Radiation Imaging Techniques Research Articles

Microstructure and growth kinetic study in Sn–Cu transient liquid phase sintering solder paste

The feasibility of the highly reliable and replicable microstructure formation of the transient liquid phase sintering (TLPS) paste during the early soldering and isothermal aging on the Cu substrate had been successfully investigated in this study. By using the Sn–0.7 wt% Cu (SC) solder paste as the base material and the Cu particles in the production of the TLPS Sn–10 wt% Cu (SC10) solder paste, the ensuing Cu6Sn5 phase from the isothermal aging process was found to have reduced the β-Sn area of the bulk SC10 solder microstructure. The growth kinetic for TLPS SC10 resulted in a 26.76 kJ/mol of activation energy level. The real-time synchrotron radiation imaging technique that was employed in studying the growth and formation of the primary intermetallic phases at the solder joints had also discovered the primary intermetallic in TLPS SC10 was not only found to have experienced an early nucleation just after the solder had melted, but its growth was also restricted prior to the solidification of the liquid solder. Therefore, the relevance of the results that were obtained from this research may offer a possible solution for aiding the future development of highly reliable solder joints in high temperature solder applications.

Practice patterns for the radical treatment of nasopharyngeal cancer by head and neck oncologists in the United Kingdom.

Objective:Advances in radiation delivery, imaging techniques, and chemotherapy have significantly improved treatment options for non-metastatic nasopharyngeal cancers (NPC). However, their impact on the practice in the United Kingdom (UK), where this tumour is rare, is unknown. This study examined the current attitudes of UK head and neck oncologists to the treatment of NPC.Methods:UK head and neck oncologists representing 19/23 cancer networks were sent an invitation email with a personalised link to a web-based survey designed to identify the influence of tumour and nodal staging on current NPC management practices.Results:26/42 (61%) of clinicians responded. Induction chemotherapy followed by concomitant chemoradiation was the treatment of choice for Stage III (69%) and IVa/b (96%), with cisplatin and 5-fluorouracil combination being the most commonly used induction chemotherapy regimen (88%). 16 centres (61%) used a geometric approach, adding variable margins of 0–10 mm to the gross tumour volume to define their therapeutic dose clinical target volume. 54% of respondents used 3 radiotherapy (RT) prescription doses to treat NPC. Retropharyngeal nodal region irradiation policy was inconsistent, with nearly one-quarter treating the entire group to a radical dose.Conclusion:Significant heterogeneity currently exists in the RT practice of NPC in the UK. A consensus regarding the optimal curative, function-sparing treatment paradigm for NPC is necessary to ensure cancer survivors have satisfactory long-term health-related quality of life.Advances in knowledge:This is the first study to highlight the significant variation in RT practice of NPC in the UK.

First attempt to visualize otolith motion in-situ using synchrotron radiation imaging techniques

Regarding the basics of ear structure-function relationships in fishes, there is still a substantial lack of knowledge of functional morphology. In particular, the actual motion of the solid otolith relative to the underlying sensory epithelium in the ear has rarely been investigated. To date, analyses of otolith motion have been mainly based on mathematical modeling which have yielded conflicting results. Outcomes of a recent modeling study suggest that otolith motion is not a simple oscillation, but depends on the 3D shape of the otolith, among other factors. Our study thus aims to provide experimental data to test previous models of otolith motion focusing on the relationship between different species-specific otolith shapes and their respective influence on otolith motion. As in-situ investigation of the basic parameters of otolith motion requires an approach with high spatial and temporal resolution, we used synchrotron radiation imaging techniques. In our comparative approach including the anatomically well studied and closely related cichlid species Steatocranus tinanti (vestigial swimbladder) and Etroplus maculatus (swimbladder-inner ear connection), we studied the relative motion of the saccular otolith and surrounding tissues provoked by sound stimuli at 0.2 and 0.5 kHz. We will discuss first results as well as methodological aspects.

A Monte Carlo simulation for the radiation imaging technique based on the Hemispherical Rotational Modulation Collimator (H-RMC)

The Rotational Modulation Collimator (RMC) is a simple and versatile tool for the radiation imaging system with low cost, makes it a reasonable selection for locating and tracking nuclear materials and radiation sources. In this paper, Monte Carlo simulation-based design studies for an alternative RMC which has an extended field-of-view will be presented. Modulation patterns for 5 different hemispherical RMC (H-RMC) designs were simulated for various source locations, and fundamental characteristics of rotational modulation patterns were investigated. Obtained patterns showed variations depending on the source location for most of the H-RMC designs, exhibiting promises for the future development of an omni-directional radiation imager based on a non-position sensitive radiation detector.

