2020 Reports

Author: Peter McClure
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
        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 
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