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July, 2017

A Road Map to Successful Thermal Testing

By David Brehmer, Technical Writer, Exatron W

hether testing one device or millions, the ultimate goals of any thermal testing opera- tion are efficiency, accuracy, and stability.

Thermal forcing, clamping, and calibration methods are all variables, but the one common denominator in all test scenarios is the socket. Exatron’s new Copperhead® thermal socket and wide range ther- mal head (WRTH) are designed to bypass the limita- tions of low-cost low-budget solutions. Most budget sockets are composed of plastic

materials, which are thermal insulators. Add to this the fact that the spring probes act as mini

The most common conduction method involves a Peltier-style thermal element driving temperature through an aluminum or copper block, which is placed directly onto the DUT. Direct contact greatly reduces many of the

frost and accuracy issues of forced air methods, but, because the thermal mass used is often too small, requires a great deal of power and lacks temperature range, especially at extreme cold. Combined with the insulating properties of

plastic sockets, these drawbacks lead to significant temperature loss, long soak times, and overall inef- ficient, unstable testing. Also, when these hand test issues are extrapolated to small-batch automation, then to engineering-level, and then to high-volume handling systems, the resultant string of inaccurate tests, repeated trial and error, and complicated, expensive changeover kits required to compensate for socket and test head flaws — all this can accu- mulate to render thermal testing an expensive and time-wasting proposition. The ideal solution would be a low-power ther-

mal head with a large thermal mass making direct contact with a thermally conductive, nearly kit- less test socket. This is how the company designed its new WRTH and Copperhead thermal socket. The new system is specifically built for thermal testing with efficiency, accuracy and stability.

Copperhead thermal test socket.

heatsinks and the result is temperature loss dur- ing testing, inability to achieve the hottest and coldest temperatures, and long soak times between each insertion. One way to combat these issues is through

the convection thermal forcing method of blowing hot or cold air onto the test socket. Convection is often chosen because it requires minimal docking and test board hardware; and, because it does not involve direct contact with the DUT, results in lit- tle to no physical stress on the test board. However, this method requires a great deal of

power in order to drive temperature and, to lessen thermal loss, requires an airtight foam seal, which can be difficult and messy to achieve. Convection testing is also often less accurate, creating an uneven and unstable temperature profile by heat- ing or cooling the entire test board, rather than only the test site. This can result in significant frosting during

cold test, generally increased thermal strain on the test board, difficulty in measuring temperatures accurately, and, because of the forced air running over the DUT board, potentially dangerous ESD concerns. In many ways conduction, the transfer of heat

through the direct contact of two materials of dif- fering temperatures, is generally the more efficient choice when used in thermal testing applications.

Reliable Vehicles The WRTH and Copperhead thermal sockets

are the perfect vehicles for the thermal testing journey because not only are they proven to increase temperature test range and stability, but the same exact hardware is also easily adaptable to every thermal test scenario from benchtop hand test to high-volume automated handlers. The Copperhead thermal socket is named for

its signature “socket topper,” a large copper ther- mal mass which, like the snake from which it derives its name, is an excellent retainer of heat and cold. By replacing thermally insulating plastic with thermally conductive copper, the Copperhead reduces soak times in between insertions drasti- cally, while also increasing temperature stability. Traditional thermal testing with plastic sock-

ets requires a great deal of power and time to over- come the constant draining of temperature. To achieve a test temperature of –55°C (–67°F) one may have to push the chiller to –80°C (–112°F). In order to achieve a test temperature of +155°C (311°F), one may have to push the heating element to +166°C (331°F). This is wasted time and energy. The Copperhead socket cuts those figures by more than half.

Another benefit of the Copperhead’s thermal-

ly conductive construction is temperature reten- tion between insertions. In forced air test setups, the socket must ramp down to near room tempera-

Frosted board after convection testing.

punched, with no thermal head changeover required and only a quick change of device guide for the Copperhead socket to match device size. However, due to the socket’s versatile design, the same device guide may be used for sawn or punched devices of the same size even if device thickness varies. The Copperhead socket depth is the same no

matter the device size; and the same WRTH tool- ing is used for a huge range of device sizes with no

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ture before loading or unloading. Testing is then delayed while the socket ramps back up to test temperature. With the Copperhead socket in a con- duction test scenario, devices can be removed while the socket is at temperature, thus nearly eliminating soak time before the next test. The socket becomes even more efficient when

paired with Exatron’s WRTH. The WRTH requires no fluid and only a few amps of 120/240 VAC to achieve a working temperature range of –75 to +175°C (–103 to +347°F) and a test range of –55 to +155°C (–67 to +311°F). Its solid copper construc- tion allows extremely fast soak times and excellent stability.

Expertise = Efficiency Of course, efficiency is not gained only during

testing. In traditional thermal testing setups, much time and energy is wasted dealing with com- plicated changeover kits, mechanical alignment, and the inability to adapt smoothly from hand test to automated systems. The Copperhead thermal socket and WRTH eliminate much of that hassle. The Copperhead’s topper is universal, mean-

ing the same exact piece may be used for a wide range of device sizes and types, and need not be purchased for each device size. Similarly, Exatron uses one type of WRTH thermal pusher tooling for a large range of sizes. For example, the same push- er can be used for 2 x 2 to 10 x 10 QFNs, sawn or

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