Compound simplifies assembly of compact capacitors

Edited by the Engineeringtalk editorial team May 31, 2002

Designing the tiny capacitor housing for a hearing aid posed a number of challenges for Accumold of Ankeny, Iowa.

Designing the tiny capacitor housing for a hearing aid posed a number of challenges for Accumold of Ankeny, Iowa.

None of the traditional electronics encapsulation materials, however, could meet their specifications.

That's when the company turned to specialty compounder RTP Company, of Winona, MN, who recommended a custom reinforced liquid crystal polymer (LCP) compound which optimised performance and increased assembly efficiency.

Designed to insulate components thermally, electrically and mechanically, electronic encapsulation materials have traditionally included polyesters, cellulosics and thermosets.

However, as Accumold discovered, all of these materials have their limitations.

According to Greg Peterson, Engineering Manager at Accumold, "Our foremost requirement was for the material to flow well and be capable of fully filling wall thicknesses of only 0.0045in (0.1143 mm).

With a part this small - 0.040 x 0.040 x 0.070in (1.02 x 1.02 x 1.78mm) - nearly any measurable change would be major.

Dimensional stability, therefore, was critical".

Additional requirements included the ability to withstand temperatures of 260C encountered when soldering the internal electronic components.

"Also essential", said Peterson, "Was a low coefficient of thermal expansion - this is important in retaining dimensional tolerances throughout the temperature variations".

Working closely with Accumold, RTP Company recommended RTP 3400 Series reinforced liquid crystal polymer (LCP) compound for the capacitor housing.

Accumold manufactured a proprietary Micro-Mold miniature injection press for the application and built a new single cavity tool specifically for use with the specialty compound.

While LCPs are inherently heat resistant, they present a challenge for the designer in an unmodified state.

"For example", Bett Weishalla, Product Development Engineer at RTP Company, said, "In the direction of flow, LCPs exhibit a negative coefficient of thermal expansion shrinking when heated - while transverse to flow, their thermal expansion is positive".

To negate these effects, RTP Company added a custom reinforcement to the compound that increases dimensional stability both out of the mould and throughout the varying temperature cycles.

With a flexural modulus of 8268MPa, and a heat deflection temperature of 260C at 1.82 MPa, the RTP 3400 Series LCP maintains adequate stiffness rise.

In addition, the material can achieve the required V0 flammability rating without the need for additional flame suppressants.

Their high degree of crystallinity gives LCP compounds the ability to withstand the wide range of chemicals encountered during the manufacturing process.

"With the previous capacitor housing", said Peterson, "The ends had to be sealed with epoxy in order to encapsulate the inserted electronic components.

However, this new housing design features a solid moulded end which allows us to reduce labour time by approximately 50%".

An important feature of the RTP 3400 Series LCP is that the compound is bondable.

Explains Peterson, "The parts present a micro porous surface which, coupled with the reinforcement's particulate substrate, provides a substantially increased surface area for the bonding agent to contact".

Peterson praised RTP Company for its ability to, "Bring real engineering knowledge directly to bear on our customer's requirements".

He added that "Response time in delivering crucial technical support as well as materials allowed us to use our expertise to provide increased value to our customer".