AREA CONSORTIUM REPORTS

2023 REPORTS

Alternative Techniques for Cross-sectioning and Polishing sTIM Parts with Soft Indium Alloys
Author:  Ali Davoodabadi

Abstract:
Preparation of smooth and defect-free sections for metallographic analysis of joints with soft metals/alloys is challenging, since the conventional cross-section preparation techniques can damage the soft interface via smearing, scratching and/or embedment of abrasive particles in the soft medium. In this report, a novel alternative technique based on microtome and ion etching is introduced for cross-section preparation. Cross-sections of joints with soft indium alloy thermal interface material prepared with this new technique are compared to those prepared with the conventional polishing technique. It is shown that sharp and particle-free sections can be prepared following the new microtome-based technique. Improvements in section quality obtained via mechanical polishing is shown to be possible. For example, application of alumina abrasive foil instead of the common silicon carbide foils is shown to reduce the number of embedded particles during polishing. Loading the abrasive paper with beeswax is found to alleviate the particle embedment issue during mechanical polishing, as well. However, the sections generated with mechanical polishing will still contain embedded particles (and any consequent potential artifacts) to a certain extent. 

Evaluation of a Transient Liquid Phase Sintering (TLPS) Material – XCAP
Author:  Alyssa Yaeger

Abstract:
In system-in-package (SiP) applications, passive components often see multiple reflows, and there is a concern that in later reflows the solder attaching the passives may melt, causing the passives to detach from the assembly. One solution to this problem is to use transient liquid phase sintering (TLPS) materials which have a low initial melting point, but higher melting point after the initial reaction. XCAP is a TLPS material consisting of mainly Sn and Cu, which processes similarly to solder, and has a final microstructure of 10-20µm pores, undissolved Cu cores, Sn-rich regions, and Cu-Sn intermetallics. DSC analysis has shown that although the Sn-rich regions will melt during additional reflows, the material does not fully melt below 400oC. There is no significant difference in shear strength of 0603 resistors attached with XCAP after one and three reflows for shear speeds of 10µm/s and 1000µm/s. In resistors annealed at 175oC for up to 1000 hours before shearing, with SAC joints retaining strength and XCAP joints decreasing in strength. The decrease in XCAP strength with aging may be due to the Sn coating on the resistors.

Optimizing the Solder Thermal Interface Material Pre/Intra-Reflow Process Variables for Minimum Initial Voiding
Author:  Ali Davoodabadi

Abstract:
The thermal performance and long-term reliability of solder thermal interface materials strongly depend on their processing conditions and, in particular, the reflow process. Lack of an optimized reflow process can result in voiding that in turn impacts the mechanical and thermal performance of the package. This report covers the initial progress made as part of a comprehensive study to investigate the effect of different component level variables on the optimized reflow process. This study will identify the optimized reflow process for each solder alloy, die dimension, bond line thickness etc., and find the relationships between different variables and the optimum reflow profile. The optimum reflow profile in this work is recognized as the one that will produce the smallest percentage of voids after the first reflow process. This study also highlights the significance of pre-reflow processes such as flux deposition on the quality of the joints. Parts are primarily made using a vacuum reflow oven, but baseline parts reflowed in a regular nitrogen oven are fabricated as well.

Thermal Cycle Reliability Assessment of Leadless Packaging Assembled with Low-Temperature Solders
Author:  Michael Meilunas

Abstract:
Low-temperature solders (LTS) were used to assemble surface mount components to printed circuit boards in order to create test vehicles for thermal cycle reliability analysis. Test results for BGA-style components have already been documented in four AREA reports [1,2,3,4] while the current report summarizes the results for the leadless packages evaluated. The previous reports [1,2,3,4] include additional details not found in this paper such as test vehicle and assembly process specifics. 

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

Thermal (IR) Imaging for Direct Observation of TIM Stability under Power Cycling – Testing on Grease and Putty
Author:  Ali Davoodabadi

Abstract:
An IR imaging technique for in-situ monitoring of TIM degradation during power cycling is evaluated. Two different types of common thermal interface material i.e., greases and pre-cured gels are tested with the setup to evaluate the applicability of this technique for different material groups and realize any potential shortcomings. IR snapshots recorded throughout the test clearly show the onset and growth of defects in the TIM layer. An overall agreement between IR and high-resolution optical images recorded at the end of the test is observed. Die temperature was monitored both locally and in average during the test. Data showed that areas with higher void population in TIM experienced the most increase in die temperature, locally. An agreement between the IR images, high-resolution optical images and die temperature data, is observed that confirms the applicability of the IR technique for grease and gel thermal compounds. 

