Review on microstructure evolution in Sn-Ag-Cu solders and its effect on mechanical integrity of solder joints

Scopus - Document details: "The use of Pb-bearing solders in electronic assemblies is avoided in many countries due to the inherent toxicity and environmental risks associated with lead. Although a number of "Pb-free" alloys have been invented, none of them meet all the standards generally satisfied by a conventional Pb-Sn alloy. A large number of reliability problems still exist with lead free solder joints. Solder joint reliability depends on mechanical strength, fatigue resistance, hardness, coefficient of thermal expansion which are influenced by the microstructure, type and morphology of inter metallic compounds (IMC). In recent years, Sn rich solders have been considered as suitable replacement for Pb bearing solders. The objective of this review is to study the evolution of microstructural phases in commonly used lead free xSn-yAg-zCu solders and the various factors such as substrate, minor alloying, mechanical and thermo-mechanical strains which affect the microstructure. A complete understanding of the mechanisms that determine the formation and growth of interfacial IMCs is essential for developing solder joints with high reliability. The data available in the open literature have been reviewed and discussed. "

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Extending the fatigue life of Pb-free SAC solder joints under thermal cycling

Scopus - Document details:

This paper reports on the dependence of the thermal fatigue life of Pb-free solder (Sn-3.0Ag-0.5Cu) joints on the points at which the printed circuit board (PCB) is fixed to a rigid support. Elastic-plastic finite element analysis under various thermal cycling conditions was performed. For the analysis, the mechanical boundary conditions around a solder joint bonded silicon chip were varied by changing the points at which the PCB was attached to its support. From the results of the analysis, it was found that the accumulated strain in the solder joints decreased when the PCB was fixed at points placed diagonally with respect to the silicon chip. The thermal fatigue life could therefore be expected to be extended by the choice of fixing points. Using a special support with a thermal expansion coefficient of less than 1.5 × 10-6 K-1, we demonstrated, experimentally, that the accumulated strain and the failure site of the solder joints could indeed be controlled by the positions at which the printed circuit board was fixed. © 2013 Elsevier Ltd. All rights reserved.

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Tests of tin-lead BGA packaging for mil-aero applications

Tests of tin-lead BGA packaging for mil-aero applications:
ntroduction

Each year the military and aerospace sector demands higher-speed systems with better accuracy at lower cost. Increasingly stringent size, weight and power constraints, together with the demanding power supply requirements of large FPGAs, have led to a decisive move towards POL (point of load) power supply architectures.

One solution to address these needs is Linear Technology’s µModule technology, providing a system-in-package solution that is claimed to simplify design and minimize external components. 

Figure 1: µModule Product Construction

The original packaging technology selected for µModule switching regulators was LGA (land grid array) and this has served the broad marketplace well. However, some applications subject to very harsh environments prefer BGA (ball grid array) interconnect, and Linear Technology has developed packaging to meet these requirements. 

Figure 2: LGA and BGA package interconnect

In this article, we will take a more detailed look at the comparative performance of LGA and BGA packaging and discuss the merits of gold or tin-lead (SnPb) alloy and lead-free (Pb-free) component finishes.

Component terminal finishes

The European military and aerospace market presents quite a fragmented demand picture when considering component finishes with some companies having adopted Pb-free finishes entirely whilst others have policies to completely avoid them, more common is a mix dictated by the specific needs of individual projects.

A major factor is the continued exclusion of military and aerospace equipment from the RoHS II (Restriction of Hazardous Substances) directive that permits the indefinite use of SnPb component finishes. Of foremost concern is the impact on reliability from the formation of tin whiskers on pure tin plating, leading to potential equipment failures from short-circuits of adjacent fine-pitch conductors. The addition of lead (Pb) to the tin plating remains the industry-standard approach for mitigation of whisker formation.

Weighed against the use of SnPb finishes are the practicalities of component availability, lack of distribution inventory and extended delivery times. By mandating use of SnPb finish companies are also sometimes foregoing access to a wider range of new, Pb-free only components. Whilst this can be overcome by third-party companies offering strip and re-plating or re-balling processes the additional heat cycles and difficultly in re-testing together with the associated costs make this a less desirable approach.

When Pb-free components are used another form of tin whisker mitigation frequently employed is the use of polymer conformal coatings such as Parylene or Arathane which have been shown to resist the penetration of tin whiskers for many years.

Interconnect considerations

Given the mix of needs discussed in the previous section, Linear Technology µModule products in LGA packages offer a universal solution as gold plated pads have been in long-standing use within military and aerospace systems and they have the advantage of also being RoHS compliant.

However, one concern of using gold plated components is gold embrittlement and this is especially true of large BTCs (bottom termination components) subjected to harsh environmental conditions. During reflow gold is dissolved into the solder joint creating weak interfaces in the crystalline structure, the CTE (coefficient of thermal expansion) differences between the gold-tin IMC (intermetallic compound) platelets and the surrounding volume of solder can result in fracturing of the solder joint and an eventual open-circuit when the assembly is repeatedly temperature cycled. A long-standing industry rule of thumb recommends not exceeding a threshold of ~3 wt% gold within the joint, and Linear Technology LGA µModule products meet this requirement for both SnPb and SAC305 paste. Some companies conduct their own trials on BTCs with representative PCBs and environmental conditions using special daisy-chain interconnect samples for this purpose.

