Variable frequency microwave curing
Advanced process finds optoelectronic applications
BY BILL GEISLER, BRUCE ADAMS AND IFTIKHAR AHMAD
Microwave ovens are found in almost every home in the United States. The two primary limitations of our home microwave ovens - hot and cold spots in our food and arcing to metals (such as utensils and aluminum foil) - are also potential concerns when the technology is applied to heating applications in the electronics manufacturing industry. However, these problems are largely related to the fixed-frequency (2.45 GHz) operation of such appliances in the home. Figure 1a shows a schematic representation of the electric field established in the cavity by a fixed-frequency microwave.
Variable frequency microwave (VFM) is a patented microwave technology developed at Oak Ridge National Laboratory that does not suffer from these limitations. VFM technology sweeps a bandwidth of frequencies rapidly to increase the uniformity of microwave energy in comparison to fixed-frequency microwaves. VFM ovens are normally operated to sweep a bandwidth of about 1 GHz around a center frequency every 100 milliseconds. The center frequency can be selected from a variety of bands - typically C-band (5.8 to 7 GHz) or X-band (7.3 to 8.7 GHz). The bandwidth of frequencies swept is broken up into 4,096 individual frequencies. Therefore, during operation of a VFM processing cycle, the microwave frequency launched into the cavity is changed approximately every 25 microseconds. Figure 1b shows a schematic representation of the microwave field in a VFM cavity. VFM technology eliminates hot and cold spots and allows the use of metals in the microwave - major benefits to fiber optic and optoelectronic processing.
VFM technology was first introduced in 1998 to the microelectronics packaging market for curing glob top, dam and fill, flip chip, and a number of other encapsulation and adhesive applications. More recently, VFM technology has also been used for curing adhesives in the fiber optics and optoelectronics industry. In these markets, VFM offers the advantages of faster curing of adhesives and selective heating. Adhesive cure is usually 2 to 10 times faster with VFM than conventional heating methods. Microwave heating of adhesives is a volumetric heating process in which strong dipole groups, such as epoxy functional groups, couple to the microwave field. This coupling of microwave energy to dipole groups in the adhesive resin leads to rotation of these groups resulting in heating and faster curing.
Figure 1. Schematic representations of microwave energy distribution in cavities for (a) fixed frequency microwave and (b) variable frequency microwave.
To better understand the factors involved in microwave heating, the following equation details the major variables involved in microwave absorption.
Pav = ωεΟe"effE2rmsV
The average power dissipated (Pav) by the material in the microwave field is proportional to the applied angular frequency (ω), electric field intensity (Erms), the dielectric constant of free space (εΟ), the dielectric loss factor for the material (ε"eff), and the volume (V), of the material being heated.
Furthermore, the depth of penetration of microwaves into the material (Dp) is governed by
Dp = λΟ(ε')1/2 / (2πe"eff)
where l0 is the wavelength of the microwaves, and ε' is the relative dielectric constant for the material. It is clear from these equations that as the frequency is swept, the penetration depth and the power absorbed in any specific portion of the total volume will also change simultaneously. During the VFM sweep cycle while higher frequencies are swept, power will be absorbed near the surface of the volume. Conversely, while lower frequencies are swept, power would be absorbed deeper in the volume of the material. This process of sweeping a frequency bandwidth provides uniform heating within the entire mass of the material.
Figure 2. DSC cure comparison for Adhesive A: Green curve = uncured adhesive; Red curve = VFM cure for 45 seconds at 120°C; Blue curve = Thermal cure for 5 minutes at 120°C.
Figure 1a shows the microwave distribution for a fixed frequency microwave. Because 4,096 frequencies are launched during the VFM sweep cycle, each frequency has a particular wave pattern. For each frequency, there are high-intensity peaks similar to those in Figure 1a. In fixed frequency systems, charge will build up at the high intensity location and will eventually discharge to a ground potential. However, with the VFM process the frequency changes every 25 microseconds, thereby changing the wave pattern continuously. The continuous micro wave frequency sweep, therefore, eliminates the charge build-up even on high conductivity metals.
Adhesive Curing Comparison
A comparison between variable frequency microwave cure and conventional thermal cure helps to underscore the differences between the methods. The data also suggests where VFM cure acceleration might be exploited to enhance a fiber optic or optoelectronic manufacturing process. A commonly used fiber optic adhesive (Adhesive A), often used for ferrule attach and fiber array attach as well as a wide variety of other fiber optic and optoelectronic applications, was used in this experiment. The goal of any curing process is to achieve the highest degree of cure in the shortest amount of time. The cure temperatures and times chosen for these experiments were based on manufacturer recommendations.
