CN116486007A - CT imaging-based welding spot electromigration damage hole distribution characteristic determination method - Google Patents
CT imaging-based welding spot electromigration damage hole distribution characteristic determination method Download PDFInfo
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Abstract
The invention discloses a method for determining distribution characteristics of electromigration damage holes of welding spots based on CT imaging, which mainly solves the problem that the internal damage of welding spots cannot be observed in the prior art. The implementation scheme is as follows: preparing a test sample according to industry standard standards; carrying out an electromigration experiment on a test sample, and respectively testing the temperature, the strain and the voltage of a welding spot of the test sample in the experiment by using a thermal imager, a strain gauge and a data acquisition system; CT scanning is carried out on the tested sample piece, and a two-dimensional gray level image of the sample is obtained; carrying out three-dimensional image reconstruction on the two-dimensional gray level image by utilizing Avizo software; and (3) utilizing Label Analysis to classify and count the volume and the surface area of the holes in the three-dimensional reconstruction image in the welding spot, and determining the distribution characteristics of the holes damaged by electromigration. The method can accurately determine the hole distribution characteristics, improve the accuracy and the applicability of the failure analysis of the welding spots, and can be used for predicting the failure life and evaluating the reliability of the welding spots in the electronic packaging structure.
Description
Technical Field
The invention belongs to the technical field of electronic packaging, and particularly relates to a method for determining distribution characteristics of electromigration damage holes of welding spots, which can be used for predicting the failure life of welding spots in an electronic packaging structure and evaluating reliability.
Background
The welding spot is used as a main electric connection and mechanical supporting structure of the electronic package, bears the coupling action of a temperature field, a mechanical stress field and an electromagnetic field, is the position of the electronic package structure which is most likely to fail, and the reliability of the welding spot directly influences the service life of an electronic circuit. At present, with the increase of the electronic packaging density, the working current on the device is increased, and the local high current density and temperature in the welding spot can cause the damage of holes in the welding spot, so that the conductivity of the device is reduced or even the device cannot work. Therefore, the damage of the internal structure of the welding spot under high current density is quantitatively determined, so that the method is beneficial to revealing the failure mechanism and the damage evolution rule of the welding spot and is beneficial to predicting the failure life of the electronic packaging structure.
At present, research on electromigration damage of welding spots is mainly performed by a scanning electron microscope means and a numerical simulation method. However, the scanning electron microscope needs to pretreat the welded joint, which can cause secondary damage to the internal structure of the welded joint, and cannot truly reflect the holes caused by electromigration. Meanwhile, the establishment of the numerical simulation model needs to combine certain assumption conditions, and the actual implementation of the internal structure of the welding spot cannot be truly combined. Compared with the three-dimensional nondestructive scanning imaging technology, the additional damage to the internal structure of the welding spot is avoided, and the internal structure change caused by electromigration of the welding spot can be quantitatively and intuitively represented.
Zhang Yueping in the study of mechanical response of the BGA package structure of the academic paper 2021 under the temperature cycle load, a DVC method for reconstructing fatigue damage of welding spots and combining X-ray CT and digital volume is provided, so that the measurement of a three-dimensional strain field of the welding spots inside the BGA package structure of the ball package array is realized, the measurement is compared with a finite element simulation result, the three-dimensional representation of the morphology of the BGA package welding spots is performed on the three-dimensional digital image obtained on the basis of X-ray CT, and the plastic deformation calculation is performed on the welding spots on the basis of a DVC module. The current finite element simulation analysis is mainly based on the simulation of the measurement of the appearance of an object, the model cannot analyze the deformation and holes in the welding spot, the size of each welding spot cannot be independently modeled for the welding spot in the circuit board of the research object, and the defect information in the welding spot and each part is difficult to obtain, so that the simulation structure is not fine, and even the situation different from the actual structure can occur. Although the method optimizes and repartitions the actual shape of the simulation object based on the Surface mesh repartition module Remesh Surface of the Avizo software to enable the welding spot finite element model mesh to be closer to the actual structure and finer, the method is only closer to the actual structure of the welding spot, the actual structure inside the welding spot cannot be obtained yet, and the method lacks damage analysis of the inner holes of the welding spot, cannot reflect the change of the actual mechanical property after the welding spot is damaged, and cannot provide reliable data support for subsequent research.
