Dipak Sengupta

Investigation of Stress in MEMS Sensor Device Due to Hygroscopic and Viscoelastic Behavior of Molding Compound

The stresses due to moisture saturation on microelectromechanical systems (MEMS) sensor devices after exposure to temperature cycling have been addressed. Moisture-, temperature-, and time-dependent material property of molding compounds for the MEMS devices were characterized. To determine the coefficient of hygroscopic swelling of a molding compound and diffusivity (D) of water in the molding compound, dimensional change and weight loss of moisture saturated samples at various temperatures were monitored by the digital image correlation method combined with a weight scale. To obtain the viscoelastic property of the molding compound, a series of stress relaxation tests was performed using dynamic mechanical analysis (DMA). To explain the moisture-induced viscoelastic behavior, a simple assumption was introduced based on the temperature of glass transition point (Tg) shift from the DMA result. The experimental data were utilized in numerical simulations to estimate the temperature- and moisture-induced stress on MEMS sensor devices subjected to temperature cycles.

Failure analysis of contact probe pins for SnPb and Sn applications.

A study has been conducted to investigate the failure mechanism of pogo pin-type probe contacts. Probe pins are used for electrical test of microelectronic components in manufacturing. A false rejection of parts due to high probe contact resistance results in a penalty in cost and yield. The probe pin contact bears distinctive characteristics of failure compared to the conventional contact systems such as mechanical switches and interconnects. Moreover, the transition to Pb-free component leads demands understanding of different probe failure mechanisms between a SnPb and Sn surface. The objective of this study is to understand this unique failure mechanism and the effect of lead coating metals on probe pin life. This has not been clearly elucidated to date in spite of its significant impact on yield and cost of electronic package manufacturing. A simulated probe tester with 3-axes actuation capability was devised to mimic the actual test process. The force required to penetrate the surface oxide layer and develop electrical contact was measured. Contact resistance history revealed that probe pins mating to Sn surfaces failed earlier than pins used on SnPb surfaces. Through periodic inspection of probe pins using microprobe/EDS as a function of probe actuations, the general root cause of probe pin failure was found to be probe pin tip wear out associated with the Sn oxide growth on its surface. The matte Sn surface wears the probe pin more than SnPb due to the rough and abrasive nature of the matte Sn surface.

Moisture Diffusion and Hygroscopic Swelling of Adhesives in Electronics Packaging

This paper presents a continuation of the previous hygroscopic characterization work on epoxy molding compounds, which aims to accurately measure the diffusivity and coefficient of hygroswelling (CHS) of a die-attach adhesive in MEMS packages. We obtained the CHS of the adhesives from 25 °C to 150 °C through modified DIC quick scanning approach [1-2], which includes strain measurement by digital image correlation method and correction of weight loss through the Finite Element (FE) analysis. The CHS shows a dependence on temperature above the water evaporation temperature and ranges from 0.4 to 0.55 (strain/percentage weight). The test accuracy of CHS was validated through a comparison with the correlation method [3] of in situ hygroswelling and weight loss. From moisture absorption and desorption curves of the adhesive samples, we obtained the diffusivities at different temperatures. A notable finding is that bulk samples follow the Ficks diffusion model while the thin-film samples demonstrate a non-Fickian behavior. By adding a concentration-dependent coefficient to the diffusivitys expression, the non-Fickian model can also be implemented in the FE models. As an outcome, a thorough guideline for measurement procedure is provided for hygroscopic characterization in electronics packaging.

A Study on the Thermomechanical Reliability Risks of Through-Silicon-Vias in Sensor Applications

Reliability risks for two different types of through-silicon-vias (TSVs) are discussed in this paper. The first is a partially-filled copper TSV, if which the copper layer covers the side walls and bottom. A polymer is used to fill the rest of the cavity. Stresses in risk sites are studied and ranked for this TSV structure by FEA modeling. Parametric studies for material properties (modulus and thermal expansion) of TSV polymer are performed. The second type is a high aspect ratio TSV filled by polycrystalline silicon (poly Si). Potential risks of the voids in the poly Si due to filling defects are studied. Fracture mechanics methods are utilized to evaluate the risk for two different assembly conditions: package assembled to printed circuit board (PCB) and package assembled to flexible substrate. The effect of board/substrate/die thickness and the size and location of the void are discussed.

