デンソーテクニカルレビュー Vol.10
No.２ 2005
特集 Condensation Simulation for ECU Package＊
青木孝司
岡本真一
伊 奈 治
杉浦昭夫
Takashi AOKI
Shinichi OKAMOTO
Osamu INA
Akio SUGIURA
成田量一
Ryoichi NARITA
Up to now, while automobile electric packages have demanded the high density for small and light products,
electric leaks, which have occurred due to condensation, have been a major problem. To prevent any electric leaks,
a conformal coating on the electric parts (ex. ECU : Electronic Control Unit ) has been needed, but in general the
design rules of the conformal coating (ex. Application area) is vague. Therefore, DENSO demands to clarify the
design rules of conformal coating electric package for higher reliability.
To meet the demand, DENSO has developed a condensation simulation method using CAE that can show the
occurrence condition of condensation fast and accurately. In the result, DENSO has been able to get the design
rules where to need the conformal coating in the electric package for automobiles.
Key words : Condensation simulation, Automobile electric package, Conformal coating, Electric leak
１．INTRODUCTION
a drop of condensation forms between terminals, current
In recent years, car electronic products are characterized
leakage will result. If the drop grows, as shown in Fig. 2,
by smaller gaps between terminal pins and high-density
there will be a short-circuit. There are generally two
packaging using CSP, as shown in Fig. 1.
methods of preventing this, as depicted in Fig. 3.
1995
2000
2003
Condensed water
Current leakage
(specific residence: 1MΩcm)
Terminal
2005
e
[CSP]
(Positive)
[QFP]
(Negative)
Circuit board
0.4mm gap CSP
Fig. 2 Leakage by condensation
0.8mm gap CSP [520P]
0.4mm gap QFP [256P]
1.0mm gap BGA
0.5mm gap QFP [208-256P]
0.65-0.8mm gap QFP [120-160P]
<Moisture-proof coating>
Moisture-proof
material
1.2mm gap PLCC [68P]
Condensation
QFP: Quad Flat Package
CSP: Chip Scale Package
Fig. 1 Trend in electronic packaging
Circuit board
These trends lead to concerns that the increased strain on
<Completely sealed structure>
Condensation
soldered points may result in shortened service life due to
Casing
thermal fatigue, and that moisture resistance may
deteriorate, as is often observed in leakage due to
Water
vapor
condensation. Conventional acrylic coating protection
Circuit board
against moisture only increases the thermal strain on
soldered points; furthermore, the use of acrylic resin itself
Fig. 3 Moisture-proof structure
is being scrutinized under the stringency of environmental
regulations. The objective of moisture-proof treatment is to
(1) Moisture-proof coating (insulation) on the circuit board
prevent current leakage and migration between terminals in
(2) Complete sealing of the circuit board to prevent the
the event of condensation on a printed circuit board. When
ingress of moisture (without moisture-proof material)
＊Reprinted with permission from SAE paper 2004-01-1696  2004 SAE International
−102−
特 集
Thus far, however, both methods have shortcomings. In
Phase change
Water vapor rises
method (1), it is still unclear which portion of the board is
Liquefaction and
vaporization
Latent heat transfer
(due to difference in
specific gravity with air)
likely to experience condensation, and therefore, where the
Water
vapor
coating must be applied. As for method (2), guaranteeing
completeness of the seal may require higher manufacturing
Air
Temperature
differential
cost. To maintain product competitiveness, it is necessary
Board wettability
Condensed
water
Board surface
to :
①
Fig. 4 Condensation process model
minimize amount of moisture-proof material by
applying it only to necessary points, and
minimize cost of casing by defining conditions for
due to difference in the speed of the temperature rise, as
preventing condensation (e.g. maximum gap allowed)
shown in Fig. 6. When the dew point is within the range
to guarantee insulation reliability equal to that of a
between air temperature and board temperature,
completely sealed structure even when the casing is not
condensation occurs. When the temperature stabilizes after
completely sealed.
a certain time, the condensation disappears. Under the
②
Given
①
and
②
above, it is essential to do the following
to improve product competitiveness.