The patent document with the application number of CN201510793413.4 discloses a point-by-point scanning temperature measurement detection method for the welding spot defect of a BGA chip, which comprises the steps of firstly adopting a smaller laser beam spot diameter to carry out point-by-point scanning temperature measurement on the detection of the virtual welding spot, fixing a thermal infrared imager on a support frame connected with a z axis of a three-dimensional moving platform, enabling the thermal infrared imager to be positioned on one side where the BGA chip is positioned, moving, focusing and positioning in the z direction, and ensuring that a welding ball area of the BGA chip to be detected is in a visual field range; fixing the laser by adopting a bracket with an adjustable incidence angle, enabling the laser to be positioned on one side of the substrate of the BGA chip, enabling a laser beam spot of the laser to be aligned with a pad to be tested on the substrate of the BGA chip for preheating until a thermal image is observed on a PC; and finally, adjusting the heating power and pulse time of the laser, heating the preheated pad to be tested, detecting the temperature rise process of the ball area of the BGA chip in real time by the thermal infrared imager, shooting a thermal image of the highest temperature rise point at the same time, and sending the thermal image to the PC, wherein when the temperature rise reaches a specified range, the welding point is a qualified welding point. Although the method is nondestructive to the welding spot structure, the judging whether the welding spot is qualified or not is visual, the damage condition of the welding spot can be judged indirectly only through the temperature rise amplitude, the damage structure in the welding spot can not be observed specifically, and the hole data in the welding spot can not be obtained intuitively for the hole in the welding spot electromigration damage.
Guo Fu, wen Tingyu, ma Limin et al, in the university of Beijing industry university school 2021,47 (11): "BGA solder joint electromigration damage under high temperature and high current density" published by 1264-1274, propose to use finite element simulation software to carry out the coupling of thermal-electric-force-concentration multiple physical fields, analyze the electromigration damage process based on hole formation and diffusion criteria, when electromigration occurs, the metal in the solder joint loses electrons to form ions and completes migration, the ion concentration in the solder joint becomes the key basis for judging hole formation in the process, when the atomic concentration in the grid is less than the critical atomic concentration, the software judges that the place is a hole, and analyzes the electromigration damage result through changing the size of the atomic concentration in the solder joint. Because the simulation software is based on the establishment of a finite element model of the welding spot and the boundary condition is an ideal condition, the internal damage of the welding spot after electric migration in the real environment cannot be restored, further data extraction and analysis cannot be carried out on the internal damage, the article also has no relevant experimental data, and the effectiveness of the simulation data cannot be verified.
Disclosure of Invention
Aiming at the defects of the prior art, the invention provides a method for determining the distribution characteristics of electromigration damage holes of welding spots based on CT imaging, so as to obtain the actual structure inside the welding spots, intuitively acquire the hole data inside the welding spots and the distribution characteristics of the holes of the damaged welding spots, and provide more real data and reliable data support for the prediction of the fatigue life of the welding spots.
In order to achieve the above purpose, the technical scheme of the invention comprises the following steps:
(1) Preparing a test sample according to industry standard standards;
(2) Electromigration experiments are carried out on the test sample, and a thermal imager, a strain gauge and a data acquisition system are utilized to respectively obtain the temperature T, the strain epsilon and the voltage U of the welding spot A2 of the test sample in the experiments A2 ；
(3) CT scanning is carried out on the tested sample piece, and a two-dimensional gray level image of the sample is obtained;
(4) Three-dimensional image reconstruction is carried out on the two-dimensional gray level image by utilizing Avizo software:
4a) Performing definition and noise reduction pretreatment on the two-dimensional gray level image by using a non-sharpening masking algorithm Unsharp masking;
4b) The processed two-dimensional gray image is subjected to contrast enhancement and threshold segmentation by using a Top-hat operator, and the result is shown in figure 3;
4c) Performing three-dimensional reconstruction on the image after threshold segmentation by utilizing Volume rendering to obtain a three-dimensional reconstruction visual image of electromigration damage in a welding spot;
(5) And classifying and counting the volume and the surface area of the holes in the three-dimensional reconstruction image in the welding spot by using Label Analysis, determining the distribution characteristics of the holes damaged by electromigration, and providing data for determining the unstable area of the welding spot in the mechanical structure.
Compared with the prior art, the invention has the following advantages:
1. the invention scans the welding spot after the electromigration experiment by using a CT scanning technology, uses the high-pass filtering property of a Top-hat operator, and utilizes the mutual combination of morphological open operation and closed operation to reconstruct the two-dimensional gray image data in three dimensions, thereby not only enhancing the contrast of gray images, but also accurately representing the hole defects and the change of fine structures in the welding spot, namely realizing the three-dimensional reconstruction of the hole damage in the welding spot.
