IPC-D-279 EN.pdf - 第71页
The preferred species in the case of copper will depend on the anion that is present. In water Cu +2 is the preferred species except when C1 − is present. In this case the forma- tion of the CuCl 2− will favor the format…
Appendix C
Design for Reliability (DfR) of Insulation Resistance
C-1.0 INSULATION RESISTANCE DAMAGE MECHA-
NISMS AND FAILURE
The damage mechanisms work generally in two distinct
regions: at the surface and in the bulk of the electronic
assemblies, particularly the printed board. It has been
reported, that surface and bulk phenomena exhibit different
time constants in the response to temperature changes [Ref.
C-7: 1]. The insulation resistance for a sample circuit is the
measured integrated effect of both surface and volume
resistivity as defined by ASTM [Ref. C-7: 2]. The mea-
sured bulk resistance will depend upon the nature of the
laminate, solder mask and/or conformal coating under
investigation. It will also depend upon the degree of cure
of the polymers and for printed boards on the quality of the
drilling process for the plated-through holes (PTHs) and
vias (PTVs), and will be affected by soldering flux/paste
residues if they dissolve into the polymeric material during
the soldering and/or cleaning processes.
Insulation resistance measurements provide important data
in the characterization of printed board laminates, multi-
layer boards (MLBs), soldering fluxes, solder masks, and
conformal coatings. Such measurements have been used to
study the effect of aging at accelerated conditions (tem-
perature, humidity and/or bias voltage) to determine any
detrimental effects on the reliability of the product.
Ohm’s law states that the magnitude of the current, I, flow-
ing through a circuit with a given resistance, R, is a linear
function of the applied voltage, V, such that
V = IR (Eq. C-1)
The resistance is an extrinsic property of the material
sample dependent on the resistivity of the material and the
geometry of the sample. The resistivity, ρ, of a material is
an intrinsic material property. The resistivity is determined
from the length, l, of the sample and its cross-sectional
area, A, and is related to R by
ρ=R
A
l
(Eq. C-2)
Resistivity that measures the resistance to current flow
through the bulk of a sample it termed volume resistivity,
ρ
v
,
ρ
v
= R
A
t
(Eq. C-3)
where t = the thickness of the bulk sample.
Surface resistivity, ρ
s
, measures the ability of an insulator
to resist the flow of current on its surface, such that
ρ
s
= R
A
s
l
(Eq. C-4)
where A
s
= the surface area and l is the length of the insu-
lating strip.
C-1.1 Surface Insulation Resistance (SIR) Surface insu-
lation resistance (SIR) measurements will depend on the
nature of the surface contamination and the amount of
moisture present during the measurement. Although SIR
readings are a combination of both bulk and surface resis-
tance, 99.9 % of the current leakage for FR-4 epoxy/glass
laminate will occur on the surface of the laminate, since the
ratio of the surface resistivity to the volume resistivity is
1:1000 [Ref. C-7: 3]. Test patterns for measuring volume
resistance will either use electrodes on the top and bottom
of the substrate (for z-axis measurements) or PTVs to PTV-
patterns for measuring the x, y-resistance. SIR patterns are
typically interdigitated comb patterns such as the IPC-B-24
coupon.
Some materials, notably polyglycol, act to reduce insula-
tion resistance by absorbing water and/or forming a mono-
layer of water at lower than saturation humidity, e.g. 75%
RH [Ref. C-7:4]
C-1.2 Electrochemical Corrosion Electrochemical cor-
rosion of metallic conductors and the migration of metal
ions between anode and cathode on a printed board can
lead to circuit failure. It is important to understand the
cause of these failures in order to select materials and pro-
cesses for printed board manufacture, soldering, and clean-
ing which will minimize the occurrence of these failures.
The tendency of the metal conductor to migrate under a
bias voltage in humid conditions has been shown to
decrease across the following series of metals [Ref. C-7: 5]
Ag >Pb>Solder>Cu (Eq. C-5)
Those metals whose hydroxides are more soluble at pH 7-9
have a higher migration rate. This is related to the pH gra-
dient between the anode and cathode when a film of mois-
ture is present.
In the case of SIR testing, the cathode is connected to the
high voltage source while the anode is connected to
ground. For electrochemical migration to occur, the path-
way must exist for ions to move from the anode to the
cathode. In the presence of moisture, the following electro-
chemical reactions can occur at the anode
H
2
O →
1
2
O
2
+2e
−
Cu → Cu
+1
+e
−
Cu → Cu
+2
+2e
−
Pb → Pb
+2
+2e
−
Sn → Sn
+2
+2e
−
Sn → Sn
+4
+4e
−
(Eq. C-6)
July 1996 IPC-D-279
59
The preferred species in the case of copper will depend on
the anion that is present. In water Cu
+2
is the preferred
species except when C1
−
is present. In this case the forma-
tion of the CuCl
2−
will favor the formation of Cu
+
rather
than Cu
+2
[Ref. C-7: 6].