transient condition in which the temperature falls, the
circuit board temperature will be relatively higher than that
1) Quantitative analysis of condensation process
of the air; therefore, condensation occurs on the inner walls
2) Establish structural design technique optimized to
of the casing. Although condensation is a familiar
practical environments in which products are used
phenomenon, it can occur transiently within a minute space
In the meantime, the solution cannot be found merely by
with very little temperature difference. Considering
evaluating actual products, due to the complexity of the
limitations in the forms and accuracy of measuring
condensation process and the difficulty of measurement in
instruments, it is difficult to solve the problem by relying
minute areas. With the aim of improving problem-solving
(ºC)
(%RH)
40
Temperature (T)
was initiated by applying thermohydrodynamic analysis
methods to quantitative analysis of the condensation
process under transient temperature and humidity
conditions in electronic products.
100
75
20
10
0
50
0
-10
25
-20
２．OBJECTIVES OF CONDENSATION
0
-30
SIMULATOR
2.1
P=1atm
30
40
90
40
90
0
0
390
900
(s)
Time (t)
Understanding condensation process
1200
Fig. 5 Condensation evaluation pattern
Condensation occurs on a circuit board when, by
convection or diffusion, warm air or moisture in the
ambient air enters via an opening in the casing around a
(ºC)
40
Dew point
Temperature (T)
circuit board and liquefies on the board surface, as shown in
Fig. 4. The parameters involved in the process are
temperature, relative humidity, and the latent heat transfer
that takes place in phase change.
In general, the condensation in car electronic products is
Condensation range
on the circuit board
-30
evaluated by testing under different combinations of
Air temperature within casing
Board surface temperature
temperature and humidity specified in Fig. 5. Under the
Time (t)
transient condition in which the temperature rises, a
temperature differential arises between circuit board and air
Fig. 6 Condensation range
−103−
(s)
Relative humidity (RH)
efficiency, the development of a condensation simulator
デンソーテクニカルレビュー Vol.10
only on the evaluation of actual products. Therefore, our
aim was to solve the problem rationally and effectively by
Studying simulation method/
procedure
combining parameters measured in theoretic product
2.2
1st
step
Verification
(Conditions / Amount of condition)
evaluation using CAE.
No.２ 2005
Defining elements of
structural optimization
Causes and effects of condensation
Condensation is a phenomenon in which the water
Calculating optimum structure
vapors in moist air liquefy upon reaching the state of
supersaturation. Condensation on a circuit board is
Fig. 8 Research procedure
governed by three factors, shown in Fig. 7: the product
structure, atmosphere and mounting conditions. Thus, the
humidity, and their transitions at respective positions.
objective was to find the optimum structure efficiently by
From this observation, the modeling involves, at least a
standardizing each parameter.
combination of structure and fluid (internal air). The
ambient air may be included in the modeling, or
represented by setting up a boundary that transfers heat at
Atmosphere
(Temperature / Humidity)
Opening
the outer surface of the casing. Judging from these, a multi-
Air
phase thermo hydrodynamic analysis is generally
Condensed
water
considered the appropriate simulation method that covers
all governing formula involved in the condensation process.
Electronic device
Circuit Board
However, it is also extremely complex and, as shown in
Mounting condition
Table 1, is probably inapplicable in terms of calculation
(Gravitational force)
time and convergence. In addition, this method is of the
structural mesh type and poses difficulty in modeling the
Product structure
Number of openings
Thermal capacity
Casing
Opening
structure
Position
Humidity
subject faithfully.
Self-heating
Area
Inner volume
Condensation
Single-phase
thermohydrodynamics Multi-phase
thermohydrodynamics
+ sub-program
Position
Profile
Temperature
Atmosphere
Table 1 Comparison of simulation methods
Direction
Mounting condition
Simulation
model
Fig. 7 Cause and effect diagram of condensation in
car electronic products
2.3
Research procedure
Figure 8 describes the research procedure adopted. This
paper focuses on the first step from the study of simulation
３．STUDY OF SIMULATION
Some limitations
(Structural mesh)
Convergence
Yes
No
Calculation
time
Calculation
precision
Cost
ratio
Ease
of use
ca.10hours
over 1000hours
(Parallel operation)
(Convergence program)
Good
NG
1
over 100
Evaluation
procedure to defining elements of structural optimization.