Increasing the volume of solder in a joint and the standoff to the PCB improves interconnect reliability by making the joint more compliant. Tests in the military and aerospace industry have concluded that BGA packages are generally preferred over LGA where harsh environmental conditions are likely to be encountered, particularly in airborne systems. An added benefit is that cleaning becomes easier thereby reducing concerns over contaminants.

For these reasons µModule regulators are now offered in BGA packaging in addition to LGA. The downside of the BGA package is slightly reduced thermal efficiency of around 0.5ºC/Watt and an overall increase in component height of 0.6mm. Linear Technology BGA µModule regulators are offered as standard in SAC305 Pb-free configurations and on specific products according to demand with tin-lead Sn63Pb37 balls.

Reliability testing and characteristics

It is important to keep the concerns in perspective, and Linear Technology has conducted extensive trials on interconnect reliability of µModule products with both LGA and BGA configurations.

By way of background, we should distinguish between component tests used in the initial qualification and subsequent reliability monitoring and interconnect tests that are really a characterization of performance with selected PCB configurations, assembly process and temperature cycle parameters.

Extensive µModule product testing at component level indicates an exceptionally high level of reliability with 0.72 FITS (1 FIT = 1 failure in one billion device hours). This article will then focus on interconnect level testing.

Three types of interconnect testing have been performed:

1) Daisy chain testing: where each pad in the µModule regulator is connected to the next to form a complete circuit, monitored in real time during temperature cycling in accordance with IPC-9701 and JESD22-A104. This approach ensures that each pad is tested and since many pads are connected in parallel in the application this is generally preferred over functional testing.

Table 1: LTM4601A daisy chain test results of LGA & BGA
Click on image to enlarge

Note (1): When the 5 failures in table 1 are analyzed with a Weibull distribution plot, it predicts a 1-percent failure point at 1780 cycles. 

Note (2): Subsequent further testing of LGA products with SAC305 (Pb-free) paste to 6000 cycles shows zero failures out of a total of 240 samples of various other µModule products.

Note (3): All BGA packages used SAC305 solder balls

2) Functional testing: where the µModule regulator is temperature cycled and tested for correct operation on an evaluation board. Much testing was performed on LGA µModule regulators comparing Pb-free SnAgCu paste with SnPb paste. However, testing was limited to a maximum of 2000 cycles, which did not produce any failures of either solder.

3) Random vibration testing: conducted in accordance with MIL-STD-202G, method 214A over 50Hz to 2000Hz frequency range and represents a very severe test. Testing of the LTM4610A in LGA-133 passed test letter C at 9.26g RMS but failed in the Z-axis with test letter F at 20.71g RMS. Trials on the LTM4601A in the BGA were conducted and the results are shown below in table 2. Both SnPb and SAC305 (Pb-free) solder gave good results, passing at 20.71g RMS.

Table 2: Random vibration test of BGA-133 package

Conclusions

It is likely that the military and aerospace sector will continue to require a mix of component finishes for the foreseeable future and that manufacturers committed to supporting this market must maintain support for SnPb component finishes.

BTCs such as Linear Technology µModule products with gold-plated LGA packaging have proven reliability and wide industry acceptance. Now BGA versions are providing an alternative to LGA for particularly harsh environment applications.

BGAs with RoHS compliant SAC305 (Pb-free) balls using both SAC305 and SnPb solder paste have been shown to be very reliable and capable of greater than 6000 temperature cycles without failure. For applications where SnPb is mandated, there is now a factory built option of SnPb BGA.

In the limited space available, only a small selection of the reliability data has been shown, for more details please refer to the Linear Technology website.

About the author
Steve Munns is Mil-Aero marketing manager at Linear Technology Corporation

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Taking a close look at voids in BGA solder balls

Taking a close look at voids in BGA solder balls:

Researchers are questioning the impact of voids in BGA solder balls. They may not cause defects, which may prompt a revision of standards that set defect levels for voids.

 
January 21, 2013

By Terry Costlow, IPC online editor

Manufacturing processes evolve as clever employees find ways to eliminate defects. Ball grid arrays (BGAs) are a case in point: Some defects like solder ball voids were once considered a major problem, but some researchers say they’re no longer an issue.

As ball sizes and spacing have shrunk and processes have improved, voids in solder balls may no longer be a problem. That’s a marked change from years past, and one that may call for a standard revision.

For years, J-STD-001, Requirements for Soldered Electrical and Electronic Assemblies, and IPC-A-610, Acceptability of Electronic Assemblies, had very specific guidelines for BGA solder balls. The standards say BGAs are defective when voids account for more than 25 percent of an X-ray image area. But many say that as more industry investigations have been completed, that criterion is no longer valid.

When those standards were written, lead spacings were around 1.27 mm and ball diameters were around 30 mils. Now spacings are 0.5-0.8 mm and ball sizes are 10-20 mils.

“At those dimensions, if you get a void in the solder ball, there is a high probability it’s going to be greater than 25 percent of the X-ray image area,” said Dave Hillman, principal materials and process engineer at Rockwell Collins.

Some say that voids degrade the solder ball joint, while others have done research showing that voids have little impact on reliability. Solder joints with voids of any size likely have other problems that would cause failure.

“There may be some confounding issues in test methodologies. When you’re seeing a wide diversity in industry BGA void test results, it can mean test vehicles, test parameters or testing condition setups are having an influence on the test results in contrast to other industry tests,” Hillman said.

