WLCSP器件焊点可靠性

Rate-dependent properties of Sn–Ag–Cu based lead-free solder joints for WLCSP

Y.A.Su a ,L.B.Tan a ,T.Y.Tee b ,V.B.C.Tan a,*

a National University of Singapore,Department of Mechanical Engineering,9Engineering Drive 1,Singapore 117576,Singapore b

Amkor Technology,Inc.,2Science Park Drive,Singapore 118222,Singapore

a r t i c l e i n f o Article history:

Received 22July 2009

Received in revised form 18January 2010Available online 24February 2010

a b s t r a c t

The increasing demand for portable electronics has led to the shrinking in size of electronic components and solder joint dimensions.The industry also made a transition towards the adoption of lead-free solder alloys,commonly based around the Sn–Ag–Cu alloys.As knowledge of the processes and operational reli-ability of these lead-free solder joints (used especially in advanced packages)is limited,it has become a major concern to characterise the mechanical performance of these interconnects amid the greater push for greener electronics by the European Union.

In this study,bulk solder tensile tests were performed to characterise the mechanical properties of SAC 105(Sn–1%wt Ag–0.5%wt Cu)and SAC 405(Sn–4%wt Ag–0.5%wt Cu)at strain rates ranging from 0.0088s À1to 57.0s À1.Solder joint array shear and tensile tests were also conducted on wafer-level chip scale package (WLCSP)specimens of different solder alloy materials under two test rates of 0.5mm/s (2.27s À1)and 5mm/s (22.73s À1).These WLCSP packages have an array of 12Â12solder bumps (300l m in diameter);and double redistribution layers with a Ti/Cu/Ni/Au under-bump metallurgy (UBM)as their silicon-based interface structure.

The bulk solder tensile tests show that Sn–Ag–Cu alloys exhibit higher mechanical strength (yield stress and ultimate tensile strength)with increasing strain rate.A rate-dependent model of yield stress and ultimate tensile strength (UTS)was developed based on the test results.Good mechanical perfor-mance of package pull-tests at high strain rates is often correlated to a higher percentage of bulk solder failures than interface failures in solder joints.The solder joint array tests show that for higher test rates and Ag content,there are less bulk solder failures and more interface failures.Correspondingly,the aver-age solder joint strength,peak load and ductility also decrease under higher test rate and Ag content.The solder joint results relate closely to the higher rate sensitivity of SAC 405in gaining material strength which might prove detrimental to solder joint interfaces that are less rate sensitive.In addition,speci-mens under shear yielded more bulk solder failures,higher average solder joint strength and ductility than specimens under tension.

Ó2010Elsevier Ltd.All rights reserved.

1.Introduction

Electronic components are shrinking in size to meet demands for lightweight and feature filled portable electronic products.This leads to decreasing solder joint dimensions,where mechanical reli-ability has become an issue [1],especially under high strain rate conditions during testing,transport and handling,impact loading under automotive [2]and consumer portable applications.

Tin lead alloy (SnPb)was commonly used as a solder material in microelectronic packaging,but it is also hazardous to the environ-ment and health.Therefore,the industry made a transition to lead-free solders,with the implementation a ban on lead (Pb)from elec-tronic products by the EU RoHS (restriction of the use of certain hazardous substances in electrical and electronic equipment)in

July 2006.The transition to lead-free solders is led by the widely adopted Sn–Ag–Cu (SAC)eutectic [3].However,some studies have shown that standard SAC alloys such as SAC 405(Sn–4%wt Ag–0.5%wt Cu)have poorer mechanical performance than eutectic SnPb under high strain rate conditions [4].Moreover,with the increasing popularity of portable devices,the performance of Sn–Ag–Cu solder joints under high strain rate and large rate ranges typical of drop impact situations is a major concern.

In this study,dogbone-shaped bulk material tensile tests were conducted to investigate the effect of strain rate and silver content on the material properties of Sn–Ag–Cu solders.Solder joint array shear and tensile experiments were conducted on WLCSP speci-mens of different alloy materials under different strain rates and loading orientations to investigate the effects of strain rate,silver content in Sn–Ag–Cu solder joints,and loading orientation on microelectronic packages.Failure analyses were also performed on the fractured dogbone-shaped bulk material test specimens and WLCSP solder joints.

0026-2714/$-see front matter Ó2010Elsevier Ltd.All rights reserved.doi:10.1016/j.microrel.2010.01.043

*Corresponding author.

E-mail address:mpetanbc@.sg (V.B.C.Tan).

Microelectronics Reliability 50(2010)

564–576

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