Reliability and prognostic monitoring methods of electronics interconnections in advanced SMD applications

Abstract In the interest of improving reliability, electrical monitoring methods were utilized to observe the degradation of electronics interconnections while simultaneously characterizing accelerated testing-induced changes in test structures by means of optical examination, X-ray, scanning acoustic microscopy and scanning electron microscopy. To improve the accuracy of lifetime prediction for the PCSB interconnections investigated in this work, a modified Engelmaier’s solder joint lifetime prediction model was recalibrated. The results show that with most of the presented lead-free (SAC387, SAC405, SAC-In) solder and structure combinations with a large global thermal mismatch (ΔCTE > 10 ppm/°C), lifetime was adequate in the presented TCT ranges of 0‒100 °C and −40‒125 °C, while the amount of non-preferred crack types, i.e. ceramic cracks, was minimized. Degradation of interconnections was characterized using RF measurements both during TCT and intermittently during TCT breaks. A grounded coplanar waveguide was arranged either in a straight back-to-back configuration or together with a filter module with a passband at 22‒24 GHz—both with two transitions—and characterized during cycling breaks up to 25 GHz and 30 GHz, respectively. Besides off-cycle measurements, in-cycle measurements were done on an antenna structure with an in-band at 10‒11 GHz, up to 14 GHz. The results show that the signal response was initially affected at some frequencies as short-duration (< 1 s) glitches in the monitored signal when measured during cycling in 0‒100 °C TCT. Later on the degradation could be observed in the whole frequency band as TCT was continued. Development of the semi-empirical lifetime prediction model for PCSB interconnections showed the temperature range dependency of the correction term to be a second order polynomial instead of a logarithmic one. For components with PCSB BGA, promising prediction results were achieved which differed from the realized lifetime by less than 0.5% at best.Tiivistelmä Elektroniikkaliitosten rikkoontumisen seurantaan tarkoitettuja sähköisiä monitorointimenetelmiä kehitettiin samanaikaisesti karakterisoimalla testauksella liitoksiin aikaansaatuja muutoksia optisesti, akustisella mikroskoopilla sekä röntgen- ja pyyhkäisyelektronimikroskoopeilla. Liitosten eliniän ennustamiseen soveltuva muokattu Engelmaierin malli kalibroitiin PCSB-liitosten elinikäennusteen tarkkuuden parantamiseksi. Tulosten perusteella useimmille tässä työssä käytetyille lyijyttömille (SAC387, SAC405, SAC-In) juotteille ja suuren termisen epäsovituksen (ΔCTE > 10 ppm/°C) rakenneyhdistelmille eliniät lämpösyklaustesteissä 0‒100 °C ja −40‒125 °C alueilla olivat riittävät ja haitallisimpien murtumien, eli keraamimurtumien, määrä saatiin minimoiduksi. RF-mittauksia käytettiin liitosten vikaantumisen seurantaan sekä lämpösyklauksen aikana että syklausten välillä. Maadoitettua koplanaarista aaltojohtoa käytettiin joko suoraan perättäiskytkennässä tai suodatinmoduulin kanssa, jonka päästökaista oli 22–24 GHz. Rakenteet karakterisoitiin syklausten välillä 25 GHz ja 30 GHz asti tässä järjestyksessä. Näiden mittausten lisäksi 10–11 GHz kaistalla toimivaa antennirakennetta karakterisoitiin syklauksen aikana 14 GHz asti. Tulokset osoittavat, että signaalivasteen muutos ilmenee aluksi joillakin taajuuksilla lyhyinä, alle 1 s mittaisina häiriöpiikkeinä, 0‒100 °C syklauksen aikana. Syklauksen edetessä vasteen huononeminen havaitaan myöhemmin koko mittausalueella. Puolikokeellista elinikäennustemallia tarkasteltaessa havaittiin, että PCSB-liitosten lämpötila-alueesta riippuvia korjauskertoimia kuvasivat logaritmisen riippuvuuden sijaan parhaiten toisen asteen polynomifunktiot. PCSB BGA ‒rakenteille saadun ennusteen ja toteutuneen eliniän välinen ero oli pienimmillään alle 0.5 %

Resonator‐enhanced radiating cable for UHF RFID readers

Abstract An antenna cable for enhancing and adjusting radio coverage is proposed. The presented solution is applied to the UHF RFID range of 866 MHz. With this leaky‐wave antenna cable, radio coverage can be efficient and customizable. A 10 × 100 mm resonator patch is attached on an opening on the cable’s outer conductor to improve its radiating properties. The effect of its displacement is studied through simulations and measurements. The alignment affects both resonance frequency and signal strength. One resonator increases the radiated power by 2 dB (37%), multipliable by adding resonators. However, alignment accuracy of millimeters is needed in many cases. The greatest effect in both operating frequency and transmission loss magnitude comes from lifting the resonator from the cable’s surface, averagely being 7.6% and 8.6% per mm in measured operating frequency and linear magnitude of S21, respectively. Smallest changes are observed when moving the resonator along the cable, being 0.8% and 0.7% per mm, respectively