Greater freedom
(Non-structural mesh)
Good
(Interactive)
NG
Good
NG
Therefore, it was decided to adopt a method in which
METHOD/PROCEDURE
moist air is regarded as a single-phase fluid whose three
In electronic products to be simulated, condensation
elements (dry air, water vapor, and condensed water) are
occurs when the moisture in the internal air, or ambient air
defined as scalars. The fraction of the masses of three
that has entered the interior, liquefies on various elements
elements is determined by a sub-program that calculates the
of circuit boards depending on the ambient temperature and
phase change from water vapor to condensed water by
−104−
特 集
incrementing time; the results are integrated into an
ordinary loop of flow field calculation.
Dry air
Relative humidity
>100%
Sub-program
Creation
Figure 9 shows the algorithm used in the calculation and
Ingress by Water
convection
Passage
Diffusion
of time
output of each process. This method creates the added task
of developing a sub-program for simulating the process of
Ingress by
convection
Water
vapor
Passage Diffusion
of time
phase change from water vapor (gas) to condensed water
<Equation of condensation>
(liquid). Calculations of enthalpy, radiation and form of
condensed water wetting are excluded here.
Single-phase
Thermohydrodynamics
simulation
START
Time
increment
Excluded
All enthalpies
Radiation
Board wetting form
Calculation of mixed
gas physical property
Applicable Equation of state
fomula
(Mixed gas)
Outputs Density
Thermal conductivity
Specific heat
Flow Calculation
Navier-Stokes
Applicable equation
fomula Equation of continuity
Equation of energy
(Mixed gas)
Outputs Flow
Temperature
Sub-program
(Condensation simulation)
Outputs
Time
increment
Equation of
mass transfer
(Mixed gas)
Fraction of
each element
Flow
Circuit board
w=
-Relative humidity
f =
mvapor
mair
w P0
(0.622 + w)Ps
-Water formation
(Condensation)
mH2O =
-Disappearance
of water vapor
mvapor = m H2O
(w- w ) P [kg sm ]
t
f =1
3
[kg
sm 3
]
Fig. 10 Formulas for gas-liquid phase change
simulation (condensation process simulation)
PRECISION VERIFICATION
Flow
Water
vapor
-Absolute humidity
４．BASIC MODEL SIMULATION AND
Dry air
Pressure
Mixed gas
Ps = Ps (T)
(T: temperature, t: time, P: density)
Gas-liquid phase
change simulation
Applicable
fomula
-Saturation pressure
of water vapor
Circuit board
4.1 Conditions for simulation using the basic
Condensed
water
model
END
Figure 11 shows the general concept of the test model.
Fig. 9 Calculation algorithm
Inputs are histories of temperature and relative humidity
under the transient condition in which the temperature rises
Next, Figure 10 shows the formulas used to simulate the
(transient boundary conditions of ambient temperature and
gas-liquid phase change for analyzing the condensation
humidity), with transient analysis of 1,200 steps at 1 second
process. Normally, condensation occurrence is represented
intervals. In addition, these inputs are defined as the
by the dew point, but here it is determined by defining the
boundary conditions for the simulation model while
water vapor saturation pressure (Ps) by an approximated
temperature, relative humidity and pressure (1 atm,
high degree function of temperature. Ps is defined by
constant) are assigned for openings (end sections of internal
formula (1).
air) and temperature and thermal conductivity (33 w/m2k)
are assigned for the boundaries with thermal transfer at the
Ps=103(6E-7X 4+1E-5X 3+1.7E-3X 2+4.28E-2X+0.6119)
respective outer walls of the casing and connectors. The
fomula (1)
transient behaviors of these parameters are defined by the
X=T-273(T: Temperature (k))
sub-program. Here, the relative humidity of the atmosphere
is defined by the fractions of the masses of three elements
Using this method, it is possible to determine
(dry air, water vapor and condensed water) of moist air.
condensation occurrence by combining default outputs of
the solver; this reduces the computational load and
4.2 Simulation results and precision verification
enhances calculation efficiency.
