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Ionic Migration on Printed-Circuit Boards
ESPEC CORP.
CONTENS
1. Introduction
2. About ionic migration
3. Test methods
4. Insulation resistance test
(Copper and silver migration)
5. Solder alloy migration
Migration occurring on electronics components
Copper migration
Copper migration
(a) Copper migration on PCB (b) Copper migration on telephone connector
Gold plated copper terminal
-20
0
20
40
60
80
100
2000/5/4
2000/5/14
2000/5/24
2000/6/3
2000/6/13
2000/6/23
2000/7/3
2000/7/13
2000/7/23
2000/8/2
Temp ( ℃) / Humi.(%RH)
-20
0
20
40
60
80
100
1999/12/21
1999/12/31
2000/1/10
2000/1/20
2000/1/30
2000/2/9
2000/2/19
2000/2/29
2000/3/10
Temp.(℃) / Humi (%RH)
Solder crack
Dew condensation
5mm
Connected deterioration
(a) Temperature and humidity in automobile (b) Failure on PCBs
winter
summer
Humi.
Temp.
Failure on PCB and environmental stress
Insulation deterioration
Migration occurring
Temp. cyclic
Factors of failure on PCB
Failure phenomena
Connected deterioration
Insulation deterioration
Fatigue – solder clack
Oxidation – contact defective
Corrosion – migration
Adhesion – water absorption
Failure mode Factor phenomena result
Increase of contact resistance
Decrease in insulation resistance
Transform – decrease in insulation distance
Abrasion
Breakdown
Breakdown
About ionic migration
Classification of migration phenomenon
(a) Ionic migration(electrochemical migration)
(b) Electro migration (c) Stress migration
Reference (b) (c ): Tsuneo Ajiki: “reliability of semiconductor device”, Nikkagiren, 1988
Reaction Classification Phenomenon Failure part
Electrochemistry Ionic migration(electrochemical migration)
The metal ionizes, a metallic ion migretesby the electric field.
Between wiringfor PCBs
Electro migration Interaction of metallic atom and electron
Stress migration A metallic atom migrates by the mechanicalstress.
Themal migration A metallic atom migrates by the thermalstress.
Physical Aluminum wiringof semiconductor
cathode(-)anode(+)
(a) Dendrite
M+ CAF
(b) CAF (Conductive Anodic Filament)
CAF
①
②
③ Resin
Copper electrode
cathode
anode
Dew or moisture
dendrite
moisture
0.5mm0.1mm
cathode(-)anode(+)
Solder dendrite on PCB Copper CAF in PCB
The forms of Ionic Migration
Phase1: metallic dissolutionPhase2: metallic ion migratePhase3: metallic deposition
Metal Oxide-film (MOX)
Insulation layer
M
Mn+
Diffusion
M
M
MOx
Dissolution
MOxMOx
M(OH)x
Oxide- film metal
Deposit
OH-H+
Anode (+) Cathode (-)
M(OH)x
O2
H2O
Mn+
Dew or moisture
Ⅰ Anodic reaction
M→Mn++ne- ・・・・・・・・・・・・・・(1)
H2O→1/2O2+2H++2e-・・・・・・・・・(2)
M+H2O→MO+2H++2e-・・・・・・・ (3)
Ⅱ Cathodic reaction
Mn++ne-→M・・・・・・・・・・・・・・・・・(4)
O2+2H2O+4e-→4OH-・・・・・・・・・・(5)
2H2O+2e-→H2+2OH-・・・・・・・・・・(6)
2M++2OH-→M2O+H2O・・・・・・・・(7)
M2++2OH-→M(OH)2・・・・・・・・・・・(8)
Ⅲ Reaction between electrodes
