Die Edge Research Articles

Mechanism of Generating Defects at Die Edge in Sequential Press Forming

Mechanism of Generating Defects at Die Edge in Sequential Press Forming

Surface stretch forming process based on collaborative control of sheet metal deformation and die contact state

In this study, we propose a novel method of surface stretch forming, named the Surface Stretch Forming based on Collaborative Control of Sheet Metal Deformation and Die Contact State (SF-CCDC). By controlling the motion paths of the discrete clamps, the contact process with the die, and the deformation process of the sheet metal can be changed. The processes of conventional stretch forming (SF-C) and SF-CCDC were numerically simulated. The results show that in SF-CCDC, the shearing effect of the die edge on the sheet metal can be reduced through the collaborative control of sheet metal deformation and contact with the die. Compared to SF-C, the maximum thinning rate is reduced by 50%, and the risk of stretch cracking is lower in SF-CCDC. Furthermore, both the center and edge areas of the sheet make contact with the die simultaneously, ensuring adequate deformation constraints and preventing wrinkling defects. Experiments were conducted to demonstrate the reliability and adaptability of the SF-CCDC method for forming double-curved surfaces.

Evaluating die wear-induced edge quality degradation in trimmed DP980 steel sheets from in situ force response monitoring

Assessing trimmed part quality involves periodic examinations of cut edges and shear affected zones to characterize rollover, burnish, fracture and burr formation. Established techniques are thorough and reliable but inherently time consuming. This study correlated die wear-induced force response evolutions and the observed transitions between shear-induced deformation zones for DP980 (C-0.15 wt%Mn-1.5 wt%Si-0.31 wt%) steel sheets subjected to trimming with AISI D2 steel dies. The findings are useful for peripheral assessment of part quality and tool wear via in situ trimming force/displacement response monitoring. Measurements were collected from a load cell and a non-contact laser displacement transducer embedded within the instrumented trimming press. The sliding wear damage and failure processes induced by die–workpiece interactions were correlated to force/displacement response phases. Trimming tests were conducted for 20,000 cycles to observe and assess transformations between rollover and burnish zones with increasing tool wear after the initial contact and at every subsequent 5000 cycle interval. The mean trimming force and maximum die penetration increased, primarily due to die edge blunting, by 1.33 kN (44.3%) and 194 μm (32.2%), respectively. An enlarged die–workpiece contact area increased the resistance force and delayed the onset of fracture with increased sliding damage between sheared surfaces and nearly doubled burnish zone depths. Quantitative attributes of the trimming force/displacement responses were linked to die impact and sliding contact induced damage features of trimmed parts, demonstrating the potential for straightforward, rapid and detailed in-field monitoring of acceptable limits to sheared edge quality.

Research on Repairing Edge of Trimming Die with Arc Welding Robot

Research on Repairing Edge of Trimming Die with Arc Welding Robot

The separation angle of the free surface of a viscous fluid at a straight edge

We rework some of the die-swell singularity analysis for Stokes flow, originally by Ramalingam (Ramalingam, 1994 Fiber spinning and rheology of liquid-crystalline polymers, PhD thesis, Massachusetts Institute of Technology) in appendix A of his PhD thesis, in an attempt to demonstrate that for capillary numbers in the range $(0,\infty )$ the curvature may enter into the normal stress balance on the free surface and lead to separation angles exceeding $180^{\circ }$ and infinite curvature at the separation point. The singular coefficients in the asymptotic solution and the free surface shape in a neighbourhood of the separation point cannot be determined by a local analysis of the Michael type (Michael, Mathematika, vol. 5, 1958, pp. 82–84) but must be found from matching with the solution valid away from the die edge. The numerical method that we use in the truncated die-swell domain is a boundary element method incorporating the singular solution near the separation point. Although there is some variation in the extrudate swell ratios at different capillary numbers reported in the numerical literature, our results for capillary numbers $Ca$ from $1$ to $1000$ are within the range of values published in earlier papers. The computed separation angles at different values of $Ca$ agree well with the range of separation angles to be found in experimental and numerical papers. The separation angle appears to converge to a value different from $180^{\circ }$ as $Ca$ increases, leading us to conclude that the case of zero surface tension ( $Ca=\infty$ , with corresponding separation angle of $180^{\circ }$ ), is a singular limit.

