Glob Top Research Articles

Survey of High-Temperature Polymeric Encapsulants for Power Electronics Packaging

Semiconductor encapsulation is crucial to electronic packaging because it provides protection against mechanical stress, electrical breakdown, chemical erosions, α radiations, and so on. Conventional encapsulants are only applicable below 150 °C. However, with increasing demand for high-density and high-temperature packaging, encapsulants that are functional at or above 250 °C are required. In this paper, five types of encapsulants, including conformal coatings, underfills, molding compounds, potting compounds, and glob tops, are surveyed. First, recommended properties and selection criteria of each type of encapsulant are listed. Second, standard test methods for several crucial properties, including glass-transition temperature (T g ), coefficient of thermal expansion (CTE), dielectric strength, and so on are reviewed. Afterward, commercial products with high-operation temperature are surveyed. However, the results of the survey reveal a lack of high-temperature encapsulants. Therefore, this paper reviews recent progress in achieving encapsulants with both high-temperature capability and satisfactory properties. Material compositions other than epoxy, such as polyimide (PI), bismaleimide (BMI), and cyanate ester (CE), are potential encapsulants for high-temperature (250 °C) operation, although their CTE needs to be tailored to limit internal stress. Fillers are reported to be efficient in reducing the CTE. In addition, fillers may also have a beneficial impact on the thermal stability of silicone-based encapsulants, whose high-temperature capability is limited by their thermal instability.

Novel encapsulation materials for High Pressure-High Temperature (HPHT) applications

A modified thermoset polymer resin (PR) is evaluated as a form of alternative packaging material to traditional epoxy. It is understood that the thermoset cross-links to a higher degree at high temperature. Hence, its mechanical properties can be improved by changing treatment duration and temperature, rendering the material mechanically stronger for application temperatures beyond 300°C. Material strength was evaluated through a modified compressive testing setup which shows neat PR is comparable to epoxy. Concurrently, adhesion to silicon die and ceramic substrate were evaluated by means of bond shear using the DAGE-4000 shear tester both at room temperature and elevated temperature. Bond shear strength of PR is observed to be higher at room temperature as compared to elevated temperatures when bonded to Silicon (Si) and Alumina (Al2O3), a phenomenon which can be attributed to mismatches in coefficient of thermal expansions (CTE). For high pressure only applications, PR is a good alternative. Pre- and post-pressure loading analysis was carried out by forming an encapsulation analogous to that of a glob top on DIP packages. Observation shows no crack formation, indicating that the material is suitable for high pressure loading up to 207 MPa. For high pressure and high temperature applications (HPHT), pure PR is good up to 250°C at 25 kPsi (172 MPa).

Investigation of open bond wires in MEMS devices

In this case study we present the failure analysis flow of a MEMS device. Because of the combination of a MEMS sensor and ASIC die within one package (system in package) new materials are used. This causes new failure modes and creates particular failure analysis challenges. In this particular case study, contamination due to glob top on the ASIC die causes reliability problems. The change of mechanical stress within the package then leads to open bonds causing functional fails in the field application. To determine the root cause of the problem multiple steps of selective decapsulation are necessary as shown in this study in detail.

Material Issues of Low Temperature Co-fired Ceramic (LTCC) Fine Pitch Chip Scale Package (CSP) Designs

ABSTRACTLTCC substrates for fine pitch (1.0 mm and 0.8 mm) CSP applications have been designed, fabricated, and assembled. The assembly process, including ball grid array (BGA) solder ball attach, die mount, wire bond, and glob top is described. The material and physical design interaction issues that emerged during development are discussed.The initial CSP design was conventional, with co-fired yellow gold (Au) vias and capture pads and post-fired solderable gold (PtPdAu) pads for solder ball attachment. Because LTCC tape shrinks during co-fire, solder pads were applied post co-fire to ensure proper mating with existing test fixtures and to provide the best alignment relative to the CSP body. Solder pad to capture pad misalignment was visible following solder pad firing. After CSP attachment to a test board, electrical tests revealed opens. Investigation led to the following conclusions. The decreased solder pad diameter necessary to accommodate the fine pitch design was significant relative to the area allocated for the underlying via and capture pad. Misalignment that would have been hidden under larger solder pads was exposed. Even when the capture pad surface was not visibly exposed, the offset solder pad meant less material between the capture pad and the solder ball, less of a barrier to solder leaching. Solder leaching into the yellow gold, observed after CSP removal from the test board, was the cause of the electrical disconnects. In the second design, the capture pad was eliminated in order to discourage leaching by reducing the volume of yellow gold available to alloy with the solder pad during co-fire. Reflow operations still resulted in leached solder pads. A third design replaced the first-layer via yellow gold with a solderable gold. This design proved to be robust.While developing designs and fabricating these prototypes, it was noted that all ball failures consistently occurred between the solder pad and the LTCC substrate. To investigate adhesion using different metallizations, shear tests were performed on LTCC substrates with either post-fired solder pads or co-fired pads. To investigate how the substrate material affects adhesion, alumina CSPs were also sheared. Shear test results are presented.

Bewertung des Korrosionsschutzes von Drahtbondverbindungen mittels Glob Top bei unterschiedlichen Umgebungsbedingungen

High reliability in electronics has to be guaranteed at worse environmental conditions. At the same time the tendency to smaller dimensions holds on. Under these circumstances the protection against corrosion plays an important role. Methods of accelerated testing are presented for COB devices under glob top. The degradation mechanisms under the influence of temperature and humidity are investigated. There are always different mechanisms superimposing. Galvanic corrosion and crevice corrosion is only working at high humidity levels and the existence of free ions. High temperature leads to the formation of intermetallics. Thermal shock is connected with mechanical stress. Both may initiate the formation of cracks. It can be stated that as a result of a careful selection of materials and technology a sufficient reliability level is reached.

