Microstructure Evolution of SnPb and SnAg/Cu BGA Solder Joints during Thermal Aging

Microstructure Evolution of SnPb and SnAg/Cu BGA Solder Joints during Thermal Aging

George J.S. Chou

Tyco Electronics Corporation M.S. 140-10, P.O. Box 3608, Harrisburg, PA 17105-3608

Phone: 7 17-986-5 108; e-mail: [email protected]

Abstract

The microstructures and performances of eutectic SnPb and near eutectic SnAg/Cu solder joints on the Au/Ni/Cu substrates of ball-grid-array (BGA) socket were evaluated and compared after reflow and thermal aging treatments at 150 and 170°C. The growth rate of intermetallic (IM) layer was characterized and analyzed using SEM and EDS line scan techniques. Ball shear tests were used to determine the failure shear stresses of the joints.

compared to the eutectic SnPb. In solid-state aging, however, a faster growth rate is found for the interfacial IM layer with SnPb solder than fo r the lead-free alloy of which no clear IM growth after aging at 150 and 1 7OoC for 735 hours . In addition, signijicantphase coalescence was found in the aged SnPb solderjoints Spheroidization and coalescence of Ag-rich phases in the eutectic regions were also observed for the aged lead-free SnAg/Cu solder joints. From the ball shear test results, the failure shear stresses decrease as aging time increases for the SnPb and SnAg/Cu joints aged at both 150 and 170°C excepting an initial increase of the failure shear stresses for the SnAg/Cu solder joints aged at 4-6 hours. Also, the decrease of the failure shear stresses is leveled around aging time of 20 hours up to the maximum aging time in the study for SnPb aged at 150°C and SnAg/Cu aged at 150 and 170°C. However, generally, better high temperature performances were observed for the lead-free SnAg/Cu solder joints compared to the eutectic SnPb joints.

It is found that the lead-free SnAg/Cu alloy has a faster IMgrowth rate during reflow,

Keywords: Ball Grid Array, Solder Joint, Microstructural Evolution, Intermetallic Layer, Thermal Aging

INTRODUCTION

and mechanical interconnections between components in electronic devices. During soldering operation, the initial formation of intermetallic (IM) layer at the interface resulting from the reaction of solder with substrate ensures a good metallurgical bonding between the solder and the substrate. The IM layer grows during both soldering and system use [ 1-61, and the overgrown IM layer is known to be detrimental to the joint strength due to its

Solder joints provide both electrical brittle nature. The trend of miniaturization and high I/O counts for electronic devices has lead to a continuous decrease of the dimension of the solder joint, while the heat generated per volume of a packaged device continues to increase [7]. Thus, the evolution of solder joint microstructures under the increasing temperature requirement is decisive for the joint strength. The mechanical properties, such as fatigue and shear strengths and creep resistance, are a crucial issue for the solder joint reliability

0-7803-7434-7/02/$17.00 (c) 2002 IEEE 39 8th International Symposium on Advanced Packaging Materials

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as well as the integrity of electronic packaging under thermal and mechanical loading during handling and system use [8- 123.

Under the expected legislation banning of the use of lead for many applications in industries involving electronic packaging, the development of lead-free solders are underway to replace the extensively used SnPb solders. However, information is still lack on the properties and the applications for the use of lead-free solders compared to the extensive database for the SnPb solders. To effectively use lead-free solders in electronic packaging to replace SnPb, better understanding and comparison are required to evaluate the difference between SnPb and lead-free solder joints. This study investigates the microstructural evolution, IM layer growth, and failure shear stresses of eutectic SnPb and lead-free near eutectic SnAg/Cu solder joints on ball-grid-array (BGA) sockets during thermal aging at 150 and 170°C.

