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−