Microstructure and strength of AlN–SiC interface studied by synchrotron X-rays

Bulk AlN crystals grown by sublimation on SiC substrates exhibit relatively high dislocation densities. The kind of defect formation at early growth stages influences the structural quality of the grown crystals. In this work, the dislocation distribution near to the interface is understood through investigation of thin (≤1.5 mm) continuous (non-cracked) freestanding crystals obtained in one process with the evaporation of the substrates. The AlN specimens were characterized using synchrotron radiation imaging techniques. We revealed by triple-axis X-ray diffraction study that, near to the former interface, randomly distributed dislocations configured to form boundaries between \(\sim \)0.02\(^{\circ }\) misoriented sub-grains (from [0001] direction). Threading dislocation structure similar to that in epitaxial GaN films was not detected. To explain these observations, a theoretical model of misfit stress relaxation near the interface is suggested.

The differential diagnosis of benign and malignant ovarian neoplasms (history)

The history of the differential diagnosis of ovarian neoplasms at the preoperative stage is presented in the review article. This is still a problem due to the high incidence of tumors and tumor-like formations of ovaries, the continuing upward trend in the index of ovarian cancer, along with a slight decrease in mortality and 5-year survival. Considerable success of surgical treatment and chemotherapy in the early stages of ovarian cancer suggests the earliest possible detection and accuracy of differential diagnosis of ovarian neoplasms. It highlights the role of the different research methods, including radiation imaging techniques, tumor markers and their potential advantages and disadvantages.

Sci‐Fri PM: Radiation Therapy, Planning, Imaging, and Special Techniques ‐ 06: Patient‐specific QA Procedure for Gated VMAT SABR Treatments using 10x Beam in Flattening‐Filter Free Mode

Purpose:To develop a highly sensitive patient specific QA procedure for gated VMAT stereotactic ablative radiotherapy (SABR) treatments.Methods:A platform was constructed to attach the translational stage of a Quasar respiratory motion phantom to a pinpoint ion chamber insert and move the ion chamber inside the ArcCheck. The Quasar phantom controller uses a patient‐specific breathing pattern to translate the ion chamber in a superior‐inferior direction inside the ArcCheck. With this system the ion chamber is used to QA the correct phase of the gated delivery and the ArcCheck diodes are used to QA the overall dose distribution. This novel approach requires a single plan delivery for a complete QA of a gated plan. The sensitivity of the gating QA procedure was investigated with respect to the following parameters: PTV size, exhale duration, baseline drift, gating window size.Results:The difference between the measured dose to a point in the penumbra and the Eclipse calculated dose was under 2% for small residual motions. The QA procedure was independent of PTV size and duration of exhale. Baseline drift and gating window size, however, significantly affected the penumbral dose measurement, with differences of up to 30% compared to Eclipse.Conclusion:This study described a highly sensitive QA procedure for gated VMAT SABR treatments. The QA outcome was dependent on the gating window size and baseline drift. Analysis of additional patient breathing patterns is currently undergoing to determine a clinically relevant gating window size and an appropriate tolerance level for this procedure.

Sci‐Fri PM: Radiation Therapy, Planning, Imaging, and Special Techniques ‐ 04: Assessment of intra‐fraction motion during lung SABR VMAT using a custom abdominal compression device

Purpose:Lung SABR patients are treated using Volumetrically Modulated Arc Therapy (VMAT), utilizing 2 arcs with Conebeam CT (CBCT) image‐guidance prior to each arc. Intra‐fraction imaging can prolong treatment time (up to 20%), and the aim of this study is to determine if it is necessary.Methods:We utilize an in‐house abdominal compression device to minimize respiratory motion, 4DCT to define the ITV, a 5 mm PTV margin and a 2–3 mm PRV margin. We treated 23 patients with VMAT, fifteen were treated to 48 Gy in 4 fractions, while eight were treated with up to 60 Gy in 8 fractions. Intrafraction motion was assessed by the translational errors recorded for the second CBCT.Results:There was no significant difference (t‐test, p=0.93) in the intra‐fraction motion between the patients treated with 4 and 8 fractions, or between the absolute translations in each direction (ANOVA, p=0.17). All 124 intra‐fraction CBCT images were analysed and 95% remained localized within the 5 mm PTV margin The mean magnitude of the vector displacement was 1.8 mm.Conclusions:For patients localized with an abdominal compression device, the intrafraction CBCT image may not be necessary, if it is only the tumor coverage that is of concern, as the patients are typically well within the 5 mm PTV margin. On the other hand, if there is a structure with a smaller PRV margin, an intrafraction CBCT is recommended to ensure that the dose limit for the organ at risk is not exceeded.