2. According to the invention, by combining three-dimensional reconstruction data of the welding spot damage, parameters such as the damage surface area, the volume, the equivalent diameter and the like are determined through statistical analysis and calculation, quantitative characterization of the welding spot hole damage can be realized, and a basis is provided for the research of a welding spot damage mechanism.
Drawings
FIG. 1 is a flow chart of an implementation of the present invention;
FIG. 2 is a three-dimensional reconstruction of a grayscale image of a test sample in accordance with an embodiment of the present invention;
FIG. 3 is a graph of test sample solder joint thresholding in an embodiment of the present invention;
FIG. 4 is a three-dimensional rendering of a test sample solder joint hole damage and substrate in accordance with an embodiment of the present invention.
FIG. 5 is a plot of the equivalent diameter of a test sample spot weld damage in accordance with an embodiment of the present invention.
Detailed Description
The present invention will be described in detail with reference to specific embodiments thereof in order to more clearly and clearly illustrate the objects, technical solutions and advantages of the present invention. It should be noted that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the scope of the invention.
Referring to fig. 1, the implementation steps for the example are as follows:
and step 1, preparing a test sample according to a standard.
In the embodiment, the target experimental sample piece adopts lead-free Sn-Ag-Cu as a welding spot material, a six-point metal oxide chip based on a P-channel field effect transistor is selected, the model is PMCM6501UPE, the basic size is 1.48 multiplied by 0.98 multiplied by 0.145mm, the target experimental sample piece contains 6 pins, the diameter of a welding ball is 0.26mm, and a wafer level chip scale package WLCSP form is adopted.
Pins A2, B1 and B2 of the chip are all Source ends and are internally connected with each other, and in order to facilitate the test of the voltage change of a single welding spot in an experiment, the welding spots A2 and B2 are selected as interconnection structures for electrifying, and other welding spots do not participate in current transmission and only play roles in supporting and balancing. Pin B1 is used as an auxiliary test point, and a lead wire of the welding point A2 or B2 is connected with a measuring instrument to record the voltage change of a single welding point. In designing the circuit, it is considered to lead out the pads A1, A2 and B2 individually through the leads. Meanwhile, in order to ensure the normal function of the selected chip, two module circuits are designed on the circuit board, one is that the chip is normally connected with the circuit, and the other is that the test circuit.
A PCB with the size of 95 multiplied by 1mm is selected as a substrate, a six-point metal oxide chip and the PCB are fixed by adopting a welding and pasting method, and the process of the welding and pasting method is performed in nitrogen by adopting flip chip welding so as to prevent solder oxidation and hole formation in the welding process. The six-point metal oxide chip and the circuit of the packaging structure are interconnected through a metal wiring layer on the PCB, copper leads with the thickness of 0.02mm and the width of 0.24mm are formed on the PCB through deposition lithography, and 6 bonding pads with the diameter of 0.25mm are formed on the PCB. And solder resist is coated around the bonding pad to prevent the solder balls from deforming, reduce the surface tension of the PCB to be soldered and ensure the insulativity.
When the chip is adhered to the substrate, the chip packaged by the WLCSP is reversely buckled on the substrate, and the solder balls are placed into a reflow device to reflow against the bonding pads, so that the whole test sample is manufactured.
The manufactured experimental sample is subjected to on-off test by using a universal meter, whether the experimental sample is intact or not is detected, whether a circuit is qualified or not is detected, and the fault that the welding is in cold joint or the circuit is not through is eliminated.
Step 2, performing electromigration experiments on the test sample, and respectively obtaining the voltage U of the welding spot A2 of the test sample in the experiments by using a thermal image +instrument, a strain gauge and a data acquisition system A2 Temperature T, line strain ε in an angle α direction to the x-axis α And according to A2 solder joint voltage U A2 And analyzing the interface area of the welding points.
2.1 Selecting test equipment:
in this embodiment, the apparatus required for the electromigration experiment includes: constant current power supply, data acquisition system, thermal imaging system, foil gage, and X ray tomography microscope, wherein:
the constant-current power supply adopts an adjustable direct-current voltage-stabilizing power supply, the model is a wanttekNPS 3010, the output current is 0-10A, the output voltage is 0-30V, the output power is 300W, the display precision is 0.5%, the constant-current power supply has the functions of stabilizing current and stabilizing voltage, and the output current and the output voltage can be continuously adjusted.