To verify the precision of simulation using the basic
model, condensation occurrences were compared with
measurements taken on actual products under transient
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デンソーテクニカルレビュー Vol.10
< ECU (Basic model) >
No.２ 2005
determined by transient changes in electric current leakage
Comb-shaped
gage
Casing
as measured using a comb-shaped gauge (with 300 µm gap)
that conforms to JIS Z 3197, placed on the surface of a
circuit board in an open casing as shown in Fig. 12. The
current leakage was measured by the method depicted in
Circuit
board
the circuit diagram, while temperature and humidity of
Section for
simulation model
atmosphere, internal air and circuit board were monitored
using thermometers and relative humidity gauges.
Boundaries at openings
(Boundaries with pressure,temperature,
humidity and mixed gas defined)
Measurement points are also shown in the figure.
Internal air
- Determining condensation on actual products
Circuit
board
Comb-shaped
gage
Constant temperature, humidity tank
Condensation leakage
(Comb-shaped gauge)
Casing walls
(Boundaries with thermal transfer defined)
Temperature measurement
(Temperature sensor
on board / in air)
- Conditions for simulation
Item
Conditions
Inputs
Histories of temperature and humidity
(Fig. 5)
Histories of temperature, humidity,
and amount of condensation
Transient analysis
Outputs
Calculation
method
Governing
formulas
• Equation of continuity
• Navier-Stokes equation
• Equation of energy
• Equation of state
• Equation of mass transfer
Pre/Post
FEMAP + PROSTAR
Solver
Circuit board
Condensed water Relative humidity measurement
(Humidity sensor)
- Circuit diagram for
condensational leakage measurement
JIS-Z-3197
RG
STAR/CD (v3100A)
4-node parallel operation)
CPU
20V
- Input boundary conditions
Item
Boundaries
at
openings
Outer
wall
Comb-shaped gauge
Conditions
100kV
Standard
pressure
boundary
(Pressure)
Pressure : P=1atm (Constant)
Temperature : T (Fig.5 equivalent)
Relative humidity: RH
(Fig. 5 equivalent)
Mixed gas
definition
(Scalar)
Water vapor
Condensed water
Dry air
V
Condensation leakage : Ia =
Gage resistance
Boundaries
with thermal Temperature : T (Fig.5 equivalent)
Thermal conductivity : 33 W/(m2.K)
transfer
(Wall)
V/10000
: RG = KR (20/IL)
Fig. 12 Determining condensation on actual products
- Physical properties of mixed gas
Properties
Dry air
Conditions
Water vapor Condensed
water
Weight
kg/kmol
28.9
18.0
18.0
Density
kg/m3
1.21
0.88
998
Specific
heat
J/(kg.K)
1006
2090
4183
Viscosity
Pas
1.85 E-5
1.05 E-5
1.00 E-3
0.026
0.0018
0.603
Thermal
.
conductivity W/(m K)
Figure 13 shows an example of actual measurement and a
simulation result. The two contors represent the temperature
distribution at respective points and the distribution of
condensed water fractions around the comb-shaped gauge
after 1,000 seconds. It was observed on actual products that,
as the ambient temperature and humidity rose, those of the
internal air and components also rose, due to heat transfer
from the outer walls of the casing and convection. After 750
Fig. 11 Conditions for simulation using basic model
seconds, condensation commenced on the surface of the
conditions in which the temperature and humidity change.
comb-shaped gauge, due to the difference in response time
First, condensation on actual products was observed and
constants between circuit board and internal air, and peaked
−106−
特 集
at around 900 seconds. Subsequently, the ambient
4.3 Application examples and verification
temperature and humidity became constant, forcing
As application examples, the effects of changing the area
component and internal air temperatures to converge at the
and direction of the opening were simulated. The focal points
ambient temperature, reducing the amount of condensation.
in these simulations were vent hole shape and direction. Two
On the other hand, the simulation shows condensation
standards were set for each factor: an open casing without a
starting after about 700 seconds, peaking at around 900
lid and a waterproof casing with a 6mm diameter vent hole
seconds, and diminishing thereafter. The results are
for the former, and vent holes on top and bottom for the
consistent with each other, with little discrepancy in starting
latter. The models of these simulation examples and the vent
time. Condensation occurrences and disappearances were
holes (location of boundaries with atmosphere) are shown in
thus successfully visualized and qualitative level precision
Fig. 14. Note that boundary conditions were set following
was achieved.
the example of the basic model.
