QFN Challenges: Second Bond Improvement to Eliminate the Weak Stitch (Fish Tail) Failure Mechanism on Pre Plated Lead Frame

Transcription

1 QFN Challenges: Second Bond Improvement to Eliminate the Weak Stitch (Fish Tail) Failure Mechanism on Pre Plated Lead Frame Jacky Lee Sinn Fah, Sreetharan Sekaran, Rameish Rao Subarmaniyan Central Process Engineering and Equipment (CPE), NXP Semiconductors Malaysia Camella Chee Guey Yong Package Innovation, NXP Semiconductors Malaysia Abstract Most of the device's technology has been moving towards the complex and produce of Nano-IC with demands for cheaper cost, smaller size and better thermal and electrical performance. One of the marketable packages is Quad Flat No-Lead (QFN) package. Due to the high demand of miniaturization of electronic products, QFN development becomes more promising, such as the lead frame design with half edge, cheaper tape, shrinkage of package size as to achieve more units per lead frame (cost saving) and etc [1]. The improvement methods in the lead frame design, such as lead frame metal tie bar and half edge features are always the main challenges for QFN package. With reduced the size of metal tie bar, it will fasten the package singulation process, whereas the half edge is designed for the mold compound locking for delamination reduction purpose. This paper specifically will discuss how the critical wire bonding parameters, capillary design and environmental conditions interact each other result to the unstable leads (second bond failures). During the initial evaluation of new package SOT1261 with rough PPF lead frame, several short tails and fish tails observed on wedge bond when applied with the current parameter setting which have been qualified in other packages with same wire size (18um Au wire). These problems did not surface out in earlier qualified devices mainly due to the second bond parameter robustness, capillary designs, lead frame design changes, different die packages, lead frame batches and contamination levels. One of the main root cause been studied is the second bond parameter setting which is not robust enough for the flimsy lead frame. The new bonding methodology, with the concept of low base ultrasonic and high force setting applied together with scrubbing mechanism to eliminate the fish tail bond and also reduce short tail occurrence on wedge. Wire bond parameters optimized to achieve zero fish tail, and wedge pull reading with >4.0gf. Destructive test such as wedge pull test used to test the bonding quality. Failure modes are analyzed using high power optical scope microscope and Scanning Electronic Microscope (SEM). By looking through into all possible root causes, and identifying how the factors are interacting, some efforts on the Design of Experiments (DOE) are carried out and good solutions were implemented. 1.0 Introduction During conventional wire bonding process optimization, fish tail defect is one of the most wanted and critical issues facing the semiconductor industry. The weak stitch so called fish tail can be defined as the poorly joint or partially lifted of a wedge bond that from the bond pad or bonding post as shown in figure 1. Lifted wedge during a bond pull test, is an indication of improper process optimization. Similar to lifted ball failure, weld lift failures, and even at very high bond pull values is a mark of bad quality and therefore, should not be accepted. Wedge bond is tested by destructive pulling the loop and measure the wire pull force required to break the bond. While this method assures the final quality, it does not necessarily minimize production stoppages which can reduce throughput significantly. Figure 1. Typical Fish tail on PPF Lead Frame 1

2 Second bond mechanism Upon the completion of first bond formation, the capillary will continue to move according to the looping profile (wire clamp remains open during the trajectory motion). Then, the second bond formation will start to form towards the lead fingers. Figure 2. Looping formation When capillary contact with the lead surface, the capillary will start to deform the wire against the lead and producing a half moon shaped bond which has gradual transition into the wire. During this process, ultrasonic power and force will transfer the energy to form the wedge bond. Figure 3. Second bond formation on lead frame discuss how the investigation and study of weak stitch (Fish Tail) for Au wire in pre-plated lead frame. Some DOEs also conducted to understand the interacting factors causing the fish tail formation. 2.0 Experimental Procedure 2.1 Weak Stitch (Fish Tail) Investigation During the initial released of SOT1260 package which was completed the characterization process, there were no bonding issues in terms of first bond and second bond. This package was qualified with normal PPF lead frame with Au wire (18um). Once the qualification completed, the parameter was then deployed and fan out to other similar packages with 18um Au wires. Eventually, when gone through the first trial of the process characterization for package SOT1261 on PPF lead frame, found high occurrence of short tail (premature tail break failure) and fish tail on the wedge bond. Tail short and fish tail failure mechanism can be happened at the same time as the process was same. It involved in second bond premature breaking for the next tail formation to the next bonding cycle. These two failure mechanisms can group into same as its form a weak stitch in terms of wire pull strength. By using the brainstorming technique, cause & effect diagram weak stitch was generated as shown in the following figure 5 below. While the process time towards the second completion, the capillary raises off the lead and leaving the wedge bond. At a pre-set height, the clamps are closed while the capillary is still rising with the bond head. The wire will break at the thinnest cross section of the wedge bond. This is where the fish tail can be form due to wedge bond formation that not completely attached on the lead and peel off on the half-moon area as shown in figure 4 Figure 5. Cause and Effect Diagram Figure 4. Fish tail formation Thus, a robust process to form a good wedge required through a process development. This includes a good capillary selection, good bonding surfaces, and process characterization and optimization, follow by validation using more machines/ materials to increase the sample size. This paper mainly With the prioritization, the highest rank input variables were listed as below. 1) Second Bond parameter 2) Capillary Geometry 3) Plasma Effectiveness 2