Sci‐Fri PM: Radiation Therapy, Planning, Imaging, and Special Techniques ‐ 08: Retrospective Dose Accumulation Workflow in Head and Neck Cancer Patients Using RayStation 4.5.2

Purpose:We have developed a semi‐automated dose accumulation workflow for Head and Neck Cancer (HNC) patients to evaluate volumetric and dosimetric changes that take place during radiotherapy. This work will be used to assess how dosimetric changes affect both toxicity and disease control, hence inform the feasibility and design of a prospective HNC adaptive trial.Methods:RayStation 4.5.2 features deformable image registration (DIR), where structures already defined on the planning CT image set can be deformably mapped onto cone‐beam computed tomography (CBCT) images, accounting for daily treatment set‐up shifts and changes in patient anatomy. The daily delivered dose can be calculated on each CBCT and mapped back to the planning CT to allow dose accumulation. The process is partially automated using Python scripts developed in collaboration with RaySearch.Results:To date we have performed dose accumulation on 18 HNC patients treated at our institution during 2013–2015 under REB approval. Our semi‐automated process establishes clinical feasibility. Generally, dose accumulation for the entire treatment course of one case takes 60–120 minutes: importing all CBCTs requires 20–30 minutes as each patient has 30 to 40 treated fractions; image registration and dose accumulation require 60–90 minutes. This is in contrast to the process without automated scripts where dose accumulation alone would take 3–5 hours.Conclusions:We have developed a reliable workflow for retrospective dose tracking in HNC using RayStation. The process has been validated for HNC patients treated on both Elekta and Varian linacs with CBCTs acquired on XVI and OBI platforms respectively.

Sci‐Fri PM: Radiation Therapy, Planning, Imaging, and Special Techniques ‐ 07: Transitioning from extended‐distance total body irradiation to optimized VMAT total marrow irradiation

Purpose:TMI targets only the bone marrow, with the intent of sparing normal tissues. The NSCC has recently implemented a TMI protocol which includes VMAT fields to treat the bone marrow from head to mid‐thigh and extended SSD POP fields to treat the lower legs. This work describes the commissioning and initial clinical results of the first reported VMAT TMI treatments in Canada.Methods:Detailed CT simulation, imaging, planning and treatment procedures were developed by a multi‐disciplinary team. Patients have 1 cm of bolus over the lower legs and 0.5 cm of bolus around the lower arms. The PTV includes all bone, except mandible, facial bones and hands, with the objective of V(12 Gy) > 90%. Detailed analysis of the influence of field overlap was performed to determine optimal field placement and image‐guidance tolerances.Results:PTV coverage was achieved for all cases as V(12 Gy) ranged from 90.4–96.3%. The minimum dose to the PTV, D(99%), ranged from 91.4–97.87% and V(90%Rx=10.8 Gy) ranged from 99.1–100.0%. The lungs, liver and heart had an average Dmean of (7.8±0.3)Gy/(65±2)%, (7.6±0.7)Gy/(63±5)%, and (6.8±0.4)Gy/(56±4)% respectively.Conclusions:Commissioning required input and collaboration from all team members. Transitioning from TBI to TMI requires additional time for contouring, treatment planning, QA, and treatment. Patient benefit can however be seen in the quality of OAR sparing.

Sci‐Fri PM: Radiation Therapy, Planning, Imaging, and Special Techniques ‐ 02: Feasibility of using multileaf collimation for stereotactic radiosurgery of arteriovenous malformation

