Solder joints on a Al and Ni based PVD-metallization for silicon solar cells

Transcription

1 4 th Workshop on Metallization of Crystalline Silicon Solar Cells Constance, Germany Solder joints on a Al and Ni based PVD-metallization for silicon solar cells V.Jung 1, F. Heinemeyer 1, M. Köntges 1, and R. Brendel 1,2 1 Institute for Solar Energy Research Hamelin (ISFH) 2 Leibniz Universität Hannover

2 Metallization of Silicon Solar Cells Screen printed metallization PVD-metallization Cu-connector 200 µm Cu-connector silicon Al-pastes well known as rear side metallization in PV Incorporated busbars made of Ag pastes enable solderability Alternative: PVD-based metallization Advantage: less Al consumption Problem: Al is not solderable Solution: Deposition of 3 metal layers

3 PVD Metallization ATON500 in-line metallization system Deposition of 3 layers without vacuum break 1 st thermal evaporation of contact /conduction layer 2 nd sputter deposition of adhesion / barrier layer 3 rd sputter deposition of antioxidation layer Si Contact / conduction layer Adhesion / barrier layer Antioxidation layer Solder Cu V. Jung, F. Heinemeyer, M. Köntges Energy Procedia 2012;21:84-91 V. Jung, M. Köntges Progress in Photovoltaics 2012 DOI: /pip.2169

4 Ni:Si as Barrier Material Cu Ni:Si slower reaction rate with Sn* than Ni:V Si diffuses together with Ni* : Solder Ni:Si Al Si 10 µm no additional interfaces during aging Today: Demonstrate the long-term solderability of an Al/Ni:Si/Ag metal stack Compare to metallization on passivated and laser contact opened samples *Y. Li, J. Chen, C. Lazik, P. Wang, L. Yang, J. Yu, T. Sun, E. Ko.. J. Mater. Res. 2005;20(10):

5 Solderability of the Al / Ni:Si / Ag stack Si Al Ag Ni:Si

6 Process Flow Metallization 2.5 µm Al / (150 or 200 or 250) nm Ni:Si / 25 nm Ag I. Direct processing II. Accelerated storage* 8 72 C, 85% rel. humidity C III. Storage 0.5 room temperature Hand soldering Solder: Sn-Ag; Sn-Pb-Ag Flux: 952T; 959S 4 Solder/Flux Combinations 180 Peel test according to DIN EN Contact resistance * accelerated storage according to IPC J-STD-003B

7 Peel Forces of 180 Peel Test Force F [N/mm] nm Ni:Si F max after preparation after accelerated storage after storage 200 nm Ni:Si 250 nm Ni:Si 4 F mean 2 0 F min L1 Sn-Ag F1 L2 Sn-Pb-Ag F1 L1 Sn-Ag F2 Sn-Pb-Ag L2 F2 959T 959T 952S 952S Sn-Ag L1 F1 Sn-Pb-Ag L2 F1 Sn-Ag L1 F2 Sn-Pb-Ag L2 F2 959T 959T 952S 952S L1 Sn-Ag F1 Sn-Pb-Ag L2 F1 L1 Sn-Ag F2 Sn-Pb-Ag L2 F2 959T 959T 952S 952S 150 nm thick Ni:Si layer: 3 /12 samples F min < 1 N/mm 200 nm thick Ni:Si layers always pass the test 250 nm thick Ni:Si layer: 2 /12 samples F min < 1 N/mm Sn-Pb-Ag / 952S always smallest difference F max -F min

8 Visual Inspection Top views after peel test Fracture in the solder No connection 150 nm Ni:Si, Sn-Ag, 952S no storage Fracture in the solder Ag, Sn Ag, Al, Ni No connection 1 mm

9 Visual Inspection Top view after peel test Fracture in the silicon, chipping, breakage Only 150 nm thick Ni:Si layers; soldered with the lead-free solder Indicate a pre-existing damage in the silicon soldering process Critical damage, these samples do not pass the test Lead-free soldering needs a higher soldering temperature Sn-Ag soldering process introduces more stress

10 Influence of Solder and Flux on Peel Modus Col 1 vs Col 2 Test passed: > 80% of area after peel test exhibits fracture in the solder Count all samples, which passed the test >80% of area fracture in solder Number of samples nm Ni:Si 200 nm Ni:Si 150 nm Ni:Si Sn-Ag Sn-Pb-Ag Sn-Ag Sn-Pb-Ag 959T 959T 952S 952S 9 of 9 solder joints with Sn-Pb-Ag 952S 8 of 9 solder joints with Sn-Ag and 952S 952S is preferred flux Performs best with Sn-Pb-Ag solder

11 Contact Resistance Contact resistace R C [mohm cm 2 ] 0,25 0,20 0,15 0,10 0,05 0, nm Ni:Si 200 nm Ni:Si 250 nm Ni:Si after preparation after accelerated storage after storage Sn-Ag L1F1 Sn-Pb-Ag L2F1 L1F2 Sn-Ag L2F2 Sn-Pb-Ag 959T 959T 952S 952S Sn-Ag L1F1 Sn-Pb-Ag L2F1 L1F2 Sn-Ag L2F2 Sn-Pb-Ag Sn-Ag L1F1 Sn-Pb-Ag L2F1 L1F2 Sn-Ag Sn-Pb-Ag L2F2 959T 959T 952S 952S 959T 959T 952S 952S Increasing Ni:Si layer thickness variance decreases 200 nm / 250 nm Ni:Si,11 respectively 12 /12 samples R C < 0.1 mohm cm 2 Smallest variance for Sn-Pb-Ag / 952S combination on > 200 nm Ni:Si Source for variance: unconnected areas or small cavities

