From the ESD Journal around 1999, Jon
Metallized Shielding Bags
Metallized shielding bags have either a metal film embedded into the bag
film construction or coated onto an existing layer. This metal film acts
as an electrical shield against electrical discharges from the outside
of the bag. Depending on the energy and duration of the discharge and
the thickness of the metal film, an ESD event is typically spread out
over the outer surface of the metal film and if fully enclosed, i.e. the
bag is sealed, then the charges current from the ESD event is contained
to the outside (outer surface) of the metal film, i.e., providing a
region of no electrostatic fields within the bag, thereby protecting the
contents within. This effect is known as the Faraday Cage Effect and is
commonly used in controlling ESD via metallized shielding bags,
conductive bags and the conductive tote box with a cover.
There are two common types of metallized shielding bags varying by
construction, the metal-in and the metal-out shielding bags. The
metal-in (buried metal) shielding bags are the most common type
currently used and are recommended for packaging of ESD sensitive (ESDS)
components. They also tend to be superior to the metal-out construction
in durability and cost. The metal-out shielding bags are also designed
to protect against static induced damage. The metal layer is closer to
the outside surface resulting in these bags having lower resistance
readings than the metal-in which can be important in some applications.
Moisture Vapor Barrier (MVB) shielding bags are a special subset of
metallized bags as they also have the property of EMI-RFI-ESD shielding.
This is mainly accomplished by using a much thicker metal layer (about
10x thicker than standard metallized bags), which inhibits the moisture
vapor transmission rate (MVTR) by a factor of over 20 times more
compared to ordinary shielding bags.
In general, if you are storing ESDS devices for prolonged periods of
time (6 months or more) or if the devices are sensitive to corrosion,
than you should package these materials in either an MVB (Moisture Vapor
Barrier) bag with a desiccant pack to absorb any moisture that was
sealed in or a metal-in shielding bag with a desiccant pack. The
difference between an MVB film and a metal-in film is about 1 magnitude
in the moisture vapor transmission rate (grams of water/100 in2/24 hours
@ 100 oF). Moisture sensitive ESDS devices should only be packaged in an
MVB bag.
In both cases, the bags MUST be sealed to properly keep moisture out.
For all other ESDS (ElectroStatic Discharge Sensitive) devices [that are
not moisture sensitive], a metal-in, metal-out or a clear dissipative
bag can be used. Heat sealing is much preferred, however, you can often
fold over the top of the bag and close with an ESDS sticker for adequate
protection.
ESD bags should be inspected before reuse by an internal statistical
sampling plan as they typically have a finite reuse life.
If an ESD program is laid out very well and is extremely disciplined so
threat the threat of ESD events are almost squelched in both the
handling and transportation processes, then a good dissipative bag may
be sufficient for the storage of most ESD sensitive devices. There are
very few actual programs out there that are implemented so effectively.
Most ESD programs use shielding bags because it increases its level of
confidence and makes it more foolproof. Shielding bags serve two
purposes, preventing the ESD sensitive components sealed within the bag
from charging up via field induction and minimizing the damage from a
direct contact (outer bag surface) with an ESD event. Some high quality
films (ESD shielding bags) can withstand up to a 30 kV discharge.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ken Bloomquist
Sent: Wednesday, September 03, 2008 11:09 AM
To: [log in to unmask]
Subject: Re: [TN] ANTISTATIC MATERIAL QUESTION
It was static dissipative!
Thinking a little bit more about this I believe that metalized static
dissipative bags are a sandwich of materials. Isn't the inside layer of
a
static dissipative bag basically nonconductive plastic similar to Jon's
pink
bubble wrap?
KennyB
-----Original Message-----
From: Roberts, Jon [mailto:[log in to unmask]]
Sent: Wednesday, September 03, 2008 8:46 AM
To: TechNet E-Mail Forum; Ken Bloomquist
Subject: RE: [TN] ANTISTATIC MATERIAL QUESTION
KennyB, was the ESD bag an antistatic or static dissipative? Thanks,
Jon
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ken Bloomquist
Sent: Wednesday, September 03, 2008 10:36 AM
To: [log in to unmask]
Subject: Re: [TN] ANTISTATIC MATERIAL QUESTION
Hi Jon,
I'm not sure what the drying time is on the AR that you are using but I
know
with the UR we use takes about a week to fully dry. We experienced a
situation where we were having some boards built and coated in Mexico.
The
boards were put into ESD bags after a couple of days and shipped to our
facility. A combination of not being fully dried and sitting in a hot
truck
in the Mexican sun at the border cause the bags to stick to the coating.
When we tried to remove the bags they had to be cut away they were stuck
so
badly.
I guess what I'm saying is to make sure that they are fully cured before
putting them into the "proper" ESD packaging.
KennyB
-----Original Message-----
From: [log in to unmask] [mailto:[log in to unmask]]
Sent: Wednesday, September 03, 2008 8:05 AM
To: [log in to unmask]
Subject: Re: [TN] ANTISTATIC MATERIAL QUESTION
Date: Tue, 2 Sep 2008 10:43:10 -0500
From: "Roberts, Jon" <[log in to unmask]>
Subject: ANTISTATIC MATERIAL QUESTION
We have been noticing if bubble wrap is allowed to come in direct
contact with a circuit card assembly (non ESD and ESD) sensitive that
the bubble wrap leaves impression in the AR coating. Do know if the
resulting impression is related to uncured coating or reaction of the
bubble wrap material? Thanks for any information, Jon
Jon,
Not to be a stickler on coating questions, but there is no such thing as
uncured AR coating. Because the coating is lacquer, it doesn't cure, it
only
dries. The solvents in which the acrylic resin is dissolved leave the
coating and all that is left is the plastic.?That plastic does not
cross-link, so it?can be redissolved at will, and that's why AR is the
strippable coating that it is.?Insufficient drying?could?contibute to
the
condition you describe.?In our experience?we believe that plasticizers
from
the bubble wrap (notoriously oily, nasty stuff, it even seems like the
antistatic is worse) are actually redissolving the coating in areas of
contact. This would be more noticeable if the coating were not
completely
dry, since it's already somewhat melted. In any case, our solution is
not to
let the bubble wrap come into direct contact with any type AR coating.
Mike Moninger
Thermospray Company Inc.
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