At the cathode the possible reactions are
1
2
O
2
+H
2
O+2e
−
→ 2OH
−
2H
2
O+2e
−
→ 2OH
−
+H
2
Cu
+2
+2e
−
→ Cu
Pb
+2
+2e
−
→ Pb
Sn
+2
+2e
−
→ Sn (Eq. C-7)
C-1.3 Dendrite Growth In normal electrochemical den-
dritic growth, electrolytic dissolution of the metals occurs
at the anode and reduction of the metal ions by plating out
occurs at the cathode. Typical dendrites associated with a
solder-coated copper comb pattern will be lead-needles
with some tin. These appear as ‘‘tree-like’’ dendrites which
begin at the cathode. As these surface dendrites grow, their
effect on the total SIR reading is minimal until they are
very close to the anode. At the point of bridging, the den-
drite will burn out quickly due to the high current density.
For this reason, the presence of dendrites is not easily
determined by the electrical SIR readings. These readings
are not taken frequently enough to insure that a measure-
ment will be taken exactly when the dendrite bridges.
Thus, it is customary to examine SIR samples under the
microscope with back lighting after the test is terminated to
visually observe if dendrites have formed.
C-1.4 Conductive Anodic Filaments (CAF) In the late
1970 ’s a new electrochemical migration failure mechanism
was reported by Bell Laboratories [Ref. C-7: 6]. During the
study of potential failure modes associated with high volt-
age switching applications, the subsurface formation of
conductive filaments along the glass/epoxy interface was
observed at high humidity conditions under application of
a high (200-500 V) voltage. This conductive anodic fila-
ment (CAF) formation involved the dissolution of copper
at the anode and the formation of a copper-containing con-
ductive filament along the glass/epoxy interface. It was
reported that these CAF contain copper associated with
chlorine or sulfur [Ref. C-7: 8]; these contaminants are
associated with the board manufacturing process. Others
have observed only chloride- or bromide-containing copper
filaments [Ref. C-7: 9]. It has been suggested that moisture
causes hydrolysis at the glass/epoxy interface [Ref. C-7: 7].
It is postulated that the absorption of moisture by the epoxy
causes swelling which can lead to a debonding between the
epoxy and the glass fiber [Ref. C-7: 10]. This moisture-
induced debonding can be accelerated with damage to the
bond between the glass fibers and the surrounding epoxy
during the drilling process [Ref. C-7: 11]. A capillary of
moisture at these damaged interfaces is available for the
electrochemical reactions described in Eqs. C-6 and C-7
when a bias voltage is applied.
C-2.0 INSULATION RESISTANCE MODELING
In order to make predictions as to the behavior and reliabil-
ity of electronic assemblies in environments with varying
levels of temperature and humidity, data from representa-
tive accelerated high stress tests are used to extrapolate to
milder operating conditions. This extrapolation requires an
appropriate valid extrapolation model. It is not a simple
task to obtain either good accelerated test data or appropri-
ate extrapolation models. [Ref. C-7: 12].
C-2.1 Insulation Resistance Degradation SIR measure-
ments are a relatively quick way to evaluate the interaction
of processing materials with a given substrate. Reliability
assessment, however, requires significantly more effort.
Assumptions must be made about the relationship of the
operating and use environments to the accelerating test
conditions with elevated temperatures, humidities, and bias
voltages chosen to accelerate the rate of degradation by
known mechanisms without introducing extraneous dam-
age mechanisms. If the assumptions are correct, extrapola-
tion of the results from the accelerated tests back to oper-
ating conditions will provide an estimate of the product
reliability.
To accomplish this task, a statistically significant number
of samples must be processed and tested to failure at dif-
ferent accelerated test conditions. The effect of the tem-
perature on the rate of failure follows for these processes
an Arrhenius relationship [Ref. C-7: 13]
k
T
= k
0
exp
(
−E
a
kT
)
(Eq. C-8)
where
k
T
= the reaction rate at a given temperature,
T = absolute (Kelvin) temperature,
k
0
= a constant,
E
a
= the activation energy of the reaction, and
k = Boltzmann’s constant.
The effect of relative humidity can be described by
k
H
=k
1
0
expC(RH)
b
(Eq. C-9)
where
k
H
= the reaction rate at a given relative humidity,
k
1
0
= a constant,
C = described as a constant but is likely temperature
dependent (see Eq. C-10), and
b = an exponent empirically observed in the range from
1 [Refs. C-7: 1,8] to 2 [Ref. C-7: 14].
