Balint Balogh

Thermal Simulation of UV Laser Ablation of Polyimide

Our aim is to provide a simulation tool for manufacturing processes that takes the influence of temperature distribution thus the underlying copper pattern and pulse repetition frequency into account. To establish such simulation software a finite element model was set up which is able to describe the thermal processes and ablation induced by laser irradiation of polymers. The etch rate description usually applied for excimer lasers was slightly altered to be adequate for Gaussian lasers by writing pulse energy instead of fluence. This theoretical consideration was proved by experiments. The experiments also revealed the temperature dependence of etch rate and the influence of pulse repetition frequency on the amount of ablated material. The experimental and simulation results are in good agreement.

Applications and Comparison of Failure Analysis Methods

As the importance of quality insurance and reliability is increasing electronic manufacturers apply more and more failure detection and analysis methods in and off the production line. The aim of this paper is to describe and compare the applicability of several up-to-date failure analysis methods. The discussed techniques are X-ray microstructure analysis, SAM (scanning acoustic microscopy), SEM (scanning electron microscopy) combined with EDS (energy dispersive spectroscopy) analysis, optical inspection, microsectioning. The analysis methods can be grouped not only by their destructiveness and whether they are in or off-line but also by the failure and defect types that can be detected by them. This paper gives an overview of the detectable failures and shows typical applications of the analysis methods through recent case studies.

Effect of patterned copper layer on selective polymer removal by 355 nm laser

Ab s t r a e t Optimization of lascr parameters is a challenge in case of microvia drilling. It is even more complicated to find the right parameters for large area scanning, where not only pulse energy and burst play important roles but also pulse repetition rate, scanning speed and overlap. In case of selective material removal the above process parameters define the amount of the ablated material (etch depth) and the processing time. This paper dcscribes results of experiments carried out to find correlation between the process parameters and the quantity of the ablated material. Flexible polyimide substrates of different thicknesses were processed by frequency trippled, Q-switched Nd:YAG laser. The samples were examined by surface profiler and their cross-sections by optical microscope. The experiments showed that the amount of the ablated polymer is influenced by the features of the patterned copper layer. Even though there is no direct interaction between the laser beam and the copper layer, the thermal conductivity of the circuit pattcm influences the temperature of the processed polymer. Because of this phenomenon there is a need to control the laser processing parameters according to the circuit pattern. The correlations we have found between the process parameten and thc etch depth make this controlled laser material removal possible.

Simulation and indirect measurement of temperature change in polyimide induced by laser ablation at 355 nm

Thermal effect plays an important role during frequency tripled nanosecond Nd:YAG laser ablation of polyimide. This paper describes simulation results and experimental investigations of laser induced temperature change in polyimide. The experimental results show that the ablation rate is temperature dependent, which means the underlying copper layer and the pulse repetition frequency (PRF) influence the etch depth. Based on our model it is possible to control the processing parameters according to the pattern geometry for a better exploitation of laser micromachining

Selective laser soldering on flexible boards using through foil heating

In our selective laser soldering experiments, we soldered surface mount (SM) chip resistors onto flexible substrates. Taking advantage of the transparency of the substrates in the red and the infrared range, we applied an interesting and unique technique by heating the solder pads exposing them through the substrate. Our previous measurements showed that the polyimide (PI) and polyester (PET) substrates absorbed less than 5% of the 1064 nm wavelength laser light, while polyethylene-naphtalate (PEN) absorbed about 30%. The laser energy which was let through was enough to melt the two lead-free alloys and formed a perfect meniscus at the solder joint.

Laser Soldering for Lead-free Assembly

Selective soldering is a process where some joints of a printed circuit assembly are prepared separately from the many reflowed ones. The most popular selective soldering processes are derivative wave-soldering processes. They are usually used to solder the pins of larger through-hole components (e.g. connectors) on a board, which is fully populated and reflowed using surface mount components. In the paper the laser soldering technology is discussed in detail, which -as the most promising alternative selective soldering technique -might replace the less energy-efficient derivative wave-soldering processes. Application oriented test boards were designed and applied for the investigation of the laser soldering process and qualification of the joints. Among the good results, the most interesting is that high quality solder joints could be prepared even when the temperature limit of the board was much lower than the melting point of the applied solder paste.

Effects of test conditions on shear strength of surface mount solder joints

In electronic industry shear test is a commonly used method to classify the mechanical properties and reliability of solder joints. The aim of this paper is to investigate how the shear test conditions (shear rate, shear height) affect the measured shear strength values. To study this, a series of experiments have been conducted on 0603 sized chip resistors. The experiments showed that increasing the shear rate results in slightly higher measured shear strength in the 60.600 µm/s range. The higher the shear height the lower the measured shear force was, thus it is advisable to perform the shear tests with the lowest reproducible shear height. In case of the tested components the fracture always formed between the ceramic body of the resistor and its metallization.

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.

Failure analysis methods in electronics assembly technology

The aim of this paper is to describe a failure analysis methodology applicable to reveal the root causes of electronic assembly failures. The most relevant properties of the applied techniques, i.e. optical microscopy, X-ray microstructure analysis, SEM (scanning electron microscopy) combined with EDX (energy dispersive X-ray spectroscopy) analysis, are given. Three recent failure analysis case studies are also presented. In case of a burnt component the microwire and the leadframe formed AlCu intermetallic compound in the length of several millimeters. The low joint strength of gullwing leads were deduced back to paste printing and wetting problems. The fracture surface morphology of a joint broken on the field showed different structure from the ones broken at room temperature. It was revealed that SnPb solder fracture surface is different at room temperature and above 100°C.

Laser applications in the field of MEMS

The paper overviews some of the most important applications of lasers in the field of microsystems (MEMS). We present applications from both technology and MEMS testing. Since these applications cover a wide range, the presentation does not aim to offer a thorough, detailed presentation of each of these, but only to underline some of the most important directions and features. We present the respective applications, their working principles, their advantages and their drawbacks. In some cases we present examples of microstructures realized by using laser techniques.