SRS using linac and cones offers steep dose fall‐off but a tradeoff exists between conformality and treatment time, which depends on the number of isocentres. Purpose of this study is to quantify planning metrics between cones‐ and MLC‐based SRS for arteriovenous malformation(AVM).Seven AVM cases treated with cones were re‐planned with MLC on Pinnacle treatment planning system. Planning target volume(PTV) was created with 1mm uniform margin to the AVM to account for MLC positional variation. Clinically‐planned prescription dose(15–25Gy) was used. Four plans were generated per case:non‐coplanar VMAT(ncV), single‐arc VMAT(saV), non‐coplanar IMRT(ncI), non‐coplanar conformal(ncC). Plans were compared for conformity(CI), heterogeneity(HI) and gradient(GI) indices and brain doses. Estimated treatment times and monitor units(MU) were compared.Cone‐based plans required 2–6 isocentres. Though CI‐RTOG was similar for plans(median=0.98), CI‐Paddick was most favourable for ncV(median=0.86) and worst for cones(0.54). HI for MLC plans(median=1.19–1.27) were lower than cone‐based plans(1.43). GI was similar for all plans. For 2/7 ncC had brainstem maximum dose>16.7Gy and therefore were clinically unacceptable. Brain V12Gy,V10Gy,V2Gy were lowest in the cones plan. ncV brain V12Gy,V10Gy,V2Gy were lowest of all MLC‐based plans studied. Treatment MUs were similar for MLC‐based plans and up to 70% lower than clinically delivered plans.ncV showed best conformality in this study. Of the MLC‐based plans, ncV also showed lowest normal tissue dose with reasonable treatment time.

Sci‐Fri PM: Radiation Therapy, Planning, Imaging, and Special Techniques ‐ 09: Impact of the distance of reflective markers from linac isocenter on the positional accuracy of an infrared tracking system

Purpose:The HexaPOD™ six degree of freedom couchtop is equipped with an optical tracking system, consisting of a stereoscopic camera and a reference frame (RF) carrying infrared reflective markers. The manufacturer recommends placing the RF within 50 cm from linac isocenter (ISO), which is a serious limitation since the RF does not fit around the shoulders of most brain patients. This study quantifies the impact of extended RF distances from ISO on positional accuracy.Methods:An in‐house tool with an estimated resolution of 0.3 mm and 0.1° was used. It is a large cube and a mathematical model of HexaPOD motion to determine the intersection of room lasers with the ruled cube edges. Combinations of translations (±1 and ±3 cm) and rotations (±2.5°) were executed on two HexaPOD couchtops for multiple RF distances from ISO (35 to 77 cm). For each combination, ten laser readings were fed into a least squares algorithm to determine the executed translations and rotations while minimizing operator reading errors.Results:The usable tracking volume is up to an RF distance of 82 cm from ISO. Positional accuracy of the HexaPOD/iGuide system is 0.6 mm and 0.1° (95% confidence). Positional accuracy variations versus RF distance from ISO are statistically insignificant (p = 0.05). Our results generally confirm recent internal estimates by the manufacturer (for future release).Conclusions:RF distances up to 77 cm from ISO are clinically acceptable, provided performing a patient safety study with a verification scan.

Sci‐Fri PM: Radiation Therapy, Planning, Imaging, and Special Techniques ‐ 11: Quantification of chest wall motion during deep inspiration breast hold treatments using cine EPID images and a physics based algorithm

Purpose:This work presents an algorithm used to quantify intra‐fraction motion for patients treated using deep inspiration breath hold (DIBH). The algorithm quantifies the position of the chest wall in breast tangent fields using electronic portal images.Methods:The algorithm assumes that image profiles, taken along a direction perpendicular to the medial border of the field, follow a monotonically and smooth decreasing function. This assumption is invalid in the presence of lung and can be used to calculate chest wall position. The algorithm was validated by determining the position of the chest wall for varying field edge positions in portal images of a thoracic phantom. The algorithm was used to quantify intra‐fraction motion in cine images for 7 patients treated with DIBH.Results:Phantom results show that changes in the distance between chest wall and field edge were accurate within 0.1 mm on average. For a fixed field edge, the algorithm calculates the position of the chest wall with a 0.2 mm standard deviation. Intra‐fraction motion for DIBH patients was within 1 mm 91.4% of the time and within 1.5 mm 97.9% of the time. The maximum intra‐fraction motion was 3.0 mm.Conclusions:A physics based algorithm was developed and can be used to quantify the position of chest wall irradiated in tangent portal images with an accuracy of 0.1 mm and precision of 0.6 mm. Intra‐fraction motion for patients treated with DIBH at our clinic is less than 3 mm.