Effect of Die Edge Roundness on Crack Initiation near Punch Edge in Blanking

In this study, the effect of die edge radius on crack initiation was discussed in finish blanking. Experiments were carried out using die with rounded cutting edge. As a result, it was confirmed that crack initiation near the punch edge occurred earlier than that near the die edge. It was also confirmed that the burnished surface increased with an increase in the die edge radius. Finite element method was carried out from the initial stage of shearing and until the punch penetration depth was achieved just before the occurrence of the cracks to investigate the distribution of the mean stress. As a result, it was confirmed that the punch stroke, where the mean stress began to increase rapidly to a positive value, increased with an increase in the die edge radius, and the punch stroke at which the mean stress increased rapidly also increased.

Effect of Tool Geometry Parameters on the Formability of a Camera Cover in the Deep Drawing Process.

The formability of the drawn part in the deep drawing process depends not only on the material properties, but also on the equipment used, metal flow control and tool parameters. The most common defects can be the thickening, stretching and splitting. However, the optimization of tools including the die and punch parameters leads to a reduction of the defects and improves the quality of the products. In this paper, the formability of the camera cover by aluminum alloy A1050 in the deep drawing process was examined relating to the tool geometry parameters based on numerical and experimental analyses. The results showed that the thickness was the smallest and the stress was the highest at one of the bottom corners where the biaxial stretching was the predominant mode of deformation. The problems of the thickening at the flange area, the stretching at the side wall and the splitting at the bottom corners could be prevented when the tool parameters were optimized that related to the thickness and stress. It was clear that the optimal thickness distribution of the camera cover was obtained by the design of tools with the best values—with the die edge radius 10 times, the pocket radius on the bottom of the die 5 times, and the punch nose radius 2.5 times the sheet thickness. Additionally, the quality of the camera cover was improved with a maximum thinning of 25% experimentally, and it was within the suggested maximum allowable thickness reduction of 45% for various industrial applications after optimizing the tool geometry parameters in the deep drawing process.

Die Placement Advancement for Prevention of Silhouetted Die Occurrence on LGA Package

In semiconductor packaging industry, silhouetted die defect is occasionally encountered in singulation process esp. on substrate land grid array (LGA) package with tight clearances. This paper is focused on the prevention of the silhouetted die occurrence as it would affect the assembly yield performance of the device. The silhouetted die is caused by the tight clearance of die edge to package edge, given the machine and process tolerances. Process enhancement and optimization were done through adjusting the die placement accordingly as per the defined measurement. Eventually, the occurrence of silhouetted die was successfully mitigated by formulating the appropriate die placement references.

Solder-Joint Reliability of BGA Packages in Automotive Applications

The solder-joint interconnect between an IC component and the PCB (printed circuit board) is a critical link in the system overall reliability. Trends in the automotive market are driving increased focus on solder-joint performance: (1) increasing electronics content for new functions, especially for ADAS (advanced driver-assistance systems), (2) use in safety critical systems and sub-systems, (3) decreasing interconnect pitches which reduces the stand-off and available solder, (4) increasing industry reliability expectations, and (5) package variations (ex. multi-die). In particular, BGA (Ball Grid Array) packages are used throughout the vehicle across various systems including engine control, braking, communication, infotainment, and radar to name only a few. Among these, under-the-hood applications often require high sustained operating temperatures and many heating/cooling cycles during the vehicle lifetime. The reliability of these interconnects is routinely assessed by cyclical thermal stress (temperate cycling) of components mounted to boards. While AEC (Automotive Electronics Council) offers no standards for solder-joint testing (for example, board level reliability criteria is not included in the AEC Q100 “Failure Mechanism Based Stress Test Qualification for Integrated Circuits”), IPC 9701A “Performance Test Methods and Qualification Requirements for Surface Mount Solder Attachments” can be followed. For automotive under-the-hood the specified cycle range is 40°C to 125°C (TC3). This paper summarizes the BL-TC (board level temperature cycle) performance of various BGA packages used in automotive applications. In all cases the test vehicle packages were daisy-chain versions of production devices, while maintaining critical features such as BGA footprint, physical dimensions, BOM (bill of materials), die size and thickness and substrate layer metal densities. All used Pb-free solders for both the BGA solder ball and the paste printed onto the PCB. The PCB designs were complementary to the packages establishing daisy-chain connections winding through the PCB, the solder-joint and package substrate. Each chain (net) was continuously monitored in situ during cycling. An event detector logged a failure when a net resistance exceeded 300 ohms. Wirebonded and flip chip packages were studied, ranging in size from 10mm to 29mm with BGA pitches including 0.65mm, 0.80mm and 1.00mm. In addition to these primary attributes, various other factors were found to alter the solder-joint lifetimes. For example, increasing BGA pad and solder sphere diameters improved solder-joint lifetime, but increasing the PCB pad diameter often did not. Among solder materials, eutectic SnAg typically showed longer lifetimes than other high Ag SAC alloys such as SAC305 and SAC405. The addition of Bi to the SAC alloy showed promise for further improvements. Other factors that were studied include die thickness, die size, and BGA pad finish. Both mechanical cross-section and dye penetrant analysis (dye-and-pry) were employed for failure analysis, enabling study of crack propagation and crack location within the solder-joint. Additionally, failure location (failing solder-joint) was identified for each as package corner, under the die edge, or package center in a predictable pattern depending on the package type. Examined in total, two opposing trends will force future innovation. Industry reliability requirements continue to drive expectations (i.e. cycles to failure) higher, while increasing package size and decreasing pitch will naturally reduce the solder-joint lifetimes. Solutions will be found in package design, package material and solder selections.

