EMPS
- UNIVERSITY OF PORTSMOUTH 17th February 2010
METALLURGY AND PCB
SOLDERING TECHNOLOGY
By Luca MOLITERNI & Gianluca PARODI
Istituto Italiano Della Saldatura
GENOVA
Difference between tin/lead
and “lead free” solder-copper intermetallic
growth
Relator:
Luca MOLITERNI
The purpose of this research was to study the difference in growth between tin/lead and
“lead-free” solder-copper intermetallics at different soldering processes temperatures.
Intermetallics are compound forming onto the interface between PBS or component
leads base material and solder alloys during the wetting time.
Solder Alloy
Intermetallic
Solder Alloy
Intermetallic
Base Material
Base Material
Metallizzazion
e
in Nichel
Figure 1: Example of intermetallic measurement
by SEM examination
Figure 2: Example of intermetallic measurement
by micrographic examination
THE DIFFERENCE BETWEEN TIN/LEAD AND LEAD FREE
INTERMETALLIC GROWTH
For this research the following equipment and materials have been used:
- Printed Circuit Board with pads 0,5 x 0,5 mm, surfaced with copper 35 m thick and
covered with an immersion tin surface finish (initial intermetalics thickness in the
order of Armstrong to avoid any interferences with final intermetallics thickness
measurements).
- Eutectic solder alloy with composition 63% Sn; 37% Pb.
- Eutectic “lead free” solder alloy with composition 96%Sn; 4% Ag (ESA approved
solder alloy for generic purposes).
- Cleanable soldering flux (pure Rosin type).
- Ethanol for solder joints cleaning.
- 80 watt soldering station with 0,5 mm tip.
- Oven
- Micrographic Microscope
- SEM (Scanning Electronic Microscope)
THE DIFFERENCE BETWEEN TIN/LEAD AND LEAD FREE
INTERMETALLIC GROWTH
The test was performed with the following results:
Sn/Pb Solder alloy
Sn/Ag Solder alloy
INTERMETALLIC TYPE [Cu/Sn]
SOLDERING
TEMPERATURE [°C]
INTERMETALLIC TYPE [Cu/Sn]
SODERING TIME
[s]
INTERMETALLIC
THICKNESS [m]
SOLDERING
TEMPERATURE [°C]
SODERING
TIME [s]
INTERMETALLIC
THICKNESS [m]
250
3
6
0,42
0,76
250
3
6
0,87
1,14
280
3
6
0,69
0,98
280
3
6
1,08
1,37
350
3
6
1,21
1,49
350
3
6
1,73
2,04
400
3
6
1,46
1,94
400
3
6
2,16
2,72
3
2,75
2,5
2,25
2
SnPb 3s
1,75
1,5
SnPb 6s
SnAg 3s
1,25
SnAg 6s
1
0,75
0,5
0,25
0
250
280
350
400
THE DIFFERENCE BETWEEN TIN/LEAD AND LEAD FREE
INTERMETALLIC GROWTH
The test was performed with the following results:
Sn/Pb Solder alloy
Sn/Ag Solder alloy
INTERMETALLIC [Cu/Sn]
INTERMETALLIC [Cu/Sn]
1 year at 20°C = 0,29
1 year at 20°C = 0,41
1 year at 60°C =2,04
1 year at 60°C =2,98
1 year at 80°C = 4,96
1 yesr a 80°C = 6,76
1 year at 100°C = 10,74
1 year at 100°C = 14,04
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
SnPb
SnAg
20
60
80
100
THE DIFFERENCE BETWEEN TIN/LEAD AND LEAD FREE
INTERMETALLIC GROWTH
Conclusions
1) As expected, Intermetallic grow rate is greater with the use of lead-free alloys than it
is with other.
This greater rate helps the quicker formation of bi-phasic intermetallic Cu6Sn5,
which, being very delicate, reduces the joint life (500 Hv)
2) It is also evident the influence of temperature in the process of intermetallic growth
depending on time as defined by the following empiric formula:
D2=D0t exp(-Q/RT)
Where:
D2= intermetallic thickness
D0= material specific diffusion coefficient
t = soldering time [s]
Q = activation energy
R = gas constant [8.314 J/mol*K]
T = soldering temperature [°K]
THE DIFFERENCE BETWEEN TIN/LEAD AND LEAD FREE
INTERMETALLIC GROWTH
Conclusions
3) While doing a material control on old military electronical equipment, we discovered
failures occurred for cracking in Tin-Copper intermetallic with an average thickness
of 20/25 µm. All the cracks were detected in Cu6Sn5 intermetallic.
