Spansion 2008 paper on asap 1

2008 ICSE Proc. , Johor, Malaysia

Improvement of Backside Emission Analysis Sample Prep Process For BGA Khairizam Abd Rani and Rameshwaren Arumugam Spansion (Penang) Sdn Bhd, Phase 2 Bayan Lepas FIZ, 11900 Penang. Malaysia Email: khairizam.abd-rani@spansion.com

Abstract Current method of BGA sample preparation repackaging process for backside emission analysis is by applying direct bonding from the exposed gold wire on the bottom of the package to the lead frame of the secondary open top package. The exposed gold wire on the package bottom is achieved through the back grinding process. The very small diameter of the exposed gold wire on the backside of the BGA which is about 1.0mil in diameter has imposed difficulty during the manual re-bonding process. The re-bonding process requires a skillful person as it requires high precision bonding. A single wire bonding itself may require several attempts in order to form a good bond/ joint on the exposed gold wire. With this scenario we will also be facing more challenges in the future as we are moving towards qualifying thinner wire for production.

The work presented here shows the alternative BGA repackaging technique for the Backside Emission Analysis. The paper will discuss the advantages of the new methodology over the current technique.

After the implementation of the new sample preparation repackaging technique, the overall sample preparation process have been improved. The cycle time, success rate and efficiency has been improved especially on the wire re-bonding process. This is due to the larger bonding area has been created by exposing lead frames at the backside of BGA package that is achieved through the selected area grinding method.

I. INTRODUCTION Backside emission analysis is the fault localization technique in detecting leakage in IC from the backside of the die.

This technique has been established and developed since the frontside tools becomes inefficient in some cases which due to the rapid development of multilevel metallization, flip chip, multi chip packaging technology and active circuits under the bond pads.

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Sample preparation for backside emission analysis are much easier on leaded type of packages (TSOP, PQFP etc) as compared to BGA packages. For leaded package, access to the backside surface can be made without further damaging the package electrical connectivity. In contrast, sample preparation process for BGA packages are much more complicated as any access to the die backside will damage the original package electrical connectivity and this will require further repackaging process.

In general, the sample preparation for BGA package involved two processes: (i) Grinding and polishing (ii) Repackaging The overall process flow of the sample preparation is shown in Fig 1.

Remove solder ball using Piranha Etch

Grinding from package bottom until die backside.

Continue grinding until die thickness ~ 80um – 120um

Polish die backside surface using polishing machine

Attach to open top package

Rewire from expose gold wire at BGA package bottom to open top package lead frame.

Fig. 1 Process flow of BGA sample preparation for backside emission analysis.

1-4244-2561-7/08/$20.00 Š2008 IEEE

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2008 ICSE Proc. , Johor, Malaysia

Although we have established a working technique, however we are faced with the difficulty at achieving precise bonding during wire re-bonding process. This is due to the very small diameter of the exposed gold wire on the backside of the BGA which is about 1.0mil. With this crucial challenge in mind, the current sample prep technique has been reviewed and improved to achieve better efficiency.

This paper describes the procedure of the improved BGA sample preparation technique for the backside emission analysis. A completed process flow and detailed methodology will be presented to display the successful deployment of the improved technique.

II. COMPARISON OF TECHNIQUE A. Current Process The major difference between current and improved technique is on the sample preparation for the wire re-bonding process. As shown in Figure 2, normal parallel grinding process expose a very small diameter of gold wire at the package bottom side. Wire re-bonding process on this exposed gold wire will take several attempts to form good bond/ joint due to very small bonding surface area (wire diameter is approximately 1.0mil). As a result, it will increase the process cycle time particularly in the wire re-bonding process. The overall cycle time of the sample preparation using current technique is approximately 4 hours. This process requires highly skilled operator as it requires high precision bonding.

Exposed gold

Exposed bottom die surface

B. Improved Process Unlike the current technique, sample preparation using the improved technique will create larger bonding surface area. This is achieved through the selected area grinding and polishing process. Instead of gold wire, 2nd bond copper trace metallization will be exposed at the package bottom through the selected area grinding process as shown in Figure 3. The exposed copper trace width is approximately 5.0mil which is five times larger than the diameter of exposed gold wire achieved through normal technique. By having a larger surface area to work on will reduce difficulty in wire rebonding process.

With this new method implemented, we could reduce the overall process cycle time. The overall cycle time of the sample preparation process using improved method is approximately 2 hours (50% faster than normal method).

Exposed 2nd bond copper trace Exposed bottom die surface

Die

Mold compound

Fig. 3 Cross section view of BGA at wire bond area upon selected area grinding and polishing processes.