3 2.1.1 Second Bond parameter Based on the prioritization, a trial run was conducted to verify the second bond parameter setting affected to fish tail symptoms. A validation run was run to check the current performance of the second bond formation using the existing parameter which qualified in SOT1260 device, with normal PPF lead frame. The materials used for the experiment were all from the same lead frame and the wafer batch as qualified at earlier characterization. All the background variables such as machine platform, capillary, wire size, operators, were defined at early stages. These are important as to eliminate those noises /outside variables disturb to the experiment. Other than that, the concept table scrubbing mechanism was utilized as it was effective to eliminate the fish tail with good tail bond formation. As refer to Figure 6, the scrubbing mechanism concept mainly used the x-y table movement from the bonder in which helped to improve the adhesion on the first bond and second bond. Apparently, this will also affect the wire pull strength and formed better interlocking to the lead fingers. Thus, several DOE was designed also to further optimize the parameter setting to acquire good stitch strength and good stitch formation without fish tail symptoms and short tail occurrence. Firstly, a comparison study is conducted to understand the scrubbing effect on the second bond, as refer to Table 1. Figure 6. X-Y Table Scrub mechanism Table 2: DOE 2- Full factorial DOE, 3 Factors with 2 levels. The comparison study shown high fish tail occurrence on existing setting when associated with high setting of ultrasonic power and base force. Further investigation and next level DOE is carried out to understand the root causes of fish tails symptoms. Next, Full factorial DOE, 2 factors with 3 levels are designed to study the factor of scrubbing mechanism associated with low ultrasonic power and base force Capillary Geometry and Selection From the capillary design selection, currently there are 2 types of capillary using in production for wire size 18um. Both types have a different geometry dimension which have significant differences on stitch formation and gave different wire pull strength level. When going to the small bond pad pitch devices, there is a need to get the optimum setting with optimizing the tip diameter, face angle and outer radius of the capillary. The tip diameter is limited by the bond pad pitch of devices, while the reduction of the face angle is able to improve the second bond formation, but the lower stitch pull readings. [2] The outer radius provides a gradual transition from the face of the capillary to the edge of the tip diameter. The capillary tip dimension is one of the major parts that comes into contact with the wire and effect on the stitch formation. Figure 7 and 8 below show the critical capillary dimension to the second bond formation [3]. In this paper, the evaluation between the different capillary with different tip diameter are used. Table 1: DOE 1- Comparison study for second bond setting (with and without scrub implementation). 3