Hillman will provide more detail at IPC APEX EXPO® in San Diego February 19-21. The title of his Wednesday morning technical conference session lays out his views on the topic: “The Last Will and Testaments of Tin/Lead and Lead-Free BGA Voids.”

Hillman, who has worked closely with Hewlett Packard’s Chris Troxel and Linda Woody of Lockheed Martin Missile and Fire Control on this issue, feels that it’s time to alter IPC standards to eliminate the defect criteria on voids.

“If all of the industry data is reviewed and BGA voids are determined to not be a defect, voids shouldn’t be in the standards as a defect,” Hillman said.

Instead of focusing on defects, manufacturers should tweak their processes when they see an increase of void occurrence or size. Changes in the appearance, size or shape of solder contacts often suggest that the process is beginning to go out of range. Monitoring the process can be more effective than testing for defects.

“J-STD-001E and IPC-A-610 use process control methodologies/protocols in a number of solder process areas rather than 100 percent visual inspection,” Hillman said.

While noting that there are many benefits to inspection with the broad range of tools now available, Hillman commented that there can be cases where visual inspections are no longer beneficial. That can be the case with solder voids, which aren’t visible without advanced inspection equipment.

“One of the negative aspects of X-ray inspection is that it lets you see things you couldn’t see before. A process engineer now needs to make a determination if the things being seen are a problem or an observation. Voids in BGAs are the poster child for this issue,” Hillman said. “You get more immediate benefits by establishing consistent solder processes than scanning for voids after the fact.”

Hillman feels that it’s important to update standards as more knowledge is gained. When J-STD-001 and IPC-A-610 were written, solder voids were thought to be a problem. But as technology has advanced and new test data is accumulated, researchers have found that they’re something of a red herring.

“A willingness to change workmanship criteria shows the flexibility of the industry and IPC,” Hillman said.

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How Lead-Free Solder (Mis)Behaves under Stress | Advanced Photon Source

How Lead-Free Solder (Mis)Behaves under Stress | Advanced Photon Source:

The reliability and longevity of electronics are critical to our interconnected world that is so dependent upon Internet and telecommunications technology. The computers, routers and switches, phones, and all the devices that link us together must be dependable down to the solder joints that connect integrated circuits to circuit boards, which can be an Achilles heel in an otherwise failure-free device.

Utilizing new synchrotron x-ray techniques and two synchrotron x-ray beamlines at the U.S. Department of Energy Office of Science’s Advanced Photon Source (APS), researchers from Cisco Systems, Inc.; Michigan State University; and the Max-Planck-Institut für Eisenforschung GmbH studied the beginnings of the thermal fatigue process in environmentally friendly, lead-free solder joints, as well as what happens toward the end of that process. These insights bring scientists a step closer to developing useful models for making reliability predictions about solder-joint failure in this new material.

"In electronic system failures, more than half are solder joint failures," said Thomas Bieler of Michigan State University, a coauthor of the two Journal of Electronic Materials articles based on these studies. "There are a lot of these connections, and a failure in any one of them could potentially kill the functionality of the entire system."

The first century of electronics used lead-based solder because it was cheap and easy to work with. Engineers thoroughly understood the electrical, thermal, and structural behavior of these solders and were able to accurately predict reliability. As the environmental dangers of lead became apparent, manufacturers looked for alternatives. In 2006, the European Union Restriction of Hazardous Substances Directive came into effect, banning lead-containing solders from all consumer electronics devices. But while protecting people and the environment, the ban created a new problem: How reliable were lead-free solders?

"Especially in the telecommunications industry, the expected lifetime of many products is long, and also a very low failure rate is needed," said coauthor Tae-Kyu Lee of the Component Quality and Technology Group at Cisco Systems. "Before 2006, the industry had a very large library of reliability data based on lead-containing material, so even if they didn't know the exact failure mechanism, they could still assess the lifetime of the product. But since we're now using a new material, we need to know more about how the material’s performance evolves."

The researchers in this study developed new techniques and employed them at the APS to track the development of the failure mechanism of lead-free solder joints in situ for the first time. Working at the X-ray Science Division 6-ID-D and 34-ID-E beamlines at the Argonne National Laboratory APS, they conducted experiments to demonstrate how synchrotron x-ray diffraction could reveal the evolution of crystal orientation and strain patterns during thermal cycling in tin-containing, lead-free solder joints, specifically in the commonly used tin-silver-copper alloy SAC 305 (Sn-3%Ag-0.5% Cu).

Older lead-tin solders tend to solidify as a polycrystal, delivering a solder joint that has a uniform microstructure, with reproducible properties. But lead-free solders usually solidify either as a randomly oriented single crystal or a tricrystal. "With the lead-free solders, the properties of solder joints are all over the map," Bieler said. "Some joints have hard orientations, others soft, some last forever, some fail at a very early time. That makes life difficult for electronics manufacturers when they have to think about warranty costs and when the first failure is going to happen."

