F.X. Che

Drop impact reliability testing for lead-free and lead-based soldered IC packages

Abstract Board-level drop impact testing is a useful way to characterize the drop durability of the different soldered assemblies onto the printed circuit board (PCB). The characterization process is critical to the lead-free (Pb-free) solders that are replacing lead-based (Pb-based) solders. In this study, drop impact solder joint reliability for plastic ball grid array (PBGA), very-thin quad flat no-lead (VQFN) and plastic quad flat pack (PQFP) packages was investigated for Pb-based (62Sn–36Pb–2Ag) and Pb-free (Sn–4Ag–0.5Cu) soldered assemblies onto different PCB surface finishes of OSP (organic solderability preservative) and ENIG (electroless nickel immersion gold). The Pb-free solder joints on ENIG finish revealed weaker drop reliability performance than the OSP finish. The formation of the brittle intermetallic compound (IMC) Cu–Ni–Sn has led to detrimental interfacial fracture of the PBGA solder joints. For both Pb-based and Pb-free solders onto OSP coated copper pad, the formation of Cu6Sn5 IMC resulted in different failure sites and modes. The failures migrated to the PCB copper traces and resin layers instead. The VQFN package is the most resistant to drop impact failures due to its small size and weight. The compliant leads of the PQFP are more resistant to drop failures compared to the PBGA solder joints.

Drop Impact Reliability Testing for Lead-Free and Leaded Soldered IC Packages

Portable electronic products such as cellular phones, PDAs, and MP3 units are increasingly designed for accidental drop. Repeated drop events can lead to solder joint failure and malfunction of these products. Hence board-level reliability drop testing is a useful way to characterize the drop durability of the printed circuit board with different soldered assemblies. Lead-free (or Pb-free) solders are replacing lead-based solders. Surface mounted electronic packages are getting smaller and with higher density (I/Os). In this study, Plastic Ball Grid Array (PBGA), Quad Flat No-lead (VQFN) and PQFP solder joint reliability characterization by drop impact testing was investigated for lead-based (62Sn-36Pb-2Ag) and lead-free (Sn-4Ag-0.5Cu) soldered assemblies. The influence of different package types and the impact of PCB surface finishes for OSP and ENIG (electroless nickel immersion gold) were studied. The drop test results showed that leadfree solder joints with ENIG finish have weaker drop reliability performance than the case for OSP surface finish. The different solder alloy-to-surface finish type combination results in different intermetallics formed and contributed to different failure sites and mode of failure. The formation of the brittle intermetallics (Cu-Ni-Sn) for Pb-free solder (Sn-4Ag-0.5Cu) with ENIG pad led to detrimental interfacial fracture of the PBGA solder joints. For both lead-based and Pb-free solders with copper-pad and OSP surface finish, the formation of Cu6Sn5 intermetallics resulted in a different failure site and mode. The failures migrated to the PCB copper traces and resin layers. The small size (VQFN) package is most resistant to drop impact failures, due to it’s small size, weight and larger solder volume. The compliant leads for the PQFP is more resistant to drop failures compared to the PBGA solder joints.

Drop impact analysis of Sn-Ag-Cu solder joints using dynamic high-strain rate plastic strain as the impact damage driving force

Board-level drop reliability test and analysis requires dynamic characterization of high strain-rate properties of bulk solder and solder joint failure tests. Drop impact analysis of board-level dynamic response (ie: G-levels and board bending strains) and over-simplification of deformation response of solder joints (ie: assuming elastic stress criteria) can lead to wrong conclusions in the physics-of-failure understanding in drop impact tests. Solder joint failures during drop testing is a complex failure interaction process between low cycle impact fatigue crack growths versus brittle fracture of the intermetallic interfaces. During a drop test event, dynamic hardening causes the yield stress in the solder to rise several times above the nominal monotonic tensile test yield stress. The increase in dynamic strength in the solder joint can cause dynamic strain cycling in the solder material and lead to progressive low cycle impact drop fatigue failures. On the other hand, when the drop loading is excessive, impact failure strength of the intermetallic interface will result in brittle fracture of the solder joint. Impact test were conducted with a split Hopkinson pressure bar (SHPB) test system to study the dynamic response of bulk solder materials and impact failure of solder joint interfaces. Solder joint reliability characterization by drop impact test with clamped-clamped boundary condition were investigated for PBGA assembly with Sn-Ag-Cu solder. FEA modeling and simulation of drop impact were conducted considering different solder constitutive models such as elastic and strain rate dependent plastic model to investigate the effect of solder constitutive model on dynamic response in the solder joint. The important finding of this study is that the constitutive model used has a major impact on dynamic response of solder joint stress and strain results. It was expected that strain rate dependent plastic model gave better correlation results than the simple elastic model. This study also investigates the IMC effect on solder strain response subject to drop impact test simulation

