I agree with Jeffrey that desiccant is a waste of time and money in PCB
packaging, but not for the same reason.
All plastics (and metals) have a vapor perm rate. The perm rate of PCB
packaging is so high that it is NOT considered a moisture barrier.
In other words, if you want a moisture barrier bag, say for moisture
sensitive components, then one must get a bag specifically made as an
actual moisture barrier bag (MBB). This is typically done by metalizing
the Mylar plastic to a substantially lower perm rate. (Please note that
there is still a perm rate associated for MBB but it is low enough to
make moisture protection practical for a couple of years).
Don't believe me that PCB plastic packaging is not a moisture barrier?
Then try this experiment yourself. Vacuum seal a crumpled up cloth rag
in your favorite plastic PCB packaging. Notice how the packaging is
stretched tightly around the rag (just like the TV commercials). Now
'time' how long it takes for the packaging to relax and allow the rag to
slide around in the bag. For the pink velostat bags, it's less than 24
hours. The Mylar Baystat static protective bags do better, but not
longer than a few weeks. Consider even the Mylar balloons sold at party
and gift shops. Even when filled with nitrogen instead of helium and
placed on a stick, the balloons lose air after a couple of months.
Point being, moisture, as a vapor, can and will flow in and out of the
bag, and the rate is so fast that the desiccant is as useful as setting
it on your desk to lower the humidity in your cubical. (I've even seen
shops put desiccant loose in the cardboard shipping box! AND many
places leave the desiccant supply in an open top 3 gallon metal bucket.
By the time it is used in packaging, it is already saturated!)
What the plastic does do, is act as a moisture 'insulator'. Moisture
will always try to find an equilibrium with its surroundings, but if the
relative humidity rises for a few days before dropping, the plastic will
slow the ingress of the moisture until the humidity goes back down. At
which point, if the humidity in the bag is higher than the sounding air,
moisture will try to get back out, so you get a rolling average moisture
content.
Not that any of this really matters. Cured FR-4 is not moisture
sensitive and its not moisture that kills solderability, it's oxidation.
Wrapping the boards in plastic does reduce the number of oxygen
'exchanges' that occur between the surface of the board and an available
oxygen molecule.
Ryan
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Jeffrey Bush
Sent: Friday, June 02, 2006 1:41 PM
To: [log in to unmask]
Subject: Re: [TN] Packaging requirements for IAg Finished PWBs
Consider a hard test.
We processed several VAC PAC packages of white tin products, 10 high
.062 boards, in 2 groups of 10 packages - 10 with no desiccant and 10
with desiccant. All packages had 30/40/50 humidity indicators
installed.
These were stored for 1 year in the surplus equipment locker - from
95F/100RF to 50F/30RF and back again as this is basically a garage.
After the 12 months the packages were evaluated via the indicator. No
packages had enough vapor to turn even the 30RF indicator - desiccant or
not.
This test was completed in 1998 and I have just pulled the packaged out
from a box I have in my office and the indicators are still all blue.
VAC PAC raw PCB's with desiccants - waste of time and money. Another
issue that was common up to this time was that the desiccant used in the
VAC PAC caused more breaches in the package due to the resulting
topography and increased stretching of the poly.
Jeffrey Bush
Director, Quality Assurance and Technical Support
VERMONT CIRCUITS INCORPORATED
76 Technology Drive - POB 1890
Brattleboro, Vermont 05302
Voice - 802.257.4571 ext 21
Fax - 802.257.0011
<http://www.vtcircuits.com/>
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Dehoyos, Ramon
Sent: Friday, June 02, 2006 3:24 PM
To: [log in to unmask]
Subject: Re: [TN] Packaging requirements for IAg Finished PWBs
Kane Amol:
Based on the fact that water as vapor or gas can travel
just as air does or any other gas, water vapor can move from wherever to
the desiccant in a vacuum pack. Roger Stoops found a very good article
that explains that air does not hold water as most of us were taught,
http://www.ems.psu.edu/~fraser/Bad/BadClouds.html. Water as a gas can
move about by itself till it condenses again into water or freezes as
ice.
Regards,
Ramon
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Jeffrey Bush
Sent: Friday, June 02, 2006 11:48 AM
To: [log in to unmask]
Subject: Re: [TN] Packaging requirements for IAg Finished PWBs
Desiccating a vacuum package is not needed but I would recommend an
indicator to ensure the vacuum has not been breached. Using desiccants
should also be of the type with a tyvek package that will not leach into
the inner bag onto the PCB's.
