A novel welding technique enabled by electron beam lithography involving laser welding
along absorber dye patterns has been developed by TWI Ltd in collaboration with
the University of Cambridge. The result is the world’s smallest weld in thermoplastic
material, bringing with it new applications for smaller scale biological analysis
chips, chemical micro-reactors and electronics products.
Laser welding is the process of choice in many industry sectors for joining plastics
due to its ability for precision of the joint location and in the amount of heat
applied. It is perfectly suited to the manufacture of complex products such as microfluidic
devices, where channels and structure resolution below 100 μm are used regularly.
As industry seeks to produce even smaller scale complex plastic components, there
is a call for new research to allow welds in plastics of 10 μm and below. Until
recently, the smallest laser welds possible measured between 10 and 20 μm in width.
However, this is at an infrared laser’s resolution limit, and smaller welds are
not possible using control of the laser focus spot size alone.
It was work carried out by TWI and the University of Cambridge’s Cavendish Laboratory
into precise patterning of laser absorber dye on a plastic surface to define weld
position, that sparked off the latest advance. Having proved that the process using
laser absorber dye resist material could allow joints to be made between plastics,
the project team studied the use of electron beam lithography to pattern the absorber
and enable welds with a width smaller than 10 μm, mimicking methods used to build
microelectronic circuits. The challenge was to generate micro-channels and infrared
absorber tracks at their edges simultaneously, and to seal the channels.
‘C2K is working on having other training sessions by TWI whenever possible.’
With polymethyl methacrylate (PMMA) thermoplastic as a base material, the team carried
out trials, following the principle of transmission laser welding using a thin coating
of infrared absorbent material at the joint interface. The coating was patterned
using electron beam lithography to the required resolution in a reproducible manner,
so it could be retained after welding. Joint strength was ratified using larger-scale
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