Elizabeth Benedetto

Characterizing mechanical performance of Board Level Interconnects for In-Circuit Test

Lead Free boards are more susceptible to mechanical stress and therefore more prone to damage during manufacturing, assembly and field use. The IPC/JEDEC 9707(Spherical Bend Test Method for Characterization of Board Level Interconnects) was developed to characterize the mechanical performance of new lead free materials (i.e. laminates), board design features (i.e. pad design), and components (i.e. BGAs). This paper describes the application of the new IPC/JEDEC 9707 standard to qualify the mechanical performance of board interconnects for manufacturing and In-Circuit Test. The efforts of the International Electronics Manufacturing Initiative (iNEMI) Board Flexure Initiative Project team drove the development of this new IPC/JEDEC 9707standard.

The effect of nickel microalloying on thermal fatigue reliability and microstructure of SAC105 and SAC205 solders

This study explores the effect of a nickel (Ni) microalloy addition on the thermal fatigue performance and microstructure of two low Ag content, Pb-free solder alloys, Sn-1.0Ag-0.5Cu (SAC105) and Sn-2.0Ag-0.5Cu (SAC205). The alloy performance was evaluated using two different area array component test vehicles, an 84-pin chip scale package (CSP) and a 192-pin fine pitch ball grid array (BGA). The baseline alloy microstructures were characterized using polarized light microscopy and scanning electron microscopy with backscattered electron imaging for phase identification. Thermal fatigue performance was assessed with accelerated thermal cycling (ATC) using four temperature cycling profiles with distinct temperature ranges (ΔT) and temperature extremes. Additionally, each temperature profile used a standard 10 minute dwell time or an extended 60 minute dwell time. A microalloy addition of 0.05% Ni was found to alter the base microstructures of the SAC 105 and SAC205 alloys. Generally, the Ni addition improved the thermal fatigue life but the improvement was not consistent in both alloys, both components, and across all thermal cycling profiles. The most consistent response was with the 84CTBGA component, which showed improved reliability with the Ni addition in all of the thermal cycles.

Thermal Cycling Reliability of Alternative Low-Silver Tin-Based Solders

Sn-3.0Ag-0.5Cu (SAC305) alloy is the most widely used solder in electronic assemblies. However, issues associated with cost and drop/shock durability have resulted in a continued search for alternative solder alloys. One approach to improve the drop/shock reliability has been to reduce the silver content in Sn-AgCu alloys. Another approach is doping Sn-Ag-Cu solder with additional elements. Moreover, conflicting results have been reported in literature on the effects of aging on Sn-Ag-Cu alloys. In 2008, International Electronics Manufacturing Initiative (iNEMI) started the “Characterization of Pb-Free Alloy Alternatives” project to provide a comprehensive study of fifteen tin-based solder interconnect compositions benchmarked against the eutectic tin-lead solder. For this study, temperature cycle durability was the primary focus and solders were selected to study the effect of varying silver content, microalloy additions, and aging. This paper reports the preliminary findings from one of the test conditions conducted under the iNEMI project. The cycles to failure for a temperature cycling test condition from -15oC to 125oC, with dwell times of 60 minutes at both extremes are presented. The test assembly consisted of sixteen 192 I/O BGAs and sixteen 84 I/O BGAs soldered on to LG451HR laminate. Preliminary findings revealed that the reduction of silver resulted in a reduction in cycles to failure. In all cases, the fifteen tin-based solders were more durable than the eutectic SnPb solder. Aging did not affect the cycles to failure in SAC105 solder; however, the cycles to failure decreased with aging in SAC305 solder. In addition, aging resulted in a wider distribution of cycles to failure in 192 I/O BGAs.

Acoustic emission detection of BGA components in spherical bend

The industry standard process for determining safe assembly bending levels uses destructive failure analysis following destructive stress testing to determine solder joint damage - a time-consuming and expensive process. Modal acoustic emission detection was used to record events during spherical bend tests on boards made from two laminate materials with a BGA component. A sudden increase in the number and magnitude of events was observed at strain levels corresponding to interconnect damage. These results suggest that acoustic emission could be used to detect damage initiation, potentially improving the quality, cost, and speed of this type of test.

