From Lucent's "Handling of Highly-Moisture Sensitive Components- An Analysis of Low-Humidity Containment and Baking Schedules" by J.P. Goodelle:
" Moisture diffusion is a thermodynamic temperature activated process that does not stop by simply placing a device in a dry storage condition. Perturbations of moisture exposure followed by dry storage will allow moisture to reach critical internal interfaces which can result in an increased risk for moisture/reflow induced damage. The best procedure for handling moisture sensitive devices is to assemble them within the recommened JEDEC floor-life times. If this cannot be accomplished, then dry containment storage is a viable option provided the duration of intitial ambient exposure is less than 8 hs. For exposures longer than 8 hs, diffusion kinetics will need to be considered. Cyclic storage can be used as an option to extend the floor life time, however, its usefulness as a manufacturable process is essentially limited to THIN packages. High temperature baking to remove excess moisture will have similar concerns if the bake times are not long enough. BAke times can be optimized only if prior moisture exposure conditions are adequately known." 289
" This happens during baking at +125C/O%RH. The result shows that the chip(die)-mold interface concentration INCREASES to a maximum after 2 hs of baking and then continously decreases with bake times longer than 2 hs. The interface moisture concentration does NOT drop below the determined Level 4 C Critical amount until after 15 hs of baking and requires a bake time of 21 hs to fall below Level 5 concentration. This calculation demonstrates that whenever a positive moisture gradient exist within the mold compound, baking will initially force moisture to diffuse INWARDS toward the die surface before the total gradient can begin to decrease. This occurs because a gradient maximum is created in the throug thickness of the mold compound, as shown. By Fick's Law, moisture cannot diffuse up a concentration gradient, therefore, moisture contained within the mold compound continues to diffuse down this imposed gradien maximum. As the time at bake increases, the gradient maximum moves INWARDS towards the die surface. The interface concentration cannot decrease until the moving maximum engages the die surface." 289
Floor life-time is not taken seriously everywhere. We are no exception. Many in the production process simply got no theoretical background to these figures. Now, back to John's exploded tantalum and Gaby's groan-bake-groan-bake-groan-bake, what do you do when you get molded tantalums or other molded beasts WITH NO FLOOR LIFE-TIME information? How do you know what to do? Baking-groan-baking or measuring the mold moisture absorption or what? As I can see, stopping the baking at inappropriate time can cause the moisture level to be at maximum on the die instead of minimized. If Lucent's theory is right.
I think I put my head in the sand instead...seen nothing, heard nothing...he-he
Ingemar Hernefjord
Ericsson Microwave Systems
-----Original Message-----
From: Dehoyos, Ramon [mailto:[log in to unmask]]
Sent: den 21 juni 2005 15:27
To: TechNet E-Mail Forum; Ingemar Hernefjord (KC/EMW)
Subject: RE: [TN] Moisture sensitivity levels...
I may be totally wet but my assumption has been that there needs to be a cavity such as in the QFPs or DIPs for enough moisture to create enough pressure to pop a device. Based on this assumption, perhaps if the cavities were potted after wire bonded there would not be a need for MSL nor the prohibition against ultrasonic cleaning. I have seen some components with a jelly like material inside over the electronic device.............. Just a thought.
Ramon
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Ingemar Hernefjord
(KC/EMW)
Sent: Tuesday, June 21, 2005 2:53 AM
To: [log in to unmask]
Subject: Re: [TN] Moisture sensitivity levels...
Steve,
how come that you jumped high so sudden? Have you used some kind of humidity issue filter
all these years...he-he..cheating. There is humidity everywhere. Mother nature infiltrates
water vapour molecules everywhere she can. We learned that there is humidity even on inside
of glass feedtrues, water penetrates deeply into kovar packages, there is water in the solder
paste, every single "plastic" has a small amount of water on inside. Water molecules are
universal and unavoidable. We had a period of humidity hysteria, digged deeply into the
mystery of water penetration mechanisms by means of proffessors and doctors, had seminars,
and the learned painted scaring scenarios about all bad things that could occur due to water in
the material. We spent a lot of money on residue gas analysis and made lots of MIL STD
tests and so on. The years pass and behold, a miracle: there has been no catastroph. Some
few reports have been noticed, but the equipments spread all over the globe seem to work,
and mounting seems to go on rather free from incidents despite there is water in every
single component. So, I wonder if all anti-humidity countermeasures are proportional to the
very few failures that occur due to water.
My thoughts do not necesserarily represent the general opinion here, I'm just curious about
the mystery of water content and others experience. Is humidity a severe problem today?
Can anyone give example?
(I don't include water that penetrates into packages from the surrounding during field use,
but just point at built-in humidity in parts before and during assembly.)
Ingemar Hernefjord
Ericsson Microwave Systems
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Stephen Gregory
Sent: den 20 juni 2005 17:26
To: [log in to unmask]
Subject: [TN] Moisture sensitivity levels...
Good mornin' all!
I hope all you Dads out there had a wonderful Fathers Day!
As I posted about a week or so ago, we're getting our stockroom
personnel to really focus on component moisture sensitivity levels.
But now I'm starting to get a little confused about how components
are generally classified with their respective MSL ratings.
I've always thought that it had to do with component body size and
thickness,
but now I'm finding out that it doesn't necessarily.
My stockroom clerk showed me an IDT SOJ28 that came in sealed with a MSL 3
rating that we now have in our dry box because the package was opened.
We have some Cypress SOJ28's and SOJ24's that are on reels that are not in
original packaging, so I called Cypress and asked what the MSL rating is
for the
two part numbers and they are a MSL 1. Cypress gave me a link to the SRAM
Qualification Report that states that.
Now my stockroom clerk brought me some Vishay tantalum caps that have
a MSL 2a rating label on them, and they are TNTC's. This is THE FIRST time
I
have ever seen a MSL label on a tantalum capacitor.
Are these ratings based on qualification reports? Or can one look at the
size and
thickness of the part to base how you will handle it?
Kind regards,
-Steve Gregory-
Senior Process Engineer
LaBarge Incorporated
Tulsa, Oklahoma
(918) 459-2285
(918) 459-2350 FAX
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