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04/05/2005
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Surface mount components are providing pressure sensor manufacturers with the ability to pack greater processing power while achieving a reduction in sensor size, writes Dean Palmer
Where pressure sensors are to be used in hazardous areas or have a T4 rating requirement, electronic components should be encapsulated. A problem with using surface mount components is the sensitivity vulnerability characteristic of specific surface mount metal film resistors.
Leicester-based pressure sensor manufacturer Druck had this very problem on its range of pressure sensors when it recently made the transitional change of electronic components. The company noticed that output drifts were occurring with time over temperature, which led to a higher than acceptable failure rate of the sensors.
After investigating its manufacturing process, engineers at Druck discovered that the catalyst within the existing encapsulant (used to seal the PCBs) was the cause of these device output drifts.
So the company conducted tests on a wide range of materials to identify the most effective one for the potting application and chose a room temperature vulcanising silicone, RTV 12, manufactured by GE Bayer Silicones but supplied by Warwickshire-based synthetic polymer specialist Techsil UK. This material was deemed to be superior in performance to all others that were tested.
Andy Briers, sales director at Techsil, told Eureka: "Analysis after potting showed an output drift factor of between 10 and 100 times better than all the other materials tested. The addition of a trace element to the encapsulant allowed for the sensor assembly to be accurately and easily visually checked. And, the oven cure capabilities of RTV 12 gave Druck the opportunity to reduce curing times should its production schedules require."
He said that cost was never an issue for Druck, since solving the drift problem was key. "Traditionally, tin- or platinum-based silicones are used for encapsulating, but these both contain metal salts that cause the output to drift. Platinum is preferred to tin nowadays as it can be heat cured. RTV 12 is unique in that it offers self-bonding at room temperature and can also be heat cured if required."
Briers added that RTV 12, along with Techsil's ability to advise Druck on dispensing methods and process improvements enabled the changeover to be completed "with the minimum of disruption". Also, since its use, Briers said that Druck had "achieved consistently high quality levels" on its ATEX-approved and flameproof sensors.
Another manufacturer, Elite Transducers based in Reading, is using RTV silicones from Techsil in its production process. The firm makes high performance transducers for a variety of sectors including refuse disposal, agricultural, crane safety and offshore. The units invariably have to withstand a wide range of operating conditions.
Elite is using RTV silicones in its production process to ensure that its transducers will effectively withstand moisture and vibration. Techsil's TSE 397 provides two critical functions: it gives a strong mechanical bond sufficient to hold the cable assembly in place prior to welding and it also provides extra protection to the ingress of moisture. Techsil also supplies a two-part gel, RTV 6196, which is poured into the assembly to fully encapsulate the transducer unit.
Techsil provides the materials in dispensers that allow small quantities to be accurately mixed, with minimum wastage and is also working closely with Elite to vary the curing speed to suit operating conditions. Once mixed, the materials are easily applied by hand allowing precise application within the transducer assembly.
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Author
Tom Shelley
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