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Solder joint reliability of different BGAs reworked using low melting point Pb-free alloys; Under testing, uniform joint microstructures and reliable joints were observed.

The initial low melt rework investigation was performed in four sequential phases on RIA2 test vehicles. The first phase was an investigation of joint microstructures resulting after rework using three different low melt alloys. Three commercially available alloys were selected (Table 1). Alloys A and C were alloys containing indium, and Alloy B was a Bi-containing alloy.

In this phase, all BGAs on a limited number of RIA2 boards with OSP finish were reworked in air. Hot gas rework profiles were created and optimized for each solder paste. The criteria for rework parameter optimization were proper shape of solder joint after rework, minimized voiding, uniform microstructure and absence of or reduced low-melt phases in the solder joint.

In the second phase, optimized rework profiles were used. This phase was performed on a limited number of RIA2 boards with an ENIG finish. Only alloys A and B were included for this part of the investigation. Alloy C was dropped off the matrix due to the unknown properties of the high In-content solder joints. (5)

The third phase compared 1X and 2X rework processes in air and nitrogen environments. In this phase, boards with immersion Ag, OSP and ENIG finishes were reworked using alloy A. The influence of rework on adjacent components, PBGA 196 and CSP 46 (Figure 1) was also investigated at this stage.

Two scenarios were analyzed. First, both PBGA 196 and CSP 46 were removed at the same time. Then the CSP 46 component was replaced using the low melt In-containing alloy A. After that, the PBGA 196 was placed using the same low melt alloy. The second scenario was more thermally challenging. The CSP 46 component was removed and a new component was placed using low melt In-containing alloy A. Then the PBGA 196, which was originally assembled with SAC 385 solder paste, was removed using high heat to melt the SAC alloy. Finally, the new PBGA 196 was placed using low melt In-containing alloy A.

In the fourth phase, the influence on thermal fatigue life of each of 1X and 2X reworked components using optimized profiles was studied on limited RIA2 boards using alloy A and alloy B. On all boards, two identical BGAs were assembled. During this phase, one of each component was reworked using low melting alloys and one remained untouched (primary attach). After low melt rework, the boards were tested using accelerated thermal cycling (ATC). Results indicated solder joints reworked using In-containing alloy A performed better or comparable to primary attach joints. Joints reworked with Bi-containing alloy (alloy B) failed before the nonreworked component; therefore, alloy B was excluded from further analysis.

Experiment

Based on results obtained in the initial investigation, In-containing alloy A was chosen for further analysis in Stage 2. In Stage 2, a comprehensive investigation of the thermal fatigue reliability of the low melt reworked solder joints was performed per IPC-9701. (1) BGAs on RIA3 boards were reworked using In-containing alloy A. The reworked solder joint performance was then compared to that of the primary attach joints, as well as to components conventionally reworked using SAC 385 paste.

The RIA3 test vehicle was 8" x 10" with 12 copper layers and available in 0.093" and 0.125" thicknesses. This TV was designed to represent a mid-range complexity product with greater assembly process challenges, and was made from a Pb-free compatible laminate material capable of withstanding the higher temperature requirements of Pb-free processing. Surface finishes used on the RIA3 test vehicles for this investigation were immersion silver (ImAg) and electrolytic nickel immersion gold (ENIG). This test vehicle is daisy-chained and permits four wire in-situ monitoring of the components during ATC (Figure 2, online).

The RIA3 TV covers a range of component technologies (Table 2). In the TV design, two PBGA 196 components were added at the bottom side for the rework cells, which were a mirror image of the two PBGA 196s populated on the topside.

Primary SMT assembly was performed with no-clean SAC 385 paste using a standard 10-zone reflow oven, in air.

One-time hot gas rework was performed on all BGAs on RIA3 TVs with different thicknesses. Site re-dressing was performed following component removal. Low melt solder (In-containing alloy A) was then applied to the re-dressed sites for all components. The rework process was performed under nitrogen for all boards. Figure 3 (online) shows locations of the reworked components. The matrix for this investigation is in Table 3.

Cell 6-1 and 6-2 were designed to investigate the thermal fatigue reliability of solder joints formed on all components reworked using low melt In-containing alloy A on 0.093" boards with ImmAg and ENIG finishes. Cell 6-3 and 6-4 were included to assess similar properties, but on 0.125"-thick boards. These low melt rework cells 6-1 to 6-4 were incorporated in a much larger experimental matrix in Celestica's RIA3 Lead-Free project. This project included conventional rework as well. A comparison between low melting and conventional rework was also performed.

Microstructure of Reworked Solder Joints

As-assembled and reworked solder joints were examined before and after thermal cycling using optical and scanning electron microscopy and x-ray analysis. Differential scanning calorimetry (DSC) analysis was also performed to study the solder joint solidification after mixing of the low melt solder with SAC 385 balls during rework. The DSC results showed, if full mixing was achieved, the low melt liquid was completely consumed and the resulting composition crystallized at a reasonably high temperature range.

