First Published in EOS/ESD Technology Feb/March 1988

Can Charged Boards Cause
IC Failure?

Although many authorities waver, the author answers yes
with a theoretical proof.

Donald Pierce
Associate, Booz-Allen & Hamilton
Alburqueque, NM

The threshold for a charged board failure is a lot lower than for a human body model failure, and the following article explains why.

My logic is based on the following assumptions:

The IC's failure energy (E_F) is independent of the pulse width, given adiabatic conditions.
An IC fails like a resistor.
All of the energy that is on a charged board will be available to cause a failed IC.

Figure 1: We assume that the IC ib the charged board acts like a 10 W resistor.

 

First of all, to estimate the energy needed to fail an IC assume the following: the human body model (HBM) threshold is 4000 V, and the IC acts like a 10 W resistor (Fig 1). Doing the following calculations the amount of stored energy can be estimated.

By calculating the amount of energy that can be delivered to the IC, one gets a number that corresponds with experimental data.

Most IC failures are on the order of microjoules (mJ).

Figure 2: How much energy is stored on a charged board?

Now, turning our attention to the charged board model (Fig 2) and noting again that all of the energy on the board will be delivered to the IC, one can estimate how much energy must be on the board to fail the device.

We know E_F to be 5m. With the following calculation one can find the capacitance of the board. (C_board).

Figure 3: When a metal board's area is 400 cm2 and the distance between the planes is 4 mm, the capacitance is 400 pF.

 

 

 

If one assumes that the board (Fig 3) is all metal then,

 

 

 

 

so,

(Note that the board failure threshold becomes lower as the distance (d) gets smaller, or the area (A) gets larger).

By inserting some sample numbers into the equation, one can estimate the capacitance for a given board:
Assume the board is 20 x 20 cm.

Secondly, assume that the board is 4 mm over ground,

The board's capacitance is,

These calculations for the board's capacitance are in line with values observed by R. N. Shaw and R. D. Enoch in their article, "An Experimental Investigation of ESD Induced Damage to Integrated Circuits on Printed Circuit Boards." (Proceeding of 1985 EOS/ESD Symposium).

Thus, the threshold voltage is approximately 300 V, which is much lower than a human body model threshold!