2M++2OH-→M2O+H2O・・・・・・・・・(9)
M2++2OH-→M(OH)2・・・・・・・・・・・(10)
Reaction mechanism of Ionic Migration
Acceleration factor of Ionic Migration
Factor Acceleration condition
Materials ( Fast) Ag>Cu>Pb>Sn-Pb Solder>Sn>Au( Slow)
Temperature High Temp
Humidity High humidity
Voltage High voltage
pH Acidity
Ionic impurities Halogen material (Chlorine, Bromine)Printed-cuircuitboard material Paper phenol > Glass epoxy > Polyimide > Ceramic
Acceleration factor : materials (Pourbaix potential - pH diagrams)
immunitypassivity
corrosion
Tin (Sn)
0 7 142
1
0
-1
-20 7 14
Ag2O3 Ag 2O2
AgAg 2 O
Ag +
pH
0 7 142
1
0
-1
-20 7 14
Sn
SnO 2
SnH 4
Sn2+
SnO
32-
pHPo
tent
ial /
SH
E
Pote
ntia
l / S
HE
0 7 142
1
0
-1
-20 7 14
CuO
2-
Cu
CuOCu2+
pH
0 7 142
1
0
-1
-20 7 14
PbO 2
Pb2+
PbPbH2
PbO
pH
Pote
ntia
l / S
HE
Pote
ntia
l / S
HE
Silver (Ag)
Lead (Pb)
Copper (Cu)
Acceleration factor : materials (Energy of oxide - film)
metal oxide-film △Gf o ( kJ/ mol )
SnO2 -515.47
Bi2O3 -496.64
Pb2O 3 -321.92
ZnO -321.92
SnO -257.32
PbO -189.33
CuO -127.19
Ag2O -10.75
Gibbs free energy change of metal oxide-film
Stability
Acceleration factor : Humidity (water – film and corrosion)
0.1
1
10
100
20 40 60 80 100
relative hum i. (%)
amount of corrosio
(mg/dm2)
(a) Relative humidity and corrosion of metal(iron)
(b) Thickness of water-film and corrosion speed
thickness of w ater-film
corrossion spee
1mm~10μm~
wetdry waterwet
Reference (a): varon, w. : Trans, faraday soc., 23, 113(1927)(b):Tomashov, N D.: corros., 20, 7t(1964)
Acceleration factor : Voltage
(a) Quantitative characteristics of elution: voltage and copper ions
(b)The relationship between applied voltage and migration occurrence
0
10
20
30
0 20 40 60
Tim e (sec)
Cu+/Cu2+decomposition amount
(mg/L)
5VD
2VD
3VD
1VD
Ta=25℃
0
100
200
300
0 1 2 3 4 5 6
applied voltage(V)
migration occurrence time(s
de-ionaizedw ater(pH6.6)
Ta=25℃
Test methods
Test standards for Insulation evaluation
Standard Test nam e Test conditions Applied voltageM easurem aentVoltage
IPC -TM -650-2.6.13
Assessm ent ofSusceptibility toM etallic DendriticG rowth UncoatedPrinted W iring
25℃De-ionized water:
60m l
0~20V/DC(M AX.15m A)
ANSI-J-STD-004
Requirem ent forSoldering Fluxes
85℃, 85%RH,168H
50V / DC 100V / DC
40℃, 90-95%RH,1000H
45 to 50V /DC
100V / DC
85℃, 85 to 90%RH,1000H
45 to 50V /DC
100V / DC
35℃, 85 to 93%RH,4days
100 V / DC
50℃, 85 to 93%RH,7days
100V / DC
Decided inconsultationwith purchaser
M oisture andInsurationResistance ofprinted boards
IPC -TM 650-2.6.3
JIS-Z-3284 Solder Paste
Migration test pattern on PCB
(a) IPC test pattern (b) IPC multi-purpose test board
Test methods : Simple migration test
(a) Water drop test
PCB
ElectrodeDe-ionized water
A
Power supply
Current meter
Resistor = 10kΩ
(b) Filter paper test
Resin
ElectrodeFilter paper
moisture
Resistor = 10kΩ