Sci‐Fri PM: Radiation Therapy, Planning, Imaging, and Special Techniques ‐ 10: Results from Canada Wide Survey on Total Body Irradiation Practice

Purpose:Total Body Irradiation (TBI) is delivered to a relatively small number of patients with a variety of techniques; it has been a challenge to develop consensus studies for best practice. This survey was created to assess the current state of TBI in Canada.Methods:The survey was created with questions focusing on the radiation prescription, delivery technique and resources involved. The survey was circulated electronically to the heads of every clinical medical physics department in Canada. Responses were gathered and collated, and centres that were known to deliver TBI were urged to respond.Results:Responses from 20 centres were received, including 12 from centres that perform TBI. Although a variety of TBI dose prescriptions were reported, 12 Gy in 6 fractions was used in 11 centres while 5 centres use unique prescriptions. For dose rate, a range of 9 to 51 cGy/min was reported.Most centres use an extended SSD technique, with the patient standing or lying down against a wall. The rest use either a “sweeping” technique or a more complicated multi‐field technique. All centres but one indicated that they shield the lungs, and only a minority shield other organs.The survey also showed that considerable resources are used for TBI including extra staffing, extended planning and treatment times and the use of locally developed hardware or software.Conclusions:This survey highlights that both similarities and important discrepancies exist between TBI techniques across the country, and is an opportunity to prompt more collaboration between centres.

Sci‐Fri PM: Radiation Therapy, Planning, Imaging, and Special Techniques ‐ 03: The Potential Benefit Of Esophageal Sparing During Palliative Radiotherapy For Lung Cancer

Puropose:Palliative radiotherapy is an effective technique to alleviate systems of disease burden in late‐stage lung cancer patients. Previous randomized controlled studies demonstrated a survival benefit in patients with good performance status at radiation doses of 35Gy10 or greater but with an increased incidence of esophagitis. The objective of this planning study was to assess the potential impact of esophageal‐sparing IMRT (ES‐IMRT) compared to the current standard of care using parallel‐opposed pair beams (POP).Methods:In this study, 15 patients with lung cancer treated to a dose of 30Gy in 10 fractions between August 2015 and January 2016 were identified. Radiation treatment plans were optimized using ES‐IMRT by limiting the max esophagus point dose to 24Gy. Using published Lyman‐Kutcher‐Burman normal tissue complication probabilities (LKB‐NTCP) models, both plans were evaluated for the likelihood of esophagitis (≥ grade 2) and pneumonitis (≥ grade 2).Results:Using ES‐IMRT, the median esophageal and lung mean doses reduced from 16 and 8Gy to 7 and 7Gy, respectively. Using the LKB models, the theoretical probability of symptomatic esophagitis and pneumonitis reduced from 13 to 1%, and from 5 to 3%, respectively. The median NTD mean for the GTV and PTV of the clinically approved POP plans compared to the ES‐IMRT plans were similar.Conclusions:Advanced radiotherapy techniques such as ES‐IMRT may have clinical utility in reducing treatment‐related toxicity in advanced lung cancer patients. Our data suggests that the rate of esophagitis can be reduced without compromising tumour control.

Sci‐Fri PM: Radiation Therapy, Planning, Imaging, and Special Techniques ‐ 05: A novel respiratory motion simulation program for VMAT treatment plans: a phantom validation study

Purpose:To develop and validate a computational method to simulate craniocaudal respiratory motion in a VMAT treatment plan.Methods:Three 4DCTs of the QUASAR respiratory motion phantom were acquired with a 2cm water‐density spherical tumour embedded in cedar to simulate lung. The phantom was oscillating sinusoidally with an amplitude of 2cm and periods of 3, 4, and 5 seconds. An ITV was contoured and 5mm PTV margin was added. High and a low modulation factor VMAT plans were created for each scan. An in‐house program was developed to simulate respiratory motion in the treatment plans by shifting the MLC leaf positions relative to the phantom. Each plan was delivered to the phantom and the dose was measured using Gafchromic film. The measured and calculated plans were compared using an absolute dose gamma analysis (3%/3mm).Results:The average gamma pass rate for the low modulation plan and high modulation plans were 91.1% and 51.4% respectively. The difference between the high and low modulation plans gamma pass rates is likely related to the different sampling frequency of the respiratory curve and the higher MLC leaf speeds in the high modulation plan. A high modulation plan has a slower gantry speed and therefore samples the breathing cycle at a coarser frequency leading to inaccuracies between the measured and planned doses.Conclusion:A simple program, including a novel method for increasing sampling frequency beyond the control point frequency, has been developed to simulate respiratory motion in VMAT plans by shifting the MLC leaf positions.