In general, in this study, the low melt solder paste was melted at temperatures below the SAC 385 solder ball melting point. The solid solder balls dissolved in the molten solder. In general, the dissolution process depends on the rework parameters of temperature and time, and may cause full or partial mixing. In addition to temperature and time, the ratio between solder ball and solder paste volume is especially important for complete mixing, and causes different microstructures for different components. Snugovsky, et al described the theory of solid solder ball dissolution in molten solder(7)

Metallurgical analysis was performed on all solder joints after rework, and their microstructure was characterized. No time-zero defects or open joints were found using a BGA scope, optical microscopy or SEM. Solder joints were properly formed, collapsed normally and wellshaped (Figure 5). It was also observed that for all reworked joints, the micro-structure was uniform and fully mixed. There was no portion of initial solder ball visible in the cross-sections of reworked solder joints. This finding confirms the component solder balls were fully dissolved in the liquefied solder paste during reflow, and the resulting liquid solidified during cooling.

[FIGURE 5 OMITTED]
Table 1. Melting Points of Low Melt Alloys

Solder Alloy Melting Temperature, [degrees]C

Alloy A 181-187
Alloy B 138
Alloy C 118


Transmissive x-ray was used to inspect the assemblies for defects and to assess the level of voiding. No assembly defects were noted, but all cells showed some voiding. In all cases, voiding was within acceptable levels per IPC-A-610-D. (9)

The DSC heating curves for reworked CBGA and PBGA196 solder joints using In-containing alloy A are illustrated in Figure 6. These curves confirm the metallurgical observations reported above.

As shown in Figure 6a, for reworked CBGAs, the melting of a joint starts at 208[degrees]C, 17[degrees]C higher than the In-containing solder paste melting point. It melts in a relatively narrow temperature range, and stops melting at 212[degrees]C. This range is much below 217[degrees]C, the melting temperature of SAC solder balls. Reworked PBGA 196 solder joints finish melting at an even lower temperature (209[degrees]C), confirming SAC 385 solder balls are completely consumed, Figure 6b. Melting begins at 178[degrees]C.
Table 2. Component Descriptions

Comp. Type I/O Pitch, mm Ball Composition (wt. %)

CBGA 937 1 SnAg3.8%Cu0.5%
PBGA 676 1 SnAg3.8%Cu0.5%
PBGA 256 1.27 SnAg3.8%Cu0.5%
PBGA 196 1 SnAg3.8%Cu0.5%
CSP 64 0.8 SnAg3.8%Cu0.5%

Table 3. Rework Matrix

Cell Thickness Surface Finish Rework ATC Profile Environ.

6-1 0.093" ImAg N2 0-100 [degrees] C
6-2 0.093" ENIG N2 0-100 [degrees] C
6-3 0.125" ImAg N2 0-100 [degrees] C
6-4 0.125" ENIG N2 0-100 [degrees] C



The final composition of CBGA joints after rework using In-containing alloy A is 3.7-3.8 % Ag, 3.8-4.0 % In, and 0.4-1.0 % Cu, depending on the board surface finish. The microstructures of these joints are very similar to those of pure Sn-Ag-Cu alloys. CBGA reworked solder joints have A[g.sub.3]Sn plates, Sn dendrites, and eutectic in inter-dendritic spaces. The number of A[g.sub.3]Sn plates is small, and they are not large in size, (Figure 7a, online). The EDX analysis revealed indium in the A[g.sub.3]Sn particles and the Sn matrix. These data are consistent with the literature on SnAgIn alloy microstructure formation. (8) The ternary liquidus projection phase diagram (9) (Figure 8, online) may be used to interpret the microstructures. It shows that for low In-content (0 to approximately 4 atom %), the compound phase is the A[g.sub.3]Sn type, with some indium substituted for tin. The matrix phase is the tin type.

PBGA 196 joints after rework contain 3.6-3.7 % Ag, 6.0-6.2 % In, and 0.4-1.0 % Cu. Tin dendrites and eutectic are present in the microstructure. Large primary intermetallic particles that have a specific flower shape, shown in Figure 7b, are detected in the PBGA 196 reworked solder joints. This compound contains a significant amount of indium and is more likely the A[g.sub.2.7] (in, Sn) type. It was found (9) that for intermediate In content in alloys (approximately 4 to 6 at %), the compound phase has the approximate composition of A[g.sub.2.7] (In, Sn), where the indium and tin contents are approximately equal. It also may be the A[g.sub.2]In type that forms in alloys with a high In-content (greater than approximately 6 at %). The occurrence and size of the particles depend on the rework parameters, and may be significantly reduced by optimizing the profile.

The reworked CSP46 microstructure is similar to PBGA 196. There are no significant differences between 1X and 2X reworks on ImAg and ENIG finishes. The influence of rework of the adjacent PBGA 196 was negligible.