A resonator enhanced UHF RFID antenna cable for inventory and warehouse applications

Abstract Structures to improve the RF properties of leaky coaxial cables (LCX) are proposed. These resonator structures are optimized for UHF RFID frequency of 866 MHz; however, they are applicable also to other frequencies of interest by redimensioning with corresponding data. Simulations of leaky coaxial cables with resonators (denoted “RCX”) show to improve the antenna gain, which is verified also by measurements. Compared to traditional LCX, RCX have increased radiating power capability while the shape of the propagated wave is also controlled. Cables are simulated with lengths 2.1 m and 6.4 m where the number of radiating slots and resonators is varied. RCXs exhibit a much higher radiation capability than LCXs. The radiation efficiency for RCXs is between 40% ⋯60 % whereas an LCX has only 0.2% efficiency. Simulations are complemented with measurements of the 2.1 m cables. A 30x improvement of the read range of passive RFID tags are reported in tests, from 0.05 m to 1.5 m along the resonator cable. The cable length can be extended so that the signal is carried to the radiating point of the cable using conventional, i.e., non-radiating coaxial cable

BGA interconnection reliability in mirrored module configurations

Abstract Interconnection dimensions are becoming more important due to electrical signal timing requirements and stray effects, such as unwanted inductance, leading to increasingly denser packaging. One way to shorten the signal path is to use mirrored structures, where the components are placed on opposite sides of the printed circuit board (PCB). This paper presents thermal cycling test (TCT) results in a temperature interval from -40 °C to +125 °C and simulation results of plastic ball grid array components mounted on one side [single side (SS) configuration] and in different mirrored configurations on a PCB. Anand’s constitutive model is used in the finite-element analysis software to calculate dissipated creep energy densities in the interconnections. Field lifetimes of the presented cases were also calculated on the basis of the dissipated creep energy densities and TCT data. In addition, supporting microscopic studies were done. The single-sided configuration had a longer lifetime than the mirrored configurations with components on opposite sides. The mirrored configurations had adverse simulated creep energies and TCT lifetimes, compared with the single-sided configuration. The simulations proved that the flexibility of the mirrored structures was hindered, thus increasing stress levels in critical interconnections

Eddy current soldering of solar cell ribbons under a layer of glass

Abstract Thermal fatigue of soldered interconnections of silicon solar cells is considered one of the key failure modes in photovoltaic (PV) modules. Due to differences in thermal expansion coefficients of the used materials, cracks and disconnected wiring can occur, causing a local increase of resistance. This can lead to hot spots and early failure of a panel. In this research, we develop eddy current soldering as a non-contact soldering technique for tabbing the ribbon of PV cells under a layer of glass. The performance of eddy current soldering was studied in detail by changing an induction coil distance to the treated sample from 2 to 4 mm and varying exposure time. An IR camera monitored temperature profiles at the soldering zone, and I–V curves of samples were extracted to validate the soldering technique and the efficiency of the tabbing process. Electroluminescence imaging was deployed to monitor cracking after the soldering process; scanning electron microscopy (SEM) and scanning acoustic microscopy (SAM) were used to investigate the microstructure and cracking within the solder joint and silicon. The results showed that the shortest exposure times at a 2 mm heating distance caused higher risks for cracks in silicon. The intermediate exposures at a 3 mm heating distance increased the likelihood of cracking in the solder joint. For the longest heating times at a distance of 4 mm, the solder joint appeared crack free, although, in some areas, poor wetting of solder was observed. The performance of eddy current soldering under a layer of glass was compared to manually soldered samples. Based on the I–V curves, all samples were functional, and differences between exposure times and manually soldered samples were not evident. Thus, only electrical verification of the solder quality was insufficient. Due to the study, it was concluded that the proper solder interconnections through glass could be made by using the eddy current soldering process and a sufficient exposure time to avoid cracking within the solder joint configuration and silicon

Effect of voids on thermomechanical cracking in lead-free Sn3Ag0.5Cu interconnections of power modules

Abstract In this article, the effect of voids on thermomechanically-induced failure in the lead-free solder interconnections of power amplifier (PA) modules is investigated. The interconnection of interest is between the module’s flange and substrate (baseplate) initially having a large contact area, and whose cracking has strong detrimental effects on the RF performance and reliability of the PA. PA modules were attached onto baseplates with lead-free SAC305 solder and put to a thermal cycling test (TCT) in the 15 °C to 95 °C range. X-ray imaging was used to characterize the number of voids in interconnections after reflow soldering. The cross-sections of the pristine reflow-soldered and tested interconnections were inspected with cross-polarized light microscopy to reveal the shapes of the voids and crack paths. It was noted that the voids forming in interconnections during the final stages of the reflow process take an elliptical shape, leaning towards the module’s outer edge, due to differences in the thermal expansions of the module and baseplate. During the TCT, high thermomechanical stresses caused localized recrystallization of the as-soldered SAC matrix in the vicinity of the elliptical voids. Finally, creep related intergranular cracks formed in these recrystallized areas. Anand’s viscoplastic model was used to model the void formation and the effect of the formed voids on crack evolution in solder. Simulations of the high creep energy of the elliptical, tilted voids and their correlation with the recrystallization behavior of the SAC305 interconnections were consistent with the observed creep related failures. Confirmed by simulations, these voids are detrimental to the reliability of solder interconnection because the creep strain is strongly localized to the sharp curvatures of the elliptic void

