-sink at Tamb
Thermal measurements and qualification using the transient method: principles and applications
29
A hypothetic example for the explanation
of the concept of structure functions 2
An ideal homogeneous rod
DL
Rth = DL/(A·l)
1D heat-flow
V = A·DL
DL
Cth = V·cv
A
Tamb
Thermal measurements and qualification using the transient method: principles and applications
Ideal heat-sink at Tamb
30
A hypothetic example for the explanation
of the concept of structure functions 3
An ideal homogeneous rod
Driving point
This is the network model of the
thermal impedance of the rod
Thermal measurements and qualification using the transient method: principles and applications
Ambient
Ideal heat-sink at Tamb
31
A hypothetic example for the explanation
of the concept of structure functions 4
Let us assume DL, A and material parameters such,
that all element values in the model are 1!
1
1
1
1
1
1
1
1
It is very easy to create
the cumulative
structure function:
1
1
1
1
1
1
n
C    Ci
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
Rth_tot
i 1
y=x – a straight line
There must be a
singularity when we
reach the ideal heat-sink.
1
1
The location of the singularity gives the
total thermal resistance of the structure.
Rth_tot
n
R   Ri
i 1
Thermal measurements and qualification using the transient method: principles and applications
32
A hypothetic example for the explanation
of the concept of structure functions 5
Let us assume DL, A and material parameters such,
that all element values in the model are 1!
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
n
C    Ci
K ( R ) 
i 1
1
1
1
1
1
1
dC
dR
It is also very easy to create the differential structure
function for this case. Again, we obtain a straight line:
y=1
Rth_tot
n
R   Ri
i 1
Thermal measurements and qualification using the transient method: principles and applications
Rth_tot
n
R   Ri
i 1
33
A hypothetic example for the explanation
of the concept of structure functions 6
What happens, if e.g. in a certain section of the
structure model all capacitance values are equal to 2?
1
1
1
1
1
1
1
1
1
1
1
2
1
2
1
1
2
1
1
1
1
n
C    Ci
K ( R ) 
i 1
1
1
1
1
1
1
1
1
1
1
1
dC
dR
a peak
2
double
slope
1
n
n
R   Ri
R   Ri
i 1
Cumulative structure function
i 1
Differential structure function
Thermal measurements and qualification using the transient method: principles and applications
34
A hypothetic example for the explanation
of the concept of structure functions 7
What would such a change in the structure functions
indicate?
It means either a
change in the
material properties…
Thermal measurements and qualification using the transient method: principles and applications
35
A hypothetic example for the explanation
of the concept of structure functions 8
What would such a change in the structure functions
indicate?
… or a change in the
geometry …or both
Thermal measurements and qualification using the transient method: principles and applications
36
A hypothetic example for the explanation
of the concept of structure functions 9
What values can we read from the structure functions?
Cth1
Cth2
Cth3
n
C    Ci
i 1
Cth3
K ( R ) 
Cth2
Cth1
dC
dR
Rth2
Rth1
Rth3
n
R   Ri
i 1
Differential structure function
Cumulative structure function
n
R   Ri
Thermal capacitance
values can be read
i 1
Rth1 Rth2 Rth3
Thermal measurements and qualification using the transient method: principles and applications
Partial thermal resistance
values can be read
37
A hypothetic example for the explanation
of the concept of structure functions 10
What values can we read from the structure functions?
V1
V2
V3
n
C    Ci
i 1
A1
V3/cv1
K ( R ) 
V2/cv2
V1/cv1
n
R   Ri
A2
A1
dC
dR
K2 = A22·cv2·l2
i 1
Cumulative structure function
If material is known,
volume can be identified.
If volume is known, volumetric
thermal capacitance can be identified.
Differential structure function
K1 =
A21·cv1·l1
n
R   Ri
i 1
If material is known, cross-sectional
area can be identified.
If cross-sectional area is known, material
parameters (cv·l) can be identified.
Thermal measurements and qualification using the transient method: principles and applications
38
Structure functions 5
Differential structure function
• The differential structure function is defined as the derivative
of the cumulative thermal capacitance with respect to the
cumulative thermal resistance
K ( R ) 
dC
dR
cAdx
K ( R ) 
 clA2
dx / lA
• K is proportional to the square of the cross sectional area of the
heat flow path.
Thermal measurements and qualification using the transient method: principles and applications
39
Some conclusions regarding structure
functions
• Structure functions are direct models of one-dimensional
heat-flow
– longitudinal flow (like in case of a rod)
• Also, structure functions are direct models of “essentially”
1D heat-flow, such as
–
–
–
–
radial spreading in a disc (1D flow in polar coordinate system)
spherical spreading
conical spreading
etc.
