Jan Felba

Effects of Printed Circuit Board Materials on Lead-free Interconnect Durability

This study investigates the effects of printed circuit board (PCB) material on interconnect durability of lead free assemblies. The assemblies involve soldering various packages (array and peripheral) on to FR4, high glass transition temperature (Tg) FR4 and Polyimide (PI) printed circuit boards using Sn3Ag0.5Cu solder alloy. The glass transition temperature of these materials ranges from 130°C to 230°C. Thermomechanical properties, such as elastic modulus and thermal expansion coefficients, of the board materials vary considerably. These properties have a direct impact on the interconnect durability. In this paper, thermomechanical properties are experimentally determined and used for solder joint durability simulation. Two kinds of environmental loadings are simulated: temperature cycling and random vibration loading. The results show that PI board provides a better solder joint durability than FR4 and high TgFR4 under temperature cycling conditions. PI assembly has better durability than FR4 assembly under random vibration. The paper also presents the effect of temperature on the vibration response of the FR4 printed circuit board assemblies. The understanding of these changes can contribute to the study on interconnect durability under combined temperature cycling and vibration loading conditions.

Thermally Conductive Nanocomposites

The heat dissipation problem is becoming a crucial barrier in the continuous process of electronic devices and systems miniaturization, and thermal interface materials play a key role in transport at all level microelectronics packaging. In this chapter, thermally conductive composites, as one of the main types of thermal interface materials are discussed. Such composites consist of the polymer base material matrix and thermally conducting filler. In modern bio- and micro-electronics, the thermal conductivity of the filler material ought to be as high as possible and its size—in the nanometer range. For this purpose, the nanosized particles of metals (mostly silver) or carbon allotropies (mostly carbon nanotubes) are used. Unfortunately, even when the fillers with thermal conductivity in the range of hundreds or more W/m×K are applied, the conductivity of nanocompostes elaborated currently are not higher than a few W/m×K. The analysis of heat transport in filled composites shows that the reason of such limitation of the heat transport is related with thermal contact resistance between filler’s particles. The way of the thermal conductivity improvement is discussed; however, in order to evaluate this progress correctly, the proper measurement methods have to be used. Currently there are many technical solutions to measure thermal conductivity which are based on either steady-state or transient techniques, but for relatively low thermal conductivity of nanocomposites, only few methods can be applied. Here the analysis of the thermal conductivity measurement is presented.

Properties of conductive microstructures containing nano sized silver particles

The properties of the structures made by ink-jet printing with the use of the ink containing nano silver sized particles are presented. After structures printing on substrate, to obtain good electrical conductivity, sintering process is necessary. It is shown, that thermal process influences strongly the resistance, and after the process the resistivity of printed structures can be only a little bit higher than the value of the bulk material. Also different electrical test proved similarity between printed and bulk silver. It was stated that the adding some polymer materials for mechanical parameters improving of printed materials up to 1.5% of total mass of the ink do not influence significantly electrical parameters of the printed layers.

Electron beam activated brazing of cubic boron nitride to tungsten carbide cutting tools

Abstract New tool materials are needed for efficient and high-grade finishing of difficult-to-work materials such as cladded and sprayed layers, cast iron and hardened steels. Advanced superhard materials based on diamond are in common use. However, cubic boron nitride (CBN), as a material for efficient tools, seems to be better than natural diamond. Obtaining reliable cubic boron nitride–tungsten carbide joints is an essential problem in the production of cutting tools with this superhard material. There are different methods of fixing CBN into a tool body. One of them, electron beam activated brazing, seems to be very promising. An electron beam, used as a heat source in this process, enables the precise control of heat delivered to the tool parts during brazing as well as high cleanliness of the joint due to lack of gas impurities in the vacuum environment. Using an experimental design method based on Taguchi techniques for quality engineering, it was found that the heating time, the electron beam current and the roughness of the CBN surface influence mostly the joint shear strength.

Nano-silver inkjet printed interconnections through the microvias for flexible electronics

In the recent years the huge demand for miniaturized, highly functional and also more and more reliable electronic equipment is observed. The development of electronic industry strongly depends not only on the scale of electronic components miniaturization but also depends on miniaturization and kind of printed circuit boards (PCBs). Nowadays, the flexible substrates plays more and more important role in electronics. Organic flexible electronics are being developed for computer displays, radio frequency identification tags, sensors and actuators, solar cells and for much other devices. The use of polymer-based substrates, coupled with an inkjet printing technique opens up the possibility of cost-effective processing in high volumes by use of roll-to-roll processing. Double-sided circuits requires the electrically conductive interconnections between both layers. The big challenge is the manufacturing of surface layouts and vertical interconnections in flexible substrates in the same process without an involvement of other technologies. In the current paper the idea of through-hole microvias metallization by using the inkjet printing technique is presented.

