Proceedings of the 32nd International Symposium for Testing and Failure Analysis November 12-16, 2006, Renaissance Austin Hotel, Austin, Texas, USA
New Approach: Sample preparation techniques for plastic small outline package (front-side and backside). Erwin Marquez and Dat Nguyen Texas Instruments Inc 12500 TI Blvd. M/S 8738 Dallas, Texas 75243 Email: erwin-marquez@ti.com, Phone: 214-8868913
To enable successful failure analysis for this package type, we developed a failure analysis sample preparation method, which facilitates backside and front-side failure analysis methods.
Abstract As device packages become smaller, the job of failure analysts becomes more difficult. Other complex configurations such as Small Outline Packages (SOP) pose unique problems. The difficulty of removing the encapsulant while preserving the integrity of the die, bond pads, bond wires and lead frame interconnects on a small outline package pose a serious problem. A new sample preparation technique is offered in order to expose the front side and backside of the die. This technique dramatically reduced the risk of damage and ensures the functionality of the device after decapsulation.
Introduction In this paper, two sample preparation techniques for Small Outline Package (SOP) devices will be presented. The first technique is for front side sample preparation. A new procedure using a jet etching system decapsulation was developed specifically for small outline package devices. The encapsulant is removed while preserving the integrity of the die, bond pads, bond wires and lead frame interconnects. In addition, we will show a method to eliminate the need for repackaging for backside sample preparation. Both techniques allow the failure analysts to do front-side or backside analysis on any small package device.
Figure 2: Plastic small outline package on US nickel.
Sample preparation procedure (Front-side) One solution to the problem is to insert a second layer between the gaskets. An Aluminum strip was inserted between the gaskets before running the automated jet etching system. Removing the plastic to expose the die usually involves bending the leads out straight and using an automated jet etching system. A small window (about the size of the die) will be cut on the Al strip using a sharp stencil under the microscope. Once set, the software enables an entire etching program to be completed. This procedure includes the following four steps:
The Small Outline Package (SOP) is a low-profile rectangular surface-mount component package. Its chip is bonded to an inner land contact area (usually a lead frame). External terminals exit parallel to the seating plane on two opposite sides of the molded, flat package. SOP uses gull wing formed leads and is a surface mount package with a fixed lead pitch.
1. 2. 3. 4.
Figure 1: Cross-sectional view of a SOP device (Cavity up). CopyrightŠ 2006 ASM InternationalŽ
253
Mounting the device in the position gasket. Placing with the Aluminum tape in order to define the area to be removed. Mounting the second gasket. Running the recipe from the automated jet etcher.
Mounting the device on a position gasket.
Mounting the second gasket.
The cornerstone of the successful acid decapsulation is the correct selection of both the position and definition gaskets. Determine the width and the length of the package base from the mechanical data or by direct measurement. The function of the position gasket is to position the package and hold the sample.
Using vacuum grease, gently apply the sample to one side of the secondary gasket and stack it above the position gasket and the Aluminum. Ensure that the alignment between the position and the secondary gasket is the same. Ensure that the window in the Aluminum tape has no contamination that will prevent the acid flow. The acid should go freely and continuously remove the defined area.
Figure 3: The position gasket firmly holds the device. Figure 5: Mount the second gasket and cover the Al strip.
Covering with Aluminum.
Figure 6: shows the complete cross sectional view of the entire sample preparation technique for front side decapsulation. The Aluminum tape offers many advantages. One advantage is to prevent the possible misalignment. Second, it allows the user to define any window size desired for exposure.
Normally, a definition gasket is used to define the area of the package that will be exposed to acid. But due to the very small dimension of the die, Aluminum tape was used. Using a sharp stencil knife create a small window in the Aluminum. Remember that the size of the window should at least match the size of the die that you want to expose. Cutting should be made under an ordinary optical microscope for accurate result. This cut will define the area that you want to be removed during the decapsulation process.