• Structure functions are "reverse engineering tools":
geometry/material parameters can be identified with them
Thermal measurements and qualification using the transient method: principles and applications
40
Some conclusions regarding structure
functions
In many cases a complex heat-flow path can be
partitioned into essentially 1D heat-flow path sections
connected in series:
IDEAL HEAT-SINK
Thermal measurements and qualification using the transient method: principles and applications
41
IC package assuming pure 1D heat-flow
P(t)
Grease
Chip
Base
T(z)
1W
Junction
Die attach
t
z = ln t
We measure the thermal
impedance at the junction...
...and create its model in form of the
cumulative structure function:
1D heat-flow
Cold-plate
Cumulative structure function:
C
Cold-plate: infinite Cth
Junction: is always in the origin
Grease: large Rth/Cth ratio
Base: small Rth/Cth ratio
Die attach: large Rth/Cth ratio
Chip: small Rth/Cth ratio
Thermal measurements and qualification using the transient method: principles and applications
R
42
IC package assuming pure 1D heat-flow
Cumulative structure function:
C
Cold-plate: infinite Cth
Grease
Base
Die attach
Chip
Junction
R
Differential structure function:
K
C
R
Die attach
interface
thermal
resistance
The RthDA value is derived
entirely from the junction
temperature transient.
Chip
Base
Junction
Die attach
The heat-flow path can be
well characterized e.g. by
partial thermal resistance
values
Grease
R
Thermal measurements and qualification using the transient method: principles and applications
No thermocouples are
needed.
43
Example of using structure functions:
DA testing (cumulative structure functions)
Reference device with good DA
Junction
Grease
Die attach
Unknown device with suspected
DA voids
Junction
Grease
Chip
Base
Chip
Base
Cold-plate
Cold-plate
Die attach
Identify its structure function:
Identify its structure function:
C
C
Grease
Base
Copy the reference
structure function into
this plot
Die attach
Chip
R
Thermal measurements and qualification using the transient method: principles and applications
This change is more
visible in the differential
structure function.
This increase
suggests DA voids
R
44
Example of using structure functions:
DA testing (differential structure functions)
Unknown device with suspected
DA voids
Reference device with good DA
Junction
Grease
K
Die attach
Junction
Grease
Chip
Base
Chip
Base
Cold-plate
Cold-plate
C
R
K
Chip
Junction
C
R
Copy the reference
structure function into
this plot
Base
Die attach
Grease
R
Junction
Thermal measurements and qualification using the transient method: principles and applications
Die attach
Shift of peak: Increased die
attach thermal resistance
indicates voids
Chip
Die attach
Base
Grease
R
45
Some conclusions regarding structure
functions
• In case of complex, 3D streaming the derived model
has to be considered as an equivalent physical
structure providing the same thermal impedance as
the original structure.
Thermal measurements and qualification using the transient method: principles and applications
46
Specific features of structure functions for
a given way of essentially 1D heat-flow
• For “ideal” cases structure functions can be given even
by analytical formulae
– for a rod:
C  const R
– for radial spreading in a disc of w thickness and l thermal
Cth2
conductivity:
1 ln(Cth2 / Cth1 )
lw 
4 Rth2  Rth1
Cth1
Section
corresponding to
radial heat
spreading in a
disk
Rth1
Thermal measurements and qualification using the transient method: principles and applications
Rth2
47
Accuracy, resolution
• Structure functions obtained in practice always differ
from the theoretical ones, due to several reasons:
– Numerical procedures
•
•
•
•
d
dz
Numerical derivation
1

wz ( z)
Numerical deconvolution
Discretization of the time-constant spectrum
Limits of the Foster-Cauer conversion
100-150 stages
– Real physical heat-flow paths are never “sharp”
• Physical effects that we can try to cope with
– There is always some noise in the measurements
– Not 100% complete transient / small transfer effect
– In reality there are always parasitic paths (heat-loss) allowing
parallel heat-flow
Thermal measurements and qualification using the transient method: principles and applications
48
Accuracy, resolution
• Comparison of the effect of the numerical procedures:
Cumulative structure functions of an artificially constructed Cauer model:
10
'cprob3.cum'
Sharp knees
become
smoother due to
the numerical
procedures
1
0.1
0
2
4
6
8
Generated directly from the RC
ladder values
10
Identified from the simulated unitstep response of the RC ladder
a(t)
NID
SPICE
ln t
• Resolution of structure functions in practice is about 1% of the
total Rthja of the heat-flow path
Thermal measurements and qualification using the transient method: principles and applications
49
Use of structure functions:
•
•
•
•
Cth values can be read