EXPERIMENTAL PROCEDURE

eutectic SnAg/Cu solder balls with a nominal diameter of 0.762 rnm were used to attach on the electric contact paddles in BGA socket. The melting temperature of the near eutectic SnAg/Cu is in the range of 2 16-2 19°C compared to 183°C for the eutectic SnPb. The base material of the paddles is a beryllium copper C 1741 0 over- plated with 1.25 pm nickel as a diffusion barrier and a 0.4 pm thin layer of gold finish to preserve surfaces for soldering. The solder balls by hot air reflow were attached to the paddles in the socket. Four socket samples were then mounted in epoxy, cross- sectioned, and polished to expose solder balls on the sectioned surfaces for evaluations. These metallurgically prepared samples in the as-polished conditions were coated with a thin film of carbon for later

Eutectic SnPb and lead-free near

thermal aging treatments to study the IM layer growth. The reasons for the carbon coating is due to its transparency to scanning electron microscopy (SEM) in backscattered electron image mode and to prevent the exposed solder balls from oxidation during thermal aging.

To conduct the thermal aging treatments, the mounted samples and the other solder-ball attached sockets were placed into two forced convection ovens set at 150 and 170°C with an air ambient. The samples were taken out of the ovens at various times to perform microstructure evaluations and ball shear tests. After the evaluations and tests, the samples were placed back to the ovens to continue the aging treatments.

SEM and energy dispersive x-ray spectroscopy (EDS) techniques were used for the study of microstructural evolution and the measurements of IM layer thickness in the solder joints. The measurements on the thickness of the IM layers formed in the solder joints were carried out using SEM/EDS in a line scan mode at a magnification of 5,000 times. The 1M layer thickness was determined by analyzing the chemistry distribution profiles of Ni, Cu, and Sn as a function of distance perpendicular to the IM layer. To make a comparable result, the same locations in the samples aged at the different times were used for conducting the EDS line scans.

The shear strengths of solder ball joints were measured using a ball shear tester at room temperature. Identical testing parameters were preset and used in all shear tests in the present study. The shearing blade was positioned at the 125 pm above the solder joint interface and moved at a speed of 152 pdsec. The values from at least 20 measurements were averaged for the failure shear stress at each condition.

40 8th International Symposium on Advanced Packaging Materials

RESULTS AND DISCUSSION Figures 1 and 2 show the

microstructures of eutectic SnPb and near eutectic SnAg/Cu solder balls in the as- reflowed conditions. Typical eutectic structures are shown in the SnPb solder. For the near eutectic SnAg/Cu solder, Sn-rich dendrites are surrounded by interdendritic eutectic Sn-Ag regions with some occasional Ag-rich acicular phases.

Figure 1. Optical image of microstructures- of a SnPb solder ball etched with 2% nital

Figure 2. Optical image of microstructures of a near eutectic SnAg/Cu solder ball etched with 2% nital

The cross-sectioned solder joints in the as-polished conditions were also

analyzed by SEM EDS in the bulk solders and along the joint interfaces to determine the chemistry distribution. No apparent gold x-ray peaks were picked up by the EDS along the joint interface and in the solders. The thin layer of gold is dissolved into the molten solder very quickly with a very low content evenly distributed in the solder. The final solder joint is actually being made to the nickel underplate, which has been protected by the thin gold finish during the handling and storage. This means that then nickel has the opportunity to react with the solder during reflow process.

Intermetallic (IM) Layer The SEM images in Figures 3 and 4

depict the IM layers in the as-reflowed solder joints of eutectic SnPb and lead-free near eutectic SnAg/Cu, respectively. A smoother and uniform thickness of IM layer is found for the IM-SnPb interface compared to a rough IM-solder interface with nodular protrusions for the lead-free solder. Apparently, during the reflow, the IM growth rate was faster for the lead-free solder than the eutectic SnPb by their distinct IM interface morphologies.

measurements of IM layer thickness using EDS line scan technique. During the aging, the growth rate of IM layer is very fast for the SnPb joint aged at 17OOC (1 3OC below the melting temperature of eutectic SnPb). Within 20 hours of exposure at 1 70°C, all the nickel underplate was reacted with and consumed by Sn. In comparison, a slower growth rate is found on the IM layer of SnPb joint aged at 150°C. However, amazingly, no clear growth was found for SnAg/Cu aged at both 150 and 17OoC.

