Micro Processing: Micro Joining (Welding and Brazing) (LiM 2015)

Sputter-free and Reproducible Laser Welding of Electric or Electronic Copper Contacts with a Green Laser
Elke Kaiser, Rudolf Huber, Christian Stolzenburg, Alexander Killi

Today’s commonly used IR-lasers suffer from two limitations: Firstly the process reproducibility can be quite low as
copper is highly reflective at 1 μm wavelength, and secondly the process parameters used today typically result in
splatters emerging from the welded region during the deep penetration welding process, leading to short circuits.

A newly developed purely green laser source meets all requirements of optimized process quality.
For a pulsed welding process at 0.5 μm wavelength, local power density distribution and pulse shape have been
examined and optimized. The results have been compared to 1 μm wavelength.

The most precise method to detect splatters is the recording of the welding process by means of a high-speed camera.
This allows to detect exactly which types of splatters exist and which process phase is responsible for their formation.

The time and the type of coupling of laser light into highly reflective materials can be determined as well.
By using green laser pulses no splatters occur from 0.1 mm to 0.8 mm penetration depth. Welding spots do not differ in
size. Surface condition of copper has no influence on the welding. In addition to tests on overlap welding and butt
welding of electric contacts, special attention has been turned to welding of DCB ́s (Direct copper bondings in power
electronic substrates).

Keywords: green laser, copper, reproducibility, process quality, sputter-free, DCB, pulsed laser


Energy efficiency in laser rod end melting
Heiko Brüning, Frank Vollertsen

The energetic efficiency of a laser based process, called “laser rod end melting” is investigated. Laser rod end melting
allows the generation of spherical geometries at the end of cylindrical rods. After the spherical geometry at the rod end
is generated in the master forming stage, this part can be used as a preform for a micro cold forming operation in a
subsequent process stage. Within the laser rod end melting process, rods with diameters less than 1 mm are used as
wrought material. The presented process is based on the size effect that when scaling down the size of a body, the ratio
between surface area and volume is increasing

In this paper, results of the laser rod end melting process concerning the process efficiency with regard to usage of
energy are presented. Therefore, the laser rod end melting process is carried out with two different laser beam
propagations relative to the rod. For both experimental setups a fiber laser and rods of chromium nickel steel are used.
The laser power is measured with a power meter on the work piece surface. A method is presented to determine the
energetic efficiency of the laser rod end melting process. It is found that the lateral orientation of the laser beam avoids
the defocusing effect so that the energetic efficiency remains constant during accumulation process allowing the total
process efficiency to take values of up to 0.45 compared to 0.19 under influence of defocusing effect. Power
measurements of the laser beam show a significant deviation between measured laser power and requested power.

Keywords: Laser micro machining; Energy efficiency; Micro forming


Laserwelding of transparent polymer films
Maximilian Brosda, Viktor Mamuschkin

In the field of packaging technology in many cases transparent polymer films are used for food but also for goods packaging. The
polymer based packaging protects the goods in transit from the environment but also against transportation typical stresses such as
shocks, etc. Conventionally the polymer films are sealed by a heat sealing jaws method or ultrasound. So far, the use of lase r sources
could be realized only for very thin or specially modified absorbing films. Polymers have a wavelength dependent absorption coefficient.
Most polymers, which a relevant to the packaging technology, have in the spectral range from 1500 nm to 2000 nm an absorption peak
or areas of increased absorption. Newly especially for polymer welding developed diode laser modules emit laser radiation precisely in
these areas of higher absorption. Therefore it is possible with the aid of an adapted beam shaping to laserseal directly commercially
available packaging films without any modification. On the basis of thin sections and tensile tests the suitability of laser-based sealing
process is demonstrated on representive samples directly from the packaging industry.

Keywords: Lasertransmissionwelding; Packaging technology; Multilayer Films; Polymer


High precise welding of transparent polymers
Frederick Vinzent, Michael Schwalme, Tobias Jaus, Manuel Sieben

Since the well established classic laser plastic welding has become a favorite standard clean serial production
proceeding, providing reliable strength and tight joints, its major field of application is still located in automotive and
adjacent industry sectors. However, LPKF Laser & Electronics now targets the sophisticated requirements of researchers
and manufactures in a modern and pioneering medical technology field. These so called BioMEMS ( Bio-
Microelectromechanical Systems) or Lab-on-a-chip devices are mainly based on complex systems of micrometer scale
canals, enabling a rapid detection method for food safety and clinical diagnostics, chemical synthesis or biological
research (Bhattacharya, Jang, Yang, Aakin, & Bashir, 2007) (Sackmann, Fulton, & Beebe, 2014).