Transient temperature distribution (1000s)
39.1ºC
Simulation models
34.1ºC
Distribution of condensed water
around comb-shaped gauge (1000s)
Top
Output point
Casing
Circuit board
Circuit board
Bottom
Opening casing
Vent hole
Vent hole
- Actual measurement
(µA)
3
(RH)
100
Temperature
humidity
80
Relative humidity
60
2
40
20
Humidity
setting
0
Leakage
1
current
Board
Temperature
-20
Temperature
setting
-40
0
1200
700 900
0
Time
Current leakage at gauge
(ºC)
Top
Fig. 14 Simulation models
Next, the condensation evaluation results are shown in
gauge for actual measurement and maximum fractions of
(s)
condensed water for the simulation. Both measurement and
(%)
simulation show that the open casing has condensation
100
10
more than 104 times that of the waterproof casing. From this
Relative humidity
60
40
Condensation
amount
Humidity
setting
20
5
0
-20
-40
0
Board
Temperature
Temperature
setting
700 900
Time
0
Fraction of condensed water
(RH)
80
Temperature
humidity
Vent hole
Table 2. Evaluated values were maximum leakage at the
- Simulation result
(ºC)
Circuit board
Circuit board
Bottom
Waterproof casing
Vent hole
1200
(s)
Fig. 13 Simulation results and precision verification
it can be concluded that the area of openings is the major
contributing factor to condensation. As for correlation
between vent hole direction and condensation, more
condensed water was observed with the bottom vent hole
for both types of casing. This is because with the vent hole
on top, water vapor in the moist air does not usually enter
the casing due to its lightness compared to dry air and
because the moisture in the internal air tends to liquefy on
the ceiling. Therefore, it is more advantageous to have a
−107−
デンソーテクニカルレビュー Vol.10
vent hole on top and the circuit board at the bottom of the
No.２ 2005
Simulation models
casing.
Vent hole
top under transient conditions in which the temperature
rises. Figure 16 shows the contour outputs at the peak of
condensation for each simulation standard. Given these
Top
examples, it can be said that quantitative analysis and
here will apply effectively to the design of optimum
moisture-proof structures and the development of moisture-
0.5%
Top
0.003%
0.005%
Circuit board
Circuit board
Bottom
product development method and procedures established
34%
Vent hole
Waterproof casing
been made possible. These examples also suggest that the
9%
Vent hole
visualization of the condensation/vaporization process,
which is difficult to measure with actual products, have
1%
Circuit board
Circuit board
Bottom
(simulation output) for both the casing with a vent hole on
Opening casing
Figure 15 shows the fractions of condensed water
0.007%
0.001%
Vent hole
proof treatment with theoretical support. In the future,
efforts will be made to propose appropriate moisture-proof
Fig. 16 Contour outputs of condensed water fraction
structures and conditions for coating, through planned
５．CORRELATION BETWEEN
research into defining elements of structural optimization.
CONDENSATION AMOUNT AND
INSULATION RESITANCE
Table 2 Condensation evaluation results
As indicated in earlier sections, it has become generally
Simulation
outputs
Gravitational
(condensed
force
(leakage at gauge) water fraction)
Actual
measurement
2 ~ 200µA
+1G
Opening
casing
-1G
+1G
Waterproof
casing
possible to simulate the extent of condensation. This section
reviews the correlation between amount of condensation
9%
and insulation resistance. Condensation leakage current is
34%
determined by the resistance of the condensed water that
bridges two conductors on the circuit board, and the
0.005%
‹ 0.002µA
difference in electrical potential between the two
0.007%
-1G
conductors. Condensation typically begins with a
hemispheric drop of condensation 1 µm diameter or less at
(%)
the core, growing with expansion of the diameter and by
Condensed water fraction
100
joining with adjacent droplets. The process is very complex
Opening casting /
top vent hole
10
and depends on the temperature, humidity, surface
conditions of the board etc. Assuming that condensed water
1
is a film of constant thickness and that the specific
0.1
resistance of condensed water is ρΩ-cm, the correlation
0.01
between amount of condensed water and leakage current is
Waterproof casting /
bottom vent hole
0.001
expressed by formula (2) in Fig. 17.
