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Feb 13th, 2014
 
Scalable test solution for MEMS microphones
 
The MEMS Microphone market has demonstrated significant growth rates over a long period of time. Volumes are constantly rising and the main differentiators are price and quality. Balancing these two key factors requires scalable and flexible test equipment, that enables frequent product launches and low volume production as well as high volume production with an outstanding Cost Of Test (COT) model.
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Package trends for MEMS microphones show a certain steadiness, but the trend to smaller devices can be observed as well in this area.

The scalability of MEMS test equipment is ruled by several key factors. Throughput is widely considered to be the main factor. The test equipment must be capable of running high volume production but must be able to run lower volumes at a reasonable cost of investment.  Modularity is a good approach to this. Having a base system that can be scaled up by just multiplying stimuli or resources can solve this issue.


Multitest InPhone High-parallel modular MEMS microphone test

But true modularity means even more.  A convertible design of the test module makes sure that more than one device type can be tested in the unit. This supports package type changes and helps to run multiple package types on one test system. A fast conversion kit exchange and an easily adjustable, recipe driven change over procedure is mandatory to accommodate a seamless exchange between handling different package types.

Achieving high throughput rates is not only a matter of the so called sprint rate. Overall equipment efficiency has a high impact on throughput and is strongly dependent on the type of handling systems employed. As device sizes become smaller and smaller, traditional handling principles such as gravity and pick and place find their limits in the range below 2x2mm devices. This limits are based on physical effects like electrostatic charge and adhesion, that become considerable when the weight and size of the devices become smaller and lighter. This causes a higher jam rate and reduces Overall Equipment Efficiency (OEE.)

Physical stress on the package itself caused by multiple picking or sliding actions can damage the device. This is extremely true for WLCSP devices. This may even make it impossible to use either of the traditional handling type systems.

Multitest’s InPhone test system for MEMS microphones offers a parallelism of by 16, scalable up to by 144, utilizing the same base handling system, just adding resources to the system. The change over time to use a conversion kit for different package types is less than one hour.

Strip test is a well-known process that can overcome most of the disadvantages of pick-and-place or gravity handlers. It provides the advantage of high parallel test and a unique way of handling even the smallest packages. One of the main concerns for MEMS devices is the required singulation of the devices after final test. Introducing stress into the device is virtually unavoidable and may influence the trimming of the tested devices.

Test in carriers is a new developed test process that is combining the advantages of singulated test and strip test. The InCarrier™ - a strip like carrier for singulated devices - mechanically clamps and registers the singulated devices reliably. This way even the smallest devices with pitches down to 0.35 mm can be handled robustly with an ‘unlimited’ test parallelism for MEMS test over all temperatures. For calibration and test binning, the test results are mapped in an electronic map file via the carrier ID. After all tests are complete the devices get sorted according to the map file without any singulation risks inherent to strip test. Also unlike strip test, the InCarrier™ is supports retest with one and the same setup.


Test in Carriers – Multitest InCarrier™ enabling highest parallel test for singulated MEMS packages.

The modular approach of the test system should not only focus on conversion kits, but on the test modules as well. The handling process usually remains the same for many different package types. The stimulus itself, however, is due to frequent changes with respect to new technical requirements in terms of new stimulus types, better stimulus quality, new material types and so on. At this point it makes sense to make the MEMS stimulus a separate module that can be exchanged. In this way, the handling module can be used for a long time period and the MEMS stimulus can be exchanged or easily modified. This guarantees the best return of investment of the handling system. An example for this is the Multitest InPhone MEMS module. A new module has been developed to support a 65db SNR. Development was done independent of field installations. New modules can easily be exchanged with systems in the field, thus achieving the shortest possible transition time from one technical generation to the next.
 

 
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