APower supply
Test methods : Environmental migration test
Temperature / humidity chamber
HAST
Dew condensation cyclic chamber
Temperature / humidity cyclic chamber
(a) Evaluation by absorption (b) Evaluation by dew
(1) specimen
(2) High Temp/High Humi. test
(3) Insulation resistance meter (HP4329)
(4) Insulation resistance meter (HP4339)
measurement method : Manual measurement
Insulation resistance test of PCBs
Results of Water Drop test (Cu and Ag, 2V/DC bias)
cathode(-)anode(+)
External appearance after testing
Copper migration
Silver migration
0.0
0.2
0 .4
0 .6
0 .8
1 .0
0 100 200 300
T im e (s e c )
Cu
rre
nt
(mA
)
0.0
0.2
0.4
0.6
0.8
1.0
0 100 200 300
Tim e (sec)
Current (mA)
Change in currentCopper (Cu)
Silver (Ag)
Surface insulation resistance (SIR) test for PCB
(a) Test condition
(b) Specimen
Grass-epoxy PCB
40℃,87%60℃,87%85℃,85%
1000HrGlass-cloth epoxy (FR-4)
IPC-B comb pattern(Gap=0.3mm)
Applied voltage 50V / DCMesurement
intervals 1 Hr
Test conditions
PCB material
Ionic migration evaluation system (ESPEC)
Wiring situation
Results of SIR test (Copper pattern PCB, 50V/DC bias)
Change in insulation resistance Change in absorption characteristic
1.E+09
1.E+10
1.E+11
1.E+12
1.E+13
0 200 400 600 800 1000
Tim e(hr)
Resistance(Ω
40℃87%RH
60℃, 87%RH
85℃,85%RH
0.01
0.1
1
0 200 400 600 800 1000
Tim e(hr)humidity absorption weight
40℃87%RH
60℃87%RH
85℃85%RH
Results of SIR test (Copper pattern PCB, 50V/DC bias)
40℃, 87% 60℃, 87% 85℃, 85%
External appearance after testing (2000Hour)
No migration No migration No migration
cathode(-)
anode(+)
Example : Results of SIR test (Paper phenolic PCB)
85℃, 85%, 50V/DC, 2000Hr
External appearance after testing
110℃, 85%, 5V/DC, 300Hr
cathode(-)
anode(+)
CAF growth CAF growth
CAF
CAF
1.0E+06
1.0E+07
1.0E+08
1.0E+09
1.0E+10
1.0E+11
0 200 400 600 800 1000
Tim e (Hour)
Resistance (Ω
Example : Results of SIR test (Flux and Dew test)
Evaluation of soldering flux( 85℃85%,50V/DC bias )
Flux A
Flux B
Flux CMigration growth
Dew cyclic test( 5℃/25℃90%,5V/DC bias )
1E+06
1E+08
1E+10
1E+12
1E+14
1E+16
0 20 40 60 80 100
Tim e (C ycle)
Resistance(Ω
Migration growth
IPC-9201 (Surface Insulation Resistance Handbook)
)1122()]
21
11(exp[)]
21
11)(
REaexp[(12
VAVA
RHRHb
TTtt ×−×−×=
The Institute of Electrical Engineers of Japan (Report No. 772)
Ea = Activation energy ( Glass Epoxy PCB = 1eV)
R = Boltzmann’s Constant (8.63×10-5 eV/k)
T1, RH1, V1, D1, t2 = Practice use Temp., Humi., Voltage, Electrode Distance, Time
T2, RH2, V2 , D2, t1= Accreted test Temp., Humi., Voltage, Electrode Distance, Time
B, A1,A2, r, n, m = constant ( r=3, n=2)
mDDn
VVr
RHRH
TTREaAF ⎟
⎠⎞
⎜⎝⎛×⎟
⎠⎞
⎜⎝⎛×⎟
⎠⎞
⎜⎝⎛×⎟
⎠⎞
⎜⎝⎛ −=
21
12
12)
21
11(exp
Life estimated formula of insulation failure
・Ea = Activation energy
(Glass Epoxy PCB = 1eV)
・R = Boltzmann’s Constant (8.63×10-5 eV/k)
