Miklos Ruszinko

Virtual laboratory support for microelectronics packaging education

The development of information technology has opened new perspectives in electronics packaging education to prepare engineers for the needs of the 21st century. A project for the creation of virtual laboratory environment in the field of microelectronics packaging is proposed and outlined in this paper. Educational courses for undergraduate students and working engineers can use the proposed virtual laboratories through the internet in order to support their training and extend their skills into emerging fields.

Recent advancements in MCM-L imaging and via generation by laser direct writing

The paper presents some preliminary results and describes the state of a research project that aims at the improvement of the quality of MCM-L circuit boards by the application of UV lasers for the following processes. 1) Pattern generation by direct writing using Nd:YAG (IR, visible or UV) laser. The copper clad laminate is covered by some protective layer, and the pattern is directly written to this layer by material removal (ablation). The pattern of the copper layer is prepared either by wet chemical etching or applying the electroplating-stripping-etching process sequence. 2) Photo-mask exposure by direct writing. In this case a photoresist protective layer is exposed by a UV He-Cd laser, and after developing it is used for the conventional image transfer process. 3) Through board via generation by contour direct writing using frequency multiplied Nd:YAG laser. It means that holes are drilled by moving the well-focused laser beam along their contour. 4) Blind via generation by controlled number of pulses using the same frequency multiplied Nd:YAG laser. In this case the whole area of the via is exposed, and the hole is deepened shot by shot.

Soldering profile optimization for vapour phase reflow technology

Vapour Phase Soldering (VPS) is an alternative method for widespread reflow soldering technologies (e.g. infrared or convection type). VPS has emerged in the last few years after the introduction of Galden fluid. The vapour of Galden is used as the heat transfer medium. The main advantages of VPS are the lower peak temperature, nitrogen/oxygen-free inert atmosphere, the improvement in solder wetting and the reduction of the profiling time [1]. One disadvantage of VPS is the non-controllable, linear soldering profile achieved by common VPS soldering method [2]. With controlled dipping of solder goods into the vapour space, non-linear soldering profiles can be obtained. This method is called Soft-VPS (S-VPS) and was developed and patented by IBL. With this method, custom temperature gradients and profiles can be realized, reducing voids, tombstoning and avoiding high thermal stress on the boards [3]. This paper presents thermal profiling of an experimental VPS station from the aspect of this method, with a successful optimization of the process. The optimized profiles for low melting point (138 °C) lead-free Sn-Bi solder paste were inspected. Soldering results of 0603 chip size resistors were then evaluated with resistance measurements and x-ray inspection.

Electrochemical migration of silver on conventional and biodegradable substrates in microelectronics

Water drop (WD) test and a contact angle measurement method were used to investigate the electrochemical migration (ECM) behavior on different microelectronic substrates with silver conductor lines in all cases. Environment friendly substrates were also investigated in this study as possible candidates of the conventional ones. ECM is kind of short failure phenomenon, which can lead to series defect in case of operating microcircuits, when moisture appears on/in a conductor-dielectric-conductor system. The results show that the novel biodegradable substrate patterns can have a significant higher ECM risk comparing to the conventional ones. The main causes can be the different wetting behavior and the different conductor line accuracy of the technological processes.

Laser via generation into flexible substrates

The microelectronics industry is moving toward smaller feature sizes. The main driving forces are to improve performance and to lower cost. From the performance point of view the small distances between chips together with the short interconnection routes have of great importance in order to achieve faster operation. The application of polymeric materials for the insulating and protective layers of interconnect substrates is beneficial to the performance and to the cost of a circuit module as well. An advanced technology for the fabrication of very high density interconnects applies microvia flexible substrates. This technology has particular significance for the interposers of chip scale packages. Laser processing of polymeric materials applied for via generation, image transfer, contour cutting, etc. has proved to be an efficient tool for the fabrication of interconnect substrates. The paper describes some results of a research project that aims at the application of CO/sub 2/ and frequency multiplied Nd:YAG lasers for drilling via holes into copper clad flexible laminates of polyimide, epoxy and polyester base materials. The effect of the application of an interfacing (adhesive) layer is also the topic of the investigations. The physics of processing using a wavelength range from the far infrared radiation of the CO/sub 2/ laser till the UV wavelengths of frequency quadrupled Nd:YAG lasers are considered to be modeled, examined and evaluated.

