EMPS - UNIVERSITY OF PORTSMOUTH - 17th February 2010 METALLURGY AND PCB SOLDERING TECHNOLOGY By Luca MOLITERNI & Gianluca PARODI Istituto Italiano Della

EMPS - UNIVERSITY OF PORTSMOUTH -

17th February 2010

METALLURGY AND PCB SOLDERING TECHNOLOGY

By Luca MOLITERNI & Gianluca PARODI

Istituto Italiano Della SaldaturaGENOVA

Difference between tin/leadand “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.

Metallizzazione in Nichel

Base Material

Solder Alloy

Intermetallic

Figure 1: Example of intermetallic measurement by SEM examination

Figure 2: Example of intermetallic measurement by micrographic examination

Base Material

IntermetallicSolder Alloy

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 test was performed with the following results:

Sn/Pb Solder alloy

INTERMETALLIC TYPE [Cu/Sn]

SOLDERING TEMPERATURE [°C]

SODERING TIME [s]

INTERMETALLIC THICKNESS [m]

250 3 6 0,42 0,76

280 3 6 0,69 0,98

350 3 6 1,21 1,49

400 3 6 1,46 1,94

Sn/Ag Solder alloy

INTERMETALLIC TYPE [Cu/Sn]

SOLDERING TEMPERATURE [°C]

SODERING TIME [s]

INTERMETALLIC THICKNESS [m]

250 3 6 0,87 1,14

280 3 6 1,08 1,37

350 3 6 1,73 2,04

400 3 6 2,16 2,72

0

0,25

0,5

0,75

1

1,25

1,5

1,75

2

2,25

2,5

2,75

3

250 280 350 400

SnPb 3s

SnPb 6s

SnAg 3s

SnAg 6s


Sn/Pb Solder alloy

INTERMETALLIC [Cu/Sn]

1 year at 20°C = 0,29

1 year at 60°C =2,04

1 year at 80°C = 4,96

1 year at 100°C = 10,74

Sn/Ag Solder alloy

INTERMETALLIC [Cu/Sn]

1 year at 20°C = 0,41

1 year at 60°C =2,98

1 yesr a 80°C = 6,76

1 year at 100°C = 14,04

0

1

2

34

5

6

7

8

9

10

1112

13

14

15

20 60 80 100

SnPb

SnAg


The test was performed with the following results:

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 thicknessD0= material specific diffusion coefficient

t = soldering time [s]Q = activation energyR = gas constant [8.314 J/mol*K]T = soldering temperature [°K]


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.


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


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)


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


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


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.

Figure 12: Example of an ideal wetting curve

AB

C


We

ttin

g F

orc

e (

mN

)

Time (s)

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


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


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


Batch n°2 – Samples tested with tin lead solder alloy


ENIG Ag Immersion Sn Immersion (not baked)

OSP (not baked)



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


Batch n°1 – Samples tested with lead-free solder alloy


ENIG Ag Immersion Sn Immersion

OSP


Batch n°2 – Samples tested with lead-free solder alloy


ENIG Ag Immersion Sn Immersion (not baked)

OSP (not baked)


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


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.

Documents

EMPS - UNIVERSITY OF PORTSMOUTH - 17th February 2010 METALLURGY AND PCB SOLDERING TECHNOLOGY By Luca MOLITERNI & Gianluca PARODI Istituto Italiano Della