Jeffrey Bush
Director, Quality Assurance and Technical Support VERMONT CIRCUITS
INCORPORATED
76 Technology Drive - POB 1890
Brattleboro, Vermont 05302
Voice - 802.257.4571 ext 21
Fax - 802.257.0011
<http://www.vtcircuits.com/>
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Franklin D Asbell
Sent: Thursday, June 01, 2006 12:59 PM
To: [log in to unmask]
Subject: Re: [TN] Packaging requirements for IAg Finished PWBs
I'm not making any recommendations, just stating our method.
Many desiccant packs contain sulfur either in the media or paper, you
want to keep sulfur of any type away from any solderable finish,
especially silver.
Vacuum sealing is good if product is going to remain on a shelf for a
length of time. In today's busy facilities product is typically
purchased when needed and used immediately.
If the product being packaged is contaminate free, handled and packaged
properly, and, not subjected to any harsh environment prior to use then
in my opinion vacuum packaging is not necessary.
That packaging method is best agreed upon between the buyer and seller.
I have no opinion on the (in)effectiveness of a dessicant pack in a
vacuum sealed package, I have seen and heard information both pro and
con. It is something that should be agreed upon depending upon how long
product will remain in stock, environment, etc etc.
Franklin
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Kane, Amol (349)
Sent: Thursday, June 01, 2006 11:42 AM
To: [log in to unmask]
Subject: Re: [TN] Packaging requirements for IAg Finished PWBs
Hi,
Why do you recommend no desiccant?
Also, are there any distinct pros and cons of Vacuum Sealing the boards
vs.
putting them in a plastic bag with proper paper between the board and a
desiccant pack?
I was of the opinion that putting a desiccant pack and then vacuum
sealing does not make sense as there is very little air inside the
package, and due to the vacuum, any moisture would never reach the
desiccant pack anyways! Is there a specific reason board manufacturers
package their boards this way??
Regards,
Amol Kane
M.S (Industrial Eng.)
Process Engineer
Harvard Custom Manufacturing
941 Route 38 Owego, NY 13827
Phone: (607) 687-7669 x349
[log in to unmask]
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Franklin D Asbell
Sent: Thursday, June 01, 2006 11:00 AM
To: [log in to unmask]
Subject: Re: [TN] Packaging requirements for IAg Finished PWBs
Yes, we use a neutral pH paper, almost like tissue paper and fold 10 bds
max into bundles, then package these in neutral pH plastic bags.
Scratches are not so much this issue, it's more of abrasion prevention
as well as protection from the environment. We have customer's however
that once the product is received they completely remove the product
from the packaging and place them open in bins until use.
As a practice we pass along assembly and handling guidelines to all
customers who request silver finish for the first time, and with new
customers order silver finish product.
Silver finish reacts to atmosphere more dramatic than tin-lead, the
tin-lead finish is also more durable and less affected by handling,
contaminates, etc so more controlled measures must be taken with silver
finish.
Also, it is best to perform the silver finish process as close to the
end of the fabrication process as possible. We typically perform this
process after rout and final inspection. Silver finish is applied, tape
testing and visual inspection is performed and the product is
immediately packaged.
The following is an excerpt from such a procedure.
==============
5.0 Handling, packaging, and storage
5.1 It is recommended that gloves be used for all handling after the
immersion silver process.
5.2 Salts and acids from fingerprints may tarnish the silver finish.
5.3 Surfactants and/or acid cleaners should not be used on silver
finish
boards. Clean only with DI water rinses.
5.4 Silver plated boards should be packaged as soon as possible to
avoid
exposure to sulfides and chlorides in the air.
5.5 Bundle wrap silver parts in clean acid/sulfur free paper.
Neutral pH
paper is sufficient.
5.6 Storage should be in sealed neutral pH plastic bags to eliminate
direct contact with the air.
5.7 Do not allow sulfur bearing plastic to contact the silver
finish.
5.8 Do not use desiccant in the packages with silver plated boards.
5.9 Do not use rubber bands on boards. Most rubber bands contain
Sulfur
compounds.
==============
Franklin
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Gary Camac
Sent: Thursday, June 01, 2006 9:47 AM
To: [log in to unmask]
Subject: [TN] Packaging requirements for IAg Finished PWBs
Good morning all,
I am struggling through a rewrite of our material specification for
PWBs. I have seen the recommendation that cover sheets and interleaf
sheets should be used to prevent scratches on IAg finished PWBs. Have
those that are using IAg seen this as necessary? We eliminated this
requirement on Sn/Pb finished PWBs some time ago without any negative
effects.
Thanks,
Gary Camac
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