III. TECHNIQUE DESCRIPTION Die

Mold compound

Fig. 2 Cross section view of BGA at wire bond area after grinding and polishing process.

Backside emission analysis sample preparation process flow for the improved method is shown in Figure 4. Before the grinding process is performed, the lead or solder ball of the package are removed using heated piranha etch solution.

This step will provide better contact interface between the package and the sample fixture.

Once the solder balls have been removed, selected area grinding process is done using the Ultratech ASAP-1 system. The system utilizing a 601

Authorized licensed use limited to: James Colvin. Downloaded on August 09,2010 at 19:13:11 UTC from IEEE Xplore. Restrictions apply.

2008 ICSE Proc. , Johor, Malaysia

Remove solder ball using Piranha Etch

Milling from package bottom into 2nd bond copper trace

Continue milling process by selecting die area only. Mill until die thickness ~ 80um to 120um.

rotating tool, controlled in the Z-direction. An XY stage hold the sample and located underneath the rotating chuck. This stage is motorized and runs independently in X and Y directions at userset amplitudes, to produce the desired dimensional ‘window’. Tool type is crucial to the production of a specific surface finish. Milling tool is used for the grinding process. The grinding process will involve two stages. At the first stage, the whole area surface at the package bottom is grinded until exposing the 2nd bond copper trace metallization. This process is shown in Figure 6.

Milling machine rotating tool

Polish die backside surface using Ultratech polishing tool

Attach to open top package

Rewire from exposed 2nd lead copper trace of BGA to open top package lead frame

Fig. 4 Process flow of improved method for backside emission analysis sample preparation.

Die

Exposed 2nd bond copper trace

Die

Fig. 6 First stage of the selected area grinding process.

Fig. 5 The Ultractec ASAP-1 system used for selected area grinding/ polishing process.

Next the polishing process is performed on the exposed 2nd bond copper trace to obtain smooth surface. This is done by changing the milling tool with a polishing tool. A smooth copper trace surface is important to achieve a good bond ball joint formation during wire bond process.

Figure 7 shows the second stage of the grinding process. At this stage, the X-Y dimension of the grinding machine is adjusted to meet the size of the die. With this, only the selected area will be thinned, so preserving wiring external to the die.

The grinding process is continued until the die backside surface is exposed. The die is then thinned and polished to a thickness range of 80um – 120um. Thinning and polishing of the die allows Near Infrared (NIR) light to penetrate the backside so that imaging of the circuitry can be done.

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2008 ICSE Proc. , Johor, Malaysia

process. This is due to larger bonding area created by exposing the 2nd bond copper trace metallization at the backside of BGA package.

The new process provides better efficiency giving consistent result and able to reduce the overall process cycle time.

ACKNOWLEDGEMENT Exposed die backside surface

The authors wish to thank Spansion Penang DA staffs TK Lim, Zainal Abidin and GC Gnoi for their enormous support and technical assistant during the project. A special thanks also to Susan Li of Global Device Analysis and Spansion Penang management (CH Lian and Rodzaki Saad) for their encouragement and valuable insight in publishing this paper.

Fig. 7 Second stage of the selected area grinding process.

Final step is the repackaging process. Sample is attached onto the secondary open top package using typical die attach paste. Subsequent wire re-bonding is then performed using manual wire bonder system. Wire re-bonding is performed between the exposed 2nd bond copper trace on the sample backside to the lead frame of the secondary open top package. The complete process is shown in Figure 8.

Secondary open top package

REFERENCES [1] Salim L. Khoury et al., “A Comparison of Copper and Gold Wire Bonding on Integrated Circuit Devices”, pp. 673-681, IEEE Trans. Comp., Hybrids, Manuf. Tech., Dec 1990.

[2] J.B. Colvin, “BGA and Advanced Package Wire to Wire Bonding for Backside Emission Microscopy”, 25th IEEE Proceedings of 25th ISTFA 1999.[ [3] G. G. Harman, Reliability and Yield Problems of Wire Bonding in Microelectronics. Reston, VA: ISHM, 1989.

Wire bonding

Secondary open top package lead frame

Fig. 8 Complete sample preparation process.

IV. CONCLUSION This method was implemented to further improve the already existing BGA sample preparation process for backside emission analysis.

The improved method is able to reduce the difficulty in performing the wire re-bonding 603 Authorized licensed use limited to: James Colvin. Downloaded on August 09,2010 at 19:13:11 UTC from IEEE Xplore. Restrictions apply.