Optimizing solder joints on biodegradable PCBs with vapour phase soldering

The paper focuses on biodegradable Printed Circuit Boards (PCB) from the aspect of PCB fabrication technology and soldering. Different biodegradable Cellulose Acetate (CA) PCBs were used during the tests (one sided and two sided), and the possible application of pre-preg was also taken into account to improve pad adhesion on the base material. The surfaces of the boards were evaluated in order to obtain information about copper adhesion. FR4 test boards were also investigated as a reference, both for soldering and surface analyses. The soldering profile setup was focused on minimizing high temperatures on the heat-sensitive biopolymer substrates. The soldering profiles were created with a custom vapour phase soldering (VPS) device. The customized solder profile setting is conducted with an adaptive temperature-tracker sample holder. The boards and the solder joints were investigated with non-destructive optical microscopy, X-ray microscopy (deformation and joint analysis), and destructive shear tests. The results are promising for future application of biodegradable PCBs.

Soldering tests with biodegradable printed circuit boards

The paper presents soldering tests on biodegradable printed circuit boards (PCB) focusing on different alternative base materials. The aim of the work was to investigate and optimize different low-temperature soldering profiles on cellulose-acetate (CA) and polylactide (PLA) biopolymer-based PCBs with the application of special lead-free solder alloy suited for the temperatures. The investigations were focused on the minimization of the heating effect on the heat-sensitive biopolymers, where on the other hand, sufficient thermal energy is required to melt the solder alloys. The soldering profiles were created with a custom vapour phase soldering (VPS) approach where the heat transfer is controlled with an adaptive height-setting, temperature tracking sample holder. The profiles were evaluated according to their heating factor values. The soldered joints were evaluated with X-Ray microscopy and Shear Tests. The deformation of the PCB tracks was also investigated. The preliminary results reveal the priority of CA for further experiments.

Electrochemical migration of copper in pure water used in printed circuit boards

Electrochemical migration (ECM) behaviour investigations were carried out on “oxide-free” and highly oxidized copper surfaces in pure water. It was shown in our previous paper that the oxidation number of copper ion mainly depends on the oxide layer thickness of the copper surface; in the case of “oxide-free” copper surface the Cu1+ ions dominated, while Cu2+ ions were mainly formed in the case of highly oxidized copper surface, which modify the standard electrode potential and also the solubility product constant of copper-hydroxides, thus can effect on the mean time to failure (MTTF) caused by ECM. In order to verify the effect of different copper ions (Cu1+ vs. Cu2+) on MTTF, water drop (WD) tests were carried out on both copper surfaces. The results showed that in the case of “oxide-free” copper the MTTF value was lower than in the case of highly oxidized copper.

UV laser based direct structuring

Conventional continuous wave and long pulse (nanosecond) laser ablation is used in many fields, such as direct PCB structuring process. In this regimes the dominant process involved is the heating of the target material through the liquid phase to the vapour phase, resulting in expansion and expulsion of the desired target material. Frequency-tripled UV Nd:YAG laser spot is used at focal point, however we can conveniently change the power, pulse repetition rate and beam scanning speed to get optimum surface energy density for selective material removing.

Vapour phase soldering on flexible printed circuit boards

The paper presents experimental results about Vapour Phase Soldering on flexible printed circuit boards, where the standard Surface Mounted Technology process was used for investigation. Test boards were prepared from FR4 type Printed Circuit Boards and single-sided flexible polyimide substrates. Lead-free SAC305 paste and different chip size surface mounted resistors were used for the test assembly. The soldering was performed with heat-level vapour phase method in a commercial oven. The tests were composed of different board setups: where the FR4 or the flexible boards were soldered without fixtures, and where the flexible boards were supported by fixtures. The results were evaluated with X-ray microscopy, cross-section optical inspection (IMC tests) and shear tests. The conclusion reveals similar quality factors in all cases, rendering vapour phase soldering a plausible alternative reflow method for soldering on flexible printed circuit boards