Xiaosong Ma

Moisture Effects on the Creep of Thermosetting IC Packaging Polymers

We have investigated the behaviour of moisture absorption in various humidity environments at constant temperature of two types of epoxy novolac polymers. In addition, we examined the effects of moisture on the viscoelastic properties. The water absorption is carried out in an adjustable thermal and humidity chamber at different temperature and humidity. The diffusion coefficients at different conditions were obtained through early stage moisture absorption tests. The moisture absorption was revealed to be of the Fickian type of diffusion for both types of samples and to shift the creep compliance to the left of that of a dried sample, as an effect of plasticization. The relationship between logaH and the equilibrium water content is analogically described by the WLF-type equation on the time-temperature superposition. The time-moisture superposition was confirmed to hold at various equilibrium moisture contents at constant temperature. The more moisture in the sample, the more plasticization will happen for the epoxy

Moisture diffusion model verification of packaging materials

The use of the non-hermetic material for electronic packaging does raise a potential concern, i.e. moisture induced interfacial delamination and pop corning during reflow. Therefore, it is very important we can correctly model the moisture absorption property. In this study, moisture absorption and desorption properties of three kinds of package materials were investigated. Moisture absorption equilibrium weight gain and diffusion coefficient at different temperature and different humidity are characterized. Moisture absorption processes are simulated using a 3D model at conditions according to the moisture sensitivity test levels. Finally moisture absorption is verified by our research carrier.

Hygroscopic Effects on Swelling and Viscoelasticity of Electronic Packaging Epoxy

One type of polymer EPN1180 is selected for the hygroscopic swelling tests and the sample is made in the thickness of 30mum . Coefficient of thermal expansion is tested by using DMA Q800 and coefficient of moisture expansion is tested by using DMA Q800 jointed with a humidity generator. Three conditions temperature, 50degC, 60degC, and 70degC, and different relative humidity with 20RH% steps are used for the tests and moisture induced expansion is characterized. The characterized equation can be used to expect the moisture-induced expansion at temperature between 0degC and 100degC, at relative humidity between 0RH% and 100RH% for this epoxy. This new method is convenient than that of the old method and easily is used to characterize other polymers. In addition, hygroscopic effect on the viscoelasticity is tested and characterized. The storage modulus is linear decreased with the increase of relative humidity at certain temperature

A new method to measure the moisture expansion in plastic packaging materials

Moisture induced failure in plastic encapsulated packages is one most important failure mechanisms in microelectronics. This failure is driven by the mismatch between different material properties, such as CTE, CME (Coefficient of Moisture induced Expansion) caused by moisture absorption in plastic packaging materials. Therefore, it is important to know moisture effects on mechanical properties of plastic packaging materials, especially CME. Moisture induced expansion can be calculated using ε = β · c, here ε is the strain, β is the CME and C is the moisture concentration. Traditionally using the combined TGA (Thermal Gravimetric Analyzer)/TMA (Thermal Mechanical Analyzer) technique, the CME of plastic packaging materials is characterized. TGA is used to measure the weight change and TMA is used to measure the length change. By combining both the TMA and TGA measurements, the CME can be determined. This method is often used in industry and it is observed that the CME value is often over estimated. In order to get precise CME values a high precision DMA (Dynamic Mechanical Analyzer) is used to measure the length change of a sample while a humidity generator is used to regulate the relative humidity. Therefore, temperature and relative humidity are controlled in the DMA chamber and can be used to measure the length change under different relative humidity conditions. CME values measured by the DMA plus humidity method are much lower than that of the TGA/TMA method. In order to find out which method is more reliable, a third experiment was done. A bi-material sample is created to verify our measured CME value. TDM equipment (oven + camera system to detect vertical displacement of the sample) is used to measure the warpage of the bi-material sample. Using our measured CME value, finite element model simulation result shows that the hygro-mechanical warpage of the model fits well with TDM test result.

Characterization and Modeling of Moisture Absorption of Underfill for IC Packaging

Underfill is a highly particle filled epoxy polymer used in flip chip to fill the gap between the leadframe and die or between die and die. In order to be able to flow in the thin gap, the silica filler particle must have a diameter below 10 mum. This epoxy underfill material mechanically couples the chip and substrate and decreases the stress in the solder joints, therefore enhancing solder fatigue life. Good adhesion between the epoxy underfill and the surface of solder ball, silicon chip, and substrate is necessary to minimize stress in a package. Use of the non-hermetic material does raise a potential concern, i.e. moisture induced interfacial delamination and moisture expansion may cause failure. In this study, moisture absorption and desorption properties were investigated, such as moisture absorption equilibrium weight gain and diffusion coefficients at different temperature and different humidity are tested and characterized, moisture absorption and desorption kinetics are tested and fitted.