Failures in lead-free solders don't always occur in areas where shear strains from thermal expansion mismatches are highest. This is a consequence of the non-cubic structure of tin and its highly anisotropic thermal expansion and stiffness. It is difficult to predict the strains that develop in an individual joint, given the random crystal orientation, so the prediction of damage is even more challenging. Furthermore, the crystal orientations evolve with thermal cycling. The experimenters examined SAC 305 joints both in a plastic ball grid array (PBGA) package sample and a wafer-level chip-scale package (WLCSP). Both types have been examined under thermal cycling, and the latter was studied in situ during melting, resolidification, and thermal cycling.

The advantages of the x-ray approach over the cross-sectional electron microscopy generally used to study solder joints were immediately apparent. "In a two-dimensional cross section, one does not necessarily know what has been removed or what is underneath the surface. With three-dimensional full illumination of the joint, we were able to prove that we did indeed primarily have single crystals in some joints and tricrystals in others," said Bieler.

"To clearly predict the lifetime of a certain joint,” said Lee, “we need to understand the failure mechanism. This study allowed us to see inside the joint while imposing thermal cycles in situ. Most of the earlier studies use destructive analysis to see the microstructure development after things happened. But the APS synchrotron lets us see the microstructure evolution during the event."

The x-ray diffraction measurements also provide a more dynamic picture of what happens inside the solder joint as it passes through different phases over time. For example, although there are only a few different tin crystal orientations in a given SAC lead-free solder joint, different orientations are always observed after melting and resolidification. Recrystallization behavior under thermal cycling also results in evolution of the tin crystal orientation.

Lee was surprised by "the overall tin material response to the external factors. I considered tin as a 'dead' material with a certain crystal structure, but the more you look into this material, it seems to react like a living thing."

“What the APS data has especially enabled us to do is get a comprehensive picture of the very beginning of the thermal fatigue process [on beamline 6-ID-D] and connect it with what happens toward the end [on beamline 34-ID-E]," Bieler said.

Observing how the x-ray diffraction peaks changed from initial solidification and proceeded to spread out over repeated thermal cycles, Bieler said, "really helped us get to the point where we could describe the sequential mechanism from the moment of solidification up to the first crack that forms. We are able to explain everything that happened and why it happened in these solder joints as a result of what we've been able to do at the APS. The APS has been very helpful for understanding the early stage as well as the stage just before cracks form."

These fresh insights into the failure mechanism of lead-free solder joints guide scientists in their development of models for making reliability predictions. Although it is too early to proclaim any definite remedy for improving reliability in lead-free solder joints, the work provides some important clues.

"To find the right mitigation, you need to find the right failure mechanism," Lee said. "Based on the study so far, we identified that the grain refinement in tin interconnects is bad for thermal cycling performance. But at the same time it is beneficial for resistance to mechanical shock. So there are no golden rules, but at least we are beginning to understand why."

As for next steps, Bieler said that "finding a reliable, automated way to index diffraction patterns from multi-crystal joints would greatly speed up our ability to mine the data we have more effectively. Analysis strategies to obtain stress and strain would naturally follow using established methods. Continuing in situ measurements in thermomechanical cycling specimens, and looking toward comparing the microstructure before and after shock conditions (or possibly in situ), could also be effective. Microbeam diffraction in solder bumps is also a desirable thing to do, as the volumes are small, and this method is well suited for exploring tiny places." — Mark Wolverton


http://www.aps.anl.gov/Science/Highlights/Content/APS_SCIENCE_20121008.php

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Key reliability concerns with lead-free connectors

The key reliability issues of lead-free connectors are presented. The electrical contact resistance of lead-free and lead-based solder-dipped contacts under various aging conditions is discussed first, followed by the fretting corrosion of lead-free contacts. Also presented is the reliability due to tin whiskers induced by mating pressure between connector contact elements.



Tadahiro Shibutani & Qiang Yu, Faculty of Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya, Yokohama, Kanagawa 240-8501, Japan

Michael Pechta, Center of Advanced Life Cycle Engineering (CALCE), University of Maryland, College Park, Maryland, United States

Ji Wuc, Intel Corporation, Chandler, Arizona, United States


Microelectronics Reliability
Volume 48, Issue 10, October 2008, Pages 1613–1627



ScienceDirect.com - Microelectronics Reliability - Key reliability concerns with lead-free connectors:

Comparison of SAC105 and SAC305 Solders

Many BGA and CSP component suppliers have begun shipment of components with a variety of “second generation” Pb-free solder ball alloys.  Much of the motivation for the alloy changes has been to improve mechanical shock resistance.  Several publications have established the improved performance of such 2nd
 level BGA/CSP sphere alloys; however, much less has been published regarding the thermal fatigue resistance of components with these new Pb-free ball alloys.  As these components and alloys become mainstream, their use in situations where thermal fatigue resistance is critical to product life will become an important consideration.  Therefore, an understanding of thermal fatigue performance for new alloys is
necessary for OEM/ EMS/ ODM companies to make design and procurement decisions, and for component suppliers to ensure the reliability of their products under a range of field use conditions.
In this study, the thermal fatigue performance under accelerated test conditions is compared for three common BGA ball alloys: SAC105, Sn-3.5Ag, and SAC305 as a control.  Accelerated thermal cycle (ATC) testing was performed using 676 PBGA components with 1.0 mm pitch and electrolytic Ni/Au finished component pads.  These components were assembled to high-temperature rated Cu-OSP coated printed circuit boards using SAC 305 solder paste, which represents one of the most common assembly practices in industry today.  ATC testing was performed using the IPC-9701A TC1 condition of 0/100°C with 10-minute dwells (nominal); 3 different failure criteria were used in constructing the Weibull failure curves.  The data indicate that SAC105 has the lowest thermal fatigue resistance among the alloys tested, with Sn-3.5Ag and SAC
305 having similar and superior performance.  The impact of failure criterion on the Weibull curves is also presented.  The implications of these findings and areas for further study are discussed.


