IR: a final 'cure'? Pure infrared systems can shorten cycle time drastically, and offer dual use for soldermask and legend curing. A primer on how it works.

Infrared heat has been used every day for thousands of years. Large amounts of clay tablets and pottery found at archaeological sites indicate that the sun was used to increase the temperature of material in order to provide molds for construction. Between approximately 2700 and 2200 B.C., the great pyramids of Egypt were built of sun-cured bricks.

In that era, due to limited access to fire, builders of pyramids and temples used the sun's IR radiation to cure bricks. Mesopotamian remains provide evidence that the Tower of Babel was constructed of bricks cured by the sun. (The face of the brick and stone have indications of burn marks.) In India, wastewater was carried through pottery pipes into covered brick drains along the streets. The wastewater was discharged into brick culverts leading to a stream. These sewer systems date to 2500 B.C.

IR heat sources transfer heat via radiation. IR is one of three methods by which heat is transferred; convection and conduction are the others. Convection and conduction methods rely on dome mediums to transfer heat. Like radio waves, infrared energy is radiated through space. Radiated energy has a specific wavelength within the electromagnetic spectrum. This ranges from the very short wavelength of gamma and X-rays, through the visible light portion of the spectrum, to the infrared, up to radio waves and beyond.

IR heat, or radiant heating, is advantageous due to the fact that the product being cured need not be in direct contact with the heating elements or circulated air. IR heaters must have a direct line of sight to the surface of the product to be heated. The intensity distribution of an IR panel varies with temperature. More energy is radiated at shorter wavelengths as the temperature increases. IR panels needed for relativity low temperatures are normally used for heating nonmetallic materials, which absorb energy at the long wavelengths.

As stated, the ultimate source of radiated heat is the sun, which generates an enormous amount of IR energy through gas combustion. This IR heat passes directly through space to warm the surface of Earth and is, in fact, the energy source that makes life possible.

How IR Works

All products emit IR radiation. Depending on the temperature an item may reach, this temperature will influence the wavelength of emitted energy. The higher the temperature reached by the product, the shorter the wavelength of IR radiation. The amount of energy radiated from a hot item, versus that of a cool item, depends on the variation between the fourth powers of their absolute temperatures. A very hot item emitting short wavelength IR radiation will transmit more energy for its size than that of a cooler item of the same size. Therefore, to achieve high-density temperatures, short wavelength emitters are used.

All products absorb IR energy differently. The absorption characteristics depend on such factors including the color and surface finish of the product. For example, a matte black surface will absorb more IR radiation than that of a glossy white surface. In addition, absorption characteristics of a given item can be modified depending on the wavelength of the IR radiation. When curing soldermask on a circuit board, an IR emitter will heat the surface under the soldermask, essentially curing it from the inside out (FIGURE 1).

[FIGURE 1 OMITTED]

IR heat transfer begins when two products measuring different temperatures are in sight of each other. The heat is transferred via electromagnetic waves that measure between 0.78 and 1000 pm wavelengths. The amount of IR energy emitted and the wavelength of the emission depend upon the temperature of the product (FIGURE 2).

[FIGURE 2 OMITTED]

* Short wave, or near IR, is defined as the area from 0.72 to 1.5 [micro]m. To do this, most of its points should be between 3016[degrees] and 6400[degrees] F.

* Medium wave, or middle IR, is defined as the area from 1.5 to 5.6 [micro]m. To do this, most of its points should be between 471[degrees] and 3016[degrees]F.

* Long wave, or short IR, is defined as the area from 5.6 to 1000 [micro]m. To do this, most of the points should be less than 471[degrees]F.

Convection and radiation are designed to transfer energy from the heat source to the product being cured, without direct contact. This is the preferred method when contact-free heating must be achieved. Because the insulating characteristics of the boundary film of air--which adheres to surfaces of products--gravity convection heating is extremely slow and inefficient for high-speed production. Forced air convection systems direct heated air onto the product and assist in breaking up the boundary film. The disadvantage is that this system requires enclosures and handling means, and if the air is not recirculated properly the spent volatiles must be discharged (with consequent heat loss).

Emittance and Emissivity 

Emittance has been identified as a critical parameter when accurately converting the output of a detector used in a radiation thermometer into a value representing object temperature.

Emittance and emissivity are terms often used interchangeably, although they have a definite technical distinction. Emissivity refers to the characteristics of a material, emittance to the characteristics of a particular object. Therefore, emissivity is only one factor in determining emittance. Other factors that must be taken into account include the shape of the object, oxidation and surface finish.

The emittance of a particular material also depends on the temperature at which it is determined, along with the wavelength at which the measurement is taken. The surface condition of the material also affects the value of an object's emittance. Polished finished surfaces will have lower values relative to matte surfaces.

IR technology as a heating source offers many advantages. It can heat portions of a process or product. Less floor space is required than for convection ovens. Process cycle times are greatly reduced. Process quality is greatly increased, giving a more thorough cure and repeatable accurate heat control. In addition, lower energy consumption is achievable.

For PCB curing, IR curing can:

* Improve curing.

* Reduce cycle times.

* Reduce electrical consumption.

* Reduce floor space.

Over the past few years, PCB makers are beginning to embrace this old--but new--curing method for thermal inks. Likewise, ink manufacturers are developing faster curing inks.

PCB manufacturers that have invested in IR systems have reported improvements in the quality of their curing processes. For final cure of liquid photoimageable (LPI) soldermask, these improvements include reduced soldermask breakdown in the nickel-gold process, stress relief of warped panels, elimination of the need for a UV bump to enhance the structural integrity of the soldermask, and a uniform, superior cure.

One board manufacturer I know of now applies legend ink via an inkjet printer. The company has improved its legend-ink curing by processing panels through a pure IR curing system after ink has been applied. In addition, tests have been performed in an attempt to reduce cycle-time for curing conductive ink products. The test results were very positive.

In independent tests performed to determine the volatiles remaining in soldermask after the final-cure process, we sent soldermask samples cured in a pure IR system, and samples cured in a tunnel oven, to NuSil Technology Laboratory. The test method used was Weight Loss, in accordance with ASTM-595, NASA SP-R0022A and ESA PSS-01-0702.

Upon completion of the test, the results indicated the panel cured in the tunnel oven resulted in a 0.20 weight loss, and the panel cured in the pure IR system resulted in a 0.13 weight loss. These data confirm that there were more volatiles left in the soldermask after final cure when a tunnel oven was used. In short, the pure IR system produced a more thorough final cure.

DAVE COREY is co-founder of ACS Technologies (acs-tech.com) and inventor of TruCure infrared curing systems. He can be reached at [e-mail].