Our medical customer uses accelerometer MMA8452QT. The product successfully passes initial functional test. Upon ultrasonic welding and re-test, we experience 50% fallout.

[arunthangavelu]

Are there any guidelines or application notes relating to ultrasonic welding and this device? Thanks.

[Martin35804 · Accepted solution]

Hi,

Ultrasonic welding is difficult to recommend for any vibration sensitive measurement device like accelerometer is. The ultrasonic frequency can easily reach the g-Cell resonance frequency considering the mass of the sensor itself. Additionally the internal wire bonds could be damaged as the g-Cell is on a soft die attach and the ultrasonic's could cause the g-Cell to vibrate weakening the wire bonds.

There has been issues at several customers in past with such type of failures.

Rather consider another technology like over-molding in case of the assembly. Unfortunately there are no recommendation regarding this technology since it has impact on the whole customers assembly with the sensor. A determination must be made if this impact is acceptable for the application. Customer would need to run their own studies and risk assessment and make their own determination.

I would recommend to consider finite element analysis of the assembly to calculate eigen-frequencies and to avoid these frequencies with the ultrasonic welding if this technology is preferred and try different frequencies with shorter time where the impact on the MEMS structure could be minimal but still with certain percentage of fallout.

In general customers are urged to conduct their own in-house evaluations and characterizations to determine if Freescale devices are suitable for production process and application.

For the accelerometer itself the recommended soldering method is reflow. I would refer to the application note below for more information.

AN3111: Soldering the QFN Stacked Die Sensors to PC Board

   http://www.freescale.com/files/sensors/doc/app_note/AN3111.pdf

[johnpritch11]

Greetings,

Thank you for your reply, I have had a similar question. Our application is for a wrist-worn wearable device with motion sensing capability, potentially using ultrasonic welding in the final assembly process.

As a follow-up, could you provide insight as to the impact of ultrasonic welding on the lifetime of the sensor? For example, if we are able to design an ultrasonic welding process that leads to good yield, what are the chances of latent failures due to the initial stress during assembly in a typically low-g application?

Thanks