Authored By:
 Gregory Henshall, Hewlett-Packard Co.
 Jasbir Bath, Bath Technical Consultancy
 Sundar Sethuraman, Jabil
 David Geiger, Flextronics International
 Ahmer Syed, Amkor
 M.J. Lee, Xilinx
 Keith Newman and Livia Hu, Sun Microsystems
 Dong Hyun Kim and Weidong Xie, Cisco Systems
 Wade Eagar and Jack Waldvogel, Motorola



http://www.ipcoutlook.org/pdf/comparison_thermal_fatigue_ipc.pdf

IPCWorksAsia 2012

IPC WorksAsia 2012 to build the South China electronics industry technology festival

PCB & electronic assembly and cutting-edge technology / best practices / Green Manufacturing

Time: 30 to 31st October 2012

Location: Shenzhen, Shihua Hotel

Organizers: IPC-International Electronics Industry Association Connecting

IPC engineers in Shenzhen, southern China, the majority of electronics companies, professional and technical personnel, functional managers are meeting to create a PCB & electronic assembly and cutting-edge technology / best practices / green manufacturing technology festival! Economic slowdown, lack of demand, rising costs, the opportunity to learn a changing set of technologies, processes, management and policy in one place, absolutely not to be missed!
•Activities: theme seminars , experienced professionals bringing together well-known domestic and foreign enterprises and academia, side by side hot topics dedicated to advanced theories on PCB & assembly, best practices, management and policy.
•Activity Two: Reliability seminar, lectures by experts from well-known companies and laboratories on ways to improve product reliability.

Seats are limited, please register reserved seats as soon as possible!


http://www.ipc.org.cn/Events/IPCWorksAsia-2012/default.htm

THE Pb-FREE IN ELECTRONICS RISK MITIGATION (PERM) CONSORTIUM

A paper has just be released describing the organization, content, and expectations of the newly formed Pb-free Electronics Risk Mitigation (PERM) consortium and its stakeholders. PERM was created as a more focussed group of the LEAP consortium.

The AIA-sponsored Pb-free Electronics in Aerospace  Project (LEAP) has been the premier industry working group in the the defense/aerospace industry since 2004.  It has worked to identify and address the risks and other challenges associated with the use of Pb-free electronics in high performance & high reliability equipment.  It has successfully provided resources such as handbooks and standards as well as templates, guidelines, and other tools to support risk mitigation.  However, the use of Pb-free materials has grown to such an extent that many components/parts are no longer available in tin-lead (either surface finishes or interconnection media).   Engineers have been forced to use some Pb-free parts in their new designs or,  in  many  cases,  as  part  of retrofits and upgrades, leading to concern over performance and reliability.

An ad hoc sub-committee of the LEAP team spent over a year producing a more focused entity called the Pb-free Electronics Risk Mitigation (PERM) consortium, to provide overarching executive leadership and coordination of Pb-free electronics risk management activities for the aerospace and defense community on both the government and industry side.   It is focussed on those activities that provide value-added results.  More information


http://www.ipcoutlook.org/pdf/pb_free_risk_mitigation_smta.pdf

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Tin whisker mitigation research to be performed by Rockwell Collins for DoD program – Military Embedded Systems

A team lead by Rockwell Collins has been selected by the Department of Defense's (DoD's) Strategic Environmental Research and Development Program to lead team a research project on the Tin Whiskers Inorganic Coatings Evaluation (TWICE) program. The research will focus on mitigating the impact of tin whiskers on high-reliability, high-performance electronic systems caused by new lead-free alloys and finishes in the defense and aerospace and defense manufacturing systems and supply chain.

The project is expected to form a better scientific understanding of the mechanisms of tin whisker formation and how they may be managed with conformal coatings. Materials and the processes necessary to generate coatings that could mitigate tin whisker growth on a variety of surfaces over a wide range of environmental conditions will be developed.


Tin whisker mitigation research to be performed by Rockwell Collins for DoD program – Military Embedded Systems:

Proportional hazards model for reliability analysis of solder joints under various drop-impact and vibration conditions

A new study on Mechanical Engineering is now available. According to news reporting out of Shanghai, People's Republic of China, by VerticalNews editors, researchers stated "This paper is devoted to the use of a proportional hazards model (PHM) for reliability analysis of solder joints through two case studies: a drop-impact test and a vibration test. The lifetime data collected under various load conditions can be integrated in establishing a PHM for reliability analysis of solder joints."

Our news journalists obtained a quote from the research by the authors from Shanghai Jiao-Tong University, "PHM has the capability of evaluating the influence of load conditions (e. g. drop height, power spectrum density in random vibration) on the life span of solder joints. Using this capability, this paper focuses on the implementation of PHM for estimating the reliability characteristics (MTTF and probability density function (p.d.f.) of failure) of solder joints whose load conditions are not contained in the training dataset of the PHM. In comparison with the naive use of lifetime data by simply ignoring the effect of load conditions, the two case studies verify the effectiveness of PHM in providing consistent reasonable estimation of the reliability characteristics of solder joints."