The data acquisition system selects a multi-channel measurement system based on LabVIEW, and mainly comprises an NI9219 data acquisition card, an NI compactDAQ machine case and PC end LabVIEW software. The NI9219 data acquisition card is a 24-bit 4-channel acquisition module, each channel comprises a 6-pin spring terminal connector, the channels are mutually independent, an access sensor can measure signals of different types such as temperature, strain, voltage, current and resistance signals, the NI9219 data acquisition card can be connected with a computer only by being inserted into an NI compactDAQ machine case, data communication transmission and real-time display of signals of each channel are carried out, and the NI9219 channels are mutually isolated so as to ensure the safety of the machine case, the computer and each channel.
The thermal imager adopts 220-series double-view-field temperature measurement thermal imager produced by FOTRIC company, the thermal imager adopts a large-aperture professional thermal imaging lens to provide more infrared energy for a detector, so that generated professional thermal imaging pictures and videos can be resolved into finer temperature differences, meanwhile, the FOTRIC provides a lens with comprehensive types, different use occasions are met, the temperature measurement precision of +/-2 ℃ can be ensured, the FORTIC 225s thermal imager is utilized to record pictures and full-radiation thermal imaging videos, images are imported into FORTIC analytical IR analysis software, the temperature of each pixel point of each frame of image can be checked, and the parameters of the thermal imager are shown in the following table 1:
table 1 parameters of thermal imager
The strain gage adopts the following structureUniversal foil-type strain gauge with the number KFGS-1-120-D17-11N30C2, and measuring strain data of the strain gauge are strains in three different directions, namely epsilon 0° ，ε 45° ，ε 90° The basic parameters of the strain gauge are as shown as strain data epsilon collected by the strain gauge in the experiment 0° And epsilon 90° Strain data corresponding to the horizontal x direction and the vertical y direction of the PCB respectively pass through epsilon 45° The strain in any direction can be obtained.
The X-ray tomography microscope adopts an Xradia high-resolution three-dimensional microscope of Zeiss, germany, the model is Zeiss Xradia 520versa, the device has submicron resolution, an objective lens turntable for providing magnification, and the specific parameters of the device are shown in the table 2:
table 2,Zeiss Xradia 520versa basic parameters
| Tube voltage and power | 30kV-160kV，1W-10W |
| Objective lens | 0.4x，4x，20x，40x |
| Three-dimensional spatial resolution | 0.7μm |
| Resolution at 50nm working distance | 1.0μm |
| Four-axis sample stage | X，Y，Z，Theta |
| Maximum weight of sample | 25kg |
| Maximum size of sample | 300mm |
2.2 Experimental equipment was used to perform the experiments and collect data:
2.2.1 Voltage U of A2 welding spot in power-on process by utilizing data acquisition card A2 The change is detected:
pins B1, A2 and B2 of the chip are led out through leads, and B2 and A2 are respectively connected with the positive electrode and the negative electrode of a power supply to be used as power-on welding spots, and the power supply is regulated to constant current output 2A; and the leads of the B1 and the A2 are connected into a CH0 channel of the data acquisition card N19219 for voltage monitoring, and the electromigration failure cannot occur due to the fact that no current is fed into the welding spot of the B1, so that the reasons of the change of the measured voltage are all caused by the electromigration failure of the welding spot of the A2.
2.2.2 Detecting the temperature T change born by the circuit board by using a thermal infrared imager:
the thermal imager is fixed by a tripod, the shooting position is arranged right above the circuit board, and proper spacing and thermal imager parameters are set. The whole experimental process is carried out in a room with a constant temperature of 20 ℃, and the thermal image file is analyzed and processed by matching with the professional thermal imager analysis software FORTIC AnalyzIR, so that the temperature distribution of the experimental sample under the action of the current of 2A is obtained.
2.2.3 Using strain gauge to monitor strain at a substrate position of the board-level circuit near the chip:
the strain gauge sensor is connected into CH1, CH2 and CH3 channels of a data acquisition card to perform signal processing, and strain signals of 0 degree, 45 degrees and 90 degrees are acquired; the N19219 acquisition card is connected into a computer through an NI USB-compactDAQ chassis module outer box connector, labVIEW software is installed at a PC end to perform continuous acquisition and measurement of signals, the whole measurement system simultaneously acquires three strain and voltage four paths of signals, a data channel is added at a LabVIEW software interface to perform input and timing setting, the sampling rate, the sampling number and the sampling mode of each path of data are determined, and strain and voltage data of each channel can be checked in real time through a waveform chart and a table in the sampling process.