Automated image analysis for evaluation of wafer backside chipping

When a silicon wafer is cut into separate dies, their front and back sides might have chipping resulting in die cracks and yield loss. To prevent defect formation, silicon wafers should undergo optical inspection for evaluation of wafer chipping, its size, and its shape. This work proposes an automated method of image processing that includes die edge detection, die street search, and determination of chipping size and shape. Die edge search was done using an Otsu’s thresholding method. This technique was chosen out as the optimal of the five ones. The choice was based on the segmentation precision evaluation of two types of images: with sharp and blurred edges. Die street search was done using a developed algorithm capable of processing images with angular displacement. Chipping shape and size were calculated through die edge displacement from the die street. Based on the numerical evaluation of chipping size and shape, a chipping danger metric that may be used for detection of defective dies has been proposed.

Numerical simulation of hot blanking for boron steel B1500HS based on ductile fracture criterion

The distribution and evolution of stress-strain in the sheared zone at elevated temperature is an important theoretical basis for studying the hot blanking mechanism. The fracture criterions and fracture thresholds are the main factors affecting the accuracy of numerical simulation for the hot blanking. To research the mechanism of hot blanking, B1500HS steel sheet with full austenitization was blanked at the different temperatures (650, 700, 750, and 800 °C). Finite element model of hot blanking was established, and five fracture criterions (normalized Cockroft and Latham, Brozzo, Freudenthal, Oyane, and Rice and Traccy) were used to simulate the hot blanking of B1500HS steel. According to the results of numerical simulation and experiment, the fracture thresholds of five fracture criterions were evaluated using the method of “regression fitting-calculation-prediction-correction.” The fracture thresholds attained by this method were used to simulate the hot blanking, and the location where crack occur initially and fracture shape was obtained. Comparing the simulation results and the experimental results, the results show that the crack occurs at the edges of die and punch as the hydrostatic stress reaches to the maximum value, and the simulation results based on the Oyane fracture criterion and Brozzo fracture criterion are well consistent with the experiment results. The fracture thresholds attained by the method of “regression fitting-calculation-prediction-correction” have the better reliability and practicability.

Assessment of Optimized Process Quality and Reliability for Wafer Level Applications

Fanout Wafer Level Packaging (FoWLP) is a very attractive solution for microelectronics applications requiring optimized performance, smaller form factor, and low cost. By utilizing such an approach where system integration is done to multiple chips on a single package frame, the need to ensure much higher levels of process integrity, quality, and reliability becomes absolutely critical, especially if the total product volume lies in the range of tens of millions of units. A single defect type may negate the benefits of such an approach because the cost of losing one FoWLP unit results in the loss of multiple devices. Thus, yield, quality, and reliability optimization using such a package solution is critical for successful large scale manufacturing. In this talk, the issue of defectivity and its impact on quality and reliability on Wafer-Level (WL) devices with regards to the issue of Die Edge Delamination (DED) and Chip Mechanical Integrity (CMI) is discussed. Through this discussion and the resulting solutions found to improve WL quality and reliability, better understanding on how to assess the quality and reliability of a given FoWLP solution for large scale production will be demonstrated.