Detecting Interconnect Damage in Shock Using Acoustic Emission Detection

Acoustic emission detection was used to detect damage events on electronic assemblies during shock testing. The boards were instrumented with an accelerometer and four acoustic transducers and dropped multiple times at acceleration levels between 100 and 250 g. High intensity acoustic events were identified and the origins of the events were located. Dye stain failure analysis was used to identify the type and location of damage between the interconnection and the circuit board, and showed good agreement with the acoustic events. The results indicate that acoustic emission detection can be applied to this test method with the potential to significantly improve its precision and throughput time.

Acoustic Waveform Energy as an Interconnect Damage Indicator

The industry standard process for determining safe assembly bending levels uses failure analysis following destructive stress testing to determine solder joint damage – a time-consuming and expensive process. Modal acoustic emission detection has been used to indicate the initiation of interconnect damage based on the energy of the waveforms, where a sudden increase in energy was observed at strain levels corresponding to interconnect damage. The test method and results are presented, and the use of event waveform energy is discussed.

Pad crater detection using acoustic waveform analysis

Initial studies have shown that acoustic emission detection may prove an effective technique for pad crater monitoring. Implementation concerns include pad crater locating accuracy and the ability to discriminate between different types of events. This study found that the direction of wave propagation in the circuit board laminate has a significant effect on the event locating calculation due to orientation-dependent acoustic velocity, and that adjustment of the acoustic velocity based on the orientation between event location and transducers can improve locating accuracy to approximately 1 mm. In addition, the use of cumulative acoustic energy is demonstrated as a simple and effective metric for determining the onset of pad cratering. An alteration to the present test method is proposed that would improve the precision, decrease the throughput time, and decrease the cost of the test.

iNEMI Pb-free alloy characterization project report: Thermal fatigue results for low and no-Ag alloys

Significant innovations in Pb-free solder alloy formulations are being driven by volume manufacturing and field experiences. As a result, the industry has seen an increase in the number of Pb-free solder alloy choices beyond the common near-eutectic Sn-Ag-Cu (SAC) alloys first established as replacements for Sn-37Pb. The increasing number of Pb-free alloys provides opportunities to address shortcomings of near-eutectic SAC, such as poor mechanical shock performance, but also introduces a variety of technical and logistical risks. Since 2008, the Pb-Free Alloy Characterization Program sponsored by the International Electronics Manufacturing Initiative (iNEMI) has been working to fill the gap in knowledge associated with thermal fatigue resistance of these new solder alloys. Results from the extensive experimental program are now becoming available and are being published through a series of publications (see References). This paper provides a summary of the overall iNEMIs program goals, the experimental structure, and the results and analysis of thermal cycling for low silver alloys, containing 1 wt.% or less Ag. Results indicated that there is a correlation between the characteristic life of short dwell thermal cycles and Ag content. Increase in the Ag content increased the characteristic life. Another important finding is that all low-and no-Ag alloys performed better than Sn-37Pb under the test conditions. Finally, as the stress levels increase during thermal cycling, the performance differences between the Pb-free alloys diminish, and their performance appears to be approaching that of Sn-37Pb.

Acceptance testing of BGA ball alloys

The industry has seen the development of a wide range of new Pb-free BGA ball alloys. A significant element of uncertainty regarding these new alloys is the lack of defined data requirements for alloy acceptance. This paper describes recent efforts at Hewlett-Packard to develop Pb-free solder alloy testing requirements. To facilitate the standardization of alloy testing, the required tests are divided into three major areas. • Material properties • Solder joint reliability • Impact to manufacturing processes This paper presents the approach to assess the risk of using new Pb-free BGA ball alloys on printed circuit assemblies.