Effect of massive spalling on mechanical strength of solder joints in Pb-free solder reflowed on Co-based surface finishes

Pb-free solder joints undergo microstructural and mechanical evolution due to alloy coarsening and growing intermetallic compounds which degrade the joint electrical performance. Electronics assemblies containing solder joints are frequently exposed to elevated temperatures for prolonged periods of time. The purpose of the study is to discover the effect of isothermal aging on the reliability of Sn-Ag-Cu (SAC) assemblies. After studied different surface finishes, we employed Immersion Ag (ImAg), Immersion Sn (ImSn), Electroless Ni/Immersion Au (ENIG) and Electroless Ni/Electroless Pd/Immersion Au (ENEPIG) which have potential for higher reliability and better performance and received increased attention for both packaging and subtracted applications. A full experiment matrix with varying aging temperatures and solder alloys was considered. Package sizes ranged from 19mm, 0.8mm pitch ball grid arrays (BGAs) to 5mm, 0.4mm pitch μBGAs and in additional, 0.65mm MLF and 2512 resistors be particularly tested. Storage condition are temperatures leveling up from 25°C, 55°C, 85°C, 100°C and 125 °C with aging over time periods of 0, 21 days, 6 months, 12 months and 24 months. Afterwards, the specimens all subjected to accelerated thermally cycled from −40°C to 125°C with 15 min dwell times at the high and low peak temperature. The paper presents the experimental data to justify the investigation of the degradation on the characteristic lifetime of SAC alloy on ImAg and ImSn surface finish in elevated temperature environments.

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Validated High Speed Pull and Shear Test Methodologies to Evaluate Pb-Free BGA Mechanical Strength

International Symposium on Microelectronics ◽

10.4071/isom-2010-ta4-paper2 ◽

2010 ◽

Vol 2010 (1) ◽

pp. 000110-000118

Author(s):

Mudasir Ahmad ◽

Ravi Assudani ◽

Drew Nelson ◽

Amir Youssef

Keyword(s):

Mechanical Strength ◽

High Speed ◽

Solder Joints ◽

Test Method ◽

Lead Free ◽

Lead Free Solder ◽

Mechanical Assembly ◽

Component Level ◽

Reliability Indicator ◽

Free Solder

The conversion to lead free Ball Grid Array (BGA) packages has raised several new assembly and reliability issues. Lead-free solder joints are generally stiffer than tin-lead solder joints, and mechanically induced failures have become more prevalent in lead-free solder assemblies. Traditionally, assembly bend and shock testing is performed to evaluate mechanical assembly reliability. However, bend and shock tests are expensive, cumbersome and not feasible for evaluating lot-to-lot variations in mechanical strength. Consequently, there is a need for a validated, component level test method that can be used as an accurate indicator of assembly level mechanical strength. Several publications have been written to evaluate the efficacy of high speed pull and shear testing as a viable reliability indicator [1 – 10]. In this study, a comprehensive Gauge R&R study was performed to evaluate the accuracy of the test equipment, including high speed video calibration. Then, test studies were performed to compare the accuracy of the results, spread across different package constructions, solder metallurgies, ball pitch and surface finish. In addition, the effect of parameters like multiple reflows and aging on specific metallurgies and surface finishes was studied. The results were generated over more than 2000 test runs. Finally, the study rank orders all critical test parameters and articulates what precise steps can be taken to generate relevant data for standard and custom devices during early evaluation stages and during high volume manufacturing.

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