Testing microwave components over their respective operating temperature ranges can be a cumbersome and complicated task, as well as time consuming and costly. Fortunately, Sigma Systems has designed a family of products that makes the task much easier to accomplish, thereby reducing both cost and time, without sacrificing accuracy. Sigma Systems Thermal Platforms are both compact in size and easy to use. Cooled with cryogenic gas or mechanical refrigeration, and heated with electric heating elements, the operating range and rapid ramp rates of these plates make them superbly qualified for many unique thermal conditioning applications not suited to a standard thermal test chamber. From MIL-SPEC profiles performing thermal testing to parametrically tuning of microwave and RF components, the Sigma Systems Thermal Platform has consistently been the specified temperature-conditioning device for the most demanding customers.

A Unique and Versatile Design

In an effort to diminish operating costs by decreasing test times and facility resources, thermal platforms were developed as a cost-effective alternative to using large chambers for thermal conditioning of flat package and micro-electronic devices. Since Sigma Systems first introduced the thermal platform over 30 years ago demand for supporting faster cycling, higher frequency tolerance, larger wattage dissipation and a host of custom fixture applications has driven this product to adapt to an industry that is constantly evolving. Sigma Systems has grown with this industry by never forgetting the fundamentals of what it takes to make a properly designed thermal platform. Properly designed thermal platforms are cooled by injecting high pressure liquid nitrogen (LN2), carbon dioxide (LCO2), R507, or R508B (SUVA95) through a small diameter tube into a coolant channel that is physically imbedded within the platform. The injection capillary tube acts as a throttling device. The refrigerant passes through the capillary tube and transitions from high to low pressure in much the same way a pressurized fluid passing through the nozzle of a spray bottle will atomize and turn to a mixture of vapor and microscopic particles. To fully understand the physics of this process, a thermodynamic analysis of the expendable refrigerant must be made.

Flow Mechanics

A properly designed thermal platform should be quiet and efficient. For example, a poorly designed unit using LCO2 will whistle and can sound like a siren. The areas that need to be considered when properly designing a thermal platform is to increase the channel volume proportional to the expansion rate of the refrigerant in order to support a decrease in particle population as the particles sublimate or migrate from a fluid to a gas (depending on the refrigerant). Another area of channel construction that significantly affects the channel geometry is designing a channel that promotes expansion while inhibiting particles bombarding the channel wall. The last area that is taken into consideration is expansion rates of up to 700:1.

The channel must be able to support as much expansion as possible in order to harness this cooling energy and at the same time harness the expansion gradient itself to help propel the vapor and remaining particles through the exhaust at the end of the channel.

It was this premise that lead to the gentle arcing double-helix channel design that Sigma Systems uses (see Figure 1).

Conversely, a poorly designed plate can have a profound negative impact on performance. For example, if the channel does not inhibit bombardment into the wall, the pressure driving the particulate stream will stabilize.

This, in turn, will promote particle accumulation. This effect can result in catastrophic failure as any significant restriction can quickly turn into an extremely high pressure condition or could result in more minimal ways by creating undesirable temperature gradients.

Construction Features

The Sigma Systems Thermal Platforms are constructed of a high grade aluminum body and feature an extremely flat mounting surface that minimizes thermal transfer losses (see Figure 2). Below the surface is the proprietary milled expanding cross section inverted double-helix design cryogenic cooling channel with a single coolant inlet for maximum cooling efficiency. Added to that is a high resolution temperature sensor and a compact design for maximum accuracy and efficiency. The resulting system provides faster thermal testing and greater accuracy and improved stability, with increased component throughput and efficient use of cryogenic gases and lower overall power consumption.


Applications for these thermal platforms include thermal conditioning and programmed thermal cycling, environmental stress testing (ESS), space temperature and pressure simulation using a bell jar in place over the thermal platform, highly accelerated life testing (HALT), highly accelerated stress testing (HAST), parametric tuning over temperature, component curing and bake out.


Many test professionals have found that a well engineered platform is the fastest, quietest and most efficient means to boost product performance while lowering operational costs. Controlling delivery and sublimation rates of the expendable coolant by optimizing path curvature, length and geometry are but some of the methodologies required to design and produce an efficient thermal platform. Constantly refining these basic premises that are over 30 years old attest to Sigma Systems’ ability to grow and adapt to the ever-changing face of the defense industry and to provide a product that is of sound design, built from quality materials and tested to ensure reliability for years to come.

Sigma Systems Corp.
El Cajon, CA (619) 258-3700, www.sigmasystems.com.

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