Core Manufacturing: ESD Flooring Selection and Requirement a Key Infrastructure Requirement

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Material handling and infrastructure required to support Electronic Manufacturing is key to ensure sensitive electronic components does not get damaged due to electrostatic discharge. Such infrastructure is designated as an ESD Protected Area (EPA) having two key requirements of low charge generation and equipotential bonding.

By: Ankan Mitra

Any area which meets the above criterions and is used for handling or storing sensitive electronic components can be designated as an EPA. The EPA can be stores, incoming quality inspection area, manufacturing shop-floor, conformal coating area, packaging area, R&D lab, designated installation site etc.

Figure 7: Key requirement for a ESD Protected Area

ESD Flooring is a primary mechanism used in electronics industry to ensure equipotential bonding of the various ESD Protective Equipment (EPE) used inside an EPA. There is significant confusion between on what type of flooring needs to be selected, what is the resistance range and various other parameters. The selection criteria of the ESD Flooring to be used is driven by the interpretation of various traders dealing with such material based on multiple interpretations.

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Materials are classified in following ways in the ESD domain based on the value of its surface resistances:

Resistance of material in ohms per square

1 to <=103

>103 to <=105

>106 to <109

>109

Material classification

Shielding materials

Conductive materials

Static Dissipative materials

Insulative materials

Table-1: Properties of materials used for ESD Control System deployment classified by resistance (Ω/square)

Following chart looks at the various parameters required for selecting the type of ESD Flooring.

Figure 8: Criteria for selection of ESD Floor Material type

The different are reviewed briefly.

  1. Type of electrical operation: whether high voltage electrical operation is being performed, low voltage test is being performed or non-electrical operation is being performed. This determines whether insulative property is chosen for flooring material or static dissipative material is selected.
  2. Type of physical and mechanical operation: what type of abrasive operation, material movement and weight of equipment kept on the ESD floor. Hardness of flooring, expansion capabilities of the material, anti-abrasion finish of the floor material.
  3. Expected duration of the project: based on the expected surface life of the flooring, it is determined whether coating-based flooring surface or tile-based floor will be used

ESD is a current-phenomenon and key mechanism for protection from an ESD event is to reduce or drain the flow of current instead of a discharge. As mentioned earlier considering the primary equipotential function of ESD Flooring it is important to ensure the flooring measures. Hence considering Ohm’s Law (I = V/R; where V is the potential difference, R is the path resistance and I is the resultant flow of current) to ensure low value of resultant current to drain levels optimum value of resistance which is typically in the dissipative range is chosen.

Dissipative range itself has a wide range hence as the sensitivity of components increases, materials which fall towards the higher range of the dissipative range are typically chosen.

For equipotential bonding the redundant ESD Earth Pits are connected to the Auxiliary ground where the EPA has to be deployed. ESD Flooring is bonded to the Auxiliary ground using carbon impregnated paints or copper strips.

Figure 9: Recommended method for copper strip layout for ESD Flooring

Verification of the ESD Flooring in working condition is crucial. One of the method to verify is to measure the Point-to-Point resistance and Resistance-to-Ground resistance using a known voltage and recorded Relative Humidity.

However, such flooring works primarily during material handling movement where personnel working in the EPA are involved. Hence it is prudent to measure the personnel with footwear standing on the ESD flooring as a complete system. This is possible to verify using Walking Test.

Regular cleaning of this very important bonding mechanism is crucial. For such purposes a neutral pH cleaning solution is used using automated or semi-automated cleaning equipment.

By: Ankan Mitra

The author is the Ex-president, SMTA INDIA CHAPTER

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