OPTIMIZATION OF INFLUENTIAL PROCESS PARAMETERS ON THE DEEP DRAWING OF ALUMINIUM 6061 SHEET USING TAGUCHI AND FINITE ELEMENT METHOD

The plastic deformation behavior of axis symmetric aluminium 6061 cups was determined by analyzing the four important deep drawing process parameters, namely blank temperature, die edge radius, blank holder force and friction coefficient. Taguchi techniques along with finite element method (FEM) were used to determine the importance of process parameters. The Taguchi method was used to analyze the influence of each process parameter. From the deformation result and analysis of variance (ANOVA), it was determined that the temperature of the blank has a major influence on the deformation characteristic of aluminium 6061 sheets followed by die edge radius, coefficient of friction, and blank holder force. The optimum levels of the four factors in determining the deformed cup heights are found to be blank temperature of 450°C, die edge radius of 14 mm, coefficient of friction of 0.60 and blank holder force of 9 KN.

An Investigation into the Package and Printed Circuit Board Assembly Solutions of an Ultrathin Coreless Flip-Chip Substrate

Flip-chip technology has been widely accepted as a solution for electronic packaging of high-pin-count devices. Due to the demand for smaller and thinner package dimensions, coreless build-up substrates will be used in industry to carry the die by solder bumps due to the advantages of shorter transmission route and lower inductance and thermal resistance. However, coefficient of thermal expansion (CTE) mismatch between the Cu trace and the laminate often causes the coreless substrate to warp, which leads to failures such as nonwetted solder bumps and interfacial cracking during assembly and reliability tests. In a previous study, assembly of a six-layer polyimide-based coreless flip-chip package was achieved by a 17 mm × 17 mm die with 4355 Sn-37Pb solder bumps, an amide-based underfill, and 1521 Sn-3.0Ag-0.5Cu solder balls. For determination of its board-level reliability characteristics, the component was mounted on a printed circuit board (PCB) using a conventional surface mount technology, and 10 test vehicles were assembled for assessment of their reliability under a temperature cycling environment. The experimental results show that the characteristic life of the PCB assembly exceeded 1500 cycles and that failure resulted from fracture of the outermost solder balls on the substrate side. This was different from the failure mode of die cracking when the package experienced hundreds of temperature cycles at the component level because the rigid PCB, through solder balls, moderated the deformation of the coreless flip-chip package. Hence, the concentrated bending stress at the die edge region was lowered. Finally, the local CTE mismatch between the stiffener and the PCB dominated the fatigue fracture of the outermost solder balls to become the main failure mode.

Microstructure evolution during 300 °C storage of sintered Ag nanoparticles on Ag and Au substrates

A silver nanoparticle based die attach material was used in a pressure free process to bond 2.5mm square Ag plated Si die to Ag and Au plated substrates. The assemblies were stored at 300°C for up to 500h and the morphology of the sintered Ag and the shear strength were monitored as a function of time. On Ag substrate it was found that die shear strength increased and that the Ag grains grew in size and porosity decreased over time. There was also a clear difference in morphology between sintered Ag at the die edge and centre. On Au substrate, it was observed that the initially high die shear strength decreased with storage time and that voids migrated away from the Ag/Au interface and into the Ag joint. This has led to the formation of a void free layer at the interface followed by a high porosity region, which weakened the joint. The microstructure reveals a high density of grain and twin boundaries which facilitate the Ag and Au atomic diffusion responsible. The grain structure of the plated Au led to diffusion of Au into the Ag via high-angle tilt grain boundaries, and grain boundary migration further dispersed the Au into the Ag layer.

Accuracy and Uph Challenges and Solutions in Flip Chip Underfill Dispensing

Flip chip configurations have changed in terms of both their package design and packaging process. These emerging flip chips have become more popular now. Compared to traditional ones, the new flip chips need different dispensing approaches to improve unit-per-hour production, and require more accurate fluid dispensing. Newly developed motion control and stiffer platforms for automatic fluid dispensers have addressed the challenges. This paper shows how the new system achieved significant improvements for both: 27% UPH improvement in packaging a certain flip chip configuration and +/-40um line dispensing accuracy for a few hundred micron tighter clearance from die edge.