The reaction intermetallic layer at the board side is [(Cu, Ni).sub.6] S[n.sub.5] with 3-5 % Ni. The intermetallic on the component side is a ternary compound, SnCuNi, with a nickel content in the range of 10-15%. For the OSP and ImAg finished boards, the thickness of the intermetallic layer on the board side is about 5 [mu]m in CBGA joints and 6 [mu]m in PBGA 196 joints, in both cases after 1X and 2X rework. The intermetallic layers on the board side of the ENIG boards is much thinner, about 1.8 [mu]m and 2.5 [mu]m in CBGA and PBGA 196 joints, respectively.

[FIGURE 6 OMITTED]

Both CBGA and PBGA 196 solder joint compositions after rework with In-containing alloy A provided an excellent balance of the desirable properties: strength, plasticity, fatigue life and superb fatigue resistance. The described microstructures are also favorable for high reliability of the reworked solder joints.

There were no differences found between the microstructures of components reworked in air and nitrogen.

Accelerated Thermal Cycling

The ATC results reported below are based on 2500 cycles completed to date. The test is currently ongoing to a target of 6000 cycles.

Weibull plots were generated for CBGA and PBGA 196 components reworked using the low melt In-containing alloy A, as well as for primary attached components and components reworked using conventional rework process (SAC 385 alloy). These are the only two components that have failed up to this cycle count.

CBGA. As can be seen in Figure 9 (online), cell 6-1, the thermal fatigue reliability of the solder joints formed after rework using low melt In-containing alloy A is significantly better than that of the primary attached joints where SAC 385 alloy was used. A similar trend was observed for the other cells. Figure 10 (online) compares the Weibull plots of reworked solder joints formed on thick (0.125") boards with different surface finishes. This illustrates CBGAs reworked using the low melt In-containing alloy performed slightly better on boards with ENIG than on ImAg. Figure 11 shows the performance of low melt reworked CBGAs on boards with different thicknesses. No difference was observed between cells 6-1 and 6-3, which had board thicknesses of 0.093" and 0.125",

respectively.

Weibull plots for all four low melt reworked cells are shown in Figure 12 (online). The thermal fatigue reliability of CBGA components reworked using low melt In-containing alloy is very similar for all cells.

Figure 13 (online) compares thermal fatigue reliability of low melt reworked CBGAs on thick boards (0.125") with ImAg and ENIG finishes with that of the conventionally reworked components on 0.125"-thick boards with OSP finish. The conventional rework was part of the original matrix and was performed on the same test vehicle using the optimized rework process parameters.

The surface finish on the conventionally reworked cell was OSP. These plots, illustrate the performance of low melt reworked components in both cells 6-3 and 6-4 is comparable to that of the conventionally reworked parts.

In general, CBGAs reworked using the In-containing alloy (with a final resultant joint composition of 3.7-3.8 % Ag, 3.8-4.0 % In, 0.4-1.0 % copper, and remainder tin) outperformed non-reworked pure SAC 385 joints.

Figure 14 (online) shows the 1% failures for reworked and primary cells 6-1 and 6-2. The 1% failure for the low melt reworked components is better than that of the primary attach. Reworked solder joints outperform as assembled joints formed on ImAg and ENIG by a factor of 2 and 6, respectively.

Conclusions

The alloy of this low melt rework process contains indium and has a melting temperature close to that of SnPb eutectic. When the low melt solder paste melts, dissolution of the SAC balls occurs. When the rework process parameters such as time and temperature are properly such as time and temperature are properly controlled, the result is full mixing of the solder ball with the solder paste, forming a uniform joint microstructure.

The reduced temperature of the process prevents component overheating, reduces risk of board warpage and pad cratering, and prevents neighboring and mirror-imaged components from thermal damage.

In general, thermal fatigue reliability of low melt reworked components is comparable to or slightly better than that of the as-assembled and conventionally reworked components.

There is not a statistically significant difference between the thermal fatigue reliability of low melt reworked components on different surface finishes and on boards with different thicknesses.

Reworking BGAs using the low melting alloy provides an excellent combination of processability and reliability.

In-containing alloy A, in combination with modified process parameters such as solder paste volume and reflow profile, may be recommended for manufacturing rework and field failure rework. Use of this alloy results in high-reliability solder joints without damage to boards, packages and adjacent components.

Acknowledgments

The authors would like to thank Joel Trudell of Celestica for rework profiling and Russell Brush of Celestica for ATC testing and data analysis.

Ed.: This paper was originally presented at SMTA Toronto 2008 and is published with permission.

Ed.: For the complete article, please see circuitsassembly.com/cms/content/view/7054.

Simin Bagheri, Polina Snugovsky, Zohreh Bagheri, Craig Hamilton and Heather McCormick are with Celestica (celestica.com); sbagheri@celestica.com.
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Title Annotation:Pb-Free Rework
Comment:Solder joint reliability of different BGAs reworked using low melting point Pb-free alloys; Under testing, uniform joint microstructures and reliable joints were observed.(Pb-Free Rework)
Author:Bagheri, Simin; Snugovsky, Polina; Bagheri, Zohreh; Hamilton, Craig; Heather; McCormick
Publication:Circuits Assembly
Date:Sep 1, 2008
Words:2616
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