Figure 18 is a chart that depicts this correlation, based
0.0001
0
300
600
900
1200
Time
on formula (2) (ρ=1 MΩ-cm), and the correspondence
(s)
between simulated condensation amount using the basic
Fig. 15 History of condensed water fraction
model and leakage measurements taken on actual products.
Through comparison of these two relations, it can be said
that the latter relation, based on measurement, tends to
indicate smaller leakage. This is presumably due to various
−108−
特 集
Condensed
water
[Factor1] Wetting form : Increased resistance
V
Condensed water
Terminal
IR
Terminal
t
Circuit board
Circuit
board
L
t
V
ρ
r
IR
W
L
[Factor2] Board temperature increased due to
latent heat from condensation
: Less difference in temperature
: Length between terminals
: thickness of condensed water
: Voltage
: Specific resistance of condensed water
: Length of terminal
: Leakage current
: Condensation amount
Heat radiation
Assumption :
condenses evenly on board surface
IR = V / R = V
tr
Lρ
=V
[Factor3] Effects of specific resistance of
condensed water
W10-3 r
Lρ
Fig. 17 Formula for calculating leakage by
condensation
(µA)
Condensed leakage IR
ρ
Large
Large
IR
Small
Fig. 19 Major factors affecting leakage by
condensation
100
1
Small
Theoretical line
(ρ = 1MΩcm)
tool has been developed, while achieving qualitative
precision. The simulator makes it possible to understand
V : 20V
L : 0.318mm
r : 160mm
0.01
and visualize the condensation process that takes place in a
minute area, which is impossible to measure in actual
Measured
condensation
leakage
0.0001
product evaluations. It will also be used in the future to
solve problems in optimizing moisture-proof structures and
0.0001
0.01
1
100
Condensation amount W (µg / mm2)
Fig. 18 Correlation between condensation and
leakage
developing new moisture-proof treatments for electronic
products.
REFERENCES
1) Nobuyuki Kato etc., The Society of Heating, Air-
real-life factors, such as wetting forms of condensed water,
Conditioning and Sanitary Engineering of Japan No.74
board temperature increase due to latent heat from
(1999.7)
condensation, and the specific resistance of condensed
2) You Matsuo, The Society of Heating, Air-Conditioning
water as shown in Fig. 19. To improve the accuracy of
simulation, it will be necessary to extract these factors,
and Sanitary Engineering of Japan C39 (1999.10)
3) Nobuyuki Kato etc., The Society of Heating, AirConditioning and Sanitary Engineering of Japan No.72-1
quantify them and reflect them in the algorithm.
(1998)
６．CONCLUSION
4) Hitoshi Takeda, The Society of Heating, Air-Conditioning
and Sanitary Engineering of Japan C40 (1990.10)
By combining the transient thermohydrodynamic
calculation feature from a general purpose program written
5) Masanori Yadoya, Architectural Institute of Japan,
(1987.10), p.921.
for single-phase fluids and an optional feature of gas-liquid
phase change simulation (condensation simulation), a
condensation simulator that is easy-to-use as a development
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デンソーテクニカルレビュー Vol.10
No.２ 2005
666666666666666666666666666666666666
＜著 者＞
岡本 真一
青木 孝司
（おかもと しんいち）
（あおき たかし）
材料技術部
材料技術部
絶縁・接着材料の研究・開発に
絶縁・接着材料の研究・開発に
従事
従事
伊奈 治
杉浦 昭夫
（すぎうら あきお）
（いな おさむ）
材料技術部
材料技術部
CAE解析に従事
絶縁・接着材料の研究・開発に
従事
成田 量一
（なりた りょういち）
材料技術部
非金属材料の研究・開発に従事
−110−