IR Imaging Technique, Spring Force Loading and Image Processing
Authors:  Alan Derevensky,  Ali Davoodabadi

Abstract:
This research paper presents the further development and application of an infrared (IR) imaging technique using spring force loading and Python image processing libraries to observe the behavior of Thermal Interface Material (TIM) during thermal cycling. The electronics industry heavily relies on TIMs for efficient heat transfer between electronic components and heat sinks. In this study, TIM 1s present unique challenges in terms of physical assembly and uniform coverage as the silicon device experiences a typical computing load. Understanding the thermal performance and reliability of TIM 1s is crucial for enhancing the overall performance and longevity of electronic devices. This research aims to leverage the combination of IR imaging, force loading, and Python image processing libraries to provide valuable insights into the behavior of TIMs under thermal cycling. By analyzing the recorded data and images, researchers and engineers in the electronics industry can gain a deeper understanding of TIM performance, enabling them to optimize electronic device designs, improve heat dissipation, and enhance overall product reliability. Ultimately, this study contributes to advancing the field of electronics thermal management and paves the way for more efficient and robust electronic devices in the future.

Observations of Durafuse-LT
Author: Alyssa Yaeger

Abstract: 
One of the main issues in Pb-free assembly is the high reflow temperature of SAC305. Durafuse LT is a mixed alloy paste containing two alloys which when reflowed forms a SAC+In alloy. Durafuse LT reflows about 30oC below typical SAC reflow, reducing warpage and damage to components due to high temperature. Unlike many other low temperature Pb-free solders, Durafuse LT does not contain bismuth, and has drop/shock and high temperature shear performance similar to SAC on Cu-OSP surface finishes. 

TIM 2 Test Vehicle Update: Delta T and IR and Heater
Author: Peter McClure

Abstract:
The reliability of thermal interface materials (TIMs) in large complex printed circuit board (PCB) assemblies is challenging to predict or model, so experimental study is needed. The AREA Consortium had be using a large board TIM 2 test vehicle at used in situ electrical capacitance or electrical resistance measurements to gain insight into TIM behavior. Gaining the indirect indication of TIM performance can be helpful, but directly measuring thermal resistance vs. cycling would allow for better TIM decision-making. This study modified the in situ test vehicle to allow for measuring the thermal performance of TIMs using a heater attached to the bottom of a lid and an IR camera monitoring the heat sink. By measuring the temperature gradient across the system the thermal resistance of the TIM could be measured. The TIM effective thermal conductivity was determined by relating the data to finite element analysis (FEA) models. This work acts as a proof of concept of the test design. 

Thermal cycling, 180 cycles from 0 and 100⁰C, of four graphite pad thermal interface materials [Allied Tpad SO2001, Sekisui Manion SC, Jones 21-1500A, and Nolato Compatherm 9611] was conducted on hardware that was mechanically representative of large PCBs with common heat sinks. A nominal bond line of 1100µm was used for this study, representative of TIM 2 pad bond lines.

Keywords: TIM 2, IR Analysis, ATC, Graphite Pad, Reliability, Failure Analysis, Electrical Resistance

TIM 1 Power Cycling with IR and Optical Monitoring
Author:  Peter McClure

Abstract: 
The reliability of thermal interface materials (TIMs) in die-level packages is critical for the performance and lifetime of electronic assemblies. Wunderle et al.1 developed a system for controlled stressing of TIMs while watching them react in real-time with infrared (IR) and optical imaging. Their technique gave insight into TIMs’ response to cyclic strains, both mechanistically and comparatively. The work in this report expands on their concept by using a system that gave more representative thermal and mechanical loading of TIMs while taking IR and optical imaging on the same samples. This work was done as proof of concept for the test design and worked with three repressive TIMs, a TIM intended for between a heat spreader and heat sink (TIM 2), a TIM intended between a die and heat spreader (TIM 1) grease, and TIM 1 phase change material (PCM). 

