The Thermal System and Vacuum Feedthrough Blog

Advantages of Epoxy Electrical Feedthroughs Over Glass-to-Metal and Ceramic Seals

Advances in semiconductor and medical device development has continually challenged manufacturing processes in ultra-clean environments. Getting power and control signals into high vacuum chambers has always been difficult. The vacuum seal has to be tight and not allow any contamination so that product quality is maintained.

Historically glass-to-metal seals for wire feedthroughs have been the choice in these industries, but are constrained in size, geometry, flexibility and electro-magnetic shielding. At the same time, semiconductor and medical device equipment have an increasing need for higher power, more control, better monitoring, and increased signal shielding. These ever changing requirements, which push the capability of glass-to-metal seals, open up opportunity for an alternative technology - epoxy electrical vacuum feedthroughs.

Engineered epoxy electrical feedthroughs offer the best of all technologies. Shapes, angles and curves are not a problem. Virtually any kind of shielded wire or cable can be used and still maintain a tight seal. And as equipment design requirements continue to challenge vacuum seals with space and shielding requirements, the advantages of epoxy vacuum seals look to be a promising solution as the technology itself continues to advance.

While glass-to-metal feedthroughs have advantages in high temperature and corrosive applications, many of todays semiconductor and medical device applications don’t see these conditions. In these lower temperature, and non-corrosive applications, the lower cost, easy prototyping and more flexible design capability of epoxy feedthroughs make them very attractive alternatives.

The epoxy's ability to flow and fill spaces completely make it an excellent choice for any special shapes and sizes a vacuum chamber may require for access. For the most part, epoxy feedthroughs can be used in most applications where glass-to-metal or ceramic feedthroughs are used (with the exception of temperature and corrosion issues outlined above). In some applications, organics are not allowed, and the epoxy feedthrough would be excluded from these as well.

One additional advantage is that custom epoxy vacuum feedthroughs can be quickly provided in very small quantities for prototyping and R&D.

For more information on epoxy feedthroughs, visit this page.

Advancements in Electric Resistance Heaters - Ceramic Heating Elements

Aluminum nitride, high performance electric heating elements using Tungsten traces on ALN. Product manufactured by Durex and Oasis Materials.

Power Densities up to 2500 Watts per square inch
0-400º C in a quarter of a second
Extreme temperature uniformity
Inert in acidic solutions
Custom line widths and resistance values available
Encapsulated Tungsten RTD trace
3D shapes and configurations
Thermal conductivity of Oasis' Aluminum Nitride (ALN) is 190 W/mK
Thermal conductivity of pure tungsten is 170 W/mK

Basics of OEM and Industrial Electric Heating Elements - Part 2

This blog entry, reproduced from an electric heating element basics white-paper from Hotwatt, a leading US manufacturer of OEM and industrial heating elements. To download the PDF version, click this link.

Basic Heat Equations

Electric Heating Elements

It would appear at first that calculating all of the heat transfers and losses in a design would be a daunting task. Fortunately a number of equations were developed that help simplify this task. First the equations were divided into three tasks: the wattage needed to heat a material to a specific temperature in a given amount of time; the wattage needed to overcome the losses at operating temperature; and a special calculation needed to reach a melting or vaporizing point.

This equation calculates the amount of wattage (W) needed to raise the temperature of a material a specific amount in °F (ΔF) in a given number of hours (T), you first need to know the mass (m) of the material being heated and its specific heat value (c):

m × c ×ΔF

W= -------------------

3.412 × T

The mass and specific heat of some materials may be found at www.hotwatt.com/table1.htm for metallic solids, www.hotwatt.com/table2.htm for solids other than metals, and www.hotwatt.com/table3.htm for certain liquids and gases.

Basics of OEM and Industrial Electric Heating Elements - Part 1

Electric heating elements
for OEM and Industry
(courtesy of Hotwatt)

The simplest definition of an electric heater is any device that changes electrical energy into heat energy. But from that simple explanation, electric heaters explode into a myriad of types, sizes, applications, and designs depending upon what’s being heated, the degree of heating needed, and the method by which the heat is applied.