According to the news editors, the researchers concluded: "The perspectives and the procedures of the use of PHM in this paper are generic and can be directly adopted for other systems/products."

For more information on this research see: Proportional hazards model for reliability analysis of solder joints under various drop-impact and vibration conditions. Proceedings of the Institution of Mechanical Engineers Part O-Journal of Risk and Reliability, 2012;226(O2):194-202. Proceedings of the Institution of Mechanical Engineers Part O-Journal of Risk and Reliability can be contacted at: Sage Publications Ltd, 1 Olivers Yard, 55 City Road, London EC1Y 1SP, England.

6th International Symposium on Tin Whiskers - Call for Partcipation

Whisker Growth on Tin Plated Lead
The Center of Advanced Life Cycle Engineering at the University of Maryland and Department of Materials Research at Loughbourugh University are pleased to announce the Sixth International Symposium on Tin Whiskers. Tin whiskers present a unique challenge to the electronics industry. There have been numbers of electronics failures in the market caused by tin whiskers since 1940s. After 2000, as a result of the global transition to lead-free electronics, the majority of the electronic component manufacturers are now using pure tin or tin-rich alloys for terminal and lead finishes. The increased used of tin based lead-free finishes and materials, focused concern and research on tin whiskers particular for long life and mission critical applications, such as space, aviation, and implantable medical devices.
 
Whisker Induced Metal Vapor Arc

A tin whisker is a conductive tin crystal, which can spontaneously grow from tin based lead-free finished surfaces even at room temperature, often in a needle-like form. Oxidation in humid atmosphere, corrosion, intermetallic formation, stress under thermal cycling, external pressure in fine pitch connectors and electromigration have been shown to promote whisker formation. However, acceleration models for whisker growth are very limited or not existent.

Listing of proceeding of prior International Symposia on Tin Whiskers can be found at http://www.calce.umd.edu/tin-whiskers/symposia.htm

This symposium presentations can cover case histories, theories of tin whisker growth experiments and results, risk evaluation methods and risk mitigation strategies. Presentations submitted for this year’s symposium may cover case histories, theories of tin whisker growth, experiments and results, risk evaluation methods and risk mitigation strategies. Free admission will be provided to symposium participants.


Novermber 27-28, 2012
Ford College, Loughborough University Leicestershire, UK LE11 3TU

Organized by

Department of Materials Research at Loughbourugh University

Center for Advanced Life Cycle Engineering ( CALCE )

Contact

Dr. Michael Osterman, CALCE, University of Maryland, osterman@calce.umd.edu

Dr Geoffrey D. Wilcox, Department of Materials Research, G.D.Wilcox@lboro.ac.uk


http://www.calce.umd.edu/cgi-bin/entityTools/webmultidbhtmlfrm.cgi?events+standard.irp+7

Rate-dependent behavior of Sn alloy-Cu couples: Effects of microstructure and composition on mechanical shock resistance

Scopus: Solders serve as electrical and mechanical interconnects in electronic packaging. The mechanical shock behavior of a Pb-free solder joint is quite complex, since the influences of solder microstructure, intermetallic compound (IMC) layer thickness, and strain rate on the overall dynamic solder joint strength need to be quantified. Dynamic solder joint strength is hypothesized to be controlled by two factors. At low strain rates it should be controlled by the bulk solder, whereas at high strain rates it may be controlled by the brittle intermetallic compound layer. In this paper, the dynamic solder joint strength of Sn-3.9 Ag-0.7 Cu solder joints was experimentally measured over the strain rate range 10 -3-12 s -1. The influences of changes in solder microstructure and IMC layer on dynamic solder joint strength were quantified, and visualized in three dimensions. Fracture mechanisms operating in the solder-controlled and IMC layer-controlled dynamic strength regimes are discussed. Finally, qualitative numerical simulations were conducted, which accurately depict the experimentally observed fracture behaviors

 Yazzie, K.E.a, Fei, H.E.b, Jiang, H.b, Chawla, N.ab 

a  Materials Science and Engineering, School for Engineering of Matter, Transport, and Energy, Arizona State University, Tempe, AZ 85287-6106, United States
b  Mechanical and Aerospace Engineering, School for Engineering of Matter, Transport, and Energy, Arizona State University, Tempe, AZ 85287-6106, United States

An approach to life consumption monitoring of solder joints in operating temperature environment

Scopus This paper elaborates the 3T-approach to life consumption monitoring of solder joints in operating temperature environment without requiring simplification of operating loads.

(Dont really know what this is - sounded interesting... - Jeremy)


2012 13th International Thermal, Mechanical and Multi-Physics Simulation and Experiments in Microelectronics and Microsystems, EuroSimE 2012
2012, Article number 6191699
2012 13th International Thermal, Mechanical and Multi-Physics Simulation and Experiments in Microelectronics and Microsystems, EuroSimE 2012;Cascais;16 April 2012through18 April 2012;Code89825
An approach to life consumption monitoring of solder joints in operating temperature environment  ( Conference Paper )

Johansson, J.ab, Belov, I.a , Johnson, E.c, Leisner, P.ac 

a  Dept. Mechanical Engineering, School of Engineering, Jönköping University, Box 1026, SE-551 11 Jönköping, Sweden
b  Saab AB, Business Area Electronic Defence Systems, Box 1017, SE-551 11 Jönköping, Sweden
c  SP Technical Research Institute of Sweden, Box 857, SE-501 15 Borås, Sweden

