Rapid curing by variable frequency microwave (VFM)

TWI Bulletin, July/August 2002

The demand for lighter, faster, smaller, lower cost and more reliable products has led to a sweeping change in many industries. As Sam Rostami reports polymeric materials, for this reason, are continually finding applications in more traditional and high tech electronics, optics, medical, and other industries.

Apart from housing and support components, where fully polymerised materials are used, numerous other applications require the use of low molecular weight monomers or oligomers that are capable of being polymerised in-situ to provide a function as well as conform to a shape. This is the case for the majority of adhesives, encapsulants and coatings that are used in electronics, optics, medical and many other applications. The search for ultra-rapid and selective curing of these reactive oligomers, has led to the development of curing techniques such as variable frequency microwave (VFM). VFM is a technique involving the use of controlled high frequency microwave energies.

Domestic microwave technology is based on a single fixed frequency of 2.45GHz while variable frequency microwave technology uses multiple frequencies from 5.85 to 7.00GHz. These multiple frequencies are continuously and selectively swept during a process, resulting in thousands of frequencies acting on a material instead of only one. By targeting the frequencies and bandwidth of the sweep a uniform energy distribution can be achieved to maximise the energy absorption by the materials to be processed.

VFM is particularly suitable in polymerising adhesives and encapsulants that require uniform heating in a short time. For this reason, the electronics industry has been quick in evaluating the potential use of VFM in device manufacturing. The microwave frequency can be swept from 5.85 to 7.00GHz in less than a second. The microwaves are delivered inside a chamber. A prescribed choice of frequency and temperature is selected to be specifically absorbed by adhesives, encapsulants or compounds without raising the temperature of the adjacent materials to anywhere near the same level.

The VFM technique uses three major parameters to provide processing or curing. These are the centre frequency, frequency bandwidth and sweep time. The bandwidth is divided into 4096 points. By cycling through the frequencies, the VFM in effect launches 4096 different frequencies into the microwave cavity with a typical sweep time of 0.1 seconds. When so many frequencies are scanned rapidly, the dynamics prevent build-up of electrical charges, leading to processing conditions that eliminate arcing or damage to metallic components that are on a circuit board. These frequencies selectively excite the polar molecular functional groups in polymeric materials to provide uniform energy absorption providing rapid curing.

Commercial and technical advantages of VFM

• Reducing curing time, providing increased throughput.
• Space saving, fewer are required.

Technically, VFM offers opportunities in:

• Replacing ultraviolet (UV), infrared (IR) radiation and/or heat cure methods with an efficient and uniform microwave cure.
• Reducing cure time, by a factor of 10 or even higher depending on the material used.
• Reducing migration of impurities by reducing exposure time to high temperatures.
• Rapid and selective heating at the molecular level, offering options for design of molecules with specific functional groups to perform a selective task.
• New cure profiles that match production needs more closely.
• Possibility of combining curing with other processes, such as moisture resistance coating.

Due to the rapid scan of frequencies, there is less chance of electrical charges that could build up around the tip of conductors used in properly designed circuit boards. However, problems may occur with poorly designed circuit boards.

Comparison between VFM, UV and IR curing

• UV curing is generally fast but requires photo initiators in the adhesive and direct radiation exposure.
• IR curing is normally slow and has limited depth of penetration that is required for uniform curing.
• VFM is rapid, selective and has high depth of cure.

The ability of VFM to cure thicker section of adhesives more selectively, uniformly and at a faster rate are the key advantages of VFM over UV or IR radiation methods. Its main disadvantage is the relatively high initial capital costs.

Applications of VFM

• Structural bonding of electronic assemblies.
• Die attach and bump cure
• Glob-top and cavity fill
• Flip chip underfill adhesives
• Coating and encapsulants
• Fibre optics devices and opto-electronic assemblies
• Coatings on wafer and flex circuits
• Packaging

Figure 1 shows some examples of applications where VFM is used.

In almost all cases, while retaining other properties of the adhesive and encapsulants, the use of VFM reduces curing time compared with the same material cured by a conventional heating process. The availability of additional heating, may anneal curing stresses too. The reduction in curing stress is mainly due to the uniform microwave heating. Moreover, in a convection oven the entire sample heats to the same temperature, therefore all different materials expand based on their CTE. The CTE mismatch may introduce stress in the structure on cooling. With the VFM heating, only the adhesive and the die will heat to the cure temperatures. The substrates adjacent to the die will only heat-up, thereby reducing the stress build-up in the assembly. The stresses built-up due to in-service thermal cycling are expected to be the same for microwave cured and conventionally cured materials.

Equipment

These equipment have features such as:

• Cycle control and data acquisition capabilities.
• Pre-set and programmable process parameters to meet the needs of a specific application.
• Infrared temperature monitoring device for closed loop temperature feed back.
• Auto ramp software to provide accurate temperature control up to 265°C.
• Microwave compatible specialised carriers and fixtures.
• Optical pyrometers to monitor temperature inside the chamber and cavity exhaust system.

Summary

Using VFM technology for processing polymeric materials, as opposed to conventional microwave ovens, provides all the process capability and advantages of microwave energy. Furthermore, the ability to operate in the presence of metals and electronic circuitry is a key advantage for the electronics and related industries. The large number of frequencies swept in less than a second excites a large number of functional groups resulting in a uniform energy distribution for rapid curing.

Interaction of microwave energy with materials

Storage: energy may be exchanged between the field and the material in a bi-directional (lossless) manner.

Loss: energy may be permanently lost from the field and absorbed in the material, usually as heat.

The electrical interactions are quantified as permitivity (

), and are often referred to as the dielectric constant. 

The real part represents storage and the imaginary part represents the loss term. A simple vector diagram below shows the relationship between permitivity storage and loss energies ( Fig.3).

Where:
The storage and loss parts form a 90° angle to each other as shown schematically. The sum vector forms an angle δ with

. This ratio is the tangent of the angle δ which is often called tan δ, loss tangent or tangent loss. The tan δ is directly related to the dissipation factor as shown below:

Generally speaking, polar polymers ( eg PMMA) have high loss tangent and non-polar polymers ( eg PTFE and PE) have low loss tangent. Each dielectric mechanism has a characteristic cut-off frequency. Water, for example, has a strong dipolar property at low frequencies but its dielectric constant drops off around 20GHz.

PTFE, on the other hand, has a low dipolar mechanism so its permitivity is relatively constant well into the microwave regions. Polyethylene is another non-polar material that has higher permitivity compared to PTFE and has lower permitivity at high frequencies compared with other polymer materials. The presence of polar additives, such as antioxidants and lubricants, increases the permitivity of these materials. Permitivity also changes with temperature and humidity as well as frequency. The heating rate of polymers in a microwave electrical field is given by:

Where the constant K is 55.61x10 ^-14, f is the applied frequency (Hz), E is the electrical field strength (V/cm), ρ is the density and C _v is specific heat. According to this equation, high-loss (high dielectric constant and tan δ ) polymers absorb microwave energy efficiently to heat up rapidly. Functional adhesives are commonly made of polar molecules with high-loss properties. They are thus ideal for microwave curing and heating. It is important to note that by adjusting temperature and the frequency regime, it is possible to maximise the energy absorption in the adhesives to achieve rapid cure.

These days, a wide range of test methods at microwave and millimetre wave frequencies is available ^[1-5] to map material properties. Most conveniently, automatic network analysers (ANA) provide fundamental materials information for all swept high frequency stimulus-response measurements.

References