Lead-Free and Mixed Assembly Solder Joint Reliability Trends

This paper presents a quantitative analysis of solder joint reliability data for lead-free Sn-Ag-Cu (SAC) and mixed assembly (SnPb + SAC) circuit boards based on an extensive, but non-exhaustive, collection of thermal cycling test results. The assembled database covers life test results under multiple test conditions and for a variety of components: conventional SMT (LCCCs, resistors), Ball Grid Arrays, Chip Scale Packages (CSPs), wafer-level CSPs, and flip-chip assemblies with and without underfill. First-order life correlations are developed for SAC assemblies under thermal cycling conditions. The results of this analysis are put in perspective with the correlation of life test results for SnPb control assemblies. Fatigue life correlations show different slopes for SAC versus SnPb assemblies, suggesting opposite reliability trends under low or high stress conditions. The paper also presents an analysis of the effect of Pb contamination and board finish on lead-free solder joint reliability. Last, test data are presented to compare the life of mixed solder assemblies to that of standard SnPb assemblies for a wide variety of area-array components. The trend analysis compares the life of area-array assemblies with: 1) SAC balls and SAC or SnPb paste; 2) SnPb balls assembled with SAC or SnPb paste. Introduction Attachment reliability being the ability of solder joints to survive the planned design life of a given product, blanket statements as to the reliability, or the lack of it, of lead-free assemblies need be put in context and generalizations should be based on extensive supporting data. Reliability is application-specific and can only be established through the careful acquisition of failure distributions under accelerated test conditions, followed by careful extrapolation of test failure times to field service conditions of interest to a particular application. To this author’s knowledge, the latter step has not been enabled, as of yet, given the lack of proven lead-free life prediction models or acceleration factors in the public domain. Numerous efforts to that effect are in progress (see references, 1-4 for example). The development of SnPb solder joint reliability models spread over three-decades starting with the landmark paper of Norris and Landzberg in 1969. It would be optimistic to expect robust lead-free reliability models to become available in a semester or two. In the mean time, it is useful to gather reliability test data for comparison purposes. Our experience working with SnPb and lead-free assemblies has been that a wide range of test data need be consolidated to establish firm reliability trends. In this paper, we present the results of a quantitative analysis of SAC and mixed assembly thermal cycling data. The main objective is to put the data in perspective, presenting lead-free versus SnPb test results for a variety of components, board finish and test conditions. Empirical Correlations of SAC Thermal Cycling Test Data Figure 1a is an attempt at correlating characteristic lives and cyclic shear strain ranges as per the classical Coffin-Manson approach for fatigue of metals. The characteristic lives (cycles to 63.2% failures) are from failure distributions for 100% leadfree SAC assemblies and component populations subject to accelerated thermal cycling under a range of conditions: 0 to 100°C, -40 to 125°C and -55 to 125°C. The cyclic shear strain range is defined as: h T L ∆ ∆ = ∆ α γ where L is the maximum Distance to Neutral Point (DNP) for the outermost joints, ∆α is the board-to-component mismatch in Coefficients of Thermal Expansion (CTE), h is the average component stand-off height or joint thickness, and ∆T is the temperature difference between the hot and cold sides of a given thermal profile. All components were leadless and mounted on FR-4 type boards: resistors (sizes: 0603, 1206 and 2512), 32 x 32 mm and 42 x 42 mm square Ceramic Ball Grid Arrays (CBGAs), 169 I/O Chip Scale Packages (CSP), 20 I/O Leadless Ceramic Chip Carriers (LCCC) and 1.27 mm pitch Plastic Ball Grid Arrays (PBGAs). The reader is referred to the original sources of data for more detailed information on components, test vehicle parameters, other conditions and test results. Twenty seven data points are shown in Figures 1a and 1b. Nominal alloy composition varied slightly (Sn3.8Ag0.7Cu or Sn3.9Ag0.6Cu), board finishes were immersion Ag, Organic Solderability Preservative (OSP) or NiAu. The board finish was unspecified for three of the 27 datasets. The correlation coefficient for the power-law trendline going through the data in Figure 1a is R = 0.64. Clearly, there is a discrepancy between the data and the basic Coffin-Manson approach. The data is re-plotted in Figure 1b with the following modifications: 1) instead of characteristic lives for a population of components, cyclic lives are given on a per joint basis as was done in the past in the development of SnPb reliability models – this allows to account for the number of most critical joints per component, 2) the joint characteristic life is then scaled for the solder joint crack area, as justified in previous work. The scaling of characteristic lives for the solder crack area is intended to account for differences in crack propagation times through joints of varied sizes. The resulting parameter characteristic life over the solder joint area is Presented at IPC Printed Circuits Expo® SMEMA Council APEX® Designers Summit 04