Table 1 shows the results of the


Table 1. Measurements of Intermetallic Layer Thickness in pm After Thermal Aging Treatments

*' All nickel underplate is consumed by Sn.

Figure 3 . SEM backscattered electron image of as-reflowed SnPb solder joint

Figure 4. SEM backscattered electron image of as-reflowed SnAg/Cu solder joint

A typical EDS line scan result is shown in Figure 5 for the measurement of IM layer thickness. In the EDS line scan result, it also shows that the IM layer is composed of Sn, Ni, Cu, and Pb. The concentration profiles of these elements are varying as a function of distance perpendicular to the thickness of IM layer. The Ni underplate is acting as a diffusion barrier which limits the diffusion of Cu and retard the reaction with Sn during reflow. Usually, the reaction of Cu-Sn is much faster than the reaction of Ni-Sn [ 131.

The SEM images in Figures 6 and 7 show the microstructures of thermally aged SnPb and SnAg/Cu solder joints, respectively. The IM layer grows to about 2.7 pm thickness from 1 pm for the SnPb solder joint after 160 hours exposure at 150°C. Depletion of Sn at the IM-solder interface results in an increase in the volume fraction of Pb-rich phase close to the IM- solder interface. Significant coalescence of Sn and Pb phases is also found in the SnPb solder by comparison to the as-reflowed SnPb microstructures shown in Figure 3 .


~ x ~~~~~

Frames 553 SnPb 15O-OA Res Fins Length 11 09 pm

1~ t.ayer 1 Thickness I it

BSE. 118741 I I I 1 lfL

/ I I I

NiKr PbHal 366 7- - -. ^C

! ! -1 ". NiKaSnLal. 340

NiKa. 269

Figure 5. A typical SEM EDS line scan used for determining intermetallic layer thickness

Figure 6. SEM backscattered electron image of SnPb solder joint at the same location as that in Figure 3 after thermal aging at 1 50°C for 160 hours

Figure 7. SEM backscattered electron image of SnAg/Cu solder joint at the same location as that in Figure 4 after thermal aging at 17OoC for 20 hours

For both SnAglCu joints aged at 150 and 1 70"C, however, no clear growths were observed for the IM layers as shown in Figure 7. This is consistent with the results in Table 1, although some variations occur in the IM thickness measurements due to the variations of the placement of the line scans perpendicular to the rough and nodular IM- solder interfaces. That means that the diffusion of Sn to form IM is very slow at both 150 and 170°C for SnAg/Cu solder joints. Also, by careful examinations on the solder microstructures, the Ag-rich phases in the interdendritic eutectic Sn-Ag regions became spheroidized and coalesced due to the thermal aging. The spheroidizaton and coalescence occur even at the aging time of 20 hours at 150 and 170°C.

Shear Strength and Fracture Surface Table 2 lists the failure shear stresses

of eutectic SnPb and near eutectic SnAg/Cu solder joints as a function of the aging time. For both SnPb solder joints aged at 150 and 1 7OoC, the failure stress decreases with the increase of aging time. A loss of 33% is for the shear strength of the SnPb joint aged at 170°C for 8 hours. This is consistent with the results of the IM layer thickness measurements of which all the nickel 8th International Symposium on Advanced Packaging Materials 43

Table 2. Failure Shear Strengths in Gram Force of the Solder Joints at Different Thermal Aging Times

Aging Time SnPb SnPb snAg/cu snAg/cu

As-Reflowed 1245 + 147 1288 f 237

underplate is consumed by Sn to form the IM layer within 20 hours aging. As a contrast, the SnPb joint aged at 150°C for 6 hours still maintain 86% of its shear strength of the as-reflowed conditions. The failure shear stress levels out beginning with a aging time of 6 hours up to the maximum aging time of 256 hours in the study. However, currently, there is no data available beyond the aging time of 256 hours. It will be interesting to know if the failure shear stress of SnPb joint aged at 150°C may drop to a much lower value once all the nickel is consumed by Sn to formed the IM layer during a prolonged aging.