12 Solderability of the Al / Ni:Si / Ag stack on passivated and laser contact opened samples Si B-diffusion passivation Al Ni:Si Ag

13 Process Flow Passivation Laser contact opening Al 2 O 3 /SiN y or SiN x /SiN y I. Direct processing Metallization 2.5 µm Al / 200 nm Ni:Si / 25 nm Ag II. Accelerated storage* 8 72 C, 85% rel. humidity C III. Storage 0.5 room temperature Passivation: Al 2 O 3 /SiN y : 10 nm Al 2 O nm SiN x (2,05) SiN x /SiN y : 10 nm SiN x (2,4) + 90 nm SiN x (2,05) Laser contact opening : Hand soldering Solder: Sn-Ag, Sn-Pb-Ag Flux: 952S, 959T 4 Solder / Flux combinations Lines Dots d = 2 mm d = 140 µm Peel Test

14 rt n Lagerung Peel Forces of 180 Peel Test VC79 SiN x /SiN x Linien VC79 Al 2 O 3 /SiN x Linien Force F [N/mm] SiN x / SiN y Lines Dots Lines Dots Force F [N/mm] 10 8 Force F [N/mm] nm Ni:Si Al 2 O 3 / SiN y after preparation after accelerated storage after storage Lines 200 nm Ni:Si Dots 0 SnAg SnPbAg SnAg SnPbAg SnAg SnPbAg 959T 959T 952S 952S 952S 952S Sn-Ag-Pb: small differences F max -F min and F min always > 1N/mm 0 L1 F1 L2 F1 L1 F2 L2 F2 L1 F1 L2 F1 L1 F2 L2 0 L1 SnAg F1 L2 SnPbAg F1 L1 SnAg F2 SnPbAg L2 F2 SnAg L1F2 SnPbAg L2F2 959T 959T 952S 952S 952S 952S Lines Dots Sn-Ag: F min > 1 N/mm for direct processing F mean and/or F min often 1 N/mm Test not passed No influence of contact opening geometry

15 Visual Inspection Fractures in the Si Unconnected area Fracture in the solder 1 mm Chipping Fracture in passivation Better: No contact openings under connector

16 Peel Modus VC79 SiNx/SiNx >80% Riss im Lot und kein Riss in Si 4 VC79 Al2O3/SiNx >80% Riss im Lot und kein Riss in Si Test passed: > 80% of area after peel test exhibits fracture in the solder SiN x /SiN y lines dots lines 4 Al 2 O 3 /SiN y lines dots Number of samples Col 3 dots Number of samples Col 3 0 SnAg L1F1 SnPbAg L2F1 SnAg L1F2 SnPbAg L2F2 L1F2 SnAg L2F2 SnPbAg 959T 959T 952S 952S 952S 952S 0 SnAg L1F1 SnPbAg L2F1 L1F2 SnAg L2F2 SnPbAg L1F2 SnAg L2F2SnPbAg 959T 959T 952S 952S 952S 952S Sn-Pb-Ag: 9 of 9 samples pass the test Only one sample passes the test 952S: 9 of 12 samples pass the test No general difference between both contact opening geometries

17 Summary Samples without Passivation Listed: Number of samples which pass the test Peel force Visual inspection Contact resistance 150 nm Ni:Si SnAg 959T SnPbAg 959T SnAg 952S SnPbAg 952S nm Ni:Si SnAg 959T SnPbAg 959T SnAg 952S SnPbAg 952S nm Ni:Si SnAg 959T SnPbAg 959T SnAg 952S SnPbAg 952S 3 3 3

18 Summary Samples with Passivation Listed: Number of samples which pass the test Peel force Visual inspection 200 nm Ni:Si Lines SnAg 959T 0 0 Al 2 O 3 /SiN y SnPbAg 959T 1 1 SnAg 952S 0 0 SnPbAg 952S 1 0 Dots SnAg 952S 1 0 SnPbAg 952S nm Ni:Si Lines SnAg 959T 3 0 SiN x /SiN y SnPbAg 959T 3 3 SnAg 952S 1 1 SnPbAg 952S 3 3 Dots SnAg 952S 2 2 SnPbAg 952S 3 3

19 Results Solderabilty of Al / Ni:Si / Ag layers demonstrated by 180 peel test and contact resistance measurement Samples soldered with 952S and Sn-Pb-Ag perform best when Ni:Si layer is 200 nm or thicker Samples with Al 2 O 3 / SiN y passivation and laser contact openings do not pass the test Samples with SiN x / SiN y passivation and laser contact openings perform comparable to only-metalized samples and perform best with Sn-Pb-Ag solder No contact openings under connector would be beneficial Each passivation system needs an extra analyses

20 Thanks to D. Münster and S. Bräunig for sample preparation and measurements. The German Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU) under Contract No A (VaCoC) for funding. You for your attention!