ASM Desk Reference - Susan Li -- Chip Scale Packges by ULTRA TEC

Microelectronics Failure Analysis Desk Reference, Sixth Edition R.J. Ross, editor

Chip-Scale Packages and Their Failure Analysis Challenges Susan Xia Li Spansion, Inc, Sunnyvale, California, USA

Abstract 8mm

Chip Scale Package (CSP) is ideal for the applications of Cellular and Portable devices that require better use of real estate on the PC boards. It has advantages of low package profile, easy routing and superior reliability. However, due to their small form factor, it is difficult to handle this type of package for both package level and die level failure analysis. In this paper, a brief overview of definitions for CSPs and their applications are included. The challenges for performing failure analysis on CSPs, particularly for Multi-Chip Packages (MCP), at package level and die level are discussed. In order to successfully perform device electrical testing and failure diagnostic on CSPs, special requirements have to be addressed on precision decapsulation for FBGA packages, and additional attention has to be paid to top die removal for MCPs. Two case studies are presented at the end of this article to demonstrate the procedures for performing failure analysis on this type of device.

14mm

(a)

(b)

Figure 1: A typical FBGA package shows the die size similar to the package size (a) and the dimension of the package is small compared to conventional BGA packages (b) To further increase Silicon density within small CSPs, the so-called Stacked Chip-Scale Packages (SCSPs) or Multi-Chip Packages (MCPs) have emerged into the market and are becoming one of the most rapidly growing sectors for CSPs. Packaging for MCPs begins by stacking two or three dice on top of a BGA substrate with an insulating strip between them. Leads are bonded to the substrate, and molding around the stack completes the device. Since a MCP is a package that may even be smaller than the enclosed die area (counting area from the stacked dice), our definition of a CSP no longer holds. The MCP example in Figure 2 shows paired Flash memory and SRAM.

Introduction There is a trend in the electronic industry to miniaturize. From tower PCs to laptops to Pocket PCs, from giant cell phones to pager size handsets, the demand for smaller featurerich electronic devices will continue for many years. In response, the demand for Chip-Scale Packages (CSPs) has grown tremendously. Their main advantage is the small form factor that provides a better use of real estate on the PC board in many applications such as cell phones, home entertainment equipment, automotive engine controllers and networking equipment. All have adopted CSPs into their systems.

Another MCP example is memory to support logic. Packageon-Package (PoP) stacking is often used to make the assembly process high yield, low cost and more flexible for the integration of memory to logic devices. Figure 3 shows an example of PoP stacking. These stacked packages are so thin (1.4mm) that wafers must be thinned to 150-200um prior to wafer saw for die separation and placement in order to fit into the MCPs.

A Chip-Scale Package is, by definition, a package about 1 to 1.2 x the perimeter (1.5 x the area) of the die (Figure 1). Within this definition, CSPs have many variations. There are more than 20 different types of CSPs in the market today, but all of them can be grouped into 4 main categories based on their technologies and features (Table 1).

Table 1: Four Main Categories for Chip Scale Packages

TECHNOLOGY

REMARKS

Flexible Substrate

It uses a flex circuit interconnect as a substrate by which the chip is connect to the circuit board. There are two ways of interconnecting the chip to the substrate: flip chip bonding and TAB style lead bonding.

Micro BGA

(a) As for wire bond type, the chip is protected using conventional molding technology

DIE 1

FBGA-PI Rigid Substrate Organic Or Ceramic

FBGA-BT

JACS-Pak

DIE 2 This type of CSPs has a ceramic or an organic laminated rigid substrate. Either conventional wire bonding or flip chip can be used to connect the die to the substrate.

BT Resin Substrate

(b) Figure 2: A two die stacked MCP package with SRAM on top of Flash configuration: (a) top view of the MCP package (b) cross-sectional view of the MCP package

The wire bond version can be considered as a downsized version of the standard PBGA even though assembly techniques could be different.

