Moisture Preconditioning Research Articles

Composite Singular Value Decomposition Analysis of Moisture Variations Associated with the Madden–Julian Oscillation: ERA-40 Analysis

Abstract The role of moisture variations in the initiation of Madden–Julian oscillation (MJO) variability is reexamined through composite singular value decomposition (CSVD) analyses using the European Centre for Medium-Range Weather Forecasts (ECMWF) 40-yr Re-Analyses (ERA-40) data. The CSVD analyses at various time lags are carried out to discern the complex space–time relationships between convection identified using outgoing longwave radiation and 1000-hPa divergence, 850-hPa specific humidity, and surface evaporation. The most striking difference from the earlier analyses using NCEP–NCAR reanalysis data is that the observed relations between 20–100-day filtered OLR and ∇ · V1000 anomalies are weaker and less significant in the current analyses. On the other hand, both analyses show increasing low-level moisture near and to the east of the developing convection. Thus, both results imply that moisture preconditioning of convective events is not totally driven by boundary layer moisture convergence.

Failure mechanism study of anisotropic conductive film (ACF) packages

Anisotropic conductive film (ACF) consists of an adhesive polymer matrix with dispersed conductive particles. In flip-chip technology, ACF has been used in place of solder and underfill for chip attachment to glass or organic substrates. The filler particles establish the electrical contacts between the interconnecting areas. ACF flip-chip bonding provides finer pitch, higher package density, reduced package size and improved lead-free compatibility. Nevertheless, the interconnection is different from traditional solder joints, the integrity and durability of the ACF interconnects have major concerns. Failures in anisotropic conductive film (ACF) parts have been reported after temperature cycling, moisture preconditioning and autoclave. The failures have not been well understood and have been attributed to a wide variety of causes. This paper investigates the failure mechanism of ACF using finite element simulation. From a failure-initiation point of view, the response of ACF packages to environmental (temperature and humidity) exposure is very different from standard underfilled packages. These differences cause the ACF package to fail in different ways from an underfilled package. Simulation results have shown that moisture-induced ACF swelling and delamination is the major cause of ACF failure. With moisture absorption, the loading condition at the interface is tensile-dominant, which corresponds to lower interface toughness (or fracture resistance). This condition is more prone to interface delamination. Therefore, the reliability of ACF packages is highly dependent on the ACF materials. The paper suggests a new approach toward material selection for reliable ACF packages. This approach has very good correlation with experimental results and reliability testing of various ACF materials.

Reliability assessment and hygroswelling modeling of FCBGA with no-flow underfill

In the flip-chip ball grid array (FCBGA) assembly process, no-flow underfill has the advantage over traditional capillary-flow underfill on shorter cycle time. Reliability tests are performed on both unmolded and molded FCBGA with three different types of no-flow underfill materials. The JEDEC Level-3 (JL3) moisture preconditioning, followed by reflow and pressure cooker test (PCT) is found to be a critical test for failures of underbump metallization (UBM) opening and underfill/die delamination. In this paper, various types of modeling techniques are applied to analyze the FCBGA-8×8 mm on moisture distribution, hygroswelling behavior, and thermomechanical stress. For moisture diffusion modeling, thermal-moisture analogy is used to calculate the degree of moisture saturation in the multi-material system of FCBGA. The local moisture concentration along the critical interface, e.g. die/underfill, is critical for delamination, because the moisture weakens the interfacial adhesion strength, generates internal vapor pressure during reflow, and induces tensile hygroswelling stress on UBM during PCT. The results of moisture distribution can be used as loading input for the subsequent hygroswelling modeling. The magnitude of hygroswelling stress acting on UBM is found to be greater than the thermal stress induced during reflow, both in tensile mode which may cause the UBM-opening failure. Underfill with lower saturated moisture concentration ( C sat) and coefficient of moisture expansion (CME) are found to induce lower UBM stress and has better reliability results. Molded package generally has higher stress level than unmolded package. Parametric studies are performed to study the effects of no-flow underfill materials, package type (molded vs. unmolded), die thickness, and substrate size on the stresses of UBM during reflow and PCT.

Processing and reliability of fast-flow, snap-cure underfills. I. Processing and moisture sensitivity

This series of articles studies the processing, moisture sensitivity, reliability, and failure mode analysis of a number of commercial fast-flow, snap-cure underfill materials. It includes data on process analysis, processing times, and reliability of the test vehicles in air-to-air thermal cycling (-55/spl deg/C to 125/spl deg/C), liquid to liquid thermal shock (-55/spl deg/C to 125/spl deg/C), and J Standard 020 Revision A Level 3 Moisture Sensitivity Preconditioning followed by reliability testing. Samples cured using a second reflow pass are compared to baseline samples cured in a standard batch oven based on the underfill manufacturer's recommended cure schedule. Results from the processing of these new materials have shown a greatly reduced flow time from earlier generation underfill materials as all of these materials had flow times of less than 30 s under 5-mm test die. Through differential scanning calorimetry analysis, it was shown that materials requiring less than 8 min to cure exhibited at least 95% conversion (percentage of material cured) through the modified second side renew profile. Some of these commercial underfills have also passed J Standard 020 Revision A Level 3 Moisture Sensitivity testing. Specifically in this paper, the flip chip processing, underfill processing, moisture preconditioning, and preconditioning failure modes are presented.