2.2.4 After the measurement is finished, processing the strain data of the substrate position of the board-level circuit close to the chip to obtain the linear strain epsilon in the alpha angle direction with the x axis α ：
The strain data obtained by sampling the strain gauge is micro-strain mu ε Because the board-level circuit contains initial strain, the strain of the strain gauge at the initial time of mounting is not 0, so that the acquired strain data needs to be preprocessed. Firstly, calculating the actual strain of a substrate, and subtracting the strain value at the initial moment from the strain value acquired in the experimental process; next, the micro-strain mu measured by the experiment ε The strain epsilon is converted into the linear strain epsilon, and the triaxial strain gage adopted in the experiment can measure the linear strain epsilon in the directions of 0 DEG, 45 DEG and 90 DEG 0° 、ε 45° And epsilon 90°
In the initial stage of energization, the thermal strain of the substrate increases rapidly, and energization is stopped after the voltage of the sample is observed to be 10% higher than the initial value, thereby obtaining ε 0° 、ε 45° And epsilon 90° The time-varying curve has stable strain amount, no obvious temperature change between the circuit board and environment, balanced test sample, and measured linear strain epsilon in the directions of 0 deg, 45 deg and 90 deg 0° 、ε 45° And epsilon 90° ；
Line strain epsilon according to three directions 0° 、ε 45° And epsilon 90° Respectively calculating horizontal x-direction strain epsilon of PCB x Strain epsilon in vertical y-direction y Shear strain gamma xy ：
According to the theory of strain state, epsilon x 、ε y And gamma xy Bringing the following to obtain the linear strain epsilon in the alpha-angle direction with the x-axis α ：
2.2.5 Using the voltage change of the A2 solder joint to determine the interface area when the solder joint fails:
in the process of electrifying, holes are formed in the welding spots, the cross-sectional area of the welding spots is changed due to the increase of the holes, so that the resistance value of the conductive solder is changed, and the voltage at two ends of the welding spots is changed due to the holes formed by electromigration to influence transmission, so that the cross-sectional area of the welding spots needs to be calculated, and the method is realized as follows:
first, the voltage U of the solder joint A2 is calculated according to ohm's law A2 ：
U A2 ＝R A2 ·I
Wherein I is constant current, and has values of 2A and R A2 The resistance value of the pad A2 is gradually increased by electromigration of the pad during energization, and when the resistance value of the pad is increased by 10%, the pad is determined to be failed.
Then, according to the proportional relation between the voltages at the two ends of the welding spot and the resistance, the voltage of the welding spot A2 is subjected to equation connection with ohm law according to the definition of the resistance, and the equation is expressed as:
wherein l is the length of the resistor; s is S A2 The resistivity ρ of the welding spot when the temperature is stable and the cross-sectional area of the welding spot is the same R And l is considered a constant value;
then, according to the inverse relation between the resistance and the cross-sectional area of the welding spot, a proportional relation expression between the welding spot voltage, the resistance and the cross-sectional area is established, and is expressed as:
in the method, in the process of the invention,and->Representing the initial electrical voltage, resistance and cross-sectional area of the solder joint;
finally, comparing the failure welding spot with the initial welding spot resistance before electrifying, when the welding spot resistance is increased by 10% compared with the welding spot resistance when the experiment is not started, obtainingAnd brings it into the inverse relation between the resistance and the cross-sectional area of the welding spot, and calculates: />
Interface area S when electromigration failure occurs in the welding spot A2 90.91% of the initial interface area is reduced by 9.09% compared with the initial area, so that the holes occupy 9.09% of the total area of the welding spot at the initial moment.
And step 3, CT scanning is carried out on the tested sample piece, and a two-dimensional gray level image of the sample is obtained.
3.1 Setting various parameters of CT scanning and adjusting the position of a sample, namely setting the scanning resolution to be 1.6 mu m/pt, using 60 kilovolts for the voltage of an X-ray tube, and selecting 15mm for the scanning field of view so as to ensure that the whole sample is scanned and ensure that the sample is stable, horizontal and vertical;
3.2 Starting the equipment to perform CT scanning, and rotating the sample for one circle to obtain X-ray projection data of the sample at different angles;
3.3 Re-sequencing and weighting the original projection data received by the CCD camera according to a set angle interval through a digital-analog converter to obtain cross-section data in three orthogonal directions;
3.4 Reconstructing the section data by using a reconstruction algorithm to obtain three-dimensional volume data;
the reconstruction algorithm is that the data obtained by the detector are converted into visual three-dimensional images, a bulb tube and the detector of CT scanning are collected around a sample, the bulb tube generates photons, the photons linearly pass through the sample, each material of the sample has different photon absorption capacities, the detector obtains residual photons, each angle generates a row of detector data, and finally three-dimensional data are obtained by combination;
3.5 Three-dimensional volume data is projected along three orthogonal directions to obtain 988 two-dimensional gray scale images of 984×1013 pixels, which as shown in fig. 2, include a chip, pads and Cu leads, wherein a and B are energized pads as main observation pads.