Figure 6: Cross-sectional view of the front side sample prep. It is very important to know whether the device has Copper on top. If there is Copper on top of the device, run a fuming acid mixture (fuming nitric acid (HNO3)/ concentrated fuming Sulfuric acid (H2SO4) 3:1 at 70-90 째C. The HNO3 is the primary enchants and the concentrated H2SO4 protects the Copper lead frames and Copper bonding wires from corrosion. Place the stacked gasket on the decapsulation head. A nozzle jets into the head from below, through which hot acid is
Figure 4: Covering with Aluminum tape. 254
sprayed against the surface of the sample, etching a hole at the milled cavity. The fuming nitric acid is taken from the acid bottle, with the aid of a pump and sent through heat exchanger. Then the fluid is circulated through the heat exchanger until it reaches a pre-set temperature. A timer is used to cut off the flow of the etchant to the device, depending upon package type and configuration. The automated jet etcher exposes the die by removing the mold compound. Different types of mold compound require different temperatures and acid. It is very important to know the exact type of mold compound, die size and FAB process (e.g. Copper on top) information. Figure 9: Close up view of the exposed die. Automated milling tool (back-side):
Sample preparation procedure (Back-side) Special milling techniques for backside sample preparation eliminate the need for the repackaging process on backside sample preparation. The development of reproducible backside sample preparation and failure analysis methods becomes more and more important because of the increasing number of metal levels within a semiconductor device and the ongoing transition to new packages. Quite often the defects are located in the lowest chip levels, which make electrical defect localization from the front side very difficult. Quite often, repackaging is both time and resource consuming which does not meet the failure analysis goals in terms of cycle time and efficiency. This paper will give an insight on providing solutions to meet these demands on failure analysis.
Figure 7: Cross sectional view of the enhanced gasket. It is necessary to perform the process on few dummy units and run it on the automated jet etcher in order to standardize times and ensure proper etching. Figures 8 and 9 show the excellent result on the SOP encapsulated device. Bond wire loop profile was preserved, lead frames are all intact, and the device is fully functional.
Photon Emission Microscopy (PEM) is a passive type of analysis. Passive techniques measures the emissions from the sample and do not alter the sample. The physical origin of the light emissions from the pn junctions can be easily understood by studying the recombination of holes and electron under forward and reverse bias conditions.
Figure 8: Optical image of the exposed die of a plastic small outline package.
Backside emission Figure 10: PEM image after backside sample preparation. 255
The Automated Selected Area Polisher (ASAP-1, commercially available in the market) is designed to thin and polish small selected areas in electronic packages and waferlevel dies. The resultant thinned component is transparent to infrared light, thus enabling backside failure analysis to be carried out on the circuitry.
Copper heat slug
The automated selected area polisher utilizes a high precision rotating chuck that holds a variety of tools, in various sizes, and is speed controllable. An adjustment is incorporated to center the motion at the correct point of the package or wafer under preparation. This chuck is attached to a Z-Stage that moves vertically on linear bearings and is controlled. The correct polishing pressure is applied. An X-Y stage holds the sample and is located underneath the rotating chuck. This stage is motorized and runs independently in X and Y directions at user-set amplitudes, to produce the desired dimensional ‘window’.
Die
Wire bond Figure 12: Scanning Electron Microscope (SEM) image of the cross-sectioned SOP device.
Selected area polishing on the backside was developed to aid the test, debug, and failure analysis engineering. (and semiconductor devices. ) Figure 12 shows the cross sectional view of the selected area polishing on the backside. ASAP incorporates adjustment for setting sweep center, along with intuitive X and Y amplitude controls. Using standard tool diameters enhances process reproducibility. This means that one straightforward setup of the X-Y parameters will be sufficient for the whole process.
Figure 13: SOP device after backside sample preparation.
Conclusion and Acknowledgements We presented a new sample preparation approach on Small Outline Package (SOP) in removing the encapsulant while preserving the integrity of the die, bond pads, bond wires and lead frame interconnects. A special milling technique which eliminates the need for repackaging process on backside sample preparation was also presented. Figure 11: Schematic of the sample preparation procedure for PEM backside analysis on a cavity down sample.
I thank my friends and colleagues from Texas Instruments (TI) for their encouragement to write this paper. Special thanks to David Maxwell, Thao To and Olivia Pambid for their inputs and sharing their Failure Analysis (FA) knowledge.
Controlling the pressure, the provision of a controllable loading system means that the correct pressure for specific tool, tool diameter and abrasive can be selected. For SOP package device, use 1mm drill bit and adjust the Y coordinate only in creating the window.
References [1] Steve Brockett, “Biased Backside Failure Analysis Techniques for Small Plastic Packages”, IEEE. [2] ISTFA Microelectronic Failure Analysis Desk Reference 4th edition ISTFA conference, pp. 55. [3] ASAP-1 milling tools literature, www.ultratecusa.com.
Knowing the package information is needed to be able to process the samples accurately. Figure 13 shows the crosssectional view of a cavity down in an SOP package. Critical information, such as the package and copper thickness are the some of the most important key parameters for backside sample preparation. 256