|
About UV Cure and Materials
The UV portion of the electromagnetic spectrum includes wavelengths from
250nm to 450nm
The visible portion extends from 400nm to 700nm.
Deep UV is from 150-300nm, mid UV is from 300-350nm, and near UV is from
365 to 435.
UV curing uses high intensity light to start a chemical reaction which
converts low molecular weight materials to high molecular weight polymers.
Why use UV light to cure polymeric materials?
UV technology is very environmentally friendly. These formulations do not incorporate VOC's to obtain low viscosity materials. Water reducible raw materials are available.
Once cured, UV formulations become thermoset polymers with good to
excellent chemical and thermal resistance.
UV light provides a nearly instantaneous cure, which enables very fast
line speeds. Cure speed depends on the intensity of the light source and
the formulation chemistry.
Most UV formulations are non hazardous and do not require special shipping considerations.
Traditional UV Applications
Coatings
Plastic
Wood
Paper
Metal
Protective coatings for tubes and pipes.
Flooring
Inks
Adhesives
Electronics
Fiber Optics
Stereolithography
New UV Applications
Fielco's New Applications
Future Trends
Dual cure technologies.
Microphotoresists for semiconductor production.
Resolution down to less than 0.1 micron for positive resists and to 5 micron for negative resists.
Non-yellowing clear coats for outdoor use.
Opaque Microencapsulants
Due to high shrinkage of rigid UV materials, particulate fillers are incorporated into the resin to reduce the chance of the material pulling away from the sides of the housing.
Cure depth to 100 mils is possible in a filled or pigmented system.
Deeper cure requires longer exposure to UV radiation, up to one minute.
Warm resin will speed cure.
Method of application is by syringe, either manual or pneumatic operation.
Usually free radical formulations due to higher reactivity.
These materials provide nearly instantaneous cure, which greatly speeds the throughput time over conventional one or two component formulations.
UV cured microencapsulants can be used on plastics which can not be exposed to high temperatures required for typical one component thermally cured materials.
Electrically Conductive Adhesives and Inks
Electrically conductive fillers are incorporated into the resin matrix.
These fillers include, gold, platinum, carbon black pigments, graphite, and most often, silver.
The required conductivity and the performance of the part determine the filler used.
Conductive Adhesives
Average volume resistivities: .09 Ohm-cm to .0001 Ohm-cm.
Typically one or two component thermoset polymers, usually epoxy based.
Uses include bonding wire terminations to circuit interface and attachment
of surface mount components to flex circuitry where the substrates involved
can not handle the high temperature required for solder.
One component formulations require heat cures of 110°C to 160°C for five
minutes to several hours.
Electrically Conductive Adhesives and Inks
Depending on the formulation, a two component material will cure at room temperature, or may need heat to cure.
When better conductivity is required, heat is used to shrink the polymer thereby packing the conductive fillers closer and increasing the conductivity of the joint.
Method of application varies. Most often these are dispensed by syringes which are loaded void free by the manufacturer.
Two component room temperature cured adhesives can be mixed and frozen then shipped to the customer on dry ice. This eliminates the need for mixing on site.
Conductive Inks
Average volume resistivities: .005 Ohm-cm to .00001 Ohm-cm.
Typically one component thermoplastic polymers dispersed in a suitable solvent.
Sometimes a solvent is dispersed in a one component epoxy to lower the viscosity.
The trace is baked at 50°C to 75°C then post baked at 120°C to 140°C.
Thermoset materials are used when chemical or thermal resistance is needed.
The thermoplastic materials can either be air dried or accelerated with mild heat. Again, heat will decrease the resistivity of the trace.
The method of application is usually by screen printing on flex circuitry, or by direct deposition on components, such as the end terminals on surface mount components.
UV Activated Tape
Two Approaches to UV-Activated Tapes
The polyester, fiber reinforced tape is coated with a liquid material where the solid resin and photoinitiators have been dispersed in a suitable solvent.
The solvent is then evaporated to form a solid film on the surface of the tape.
The tape is exposed to UV light, placed on the splice, then heated on a press for 15 seconds at 250°F at 1500 psi.
Considerations:
Once exposed to UV light, the material has a limited open time of up to one minute before the tape must be applied to the splice.
Fast cure once the material reflows.
Unidirectional tape is coated with a 100% solids dual cure adhesive.
The polyester, fiber reinforced tape is coated with a liquid material where the resins, latent curing agents, and photoinitiators have been predispersed in a liquid resin.
The tape is exposed to UV light where the UV activated portion of the formulation forms a loose, solid matrix, which binds the heat activated portion of the formulation.
The tape is then cured for the specified time, which is determined by the reactivity of the curing agent used to cure the heat activated portion of the material.
Considerations:
No VOCs are involved in the manufacture of this adhesive.
Once exposed to UV light, the adhesive has an open time or shelf life equal to the reactivity of the curing agent used. This may be up to 5 to 6 months at room temperature.
Variable cure speeds depending on the latent cure rate.
Another new process that is proving very beneficial is "Dual Cure" technology. Dual Cure involve the use of two-different technologies, such as UV for an initial cure and conventional adhesives technology for the final cure. The first cure stage provides the green strength (holding, but not fully cured) that's often necessary for the assembly process. This can be especially helpful in electronics assembly, where you would need a fast cure for the assembly line, and then the product could cure fully over time. If you would like to find out more about Dual Cure technology contact us by e-mail.
Electronics encapsulation materials. Typically used to protect delicate assemblies from shock and severe climate conditions, the demands on these formulations have increased with increased emphasis on severe climate conditions of electronics equipment, and product miniaturization.
For years we have been continually improving encapsulations formulations, to insure they meet and exceed the new industry requirements.
Fielco has a product selector guide section on encapsulation materials, but we also conduct ongoing research into the frontiers of encapsulation technology. If you would like to find out more about encapsulation materials, contact us by e-mail.
|