Lead Frame Type The most common CSPs of this type are the LOCs (Lead on Chip) where the lead frame extends over the top of the chip. LOC/SON Wafer Scale

Ultra CSP

This type of CSPs has the chips that are processed in wafer form before singulation. A redistribution layer is created in the wafer scribe to pin out the bond pads to a standard ball grid array footprint.

Figure3: PoP stacking provides high yield, low cost and more flexible assembly process for the integration of memory to logic devices

Failure Analysis Challenges The many advantages that CSPs bring to the IC applications also bring new challenges for device testing and failure analysis at both the package and die levels. Since they are small, device handling for CSP operations is difficult. The following discussion mainly focuses on one type of CSPs, the

FBGA (Fine-Pitch Ball Grid Array) package, which illustrates the issues that we have encountered during failure analysis on CSPs.

Cu Trace Cra

A typical wire-bonded CSP, such as an FBGA package with a BT (Bismaleimide Triazine) resin substrate, uses a standard die attach material and is gold wire bonded to external I/O lead frames. The entire package has a straight edge, since the dicing process makes it so. Its package height is normally less than 1.2mm, with 0.8mm ball pitch, and 0.3mm solder ball size, which maintains a nominal stand-off of 0.25mm (Figure 4).

(b)

(c) Figure 5: Typical package level failures on FBGA packages: (a) Cu trace cracks in the package (b) Through-hold via cracks after temp cycling (c) Solder ball cracks at the interface of the FBGA package to the system PCB board.

Figure 4: Single Die FBGA Package Build-up Structures Package Related Failures Like any other BGA packages, the typical failures at the package level for FBGA include open and short circuits after reliability stress testing. Most of common open circuit failures are related to package and/or die cracks, Cu trace cracks or through-hole via cracks (Figures 5a and 5b). The failures may also involve wire bond cracks and solder ball cracks (Figure 5c). Most common short circuit failures involve Cu trace shorts due to contamination, wire bond touching silicon die edge, or adjacent wire bonds touching each other. Traditional failure analysis techniques still apply to FBGA packages except more attention is required due to their smaller feature sizes. Non-destructive analysis, such as X-ray micrographs, Scanning Acoustic Microscopy (SAM), or even mechanical probing should always be done first to isolate possible failure site(s). Destructive analysis, such as, mechanical cross sectioning or dry and wet chemical etching for exposing the failure site(s) at the package level, must then be performed to collect physical evidence.

Die Related Failures The real challenges for failure analysis of CSPs come from analysis of die related failures. Since a CSP device has a package size about the same as the die size, it is difficult to decapsulate the package and to expose the die for circuit diagnosis and analysis without destroying device connectivity to the package. A traditional plastic package usually has a package much bigger than the die, so decapsulation can be done easily using an automatic package-etching tool with a proper rubber gasket since there is a lot of room for error tolerance. For a CSP, precision decapsulation is required since there is no room for error. The etching window on the gasket needs to precisely correspond to the die size, requiring that the gasket be more rigid and have no deterioration, even under long exposures to H2SO4 acid (>60 seconds) at high temperature (>220C). Any over-etching could damage the package PCB substrate, break Cu traces, and even lift wire bonds (Figures 6a and 6b). Without package integrity after decapsulation, no electrical testing and subsequent diagnostic work can be done on a die related functional failure. Furthermore, the CSP devices have very thin dice that can be easily cracked by applying extra force on them during the decapsulation process.

Cu Trace Crack

Crack Flash

Di e Cu Trace Crack

(a)

solutions for this challenge will be discussed in the following sections.