Effect of High Tg Mold Compound on MEMS Sensor Package Performance

A signal shift following solder reflow or temperature cycle stress testing can severely affect micro-electro-mechanical systems (MEMS) product performance in encapsulated package. This is mainly attributed to nonlinear material property of the mold compound, a key low cost packaging material. Due to viscoelastic characteristic, the mold compound exhibits a temperature and time dependent behavior. Above the temperature of glass transition (Tg) of the mold compound, the viscoelastic behavior is more significant. Thus, when the MEMS package is subjected to high temperature such as solder reflow process, a package induced internal stress on the MEMS sensor is generated depending on the maximum temperature reached and cooling rate. This results in the signal shift. Minimizing this signal shift plays an important role in MEMS product quality and performance. In this study, high Tg mold compounds (Tg: 200°C) were evaluated to alleviate the signal shift. Material property characterization tests were performed and test vehicles were assembled with the selected molding compounds. The MEMS sensor signal was measured before and after the temperature cycle test. Also, the viscoelastic material property of the mold compound was used in finite element analysis to investigate the resulting deformation and stress of the MEMS sensor. Compared to a baseline mold compound (Tg: 110°C), the high Tg molding compound did show about 10% less signal shift in the temperature range of -40°C to 125°C.

Optimizing System-on-Film (SOF) Design Using Finite Element Analysis

System-On-Film (SOF) module is a complex integration of a fine pitch high density die and surface mounted discrete devices on a polyimide (PI) film laminate. The die is connected to the film using a thermo-compression flip-chip bonding (TCB) process which is capable of providing a very high density interconnect at less than 50um pitch. Several design and bonding parameters have to be controlled in order to achieve a reliable bond between the Au bumps on the die and the Sn plated Cu traces on the PI film. In the current work, the TCB process is studied using Finite Element Analysis (FEA) to optimize the design parameters and assure proper process margins. The resultant forces acting on the bump-to-trace interfaces are quantified across the different potential geometrical combinations. Baseline simulations showed higher stresses on specific bump locations and stress gradients acting on the bumps along the different sides of the die. These observations were correlated to both the failures and near failures on the actual test vehicles. Further simulations were then utilized to optimize and navigate design tradeoffs at both the die and flexible substrate design levels for a more robust design solution. Construction analysis performed on parts built using optimized design parameters showed significant improvements and correlated well with the simulation results.Copyright

Failure Analysis of Contact Probe Pins for SnPb and Sn Applications

A failure mechanism of pogo-type probe pin is investigated. A probing tester with actuation capable in three-axes is used to simulate the actual inspection process experimentally. Force required to break in surface oxides and develop electrical contact is measured. Contact resistance history reveals that pins mating to Sn surfaces fail earlier than SnPb surfaces. Through periodic inspection of pin using microprobe/EDS as a function of probing count, the general root cause of pin failure is turned out to be pin tip wear out associated with Sn oxide growth on its surface. The cause of earlier failure of the pin probing matte Sn surface is identified as severe wear out by a rough and abrasive characteristic of matte Sn.Copyright

Die Stress Analysis in Stacked Die Chip Scale Packages (SCSP)

As board real estate becomes more precious and market demand for increased functional density in modules rises, engineers are looking at 3-dimensional packaging to provide the solutions. Stacked die technology, which involves stacking the die one above another, is fast becoming the preferred packaging option for memory applications in handheld consumer products where board space is a premium. The current study focuses on the stacked die wire bonded CSP (SCSP). The CSP offers reduced package size while the vertical stacking provides a smaller form for multi-chip integration compared to a horizontal layout. In some test chip designs, it has been observed that passivation cracks occur on the functional surface of the top die as a result of thermal cycling. Stress analysis using the finite element method has been carried out to understand the effect of package parameters on die stress under cycling conditions. The distribution of the stress components that may cause this passivation cracking are discussed. The stress magnitudes observed in the SCSP are compared with those of a single die package where no passivation cracks have been observed to obtain a quantitative perspective of the stress. Results reported for parametric studies include the effect of variation of die thickness, spacer size and thickness, and thickness of overmold compound above the die. Finally, design considerations for two-die, over-molded, wire-bond, stacked die packages are presented based on the above study.Copyright