・ Practice Temp.(T1)=45℃,Humi.(RH1)=85%
Voltage(V1)= 5V
・Test Temp(T2)=85℃,Humi.(RH2)=85%
Voltage(V2)=5V, 2000Hr
・r, n = constant ( r=3, n=2)
・AF1= exp[1.16×104×(1/318-1/358)]
= exp(4.1) = 59
・AF2 = (85/85)3 = 1
・AF3 = (5/5)2 = 1
※AF=59×1×1 = 59
Life Time = 2000Hr×AF
= 118000Hr = 13.4 years
mDDn
VVr
RHRH
TTREaAF ⎟
⎠⎞
⎜⎝⎛×⎟
⎠⎞
⎜⎝⎛×⎟
⎠⎞
⎜⎝⎛×⎟
⎠⎞
⎜⎝⎛ −=
21
12
12)
21
11(exp
Life estimated formula of insulation failure
Reference: The Institute of Electrical Engineers of Japan (Report No. 772)
Solder alloy migration
(a) SEM image of solder surfaces
(b) Sn-0.75Cu
(a) Sn-3.5Ag (c) Sn-9Zn
Ag3Sn
Zn
Pb
20μm
CuxSnx
(b) Standard potential of solder composition materials
Factors of Solder alloy migration
(d) Sn-37Pb
E ゚(V vs. SHE)
base Zinc Zn2++2e-=Zn -0.763
Tin Sn2++2e-=Sn -0.138
Lead Pb2++2e-=Pb -0.126
Hydorogen 2H++2e-=H2 0.000
Bismath Bi3++3e-=Bi 0.215
Copper Cu2++2e-=Cu 0.337
Nobl e Silber Ag++e-=Ag 0.779
m aterials Reaction
-1000
-800
-600
-400
-200
0
200
400
Pote
ntia
l / m
V vs
. SC
Electrochemical characteristics (Static characteristic)
Zn
Sn-37Pb
Sn-0.75Cu
Sn-58Bi
Cu
Ag
Sn-3.5AgSn
base (active)
Noble
Pb
Sn-9Zn
Rest potential of each type solder (in 0.1M KO3 aqueous solution)
Sn-8Zn-3BiSn-3Ag-0.5Cu
Electrochemical characteristics (Dynamic characteristic)
Simple metals Solder Alloys
Sn-3.5Ag Sn
Zn
Sn-9Zn
Sn-37Pb
-2000 -1500 -1000 -500 0 -2000 -1500 -1000 -500 0 500
Potential / mV vs. SCE
Cur
rent
den
sity
/m
Acm
-2
Pb
500- 10
0
10
20
30
- 10
0
10
20
30
- 10
0
10
20
30
Current - potential curves = dissolution characteristic (in 0.1M KO3 aqueous solution)
Dissolution is easy
0
5
10
15
20
0 25 50 75 100
0
0.1
0.2
0.3
0.4
0.5
0 100 200 300 400
20 sec 90 sec 123 sec
Water Drop test - 1(5V/DC bias)
250 sec 332 sec 350 secSn-Zn
Time (sec)
Cur
rent
(mA
)Sn-Pb 10 sec 20 sec 30 sec
Change in current
0
0.01
0.02
0.03
0.04
0.05
0 100 200 300 400
Sn-Ag
Process of migration depositsSn-Pb
Sn-Ag
Sn-Zn
cathode(-)
A
B
C
200μm
Water Drop test - 2(SEM analysis)
SEM image of migration deposits
Sn-Zn
Sn-Pb
Sn-Ag cathode(-)
anode(+)
Compositional analysis of migration deposits
A
B
C
A
B
C
solder Point A Point B Point C
Sn = 63% Sn = 46% Sn = 47%
Pb = 37% Pb = 54% Pb = 53%
Sn = 100% Sn = 100% Sn = 100%
Ag = 0% Ag = 0% Ag = 0%
Sn = 98% Sn = 93% Sn = 97%
Zn = 2% Zn = 7% Zn = 3%
Sn-Pb
Sn-Ag
Sn-Zn
1.0E+08
1.0E+09
1.0E+10
1.0E+11
0 250 500 750 1000
1.0E+08
1.0E+09
1.0E+10
1.0E+11
0 250 500 750 1000
Sn-Ag-Cu
Sn-37Pb
Solder migration
Breakdown
Surface insulation resistance test (50V/DC bias, 85℃85%)
No migration
Migration occurringTime (Hour)
Res
ista
nce
(Ω)
Time (Hour)
Res
ista
nce
(Ω)
Change in insulation resistance
Evaluation using mounting PCBs(THB Testing, 5/DC bias,80℃90%)
Terminal of QFP (spacing:0.5mm)
Chip capacitance (spacing:0.8mm)
No migration
No migrationSn-Ag-Cu solder
5V GND
5V GND
END