(a)

Figure 7: A MCP package has stacked dice sharing most power supply and data pins

(b) Figure 6: Comparison of good (a) and bad (b) decapsulation of FBGA package

Corresponding Solutions

Compared to a CSP that has a single die within the package, a MCP has additional challenges due to its stacked die configuration, particularly for failure analysis of the bottom die or dice. Since the stacked dice share power supply pins and most address pins, there are fewer connections to the package with a MCP than if the enclosed dice were in separate packages. Normally, a Chip Enable (CE) function activates each die to run the diagnostic testing, so that the functional failures can be isolated to certain suspect die. However, for a high IccSSB (Icc Super Standby) current failure, multiple dice often share the same power supply pins (Figure 7), so it may not be easy to determine the source of the IccSSB leakage. To find out which die is responsible for the failure, the bond wires for power supply pins must be removed from one die at a time, then the leakage re-checked until the source is found.

There are no easy solutions to address all issues encountered for failure analysis of CSPs. However, many new techniques and special tools have been developed around CSPs to overcome the difficulties. Precision Decapsulation Precision decapsulation is required for CSPs, particularly for most FBGA packages. The key for successful decapsulation is to use well-defined gaskets. Rubber gaskets are currently used for most automatic package etching systems. These gaskets are made of acid-resistant viton rubbers, but they easily deteriorate with long exposure to H2SO4 at high temperatures. As a result, a rubber gasket can only be used once or twice before high precision decapsulation cannot be achieved. Furthermore, the debris from a deteriorated rubber gasket often get into the etch head of a package etching tool, preventing proper acid flow and causing severe system problems.

The analysis can proceed once the failing die is identified. If the failing die is on top, further analysis on the device is similar to a single die CSP. However, if the failing die is on the bottom, it is most likely partially or entirely covered by the top die. Circuit analysis and fault isolation cannot proceed until the top die is removed and the failing die is exposed. The most challenging part for MCP failure analysis is removing the top die or dice while preserving electrical connectivity of the bottom die to the package. Some of the

One solution to this problem is the use of a different type of gasket, such as those made from Teflon materials [1]. As we know, Teflon materials have acid resistance and heat endurance. They also can have well-defined cuts that can form etching windows for precision gasket. However, they

milling [2]. A micro drill with computerized control can mill off the plastic encapsulation material and the top die or dice from a pre-defined area (Figure 9). However, the mechanical force applied to the die is a concern during the milling. The MCP dice are very thin, so the milling process has to be well controlled to ensure that the force applied to the device will not crack the die or the package.

may not have as much flexibility as rubber materials to provide an airtight environment for the automatic package etching tools. The combination of a Teflon gasket and a rubber gasket has proven to be the ideal solution for precision decapsulation (Figure 8). In addition, using a less aggressive acid, such as, a mixture of HNO3 and H2SO4 acid also reduces the damage on the FBGA packages but requires a longer etch time. Positioning Fixture

Rubber Gasket

Original Method

Teflon Sheet

(a) (b) Figure 9: The top die was removed using a micro drill from the MCP package with two same die stacking (a) After molding compound and top die removal (stopped at the adhesive layer) (b) After bottom die exposure (etched off the adhesive layer)

Etch Window

New Method

Teflon Sheet

The chemical and mechanical approaches for MCPs with more than three stacked dice may reach their limitation. An alternative approach should be considered. One approach loosens the package with a wet chemical etch to get the dice out for repackaging. A mixture of nitric acid and water can free the plastic FBGA packages and leave the silicon dice in good condition after the etching. However, prolonged etch with this mixture will also remove the Al metal within the bond pad areas, resulting in wire-bonding problem afterwards. Another approach mechanically polishes off undesirable top die/dice as well as the solder balls on the backside of the package, and then repackages the remaining piece with exposed failing die into another open top package for analysis. However, repackaged device with longer wire loop could have noise issues during electrical testing so that this approach may not apply to the devices sensitive to noise. Experimenting and practicing on the suggested techniques are the keys to success on MCP analysis.

