User guide

30 Document No. 001-64846 Rev. *A Getting Started with CapSense®
3.1.2 Bonding Overlay to PCB
Since the dielectric constant of air is very low, an air gap between the overlay and sensor degrades the performance
of the sensor. To eliminate the air gap, an adhesive is typically used to bond the overlay to the CapSense PCB. The
adhesive must be nonconductive. A transparent acrylic adhesive film from 3M™ called 200MP is qualified for use in
CapSense applications. This special adhesive is dispensed from paper-backed tape rolls (3M™ product numbers
467MP and 468MP).
3.2 ESD Protection
Robust ESD tolerance is a natural byproduct of thoughtful system design. By considering how contact discharge will
occur in your end product, particularly in your user interface, it is possible to withstand an 18 kV discharge event
without incurring any damage to the CapSense controller.
CapSense controller pins can withstand a direct 2-kV event. In most cases, the overlay material provides sufficient
ESD protection for the controller pins. Table 3-1 on page 29 lists the thickness of various overlay materials required
to protect the CapSense sensors from a 12-kV discharge as specified in IEC 61000-4-2. If the overlay material does
not provide sufficient protection, ESD countermeasures should be applied in the following order: Prevent, Redirect,
Clamp.
3.2.1 Preventing ESD Discharge
Preventing the ESD discharge from reaching the CapSense controller is the best countermeasure you can take.
Make certain that all paths on the touch surface have a breakdown voltage greater than any voltage to which the
surface may be exposed. Also, design your system to maintain an appropriate distance between the CapSense
controller and possible sources of ESD. In the example illustrated in Figure 3-1, if L1 and L2 are greater than 10 mm
the system will withstand 12 kV.
Figure 3-1. ESD Path
Air-Filled Space
PCB
CapSense
Non Conductive
Material
ESD Event
Mechanical Structure
Exposed
mounting
hardware
ESD Event
L2
L1
If it is not possible to maintain adequate distance, place a protective layer of a high breakdown voltage material
between the ESD source and CapSense controller. One layer of 5 mil-thick Kapton
®
Table 3-1
tape will withstand 18 kV. Refer
to on page 29 for other material dielectric strengths.