Components can be easily damaged with ESD. Many things can touch and discharge into the component as it makes its way to its final destination, including people and the packaging they come in. These models along with some tests were created to protect the components from the harsh environment and determined how sensitive a component is.
CDM: one of the most neglected models. This ESD event occurs when a device slides down a tube, bag or whatever is holding the component and causes a rapid discharge. This also can occur when connecting the device to a ground. This ESD event is totally part-dependent. A common discharge network consists of a 4 or 30pF capacitor.
Classes of CDM:
Test procedures involve placing device leads up on a field plate and then charging and discharging device. The image below shows the process.
MM: Similar to the HMB is the Machine Model. Considered as a worst case HBM. The prime source of damage for the MM comes from charged machine or device. An example of this can be found in the pins of an automatic tester. A common discharge network consists of 200pF capacitor, no resistor, and discharging through a 500nH series inductor.
Classes of MM:
Testing for MM model uses this circuit:
HBM: the oldest and most common form of ESD. As stated in Mil-Std 1686 section 220.127.116.11, the principal source of ESD damage is the human body, as modeled by the HBM standards. Although this is called the human body model, it really represents the discharge from the human body or any charged device into an ESD sensitive (ESDS) device. This is commonly represented as the discharge from a fingertip to the device. Companies combat this by wearing ground straps, wrist straps, and special mats that parts can be placed on that discharge the static.
Classes of HBM:
For testing there is a human body model circuit. The device is placed in the circuit and the part is failed if it does not meet the datasheet parameters using parametric and functional testing.
The MIL-STD 1686 manual