By combining the weldability of two transparent polymers using a 2 μm fiber laser (Mingareev et al., 2012) with a high
precise mechanical and optical positioning system the LPKF PrecisionWeld now provides an effective way for prototype
or serial production of microfluidic devices. The heat generation for the welding process of the transparent polymers,
typically polycarbonate, PMMA or COC, is based on intrinsic absorption of ~ 30 %/mm at the laser wavelength of 2 μm.
The TEM00 shaped beam profile from the fiber laser can be focused down to ~ 60 μm spot diameter enabling the
generation of extreme fine weld seams sealing the micro canals. By using a galvanometer scanner and a high precision
mechanically moveable holding fixture the weld seams can be positioned with accuracy of 10 μm, besides this the
treatment area can be increased by stitching scan fields based on automatic fiducial detection. With this new technique
no critical absorbing additives or glue are needed for the joining procedure avoiding any toxic risk to high sensitive
biological samples and it also pushes forward the existing canal diameter limits of previously known manufacturing

Keywords: laser plastic welding, welding of transparent polymers, LPKF ClearJoining, fabrication of microfluidic devices, LPKF


Direct bonding of transparent PMMA using an ultrafast fiber CPA laser system
Annalisa Volpe, Caterina Gaudiuso, Andrea De Rosa, Rebeca Martínez Vázquez, Antonio Ancona, Pietro Mario Lugarà, Roberto Osellame

Laser welding of transparent materials with micrometer precision is attracting growing interest in several application
fields, especially for the assembly of biomedical devices.

In this work, we exploited the flexibility of focused laser light to weld two transparent 1-mm-thick layers of polymethyl-
methacrylate (PMMA) in a lap-joint configuration. A high repetition rate ultrafast fiber CPA laser system delivering pulses
ranging from 15 ps to 650 fs at the wavelength of 1030 nm was used for our experiments. Non-linear absorption and heat
cumulative processes originated in the focal volume by the high repetition rate (> 200 kHz) produced localized melting of
the polymer at the interface between the two layers.

The influence of the pulse width on the morphology of the laser-induced modifications in the bulk PMMA was evaluated
exploring a wide range of repetition rates and pulse energies.

An appropriate set of process parameters was found able to generate continuous and localized melting of the material.
Based on these results, ultrashort pulsed laser lap welding of two 1-mm-thick PMMA layers was demonstrated. A simple
microfluidic polymeric device has been then assembled taking advantage of this novel joining technique and the
effectiveness of the sealing has been proved by a static leakage test with injected fluid pressures up to 1 bar.

The intrinsic flexibility of the proposed ultrashort-laser micro-welding technique to seal microfluidic devices with complex
geometries without the need for any absorbing layer or chemical additive which could in principle contaminate the biological samples, make this technology very promising for the direct laser fabrication of transparent polymeric
microfluidic devices.

KeyWords: Fiber laser, Welding, Ultrafast laser micromachining, Transparent Materials, PMMA


Influence of Welding Parameters and Stack Configuration on Pore Formation of Laser Welded Aluminum Foil Stacks
Thomas Engelhardt

Ultra-thin copper and aluminum foils with a thickness of 10 μm and 20 μm respectively are used as base material for the
active layers in lithium ion batteries. In a pouch cell design, several layers of coated copper (anode) and aluminum
(cathode) electrode foils are stacked alternately. All layers are separated by an ion conductive synthetic material. The
uncoated aluminum foil contact tabs of all cathode layers must be welded in a stack configuration to an aluminum
terminal sheet. On the anode side, the copper foil stack has to be welded to a copper terminal sheet.

Continuous wave laser welding of such aluminum foil stacks with up to 30 layers with a 1 μm wavelength fiber laser was
investigated as presented in the following. Advantages in comparison to state of the art ultrasonic welds can be found in
less mechanical stress from the joining process. However, especially this cathode laser weld shows imperfections such as
porosity and sporadic separations of foils from the weld seam.

Seam porosity was characterized by computer tomography and image analysis of cross sections to investigate the
influence of the welding parameters and the number of welded foils in the stack. It was noticed that the variation of
these parameters affects the pore volume and distribution.

Keywords: Aluminum Laser Welding, Ultra-Thin Foils, Foil Stack, Lithium-Ion Battery, Pore Formation


Camera based closed-loop control of laser micro-welding processes by observation of the full penetration hole
Andreas Blug, Volker Jetter, Daniel Carl, Simon Gutscher, Jan Nekarda

In macro welding-processes, monitoring and closed-loop control by using the so-called “full penetration hole” is well
known. This paper reports first results for the transfer of this technology to laser micro -welding processes for lap joints
as they are used for housings or fuel cells. The laser source was a 400 W cw fiber laser with a Gaussian beam profile and
a spot size of 28 μm. A cellular neural network (CNN) camera was used to measure the image feature of the full
penetration hole within the thermal image of the welding process. For full penetration weldings on stainless steel
samples, the laser power was controlled by the rate of full penetration hole detection. The effect of the feedback system
is that the laser power is automatically adapted to changes in sheet thickness or feeding rate. The sheet thickness was
varied between 220 and 350 μm and the feeding rate between 10 and 40 m/min without significant change in the weld
seam quality. The closed-loop system increases the robustness of the process against perturbations and the process is
always guided at the minimum laser power necessary for full penetration thus reducing spatter and smoke residues.