And 4, carrying out three-dimensional image reconstruction on the two-dimensional gray level image by utilizing Avizo software.
4.1 Using a non-sharpening masking algorithm Unsharp masking to perform sharpness and noise reduction pretreatment on the two-dimensional gray image:
opening Avizo software and importing a two-dimensional gray image;
selecting "Filter" in the "Segmentation Editor" toolbar;
selecting "Unsharp Masking" in a popup "Filter Selection" window;
adjusting parameters of Sigma and Weight in the Unsharp Masking to perform definition and noise reduction treatment on the parameters, and clicking an Apply button to complete pretreatment;
4.2 Using Top-hat operator to carry out contrast enhancement and threshold segmentation on the preprocessed two-dimensional gray image:
4.2.1 Determining a threshold boundary of the material based on the edge characteristics of the medium;
4.2.2 The holes are accurately qualitative by using a local brightness difference segmentation method integrated in software, and the operation is as follows:
selecting "Morphology" in the "Segmentation Editor" toolbar;
selecting "Top-Hat" in a pop-up "morphism window;
adjusting the Size parameter in Top-Hat to realize contrast enhancement and threshold segmentation, clicking an Apply button to Apply the operator to complete the contrast enhancement and threshold segmentation;
through the operation, the brightness of the background of the damaged welding spot and the color of the hole structure are adjusted, the main body part of the research can be highlighted, and the accuracy and the qualitation of the holes are realized.
4.3 Three-dimensional reconstruction is carried out on the image after threshold segmentation by utilizing Volume rendering to obtain a three-dimensional reconstruction visual image of electromigration damage inside a welding spot:
selecting "Volume" in the "Data" toolbar;
selecting "Rendering" in the pop-up "Volume" window;
adjusting a Threshold parameter in a Rendering window, and screening an object to be presented;
selecting a Volume tab, and adjusting an Opacity parameter and a Color Map parameter to realize three-dimensional reconstruction;
clicking the "Render" button creates a three-dimensional reconstructed visual image, i.e., a matrix model of the weld spot and a geometrical model of the hole defect are created, as shown in fig. 4.
And 5, carrying out classification statistics on the volume and the surface area of the holes in the three-dimensional reconstruction image in the welding spot by using Label Analysis, and determining the distribution characteristics of the holes damaged by electromigration.
5.1 Statistical analysis is carried out on the welding spots of electromigration damage, the three-dimensional volume and the surface area of the individual hole defects are counted, classification statistics is carried out according to the size of the hole volume and the surface area, and the statistical result of random distribution of the hole volume and the surface area is obtained, as shown in table 3:
TABLE 3 statistical results of random distribution of pore volume and surface area
| Pore volume range/μm 3 | Various pore volume ratios | Pore surface area range/. Mu.m 2 | Surface area number fraction |
| 0～350 | 79.29％ | 0～200 | 75％ |
| 350～700 | 7.86％ | 200～400 | 10.71％ |
| 700～1050 | 4.29％ | 400～600 | 2.14％ |
| 1050～1400 | 2.14％ | 600～800 | 4.29％ |
| 1400～1750 | 1.43％ | 800～1000 | 2.14％ |
| 1750～2100 | 0.71％ | 1000～1200 | 0.71％ |
| 2100～2450 | 2.14％ | 1200～1400 | 1.43％ |
| 2450～2800 | 0.71％ | 1400～1600 | 0.71％ |
| 2800～3150 | 0.71％ | 1600～1800 | 1.43％ |
| 3150～4420 | 0.71％ | 1800～2920 | 1.43％ |
5.2 Equivalent pore volume of all individual pores generated in the welds in table 3 as a pore system consisting of spherical pores, converting the statistical distribution of individual pore volumes into a statistical distribution of pore diameters, determining the equivalent diameter d of the pores from each pore volume V:
5.3 Counting the projection area of the individual hole defects according to the equivalent diameter d, and counting the total area of the hole defects of the projection surface to obtain the following three hole distribution characteristics:
first category: the diameter distribution of the holes is concentrated, and the maximum diameter can reach 21 mu m, as shown in figure 5;
the second category: the proportion distribution of the hole sizes is uneven, the number of small holes is more, most of the small holes are concentrated between 1 and 4 mu m, and the number of large holes with the diameter exceeding 10 mu m is obviously reduced, as shown in figure 5;
third category: the total area of the hole defects of the projection surface is 5445.67 mu m 2 The total area of the cathode interface was 41547.57 μm 2 The cathode interface pores account for 13.11%.
