Thermal Cycling Reliability of Chip Resistor Lead Free Solder Joints

The solder joint reliability of ceramic chip resistors assembled to laminate substrates has been a long time concern for systems exposed to harsh environments such as those found in automotive and aerospace applications. This is due to a combination of the extreme temperature excursions experienced by the assemblies along with the large coefficient of thermal expansion mismatches between the alumina bodies of the chip resistors and the glass-epoxy composites of the printed circuit boards (PCBs). These reliability challenges are exacerbated for components with larger physical size (distance to neutral point) such as the 2512 resistors used in situations where higher voltages and/or currents lead to power dissipations up to 1 Watt. In this work, the thermal cycling reliability of several 2512 chip resistor lead free solder joint configurations has been investigated. In an initial study, a comparison has been made between the solder joint reliabilities obtained with components fabricated with both tin-lead and pure tin solder terminations. In the main portion of the reliability testing, two temperature ranges (-40 to 125 C and -40 to 150 C) and five different solder alloys have been examined. The investigated solders include the normal eutectic SnAgCu (SAC) alloy recommended by earlier studies (95.5Sn-3.8Ag-0.7Cu), and three variations of the lead free ternary SAC alloy that include small quaternary additions of bismuth and indium to enhance fatigue resistance. For each configuration, thermal cycling failure data has been gathered and analysed using two-parameter Weibull models to rank the relative material performances. The obtained lead free results have been compared to data for standard 63Sn-37Pb joints. In addition, a second set of thermally cycled samples was used for microscopy studies to examine crack propagation, changes in the microstructure of the solders, and intermetallic growth at the solder to PCB pad interfaces. INTRODUCTION Legislation that mandates the banning of lead in electronics, due to environmental and health concerns, has been actively pursued in several countries during the past 15 years. Although the products covered by and implementation deadlines for such legislation continue to evolve, it is clear that laws requiring conversion to lead-free electronics are becoming a reality. Other factors that are affecting the push towards the elimination of lead in electronics are the market differentiation and advantage being realized by companies producing so-called “green” products that are lead-free. A large number of research studies have been performed and are currently underway in the lead-free solder area. Detailed reports on multi-year studies have been published by the National Center for Manufacturing Sciences (NCMS) and the National Electronics Manufacturing Initiative (NEMI), as well as other consortia. Although no “drop in” replacement has been identified for all applications; Sn-Ag, Sn-Ag-Cu (SAC), and other alloys involving elements such as Sn, Ag, Cu, Bi, In, and Zn have been identified as promising replacements for standard 63Sn-37Pb eutectic solder. There have been many reports that solder joint reliability can actually be increased for a given application by using a lead-free replacement alloy such as Sn-Ag-Cu instead of conventional Sn-Pb. However, this conclusion is not universal, and the degree of reliability improvement or degradation is package/design and environment dependent. In thermal cycling reliability environments, Sn-Ag-Cu alloys appear to often outperform Sn-Pb. This has been found for solder joints in more compliant package-board assemblies such as leaded components (e.g. QFPs) and Plastic Ball Grid Array (PBGA) applications [1], and also for more stiff components (CBGA) subjected to small temperature changes [2]. However, for very stiff components with a high coefficient of thermal expansion (CTE) mismatch with the substrate (e.g. CBGA on FR-4, and non-underfilled flip chip on laminate), the solder joint reliability is typically poorer for lead-free SnAg-Cu alloys in thermal cycling tests with large swings between the temperature extremes [3-4]. Another common Surface Mount Technology (SMT) configuration involving stiff components with high CTE mismatch to the substrate is the use of ceramic (alumina) bodied chip resistors on organic substrates (e.g. glass-epoxy laminates). The solder joint reliability of such components has been a long time concern for harsh environments with extreme temperature excursions, such as those found in automotive and aerospace applications. These reliability challenges are further exacerbated for components with larger physical size (distance to neutral point) such as the 2512 resistors used in situations where higher voltages and/or currents lead to power dissipations up to 1 Watt. Crack growth and the resulting shear strength degradation for both Sn-Ag-Cu and Sn-Pb-Ag chip resistor solder joints have been examined during thermal cycling [5]. It was found that the fatigue resistances between lead bearing and SAC solder joints were not significantly different, but were influenced by the component type and board metallization. In addition, crack length variation has been measured in chip capacitor solder joints as a function of thermal cycling [6]. Cracks were found to grow fastest in the Sn-Pb-Ag solder joints relative to several lead free alternatives. Finally, the changes in microstructure occurring in Sn-Ag-Cu chip resistor solder joints during thermal cycling have been recently examined [7]. A power law relationship was established between the number of cycles to crack initiation and the average β -Sn phase growth parameter. In this work, the thermal cycling reliability of several 2512 chip resistor lead free solder joint configurations has been investigated. In an initial study, a comparison was made between the solder joint reliabilities obtained with components fabricated with both tin-lead and pure tin solder terminations. In the main portion of the reliability testing, two temperature ranges (-40 to 125 C and -40 to 150 C) and five different solder alloys have been examined. The investigated solders include the normal eutectic SnAgCu (SAC) alloy recommended by earlier studies (95.5Sn-3.8Ag-0.7Cu), and three variations of the lead free ternary SAC alloy that include small quaternary additions of bismuth and indium to enhance fatigue resistance. For each configuration, thermal cycling failure data has been gathered and analysed using two-parameter Weibull models to rank the relative material performances. The obtained lead free results have been compared to data for standard 63Sn-37Pb joints. In addition, a second set of thermally cycled samples was used for microscopy studies to examine crack propagation, changes in the microstructure of the solders, and intermetallic growth at the solder to PCB pad interfaces. TEST BOARD A test board was developed for examining the thermal cycling reliability of five sizes of chip resistors (2512, 1206, 0805, 0603, 0402). The industry standard naming convention for these chip resistor sizes describes the length and width of the resistor body in hundredths of an inch. For example, the 2512 resistor has a nominal length of .25 inches (6.35mm), and a nominal width of .12 inches (3.05mm). Each board contained 15 of each of the 5 sizes of chip resistors. The 15 resistors in each size were accessible electrically as 3 daisy-chain sets (5 resistors and thus 10 solder joints per chain). The resistor daisy chains were routed to plated through holes at the edge of the board where soldered wire connections could be made for use in resistance monitoring during the thermal cycling tests. Figure 1 is a photograph of an assembled test board. The fabricated test boards included four copper conductor layers, FR-406 glass/epoxy laminate material and had a thickness of 1.57 mm. The external copper traces had either a Hot Air Solder Leveled (HASL) Sn-Pb or an Electroless Nickel Immersion Gold (ENIG) finish. Figure 2 shows a top view of one of the 2512 resistors and a typical uncycled solder joint cross-section.