Sci‐Fri PM: Radiation Therapy, Planning, Imaging, and Special Techniques ‐ 01: On the use of proton radiography to reduce beam range uncertainties and improve patient positioning accuracy in proton therapy

To present two related developments of proton radiography (pRad) to minimize range uncertainty in proton therapy. The first combines a pRad with an X‐ray CT to produce a patient‐specific relative stopping power (RSP) map. The second aims to improve the pRad spatial resolution for accurate registration prior to the first. The enhanced‐pRad can also be used in a novel proton‐CT reconstruction algorithm.Monte Carlo pRad were computed from three phantoms; the Gammex, the Catphan and an anthropomorphic head. An optimized cubic‐spline estimator derives the most likely path. The length crossed by the protons voxel‐by‐voxel was calculated by combining their estimated paths with the CT. The difference between the theoretical (length×RSP) and measured energy loss was minimized through a least squares optimization (LSO) algorithm yielding the RSP map. To increase pRad spatial resolution for registration with the CT, the phantom was discretized into voxels columns. The average column RSP was optimized to maximize the proton energy loss likelihood (MLE).Simulations showed precise RSP (<0.75%) for Gammex materials except low‐density lung (<1.2%). For the head, accurate RSP were obtained (µ=−0.10%1.5σ=1.12%) and the range precision was improved (ΔR80 of −0.20±0.35%). Spatial resolution was increased in pRad (2.75 to 6.71 lp/cm) and pCT from MLE‐enhanced pRad (2.83 to 5.86 lp/cm).The LSO decreases the range uncertainty (R80σ<1.0%) while the MLE‐enhanced pRad spatial resolution (+244%) and is a great candidate for pCT reconstruction.

On the increase of intermetallic compound's thickness at the cold side in liquid Sn and SnAg solders under thermal gradient

The thermal gradient induced intermetallic compound growth at the cold side of the Cu/Sn/Cu and Cu/Sn–3.5Ag/Cu solders heated at 350° has been in situ studied using synchrotron radiation imaging technique. Finite element model for thermotransport of Cu from hot end to cold end in Cu/Sn/Cu solder is implemented for the description of the associated growth of the compound. Higher temperature and bigger solder volume are observed as favorable parameters for the rate of thickness increase in both Sn and SnAg solders. In case of SnAg system, the formation of Ag3Sn particle, causing retardation effects in the growth of Cu6Sn5 compound, subsequently induces a lowered resulting thickness.

Direction-resolved radiation from polarization-controlled surface plasmon modes on silver nanowire antennas.

Metallic nanowires (NWs) support multiple surface plasmon (SP) modes, which lead to extraordinary SP propagation behaviors. The leaky SP modes in metallic NWs connect the guiding and radiation of light at the nanometer scale. Understanding and controlling these modes are of vital importance for various nanophotonic applications. Here, we investigate the radiation from two polarization-controlled SP modes on supported silver NWs by using leakage radiation imaging and Fourier imaging techniques. The radiation directions from these modes can be clearly resolved from the Fourier images. The radiation polarization of the SP modes is related to the polarization of the excitation light. By depositing thin Al2O3 films onto silver NWs or decreasing the excitation wavelength, the radiation angles and wave vectors of the two modes are increased, and the longitudinal mode is more sensitive to Al2O3 thickness. Moreover, the propagation length of the longitudinal mode is obtained by analyzing the leakage radiation images, which is decreased with the decrease of the excitation wavelength and the increase of the Al2O3 layer thickness. These results show that leakage radiation from different SP modes on silver NWs can be resolved directly and controlled effectively. The supported silver NWs can thus be applied to designing plasmonic circuits, nanoantennas and nanosensors.

Modeling the diffusion-driven growth of a pre-existing gas bubble in molten tin

Finite element method is utilized to solve the diffusion equation and model the diffusion driven growth of a pre-existing spherical gas bubble in molten tin at the solder/substrate interface for reflow time of 120 s and temperature of 250 °C. The gibbs free energy change required for determining the equilibrium concentration at liquid solder/gas bubble boundary was calculated using the thermodynamic polynomial coefficients. The rate of change of radius, as function of concentration flux, is calculated using the lagrangian mesh update methodology. With an initial diameter of 20 μm, the bubble growth is calculated as a function of contact angle. When the wetting angle is varied from a value of 30° to 135°, the numerical calculation has yielded the final sizes for the bubble to change from 62.87 μm to 82.8 μm respectively. The effect of wetting transition in the growth of bubble was studied by the in-situ observation of bubble dynamics through synchrotron radiation imaging technique. The scanning electron microscopy images of the morphologies of intermetallic compounds influenced by growing bubble in Sn/Cu solder joint and bubble pictures obtained through synchrotron radiation are utilized to get the experimental size of the bubble. The mean experimental bubble diameter has been obtained as 76.39 μm. The growing bubble inhibits the growth of intermetallic compound at its vicinity and thereby reduces the strength of solder joints.