Table 1. Adhesive cure data for several samples, comparing standard convection curing and microwave curing.
Differential scanning calorimetry (DSC) was used to determine the degree of cure. It was necessary to run DSC on the uncured adhesive to establish a baseline exotherm value. The degree of cure is then determined by comparing the baseline exotherm value of the uncured adhesive to the residual exotherm of the adhesive after various curing profiles. Figure 2 provides an example of DSC data for Adhesive A. The data demonstrates that a VFM cure for 45 seconds at 120°C is equivalent to a thermal cure of 5 minutes in a 120°C convection oven.
The cure results for experiments run with a 95°C cure temperature are shown in Figure 3. A commonly used thermal cure profile for Adhesive A is 30 minutes at 95°C. Note that after 30 minutes of convection oven cure, the degree of cure is 90 percent. For VFM cure at 95°C, a 90 percent cure is achieved after 7 minutes of processing. The data in Figure 3 also demonstrates that full cure of Adhesive A is not achieved even after long cure cycles. The best that can be expected for 95°C processing and reasonable cure times is 90 to 95 percent of full cure. Bonds prepared at less than full cure have lower adhesion strength and are prone to moisture absorption.
Figure 3. Cure comparison for Adhesive A at 95°C: Red curve = VFM; Blue curve = Thermal cure.
Figures 4a and b show the results with a 120°C cure temperature. The manufacturer recommends a 5-minute thermal cure at 120°C. The results show that the adhesive is approaching full cure after 5 minutes of convection cure at 120°C. VFM cure of Adhesive A is quite rapid at 120°C in comparison to convection oven cure. Figure 4b gives an expanded view of the VFM cure data at 120°C.
In addition to Adhesive A, a number of adhesives and encapsulants are used in the manufacture of fiber optic and optoelectronic components. These include epoxy adhesives, dual UV/thermal cure adhesives and silicone encapsulants. A comparison of VFM to convection cure schedules is given in Table 1 for a range of commonly used products.
DWDM: For many dense wavelength division multiplexer (DWDM) devices, large die must be placed onto an assembly substrate and aligned with fiber arrays with an accuracy of a few microns. Traditionally, yields of around 70 percent have been experienced during fabrication of DWDM devices.
VCSEL: Multi-channel transmitter and receiver vertical cavity surface-emitting laser (VCSEL) devices are challenging to manufacture. Alignment accuracy must be maintained on each channel to within a few microns. Devices are usually held in place with a nano-positioning device and then a dual cure (UV/thermal) adhesive is used to tack the device in place. The device is then removed and post-cured to fully crosslink the adhesive and render the material stable. During this process, the VCSEL device is vulnerable to component movement which results in yield loss. Post-curing is typically accomplished using a convection oven, but it can also be done using variable frequency microwave. The VFM curing technique has been shown to significantly reduce misalignment issues.
Figure 4. (a) Cure comparison for Adhesive A at 120°C: Red curve = VFM; Blue curve = Thermal cure. (b) More detail of the VFM cure of Adhesive A at 120°C.
Fiber Connector Attach: For many of the same reasons mentioned above, VFM is being used in ferrule attach, fiber array attach, and DWDM-to-fiber block assembly processing. It has also been realized that VFM's volumetric heating can cure adhesives quick and uniformly. Convective heating, on the other hand, cures adhesives from the surface inward, which ultimately contributes to component misalignment and stress generation. Additionally, heating can be isolated to the ferrule or fiber array block so that curing can be completed without heating the optical fiber or ribbon cable.
Transceivers: VFM is also being used for die and fiber attach in the assembly of transceiver devices. Transceiver assemblies are one of the fastest growing segments of the telecommunications network industry. VFM has greatly increased cure speed for adhesives and encapsulants used in the manufacture of transceivers. In addition to fast cure speed, VFM has brought increases in yield to transceiver manufacturers by maintaining component alignment during cure.
VFM technology offers several benefits to fiber optic and optoelectronic manufacturing processes. The most prominent feature of VFM cure is acceleration of cure, which increases product throughput. Early adopters of VFM cure technology have benefited from the selective nature of VFM heating. Additionally, other important benefits such as post-cure yield improvement have been obtained with VFM technology due to the acceleration of adhesive curing, which locks components in place and leads to higher yields.
Bill Geisler, applications manager; Bruce Adams, process engineer; and Iftikhar Ahmad, applications manager, can be contacted at Lambda Technologies, 860 Aviation Parkway, Suite 900, Morrisville, NC 27560; 919-462-1919; Fax: 919-462-1929; E-mail: email@example.com; firstname.lastname@example.org; email@example.com.