A comparison of the creep behaviour of joint-scale SAC105 and SAC305 solder samples under shear conditions

Scopus: This study compares the creep behavior of joint-scale Sn1.0Ag0.5Cu (SAC105) and Sn3.0Ag0.5Cu (SAC305) solder samples under shear loading. The basis of the comparison is experimentally derived Anand viscoplastic constitutive models for the alloys. A series of monotonic constant shear stress and constant shear strain rate tests was conducted at temperatures of 20'C, 50'C, 75'C and 100'C for SAC105 in order to provide data to extract the constitutive model parameters. The predictions of the Anand model are compared graphically with the experimental data for SAC105 in order to illustrate goodness-of-fit. The Anand parameters are shown to capture the creep performance of the SAC105 solder under shear loading very well, with the experimental data being tightly bound to the Anand predictions. For comparison, model parameters for SAC305 are taken from previously published work at the authors' institution. In terms of practical usage, SAC105 is noted to be preferable for applications which may experience high strain rate stimuli (portable electronic devices) as the alloy undergoes plastic flow at lower stress levels than SAC305, minimizing the risk of interfacial failures. Conversely, SAC305 is superior for applications which feature thermo-mechanical fatigue (desktop computers, for example, or servers) as it accumulates less strain than SAC105


2012 13th International Thermal, Mechanical and Multi-Physics Simulation and Experiments in Microelectronics and Microsystems, EuroSimE 2012
2012, Article number 6191722
2012 13th International Thermal, Mechanical and Multi-Physics Simulation and Experiments in Microelectronics and Microsystems, EuroSimE 2012;Cascais;16 April 2012through18 April 2012;Code89825
A comparison of the creep behaviour of joint-scale SAC105 and SAC305 solder samples under shear conditions  ( Conference Paper )

Burke, C. , Punch, J. 

CTVR, Stokes Institute, University of Limerick, Ireland

The bulk alloy microstructure and tensile properties of Sn-1Ag-0.5Cu-xAl lead-free solder alloys (x = 0, 1, 1.5 and 2 wt.%)

Scopus : The purpose of this paper is to investigate the effect of Al addition on the bulk alloy microstructure and tensile properties of the low Ag-content Sn-1Ag-0.5Cu (SAC105) solder alloy.

Design/methodology/approach - The Sn-1Ag-0.5Cu-ITx/ITAl (ITx/IT=0, 1, 1.5 and 2?wt.%) bulk solder specimens with flat dog-bone shape were used for tensile testing in this work. The specimens were prepared by melting purity ingots of Sn, Ag, Cu and Al in an induction furnace. Subsequently, the molten alloys were poured into pre-heated stainless steel molds, and the molds were naturally air-cooled to room temperature. Finally, the molds were disassembled, and the dog-bone samples were removed. The solder specimens were subjected to tensile testing on an INSTRON tester with loading rate 10UP-3/UP?sUP-1/UP. The microstructural analysis was carried out using scanning electron microscopy/Energy dispersive X-ray spectroscopy. Electron Backscatter Diffraction (EBSD) analysis was used to identify the IMC phases. To obtain the microstructure, the solder samples were prepared by dicing, molding, grinding and polishing processes. Findings - The addition of Al to the SAC105 solder alloy suppresses the formation of AgDN3/DNSn and CuDN6/DNSnDN5/DN IMC particles and leads to the formation of larger Al-rich and Al-Cu IMC particles and a large amount of fine Al-Ag IMC particles. The addition of Al also leads to refining of the primary -Sn grains. The addition of Al results in a significant increase on the elastic modulus and yield strength. On the other hand, the addition of Al drastically deteriorates the total elongation. Originality/value - The addition of Al to the low Ag-content SAC105 solder alloy has been discussed for the first time. This work provides a starting-point to study the effect of Al addition on the drop impact and thermal cycling reliability of the SAC105 alloy.


Microelectronics International
Volume 29, Issue 2, 2012, Pages 108-116
The bulk alloy microstructure and tensile properties of Sn-1Ag-0.5Cu-xAl lead-free solder alloys (x = 0, 1, 1.5 and 2 wt.%)

Shnawah, D.A.-A.a , Sabri, M.F.M.a, Badruddin, I.A.a, Che, F.X.b 

a  Department of Mechanical Engineering, University of Malaya, Kuala Lumpur, Malaysia
b  Institute of Microelectronics, A STAR (Agency for Science, Technology and Research), Singapore, Singapore

Characterisation of lead-free solders at high strain rates considering microstructural conditions

Scopus: This work focuses on the mechanical behaviour of lead free solder alloys under high strain rate conditions. The knowledge about this specific material behaviour and the ability of consideration in finite element modelling are especially important for reliability investigations on mobile communication and entertainment devices. Since the mechanical behaviour of solders is strongly dependent on their micro- and grain structure two solder alloys SnAg1.3Cu0.5 (SAC) and SnAg3.5 (SA) have been considered in this work. After solder joint reflow changes of the micro- and grain structure occur due to thermal loading of the joints. Therefore isothermal ageing has been conducted to enable the comparison of as cast and aged solder condition. The strain rate dependent mechanical behaviour of the named alloys has been investigated by the usage of tensile experiments on miniature dog bone specimens. The effective specimen part with a diameter of 1 mm allowed to stay close to the dimensions of real solder joints and hence to their micro- and grain structure. Experiments have been accomplished at room temperature. The isothermal ageing at 150°C lasted for 1000 h to reach considerable structural changes. The applied strain rates covered a range of 25 s -1 to 870 s -1. The strain rate depending material behaviour of both solders was incorporated in a finite element model and used to analyse the solder joint stress during a standard drop test experiment.


