Influence of Solder Reaction Across Solder Joints

Influence of Solder Reaction

Across Solder Joints

Kejun Zeng

FC BGA Packaging Development

Semiconductor Packaging Development

Outline

Introduction

Solder reactions of metals

− Dissolution of metals in molten solders

− IMC formation during reflow

− IMC formation during solid state aging

Solder reactions across joints

– Cu

Sn

appears on Au/Ni(P) pad of substrate

– AuSn

forms throughout solder joint

Influence of metals on reliability across joints

– Au enhances consumption of Ni(V)/Cu UBM

– Ni plating is consumed by Cu

Sn

Introduction

With the electronic devices being continuously scaled down, solder

reaction is becoming one of the major concerns for packaging reliability.

Due to the Pb-free requirement, new surface finishes have appeared in

the market and more are being studied.

Persistence of the Black Pad problem results in the application of

OSP-Cu or bare OSP-Cu.

Local effects of solder reaction on joint reliability has been extensively

studied, but its effects on the other side of the joint is relatively new to the

industry.

Theories for the study of solder reactions:

− Local equilibrium

− Reaction path

Dissolution of metals in molten solders

W

Stable phase diagram

Metastable phase diagram

Sta ble M eta sta ble

Actual concentration of Cu in molten solder at the interface can be higher than what the stable phase diagram indicates, depending on the surface

0.0001 0.001 0.01 0.1 1 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 1000/T(K) S ol ub ili ty (M ol e fr ac tio n) Au Ni Pd Cu 0.0001 0.001 0.01 0.1 1 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 1000/T(K) S ol ub ili ty (M ol e fr ac tio n) Cu Ag Au Ni Pd

Metal solubility in Sn-Pb Metal solubility in Sn-Ag

Thermodynamically calculated equilibrium saturation solubility of metals in molten solders. (1) Ni has the lowest solubility in solders. (2) Solubility of metals in Sn-Ag, SnCu, and

Dissolution rates of various metals in the 60Sn-40Pb solder as a function of T [Bader, 1969]. Note that the latest experimental results showed that the solder reaction of Pd was very fast [Wang & Tu, 1995]. Schematic concentration profile of metal in

moleten solder. Higher solubility greater

gradient higher diffusion rate higher

dissolution rate.

Solder

M

eta

IMC formation during reflow

350°C

Reaction path (arrows exaggerated for readability). After saturation solubility is

reached, Cu₆Sn₅ forms at interface. Because

Cu₆Sn₅ is not in equilibrium with Cu, Cu₃Sn

appears between Cu₆Sn₅ and Cu.

400°C

Following the same procedure, it is

predicted that in high Pb solder joint Cu₃Sn

is the first IMC to form. This is in

agreement with experimental results by Grivas et al., 1986.

220°C

After reflow process, Cu₆Sn₅ is usually

observed in the eutectic solder joint. A thin

layer of Cu₃Sn forms but is not easy to see.

Eutectic SnPb/Cu joint after 1 reflow. SEM image (Texas Instruments).

Cu₆Sn₅

Cu₃Sn

Cu

Eutectic SnPb/Cu/Ni(V)/Al joint after 3 reflows. Voids are marked by red arrows. TEM image. (Courtesy of K. N. Tu, UCLA)

1µm

IMC formation during solid state aging

175°C

During solid state aging, both Cu₆Sn₅ and

Cu₃Sn grow thicker.

SnPb/Cu joint after 40 days at 150°C. Since

Cu is the dominant diffuser in Cu₆Sn₅,

Kirkendall voids form in the Cu₃Sn layer.

SIM image (Texas Instruments).

Cu

Sn

Cu

Sn

Cu

Solder

SnPb/Au/Ni/Cu BGA joint after 500 hours at 160°C. During reflow, Au

plating was totally dissolved away. After aging, Au diffused back to the interface

and form (Au,Ni)Sn₄. SEM image.

(Courtesy of C. R. Kao, National Central University, Taiwan).

Ni can be dissolved into AuSn₄ by replacing

gold atoms. This is because the dissolution of

Ni can lower Gibbs energy of AuSn₄.

Thermodynamic calculation shows that the max. solubility is 10 at.% or 4.9 wt.% and the max. energy change is 3 kJ/mole of atoms.

Cu

₆

Sn

₅

appeared on other side

SnPb flip chip bumps on Ni/Au pad. Cu in the interfacial compounds came from Cu UBM

on the die side. (Cu,Ni)₆Sn₅ composition (at.%): 46.7Cu, 8.2Ni, 45.1Sn. (Texas

Instruments)

This is an indication that Ag and Pd can also reach the other side of a joint after reflow.

