4 Reliability Recommendations

To maximize device reliability, TI recommends placing a drop of epoxy over the top-side of the device. The drop needs to be large enough to cover the device and make contact with the FPCB on all sides of the device. The amount of epoxy applied needs to be controlled to make sure that the epoxy does not flow near the device sensing area on the bottom of the device. A pneumatic epoxy dispenser enables control over the amount of epoxy deposited and makes sure of uniformity across units. This drop is meant to both protect the device from direct contact from outside objects and increase the board level reliability by providing additional stability between the device and the flex PCB.

Figure 4-1 Epoxy Application Example Side View

Epoxy needs to be rated above the maximum temperature range expected by the application. The viscosity of the epoxy needs to be chosen to allow the epoxy to flow to contact with the solder joints between the device and flex-PCB. However, the epoxy needs to not flow to obstruct or intrude into the field of view of the sensing area. Figure 4-2 illustrates an example of proper epoxy application as seen through the flex-PCB cutout.

Figure 4-2 Epoxy Application Example Bottom View

Epoxy application guidelines are similar for different cutout shapes (plus sign, circular, rectangular, and so on.). Care needs to be taken that the epoxy does not flow through the cutout to the other side of the FPCB or sensing area. For example, in the plus cutout image gaps can be seen between the device and flex that enables very large field of view. Make sure the epoxy does not flow through this gap into the field of view of the sensor.