Comparative study of thermally conductive fillers for use in liquid encapsulants for electronic packaging

Thermal management plays a very vital role in the packaging of high performance electronic devices. Effective heat dissipation is crucial to enhance the performance and reliability of the packaged devices. Liquid encapsulants used for glob top, potting, and underfilling applications can strongly influence the package heat dissipation. Unlike molding compounds, the filler loading in these encapsulants is restrained. This paper deals with the development and characterization of thermally conductive encapsulants with relatively low filler loading. A comparative study on the effect of different ceramic fillers on the thermal conductivity and other critical properties of an epoxy based liquid encapsulant is presented.

Temperature cycling effects in some glob top polymer materials

Three common glob top encapsulant materials, two epoxy‐based, and one silicone‐based, were characterized prior to temperature cycling using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), dynamic mechanical spectroscopy (DMS), gas chromatograph‐mass spectrometry (GC‐MS), and Fourier transform infrared spectroscopy (FTIR). After cycling between ‐55 to +125°C, for 1,000 cycles, the same samples were again analysed using DMS and FTIR. For the epoxy‐based samples, the DMS results indicated that temperature cycling in a humid environment can seriously affect the physical and mechanical properties of these samples. FTIR data also indicated that the molecular changes in the epoxy‐based samples appeared quite extensive after cycling, indicating a high level of degradation on the molecular scale. On the other hand, the silicon‐based glob top appeared to have survived the temperature cycling quite well.

Structures for piezoresistive measurement of package induced stress in transfer molded silicon pressure sensors

The package induced stress in a transfer molded micromechanical pressure sensor has been measured by use of silicon piezoresistors. Measurements have been carried out by monitoring the individual piezoresistors in the measuring bridge of an existing sensor, and by measurements on a modified sensor structure with specially designed stress sensing structures. The induced difference ( σ x – σ y ) between x- and y- normal stress in the sensor diaphragm has been measured to 50 ± 10 MPa at 25°C. The induced stress creates a thermal zero shift of the sensor signal. By covering the sensor chip with a soft glob top before molding, the stress may be considerably reduced. For glob-topped samples the measured value of ( σ x – σ y ) at 25°C is 20 ± 20 MPa.

Reliability of electrically conductive adhesive joints for surface mount applications: a summary of the state of the art

Electrically conductive adhesives are of large potential interest for the bonding of surface mount devices, especially in those cases where the use of soldering processes is restricted or impossible. Among the claimed benefits are mild processing conditions, process simplicity, flexibility, high resolution potential, and lead omission. Most of the presently available knowledge on the utility and reliability of conductive adhesives has been gathered with silver filled isotropic conductive epoxy adhesives (ICAs). In addition, the use of gold or nickel filled anisotropic conductive film or paste adhesives has been investigated quite intensively. Even conductive adhesive joints made with unfilled nonconductive adhesives are being considered for flip chip applications. To date, quite some work has been published demonstrating the usefulness and limitations of isotropic adhesives to bond a variety of IC (e.g., QFP) discrete (e.g., SOT) and passive components (R and C) under different climate conditions using various circuit and component-metallizations. Most of the isotropic conductive adhesives require noble metallizations (e.g., An or AgPd) to survive harsh environmental conditions as for instance 85/spl deg/C/85%RH and temperature cycling from -40 to +125/spl deg/C. Most of the adhesives give bad results on SnPb surfaces, a few special types show better results in 85/spl deg/C/85%RH. Deterioration of the electrical properties is due to an increase in the contact resistance. The bulk resistance of the adhesive, although considerably higher than that of solder, usually remains quite constant. On passivated Cu substrates reasonable good results are obtained. With Au finished surfaces large components may give problems in drop tests and temperature cycling due to insufficient bond area, unfavorable bond geometries or failures due to thermal expansion differences. The use of a coating or glob top on the leads may overcome these problems. With Au coated plastic spheres filled anisotropic conductive adhesives [ACA(F)] results opposite to the ICAs were obtained. For QFP80 (Au) joints with ACAF on FR-4 (Au) considerable increase in R-value was found after Temperature Humidity (TH) testing at 85/spl deg/C/85%RH and Temperature Cycling (TC,-20/100/spl deg/C), while with QFP80 (SnPb) R-values did not increase. This was found to be due to the formation of microsolder bridges. Microsolder joints can also be made with commercially available SnBi filled ACA. Some insight has been developed into the possible failure causes with ICAs. Surface oxidation of SnPb and interfacial cracks and delamination have for instance been observed, although the extent of their contributions under various conditions still needs to be resolved. The danger of Ag migration seems to be low as long as no liquid water is present. In summary, present results indicate that reliable connections with conductive adhesives even under harsh climate conditions are possible with the right choice of adhesive and metallizations of components and boards.

Measurement of package-induced stress and thermal zero shift in transfer molded silicon piezoresistive pressure sensors

The package-induced stress in a transfer molded piezoresistive pressure sensor has been measured by monitoring the individual piezoresistors in the measuring bridge. Based on measurements on six different samples, the induced difference between longitudinal and transversal stress in the sensor diaphragm has been measured as MPa at room temperature. The induced stress creates thermal zero shift and long term drift of the sensor signal. By covering the sensor chip with a low modulus glob top before molding, the stress and thermal zero shift may be considerably reduced.