Adhesive bonding is the method of choice for assembling high-performance photonic components. Adhesives enable the process of active alignment, in which measurements of part function are used to guide the actuators as they position those parts for assembly.
The process works like this: Adhesive is applied to the parts, then cured to a gel stage using heat or ultraviolet light. The parts are adjusted until the function of the assembly is maximized. Then, the adhesive is rapidly brought to a full cure, locking the parts in place. Often, the final cure is done at a relatively low temperature. This safeguards temperature-sensitive components and prevents thermal stresses that might distort alignment of the parts.
To keep throughput high and thermal distortion low, engineers are often forced to compromise on adhesive selection and cure schedules. In turn, these compromises can lead to long-term reliability problems with the component, such as failures at the fiber-ferrule interface during temperature cycling.
With a new curing technique that uses variable frequency microwave energy (VFM), engineers may be able to avoid these compromises. VFM applies heat energy rapidly and directly to the adhesive material and package interior. VFM can cure adhesives as much as 20 times faster than convection heating, without overheating the package exterior. In fact, the speed and control of VFM systems are good enough for full in-situ cure of the adhesive during active alignment, without compromising adhesive cure strength.
The Need for Speed
Adhesive cure profiles vary based on the cure temperature and the properties desired from the bonded joint. Generally, bond strength and completeness of cure will increase as cure time and cure temperature increase. For example, a popular adhesive for fiber optic assembly is Epo-Tek 353ND. The adhesive can be fully cured at 80 C for 1 hour or 150 C for 30 minutes. However, the best performance from the adhesive is typically obtained with the higher temperature cure.
If an assembly contains heat-sensitive components, engineers simply extend the cure time at a lower temperature. While this strategy has merit, it is still critical to heat the adhesive at a sufficient rate to ensure good chemical conversion. Low temperatures may be insufficient to completely cure the adhesive, even with very long cycles.
There?s another reason why fast cure times are important: Rapid curing minimizes the time period when adhesive relaxation (a temporary lowering of the adhesive?s viscosity and strength) can degrade component alignment. Most adhesives that have been gelled will relax and pass through a period of lower viscosity as they complete the transformation into a fully cross-linked polymer. By reaching full cure as rapidly as possible, there is less time for the adhesive to relax, and thus less time for the fibers to move and degrade signal strength.
VFM for Rapid Cure
A rapid cure at the highest acceptable temperature is desirable for strength and reliability. VFM satisfies both requirements.
Microwave energy has long been recognized as a way to achieve high heat flows without imposing a high temperature gradient. This is because microwave energy heats a material directly, through forced vibration of polar molecules. While direct absorption of heat can be beneficial, conventional single-frequency microwave systems are inherently nonuniform and can easily cause arcing and part damage.
VFM sources scan rapidly through a broad range of microwave frequencies. This produces a highly uniform energy distribution without the arcing-related damage observed with single-frequency microwave sources.
In addition to curing adhesives in photonic assemblies, VFM technology can be applied in heating processes for complex electronic components, including metal-encased components, sensitive semiconductor circuits and flex circuits. The technology has also been incorporated into complex alignment machines.
VFM technology offers many benefits for curing adhesives used in fiber optic and electronic components.
First, VFM gives engineers agile control over heat flow. Direct absorption of energy translates into high heating rates without the temperature gradients and propagation delays associated with thermal conduction. The flow of heat to the parts can be rapidly modulated, allowing engineers to devise heating profiles that include both a gel stage and a final curing stage.
Another advantage of VFM is selective heating. Material selection and material doping can be used to tune the absorption of microwave energy, allowing certain areas to be heated more than others. This technique is used in the production of smart cards and radio frequency identification tags. For example, the chip and underfill material can be heated to 100 C, while the substrate remains below 40 C. With selective heating, smart cards can be cured in 4 minutes, with superior adhesive properties. In contrast, a low-temperature curing cycle in a conventional oven could take as long as 4 hours.
A third benefit of VFM is the ability to control thermal strain. With microwave heating, the temperature gradient in the assembly is the opposite of other forms of heat transfer. Heat can be delivered to the interior of a part in a highly controlled manner while minimizing stress due to mismatches in coefficients of thermal expansion. That?s important, because many adhesives absorb less energy as they cure. This allows the same assembly to undergo subsequent cure cycles without overheating pre-existing cured joints.
For even more control over thermal strain, VFM can be combined with conventional heating methods to produce heating rates with very low inside-to-outside temperature gradients.