Modeling stress strain curves for lead-free 95.5Sn-3.8Ag-0.7Cu solder

Mechanics of materials characterization of lead-free solder mechanical properties and stress strain curve properties are needed for applications in finite element analysis. In this study, tensile properties for lead-free 95.5Sn-3.8Ag-0.7Cu solder alloy were obtained from bulk tensile specimen tests. A modified Ramberg-Osgood model was developed to describe the temperature and strain rate dependent stress-strain curves for the solder alloys. The modified Ramberg-Osgood model is able to predict the temperature and strain rate effects on the stress-strain properties for lead free 95.5Sn-3.8Ag-0.7Cu solder alloy. Comparison between the proposed modified Ramberg-Osgood model and the Anand viscoplastic model is made in predicting the stress-strain data obtained from the constant strain rate tensile tests.

Fatigue Reliability Analysis of Sn–Ag–Cu Solder Joints Subject to Thermal Cycling

In this paper, thermal cycling tests and finite-element analysis (FEA) for a plastic ball grid array package with Sn-3.8Ag-0.7Cu lead-free solder joints have been performed. The solder joint fatigue lives were predicted and compared by using different 2-D and 3-D FEA models. The effects of solder constitutive models and fatigue life models on solder fatigue life prediction have been investigated. In order to obtain fatigue parameters, new averaging volumes were proposed for the solder fatigue life prediction. Different reference temperatures were simulated to investigate its effect on the solder fatigue life prediction. The effect of intermetallic compound thickness on solder joint fatigue life prediction was also investigated.

Lead free solder joint reliability characterization for PBGA, PQFP and TSSOP assemblies

In this study, thermal cycling test from -40/spl deg/C to 125/spl deg/C with 1 hour per cycle for Sn/sub -3.8/Ag/sub -0.7/Cu solder joint electronic assemblies was conducted for PBGA316, PQFP208, PQFP176, and TSSOP48 components. Two PCB surface finish conditions were investigated for ENIG and OSP. Daisy chain, in-situ resistance monitoring with data logger, was used for failure detection when the resistance value is larger than 300/spl Omega/. A two-parameter Weibull distribution model was used to determine the mean time to failure (MTTF) for different components. The Weibull parameters such as characteristic life and slope are compared for PBGA assembly with Ni/Au and OSP surface finishes. Microscopy was used to determine the crack path and failure mode. It was shown that components with Ni/Au surface finish have higher thermal fatigue life than those with OSP surface finish. PBGA solder joints are more sensitive to thermal fatigue failure than solder joints in PQFP and TSSOP components. More failures occur at the package side than board side for PBGA assembly. Fatigue life prediction was conducted using fatigue life prediction model and FEA analysis results for different component types. Quarter model with submodeling technique was used in FEA simulation due to symmetric geometry and uniform loading for PBGA, PQFP and TSSOP assemblies. Effect of solder joint location on fatigue life was studied based on FEA result for different assemblies. It was shown that test and predicted fatigue life results have a good agreement for different component types.

Comprehensive Modeling of Stress-Strain Behavior for Lead-Free Solder Joints under Board-Level Drop Impact Loading Condition

Board level drop testing is an effective method to characterize the solder joint reliability performance of miniature handheld products. In this study, some drop testing results were summarized based on our previous board-level drop tests. And then the finite element modeling and simulation were conducted to investigate and understand the drop reliability of lead-free solder joints by considering different factors. The strain-rate dependent material properties for lead-free solder has been developed by us and successfully applied in FEA simulation. The important finding of this study is that the constitutive model used has a major impact on dynamic response of solder joint stress and strain results. It was expected that the strain-rate dependent plastic model gave better correlation results than the simple elastic model or bilinear plastic model. In addition to solder material properties, many other factors, including package locations on the PCB, boundary conditions, input-G level, PCB thickness and solder materials, were also simulated to investigate their effects on stress strain performance of solder joint. Comparing to clamped boundary, the 4-screw support condition leads to higher stress level in solder joint. Higher input G-level results in higher solder stress due to larger inertial force and deflection effects on solder joint. The thinner PCB and softer solder can improve the drop performance of board-level electronic assembly.

