Markus P. K. Turunen

Pull-off test in the assessment of adhesion at printed wiring board metallisation/epoxy interface

It is shown that the value of adhesion strength at the metallisation/polymer interface can be determined directly from a printed wiring board (PWB) by a pull-off method. However, careful optimisation of the test geometry is required. In particular, the mechanical properties of the substrate are important. The use of flexible substrate in the test results in severe underestimation of the adhesion strength. There was a considerable stress peak at the edge of the test area, as shown by a finite element method (FEM) calculation with the flexible construction. This unevenly distributed stress decreases the force needed to detach the coating. In addition, if the test area is not defined to match the stud of the test equipment, the stress peak appears at the adhesive/coating interface rather than at the coating/metallisation interface. The experimental data and the examination of the fracture surfaces by scanning electron microscope (SEM) were consistent with the FEM results. In addition, the effect of environmental stresses on the adhesion was investigated. The environmental tests consisted of exposures to (1) thermal shock, (2) elevated temperature and relative humidity and (3) corrosive gases NO2, SO2, H2S and Cl2. The employment of embedded capacitors during the exposures revealed the diffusion of water into the epoxy, which weakened the adhesion. The corrosive gases induced irreversible deterioration of the dielectric material. An adhesion promoter enhanced the durability of the coating/Cu interface in the environmental tests. X-ray photoelectron spectroscopy was used to analyse the chemical states of the pre-treated copper surfaces.

Surface Modification and Characterization of Photodefinable Epoxy/Copper Systems

The effects of the wet-chemical and plasma treatments on the adhesion of electroless and sputter-deposited copper to the photodefinable epoxy have been investigated. The wet-chemical treatment increased the amounts of oxygen-containing functionalities on the epoxy surface mainly during the early stage of etching. With the longer etching the pronounced surface roughness was revealed in the form of microcavities. The plasma treatment increased significantly the polar component of surface free energy of the epoxy and produced low surface roughness. The physicochemical changes of the epoxy combined with the experimental adhesion results indicated that in the case of the electroless deposition the mechanical interlocking was the main adhesion mechanism. Sputter-deposited copper exhibited the highest pull strengths on the epoxy when the plasma pretreatment with oxygen was employed. The enhancement of the surface polarity of the epoxy and the enlarged surface contact area due to the increased roughness were the most important factors in the adhesion.

Effects of surface treatment on the adhesion of copper to a hybrid polymer material

The effects of various surface pretreatments on the adhesion of electroless and sputter-deposited copper metallizations to a hybrid polymer material were investigated. Without pretreatment, the adhesion between copper and the polymer was virtually zero. The adhesion of electroless copper to the polymer was poor regardless of the pretreatment used. However, the wet-chemical pretreatment of the polymer surface markedly increased the adhesion of sputtered copper to the polymer. It preferentially removed the inorganic part of the polymer and formed micropores on the surface. The plasma and reactive ion etching pretreatments, in turn, selectively etched away the organic part of the polymer and noticeably increased the hydrophilicity. Although this resulted in even higher increase in the surface free energy than was achieved with the chemical treatment, the granular surfaces became mechanically brittle. With the help of x-ray photoelectron spectroscopy, scanning electron microscopy, atomic force microscopy, and contact-angle measurements and with the recently developed pull test, the physicochemical changes of the wet-chemically pretreated polymer surfaces were demonstrated to have significant effects on the adhesion.

Evaluation of electrolessly deposited NiP integral resistors on flexible polyimide substrate

Abstract Integral nickel–phosphorus (NiP) resistors were fabricated on flexible polyimide (PI) substrates by electroless NiP deposition. The deposition process was first set up for standard rigid epoxy substrates and then modified for the flexible substrates. The effects of the PI surface modifications on the interfacial adhesion (NiP/PI) were measured experimentally by the pull-off method. The process parameters were optimised to give good adhesion. The mechanical durability of the electrolessly deposited thin film NiP resistors was tested by measuring the electrical resistance during cyclic loading. The results showed the resistors to be mechanically stable. The electrical resistance was also monitored continuously during exposure to corrosive gas environment. The corrosive environment had no significant effect on the resistance of either the electrolessly deposited resistors or the commercial integral resistors used as a reference. The results show that resistors can be fabricated on flexible PI substrate by the described method.

A Wireless Biosensing Device

In this work we have evaluated the feasibility of magnetostrictive materials in the realization of a wireless biosensing device. The measuring concept is composed of two major components that are the miniaturized sensor element and the external instrumentation for the electromagnetic excitation of the sensor and for receiving the bioinformation. For evaluation purposes a suitably cut piece of a magnetostrictive metal ribbon was used as the sensors transducer element. To wirelessly excite this piece and to read its resonance frequency a novel instrumentation was designed and manufactured. This instrumentation provides means to realize the measurement with a much less expensive construction than those reported in the literature. To excite the sensor a magnetic field impulse is sent towards it. This sets the sensor to a vibration and as a vibrating magnetostrictive piece it creates a dumping magnetic field, which then can be collected with a receiver coil. The given concept was demonstrated and is presented here. The biosensing property for the sensor element can be achieved by utilizing another-environmentally responsive-material in connection with the magnetostrictive material as a coating. From the measurement point of view the interaction between a physiological environment and the coating results in a change of the resonance frequency of the sensor element that can then be detected and taken as the representative signal. The instrumentation can be used to measure for example temperature, viscosity, mechanical properties of coatings, pH of a medium, and several other physical, chemical and thus environmental and physiological parameters-wirelessly. The wireless sensor technology makes it ideal for monitoring parameters from inside sealed, electromagnetically opaque containers - such as human. A wireless micro-sensor array would improve patient convenience remarkably in many medical analyses and the technology truly enables the realization of the measurement on a low cost basis. A patient undergoing testing with the presented technique would only have to swallow a pill containing the sensor element, which could then be monitored from a pickup coil embedded within a belt worn around the body-home healthcare