Comparing the experimental value of the cathode interface hole ratio with the theoretical value of the cathode interface hole ratio of 13.11% when the welding spot fails and the theoretical value of the cathode interface hole ratio of 9.09%, the experimental value is only 4.02% higher than the theoretical value, and the error is caused by the fact that the welding spot already contains 4.02% of holes when the welding spot is not electrified, so that the accuracy of the experiment is not affected.
The above description and examples are given by way of illustration only of preferred embodiments of the invention and are not to be construed as limiting the invention in any way, since various modifications and changes in form and detail are possible for those skilled in the art upon attaining an understanding of the principles and principles of the invention, and yet remain within the scope of the present claims.
Claims (10)
1. The method for determining the distribution characteristics of the electromigration damage holes of the welding spots based on CT imaging is characterized by comprising the following steps of:
(1) Preparing a test sample according to industry standard standards;
(2) Electromigration experiments are carried out on the test sample, and a thermal imager, a strain gauge and a data acquisition system are utilized to respectively obtain the temperature T, the strain epsilon and the voltage U of the welding spot A2 of the test sample in the experiments A2 ；
(3) CT scanning is carried out on the tested sample piece, and a two-dimensional gray level image of the sample is obtained;
(4) Three-dimensional image reconstruction is carried out on the two-dimensional gray level image by utilizing Avizo software:
4a) Performing definition and noise reduction pretreatment on the two-dimensional gray level image by using a non-sharpening masking algorithm Unsharp masking;
4b) Performing contrast enhancement and threshold segmentation on the processed two-dimensional gray image by using a Top-hat operator;
4c) Performing three-dimensional reconstruction on the image after threshold segmentation by utilizing Volume rendering to obtain a three-dimensional reconstruction visual image of electromigration damage in a welding spot;
(5) And classifying and counting the volume and the surface area of the holes in the three-dimensional reconstruction image in the welding spot by using Label Analysis, determining the distribution characteristics of the holes damaged by electromigration, and providing data for determining the unstable area of the welding spot in the mechanical structure.
2. The method of claim 1, wherein the test sample prepared according to the industry standard in the step (1) is formed by using lead-free Sn-Ag-Cu as solder joint material on a monolithic PCB board with a size of 95 x 1mm, coating solder resist around the solder joint, selecting a six-point metal oxide chip based on a P-channel field effect transistor with a basic size of 1.48 x 0.98 x 0.145mm and a solder ball diameter of 0.26mm, interconnecting the chip and the circuit of the package structure through metal wiring, and forming copper leads with a thickness of 0.02mm and a width of 0.24mm through deposition lithography.
3. The method of claim 1, wherein the electromigration experiment of step (2) is performed as follows:
firstly setting working current, attaching foil strain gauges at specified positions, respectively connecting two pins B2 and A2 of a sample chip with the anode and the cathode of a power supply, connecting the pins and the strain gauges to a data acquisition system, turning on the power supply, capturing the temperature T of the sample in real time by using a thermal imager and carrying out analysis treatment by matching with professional thermal imager analysis software FORTIC AnalyzIR;
in the process of electrifying, the increase of the holes can cause the change of the cross section area of the welding spot A2, the resistance value of the welding spot is changed, the evolution rule of the holes is reflected through the voltage change at the two ends of the welding spot, and therefore the failure time of the welding spot is obtained.
4. The method of claim 1, wherein the strain ε of the test piece is measured in step (2) using a strain gauge,
based on the measured strain results in the directions of 0 degree, 45 degree and 90 degree, the plane strain epsilon in the three directions is completed by the following method x 、ε y And gamma xy Is calculated by (1):
wherein ε 0° ，ε 45° ，ε 90° Respectively, strain data, epsilon measured in three directions of the sample x 、ε y And gamma xy Is plane strain.
