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A novel test circuit for automatically detecting electrochemical migration and conductive anodic filament formation 1158
Ready, Turbini, Nickel, and Fischer
1158
Special Issue Paper
Journal of ELECTRONIC MATERIALS, Vol. 28, No. 11, 1999
(Received March 1, 1999; accepted June 17, 1999)
INTRODUCTION
Surface Insulation Resistance Testing
Insulation resistance measurements are important
test data for characterizing printed wiring board
laminates, soldering fluxes, solder masks and confor-
mal coating. They are used to study the effect of aging
a sample at accelerated conditions (temperature,
humidity and/or bias voltage) to determine if there
are any detrimental effects to the electrical proper-
ties. Although the insulation resistance readings are
a combination of both bulk and surface resistance, for
the case of FR-4, 99.9% of the current leakage will
occur on the surface of the laminate.
1
A Novel Test Circuit for Automatically Detecting
Electrochemical Migration and Conductive
Anodic Filament Formation
W. JUD READY,
1
LAURA J. TURBINI,
1
ROGER NICKEL,
2
and JOHN
FISCHER
2
1.School of Materials Science and Engineering, Georgia Institute of Technology, 778 Atlantic
Drive, Atlanta, GA 30332-0245. 2.Naval Air Warfare Center, China Lake, CA
The rapid growth of the global electronics manufacture environment has brought
about the onset of a variety of new, untested materials and processing chemicals.
The interactions between substrates and processing chemicals that can occur
during manufacture, storage and use must be assessed in order to determine
long-term reliability. Surface insulation resistance (SIR) testing is a standard
industry technique used to assess the interactions between processing chemicals
(e.g., soldering fluxes) and substrates. SIR test method conditions vary in terms
of the temperature and the humidity used to accelerate the normal failure modes.
Typically, a 45 to 50 volt bias is applied to an interdigitated comb pattern, and
periodic SIR measurements are made using a 100 volt test. Pass/fail criteria
based solely on SIR electrical values, (e.g., 100 M ) however are inadequate.
Often the electrical measurement fails to reveal the presence of surface dendrites
due to contaminants related to processing chemicals. This failure occurs because
the dendrite burns out between electrical readings when the circuit continues to
be biased at 50 volts. A new linear test circuit has been developed to overcome
this deficiency. The circuit uses an operational amplifier to detect the formation
of a surface dendrite between electrodes on the comb pattern. When the dendrite
shorts the circuit, voltage to the comb pattern is removed. Thus, the presence of
the dendrite is captured electrically, and the dendrite is preserved for further
analysis. This paper will present the circuit used and data showing its effective-
ness at detecting both surface dendrites and subsurface conductive anodic
filament formation.
Key words: Conductive anodic filament formation (CAF), electrochemical
migration (ECM), surface insulation resistance (SIR),
reliability, dendrite
Surface insulation resistance (SIR) testing is per-
formed in order to accelerate normal failure modes
through the use of elevated stress conditions. Joint
Industry Standard (J-STD-004) Requirements for
Soldering Fluxes,
2
paragraph 3.2.4.5 specifies that a
SIR test be performed in accordance with Test Method
IPC-TM-650 section 2.6.3.3. In this test, processed
interdigitated comb patterns on an IPC-B-24 board
(Fig. 1) are placed under a bias voltage of 45V to
50V. Periodically (at 24, 96, and 168 h) the bias is
removed and the resistance between the comb fingers
is measured using a test voltage of 100 V. This reading
is taken while the test coupons are in an environmen-
tal chamber at 85
°
C
±
2
°
C, and 85%RH
±
2%RH.
Bellcore
3
requires an aging condition of 35
°
C and
95%RH in their SIR test method. SIR measurements 1159
A Novel Test Circuit for Automatically Detecting Electrochemical
Migration and Conductive Anodic Filament Formation
are a relatively quick way to evaluate the interaction
of processing materials with a given substrate.
4
SIR is an extrinsic property of the material system
under investigation. This property will be affected by
the test pattern chosen, temperature, humidity, bias
voltage and duration of test, as well as the contamina-
tion associated with previous processing steps. This
contamination may result in electrochemical corro-
sion.
SIR electrical data along with microscopic exami-
nation of comb patterns after testing provides impor-
tant information on the flux/substrate interactions.