•
Rth values can be read
•
•
•
Plateaus correspond to a certain
mass of material
Cth values can be read
material  volume
dimensions  volumetric thermal
capacitance
Peaks correspond to change in
material
corresponding Rth values can be
read
material  cross-sectional area
cross-sectional area  thermal
conductivity
Thermal measurements and qualification using the transient method: principles and applications
50
Use of structure functions:
partial thermal resistances, interface resistance
Rthjc
•
•
•
Thermal measurements and qualification using the transient method: principles and applications
Origin = junction, singularity =
ambient
Rthja and partial resistance
values
interface resistance values
(difference between two peaks)
51
Some examples of using
structure functions
Thermal measurements and qualification using the transient method: principles and applications
52
Measurement of the package/heat-sink
interface resistance
Four cases have been investigated:
1. Direct mounting, with
heat-conducting
grease
2. Direct mounting,
without grease
3. Mica, screw strongly
tightened
4. Mica, screw medium
tightened
We obtain partial thermal resistance values (interface
resistance) and properties of the heat-sink
Thermal measurements and qualification using the transient method: principles and applications
53
Measurement of the package/heat-sink
interface resistance
The transient responses:
T3Ster: record=demo11
??
STRUCTURE FUNCTIONS WILL HELP
Curves coincide:
transient inside the
package - no problem
Thermal measurements and qualification using the transient method: principles and applications
54
Measurement of the package/heat-sink
interface resistance
The structure functions
Inside-package part
See details in: A. Poppe, V. Székely: Dynamic Temperature Measurements: Tools Providing a
Look into Package and Mount Structures, Electronics Cooling, Vol.8, No.2, May 2002.
Thermal measurements and qualification using the transient method: principles and applications
55
Example: The differential structure function
of a processor chip with cooling mount
Cooling mount
Chip
Al2O3 beneath the
chip
Intel mP
powered and measured
via the chip
• The local peaks represent usually reaching new surfaces
(materials) in the heat flow path,
• their distance on the horizontal axis gives the partial thermal
resistances between these surfaces
Thermal measurements and qualification using the transient method: principles and applications
56
Example: FEM model validation with
structure functions
T3Ster: differential structure function(s)
100000
Differential structure function - H67
Differential structure function - inf_dcp1_sim
Differential structure function - flom_dcp1_grid_g2t3
10000
From FLOTHERM
simulation
1000
From
MEASUREMENT
K [W2s/K2]
100
From ANSYS
simulation
10
1
0.1
Courtesy of
D. Schweitzer (Infineon AG),
J. Parry (Flomerics Ltd.)
0.01
0
1
2
3
Thermal measurements and qualification using the transient method: principles and applications
4
5
6
7
Rth [K/W]
57
Structure functions summary
• Structure functions are defined for driving point
thermal impedances only. Deriving structure
functions from a transfer impedance results in
nonsense.
• Structure functions = thermal resistance & capacitance
maps of the heat conduction path.
• Connection to the RC model representation as well as
mathematically derived from the heat-conduction
equation.
• Exploit special features for certain types of heatconduction (lateral, radial).
Thermal measurements and qualification using the transient method: principles and applications
58
SUMMARY of descriptive functions
• Descriptive functions of distributed RC systems (i.e. thermal
systems) are
– the a(t) or a(z) step-response functions
– the R() time-constant spectrum
– the structure functions
• C(R) cumulative
• K(R) differential
• Any of these functions fully characterizes the dynamic behavior
of the thermal system
• The step-response function can be easily measured or
simulated
• The structure functions are easily interpreted since they are
maps of the heat flow path
Thermal measurements and qualification using the transient method: principles and applications
59
SUMMARY of descriptive functions
• Descriptive functions can be used in evaluation of both
measurement and simulation results:
• Step-response can be both measured and simulated
– Small differences in the transient may remain hidden, that is why other
descriptive functions need to be used
• Time-constant spectra are already good means of comparison
– Extracted from step-response by the NID method
– Can be directly calculated from the thermal impedance given in the
frequency-domain (see e.g. Székely et al, SEMI-THERM 2000)
• Structure functions are good means to compare simulation
models and reality
• Structure functions are also means of non-destructive structure
analysis and material property identification or Rth measurement.