Processing and reliability of fast-flow, snap-cure underfills - Part I: processing and moisture sensitivity

This series of articles studies the processing, moisture sensitivity, reliability, and failure mode analysis of a number of commercial fast-flow, snap-cure underfill materials. It includes data on process analysis, processing times, and reliability of the test vehicles in air-to-air thermal cycling (-55/spl deg/C to 125/spl deg/C), liquid to liquid thermal shock (-55/spl deg/C to 125/spl deg/C), and J Standard 020 Revision A Level 3 Moisture Sensitivity Preconditioning followed by reliability testing. Samples cured using a second reflow pass are compared to baseline samples cured in a standard batch oven based on the underfill manufacturer's recommended cure schedule. Results from the processing of these new materials have shown a greatly reduced flow time from earlier generation underfill materials as all of these materials had flow times of less than 30 s under 5-mm test die. Through differential scanning calorimetry analysis, it was shown that materials requiring less than 8 min to cure exhibited at least 95% conversion (percentage of material cured) through the modified second side renew profile. Some of these commercial underfills have also passed J Standard 020 Revision A Level 3 Moisture Sensitivity testing. Specifically in this paper, the flip chip processing, underfill processing, moisture preconditioning, and preconditioning failure modes are presented.

A novel high performance adhesion enhancing Zn-Cr leadframe coating for popcorn prevention

A novel adhesion enhancing Zn-Cr (A2) leadframe coating has been reported to be a highly effective solution for popcorn prevention in plastic surface mount packages. In this paper, we report our recent understanding of the adhesion and degradation mechanisms of such a coating system. TEM and TOF-SIMS were employed to study the structure of A2-coating and the molding compound/leadframe (MC/LF) interface. The A2-coating was found to be a partially thin, crystalline layer comprised of a continuous interfacial layer and a network of whiskers. Zn silicate compound, ZnO and Cr oxide(s) were identified as possible phases present in the A2-coating. The adhesion enhancing and degradation mechanisms of the A2-coating were investigated by lead pull test to study the influence of temperature cycling, pressure cooker testing, moisture preconditioning, and leadframe oxidation. The adhesion data showed that the A2-enhanced MC/Cu LF interface is not susceptible to thermomechanical stress and moisture degradation. The wetting of the molding compound coupled with mechanical interlocking mechanism offered by the whiskers are believed to be key contributing factors of adhesion enhancement. Degradation of the A2-enhanced MC/Cu LF adhesion was observed after 100 min exposure at 300/spl deg/C by Cu oxide formation from Cu outward diffusion through the A2-layer and was validated by AES analysis operated in the depth profiling mode. Lead pull test specimens were cleaved to have access to the MC/Cu LF interface for RBS analysis to identify the locus of failure.

The effects of flux materials on the moisture sensitivity and reliability of flip-chip-on-board assemblies

This paper discusses the nonproprietary aspects of the work-in-progress regarding developments in flux materials to improve the moisture-induced stress sensitivity of flip chip plastic ball grid array devices. Studies addressing the moisture characterization and preconditioning, using JEDEC Level 3, of assemblies built with four proprietary fluxes and two alternative underfill materials will be presented. This report includes the evaluation of the weight loss during a simulated die attach reflow profile for each flux type using thermogravimetric analysis. Also, the underfill adhesion strength to the die surface after assembly, after moisture preconditioning and after 48 h of autoclave stressing will be discussed. The integrity of the solder joint corresponding to each flux type and measured using die pull techniques will also be presented. The effects of the experimental variables on underfill to die/substrate adhesion will also be reviewed using C-SAM imaging methods. Proprietary aspects of Motorola's FC-PBGA materials and assembly technology will not be disclosed.

Moisture diffusion and heat transfer in plastic IC packages

This paper deals with finite-element (FE) simulations of moisture diffusion during preconditioning of plastic integrated circuit (IC) packages as well as the simultaneous diffusion of heat and moisture during vapor phase reflow (VPR) soldering. More realistic 2-D analyses are described. The results show good agreement with experimental data. The limitations of hitherto one-dimensional (1-D) analyses are also examined. The transient build-up of water vapor pressure in the delaminated pad-resin interface during vapor phase reflow soldering is also obtained. It is found that, irrespective of the manner of moisture preconditioning (moisture absorption or desorption), the same critical water vapor pressure level of about 1 MPa is obtained for the plastic IC package under study. It is also shown that the water vapor in the delaminated interface never reached saturation during the short period when solder is reflowed and re-solidified.

Accelerated environmental testing of composites

The strengths and absorbed moisture obtained from eight different accelerated environmental test methods are presented and compared with static, ambient fatigue and environmentally exposed real time data. The test parameters included 0.52% and 1.17% moisture targets, the presence and absence of 127°C thermal spikes, moisture preconditioning and concurrent conditioning, six life times of spectrum loading and four types of graphite/epoxy specimens. Compression strength is reduced by increased moisture and elevated temperature, but is not significantly affected by moisture conditioning method or spectrum loading. Accelerated environmental testing, subject to the limitations of test variable severities, can determine the effects of real time environmental testing. If temperatures above glass transition temperatures cannot be avoided, their potentially detrimental effect must be accounted for in accelerated simulations.

Strain Rate, Temperature, and Humidity Influences on Strength and Moduli of a Graphite/Epoxy Composite

Results of an experimental study of the influence of strain rate, temperature, and humidity on the mechanical behavior of a graphite/epoxy fiber composite are presented. Three principal strengths (longitudinal, transverse, and shear) and four basic moduli (longitudinal, transverse, axial shear, and major Poisson's ratio) of a unidirectional graphite/epoxy (T300/5208) composite were studied as functions of strain rate, temperature, and humidity. Each test was performed at a constant tensile strain rate in an environmental chamber providing simultaneous temperature and humidity control. Before being tested, specimens were given a moisture preconditioning treatment at 60°C. Values for the matrix-dominated moduli and strength were significantly influenced by both environmental and rate parameters, whereas the fiber-dominated moduli were not. However, the longitudinal strength was significantly influenced by temperature and moisture content. A qualitative explanation for these observations is presented.