5
For example, rosin-based soldering fluxes frequently
show a higher SIR value than unprocessed control
coupons. This is due to the fact that unremoved rosin
acts as an insulator, trapping small amounts of ionic
residue. In addition, rosin residue, which is hydro-
phobic, inhibits moisture absorption and thus poten-
tial electrochemical migration is reduced.
Electrochemical Migration
In most SIR testing the comb pattern is placed
under a bias voltage. Periodically this voltage is
removed and a test voltage applied to measure the
resistance between traces. Under bias voltage, elec-
trochemical migration may be induced. However, it is
only detectable electrically during the test phase of
the SIR measurement. Electrochemical corrosion of
metallic conductors and the migration of metal ions
between anode and cathode on a printed wiring board
can lead to circuit failure. It is important to under-
stand the cause of these failures in order to select
materials and processes for soldering and cleaning
which will minimize the occurrence of these failures.
For electrochemical migration to occur, a pathway
must exist for ions to move from the anode to the
cathode. Surface moisture can provide that pathway.
This may come from the high humidity used in test-
ing, or may occur if the humidity generator in the
environmental chamber overshoots its setting and
condensation occurs. In the presence of moisture, the
following electrochemical reactions can occur at the
anode:
H
2
O
1
/
2
O
2
+ 2 H
+
+ 2 e (1)
Cu Cu
+1
+ e (2)
Cu Cu
+2
+ 2 e (3)
Pb Pb
+2
+ 2 e (4)
Sn Sn
+2
+ 2 e (5)
Sn Sn
+4
+ 4 e (6)
The preferred species in the case of copper will depend
upon the anion that is present. In water, Cu
+2
is the
preferred species except when Cl is present. In this
case the formation of the CuCl
2
will favor the forma-
tion of Cu
+
rather than Cu
+2
.
6
At the cathode:
_ O
2
+ H
2
O + 2 e 2 OH (7)
2 H
2
O + 2 e
2 OH

+ H
2
(8)
Cu
+2
+ 2 e Cu
(9)
Pb
+2
+ 2 e Pb
(10)
Sn
+2
+ 2 e Sn
(11)
Sn
+2
+ 2 e Sn
(12)
In electrochemical dendritic growth, electrolytic dis-
solution of the metal occurs at the anode and reduc-
tion of the metal ions occurs 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 dendrite will burn
out quickly due to the high current density. For this
Fig. 1. IPC-B-24 test coupon.
Fig. 2. Two CAFs ( and ) that formed on the tip of a comb pattern
processed with a water-soluble flux. 1160
Ready, Turbini, Nickel, and Fischer
reason, the presence of dendrites is not easily deter-
mined by the electrical SIR readings. The readings
required by J-STD-004 are not taken frequently
enough to insure that a measurement will be made
exactly when the dendrite bridges. Thus, it is neces-
sary to examine SIR samples under the microscope
with back lighting after the test is terminated to
visually observe if dendrites have formed.
Conductive Anodic Filament Formation
In the mid 1970s a new failure mode was observed
as a result of increased wiring density and hostile
environments in which circuits were required to per-
form. This failure mode is characterized by an abrupt,
unpredictable loss of SIR between conductors that are
held at a potential difference.
7
The loss of resistance
is caused by the growth of a subsurface filament from
anode to cathode between the two conductors. The
filament, termed conductive anodic filament (CAF), is
a result of an electrochemical corrosion process that
initiates at the anode and proceeds along separated
fiber/epoxy interfaces.
811
CAF is easily differentiated
from dendrite growth. In dendritic growth, metal ions
Fig. 3. Daily SIR data for IPC-B-24 test coupon processed with a water-
soluble flux. Note that there is no electrical indication of the CAF
formation, which was detected optically on all comb patterns.
Fig. 4. Hourly SIR data for IPC-B-24 test coupon processed with a
water-soluble flux. Note that there is no electrical indication of the CAF
formation, which was detected optically on all comb patterns. The
spike on comb B was due to a localized dendrite.
go into solution at the anode, but plate out at the
cathode, growing in tree-like dendrites across the
surface of the PWB. In contrast, CAF growth ema-
nates from the anode. Furthermore, the filament is a
copper salt containing chlorine or bromine in addition
to copper rather than pure metal as in the case of
dendrites. Additionally, dendrite growth is a surface
phenomenon, while CAF is a subsurface phenom-
enon.
Problems with SIR Testing
Ready
12
sh