2020 Reports



Author: Pericles A. Kondos
Abstract: 
        The TB2019-U test vehicle was populated with six types of electrically testable dummy BGA components, varying in size, thickness, pitch and number of I/O. The smallest-pitch BGA had some issues with Head in Pillow solder defects which were corrected during assembly. The assembled PCBs were cleaned and then divided into six sets of 16 or 17 boards. Two sets were fully underfilled with the Namics SUF1570-2 and the United Adhesives UF1230. These two sets, together with one without underfill for reference, were to be reliability tested in accelerated thermal cycle, while the remaining three were to be used for edge-bonding studies.



Authors: Xuanyi Ding, Luke Wentlent
Abstract: 
        Next generation SnAgCu based solder alloys often depend on solid solution strengthening by bismuth additions.  Bismuth contents vary by alloy, some at levels above the room temperature solubility limit, some below.  However the solubility limit of bismuth in tin increases significantly with temperature.  Of interest is the effect of this varying bismuth solubility over the range of solder joint operating temperatures.  Within any given thermal cycle, some amount of bismuth will be driven into solution and then precipitated out again on cool down. This should have a dynamic impact on the solder strength.  This experiment attempts to capture such strength changes with solder joint shear testing after varying thermal cycling exposures.  Bismuth bearing alloys M758 (3% Bi) and Violot (6%Bi) are seen to exhibit the same power law dependency of shear strength on shear strain rate.as the baseline SAC305 alloy albeit stronger. After thermal cycling exposure however, the strength of the Bi strengthened alloys show a decline at higher strain rates while retaining their monotonic dependency on strain rate at lower shear rates.  The SAC 305 control shows very little effect of thermal cycle exposure, retaining its power law dependency of strength on shear strain rate over the full range of shear rates tested.




Authors: Michael Meilunas, Daniel Goldring, Arvind Srinivasan
Abstract: 
        Electroless Palladium / Autocatalytic Gold (EPAG) provides a wirebondable and solderable finish for printed circuit boards ostensibly without the need for a nickel diffusion barrier over the base copper layer.  This study investigates the interconnect reliability of SAC305 solder joints on an EPAG surface finish using mechanical drop shock and -40 to 125°C thermal cycle.  In both instances reliability results were compared to those with ENIG and copper board finishes. Drop shock testing was performed at 600G with an attached BGA196 component.  EPAG produced drop lifetimes far superior to ENIG but not as good as copper.  Thermal cycle testing was performed on nine package types and four surface mount resistor formats.  EPAG performed poorly in thermal cycle relative to ENIG or copper for those BGA components given to temperature induced warpage.  In parts with minimal warpage (i.e., BGA360) the EPAG thermal cycle reliability performance was superior to both ENIG and copper finishes.



Authors: Peter McClure, Dave Shaddock 
Abstract:
Printing conductive ink circuity is a key enabler for flexible electronics and could be used in other electronics assembly applications as well.  Knowing what printing process parameters to change for optimized resolution and knowing the limits of what can be printed are important for board design and processing.  In this study the line and spacing limitations of screen printing were examined by changing key processing variables (squeegee speed, squeegee pressure, snap off gap, screen mesh, mesh angle, emulsion thickness, and ink viscosity) and examining their impact on resolution of printed features with different orientation angle vs. print direction.  Relations between printing conditions and resolution were found using analysis of variance.  Suggested ideal conditions for printing were determined 

Key words: Screen printing, ANOVA, analysis of variance, conductive ink, flexible electronics