2012 13th International Thermal, Mechanical and Multi-Physics Simulation and Experiments in Microelectronics and Microsystems, EuroSimE 2012
2012, Article number 6191786
2012 13th International Thermal, Mechanical and Multi-Physics Simulation and Experiments in Microelectronics and Microsystems, EuroSimE 2012;Cascais;16 April 2012through18 April 2012;Code89825
Characterisation of lead-free solders at high strain rates considering microstructural conditions  ( Conference Paper )

Meier, K.a , Kraemer, F.b, Roellig, M.c, Wolter, K.-J.a 

a  Technische Universität Dresden, Electronics Packaging Laboratory, D-01062 Dresden, Germany
b  Saarland University, Saarbruecken, Germany
c  Fraunhofer IZFP-D, Dresden, Germany

Acceleration of the growth of Cu 3Sn voids in solder joints

Scopus: Soldering to Cu surface finishes usually leads to the formation of a bi-layer intermetallic structure, Cu 3Sn/Cu 6Sn 5, that provides a more robust bond than common alternatives. Occasionally, and so far unpredictably, voids may however grow within the Cu 3Sn over time and allow for premature failure of microelectronics products in service. A quantitative assessment of the reliability risk of voids observed after accelerated aging requires the knowledge of the variation of void growth with temperature and time. It is argued that in the case of realistic solder joints diffusion controlled void growth kinetics are unlikely to follow simple Arrhenius and parabolic dependencies, respectively. Nevertheless, three very different sets of samples were all shown to exhibit void growth that could be well approximated by a parabolic time dependence and an effective activation energy of 0.65-0.80 eV.



Microelectronics Reliability
Volume 52, Issue 6, June 2012, Pages 1121-1127
Acceleration of the growth of Cu 3Sn voids in solder joints

Borgesen, P.a, Yin, L.b , Kondos, P.b 

a  Department of Systems Science and Industrial Engineering, Binghamton University, Binghamton, NY 13902, United States
b  Universal Instruments Corporation, Conklin, NY 13748, United States

Drop impact reliability of Sn-1.0Ag-0.5Cu BGA interconnects with different mounting methods

Scopus: The poor drop-shock resistance of near-eutectic Sn-Ag-Cu (SAC) solder interconnects drives the research and application low-Ag SAC solder alloys, especially for Sn-1.0Ag-0.5Cu (SAC105). In this work, by dynamic four-point bend testing, we investigate the drop impact reliability of SAC105 alloy ball grid array (BGA) interconnects with two different surface mounting methods: near-eutectic solder paste printing and flux dipping. The results indicate that the flux dipping method improves the interconnects failure strain by 44.7% over paste printing. Further mechanism studies show the fine interfacial intermetallic compounds (IMCs) at the printed circuit board side and a reduced Ag content inside solder bulk are the main beneficial factors overcoming other negative factors. The flux dipping SAC105 BGA solder joints possess fine Cu 6Sn 5 IMCs at the interface of solder/Cu pads, which increases the bonding strength between the solder/IMCs and the fracture resistance of the IMC grains themselves. Short soldering time of flux dipping joints above the solder alloy liquidus mitigates the growth of interfacial IMCs in size. In addition, a reduced Ag content in flux dipping joint bulk causes a low hardness and high compliance, thus increasing fracture resistance under higher-strain rate conditions.


Microelectronics Reliability
Volume 52, Issue 7, July 2012, Pages 1475-1482
Drop impact reliability of Sn-1.0Ag-0.5Cu BGA interconnects with different mounting methods

Wang, B.ab, Li, J.a, Gallagher, A.c, Wrezel, J.c, Towashirporn, P.c, Zhao, N.a  

a  School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China
b  Motorola Mobility Technologies Co. Ltd., Tianjin 300457, China
c  Motorola Inc., Libertyville, IL 60048, United States

Gap Size Effects on the Shear Strength of Sn/Cu and Sn/FeNi Solder Joints

Scopus: The effect of the gap size on the shear strength of a solder joint was investigated in both Sn/Cu and Sn/FeNi lap solder joints by varying the gap size from 20 μm to 300 μm. In the Sn/FeNi joints, the shear strength remained relatively constant, around 14 MPa, independent of the gap size. In contrast, the shear strength of the Sn/Cu joint decreased about 20% from 25 MPa to 20 MPa. The decrease was shown to result from Cu 6Sn 5 precipitation in the thicker Sn/Cu joint, which was absent in the thinner joints


Journal of Electronic Materials
2012, Pages 1-8
Gap Size Effects on the Shear Strength of Sn/Cu and Sn/FeNi Solder Joints  ( Articles not published yet, but available online Article in press About articles in press (opens in a new window) )

Chen, C., Zhang, L. , Zhao, J., Cao, L., Shang, J.K. 

Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of, Shenyang, 110016, China