How a novel material solved an electronics assembly dilemma deep in the Indian Ocean.

In the competitive oil and gas drilling industry, a project's success – and a company's reputation – can hinge on seemingly insignificant variables such as a few degrees of temperature on a circuit board. That was quite literally the issue facing a major oil-exploration company as it performed data-collection tests at a client's wells sunk deep in the Indian Ocean.

Traditional, commercially available circuitry used in deep-sea wells can handle temperature spikes of up to 350°F and pressures of 20,000 psi or more, perfectly adequate for 90% of deep-sea wells worldwide. However, explains project manager Tony Jones, "as oil gets harder to find and the technology improves, the economics of looking for crude becomes more viable in increasingly hostile environments."

For the Indian Ocean project, Jones and his team were working with temperatures up to 400°F in a geothermal "hot spot" – where magma, steam, and other geological phenomena contribute to extremely high temperatures. When Jones' team introduced a new testing service in the area, they feared standardized components wouldn't be able to take the heat.

With such high temperatures, the components would run at or exceed their limits for extended periods of time. During the 12- to 24-hr. tests, the equipment would be gradually subjected to increasing temperatures and would need to perform at sustained temperatures of 350°F or higher for four to 12 hrs.

"The reliability of the components is dependent on time and temperature," Jones explains. "The longer you stay at higher temperatures, the more the life of the electronics deteriorates. And as the life of the electronics deteriorates, reliability deteriorates as well."

Under typical deep-sea conditions, a component is used until it reaches the end of its expected life, and then is replaced. If a component fails during testing, the test apparatus must be removed from the well and the component replaced before testing can resume, at a possible cost of tens of thousands of dollars per day in lost production.

"This industry requires less than 5% lost time on an operation," Jones imparts. "The industry standard is 95% efficiency or better. So if you can't maintain a high level of efficiency, you might not get chosen to do the work."

Many projects dealing with temperatures exceeding 350°F would call for custom components or components sealed in Dewar flasks, a kind of protective sleeve. But Jones was happy with the functionality of the components on hand – provided they could find a way to shield the circuitry from the worst temperature spikes.

Because of the depths at which the components would be used, they had to be enclosed in a special housing to protect them from the pressure. But the sealed housing acted like an oven, baking the circuits inside. The pressure housing precluded cooling apparatus, such as fans.

An overheated component can stop working for a while and then resume working when temperatures cool – or it can die altogether. To avoid both scenarios meant finding a way to keep the components safely under the 350°F threshold.

Avoiding a Redesign

Without a suitable solution, the team would be faced with redesigning the chassis to better dissipate the heat. They would also have to redesign the circuit boards to convey the heat to the chassis instead of to the atmospheric cavity or the external pressure housing. The in-depth redesigns would have added time and expense that neither Jones nor the client wanted.

Another alternative Jones considered was for his team to pot or encapsulate the components using commercially available materials. "While that solution might have worked," Jones indicates, "it's a time-consuming, labor-intensive operation that requires a lot of processes to ensure consistency."

Jones began searching for possible solutions, eventually contacting Dow, which brought in Ultimate Solutions (ultimatesolutions-inc.com). Ultimate Solutions makes a patent-pending technology called preforms, a highly filled silicone material that protects electronics components from shock, vibration and temperature extremes.

The 3-D preforms wrap around circuits like a glove. The high thermal conductivity of the silicone shunts heat away from components. It also insulates components from shock and vibration.

Jones' team wanted reassurance that the initial expense to prototype the preforms would be worthwhile. "You could pay a company to develop the parts for you," he suggests, "but in the end, you might throw them away." He wanted some assurance that they would conduct heat well enough to make a difference in a harsh environment.

To get a better idea of the preforms' effectiveness, Jones' team set up a computer model comparing the heat buildup within the component chassis both with and without the material.

The team entered the thermal coefficients of aluminum and the other materials used in the components, as well as that of the air between the components and the housing, to generate a model of the heat buildup in the untreated components (Figure 1).

Next, Jones changed the model slightly to include the thermal coefficient of the highly filled silicone, rather than air surrounding the components (Figure 2). The model indicated that the preforms would have a significant cooling effect on the components.

Reassured that testing the preforms would be worth the effort, Jones' team sat down with Ultimate Solutions to discuss their design requirements. Typically, a customer provides a circuit board and the chassis to house it, and Ultimate Solutions creates the finished preforms for an exact fit.

This case had a wrinkle: Because end-of-life components would need to be replaced, the preforms had to permit easy separation and removal of the components from the pressure housing. The preforms also needed to connect to the housing to provide a path to dissipate heat. This was accomplished by manipulating the preform geometry to ensure a snug fit, while allowing clearance around the vertical sides of each component to accommodate thermal expansion of the silicone (Figure 3).