4) Considering the collected data presented before, it is clear lead free alloys have
reliability issues. Even in the automotive sector, the main brands obtained a 2012
delay to the ROHS promoted by EU and are organizing a revision to avoid using
lead free alloys for the issues mentioned above. Seeing the problems, it is hoped
ESA will never use lead free, to always uphold the high standards requested by
space industry.
THE DIFFERENCE BETWEEN TIN/LEAD AND LEAD FREE
INTERMETALLIC GROWTH
Solderability characterization
for main PCB surface finishes
Relator:
Gianluca PARODI
Good solder wetting is essential for space and non-space quality PCB assemblies!
The purpose of this research was to evaluate wettability for different kinds of PCBs
surface finishes in different storage conditions.
These tests were considered necessary by IIS to analyze the trend followed by some
electronic assembly industries to move from tin-lead to lead-free soldering process.
Figure 1: Wetting Condition
Figure 2: No Wetting Conditions
SOLDERABILITY CHARACTERIZATION FOR MAIN PCB
SURFACE FINISHES
Tests were executed employing suitable samples, constituted by PCBs copper pads
coated with seven different kinds of surface finishes.
The surface finishes adopted were the following:
- Sn/Pb electroplated and reflowed
- Sn/Pb hot air solder levelled (H.A.S.L.)
- Sn100 (99,3 Sn - 0,7 Cu - 0,05 Ni + Ge) hot air solder levelled (H.A.S.L.)
- Electroless Nickel Immersion Gold deposition (ENIG)
- Ag Immersion deposition
- Sn Immersion deposition
- Organic Solderability Preservative (O.S.P.)
Figure 3: Preparation of the samples
Figure 4: ENIG sample after wetting test
SOLDERABILITY CHARACTERIZATION FOR MAIN PCB
SURFACE FINISHES
Each sample was previously fluxed with a tested ROL 0 type flux.
Figure 7, 8: Copper mirror test execution
Figure 9, 11: Evaluation of flux activity level before and after cleaning operation (Figure 10)
SOLDERABILITY CHARACTERIZATION FOR MAIN PCB
SURFACE FINISHES
The samples investigated were obtained by arranging two different batches, which
differed for their surface conditioning prior to the wettability test.
The two surface conditionings were the following:
Batch 1 - Fresh finishes, tested within two weeks from their manufacturing,
without baking.
Batch 2 - Aged finishes, tested after a storage of 6 months, with baking.
Ageing conditions: the PCBs have been packaged with heat shrinkable film
and stored in a room at 20°C - 25°C and 50% - 55% R.U.
Baking conditions :
- Sn/Pb electroplated and reflowed, HASL Sn/Pb
and HASL Sn100  4 hours at 120°C
- ENIG  4 hours at 105°C
- Ag Immersion  2 hours at 120°C
- Sn Immersion and OSP  not baked because their
solderability is irremediably compromised when baked
SOLDERABILITY CHARACTERIZATION FOR MAIN PCB
SURFACE FINISHES
The tests were performed accordingly to IPC/EIA J-STD-003B Standard, by dipping a
sample of the surface finish under investigation in a melting pot of tin-lead (63% tin) or
lead-free (SAC 305) solder alloy employing Metronelec MENISCO ST 60
Figure 5: Detail of the test apparatus
Figure 6: Immersion of a sample in the melting pot,
during a test
SOLDERABILITY CHARACTERIZATION FOR MAIN PCB
SURFACE FINISHES
The wetting balance test permit us to obtain a graphic where in abscissae axis is shown
the immersion time of the sample in the melting pot, while in the ordinate is represented
the related wetting force.
Wetting Force (mN)
C
B
A
Time (s)
Figure 12: Example of an ideal wetting curve
SOLDERABILITY CHARACTERIZATION FOR MAIN PCB
SURFACE FINISHES
The basic equation that rules wetting balance test is the following:
F Measured = F Wetting - F Buoyancy
At the beginning the force value is negative because the main action is applied by the
Archimedes buoyancy force: this force is the first one to intervene and is opposed to the
wetting action  See Figure 12, Point A
Later on, the wetting force begin to intervene as well, allowing the sample immersion
and contrasting the Archimedes action: typically the profile of the graphic changes
direction and starts to rise  See Figure 12, Point B
During the dipping the wetting force increases reaching the zero axis, then it grows
steady on a positive value till the end of the test (coupon emersion from the melting pot)
 See Figure 12, Point C
SOLDERABILITY CHARACTERIZATION FOR MAIN PCB
SURFACE FINISHES
Depending on surface conditions of the materials to be tested, appearance of curves
may differ from the ideal one, and resemble the ones represented hereafter.