Thermal measurements and qualification using the transient method: principles and applications
60
SUMMARY of descriptive functions
• The advanced descriptive functions (time-constant
spectra, complex loci, structure functions) are obtained
by numerical methods using sophisticated maths.
• That is why the recorded transients
– must be noise-free and accurate,
– must reflect reality (artifacts and measurement errors should be
avoided),
– must have high data density.
since the numerical procedures like
– derivation and
– deconvolution
enhance noise and errors.
Besides compliance to the JEDEC JESD51-1 standard, measurement tools
and methods should provide such accurate thermal transient curves.
Thermal measurements and qualification using the transient method: principles and applications
61
PART 3
APPLICATION EXAMPLES
Failure analysis/DA testing
Study of stacked dies
Power LED characterization
Rthjc measurements
Compact modeling
Thermal measurements and qualification using the transient method: principles and applications
62
TESTING OF DIE
ATTACH QUALITY
basics
Thermal measurements and qualification using the transient method: principles and applications
63
Die attach quality testing
The die attach is a key element in the junctionto-ambient heat-conduction path
Plastic package
Forced air
cooling
pn junction
Silicon chip
Die attach solder
Heat-sink
Chip carrier (Cu)
Thermal interface material
Leads
Thermal measurements and qualification using the transient method: principles and applications
64
Detecting voids in the die attach of single
die packages
Experimental package samples with die attach voids
prepared to verify the accuracy of the detection
method based on thermal transient testing
(acoustic microscopic images, ST Microelectronics)
See:
M. Rencz, V. Székely, A. Morelli, C. Villa: Determining partial thermal resistances with transient measurements and using the
method to detect die attach discontinuities, 18th Annual IEEE SEMI-THERM Symposium, March 1-14 2002, San Jose, CA,USA,
pp. 15-20
Thermal measurements and qualification using the transient method: principles and applications
65
Main time-constants of the experimental
samples
Thermal measurements and qualification using the transient method: principles and applications
66
Measured Zth curves of the average
samples
Already
distinguishable
Thermal measurements and qualification using the transient method: principles and applications
67
Differential structure functions of the
experimental samples
Thermal measurements and qualification using the transient method: principles and applications
68
The principle of failure detection
• Take a good sample as a reference
– Measure its thermal transient
– Identify its structure function
• Take sample to be qualified
–
–
–
–
–
–
Measure its thermal transient
Identify its structure function
Compare it with the reference structure function
Locate differences
A difference means a possible failure
If needed, quantify the failure (e.g. increased partial thermal
resistance)
Thermal measurements and qualification using the transient method: principles and applications
69
The principle again
Unknown device with suspected
DA voids
Reference device with good DA
Junction
Grease
K
Die attach
Junction
Grease
Chip
Base
Chip
Base
Cold-plate
Cold-plate
C
R
K
Chip
Junction
C
R
Copy the reference
structure function into
this plot
Base
Die attach
Grease
R
Junction
Thermal measurements and qualification using the transient method: principles and applications
Die attach
Shift of peak: Increased die
attach thermal resistance
indicates voids
Chip
Die attach
Base
Grease
R
70
TESTING OF DIE
ATTACH and SOLDER
QUALITY: case studies
A power BJT mount
Stacked die packages
Thermal measurements and qualification using the transient method: principles and applications
71
Measurement of a power BJT mount:
failure analysis
The measurement setup
Mo n Oc t 23 2 2:1 5:37 20 00
miTTT: R EC O R D = C 1 D
35
" C 01 d.MR 1"
" C 02 d.mr1 "
" C 03 d.mr1 "
" C 04 d.mr1 "
" C 05 d.mr1 "
" C 06 d.mr1 "
30
25
20
The measured transient responses
15
10
5
0
1e - 06
1e - 05
0.0 00 1
0.0 01
0.0 1
Time [s ]
0.1
1
10
10 0
The transistors are soldered to the Cu platform of the mount
Problems: imperfect soldering, chip delamination
Thermal measurements and qualification using the transient method: principles and applications
72
Measurement of a power BJT mount:
failure analysis
T3Ster: differential structure function
Rth=3.2 K/W
The “good” structure function
Thermal measurements and qualification using the transient method: principles and applications
73
Measurement of a power BJT mount:
failure analysis
T3Ster: differential structure function
Die attach delamination
inside the package
Thermal measurements and qualification using the transient method: principles and applications
74
Measurement of a power BJT mount:
failure analysis
T3Ster: differential structure function
Imperfect soldering of the
package
Thermal measurements and qualification using the transient method: principles and applications
75