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

2. Difference between tin/lead and lead free solder-copper intermetallic growth Relator:Luca MOLITERNI

3. 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. Metallizzazion e 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 Intermetallic Solder Alloy THE DIFFERENCE BETWEEN TIN/LEAD AND LEAD FREE INTERMETALLIC GROWTH

4. 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

5. 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] 250360,420,76 280360,690,98 350361,211,49 400361,461,94 Sn/Ag Solder alloy INTERMETALLIC TYPE [Cu/Sn] SOLDERING TEMPERATURE [°C] SODERING TIME [s] INTERMETALLIC THICKNESS [ m] 250360,871,14 280361,081,37 350361,732,04 400362,162,72 THE DIFFERENCE BETWEEN TIN/LEAD AND LEAD FREE INTERMETALLIC GROWTH

6. 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 THE DIFFERENCE BETWEEN TIN/LEAD AND LEAD FREE INTERMETALLIC GROWTH The test was performed with the following results:

7. 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 Cu 6 Sn 5, 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: D 2 =D 0 t exp(-Q/RT) Where: D 2 = intermetallic thickness D 0 = 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

8. 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 Cu 6 Sn 5 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

9. Solderability characterization for main PCB surface finishes Relator:Gianluca PARODI

10. 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 ConditionFigure 2: No Wetting Conditions SOLDERABILITY CHARACTERIZATION FOR MAIN PCB SURFACE FINISHES

11. 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

12. 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

13. 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

14. 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

15. 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 A B C SOLDERABILITY CHARACTERIZATION FOR MAIN PCB SURFACE FINISHES Wetting Force (mN) Time (s)

16. 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

17. 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

18. Batch n°1 – Samples tested with tin lead solder alloy Sn/Pb Electroplated and ReflowedSn/Pb HASLSn100 HASL ENIGAg ImmersionSn Immersion OSP SOLDERABILITY CHARACTERIZATION FOR MAIN PCB SURFACE FINISHES

19. Batch n°2 – Samples tested with tin lead solder alloy Sn/Pb Electroplated and ReflowedSn/Pb HASLSn100 HASL ENIGAg ImmersionSn Immersion (not baked) OSP (not baked) SOLDERABILITY CHARACTERIZATION FOR MAIN PCB SURFACE FINISHES

20. SOLDERABILITY CHARACTERIZATION FOR MAIN PCB SURFACE FINISHES TIN LEAD SOLDER ALLOY TESTS Surface finish typeMaximum wetting force recorded Batch 1Batch 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 OSP0 mN- 3,10 mN

21. SOLDERABILITY CHARACTERIZATION FOR MAIN PCB SURFACE FINISHES Batch n°1 – Samples tested with lead-free solder alloy Sn/Pb Electroplated and ReflowedSn/Pb HASLSn100 HASL ENIGAg ImmersionSn Immersion OSP

22. SOLDERABILITY CHARACTERIZATION FOR MAIN PCB SURFACE FINISHES Batch n°2 – Samples tested with lead-free solder alloy Sn/Pb Electroplated and ReflowedSn/Pb HASLSn100 HASL ENIGAg ImmersionSn Immersion (not baked) OSP (not baked)

23. SOLDERABILITY CHARACTERIZATION FOR MAIN PCB SURFACE FINISHES LEAD FREE SOLDER ALLOY TESTS Surface finish typeMaximum wetting force recorded Batch 1Batch 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 Immersion0 mN- 3,3 mN OSP- 0,01 mN- 3,5 mN

24. 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. Its 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.