Power module interconnection reliability in BTS applications

Abstract In this paper, the reliability of RF power transistors’ solder attachments is characterized through experiments and simulations. Test cases consisted of power amplifier (PA) modules on AlSi10Mg substrates with either a low or high mutual thermal mismatch. The module’s flange interconnections were stressed by means of thermal cycling testing (TCT) in the 15 °C–95 °C range. Scanning acoustic microscopy (SAM) was used intermittently to inspect the interconnections of selected structures during cycling breaks. Optical cross-polarization microscopy and scanning electron microscopy were used in the failure analysis of the solder joints. Different materials and dimensional variations were tested in simulations to observe differences in thermal stress. The viscoplastic behavior of lead-free solder in the interconnection was modeled using Anand’s constitutive equations. The first cracks could be observed with SAM after 100 cycles. SAM imaging showed that in the worst case, 72% of the interconnection area had cracked at the end of the 1100-cycle TCT. Only a marginal amount of cracks could be observed in PA modules with a better coefficients of thermal expansion match to the substrate. Simulations indicated that it is possible to decrease creep energies significantly and thereby increase the lifetime expectancy of interconnections by selecting the correct materials and structures

Capability assessment of inkjet printing for reliable RFID applications

Abstract In this paper, inkjet-printed silver traces and interconnections produced with the print-on-slope technique were used in an radio-frequency identification (RFID) structure operating in the ultra-high-frequency range. Underfill material was used to attach silicon RFID chips onto flexible, 125-μm -thick polymer substrates. The cured underfill was also used as a sloped surface for printing interconnection traces from the chip to the plastic substrate’s radiators. Inkjet printing was performed in one phase, producing both the interconnections to the chip and the radiators. This enables the use of a single-phase continuous roll-to-roll compatible process instead of the commonly used two-phase stop-and-go process. To further investigate the behavior of the printed low-temperature nanoparticle ink and its compatibility with different substrate materials, basic conductive traces were printed onto the substrates. Thereafter, the structures were exposed to thermal/humidity tests at 85 °C temperature/85% relative humidity (“85/85”) for up to a 2000-h period. To gain an understanding of the response of the structures under stress, the samples were intermittently characterized by using a read range measurement device, followed by the removal of failed samples from the test. The samples were characterized also by optical imaging and field-emission scanning electron microscopy. The bulk conductive traces were characterized electrically by measuring their resistances during test breaks. The results point out that although some challenges are still to overcome, inkjet printing is a feasible way of producing conductive traces for RFID structures, and that the print-on-slope technique is utilizable also in practical applications as a cost-effective method with adequate reliability for producing interconnections between chip and substrate

Dual-polarized dual fed vivaldi antenna for cellular base station operating at 1.7-2.7 GHz

Abstract The paper presents a novel dual polarized dual fed Vivaldi antenna structure for 1.7–2.7 GHz cellular bands. The radiating element is designed for a base station antenna array with high antenna performance criteria. One radiating element contains two parallel dual fed Vivaldi antennas for one polarization with 65 mm separation. Both Vivaldi antennas for one polarization are excited symmetrically. This means that the amplitudes for both antennas are equal, and the phase difference is zero. The orthogonal polarization is implemented in the same way. The dual polarized dual fed Vivaldi is positioned 15 mm ahead from the reflector to improve directivity. The antenna is designed for 14 dB impedance bandwidth (1.7–2.7 GHz) with better than 25 dB isolation between the antenna ports. The measured total efficiency is better than 0.625 dB (87%) and the antenna presents a flat, approximately 8.5 dB, gain in the direction of boresight over the operating bandwidth whose characteristics promote it among the best antennas in the field. Additionally, the measured cross polarization discrimination (XPD) is between 15 and 30 dB and the 3 dB beamwidth varies between 68° and 75° depending on the studied frequency

Wide band frequency measurements of fungal species using laser patterned finger electrodes on LTCC

Abstract High frequency measurements at 50 MHz – 10 GHz were performed for the first time using interdigitated electrodes on a low temperature co-fired ceramic substrate to analyze fungal spores. Wet and dry spore generation methods were evaluated and tested with two different fungal species. The dry generation method was found feasible for RF measurements, since the component capacitance increased 14–21% in the 2–6 GHz range, but for the wet generation method the capacitance decreased only slightly (<1%). Based on these initial results the RF measurements have the capacity to evaluate the quantity of fungal spores but not to identify their species