For tests with a bias voltage of 52 V, the insulation resis-
tance, IR, was found to have the following dependence on
temperature and humidity [Ref. C-7:1]
IPC-D-279 July 1996
60
1nIR = 1nIR
∞
+
E
T
kT
−
RH
100
E’
RH
kT
epx
[
E’’
RH
kT
]
= 1n 3.7x10
−29
+
2.51
kT
−
RH
100
0.31
kT
exp
[
0.12
kT
]
(Eq. C-10)
where
IR∞= an empirical constant interpreted as the insulation
resistance of the test sample at infinite tempera-
ture and zero humidity,
E
T
= the activation energy for the temperature depen-
dence, and
E’
RH
,E’’
RH
= activation energies for the humidity
dependence in eV
k = 8.62x10
-5
eV/°K
The form of Eq. C-10 is the consequence of the fact that
the driving parameter, the vapor pressure of water, p
v
,is
dependent on temperature. The relationship of the vapor
pressure of water with RH can be approximated in the
temperature range from 0 to 75°C by
p
v
=
RH
100
exp
[
16.82 −
5250
T[K]
]
(Eq. C-11)
Under the assumption that the effects of temperature and
humidity are independent of each other and that the rela-
tionships can be validly extrapolated, the expected life can
be calculated from results of the accelerated tests using an
acceleration factor, A.F., using
A.F. = exp
[
E
a
k
(
1
T
life
−
1
T
test
)
− C(RH
b
life
− RH
b
test
)
]
(Eq. C-12)
where the subscript ‘life’ refers to the normal operating
conditions and the subscript ‘test’ to the accelerated test
conditions.
The failure mechanism is highly temperature dependent
with activation energies, E
a
, variably reported as a very
low 0.02 eV for some samples below 60°C and 0.6 to 2.5
eV for all samples above 60-65°C [Ref. C-7: 15], 0.6 eV
[Ref. C-7: 14], 0.9 eV [Ref. C-7: 8] and 2.51 eV [Ref. C-7:
1].
The value of C associated with humidity can be taken from
Eq. C-10 with b=l; for b=2 and RH expressed in percent C
has been reported as 4.4x10
-4
[Ref. C-7: 14].
C.2.2 Conductive Anodic Filament Failure The determi-
nation of the mean-time-to-failure (MTTF) for the CAF
failure mode is more complicated.
The CAF-MTTF is not only dependent temperature,
humidity, and bias voltage, but also on the PCB materials
[Refs. C-7: 8,16], and the quality of the manufacturing
processes, particularly drilling [Ref. C-7: 11].
Further, preconditioning in terms of both thermal shock
[Ref. C-7: 17] and exposure to high humidity [Refs. C-7:
11,17] has been observed to reduce CAF-MTTF. There
appears to be a two-step process in which pre-conditioning
at an accelerated relative humidity will reduce the time for
CAF failure to occur once voltage is applied. (1) Moisture
absorption by the substrate leads to interfacial degradation
at the glass/epoxy interfaces. This interfacial degradation
can also be enhanced by prior thermal cycling, thermal
shock or, by mechanical stresses. (2) Electro-chemical cor-
rosion and oxidation of Cu to Cu
n+
creates the ions which
migrate under a bias voltage. The time to failure can be
expressed as the sum of these two steps
t
F
=t
1
+t
2
(Eq. C-13)
Studies where the application of the bias voltage was
delayed indicate that t
1
»t
2
[Ref. C-7: 10].
There is evidence of a threshold relative humidity below
which this degradation mode will not be observed. It has
been proposed that for a given voltage, V, and a tempera-
ture T, the relative humidity, in percent, corresponding to a
constant failure probability is [Ref. C-7: 7]
RH =
[
2.3975 + 1n (c)+
0.9
kT
− 1.52 1n (V)
5.47
]
(Eq. C-14)
where
c = 6.9x10
-4
for a failure probability of 0.5%.
C-3.0 DfR-PROCESS
For insulation resistance, DfR should perhaps stand for
‘Design for Robustness’ rather than ‘Design for Reliabil-
ity,’ because reliability assurance implies some numerical
certitude which is not attainable in this context.
Thus, a successful DfR-process is one that assures the
highest level of robustness that is practically and economi-
cally achievable. This requires that a number of issues be
addressed at the design stage. The generally applicable
guidelines to maximize robustness are:
1) Keep conductor lines and spaces on the printed
board as wide as possible;
2) Provide a controlled operating environment;
3) Conformally coat electronic product if it is subject to
temperature fluctuations such that condensation can
occur (the degree of protection will depend on both
the type and method of application of the coating);
4) Provide a controlled storage environment;
5) Avoid water-soluble fluxes and fusing fluids contain-
ing polyglycols for high humidity operating environ-
ments and high DC voltage gradients, since some of
these fluxes have been implicated in enhanced CAF
formation;
6) Utilize CAF-resistant PCB materials for assemblies
with high humidity operating environments and high
July 1996 IPC-D-279
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