Radu Bunea

Advanced investigations on PCB traces fusing in high power applications

The paper presents the virtual and real investigations related to the fusing of PCB traces in high power applications. The reason of performing the research and related tests is that in real applications the current carrying capacity of PCB traces is different than the value presented in standards and datasheets or obtained after solving thermal equations. Based on the experimental results, the authors want to offer a first practical resource in the case of traces/tracks fusing, in order to avoid failures of electronic systems during the operation and, why not, to offer a design guide of developing PCB fuses, which could be interesting in some specific or low cost applications. In addition, the contribution introduces finally a few “rules of thumb”, useful to designers, fabricators and hardware engineers, which will deliver practice oriented advices for specialists involved in electronic design and manufacturing.

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.

PCB tracks thermal simulation, analysis and comparison to IPC-2152 for electrical current carrying capacity

Along with the introduction of the new IPC-2152 standard came several questions: What has changed? How does this new standard affect the PCB design? Are the simulation programs up to date with the standard? And the most important: In practice, how close are we to the standard? In this research, we developed a series of tests to analyze the real thermal behavior of tracks when charged with different constant currents. We used boards with different thicknesses (FR4 0.8mm, FR4 1.6mm, FR2 1.6mm, CEM 1.6mm) and 35um copper thickness and 1mm track width. We also performed simulations of the structures using Ansys. All the results were compared to IPC-2152 standard.

Thermal investigations of solder joints used in power applications

A good way to characterize the quality of a solder joint is its thermal behavior at different currents and the current capabilities. The electrical current capabilities have been discussed in many other studies, but thermal investigations were not performed very often. The team developed a series of tests on boards with different configurations, and in all cases was proven that the solder joints heat up much less than the components or the tracks, and their temperature increases due to thermal conductivity from the surrounding elements.

Investigation on the efficiency of thermal relief shapes on different Printed Circuit Boards

As known, thermal relief is a technique used to separate soldering pads or plated through-hole vias from large copper planes in order to reduce soldering dwell time by providing thermal resistance during the process, thus minimizing the heat transfer to the plane. However, during normal operation, the same thermal resistance limits the heat transfer for power component dissipation. The narrower the thermal pad, the greater temperature increasing during power dissipation. Hence, the dimensions of the thermal pads are critical. The work presents the results of a practical approach not for a hole via or a pad for an ordinarily component connected to a large copper area, but a medium power Surface Mounting Device, SMD, using the Printed Circuit Board, PCB, as heat sink. There are taken into consideration different thermal pads and there are investigated by temperature measurements with thermocouples and thermovision. The variables are the thermal pad dimensions, the area of the radiant surface, the base material of PCB, and the reflow soldering process. It is analyzed the effect of the thermal pad versus the temperature on the soldering pad and beyond the thermal pad and on the case of the device during reflow soldering process and during operation

Modeling of heat dissipation in SMD chip components

The ever-rising pressure to reduce circuit board size to fit the ever-shrinking chassis challenges design engineers not just with size constraints, but power and thermal constraints concurrently. The power dissipation of a circuit is not necessarily reduced as technologies advance and circuits become smaller. It may not be an acceptable option to reduce the power dissipation and there may even be a drive to increase it. Reliability is a major concern of all electronic products manufacturers, and it is given by the reliabilities of the electronic components. One factor that determines the operation time of electronic components is temperature, and to be more precisely heat dissipation. There appears the problem of how heat is dissipated from SMD components, especially when no forced cooling is available.

Laser soldering of small pitch BGAs on Plexiglas substrate PCBs

The paper presents the results of the study and the procedure of soldering 200µm pitch BGAs on circuits on PMMA (Poly-methyl-methacrylate or Plexiglas) substrate, starting from the paste deposition process, fine placement of the BGAs and finally soldering. The novelty in this research is that it had to face two restrictions. The combination of PMMA substrate, transparent to laser, and SnBi, ecologic solder paste, which required a low and very well-controlled soldering temperature, and the small size of the BGAs which needed a punctual soldering. These particularities led the team to use laser for soldering, due to the possibility of highly accurate energy calculation and very small size of the focused beam.

Optimizing technological process for laser soldering of fine-pitch flip chips

In the frame of EC sponsored RTD projects as well as industrially requested innovation developments the activities of BME-ETT cover the full technology of electronics design, assembling, soldering, process monitoring and testing. This paper presents some important details how a team of BME-ETT has been developing and optimizing the solder paste printing, component placement and laser soldering sequence of processes to assemble flip-chips with 80µm size bumps onto a polymer substrate with 200×200 µm size pads. In particular, the problems and solutions of working out the stencil printing, chip placement, and their testing methods are illustrated and explained in the paper.

Optimizing laser soldering of SMD components: From theory to practice

The paper presents the results of a study of the procedure of laser soldering chip SMD components, taking into consideration various parameters of the soldering process: type and quantity of solder paste used, energy pumped into the system, etc. There are many situations in modern electronics where the heat for soldering must be controlled very accurately, for example for soldering temperature sensitive components or using different substrates that require a soldering temperature different than the usual ones (PMMA substrates need low temperature soldering). We studied the energy necessary for realizing high quality solder joints even from the first soldering, thus reducing the number of destroyed samples and decreasing the time and costs of development of the electronic module. Based on the theoretical model, we soldered components and performed electrical and mechanical tests, cross-sections, optical and X-ray inspections.

Multicriterial approach correlation of the solder joints functional tests results

The assembling of the electronic components on PCBs becomes a key issue in electronic packaging reliability. The solder joints resulting from the soldering process represent an important outcome for the PCB assembling. Moreover, the Lead-Free soldering requests, imposed by the RoHS Directive, can cause difficulties in obtaining a high quality/reliable soldering process. Considering the reliability of electronic modules as expression of solder joints functionality, could be emphasized the lack of tests over solder joints functionality on assembling line. Usually are realized optical inspections and, in precise situation, X-Ray analysis are done over assembled PCBs being followed by “in circuit” electrical functional tests. The defects type “hide” or “cold/dry joints” could be not found. In the paper are presented the results of the functional tests of solder joints realized in two soldering technology, infrared-convection and vapor phase. Regarding to thermal functionality, infrared analysis of temperature distribution on the surface of a populated PCBs using thermovision were done. A correlation among functionality, reliability and thermovision functional analysis will be realized based on the experiments results. In consequence, the authors emphasize thermovision as available in circuit indirect test method for “hide” defects of solder joints.