Rubber Gasket

New Combined Method Figure 8: A combination of Teflon gasket and Rubber gasket provides a better etching result for precision decapsulation. Top Die Removal If the package in a MCP has two stacked dice and the failing die is identified to be the bottom one, then top die removal becomes necessary for further analysis. There are several approaches for top die removal. One uses a chemical etch after polishing off the active region of the top die. KOH or TMAH are good selective etchants that etch bulk silicon with a high etch rate. The etch stops at the adhesive layer before reaching to the bottom die. One concern for this approach is that the adhesive layer may not completely stop the chemical from etching the bottom die if the adhesive layer does not cover the bottom die entirely. Another concern is that the entire package is subjected to a chemical environment at an elevated temperature (~60-80C), and the solder mask from the FBGA package substrate is often etched off. Therefore, the Cu traces in the package could be easily damaged during subsequent handling. To avoid a chemical etch for top die removal, another approach uses mechanical

For more advanced process technology, copper material is used for circuit interconnection. A traditional chemical etch process becomes incompatible with this new metallization. An increased number of stacked dice in MCP packages also requires each die to be thinner (individual die thickness to be 50um for a 6 stacked-die MCP). Mechanical milling for top die/dice removal for MCP analysis becomes impossible. Laser ablation using next generation multi-wavelength ND:YAG laser technology is being developed to handle decapsulation of advanced MCP and copper metallization devices [3]. This new technique avoids chemical reaction to the copper materials and eliminates mechanical force applied to the packages during decapsulation. However, controlling a laser to mill off materials with very different properties is

straightforward in principle, but more difficult in practice. The optimal process settings for bulk decapsulation of the package may be significantly different from those required at the device surface. Currently users are still experiencing laser damage on the die surface if the process parameters are not properly set.

Case Study I One Flash device failed after endurance cycling. Electrical testing results showed a cluster of sectors failed for embedded programming and erase operations. Bitmapping of the failing bits showed a hairline crack-like pattern across some of the failing sectors (Figure 11). The cell Vt tests also showed higher leakage values at the failing bits.

Backside Accessibility For some CSPs, such as FBGA package, it becomes difficult to access the device from the backside due to the solder ball attachment from the backside. With multi-level metallization on the IC devices, emission analysis is not always successful from the front side. The upper level power buses often times cover the emission site at the lower level to make emission detection impossible. In addition, backside emission analysis is more capable to detect the emission from low level leakages. In order to perform backside analysis on FBGA packages, a special sample preparation is required. One example of sample preparation for backside emission analysis is illustrated here:

Figure 11: Bitmapping results using a memory tester showed Hairline crack-like pattern of the bits (red colored) that could not be programmed and erased at the failing sector(s) With the speculation of a hairline crack on the failing device being present, careful SAM scan was performed on the unit, and C-scan (x/y plane) and B-scan (x/z cross-sectional) results indeed showed the possible die crack at the failing sectors (Figure 12). With this information, the FBGA package was re-examined again, and a faint crack line could be seen on the package surface (Figure 13).

Figure 10: the basic steps of sample preparation for backside emission analysis on FBGA packages

As illustrated in Figure 10, the steps for sample preparation are: 1) polish the package to expose the backside of the failing device for analysis; 2) attach the polished the device onto a ceramic PGA package; 3) use a manual wire bonder to connect the polished device to the PGA package. After this sample preparation, it is ready to perform emission analysis on the repackaged device.

Figure 12: SAM C-scan (left) and B-scan (right) results showed a possible die crack at the failing sectors location (circled)

Case Study II One MCP device (one SRAM + two FLASH) failed for the top flash erase (FLASH1). Further electrical testing showed that one of the sectors on the failing device had preprogramming problem at a single column (Figure 15). The column leakage test showed high leakage at the affected column. The row leakage test also showed a single row with high row leakage. The intersect cell of the leaky row and column had an abnormal Ids curve (Figure 16). Based on this electrical results, the focus was at the intersect cell of the affected row and column for physical defect location. Since this is a MCP device with one SRAM on top of two Flash devices, and the top Flash (FLASH1) is the one identified with the failure, mechanical polishing using the milling tool was done to remove the top SRAM and the Si spacer to expose the top Flash die (Figure 17).