The measure of electrical energy is called the Joule after its discoverer, James Prescott Joule. Through numerous experiments, Joule determined that the quantity

(Q) of heat transferred from electrical energy is proportional to the square of the current (I² ) multiplied by the resistance (R) for the period of time (t) through which it passes:

Q ∝ I² × R × t

However, one seldom sees a reference to Joules used in modern electric circuits. Instead, the controlling factor becomes that of power (P):

P = I² × R

You’ll note the only difference between the formula for determining power and that of determining Joules is the time component. The time factor in heating becomes readily apparent in any device that gets hot when an electric current flows through it: its temperature rises as time passes.

Semiconductor Gas & Pump
Line Heaters (courtesy of Durex)

Low vapor pressure gas delivery lines must be held at elevated temperatures (higher than the gas vaporization point) in order to prevent condensation that adversely effects process yields. Similarly, sublimation (transition of a substance directly from the solid to the gas phase) occurs when the vapor phase materials are allowed to cool in a vacuum line. The most common types of sublimation in the semiconductor process is of ammonium chloride (AlCl2) and nitrides (NH4) (NH4Cl).

The more common semiconductor processing applications requiring unique thermal solutions for condensate and sublimation prevention are PECVD, LPCVD, MOCVD, ALD, plasma etch and other vacuum applications.

Keeping temperatures elevated along the vacuum lines, and in the vacuum pump, assures gas temperature above the vapor condensation point, thus keeping condensate at bay. Heating the vacuum lines, vacuum pumps, and forelines, also substantially reduces sublimation in these areas.

By controlling condensate and sublimation, the need for frequent preventive maintenance is dramatically reduced and subsequently, the costs. Additionally, the life of associated valves and vacuum pumps is increased as well.

The ideal heater should be self contained, be easy to install and remove, fit tightly on the lines and pumps, and provide optimum heat transfer. It should be powered by readily available voltages (120, 240), have built-in fasteners, and provide over-temperature limit control. It should provide process temperatures up to 200°C and have the ability to distribute wattage along the length of the process line to compensate for colder line sections. The backside of the heater should include thermal insulation that can withstand the operating temperatures, while still providing good thermal insulation.

For more information on semiconductor line, pump and valve heating contact Belilove Company-Engineers at (510) 274-1990 or at sales@belilove.com.

2-Wire and 4-Wire Transmitters and Control Loops

Digital Meters

Industrial process control transmitters commonly provide analog signals, such as 4-20 milliamps (mA), 1-5 volts DC, and 10-50 milliamps as outputs which can be scaled to the control range of the process variable they are sensing. Industrial transmitters are either “4-wire” or “2-wire” which defines how the transmitter gets its supply or “excitation” voltage.

4-wire transmitters have four wires exiting the device. Two wires are for the supply power of the device, typically 120 volts AC, 240 volts AC or and external 24 volts DC supply. The other two wires provide the analog output signal provided by the transmitter circuitry.

2-wire, loop power example

2-wire transmitters have only two wires exiting the device and rely upon the control “loop” for the excitation voltage - typically 24 volts DC which normally comes from the loop controller, PLC or DCS.

4-wire devices are also classified as “active” (supplying power) devices, while 2-wire devices are classified as “passive” (loop powered) devices.

For example, a digital meter (active) may provide loop power to a pressure transmitter. The pressure transmitter regulates the current on the loop to send the signal back to the digital meter, but since the transmitter does not provide power to the control loop, it is deemed passive. In another example, a passive (loop powered) digital meter and a passive pressure transmitter may be used in the same loop, but uses a 24 V battery as the active device to power the loop.

Industrial Electric Immersion Heaters

Screw plug and flanged
immersion heaters
(courtesy of Durex)

Electric immersion heaters are used in a myriad of industrial applications. From drying industrial gasses, to freeze protecting cooling tower sumps, to heating acids in plating applications, the versatility of electric heating element can save time, energy and space.

Industrial immersion heaters are used to directly heat a standing or moving fluid by using electric heating elements. There are three primary types of industrial electric immersion heaters; screw-plug heaters, flanged immersion heater, and over-the-side heaters.

At the heart of industrial immersion heaters are the individual heating elements, normally constructed from a stainless steel or Inconel tube containing a magnesium oxide filler and a nichrome resistance wire. Current is applied to the wire which produces the heat, while the compacted magnesium oxide powder provides the electrical insulation, and the metallic tube provides the physical protection.