5. The method of claim 1, wherein the step (2) uses the data acquisition system to test the sample for the experimental solder joint A2 voltage U A2 The expression is as follows:
wherein R is A2 The resistance of the welding spot is that I is the energizing current, and the resistivity ρ of the welding spot R And the resistance length l is regarded as a constant value, S A2 The resistance and cross-sectional area S of the welding spot A2 are the cross-sectional area of the welding spot A2 Inversely proportional relation, namely:
in the middle ofAnd->Represents the initial voltage, resistance and cross-sectional area of the solder joint A2, when the resistance of the solder joint A2 is increased by 10% from the initial value, ">Bringing it into the inverse relationship to obtain +.>That is, when the electromigration failure occurs in the welding spot A2, the contact area between the welding spot A2 and the welding pad is reduced to 90.91% of the initial area, and the voltage of the welding spot A2 is increased by 10% compared with the initial voltage.
6. The method according to claim 1, wherein in the step (3), the sample after the test is subjected to CT scanning to obtain two-dimensional gray scale image data of the sample, and the following steps are implemented:
3a) Setting the resolution of CT scanning to be 1.6 mu m/pt, using 60 kilovolts for the X-ray tube voltage, selecting 15mm for the scanning visual field to ensure that the whole sample is scanned, and ensuring the stability, the horizontal and the vertical of the sample;
3b) Starting equipment to perform CT scanning, rotating a sample for one circle, acquiring X-ray projection data of the sample under different angles, and re-sequencing and weighting original projection data according to set angle intervals by a digital-analog converter on two-dimensional projection data received by a CCD camera to obtain section data in three orthogonal directions;
3c) Reconstructing the section data by using a reconstruction algorithm to obtain three-dimensional volume data;
3d) And projecting the three-dimensional data along three orthogonal directions to obtain three views of a two-dimensional gray level image of the sample, and clearly observing damage caused by electromigration in the welding spot.
7. The method according to claim 1, wherein the sharpness and noise reduction preprocessing of the two-dimensional gray-scale image in step 4 a) is performed by using a non-sharpening masking algorithm, namely, un-sharpening masking, as follows:
4a1) Opening Avizo software and importing a two-dimensional gray image;
4a2) Selecting "Filter" in the "Segmentation Editor" toolbar;
4a3) Selecting "Unsharp Masking" in a popup "Filter Selection" window;
4a4) The parameters "Sigma" and "Weight" are adjusted in the Unsharp mask to perform sharpness and noise reduction processing on the parameters, and the pretreatment is completed by clicking the "Apply" button.
8. The method according to claim 1, wherein the step 4 b) uses a Top-hat operator to perform contrast enhancement and thresholding on the processed two-dimensional gray scale image, and is implemented as follows:
4b1) Selecting "Morphology" in the "Segmentation Editor" toolbar;
4b2) Selecting "Top-Hat" in a pop-up "morphism window;
4b3) The "Size" parameter is adjusted in Top-Hat to achieve contrast enhancement and threshold segmentation, and clicking the "Apply" button applies the operator to complete contrast enhancement and threshold segmentation.
9. The method according to claim 1, wherein the three-dimensional reconstruction of the thresholded image using Volume rendering in step 4 c) is performed as follows:
4c1) Selecting "Volume" in the "Data" toolbar;
4c2) Selecting "Rendering" in the pop-up "Volume" window;
4c3) Adjusting a Threshold parameter in a Rendering window, and screening an object to be presented;
4c4) Selecting a Volume tab, and adjusting an Opacity parameter and a Color Map parameter to realize three-dimensional reconstruction;
4c5) Clicking on the "Render" button generates a three-dimensional reconstructed visual image.
10. The method of claim 1, wherein determining the pore distribution characteristics of the electromigration damage in step (5) is performed by:
5a) Counting the three-dimensional volume and the surface area of the individual hole defects, and carrying out classification statistics according to the size of the hole volume and the surface area;
5b) All individual holes generated in the welding spots are equivalent to a pore system consisting of spherical holes, the statistical distribution of the volumes of the individual holes is converted into the statistical distribution of the pore diameters, and the equivalent diameter d of the holes is determined:
wherein V is the pore volume;
5c) And counting the projection area of the individual hole defects according to the equivalent diameter, and counting the total area of the hole defects of the projection surface to obtain the hole distribution characteristics that the cathode interface cavity ratio is 13.11% and the matrix ratio is 86.89%.
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| CN117274365B (en) * | 2023-11-22 | 2024-02-13 | 武汉罗博半导体科技有限公司 | Wafer solder ball diameter detection method, device, equipment and storage medium |
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