Nanomaterials and technology for conductive microstructures

Production of modern microelectronic devices needs printing technologies with the highest level of resolution and repeatability. Ink-jet technology makes it possible to dispense small volumes of a material in the range of tens picolitres. As the results the printed matrix of dots, lines and more complicated shapes have tens to hundreds micrometers scale. The printing of electrically conductive microstructures requires dispensed materials containing conductive particles as a filler. In the paper the systems for printing, possible results as well as background of nano silver production and its properties are presented. Just after ink-jet printing process the structures have the form of molecular fluid without any electrical conductivity hence special technologies for obtaining the printed microstructures with very high conductance have to be applied. As the result it is possible to make lines and complicated shapes with resitivity in range 10-6 Ohm cm.

Application of FPGA units in combined temperature cycle and vibration reliability tests of lead-free interconnections

According to the European Directive 2002/95/EC RoHS solders that contain lead have to be replaced with lead-free equivalents. Replacing well-known materials with new materials not only force technological adaptations but also force necessity of reliability investigations, especially new failure modes. In case of lead-free solders, the main problem is reduction of solder joint strength. Lower strength of lead- free solder joint in comparison with SnPb is the result of defects that occur in the joint structure. Because of local defects, the joints are much more sensitive for stresses, Stress inducted within the area of solder joint can lead to joint fractures. Despite the crack occurrence, the solder joint can still conduct electrically. Nevertheless cracked joint should be treated as failed because it does not fulfill its constructional function. Such failures can be difficult to detect in static conditions, but can be easy detected, when the tested joint is subjected to vibrations. During vibrations short opens of the joint can occur. Additional high temperature or temperature cycles loadings can make detection easier by acceleration of crack occurrence. To detect short opens of solder joints subjected to vibrations, continuous fast measurements of resistance changes during the test are required. Within the article, the essence of fast multi-channel measurements and the construction of dedicated data storage system will be described. As a summary expected results of measurements will be shortly presented.

Molecular dynamics study of the chiral vector influence on thermal conductivity of carbon nanotubes

Current paper focuses on the influence of chiral vector on thermal conductivity of carbon nanotubes. The non-equilibrium molecular dynamic technique was implemented in commercially available software. The eleven single-walled CNTs of various chirality (from zigzag to armchair) was investigated. Moreover, the influence of length on CNTs thermal conductivity was examined.

Influence of nano silver filler content on properties of ink-jet printed structures for microelectronics

The influence of nano silver filler content on properties of ink-jet printed structures for microelectronics were investigated. Samples were prepared by using ink with different nano silver filler content: 44 %, 41 % and 38 %. The electrical measurements were performed during and after heating process. It was shown, that filler content does not have a strong influence on sintering time, but it changes the final resistivity of printed and sintered structures. The enhancement of sintering process by UV exposure was also investigated. Preliminary results are promising and the further study will be conducted.

Characterization and performance of electrically conductive adhesives for microwave applications

The main advantage of isotropically conductive adhesives used for assembling electronic circuits is lack of lead and other toxic metals in resultant joints. The objective of this paper was to find the best formulation of isotropically conductive adhesive for solder replacement in microwave applications. As a result of the screening experiment the different adhesive formulations were investigated in order to identify the significant factors, which influence the electrical resistance of joints. In these formulations the adhesive base material as well as type of main and additional filler materials were changed. Silver, nickel and graphite were used as a main filler material, whereas soot and heavy silver were filler additives. The adhesive formulations were tested in standard microstrip bandpass filters, which were supplied with an additional gap in the gold strip and bridged by adhesive bonded silver jumper. As the figure-of-merit the quality factor Q and loss L of such a microwave circuit have been investigated. Both the Q-factor and loss of the filter with bonded jumper were measured at the frequency of 3.5 GHz in preliminary experiment and at 3.5 GHz as well as 14 GHz in final experiment. For identifying the best adhesive formulation the experimental design method based on Taguchi techniques for quality engineering has been used. It was stated that to each adhesive, which is prepared on the base of specific type of resin, the strictly defined type of silver filler should be added and strictly defined volume content of the filler should be selected.