AREA CONSORTIUM REPORTS

2022 REPORTS

Low Temperature Solder: BGA Drop/Shock Reliability after Current and Temperature Stressing (Phase I)
Author: Michael Meilunas

Abstract:
BGA components constructed with SnBiSbNi solder spheres were assembled using a matching alloy paste and then subjected to simultaneous direct current and temperature stressing in order to generate electromigration of the bismuth ions. The samples were then drop tested in order to evaluate solder joint lifetime and the results were compared to a non-stressed control population. Solder joint failure analysis was performed through x-ray and cross-sectional examination. The experiment described indicates that the current densities, joint temperature and stress durations examined could result in a significant reduction in drop reliability -even when minimal bismuth electromigration was observed.

Thermal Cycle Reliability Assessment of Homogeneous Low-Temperature Solder and Hybrid BGA Devices
Author: Michael Meilunas

Abstract:
Two low-temperature solder (LTS) alloys containing 37 and 57 weight percentage bismuth were acquired in solder sphere and paste forms. The solder spheres were attached to bump-less BGA devices which were then soldered to test boards using the LTS pastes to form homogenous low-temperature test vehicles. SAC305 BGA were also assembled on the same date using the 57% bismuth paste in order to create Hybrid assemblies while yet another population was created using SAC305 paste to create more traditional SAC305 assemblies. The various sample populations were then subjected to -40/105C or 0/105C accelerated thermal cycling in order to fatigue the second level solder joint interconnections and the resulting failure data was compared using the SAC305 samples for general baselining purposes.

The experiment described in this report is the third accelerated thermal cycle reliability analysis of lead-free surface mount componentry assembled using near-eutectic tin-bismuth based solder paste performed by Universal Instruments’ Advanced Process Laboratory AREA consortium. This project was unique for several reasons, including the fact that homogenous LTS BGA were available for analysis. A third thermal cycle, from 20 to 80C, was also undertaken for the experiment, however test results for such a mild cycle were slow in coming and the results of the milder cycle will be reported on in later publications.

This paper is the third in a series of reports addressing the accelerated thermal cyclic reliability of surface mount devices assembled using low-melt bismuth bearing solders.

Thermal Cycle Reliability Assessments of SAC305 BGA Devices Assembled with Bismuth containing Low-Melt Solder Pastes
Author: Michael Meilunas

Abstract:
Lead-free solder alloys containing high bismuth concentrations have been developed for low-temperature reflow assembly. These alloys are available in solder paste form and may be used to assemble electronic components with reflow temperatures as low as 150˚C as measured at the solder joint positions. Although the process procedures required to create such assemblies are straight-forward, the reliability of the bismuth bearing alloys are not well documented and require significant study before the materials can be adopted by industry. This concern is compounded by the fact that the microstructural characteristics of the bismuth based alloys are highly dependent upon processing parameters such as reflow time, peak temperature and solder paste volume when assembled in a hybrid, or “mixed alloy” system (i.e. used in conjunction with SnAgCu solder based components) and it is theorized that such microstructural variability may significantly impact the thermomechanical reliability of the assemblies.

The experiment described in this report is the second accelerated thermal cycle reliability analysis of lead-free surface mount componentry assembled using near-eutectic tin-bismuth based solder paste performed by Universal Instruments’ Advanced Process Laboratory AREA consortium. To that extent, four lead-free daisy-chained BGA component designs were assembled to printed circuit boards with SnBiAg solder paste using a forced convection reflow process producing ~190˚C peak solder joint temperatures. The samples were inspected after assembly using x-ray imaging and selective cross-sectional analysis. Test vehicles were then subjected to accelerated thermal cycling with in-situ monitoring in order to stress the assemblies in an attempt to generate solder joint fatigue failure. Lifetime data was then compiled and compared using Weibull plots. The results were also compared to equivalent SAC305 assemblies which were concurrently tested.

This paper is the second in a series of reports addressing the accelerated thermal cyclic reliability of surface mount devices assembled using low-melt bismuth bearing solders.

Henkel Loctite® Ablestik BP 8068TB and Namics Unimec H9890 7 Silver Semi-Sinter Pastes: Evaluation and Comparison
Author: Pericles Kondos

Abstract:
The Henkel Loctite® Ablestik BP 8068TB and the Namics Unimec H9890-7 silver semi-sinter pastes were used to attach Au-metallized die onto EPAG-finished pads on an organic PCB. Detailed observations of the appearance and texture of the materials were made before and after curing. Shear strength tests were conducted on time-zero samples as well as on samples that had undergone further post-attach treatment, namely high humidity exposure, high temperature storage, and accelerated thermal cycling. In addition to the strength values, the location and appearance of the fracture surfaces were studied with high-magnification optical and SEM images as well as with EDX.

