Solder joint formation and testing on cell busbars

Transcription

1 Solder joint formation and testing on cell busbars Harry Wirth Fraunhofer Institute for Solar Energy Systems ISE 2nd Workshop on Metallization for Crystalline Silicon Solar Cells Constance Thursday, April 15th,

2 Outline 1. Introduction 2. Soldering process 3. Stress induced by solder joint cooling 4. Testing of solder joints 50 μm

3 1. Introduction Module Technology Center MTC Module concept development Process development Material analysis and proving linked with ISE PV-TEC and TLPV details: MTC, interior view

4 2. Soldering process heating to T m C activation of flux melting temp./range [ C] Sn SnAg3.5 SnAg3.5Cu0.9 SnPb37 BiSn42 Eutectic solder melting points

5 Soldering process heating to T m C activation of flux wetting diffusion (morphology, consumption, solder alloy change) Dissolution of metals in liquid SnPb solder Klein-Wassink, 1991

6 Soldering process heating to T m C activation of flux wetting diffusion cooling, solidification phase growth (IM, primary) Sn... Cu Sn... Ag Cu 5 Sn 6,Cu 3 Sn Cu 5 Sn 6,Cu 3 Sn Ag 3 Sn Si Solder joint layers

7 3. Stress induced by solder joint cooling Stress visualisation on unilateral soldered 160 μm cells caused by CTE difference

8 Stress induced by solder joint cooling FE analysis, model objects: interconnector with local contacts to cell materials: silicon, copper 200 C temp. difference for cooling no plastic deformation ribbon wafer Wafer with 2 interconnector

9 Stress induced by solder joint cooling y x Calculated normal stress in cell after cooling (red: tensile, blue: compressive)

10 Stress induced by solder joint cooling Ribbon yield strength (R p0.2 ) total cross section aspect ratio (H/W) design Cell thickness metallisation Cell stress Joint configuration continuous/discontin. location on busbar Solder solidification temp. yield strength (R p0.2 ) layer thickness Thermal treatment temperature profile cooling speed storage time

11 4. Testing of solder joints Precursor materials initial cell condition solder/flux system (wetting angle) ribbon (solder coating, softness) Soldering process semi-automated plattform fully automated tabber-stringer Joints tabbed cell level module level

12 Testing of solder joints Initial cell condition integrity (E-luminescence) initial stress (bending) busbar profile busbar surface chemistry (EDX) Busbar surface profile

13 Testing of solder joints Solder/flux system wetting angle Wetting angle measurement

14 Testing of solder joints Ribbon solder coating thickness R p0.2 as delivered R p0.2 before soldering stress [N/mm2] % 1% 2% 3% 4% 5% 6% 7% 8% strain [%] Stress-strain curve for a ribbon with R p0,2 =145 N/mm 2

15 Testing of solder joints Semi-automated plattform precise contact soldering process contact force controlled single spots highly reproducible conditions Precise soldering plattform

16 Testing of solder joints Semi-automated plattform solder melting point visible in force profile Kraft [N] Zeit [s] Contact force profile for soldering process

17 Testing of solder joints Tabbed cell level E-luminescence X-Ray E-lum, module after accel. aging X-Ray, cell after soldering

18 Testing of solder joints Tabbed cell level E-luminescence X-Ray Peel force Stress-strain curve for a ribbon

19 Testing of solder joints Tabbed cell level E-luminescence X-Ray Peel force Peel fracture analysis Peel fractures, from left: wafer perforation, wafer chipping, 2x paste breakage)

20 Testing of solder joints Tabbed cell level E-luminescence X-Ray Peel force Peel fracture analysis Metallographic preparation Microscopy SnPb solder joint

21 Testing of solder joints Tabbed cell level E-luminescence X-Ray Peel force Peel fracture analysis Metallographic preparation Microscopy SEM/EDX SEM/EDX images of solder joint

22 Testing of solder joints Tabbed cell level E-luminescence X-Ray Peel force Peel fracture analysis Metallographic preparation Microscopy SEM/EDX Accelerated aging peel force [N] 5,0 Sn#1 4,0 Sn#2 3,0 2,0 1,0 0,0 initial HS 15h, 130 C HS 25h, 130 C Peel force reduction after heat storage test

23 Testing of solder joints Module level full IEC test (>3 months, T ) retesting guideline for changed metallization (IEC) IEC full test and retesting program

24 Thank You Very Much for Your Attention! Fraunhofer Institute for Solar Energy Systems ISE Harry Wirth