Inge,
I absolutely agree that you can generate a 10KV+ static field with a
simple piece of plastic.
If you are going to damage a semiconductor junction you need power, volts
x amps.
If you touch a piece of plastic that has a 10KV field because of
mechanical action do you get a spark? (ESD event.)
Try it, the answer is no. There is no current flow. Actually there are a
couple of electrons that will flow but the total charge is zero for
practical purpose.
The ESD meter you mention is telling you about electrostatic field. In the
presences of charged conductive bodies (like people) this is a good
indication of a ESD possibility.
Volts along does nothing, you need current and volts to damage a junction.
Why do so many people not believe in H Field like they believe in E field?
Bob Kondner
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Hernefjord Ingemar
Sent: Friday, May 15, 2009 3:18 AM
To: [log in to unmask]
Subject: Re: [TN] Bare PCB Packaging Requirements
Dear Bob,
don't you need read a little about ESD basics? If you remove electrons from
an insulator (plastic comb, acrylic clothings, cat etc) by rubbing it, the
loss of electrons will make the insulator positive. Or you may get a charge
in the insulator by means of outer electric fields, that can move electrons
in the insulator in a way that you get a potential difference. Pulling a
plastic comb through dry hair (if you get any)can charge your comb up to a
thousand volts. Pulling off your acrylic pullover can generate 10,000 Volts!
However, you are right in a way, the PE bag is not dangerous IF THE
SURROUNDING ALWAYS GOT THE SAME POTENTIAL. But that is seldom the case. So,
do this for me. Get a PE bag, wave it in the air (won't work if you have a
high RH or ionizers)and keep it close to your ESD charge meter and see what
happens. I'm afraid you will be bombarded with more about this topic.
Good Luck
Inge
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Robert Kondner
Sent: torsdag 14 maj 2009 22:58
To: [log in to unmask]
Subject: Re: [TN] Bare PCB Packaging Requirements
Richard,
Your wrote:
"Non-ESD polyethylene bags are notorious ESD generators. I wouldn't want
them in my factory."
You will NEVER get a ESD event from polyethylene because it is an
insulator, right?
However I see lots of ESD events with grounded metal objects. Since UL
requires metal machines to be grounded I guess you don't want any metal
machines in your factory?
What is going on here?
Bob Kondner
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Thursday, May 14, 2009 3:12 PM
To: [log in to unmask]
Subject: Re: [TN] Bare PCB Packaging Requirements
1. The same moisture barrier bags that are ESD-safe Faraday bags also work
well as moisture barrier bags for shipping and storing PWBs. These Faraday
bags often work well for storage of the CCA during assembly. Why use two
different types of bags if one fits the bill? Non-ESD polyethylene bags are
notorious ESD generators. I wouldn't want them in my factory.
2. Many CEMs and OEMs have requirements for the PWBs to be shipped in MBBs,
in order to prevent against surface finish oxidation in the case of IAg,
ISn, HASL and FTL finishes. Some invoke a requirement such as IPC 4552 or
4553 in order to ensure the boards are dry and hermetically sealed.
3. All PWBs absorb moisture from the air. Having the boards sealed prevents
excess moisture ingress, in some cases if the boards are shipped in sealed
bags with desiccant, the user may not need to pre-bake if they are able to
successfully perform Kanban pull from the fabricator within a short time
frame. This, of course, requires qualification and validation of the process
and procedures.
So, if a bag is required, use one from start to finish that meets multiple
requirements.
I assume you understand why moisture within the PWB is not a desirable
condition.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of John Parsons
Sent: Thursday, May 14, 2009 1:37 PM
To: [log in to unmask]
Subject: [TN] Bare PCB Packaging Requirements
Tell us, how many CEM's out there are demanding that bare PCB's be received
in vacuum sealed moisture barrier anti-static bags. If so, what is your
reasoning? Just to be clear my focus here is "anti-static" not the moisture
barrier properties of the packaging.
Regards
John Parsons
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