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nanotimes Companies

10-10/11 :: October/November 2010

Sensors and Actuators // A New Method for Measuring Pressure Distribution during Encapsulation

by Dr. Thomas Schreir-Alt, Fraunhofer IZM and Jeffrey G. Stark, Sensor Products Inc. (Reprint)

T

he integration of sensors and actuators into encapsulated multi-chip modules creates for-

midable challenges to characterizing and controlling process pressure. However, a new measurement method developed by Fraunhofer IZM shows that pressure-indicating sensor film such as Pressurex® can fully characterize the area pressure distribution on the cavity and on substrate surfaces within a mold cavity during encapsulation.

Thermo-mechanical stress from non-uniform encap- sulation pressures on flip chips, quad flat no leads, sensors, and thin substrates can lead to hidden damage or to long-term reliability problems. Until now, pressure differences during molding were as- sumed to be negligible, despite the presence of filler particles in the epoxy molding compound (EMC). The Fraunhofer results show that substantial pressure differences can occur, with potential damage to com- ponents and substrates.

Conventional pressure sensors measure pressure at only a single point, not simultaneously across the entire area. They also require modifying the metal

molding tool to bring out their data without short- circuits. Present sensors also leave markings on the device surface that can later interfere with pick-and- place assembly, or with laser labeling.

Commercially available Pressurex® sensor film placed

within the mold cavity produces a quick, permanent image of pressure distribution across the area during the flow of the EMC. The film requires no external connections. It neither marks nor mars the device surface. When placed between contacting surfaces the film changes color in direct proportion to the actual pressure applied.

The Fraunhofer method creates a multilayer stack consisting of the Pressurex®

sensor film, an adhesive

layer, and a protective p olymer layer. The protective polymer mechanically restrains the film, preventing movement, swelling, or degassing during measure- ments. This restraint allows several minutes of testing at 180° C (356° Fahrenheit), the maximum anticipa- ted temperature during the exothermic cure of the EMC.

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