Influence of Tool Geometry Variation on the Punch Force in Micro Deep Drawing

Micro forming processes are very well suited for manufacturing of small metal parts in large quantities and micro deep drawing provides a great application potential for the manufacturing of parts with complex shapes. But size effects like changed tribology and material properties usually result in smaller process windows for micro forming operations. Process caused wear as well as large inaccuracy in manufacturing of micro forming tools is responsible for geometrical deviation of the tools from nominal size. Both influences can have essential impact on the process window size and process stability. A better understanding of the influence of tool geometry on process stability can help to improve and optimize process control in micro forming. In addition, a quantitative judgment of the impact of wear and manufacturing inaccuracy will be possible. Therefore, in this study, the impact of different tool geometries on the punch force in micro deep drawing was investigated. Significantly varied tool geometries were punch diameter, drawing gap, punch and drawing die radius and shape of the die edge. FEM simulations as well as experiments were used to determine tool geometry influence on the punch force of a micro deep drawing process. Hereby, it was possible to classify each geometry variation regarding its impact on the punch force and therefore on one important parameter of the process stability. Results show that the greatest impact on the punch force was caused by modifications of the punch diameter and variation of the drawing gap. Changes in punch or drawing die radii proved to be of minor importance.

Evaluation of Pressure Free Nanoparticle Sintered Silver Die Attach on Silver and Gold Surfaces

A commercially available silver nanoparticle based die attach material was used in a pressure free process to bond 2.5 mm square Ag plated Si die to Ag and Au plated substrates. The two substrate types were 5mm square Ni/Ag plated silicon substrate and a W/Ni/Au metallised cofired alumina package. The assemblies were stored at 300 °C for up to 500 h and the morphology of the sintered Ag and the shear strength was monitored as a function of time. Bondline thickness measurements were carried out after following the paste manufacturer's drying and sintering temperature profile. On Ag substrate it was found that die shear strength increased with storage time. The fracture surfaces of the sheared die and substrate as well as cross sections of untested die were examined using electron microscopy. It was found that the Ag grains grew in size and porosity decreased over time. There was also a clear difference in morphology between sintered Ag at the die edge and centre. During shearing the Ni layer was found to separate from the chip at the edges of the die after ageing. On Au substrate, it was found that die shear strength decreased with storage time. It was found that the Au diffused into the Ag, creating a low porosity Au-Ag layer. Ag also migrated towards the Au surface, leaving behind a layer of voids which contributed to weakening of the joint. Rapid Au diffusion was associated with the high density of grain boundaries arising from the sintering process.

穴抜きダイ刃先面取りによる穴広げ率の向上

In this study, a piercing method using a large chamfered die is proposed for the improvement in the hole expansion ratio. To validate this method, high-strength steels pierced by a large chamfered die were investigated. The result shows that holes pierced by the large chamfered die present a much higher hole expansion ratio than holes pierced by a conventional die, whereas a small chamfered die does not improve the hole expansion ratio. Additionally, as long as the chamfer value is above a specific value, the effect of the large chamfered die on the hole expansion ratio is independent of the chamfer value and the clearance between the punch and die. To clarify the mechanism of this improvement, the hardness close to the pierced surface and the material deformation process were analyzed. The hardness measurement indicated a decrease in the extent of work hardening when the hole was pierced by the large chamfered die. This contributes to the improvement in the hole expansion ratio. The decrease in the extent of work hardening is caused by material deformation without contact between the material and die, which leads to the suppression of shear deformation around the die edge.

FEM によるPW パンチの穴抜き面凹凸の改善メカニズムの解析

Numerical simulation by a finite element method investigated the mechanism of roughness improvement in pierced holes using a press working (PW) punch. Two types of simulation, i. e. piercing and crack conjunction, were performed. Piercing simulation suggested that the PW punch suppresses both the generation and propagation of a crack in a sheet material around the punch edge because of the suppression of plastic strain localization, but promotes the generation and propagation around the die edge because of the high stress triaxiality induced. This result implies the suppression of the mismatch of the two cracks by the dominant crack from the die edge. Crack conjunction analysis based on bifurcation and stress triaxiality showed that crack mismatch increases the unevenness of a fracture area of pierced surfaces.