Keywords: TIM 1, IR Analysis, Power Cycling, Reliability, Proof of Concept

Liquid Crystal Polymer Flex-to-Flex Thermal Compression Bonding: Fine Pitch Design Considerations
Authors:  Peter McClure,  Dylan Richmond

Abstract:
Flex-to-flex bonding is crucial for producing long and complex flexible electronic cables. Liquid crystal polymers (LCP) are of interest for flexible cable material, as they are mechanically, chemically, and thermally robust. Making long (>1m) LCP cables is a complicated process, often requiring thermal-compression bonding of multiple shorter cables. This report looks at the thermal compression bonding of LCP cables with small features and pitches (<400µm), which are typically avoided due to various complications. The roles of design and processing conditions on fine-pitch assemblies were examined. Guidelines in flexible cable designs and processing conditions have been made. 

BGA TIM 1 Testing with Laser Pump and IR Probe: Proof of Concept Study
Author:  Peter McClure

Abstract:
This study proposes a pump and probe method for examining ball grid array (BGA) samples for thermal interface material (TIM) defects. A high-power laser was used to heat a large area of a BGA’s integrated heat spreader (IHS). The heating and cooling were monitored with an infrared (IR) camera. Changes in thermal profiles and heat maps were related to defects in the TIMs. This method offered a quick and non-invasive method to test BGAs for TIM defects before being taken out of matrix trays. Experiments and finite element analysis (FEA) were used for proof of concept testing.

Experimental testing of off-the-shelf thermal test vehicles (TTVs) was used to establish the system sensitivity and noise. Then TTVs were de-lidded and rebuilt with various large areas of TIM missing. The system could reliably detect large amounts of missing TIM. For testing sensitivity to more minor TIM defects, FEA was used. FEA results were compared to the measured experimental noise and showed that TIMs with 20% lower than expected conductivity and voids down to 5% of the die area should be detectable.

Key Words:  TIM 1, Screening, FEA, Laser, Proof of Concept

Mixed Via-in-Pad-Plated-Over (VIPPO) Defects:   A Comprehensive Review of the AREA Research on the Issue
Author:  Pericles Kondos

 Abstract:
This report summarizes the research performed over a period of years on a solder joint defect observed after two reflows in PCBAs that have a mixture of Via-in-Pad-Plated-Over (VIPPO) and non-VIPPO pads in a given BGA site. This defect has been seen only in joints on the boundary between VIPPO and non-VIPPO pad regions and appears as a clean separation between the solder and the intermetallic layer of the component side pads that leaves a relatively smooth solder-side failure surface not mirroring the shape of the intermetallic. The research used mostly a special test vehicle that was designed to use a variety of components with different patterns of VIPPO pad distribution and different types of non-VIPPO pads. Its principal purpose was to seek design rules, if any, that would lead to defect-free boards. The same test vehicle was later used in numerous “mechanistic” experiments where the effect of changing various process variables on the VIPPO defect was investigated, with the aim of not only eliminating the defect but also understanding the mechanism that created its characteristics as well as when during the second reflow the defect appeared.

Thermal Cycle Reliability Assessment of Low-Temperature Solder Hybrid BGA Subjected to Double Reflow Assembly 
Author:  Michael Meilunas

Abstract:
A low-temperature solder (LTS) containing 57 weight percentage bismuth was used to assemble SAC305 BGA to printed circuit boards to create Hybrid BGA test vehicles. Half of the samples were assembled using a single-reflow soldering process while the other half were assembled using two-reflow passes to simulate a double-sided circuit board assembly process. The test samples were subjected to -40/105C accelerated thermal cycling in order to fatigue the second level solder joint interconnections and the resulting failure data was compared to determine if the second reflow process affected component lifetime. The results were also compared to single-reflow 63Sn37Pb BGA and SAC305 BGA data generated with the same test vehicle and thermal cycle parameters as a means to understand the relative Hybrid performance with respect to more traditional assemblies.  

The experiment described in this report is the fourth 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 and was the first to examine the effects of two reflows on the Hybrid BGA.  

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