A fast moisture sensitivity level qualification method

In this paper, a fast moisture sensitivity level (MSL) qualification method and a fast moisture characterization method are discussed. The fast moisture characterization uses a stepwise method to obtain more reliable and more material moisture properties. The established relationships for moisture diffusion coefficients and moisture saturation levels with respect to the temperature and relative humidity can be used to predict moisture properties in the MSL range. Fast moisture sensitivity level qualification is accomplished with the aid of simulation combined with the characterized moisture diffusion properties. Moisture absorption processes at different conditions are simulated using a 3D model at conditions according to the moisture sensitivity test levels. Simulation of weight change at different condition and simulation of local moisture concentration are performed and compared between different conditions. Simulations show that at 696 h preconditioning time at 30 °C/60%RH for MSL level 2a can be decreased to 42 h at 85 °C/85%RH. Time required for package reliability and moisture sensitivity analysis is largely shortened.

Moisture effects on a system in package carrier

Moisture induced failures in the plastic encapsulated packages are one most important failure mechanisms in microelectronics. These failures are driven by the mismatch between different material properties, such as CTE, CME (Coefficient of Moisture induced Expansion) and degradation of interface strength caused by moisture absorption of polymer materials. Therefore, it is critical to know how much moisture exists in packaging materials, the moisture distribution in the package and hygro-mechanical effects on the package. In this paper moisture diffusion, moisture distribution and hygro-mechanical effects are simulated at the following conditions: 85°C/85%RH, 60°C/60%RH and 85 °C/dry, 60 °C/dry using 2D SiP(System in Package) finite element model.

Fast qualification using thermal shock combined with moisture absorption

Time to market is becoming one of the most important factors because of the fierce market competition. However, traditional reliability and interface toughness characterization tests take very long time. For example, moisture sensitivity level assessment (MSL1) will take 168 hours pre conditioning at 85°C/85%RH and tradition thermal cycling takes even longer time. The long preconditioning times are chosen to ensure that also the thicker sections of a package are completely saturated. Thinner package, however, are already saturated after one to two days. In this study, we therefore investigated whether it would be possible to speed up the qualification process by shortening the preconditioning time. We focus in particular on the interface toughness. From our four point bending test and analysis, it is found that temperature has great effects on the interface toughness and moisture also has small effects on the interface toughness. In order to do the fast qualification test, thermal shock cycling tests combined with moisture absorption are performed. Experiments show that moisture can speed up the delamination.

Effect of aging of packaging materials on die surface cracking of a SiP carrier

Generally, the viscoelastic properties of packaging materials used in the simulation models are obtained from the materials after postcuring. However these properties were observed to change during humidity conditioning and the thermal cycling. Two kinds of packaging materials are tested, one is molding compound and another is underfill. All samples are cured according to the curing procedure, postcured at 180degC. Before the test, first the samples are pre-dried at 125degC for 24 hours and then preconditioned at 60degC/60%RH for 40 hours. Secondly, one reflow at 260degC. Finally, all samples are subjected to thermal cycling. Thermal cycling temperature range is from -65degC to 150degC and every cycle is finished in 30 minutes. For the DMA test, a TA Instrument Q800 is used. Test results show the glass modulus, rubber modulus and glass transition temperature increase with the number of thermal cycles. This change in materials after humidity and thermal treatment is here referred to as aging. The finite element software Marc is used to simulate the internal change of stress and displacement. The simulation result shows that the total warpage has increased a little at the corner of passive die, which is where the critical cracks and crazes were found in our qualification tests. And the Von Mises stresses increase after thermal cycling.

Die attach interface property characterization as function of temperature using cohesive zone modeling method

Interface delamination is one of the most important issues in the microelectronic packaging industry. Silver filled die attach is a typical adhesive used between the die and copper die pad for its improved heat dissipation capacity. Delamination between die attach and die pad will severely impact the heat conduction and result in product failure. In order to predict this delamination, interface properties should be characterized. Tri-material, copper-die attach-EMC, samples are made according to the package processes. A four point bending test system is established in order to perform delamination tests at different temperatures using a universal tester Zwick/Roell Z005. In addition, a Keyence optical system is mounted to capture a series of pictures during the delamination processes. This will provide the delamination geometry information needed for determining the interface properties. Four point bending tests have been performed at room temperature, 40, 60, 85, and 150?C respectively. In addition pre conditioning sample are also tested at room temperature and 85?C respectively after 48 hours pre conditioned at 85?C/85%RH. Experiments show that the ?critical delamination load? decreases steadily with temperature increasing. Experiments also show moisture has no effects on the ?critical delamination load? compared with the dry samples tested at the same temperatures. This means that moisture has no effects on the interface toughness between copper and die attach. To quantify the interface properties, numerical simulations of the four point bending test have been performed by using a finite element model comprising cohesive zone elements which will describe the transient delamination process during the four point bending tests. Correspondently, the interface toughness decreases from 26.5J/m2 at room temperature to 1.9J/m2 at 150?C as calculated from the cohesive zone element model. These results show that temperature has a large effect on the interface toughness. By means of an extensive model parameter sensitivity study, combined with the measured delamination length in horizontal direction along the copper-die attach interface at room temperature critical opening value has been determined.