Light/UV Curing Through Polymers
"I have an application where we would like to cover the edge of a silicon die (that has been first tacked in place with epoxy that has been cured to a Valox/PBT base) with an uncured epoxy. Then add additional epoxy to a second bond location that ultimately bonds the Valox base to a Valox cover that has a slight press fit, covering the silicon die. After the press fit assembly, I would like to light or UV cure through the cover plate, causing both applications to cure. By both applications I am referring the bond line between the Valox cover plate & Valox base as well as the edge coating around the periphery of the silicon die, bonding it to the Valox base.
Are there any polymers that can transmit UV or light sufficiently to allow these types of cures? Is there a special light source required? Can the polymer be anything other then clear? Lastly, is there a medical-grade polymer that can achieve this?"
There are a couple of ways to handle this application.
There are a number of polymers that can be used for a cover plate that will let light through, whether UV or visible light. One way is to use a tinted plastic like polycarbonate or acrylic. Tinting the plastic with a dark blue dye will create the "illusion" of being opaque, but will let the proper wavelengths through the plastic to allow it to cure. Finding the right balance of dye will be critical. One way to judge the impact of the dye in the plastic, or the transmission through the plastic, is to measure the intensity of light coming through the plastic from the light source. You can calculate a % loss of transmission through the plastic in both the UV and visible regions. Most adhesives want to see a minimum of 200 mW/cm2, and some of the light sources on the market are emitting light of 20,000 mW/cm2. Even if the plastic blocks off 90% UV light, and 70% visible light, this might be enough to allow it to cure. If the plastic compounder uses a pigment to augment the tint, make sure the level of pigment is low, and they use a white pigment like Titanium Dioxide. A good field test – if you hold the plastic up to the light and can see a shadow as you wave your hand behind it – you might have enough light coming through to cure the adhesives. Stay away from yellow-orange-red colors. Whites, blues, and greens are better. There are a number of medical grade adhesives that can be used for this application, and various curing lamps. I might recommend contacting DYMAX Application Engineering to arrange for some free samples and to discuss our Trial Rental/Lease Equipment program. www.dymax.com.
Will UV light pass through polycabinate and acrylic to cure properly?
It will depend on the grade and manufacturer of the polycarbonate or acrylic. Some grades will transmit UV light and visible light through the plastic easily, while others will block off 100% UV light (due to the stabilizers used in that particular grade). Polycarbonate does block off a little bit of UV light, but not enough to be concerned with. With a radiometer, you can check light transmission through a piece of the plastic and determine if there is adequate light intensity to cure the selected adhesive.
I would like your advice. Can you recommend a name or grade of plastic that has a high transmission of light through it. Please help me in this issue.
Acrylic (also called Polymethyl methacrylate or PMMA) has excellent transmission for many wavelengths, and is used in fiber optics or light pipes to transmit visible light through the plastic.
Thank you Kyle for your information, but I have another question. Can light pass through this material? For example, if I have a cylindrical solid with a radius of 3 mm, can I shine the light through the path from somewhere other than the ends of the cylinder?
If you have a acrylic fiber of 3 mm diameter, you can shine a light down from one end of the fiber to the other, but internal reflection within the acrylic will cause the light to stay within the fiber, and it will not go through the side wall of the fiber (for example - it will not come out at a 90 degree angle). Anytime you have a interface between plastic and air, or plastic and water, or air and water, the angle of incidence becomes important. If you shine a flashlight at a pool from right above it, the beam will enter the water and bend slightly. If you shine a flashlight at a pool while at a shallow angle (almost horizontal), the light beam will instead bounce off, and not go into the pool at all. This is the same for light shining through plastic rods or fibers. There are tricks to use to get light to come out the side, but this is more of a question for an Optical Engineer. If you would like to contact our Application Engineering group by phone or email to describe what you are trying to do - we may be able to help further. Thank you for using the DYMAX Blog!