mohammad mahdi kiaee, aïcha hessler-wyser, … · introduction electromigration sem and edx...
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Introduction
Electromigration
SEM and EDX Analysis of Selected Samples
Conclusion
The demand for low-priced solar cells with high efficiency became more necessary in recent years. New contacting technology has been introduced in order to lower the consumption of costly materials. Reliability of this technology is of great importance for PV technology as contact between smart wires and front silver metallization could be the bottle neck for this technology. The present study investigates the possibility of electromigration-generated failure in these contacts. Electromigration (EM) is a mass-transport phenomenon in a conductor under the influence of electrical field. It becomes a reliability issue when high current density is applied on a small contact. In this project Cu wire coated with InSn or BiSn in contact with low temperature (LTP) or high temperature (HTP) silver paste, that are used in silicon solar cell technology, were investigated under the influence of current stressing.
Phase separation in eutectic two-phase solder joints
Phase coarsening
Nucleation and growth of voids in the interface
Effect of current stressing on the formation of IMCs
Dedicated Sample
EM in lead-free solder
Formulation
𝐹 𝐸𝑀 = 𝐹 𝑤𝑖𝑛𝑑 + 𝐹 𝑑𝑖𝑟𝑒𝑐𝑡
References
In Situ Electromigration in a TEM
1 µm 0.5µm
Reference @ 70˚C
Evolution of Resistance
30 mA @ 70˚C
Anode Cathode
250 mA @ 70˚C Anode Cathode
EDX mapping of selected samples show microstructural changes as a result of current stressing
Test was conducted at 70˚C. After 800 h current was increased from 250 mA to 750 mA on a selected contact to further accelerate the EM test
Deposition of Cu/In/Sn
Reflow process Deposition of
Ag/Cu Preparation of TEM lamella
Current stressing with a special sample
holder
Observation of microstructural evolution with
TEM
1. During electromigration resistance increases in LTP-BiSn 2. A gradual increase is observed in LTP-InSn 3. EDX mapping shows microstructural changes associated with EM i.e.
separation of In-rich and Cu-rich regions in the coating at cathode side and accumulation of material in contact at anode side.
4. At higher currents, thickness of intermetallic layer in Ag paste increases 5. Increase of resistance can be linked to microstructural changes as a result of
changes in thickness and composition of intermetallic layers
Microstructure of the sample did not change at all under high current densities. Sample was incinerated at 12300 A/cm2 before any sign of EM was observed.
Mohammad Mahdi Kiaee, Aïcha Hessler-Wyser, Antonin Faes 1 1 2
Photovoltaics and thin film electronic laboratory (PV-LAB), EPFL, Rue de la Maladière 71b, CH-2002 Neuchatel 1
CSEM, Rue Jaquet-Droz 1, CH-2002 Neuchatel 2
[1]T. Soderstrom, P. Papet, Y. Adda, and J. Ufheil, “Smartwire connection technology.” Meyer Burger AG;b Roth & Rau Research. [2] J. R. Black, “Electromigration #8212;A brief survey and some recent results,” IEEE Transactions on Electron Devices, vol. 16, no. 4, pp. 338–347, Apr. 1969. [3]Y. C. Chanele and D. Yang, “Failure mechanisms of solder interconnects under current stressing in advanced electronic packages,” Progress in Materials Science, vol. 55, no. 5, pp. 428–475, Jul. 2010.
HTP
Cu Cu
Ag
InSn
𝑒−
Cathode Anode
In
Cu Ag
Sn
Reference @ 70˚C
Sn In
Cu Ag
500 mA @ 70˚C
𝑒−
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