Authors: Arvind Srinivasan Karthikeyan, Sa’d Hamasha
Abstract:
        A thorough investigation of the influence of solder paste alloy on the fatigue life of solder joints between ball grid arrays and PCB undergoing vibration fatigue test is performed. The test matrix is comprised of three alternate Pb-free solder alloys along with traditional SAC305 solder as the control. These alloys, Innolot, Cyclomax and Violet, represent examples of bismuth strengthened alloys intended for high reliability applications. Initially, modal analysis was performed on the vibration test boards to identify the natural frequencies of the assembled board.  A sinusoidal vibration test was conducted on the test assemblies at room temperature using an electrodynamic shaker. Test boards are excited at their first natural frequency and with a constant amplitude of acceleration until failure of all BGA packages. A strain gauge was mounted on the board to measure and monitor the maximum principal strain throughout the test. Weibull analysis is performed to compare the vibration fatigue life of the various solder alloys and failure analysis is performed to reveal the failure modes. Failure analysis determined that the dominate failure mode of the tested samples was copper trace failures.  The high reliability solder joint samples were only rarely found to fail in the solder and so their relative vibration fatigue performance could not be compared in this study. 

Key words: Sinusoidal Vibration testing, Modal analysis, Lead-free solder alloys



Author: Peter McClure
Abstract:
The reliability of thermal interface materials (TIMs) in large complex printed circuit board (PCB) assemblies is difficult to predict or model, so experimental study is needed.  Thermal cycling, 500 cycles from 0 and 100⁰C, of four thermal interface materials [Jones 21-430SF(E), Laird Tgrease 2500S, Nolato 9540, and Laird Tpcm 780SP] was conducted on hardware that was mechanically representative of large PCBs with common heat sinks, like server boards.  Electrical capacitance of the TIMs was monitored during thermal cycling giving insight into the mechanical response of the TIMs.  In combination with postmortem failure analysis, the reliability of the TIMs was evaluated.  This informed the TIMs’ reliability under realistic accelerated environmental stress conditions.  A nominal bond line of 100µm was used for this study, representative of thin TIM 2 bond lines.

Key words: TIM 2, ATC, Shear Strain, Grease, Phase Change Material, Failure Analysis, Capacitance


Strain Rate Sensitivity of High Bismuth Pb-free Solder Joints

Author: Xuanyi Ding
Abstract:
        There is a growing industry interest in the integration of Pb-free solder alloys with melting points lower than that of the conventional SnAgCu solder alloys (Tm = 217°C). The primary motivators for reducing solder reflow temperature are increased material selection options (more temperature sensitive materials and components can be assembled), increased product reliability (associated with a less damaging reflow) (Morgana Ribas 2013), and increased assembly yield (fewer component warpage induced solder defects). The most popular reduced melting point alloys being investigated are those with compositions near the 57Bi43Sn eutectic. Their popularity is due to low cost, good solderability, and melting points near 139°C (Yang Liu 2017). In this project, the strain rate sensitivities of SnAgCu (SAC), near eutectic Bi42.0Sn1.0Ag (SnBiAg), and mixed SAC-SnBiAg solder joints were measured in joints produced at different solder reflow temperatures. Shear strength and fracture mechanisms were examined over a strain rate range of five orders of magnitude. The resulting fracture surfaces were examined using SEM/EDX. Results showed that mixed solder assemblies and homogenous SnBiAg showed markedly different response to strain rate than homogenous SAC solder joints.


Author:  Peter McClure 
Abstract:
Copper coins embedded into printed circuit boards offer a higher thermal conductivity heat dissipation solution.  Copper coins are monolithic slugs of copper embedded into printed circuit boards such that components can be soldered directly to them thereby enhancing heat transfer through the board thickness.  While the thermal advantages of copper coins are clear, the reliability of embedded copper coin systems has not been thoroughly examined.  It is possible that the thermal expansion mismatch of the solid copper and surrounding PCB could drive cyclic stress induced failures.  This study examines T shaped copper coin circuit boards with attached quad-flat no-lead (QFN) packages subjected to thermal cycling.  These boards were examined for failures in the board circuity, coin attachment epoxy, electroplated copper, or solder joints/pads.

Key words: Copper Coin, Reliability, QFN, Thermal Management 

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