Hi Chris,

I think that there is no such thing as a stupid question, but there is
certainly a general lack of understanding in the industry on the subject of
moisture sensitivity. There are good reasons why people need to bake parts
and it really depends on many factors, including material logistics, product
and component mix, the frequency of changeovers, ambient conditions, etc.
For example I have seen a situation with a reel of BGAs, MS level 5A (24
hours), containing 2000 components and for that specific product, at the
production rate of one board every minute and one BGA per board, it was
physically impossible to place all the parts before they expired even if the
line never stopped. This is probably an extreme situation but if you start
to account for dry storage and double-side reflow the process window gets a
lot smaller. The problem is that most internal component defects created by
excessive moisture prior to reflow cannot be detected during test and
inspection. As a result many components that should have been baked will
escape with latent defects and eventually will fail in the field.

It is also important to point out that many people do not bake as much as
they should because their internal procedures for handling
moisture-sensitive devices are based on obsolete standards (IPC-SM-786). In
the most recent standard (J-STD-033A) there have been many significant
changes because of the improved understanding of the moisture diffusion
process inside components. The most significant change is related to the
impact of dry storage on the remaining floor life. In the old way of
thinking, it was assumed that the floor life clock temporarily stops when
previously exposed components are returned to dry storage (dry storage being
defined as either a re-sealed dry bag with desiccant or a dry cabinet at
less than 10%RH). Since then it was demonstrated that the moisture gradient
that is already absorbed will continue to diffuse towards the critical
interface and eventually some components will expire while they are placed
in dry storage. Ref : J-STD-033A, section 4.1 "Placing SMD packages, which
have been exposed to factory ambient conditions for greater than one hour,
in a dry cabinet or dry pack does NOT necessarily stop/pause the floor life
clock...". If you want more background on this topic I recommend that you
read the following paper : Handling of Highly-Moisture Sensitive
Components - An Analysis of Low-Humidity Containment and Baking Schedules by
Shook and Goodelle from Lucent. You can find a link to this paper in the MSD
Knowledge base at www.cogiscan.com, click on "MSD Knowledge Base", then
click on "List of publications and other references" and scroll down the
list. The list is in chronological order and this paper was published in
1999.

In general the best practice is to open the dry bags as late as possible and
then track the exposure time for every component with a focus on the partial
trays and reels that are removed from the SMT line during a changeover. When
parts are returned to dry storage, the standard provides a series of
guidelines to account for the desiccating effect in terms of remaining floor
life.

The comment from Jason is also very appropriate. The tape and reel packaging
itself (pocket tape and cover tape) does not offer any significant
protection from moisture, whether or not there is a hole in the pocket. If
you look at the specifications for the moisture barrier bags it requires a
Water Vapor Transmission Rate (WVTR) of less than 0.002 gm/100 in2 in 24
hours at 40C. This requires a multi-layered construction with at least one
metalized barrier.

Francois Monette
Cogiscan Inc.


Date:    Thu, 10 Jul 2003 15:48:28 -0500
From:    "Jason W. Gregory" <[log in to unmask]>
Subject: Re: SOT 23 placed upside down

Chris,

Most tape pockets have holes in them on the bottom of the pocket. Some older
feeder designs had push up pins to push the part up for the nozzle to grab
the part. I don't know of many modern machines that use this technique, but
the tape still employs this feature. The fine pitch pockets may not have
these holes (that could allow moisture) , but, nonetheless, I
would still wonder about moisture "leaching" through the seal (if it's not
absolutely sealed). This assumption is that the reel was left out of a
controlled environment.

Jason Gregory
SMT Production Supervisor/
Integrated Product Team Leader
LaBarge Inc.
(918)459-2367
(918)459-2221 fax
[log in to unmask]
http://www.labarge.com


Hi Howard,

we have been picking fine pitch QFPs up to QFP100 size from tape for years
with no coplanarity or moisture issues. There are no problems with lead
damage caused if the tray is knocked by an operator when loading the tray
into the tray feeder and parts jump out, there is no possibility of
incorrect rotation caused by an operator putting the tray in the
wrong way around, less downtime from reel / tray changeovers as we can fit
many more parts onto a reel than a tray, no reduced placement efficiency if
you have a placment machine with a slow tray feeder.

I am curious as to why people are baking their parts?  Surely if you keep it
in it's vacuum package until it is needed and store it in dessicant cabinets
when off line then it should be OK?

(Sorry if this is a stupid question.)

Regards,

Chris Murphy,

Production Engineering,
Tait Electronics


Francois Monette wrote:

Howard,

For your second question, another important consideration for Tape Vs Tray
is whether the components are classified moisture-sensitive and what MS
level. Even for robust components like BGAs, keep in mind that the benefits
you get from tape and reel can be more than offset by the fact that you will
have more parts per unit resulting in longer exposure time and especially
the longer bake time required. In a high temp tray the default bake cycle
is 48 hours at 125C and in a reel it is 79 days at 40C...(ref. J-STD-033A).

Francois Monette
Cogiscan Inc.

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