Cu

Sn

Ni

Sn

Ni

Cu

Solder

(Cu,Ni)

Sn

Cu

Ni

a) After 3 reflows

b) After 150°C/1000 hour baking

AuSn

₄

formed throughout joint

Eutectic Sn-Pb solder cap on Au at 200°C. a) after 5 seconds, b) after 60 seconds. AuSn₄

compound has extended all the way to the surface of the cap. With a diffusivity of 10-5 _cm2_/s,

Au atoms can diffuse a distance of 100 µm in 5 seconds to the cap top. (Kim & Tu, 1996).

Au enhances consumption of Ni(V)/Cu UBM

50% after 1 reflow; 100% after 3 reflows Sn-3.5Ag-1.0Cu

30% after 10 reflows; 100% after 20 reflows Sn-37Pb

Ni(P)/Au/solder/Cu/Ni(V)/Al

60% after 10 reflows Sn-3.5Ag-1.0Cu

No dissolution after 20 reflows Sn-37Pb

Solder/Cu/Ni(V)/Cu/Al

Fraction of Ni(V) dissolved by molten solder Solder

Au/Ni(P) pad Cu/Ni(V)/Al UBM SnAgCu bump (a) Au/Ni(P) pad SnAgCu bump Cu/Ni(V)/Al UBM (b)

SEM images of cross-section of a eutectic SnAgCu flip chip joint. (a) As-bonded. (b) After 10 reflows, IMC crystals have been spalled into bulk solder. Ni(V) layer has been completely dissolved. The presence of Au has enhanced the dissolution of Ni(V) layer. (Courtesy of M. Li, Institute of Materials Research and Engineering, Singapore)

Cu6Sn5

compound

Cr Si

5 µm 1µm

(a) SnPb on Au/Cu/Cr UBM. Cu₆Sn₅

scallops were decorated with small particles. [Liu&Tu, 1996]

(b) SnPb on Cu/Ti UBM. Surface of

Cu₆Sn₅ scallops were smooth.

[Kim&Tu, 1996] Ni(V) r Cu₆Sn₅ Ni(V) R Cu₆Sn₅

Ni plating is consumed by Cu

₆

Sn

₅

If one side of a joint has Ni plating and the other side has bare Cu or OSP-Cu, the Cu

may decrease the reliability of the Ni plating. Here is a such an example. After 1000

hours of baking at 150°C, Cu

Sn

had dissolved a great amount of Ni from the Ni

plating so that the Ni plating became porous.(Texas Instruments)

Ni

(Cu,Ni)

Sn

Thermodynamically calculated phase diagram showed that the maximum solubility

of Ni in Cu

Sn

is 6Cu:4Ni, corresponding to 21.8 at.% Ni. Experimentally, the Ni

content in the interfacial (Cu,Ni)

Sn

layer in page 16 was measured by EDX to be

21 at.%. The driving force for the dissolution of Ni into Cu

Sn

is the Gibbs energy

Summary

Ranking of dissolution rate of metals corresponds to that of saturation

solubility of the metals, with Au being the fastest and Ni the slowest.

Dissolution rate of metals in the eutectic SnAg solder is higher than in

the eutectic SnPb solder.

Dissolved metals may diffuse across the solder joint during reflow

process, altering the solder reactions on the other side of the joint and

thus influence the joint reliability.

When assessing solder joint reliability, influence from the other side of

the joint should also be considered if the surface coatings are different

on the two sides.

Interfacial reaction is controlled by thermodynamics (Gibbs energy

change) and diffusion kinetics.

References

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Weld. J. Res. Suppl., 1969, 28, 551s-557s.

D. Grivas, D. Frear, L. Quan, and J. Morris,

J. Electr. Mater., 1986, 15, 355-359.

Y. Wang, H. K. Kim, H. K. Liou, and K. N. Tu,

Scr. Metall. Mater., 1995, 32,

2087-2092.

P. G. Kim and K. N. Tu,

J. Appl. Phys., 1996, 80, 3822-3827.

H. K. Kim, K. N. Tu, and P. A. Totta,

Appl. Phys. Lett., 1996, 68, 2204-2206.

A. A. Liu, H. K. Kim, K. N. Tu, and P. A. Totta,

J. Appl. Phys., 1996, 80,

2774-2780.

C. Y. Liu, K. N. Tu, T. T. Sheng, C. H. Tung, D. R. Frear, and P. Elenius,

J. Appl.

Phys., 2000, 87, 750-754.

C. E. Ho, W. T. Chen, and C. R. Kao,

J. Electr. Mater., 2001, 30, 379-385.

K. Zeng and J. K. Kivilahti,

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K. N. Tu and K. Zeng,

Mater. Sci. Eng. Reports, 2001, 34, 1-58.

Acknowledgements

The thoughts on this topic and the approaches taken in the study

were initiated and developed when the author was with

• Lab of Electronics Production Technology, Helsinki University

of Technology, Finland

• Electronic Thin Film Lab, University of California at Los

Angeles, USA

The author would like to thank Prof. J. Kivilahti and Prof. K. N. Tu

for inspiring discussions. Support from Texas Instruments is