Can Inverse Electrostatic Charge damage Semiconductor?

[Mechatrommer]

a riddle crossed my mind. we know electrostatic charge can damage semiconductor device and chips. it happen when our body build up enough ES voltage up to KV level and then touch one of the chip's sensitive pin. but what happen if we are securely grounded, but the floating chip/circuit build up ES charges, should it damage the chip as well right? its just the charges flowing the other direction?

[PeterG]

The way i see it, ESD is bad in either direction.

Regards

[ejeffrey]

Well, the reason that a human body is effective at causing ESD damage is that it forms a large capacitance to ground.   An IC package is much smaller and has very little capacitance to ground, so it cannot store a large amount of charge itself.  On the other hand, if the IC is connected to a much large conductor with a large self capacitance, then you could indeed damage it with 'reverse' ESD.

Your more general point, however, is correct.  It doesn't matter which direction the current flows.

[Fraser]

One word.....equipotential

It is important that the potential difference between a PCB/equipment and the human/repair equipment is minimised.

There would be nothing wrong with a human with 1000V positive potential working on a PCB provided that is at +1000V also.
Not a practice I would recommend but it's all about potential difference.

An ESD workstation has desk and floor mats at the same potential as the humans wrist strap plus the soldering equipment is also bonded to the ESD mats. The PCB potential on the desk mat should therefor equal that of the human and soldering iron etc.

P.D. is the true killer of semiconductors

[A Hellene]

An electric charge, no matter of what polarity it is, is an amount of energy waiting for a return path to return its excess or lack of electrons, in order to be discharged (to become electrically neutral).

The amount of energy stored in an isolated mass medium (that forms a capacitor in respect to the ground, due to its own mass) is given by the formula E=1/2(C*U^2), where the C is the electrical capacitance of the charged object and U is the static potential in volts. The HBM (human-body model) is defined as a 100pF capacitor in series to a 1.5Kohm resistance charged to 3KV; but a human body can easily reach a capacitance amount of 500pF charged to 50KV or more. On the other hand, the capacitance of an IC chip alone is in the low end of the sub-pF range, so the total capacitance of the circuit formed by the two in-series capacitors (the human body and the chip) becomes insignificant and the energy transferd is minimal; but if the little chip is grounded (placed on a grounded bench, for example), it will receive the full amount of the human static discharge energy.

This is where the antistatic wrist straps (with an in-series discharge resistor of 1Mohm, typically) enter the picture, in order to protect the ESD sensitive devices from violent electrical discharges by discharging first the human capacitive energy to the system ground before the human body closes any circuit to it through the ESD sensitive chip.


-George

[Mechatrommer]

ok its possible. now assuming the circuit have enough capacitance and built up ES charges, how do we bring it down safely to ground level? by carefully placing it on ESD mat? because we cannot touch the circuit with our ESD wrist strap on, we are already at ground, and the circuit is at HV static, touching it means inverse ES discharge and will damage the circuit. what feature or mechanism of ESD mat that can do this job safely?

[A Hellene]

In a few words, do not let the ESD sensitive device be in the electrical path of your discharge.

For example, by firstly touching the person that holds the device in his hands, in order to bring him down (or up) to your potential before he hands the device to you. If the device is sitting somewhere alone, touch the least resistive object in its neighbour to discharge yourself (in respect to the potential of the specific place) before touching the device's ground (or grounded parts/pads) to equalise both your potentials. The same applies when reaching to your bench while holding something ESD sensitive in your hands: First touch your desk mat or any ground line available at the bench to ground yourself in respect to the bench potential before placing the PCB/device onto it.


-George