Evaluation on Influencing Factors of Board-Level Drop Reliability for Chip Scale Packages (Fine-Pitch Ball Grid Array)

Board-level drop testing is an effective method to characterize the solder joint reliability performance of miniature handheld products. In this study, drop test of printed circuit boards (PCBs) with a four-screw support condition was conducted for a 15 mm times 15 mm fine-pitch ball grid array (FBGA) package assembly with solder ball compositions of 36Pb-62Sn-2Ag and Sn-4Ag-0.5Cu on printed circuit board (PCB) surface finishes of organic solderability preservative, electroless nickel immersion gold, and immersion tin. Finite element modeling of the FBGA assembly was performed to study the stress-strain behavior of the solder joints during drop test. The drop test results revealed a strong influence of different intermetallic compound formation on soldered assemblies drop durability. The lead-based solder supersedes the lead-free composition regardless of the types of surface finish. Joints on organic solderability preservative were found to be strongest for each solder type. Other factors affecting drop reliability such as component location on the board and thermal cycling aging effects are reported. Finite element modeling results showed that a solder joint is more prone to failure on the PCB side, and the predicted solder joint stresses are location dependent. Predicted failure sites based on simulation results are consistent with experimental observations.

Isothermal Cyclic Bend Fatigue Test Method for Lead-Free Solder Joints

Isothermal three-point and four-point cyclic bend fatigue test methods have been developed for Sn-Ag-Cu solder joints. Reported bend tests from the literature were conducted at room temperature (25°C) and there is lack of data for lead-free solder joints. In this study, very-thin quad flat no-lead (VQFN) assembly with Sn-Ag-Cu lead-free solder was tested under three-point and four-point cyclic bending loads at both room temperature (25°C) and high temperature (125°C). The correlation between three-point and four-point bend tests was developed. Two different board surface finishes of electroless Ni and immersion gold (ENIG) and organic solderability preservatives (OSP) were investigated. Bending fatigue resistance of VQFN with OSP finish is slightly better than ENIG finish case. The acceleration factor of failure at high temperature (125°C) is higher than that at room temperature (25°C). Finite element analysis modeling and simulation were performed for different test conditions to investigate the solder joint stress-strain behavior. Volume-averaged energy density was used as a fatigue damage parameter and energy-based bending fatigue models were developed for VQFN with Sn-Ag-Cu solder joint under cyclic bending load at both 25°C and 125°C.

Cure shrinkage characterization and its implementation into correlation of warpage between simulation and measurement

In this work, a new approach was proposed to characterize the cure shrinkage of EMC by using the EMC/Cu bi-layer strip specimens. The warpage of bi-layer strip was measured at different temperature using Shadow Moire. The results show that warpage at molding temperature was non-zero and zero-warpage temperature shifted from molding temperature (175 degc) to higher temperature due to cure shrinkage effect. From Timoshenkos beam theory, the cure shrinkage was calculated as 1st order approximation theoretically either from the warpage at molding temperature or from zero- warpage temperature. The determined cure shrinkage together with thermal shrinkage obtained from TMA tests was used to predict the warpage of the different EMC/Cu strips. Good correlation was observed in the wide temperature range. As comparison, direct measurement of the cure shrinkage was also done using long rectangular bar specimens. Cure shrinkage was determined by extracting thermal shrinkage from total shrinkage. Cure shrinkage of 2 EMCs were characterized and then applied to PBGA matrix. Warpage of the PBGA EMC/substrate maps was measured using Shadow Moire and simulated as well for the molding compounds (EMCs) after 3 different processes, i.e. after transfer molding (TM), post mold cure (PMC) and PMC + Reflow at 260 degc for 3 times (RF260X3). Consistence between simulation and experiments was found when cure shrinkage was considered. The presented data show the necessity and importance of cure shrinkage in warpage prediction simulation.