Figure 13: Summarising different wetting behaviour
SOLDERABILITY CHARACTERIZATION FOR MAIN PCB
SURFACE FINISHES
Batch n°1 – Samples tested with tin lead solder alloy
Sn/Pb Electroplated and Reflowed
Sn/Pb HASL
Sn100 HASL
ENIG
Ag Immersion
Sn Immersion
OSP
SOLDERABILITY CHARACTERIZATION FOR MAIN PCB
SURFACE FINISHES
Batch n°2 – Samples tested with tin lead solder alloy
Sn/Pb Electroplated and Reflowed
Sn/Pb HASL
Sn100 HASL
ENIG
Ag Immersion
Sn Immersion (not baked)
OSP (not baked)
SOLDERABILITY CHARACTERIZATION FOR MAIN PCB
SURFACE FINISHES
TIN LEAD SOLDER ALLOY TESTS
Surface finish type
Maximum wetting force recorded
Batch 1
Batch 2
Sn/Pb Electroplated and Reflowed
+ 1,08 mN
+ 0,91 mN
Sn/Pb HASL
+ 0,81 mN
+ 0,73 mN
Sn100 HASL
+ 0,76 mN
+ 0,60 mN
ENIG
+ 0,51 mN
+ 0,48 mN
Ag Immersion
+ 0,10 mN
- 1,30 mN
Sn Immersion
+ 0,05 mN
- 2,40 mN
OSP
0 mN
- 3,10 mN
SOLDERABILITY CHARACTERIZATION FOR MAIN PCB
SURFACE FINISHES
Batch n°1 – Samples tested with lead-free solder alloy
Sn/Pb Electroplated and Reflowed
Sn/Pb HASL
Sn100 HASL
ENIG
Ag Immersion
Sn Immersion
OSP
SOLDERABILITY CHARACTERIZATION FOR MAIN PCB
SURFACE FINISHES
Batch n°2 – Samples tested with lead-free solder alloy
Sn/Pb Electroplated and Reflowed
Sn/Pb HASL
Sn100 HASL
ENIG
Ag Immersion
Sn Immersion (not baked)
OSP (not baked)
SOLDERABILITY CHARACTERIZATION FOR MAIN PCB
SURFACE FINISHES
LEAD FREE SOLDER ALLOY TESTS
Surface finish type
Maximum wetting force recorded
Batch 1
Batch 2
Sn/Pb Electroplated and Reflowed
+ 0,48 mN
+ 0,32 mN
Sn/Pb HASL
+ 0,35 mN
+ 0,20 mN
Sn100 HASL
+ 0,30 mN
+ 0,12 mN
ENIG
+ 0,15 mN
- 0,40 mN
Ag Immersion
+ 0,08 mN
- 1,0 mN
Sn Immersion
0 mN
- 3,3 mN
OSP
- 0,01 mN
- 3,5 mN
SOLDERABILITY CHARACTERIZATION FOR MAIN PCB
SURFACE FINISHES
Conclusions
1) While trying to define a quality classification of the surface finishes from the
preceding tables, it appears the wettability of Sn/Pb surfused & reflowed and
HASL surface finishes (tin lead and lead free) is better than chemical surface
finishes as ENIG, Sn Immersion, Ag Immersion or organic as OSP.
Data show furthermore how this gap (related to wetting force and wetting
speed) rises in Batch 2, after ageing and baking process.
2) ENIG and Ag Immersion deposits give, during six months of storage, a good
opposition to copper intermetallic diffusion so, after ageing and baking
process, they maintain a sufficient wetting level.
In particular IIS experience is that ENIG offers more resistance to copper
diffusion and consequently higher wetting level than Ag Immersion.
3) The wetting balance tests conducted using a lead free alloy confirm the surface
finishes hierarchy established by tin lead alloy tests.
It’s interesting to observe how each lead free test, if confronted with the tin lead
test corresponding in Batch 1 and Batch 2, typically shows a lower wetting
speed and force recorded.
SOLDERABILITY CHARACTERIZATION FOR MAIN PCB
SURFACE FINISHES