Page 14 - Industrial Technology April 2021 issue

DESIGN ENGINEERING

OPTICAL BONDING

FASTENERS AND ADHESIVES

THE COMING CHANGE IN UV
SOURCES COULD LEAD TO
CONSIDERABLE PROBLEMS
GLOBALLY. KLAUS WAMMES, CEO
AT OPTOELECTRONICS SPECIALIST
WAMMES AND PARTNER, LOOKS AT
WHAT ENGINEERS NEED TO
CONSIDER WHEN SWITCHING TO
NEWER LED TECHNOLOGIES.
he march of technology means the curing of the
adhesive in optical bonding must change. The
time of vapour lamps has come to an end and
TLEDs will sooner or later replace the old mercury
UV sources. There are good reasons for this, but
unfortunately also sources of error. Integrators who simply
replace the light source may find they receive either
Indeed, this is exactly what is currently happening quite
regularly. This increases the probability that even large
batches of optical bonds will be set up incorrectly or have
already been set up – a time bomb. This is quickly used up and is naturally shortened even Defective UV cure bond on a flat panel display: incorrect colour
In both self-performed optical bonding and the further by frequent switching on and off. Many systems display density gradient bonding adhesive
purchase of ready-bonded semi-finished products, looking reach this lifetime limit earlier than expected.
to achieve the lowest possible cost has long been mercury and thus the calibrations, measurements, tests
established policy. And as long as the trip wires that such A change with impact and all empirical values were also based on it. This
optical bonding entails are taken into account, there is So a UV source change is on the agenda. But It is not easy resulted in concrete rules, standards, estimations and
nothing to be said against this in principle. Unfortunately, to exchange an old UV source for an LED. There are instructions for action depending on mercury spectra.
though, those tripwires are still regularly overlooked. several reasons for this. One is the different spectra of the Thus, in addition, misinterpretation can occur if test
At the same time, as recent examples show, new UV sources. Despite identical average wavelengths, the results are simply compared after and before the change
problems are also being added that did not exist before. spectral distribution of mercury-based UV lamps differs and not re-verified.
Recently, for example, one new trip wire in particular has from that of LEDs. Old lamps do not have a comparable
been added: changing the UV source. Unfortunately, many spectrum to an LED, but a so-called line spectrum. This Catastrophic consequences
system integrators are simply replacing the old UV source, means that they shine in many emission maxima in To make everything a little more tricky, the consequences
mercury vapour lamps of various kinds, with LEDs. As is different, non-continuous intensities, which are also do not always occur immediately after curing and,
so often the case, it’s ‘well thought, badly done’. distributed over the spectral range in which they are moreover, in a wide variety of guises. The best option for
The idea behind the UV source change is right for active. Apart from small deviations, LEDs, on the other integrators is to be able to see straight away that
several reasons. LEDs are – or could be – actually better hand, shine only in a very narrow continuous spectrum – something is wrong. In some cases, the chemical
suited as UV sources for curing. For example, they are smaller, finer and more precise. composition of the adhesive simply does not react to the
easier to handle, and save energy because they are more At the same time, many measuring instruments were LED light and does not cure at all. In other cases, various
efficient – they can be turned on and off more quickly and and still are calibrated or designed for the UV sources visible optical or mechanical artifacts occur. Optical
generate less heat due to their narrow-band spectrum. used to date. If, after a UV source change, the measuring artifacts can, for example, be seen over the entire display
At the same time, the old UV vapour lamps all contain instrument is not calibrated, replaced or adapted, some or only in local shifts of the colour temperature, in spots,
mercury. This chemical is harmful to humans and the lines from the spectra – often the most relevant lines – will stains and clouds of very different kinds, in contrast
environment, and more and more manufacturers are be completely or partially overlooked by the measuring changes, in changes of the gamma curve or similar.
discontinuing production for precisely this reason. This instruments. This happens already today, but so far due to Mechanical artifacts might include abnormalities in shock
creates – from the point of view of the mercury vapour the principle (many emission lines over the whole and vibration tests.
lamp – the typical vicious circle: the toxic lamps are spectrum) it has been without big effects. In some cases, the adhesive lasts only briefly or only
produced less, procurement becomes more difficult, The move to LEDs this changes drastically. As a result, long enough to detect failed bonding. In most cases,
systems are then converted, demand decreases, it only the detected area can be measured. Other measuring however, noticeable or visible effects only occur much
becomes even less profitable to produce the lamps, and so devices only measure the defined ‘dominant wavelength’, later – up to many months later, perhaps in the field with
on. So integrators can no longer simply swap bulb for ie the wavelength with the highest intensity and not the the end customer and thus only after many further stages
bulb. wavelength that is actually most relevant for the curing of in the value chain. The latter scenario in particular is
For those who might have wanted to delay the change the respective bonding adhesive. critical, since it can no longer be repaired, and thus
as much as possible, there is bad news. Vapour lamps do Finally, tests and test results cannot simply be causes considerable problems and costs. These quickly
not have a very long useful life – only around 1,000 hours. adapted. Historically, almost all UV sources were based on rise to dramatic dimensions when it comes to systems

14 INDUSTRIAL TECHNOLOGY • March/April 2021