First Evaluation of the Henkel Loctite® Ablestik ABP 8068TB Semi Sintering Silver Paste
Authors: Pericles Kondos, Xinyu Zhang

Abstract:
The Henkel Loctite® Ablestik ABP 8068TB semi sintering silver paste was used to attach Au-metallized die onto ENIG substrates using two different curing schedules. A secondary effort looked at Ag-metallized die on immersion silver (ImmAg) substrates. The quality of the bonding was tested with die shear, and the fracture surfaces were examined in detail with optical microscopy and SEM as well as with EDX. The effect of the curing profile on both strength and failure surfaces was investigated.

Indium Thermal Interface Material Microstructure as a Function of Thermal History and Bonding Metallization
Authors: Peter McClure, Yujia Wang

Abstract:
Indium has gained attention as a thermal interface material (TIM) in high-power electronics due to its high thermal conductivity and mechanical compliance. However, there is minimal understanding of the indium microstructure in this application as it is challenging to prepare for imaging due to its softness. Samples of indium metal are difficult to cross-section; conventional mechanical polishing is considered impractical so slow and expensive focused ion beam (FIB) techniques have been required. In this study, a cross-section procedure for components containing indium was developed based on mechanical polishing, ultrasonic cleaning, etching, and ion milling. With this technique, indium cross-sections that showed microstructural details could be made and studied for changes with different bond metallizations and thermal histories. Electronic packages with indium TIM bonded to Au or Ag metallized Si chips and Ni plated Cu lids were examined. Intermetallic compounds of In-Au, In-Ag, and In-Ni were investigated using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). The effects of bond metallization, doping, and solder reflow cycles (thermal history) on the indium microstructures were examined.

Keywords: TIM, Indium, Cross-section, Microstructure, Reflow

Capillary Underfills and Edgebond Materials on TB2019-U: ATC Reliability and Failure Analysis, Part II
Author: Pericles Kondos

Abstract:
Assembled TB2019-U boards with their parts either underfilled with one of two capillary underfills or edge-bonded with one of three edgebond materials were subjected to ATC. In Part I of this report their reliability was compared with that of a non-underfilled, non-bonded group and the location of the failure(s) in each failed part was approximately determined by electrically probing chains of joints. This Part II of the report looks in more detail at the failures themselves by means of cross-sections or dye-and-pry analysis. Numbers, sizes and location of observed cracks are discussed as well as their width, when it appeared to follow a trend.

Capillary Underfills and Edgebond Materials on TB2019-U: ATC Reliability and Failure Analysis, Part I
Author: Pericles Kondos

Abstract:
Assembled TB2019-U boards with their parts either underfilled with one of two capillary underfills or edge-bonded with one of three edgebond materials were subjected to ATC and their reliability was compared with that of a non-underfilled, non-bonded group. The location of the failure(s) in each failed part was approximately determined by electrically probing chains of joints and common trends were sought. More detailed examination of the failures themselves by means of cross-sections or dye-and-pry is presented in Part II, published separately.

Reliability Comparison of Underfilled WLCSP Assembled using No-Clean Flux and No-Clean Paste Dipping Processes
Author: Michael Meilunas

Abstract:
Wafer Level CSP components of various sizes were assembled to printed circuit boards using either a no-clean flux dip or a no-clean paste print process prior to underfill operations. The samples were then subjected to -40 to 125°C accelerated thermal cycling in order to stress the assemblies and drive solder joint fatigue failures. The failure data was compiled and the subsequent analysis indicated that the samples soldered using the no-clean flux survived longer than the samples soldered with the no-clean paste. The data was then compared to previous research in which similar test vehicles had been assembled using a cleaning process prior to underfilling while other samples had been assembled without underfill. These comparisons would show that underfilling the components dramatically improved lifetime and that cleaning the samples prior to underfilling also had a positive impact on lifetime.

Evaluation of the Heraeus DA320 Silver Sinter Paste
Author: Pericles Kondos

Abstract:
The Heraeus DA320 sinter silver paste was used to attach Au-metallized die on T-clad substrates with ENIG, ImmAg and Cu/OSP pads (despite the fact that Heraeus does not mention the latter as compatible with the paste). Attached die were tested for shear strength at time zero, after high temperature storage, and after accelerated thermal cycling. The failure surfaces were examined in detail, both with a high-magnification optical microscope and with a scanning electron microscope. Based on the results, recommendations for when to use this paste are made.