Techniques In Using UV Adhesives
for Optomechanical Designs
Eric A. Norland
Norland Products Inc.,
New Brunswick, NJ 08902
Optomechanical bonding or mounting with adhesives has demanding
requirements which are more severe than most other bonding
applications. Stress must be minimized to prevent optical
distortion and must be considered over the entire operating temperature.
are desired along the bondline and bonded components must
remain precisely located over their entire lifetime.
Ultraviolet Curing (UV) adhesives can offer advantages for fastening
or mounting of optics if the mechanical design is configured to
advantage of the handling characteristics of the adhesive. Use of
these materials simplifies any process that requires critical alignment
This paper describes some of the considerations which must
be taken in order to chose the best UV adhesive and how to control
maximum performance in a system.
and size of optics
In general, only small, low mass, optics up to about 2"
or 3" diameter are suitable for mounting with UV adhesives.
This is due to the
limitations in acceptable bondline thickness which can be easily
cured without creating an excessive amount of shrinkage and inherent
Common bondline thickness can range from 0.010" to 0.250".
Typical optical components which are good candidates for UV adhesive
mounting are lenses, prisms, filters, mirrors, as well as
Essentially any bondable substrate can be designed into the
system. Best choices are glass ,metal, glass filled plastics, and
engineering plastics now available. Undesirable plastic substrates
are unfilled nylon, delrin®, teflon®, polyethylene and polypropylene.
Physical properties of adhesive
With the wide variety of UV adhesive types available today, it is
important to consider the physical properties desired for optimum
optomechanical performance. Among the many properties, the
most important are shrinkage, adhesion, modulus of elasticity, and
These physical properties are all interrelated. It is not possible
to look at one property alone to determine suitability. It often
will be a trade off
of decreasing one property to increase another more desirable
property. In the end, a balance of properties is typically needed.
To minimize stress and provide long term stability, the lowest possible
shrinkage is desirable. It is impossible to completely eliminate
shrinkage. Fortunately, stress can also be minimized by the
cure configuration discussed later in this paper.
Adhesion, modulus of elasticity, and elongation must have the proper
balance for optimum performance. Good adhesion is necessary to stand
up to shock, temperature cycling, and differential expansion,
but a more resilient product, ie. lower modulus with higher elongation
more desirable to withstand these same conditions. A lower
modulus adhesive can provide a certain amount of stress relief when
stretches to eliminate the stressed condition.
Glass transition temperature (Tg) is sometimes considered to be
a critical property to indicate maximum temperature range. This
temperature where the physical properties change from 'glassy'
state to a 'rubbery' state. Tg can be critical for rigid materials,
sudden change in the physical properties can affect performance.
It is much less critical for resilient materials, which are already
and do not change as drastically.
An understanding of how shrinkage occurs is important in order to
minimize stress. Because UV light enters at the surface and penetrates
into the adhesive, the curing and shrinkage occur from that
surface down. If substrates are allowed to move closer together
occurs, stress can be minimized. Alternately, in mounting
applications, if a wide bead of adhesive is used around the edge
of the optics, the
adhesive can shrink from the surface down which helps to minimize
Another factor is the speed of cure (i.e. light intensity). Higher
intensity UV light provides a faster, more penetrating, depth cure
but can lead
to more stress. Lower intensity light allows for slower cure.
This more gradual cure and shrinkage from surface down appears to
UV curing is so quick and simple, it is easy to think of the light
as a magic wand which can be waved over the component and everything
magically cure. It must be understood that UV light behaves
identically to visible light. It will follow the available optical
path but will not bend
around corners and will not appreciably scatter sideways
into small gaps beside the optics. There are products which have
a 'shadow cure'
where exposed adhesive can propagate the cure sideways to a small
degree and other products that can be heat cured to complete the
areas unexposed to light, but care must be taken to check
for the stresses after cure.
The key to UV curing for optics is uniformity of illumination. Be
sure to have even intensity, with no shadows over the bondline.
change is acceptable but any sudden changes lead to internal
stress in the adhesive itself. This will always cause instability
over long term.
It is important to be aware of light source position. It must be
located where it will see the adhesive. A simple check is the 'eyeball'
test. If the
adhesive bondline can be seen by the eye, the UV light source
can see it from that same location, too. If the bondline cannot
be seen with the eye,
UV curing will be a problem.
Theoretical calculations can only supply an indication of an adhesives
performance. The greatest assurance is provided by testing it in
actual application. After an initial screening for adhesion and
testing for stress or optical distortion, running samples through
testing is the best way to evaluate the long term performance
of an adhesive. Comparative tests among different materials with
cycling, shock, and accelerated heat aging will quickly indicate
any changes in the adhesive bondline and determine the acceptable
There are a multitude of techniques used to mount optical components
with adhesives. Here are a variety of illustrations for mounting
filters, windows, prism and mirrors. Note how the configurations
allow for efficient UV cure.
UV adhesives provide an efficient method of providing fast precision
alignment or exact positioning of optical components. They are suitable
for use in optomechanical designs when ease of assembly, reduction
of weight and long performance are desired. To take advantage of
curing adhesives, proper understanding of the capabilities
and limitations are needed.
Choosing the best adhesive for mounting of optics requires analyzing
the key physical properties such as adhesion to substrates, shrinkage,
modulus and elongation. The adhesive must have the correct
balance of elasticity to take differential expansion yet not allow
any sag or
Care must be taken to provide optimum curing of UV adhesives. The
mounting configuration must be designed to maximize uniformity of
exposure and minimize stress during cure.
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