GREG SHARP, TIM SIGELKO, AND SYDNEY KOCSIS, LUMENFLOW
Surface mount optics (SMOs) are the optical version of surface mount technology (SMT) components such as resistors and capacitors that are ubiquitous in electronics. Like their electronic counterparts, SMOs are delivered on a standard tape and reel, allowing users to align them onto printed circuit boards (PCBs) with speed and precision using pick-and-place equipment (Figure 1).
As is true of all industrial optics, application conditions and requirements determine the optimal optical material. Due to their high heat resistance, silicone SMOs withstand the solder reflow process without melting or deforming, and they seamlessly integrate with the PCB assembly process to reduce time and cost. Thermoplastic optics are also well known and widely used in industry, offering their own advantages including durability and electrical insulation.
However, distinct material differences expand the use potential of low-cost polymer optics to applications beyond those for which thermoplastics are viable. For example, the silicone polymer polydimethylsiloxane (PDMS) has higher energy in the siloxane bond than the carbon and oxygen bond of epoxies and common thermoplastics. This high-energy bond results in superior thermal endurance as well as UV stability — advantages that make optical silicone an optimal material in challenging environments.
Figure 1. Surface mount optics (SMOs) are
aligned onto printed circuit boards (PCBs) using
pick-and-place equipment. This assembly step
is one of many shared similarities that SMOs
share with surface mount technology (SMT)
components — their electronics counterparts. Courtesy of LumenFlow.
Broadly speaking, though there is generally no need to switch to silicone in application environments that do not expose the restrictions of plastic optics, the design freedoms enabled by silicone open possibilities beyond many common design spaces.
Processing comparisons
Silicone and thermoplastics have different molding processes and require different processing skills, tooling, and equipment. Thermoplastics are melted pellets that are injected into a mold with high pressure and are cooled to cure. Silicone is a two-part liquid thermoset material injected at low pressure and flows into a mold as a low-viscosity liquid. Its low viscosity and high wettability allow it to flow into very small details during processing and make it an excellent candidate for replicating small features such as diffractive/holographic (nano)structures.
But this wettability creates certain processing challenges. For example, since silicone enters the mold as a liquid, the mold must be water-tight to prevent leaks that can produce excess flashing around the parting lines of the mold. Precision parting lines must therefore be implemented to prevent unnecessary rework of parts post-molding.
Injection molding thermoplastics creates molded-in stress due to the high pressures required to fill the part. This pressure may influence the shrink rates during cooling, causing parts to sink and/or warp. The injection speeds used with thermoplastics are also much faster than silicone, affecting molecular chains. The materials’ molecular orientation may further contribute to deferential shrinkage and consequent warpage. Further, the stresses may create birefringence in thermoplastics. The slow fill speeds and pressures that silicone requires means that molded-in stress is not a by-product of injection molding, and parts shrink uniformly as they cool.
Figure 2. This full silicone lens features
undercuts that are designed into the optic.
A central undercut enables off-axis, uniform
plane illumination using a light source.
The addition of solder clips makes the optic
a surface-mount solution. Courtesy of LumenFlow.
These thermal and molding properties, compared with optical thermoplastics, make silicone the ideal material for molding flatter, wider parts. Large, thick cross-sectional parts can be molded in a single shot, which eliminates the need for multilayer or multi-shot plastic injection molding (which may be used for automotive headlamps).
Thermal considerations
Thermal management in optical systems is a growing concern on multiple fronts, as packages become smaller and LEDs become brighter, and hotter as a result. Silicone presents higher thresholds for softening and deforming in hot environments than thermoplastics. For applications in which the photon density is high, such as automotive headlamps, stadium lighting, and street lighting, silicone’s high operating temperature of up to 200 °C offers a necessary benefit. Silicone will retain its shape and avoid any loss to its optical properties. And short exposures to higher temperatures, for example, in solder reflow ovens, yields no loss to mechanical or optical properties of the part.
Standardized Surface Mount Optics for Initial Evaluation
FWHM: full width at half maximum; FWTM: full width at tenth maximum; OD: outer diameter. Courtesy of LumenFlow.
It should be noted that designers who are experienced with thermoplastics no longer need to increase the size of systems to physically isolate their optics from the heat sources. They can also create exotic shapes that are impossible to mold in rigid materials to condense the size of the optical system. The full silicone optic in Figure 2, for example, uses multiple aspherical optical surfaces and a central undercut to achieve off-axis, uniform plane illumination using a VCSEL with two off-axis peaks.