Keywords: Micro-Joining; Process Monitoring and Control


Laser welding simulation of microfluidic devices
Arnaud Francois, Anne Henrotin, Jose A. Ramos

In this study, a numerical approach is presented for the simulation of the transparent laser welding process
of thermoplastic polymers. In particular, the numerical tool is used to develop the welding process of a
microfluidic device. The studied microfluidic device consists of two thermoplastic sheets, one of them has
been previously micro-machined by a specific UV femtosecond laser setup. The assembly of the microfluidic
device is then obtained by scanning a laser beam over its entire surface.

The weld quality of laser welded thermoplastics is strongly influenced by the amount of laser energy that is
converted into heat, which also depends on the optical and thermal properties of the materials. In practice,
finding suitable processes parameters for new products is often a difficult task given that temperatures at
the interface need to reach the melting point without exceeding the degradation temperature of the
polymer. Furthermore, the presence of micro channels in microfluidic devices modifies the heat absorption
and heat transfer resulting in inhomogeneous temperature distributions at the weld interface.

The numerical approach followed here gives access to values that are difficult to measure experimentally,
in particular the temperatures in the melted zone. The development of the process was carried out in steps
of increasing complexity, from the study of the assembly of two plates without any micromachining to the
analysis of the microfluidic device. Experimental welding tests are also presented and were carried out to
validate the simulation observations and are also presented.


Thermal analysis of Laser Transmission Welding of thermoplastics: Indicators of weld seam quality
Adhish Majumdar, Benjamin Lecroc, Laurent D'Alvise

This work presents a thermal analysis of the process of laser transmission welding of thermoplastics using the finite
element method. In addition to the heat transfer equations, an original approach was used to calculate phase
transformation phenomena such as melting, evaporation or decomposition, and solidification of the polymer. This yields a
prediction of the size and shape of the weld seam as well as eventual porosity that may arise due to heating of the plastic
beyond its decomposition temperature.

By defining the weld bead quality as a function of its shape, size, and porosity, the results of the simulations with the
variation of process parameters can be used to create a mapping between these parameters and the resulting weld quality.

Keywords: joining; laser transmission welding; process simulation;


Effect of processing parameters in welding of biocompatible polymer film to metal sheet using an infrared laser source
Hui-Chi Chen, Guijun Bi, Juan Carlos Hernandez Castaneda, Hong Xie

Miniaturisation and light weight design has become one of the key trends in today’s manufacture industry. This
transformation is also happening in the medical instrumentation industry. In addition to size and weight reduction,
biocompatibility and hermetic sealing are other common quality requirements for medical applications. Laser welding is
a unique non-contact joining process which yields a high precision weld with small heat-affected zone. This work
employs an infrared laser source for generating micro-scale welds. Process development of micro laser welding of
dissimilar biocompatible materials is reported herein. The optimisation of parameters and different weld designs are
investigated. The micro laser welding process developed targets the joining of heat-sensitive miniature devices that
require air-tight joining quality.

Keywords: Laser welding; biocompatible; dissimilar materials.


Adjustment and Impact of the Thermoplastic Microstructure of the Melting Layer in Laser-based Joining of Polymers to Metals
Klaus Schricker, Martin Stambke, Jean Pierre Bergmann

In this paper, the influence of specific time-temperature parameters on the melting layer and mechanical properties is
shown for laser-based joining of thermoplastics with metals. Based on a differential scanning calorimetry, the base
material is characterized by parameters of melting temperature range and solidification range. These results were
transferred to time-temperature profiles and correlated to the melting layer within the thermoplastic joining.
Considering these results, the change in morphology and the influence on mechanical properties can be shown and
correlated to fracture behavior within the plastic base material as well as the energy absorption of the joint during
mechanical testing.

Keywords: joining, fundamentals and process simulation, metal-plastic hybrid Joints, thermal joining of polymers to metals


Laser hybrid joining of plastic and metal components for lightweight assemblies
Jens Rauschenberger, Asier Cenigaonaindia, Jan Keseberg, Ulrich Gubler, Fernando Liébana

Plastic-metal hybrids are replacing all-metal structures in the automotive, aerospace and other industries at an
accelerated rate. The trend towards lightweight construction increasingly demands the usage of polymer components in
drive trains, car bodies, gaskets and other applications. However, laser joining of polymers to metals presents
significantly greater challenges compared with standard welding processes.

We present recent advances in laser hybrid joining processes. Firstly, several metal pre-structuring methods, including
selective laser melting (SLM) are characterized and their ability to provide undercut structures in the metal assessed.
Secondly, process parameter ranges for hybrid joining of metals (steel, stainless steel) and polymers (MABS, PA6.6 -GF35,
PC, PP) are given. Both transmission and direct laser joining processes are employed. Lap-shear test results are shown
that demonstrate that joint strengths exceeding the base material strength (cohesive failure) can be reached routinely
with metal-polymer joining. Weathering test series prove that such joints are able to withstand environmental
influences typical in targeted fields of application. The obtained results pave the way toward implementing metal -
polymer joints in manufacturing processes.

Keywords: Laser hybrid joining, polymer metal joining, plastic metal joining, metal surface texturing, selective laser melting