4 Figure 7. Critical capillary dimensions. (Cross-sectional view) Figure 8. Critical capillary dimensions. (Top view) Dimension Capillary A (R33) Capillary B (R35) Tip Diameter 3.6mil 3.8mil Hole Diameter 1.0mil 1.0mil Chamfer Diameter 1.9mil 1.8mil Face Angle 8 8 Outer Radius 0.5mil 0.5mil Table 3: Capillary Dimension The comparison study with these two capillary are shown as above table 3 above. In this experiment, both will use the same parameter setting during wire bonding. Stitch pull measurement for each bonding was carried out using DAGE 4000 wire pull tester. An average of 30 wire pull readings was taken for comparison. The stitch formation was analyzed using a high power scope and SEM (Scanning Electron Microscope) to the check for the gold remnant and any weak stitch (fish tail) observed Plasma Cleaning Effect Plasma cleaning is one of the middle process in between die bond and wire bond process where now is widely used in the semiconductor industry especially QFN packages. The purposes of plasma cleaning is to ensure contaminants free and bonding surface activation with energy. Consequently, it will provide the clean surfaces to ensure better stability and wire bonding quality. [2] In this paper, there is a study of the plasma effectiveness on the normal PPF lead frame and rough PPF lead frame. Comparison study within the plasma effectiveness (after plasma) was studied to understand the plasma effects towards the second bond formation and stitch strength. Figure 9. Plasma Cleaning Schematic Diagram 3.0 Results and Discussion: Experimental results from DOE 1 and DOE 2, both showed that second bond parameter with Scrubbing mechanism can improve the stitch formation and stitch strength significantly. From the existing parameter, found fish tail occurrence at random position where as the new parameter with scrubbing, no fishtail was observed as shown in figure 10. In terms of wire pull strength performance, figure 11 shown significantly improved with increased of 2g stitch pull strength. Further experiments also conducted to determine the optimum setting and process window for the second bond parameter. From DOE 2 results, as shown in figure 12 below, there are no significant differences for run 2 to run 6 as the wire pull strength above 5gf. No fish tails and short tails observed across all 8 runs with sample size of 456 wire count. Figure 10. Stitch Bond Formation Figure 11. Wire Pull Strength Comparison (DOE1) 4

5 Oneway Analysis of Stitch Pull Readings (g) By Capillary Type Stitch Pull Readings (g) Figure 12. Wire Pull Strength for 8 runs (DOE2) Quantiles Level Capillary A Capillary B Capillary A Minimum Capillary Type 10% Capillary B 25% All Pairs Tukey-Kramer 0.05 Figure 15. T-test wire pull strength comparison between capillary A and capillary B Means and Std Dev iations Median % % Maximum For the plasma effectiveness, the stitch pull also performed to Level Number Mean Std Dev Std Err Mean Lower 95% Upper 95% check Capillary the A stitch 12 strength Comparison study of Plasma time Capillary B (lagging 8 hours) vs Plasma time (without lagging) was investigated to the check for the stitch pull strength performance. From the figure 16 shown below, it's shown that the plasma time was significant improve on the stitch strength. Figure 13. Stitch appearance from DOE2, No fish tail observed. Besides that, for the experimental results shown for both capillary A and capillary B, figure 15 below shown capillary B have better stitch pull strength compared to capillary A. This proved that the bigger tip diameter provides more adhesion contact surfaces towards the second bond formation and better the imprint. However, both capillaries shown Fish Tail failures as seen in figure 14 but for capillary B, it is slightly better than capillary A as the stitch width is larger observed in a capillary B while used the same parameter settings (before implemented with scrub mechanism). Figure 14. Fish tail failures on both capillaries. Figure 16: Wire pull strength of Plasma Time (lagging 8hours) vs Plasma Time (no lagging) Conclusion In this paper, the weak stitch (fish tails) can be eliminated by optimized several variables which affected the second bond process formation. With the optimized second bond parameter setting, and with controlled performance like plasma cleaning effectiveness and suitable capillary selection, the quality can be improved with good stitch formation and enhanced stitch strength. Some other factors like lead frame design and taping condition may take into account when a new package is introduced. These interact factors may have affected to the current optimized process window and result to Fish Tail. 5

6 References 1. Tan Chee Eng, Pan Yen Jeat, Geale Fonseka QFN Wire Bond Dilemma, Stress Neck & Stress Heel, 9 th Electronics Packaging Technology Conference, Dec., 2007, Singapore, pp D.R.M. Capilto, Tirtonady A, Alcala D, Tail Lift-off Solution For Fine Pitch Applications, Semicon Singapore Nurul Hidayah Mohamad Nor, Suhaimi Taib, Ibrahim Ahmad, Ultra Fine Pitch 20 micron 2N Second Bond Improvement with New Capillary Surface morphology, Craig R. Barrett, William D. Nix, Alan S. Tetelman, The Principles of Engineering Materials, PERINTICE-HALL, INC Acknowledgments The author would like to thank Camella, Rameish and Sreetharan for working closely to study the Fish Tail mechanism and also special thanks to ASM machine supplier, CH Lim for giving strong supports. 6