Here, plastic optics require separate parts to achieve comparable performance to silicone, with its high flexibility and elongation to twist and stretch out of a mold. Thermoplastics are more rigid, and complex geometry, including undercuts, can crack or break upon removal from the mold.
Commercial implications
Combining the material properties of silicone with enhanced design freedoms and a metal solder clip, SMOs have been developed to further reduce assembly costs. Today, a variety of silicone SMOs are commercially available for proof-of-concept testing (see the table). These offerings target general applications. The current designs are singlet lenses and doublet compound lenses. Singlet lenses create more tightly collimated beams with varying spots. Compound lenses will spread and collimate the light for uniform area illumination and a relatively sharp edge roll-off.
Tests to evaluate the performance of the developed optics were performed using commercially available white LEDs. The results indicated that the developed optics may be paired with any LED of a similar footprint and yield similar results. Additionally, the optics can be redesigned to better suit a particular LED model and/or reach desired performance targets as determined by the target application. Beyond optical design, to control the surface profile, various materials can be mixed with the silicone to produce different optical effects, such as scattering centers, coloring agents, phosphors, and reflective and absorptive materials.
Thermal management in optical systems is a growing concern on multiple fronts, as packages become smaller and LEDs become brighter, and hotter as a result. Silicone presents higher thresholds for softening and deforming in hot environments than thermoplastics.
One of the most evident benefits of using SMOs is in assembly, eliminating secondary processes and saving time and labor costs. Unlocking this advantage relies on an attachment method featuring a six-tab clip, stamped from tin-plated metal, which is crimped to the optic. Three of the tabs cradle the side walls of the surface-mount optic, and the other three remain flat for soldering. Alternative solder clip designs are being developed for different applications as shown in Figure 3. Typically, optics are manually glued or clipped in place after the PCB has been fully assembled. Silicone SMOs withstand reflow conditions, meaning they can be bonded to the PCB through a single pass in the reflow oven in the same way as a standard SMT component.
Figure 3. Alternate solder clip configurations
for surface mount optics (SMOs) packaging. Courtesy of LumenFlow.
Reflow studies showed that both the LED and the optic can be placed at the same time, removing the need for a second pass through the SMT process. Post-soldering placement accuracy of SMOs is on the order of 0.25 mm. The benefits in assembly using silicone SMOs eliminate secondary processes, saving manufacturers time and reducing labor costs.
Outlook
Several applications are starting to ramp up volume production, though most potential SMO products are still in the early stages of design, productization, and exploration with commercial companies. As product sizes shrink and optics sit closer to the light sources, designers need more heat-tolerant materials. If the system can be designed with larger distances from the source and the optics, there may not be an overt need to convert to silicone.
However, a path for attachment — and costs of attachment — can drive the design to an SMO style of optic.
In this context, SMOs represent a continuing component innovation offering competitive efficiency. Enabling automation and seamlessly integrating with existing PCB processes, SMOs deliver a fast and precise solution for optical alignment and assembly. And in this context, silicone optics provide superior heat resistance, weatherproofing, vibration dampening, greater transmission, and design freedom. These material advantages should be leveraged to face challenging thermals, complex geometries, and/or assembly costs.
Silicone Material and Surface Mount Optics: Benefits and Advantages
Silicone
• High heat resistance: 200 °C operating temperature.
• UV stable: will not yellow.
• Waterproof: used for higher ingress protection rating.
• Vibration dampening: no buzz, squeak, or rattle.
• Greater transmission into the UV: 50% transmission
at 265 nm.
• Mechanical features include undercuts, voids, and
negative draft angles.
Surface mount optics
• Labor savings: Optics can be secured in place through the same solder reflow process as other surface mount technology (SMT) components, and these optics do not require secondary processing to secure to a printed circuit board (PCB), whereas traditional optics require an operator to glue and/or clip an optic in place after PCB assembly.
• Surface mount optics (SMOs) are placed by automated
assembly equipment providing better alignment of the LED and the optic, resulting in better optical efficiency.
• Manual optical mounting may require a dispensed liquid that could interfere with other components, resulting in rework
or scrap of materials. SMOs eliminate manual processes and offer improved precision to reduce the scrap costs.
• Easily switch from one optical design to another by swapping reels on pick-and-place equipment.