Overmolding eases electronics assemblyOvermolding eases electronics assembly

Printed circuit boards and their delicate electronic components often require encapsulation to protect them from harsh environmental conditions and, sometimes, to shield design details from prying eyes. But conventional encapsulation methods sure don't make electronics any easier to assemble. Working with pre-molded housings, seals, gaskets, and potting compounds drives up assembly times and costs. Now AVX Corp. has found a way to drive them back down.

The company's Polytect process overmolds fully populated circuit boards with reaction-injection-molded (RIM) urethanes from Bayer Corp. Suitable for boards from 4 to 150 mm², the process takes only one manufacturing step, lasting between 30 and 120 secs. "Conventional encapsulation methods require a minimum of three assembly steps and three or more parts," says Jennifer Hammond, principle engineer at AVX. She adds that potting compounds can take as much as five hours to cure, while manually joining a typical gasketed housing takes more than five minutes. If the RIM method does have a downside, it's that tooling has to be built. Yet Hammond says these RIM tools take less than two weeks to build and cost less than $10,000. What's more, AVX uses multi-cavity tools to increase productivity. Add up these cost factors, and the tooling can still be justified with production volumes in the low-thousands, Hammond says.

Aside from the productivity edge, RIM has other attributes that make it well suited to electronics encapsulation:

Solder friendly. Unlike thermoplastic injection molding, RIM processing temperatures and pressures don't pose an inherent threat to common solder. According to Hammond, the RIM process that AVX has developed subjects electronic components to a maximum temperature of 150C as the urethanes solidify. "This temperature range is well below the 183C melting point of solder," she notes. And RIM cavity pressures of 10 to 300 psi also come in under solder's 500-psi minimum shear stress, she reports. Contrast these solder-friendly conditions with injection molding. For components of the size and shape that AVX encapsulates, melt temperatures would range from 218 to 382C, and cavity pressure would fall between 1,200 and 2,400 psi, Hammond estimates. "It's clear that solder would fail in an injection molding environment," she says.

Keeping components together. RIM urethanes also have some ability to overcome thermal expansion problems between epoxy-laminate (FR4) boards and the components mounted on them. "Typical boards have a coefficient of thermal expansion of 12-18 ppm/ degrees C, while the components on the board can have very different expansion coefficients," Hammond says. In cases of temperature extremes, this expansion differential can result in solder fracture and failure. The RIM encapsulation, however, may help. AVX has formulated its urethanes so that they adhere to the board and its components. "Because of this adhesion, the overmolding material helps to hold the board and components together and limit the movement of the components."

Made to order. Finally, the ability to custom formulate two-component urethanes has also given AVX some flexibility in tailoring encapsulation materials to different application requirements. The company currently works with about five different materials. They cover an operating temperature range from -40 to 85C, offer tensile strengths from 2,500 to 9,000 psi, and have elongation values from 20 to 300%. Hammond adds that all have low moisture and chemical absorption.

So suitable is RIM for encapsulation that it's the only method that AVX has used for the past two years. "We haven't found any limitations yet," she says, noting that the company has subjected encapsulated circuit boards to extensive shock, vibration, drop, humidity, chemical, and thermal testing.