Per ANSI/ESD S20.20 Paragraph 6.2.2.1 Personnel Grounding Requirement
"All personnel shall be bonded or electrically connected to ground or
contrived ground when handling ESD sensitive items. When personnel are
seated at ESD protective workstations, they shall be connected to the
common point ground via a wrist strap system."
ESD protective Seating is an optional implementing process per Table 1of
ANSI/ESD S20.20; if used the minimum recommended technical requirement
range
is < 1 x 10^9 ohms.
Included in the ESD Handbook TR20.20 section 5.3.5 is Static Protective
Seating "As mentioned in the floor materials section of this handbook, the
principle cause of static electricity in the workplace is cited as the
movement of people and materials. This routine movement, particularly from
a person seated in a chair or movement of the chair itself across the
floor, can generate significant voltages. This section covers the use of
seating in an ESD protected area for dissipation of charge. Although not
required in ANSI/ESD S20.20 as a primary means of grounding personnel in
the workplace, the use of chairs that meet resistive requirements from the
seat contact surface to the castors or leg ends, may be an effective means
of grounding personnel if a reliable method can be found to bond personnel
to the chair. In addition, if the chair is used to connect personnel to
ground than the maximum resistance to ground from the person through the
chair and flooring system should be less than 35 megohms."
However, it is mandated per EN 61340-5-1. Per section 5.2.4 Seating
"The resistance from all areas of seating which may have human body
contact when in normal use, to either a floor contact point or a groundable
point, shall be as specified in table 1. [Rg =/< 1 x 10^10 ohms]
When the floor is used as part of a grounding system, a minimum of two
wheels or two feet shall provide a path to EPA ground."
Its EN 61340-5-2 User Guide states:
5.2.4 Seating
"It is important that low charging and grounded seating is used, as
although seating should never be used as a primary means of grounding there
are occasions where, through human error, equipment breakdown or bad design
it can provide an important resistive path to ground.
Although not recommended, in some EPA seats may be used as impromptu
working surfaces or storage surfaces. If nonconforming seating is used then
ESIDS may be damaged in this way.
Where grounded, if footwear is used rather than wrist straps, when seated,
many operators will spend periods when footwear is not in contact with the
ground. Charges and potential can build up where there is no path to
ground. In this case, a path may be provided through the garments (see
5.2.5), provided they are in contact with the seating.
It should be emphasized that, where operators are seated, a wrist strap
approach should be used and seating should form a secondary means of
discharge. In the event that an ESID coordinator should decide that there
is no suitable alternative to using seating as the primary grounding
approach, resistance to ground should comply with the same requirements as
wrist strap systems, or for flooring and footwear (i.e. 750 kQ to 35 MQ)
(see 5.2.8).
Seats in compliance with IEC 61340-5-1 should carry suitable labels (see
clause 4) to help with identification.
It is permissible for those parts of seats which are unlikely to come into
contact with the body or with ESIDS, for example the underside of the seat,
to be made of non-compliant materials."
Gene Felder
[log in to unmask]
-----Original Message-----
From: Dale Ritzen [SMTP:[log in to unmask]]
Sent: Monday, March 21, 2005 2:53 PM
To: [log in to unmask]
Subject: [TN] ESD Issue - non ESD Chairs on Manufacturing Floor
A sister company of ours has just undergone an ESD audit by one of their
customers. During the audit, they were cited for allowing non ESD chairs
(i.e. no drag chains and/or conductive wheels/legs) in the manufacturing
areas. I looked in IPC-A-610, in section 3.2 (ESD/EOS Safe Workstation) and
find no reference to non-ESD chair use. Simply put, if a solder operator,
assembler, etc. is seated in a non ESD chair, but wearing heel straps (on a
dissipative floor) and is jumpered to a grounded jack via a wrist strap my
take is that they are sufficiently grounded to perform their work. I have
never had any external ESD audit fail for this reason.
Anyone else have any experience with this type of call? I don't have a copy
of ESD S2020 so I can't refer to it for help in this case. Thanks!
Dale Ritzen, CQA
Quality Manager
Austin Manufacturing Services
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