Figure 13: Re-examination on the FBGA package surface showed a faint crack line at the suspected area (circled) The unit was then carefully decapsulated to expose the die surface for inspection. Optical inspection at the failing sector locations showed hairline cracks that match the patterns shown on the bitmap and SAM scan (Figure 14). Further investigation on the root cause of the failure concluded that the device had been mechanically damaged during the read point testing.

Figure 15: Bitmapping results using a memory tester showed a leaky column (unable to be programmed) and a leaky row at the failing sector. The arrow pointed to the intersect cell which was suspected to be the cause for the failing row and column.

(a)

(b)

Figure 16: Cell Vt analysis on the intersect cell showed abnormal Ids curve (a) compared to a good cell (b) After exposing the top Flash die (FLASH1), emission microscope analysis was performed on the device; however, no emission site was detected. Since the intersect cell of the failing row and column was the suspected failure site,

Figure 14: Optical inspection at the failing sectors showed hairline cracks that match the pattern shown on the bitmap and SAM scan.

deprocessing was done with dry etching and mechanical parallel polishing down to the ILD0 level (after metal 1 removal). Passive voltage contrast analysis showed the illuminated drain contact for the failing column and poly2 wordline contact for the failing row, indicating that the affected row and column had a leaky path to the substrate through the intersect cell (Figure 17). FIB cross-section through the intersect cell showed a CoSi residue bridging the poly2 wordline (the failing row) to the drain contact connected to the failing column (Figure 18). The information was then fed-back to the fab for process improvement. SRAM

(b) Figure 18: Passive Voltage Contrast Analysis showed illuminated drain contact of the failing column (a) and the poly2 wordline contact of the failing row (b)

Spacer

FLASH1 FLASH2

SRAM Spacer FLASH1 FLASH2

Figure 17: The top SRAM die and the Si spacer was removed using a mechanical milling tool to expose the top Flash die for further analysis (the milling window was setup to such that the wire bonds for FLASH1 were intact). Note that SRAM and FLASH 2 were bonded 90-degree rotation from the FLASH 1 device.

Figure 19: FIB cross-section through the intersect cell showed a CoSi residual bridging the failing wordline and the drain contact for the failing column

(a)

Summary CSPs are getting increasingly more attention due to their advantages of small form factor, cost effectiveness and PCB optimization. They are widely utilized for hand-held devices, automotive engine controller and networking equipment. Their small size creates new challenges for testing and failure analysis. Special requirements, such as precision decapsulation for FBGA packages, top die removal for MCP packages, must be addressed in order to successfully perform device testing and diagnostic analysis. In this article, some new challenges and corresponding solutions were discussed to increase the awareness of particular issues associated with CSPs. Two case studies are used to demonstrate how a typical CSP failure analysis is done.

References [1] Xia Li, Joseph Vu, Mohammad Massoodi and Jose Hulog, “Automatic decapsulation system utilizing an acid resistant, high heat endurance and flexible sheet coupled to a rubber gasket and a method of us”, US Patent Number 6409878 (2002) [2] Ultratec Application Note, “Enabling Backside Analysis, an overview of Selected Area Preparation”, 2002 [3] Control Systemation, Inc, Application Note by Jon W. Heyl, “Laser Based Failure Analysis, including Decapsulation, Cross-sectioning and Materials Characterization”, 2004

Acknowledgements The author would like to acknowledge following people for their contributions to this publication:  Joseph Vu and Shamsual Mohamed for their device anlaysis data  Jose Hulog, Mohammad Massodi and Joseph Vu for codevelopping the precision decapsulation technique  Mario Vargas for providing precision decapsulation images  Gene Daszko and Andy Gray for reviewing this paper and providing valuable input