Several rounds of lab tests were then conducted to demonstrate the cooling effects of the preforms. First, to get an idea of which circuits on the components were generating the most heat, a heat-sensitive camera was used to generate a thermal image of each of the components while they were running at room temperature (Figure 4).

Next, the team attached heat sensors at those hot spots and at numerous other points along the preformed components (Figure 5). The placement of the sensors was repeated on another set of components without preforms, which was used as a control group. The components were fully enclosed in a sealed housing and placed in an oven at 350°F for 12 hrs.

Without the preforms, surface temperatures during the test hovered between 375° and 430°F. The components inside the preforms stayed, on average, 50°F cooler on all parts monitored in the assembly (Figure 6).

Next, the components were field-tested in several locations over the course of three months. Again, thermal sensors were attached at various points on the components, and the resulting temperature profile (Figure 7) was compared to historical data recorded before the preforms were added. The data indicated that the temperature of the components without preforms averaged 77°F hotter than the temperature of the pressure housing. With the preforms, the temperature of the components averaged 50°F hotter.

Although a 27°F temperature reduction may not seem significant, it's enough to keep electronics below a critical threshold. As Jones notes, "When you consider that the component is rated for 365°F, a decrease in temperature from 392°F down to 365°F makes a big difference in lifespan and performance."

And although the team didn't specifically test for shock and vibration effects, Jones felt that the preforms' vibration-damping qualities helped to protect the components during transport and deployment.

What began as a minimal implementation to solve a specific problem ended up being deployed in all the company's assemblies, regardless of operating temperature. The preforms provided a drop-in solution that eliminated the need to modify, pot or encapsulate components. "It's plug-and-play," Jones remarks. "This gives us the best possible performance."

 

Tony Jones is a pseudonym. Because of competitive concerns, we've agreed not to identify Jones or his company by name.

Cheryl Ross is a technical writer at Market Dimensions, and wrote this on behalf of a major OEM.

 

Sidebar: An Alternative to Potting and Encapsulation

Heat and vibration are big enemies of electronics. Even specially designed electronics can melt when exposed to extremely high temperatures. And it often doesn't take much energy in the form of shock or external vibration to damage sensitive electronics.

Traditionally, potting and encapsulation have been used to protect electronics in harsh environments. These techniques involve placing the component in a potting vessel, pouring a potting compound over it, and then curing the potting compound for a period that can range anywhere from one to 48 hrs. This procedure produces a component that's permanently encased in the potting medium.

Potting and encapsulation are useful in some applications, but they have several shortcomings that make them unsuitable for many other uses, and they can significantly increase the total cost of a project. Preforms address a number of these problems.

Manufacturing cost. Potting and encapsulation are labor-intensive procedures that can significantly increase the time and expense of manufacturing. Preforms take just a few seconds to apply during the electronics assembly process, greatly reducing the time and cost involved.

Product integrity. With potting and encapsulation, impurities on the electronics can prevent the potting compound from curing properly. While the resulting device may look fine from the outside, the interior may retain a pasty consistency. This compromised physical structure diminishes the potting compound's ability to protect the electronics, and performance can suffer. Preforms permit physical inspection of the protection in just a few seconds, as it is applied during assembly.

Thermal expansion. Many potting or encapsulation materials expand at high temperatures, so they can actually damage the electronics they were intended to protect. Preforms protect fragile components by surrounding them with minimal clearances that compensate for thermal expansion.

Access to protected electronics. Once an electronics device is potted or encapsulated, regaining access to calibrate or repair the device can be expensive and time-consuming. Re-entry into the potting or encapsulating materials often comes with the risk of damage to the components inside, and the process to reapply this type of protection is labor-intensive and costly because it must be started over from the beginning. Preforms can be removed easily, so parts can be serviced, calibrated or replaced. This can be accomplished in a fraction of the time and with less risk to the devices inside. And preforms can usually be re-applied and reused.

Preforms can be used with a wide range of hardware, from circuit boards to sensors, detectors, and battery packs, to name a few. PCB protection includes 2-D and 3-D geometries. Often, problems arise when using 2-D die-cut flat sheets to protect high value electronics. If the application calls for a nonstandard thickness, or if clearance is required to accommodate fragile chips and components, preforms may be the solution.

3-D PCB configurations are handled by encasing the PCBs in single-molded or co-molded preforms. Single-molded preforms (Figure 8) are also useful for protecting PCB assemblies intended for environments with high vibration, or for regaining access to the electronics. Co-molded preforms are for electronics assemblies operating inside chasses, hatches and other restricted spaces. Co-molded versions (Figure 9) feature a hard, thin outer shell molded to the soft, inner material that surrounds the electronics. The shell promotes rigidity, durability and easy insertion. – Lauren Groth, Ultimate Preforms, and Sharon Brass, Market Dimensions

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