Coating Adhesion Testing

Knowles Corp. has been producing SiSonic Surface Mount MEMS microphones since 2002 and is in its fourth generation of development. Over 10 billion of these units have been shipped to date. These microphone assemblies contain a MEMS device and

an ASIC mounted to a PCB. They all use

a metal can to protect the electronics, act as an RF shield and

establish either the front volume (with a port hole in the can) or the back volume (with a port hole in the PCB under the MEMS device) for the

microphone. These cans are generally

bathtub shaped. During the first level

assembly process, the cans are soldered down to the PCB and laser marked. The assembled units are then diced, tested and placed into tape and

reel in preparation for the second level assembly.

Recently, several customers have

requested improved RF performance, due

to the application environment of the

microphones. One customer has required a coating on the inside of the can to achieve this decrease in the radiated RF of the microphone. This coating may be applied using several different methods: direct injection molding, offline molding and gluing into the can or spray coating. The coating can be one of several different materials.

Part of the requirement for this new can with coating configuration is to

quantify the level of adhesion between

the coating and the can. There are a

few standard methods currently used to determine the level of adhesion

between a film or coating and a

substrate: peel testing, shear testing

and tensile pull testing. All of these

methodologies work well for planar

geometries, but are not well suited

for variable geometries like a can.

Drop testing is also used to check adhesion, but it does not quantify the

level of adhesion between two materials. A new test must be developed to support the coating

technology needed for this process

improvement.

This paper will describe the materials, tooling, the process of sample preparation and the test procedures that were developed to

support the adhesion test. Several different materials and several

different application methods for this

material were tested. This test procedure is currently in production.

The advantages to this procedure are

that the test jigs can be reused and the materials are readily available.

The test jigs can be modified for application to different geometries,

different coating materials and

different coating methods.