SnAg/Cu joints aged at 150 and 170°C increase to a peak value at the aging time of 4-6 hours and then decrease to a steady value after 20 hours aging. This increase might be due to the improvement of the strength from the spheroidization of the Ag- rich phases in the interdendritic eutectic region and/or chemistry homogenization during thermal aging. But, the failure shear

The failure shear stresses of both

stress decreases about 16% after 20 hours aging at both 150 and 170°C and then keeps at this level up to the maximum aging time of 400 hours in the current study.

Figures 8 and 9 show the fracture surfaces of the SnPb and SnAg/Cu solder joints in as-reflowed conditions after ball shear tests. Shear failure is the main failure mechanism for the as-reflowed SnPb solder joints as shown in Figure 8. In contrast, ductile dimple failures of SnAg/Cu solder are contributing to the breakage of the solder joints as shown in Figure 9.

failures at IM layer and/or IM-solder interface shown in Figure 10 contribute a large part for the total breakage of the SnPb solder joints. This is an expected result due to the significant growth of the IM layer and the brittleness associated with the layer. For the lead-free SnAg/Cu solder joints, the failures at IM layer and/or IM-solder interface are also taking a part in the failure

For the thermally aged samples, the


of the solder joints (after the exposure at 150 and 17OoC) as shown in Figure 1 1.

Figure 10. SEM backscattered electron image of a fracture surface of SnPb solder joint thermally aged at 15OOC for 20 hours

Figure 8. SEM secondary electron image of ductile shear failure of as-reflowed SnPb solder joint after ball shear test

Figure 1 1. SEM secondary electron image of a fracture surface of SnAg/Cu solder joint thermally aged at 1 7OoC for 400 hours

Figure 9. SEM secondary electron image of ductile fracture surface of as-reflowed SnAg/Cu solder joint after ball shear test

SUMMARY

investigated and compared the microstructure evolution and shear strengths of thermally aged eutectic SnPb and near eutectic SnAg/Cu solder joints. It is found that the lead-free SnAg/Cu alloy has a faster IM growth rate during reflow, compared to the eutectic SnPb. In solid-state aging, however, a faster growth rate is found for the interfacial IM layer with SnPb solder than for the lead-free alloy of which no clear IM growth after aging at 150 and 170°C for 735 hours . In addition, significant phase coalescence was found in the aged SnPb

In summary, this study has


solder joints. Spheroidization and coalescence of Ag-rich phases in the interdendritic eutectic regions were also observed for the aged lead-free SnAg/Cu solder joints.

For the aged SnPb joints at 150 and I 7OoC, the failure shear stresses decrease as the aging time increases. But, the decrease is leveling out around 6 hours aging time up to 256 hours in the current study for the SnPb joints aged at 15OOC before all of the nickel underplate is consumed. For the SnAg/Cu solder joints aged at both 150 and 17OoC, the failure shear stress increases to a peak value at the aging time of 4-6 hours and decrease to a steady value at the aging time of 20 hours. This steady value is retained through the maximum aging time of 400 hours for SnAg/Cu at 150 and 17OOC. When comparing both SnPb and SnAgICu solder joints, better high temperature performances were observed for the lead- free SnAg/Cu.

ACKNOWLEDGEMENT

Robert Hilty for helphl comments and review of the manuscript.

The author would like to thank Dr.

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Microstructure Evolution of SnPb and SnAg/Cu BGA Solder Joints during Thermal Aging