Steve,
I could not find any information on what studies were done to come up
with or justify the MSD levels when I checked on this. I was told that
the levels were established by component vendor's recommendations.
The reason I wanted this same background information that you are
looking for, was because I had a very big concern with baking parts at
the recommended time and temperature of 24 hours @ 125C for a level 5a
part, for example. I know there are alternate bake times at 90 deg. C,
but they require a much longer time.
Baking at 125 C. for 8, 12, or 24 hours will definitely degrade
solderability of most components with a tin-lead finish. It can also
harm many components from a reliability standpoint.
I also know these are supposed to be "guidelines", or so it is stated in
J-STD-033.
So I went and baked a number of components and weighed the parts before
and after baking to get the moisture content using a precision scale
that I leased. I did this for a number of components, but mostly for
some class 5a parts, at a company I used to work at. I no longer have
this data, but it is still the basis of their MSD handling program.
I found that on the time/temperature curve, nearly 90% of all moisture
is removed if I baked a large variety of plastic components for 6 hours
at 105 deg. C. I baked components from small PSOICs to 40mm square QFPs.
If I am at the 10% moisture content level, those parts are safe for
"normal" reflow temperatures (up to about 220 C).
I also found that some of the most moisture-sensitive components are
silicon dies, especially silicon flip-chips. The component vendors for
the small silicon flip-chips we were using urged extreme caution in
handling them, and recommended a bake time in a vacuum bake (1/2
gravity) of 90 deg. C for 72 hours if they were exposed past their open
floor life.
One would not expect a small piece of glass to be that moisture
sensitive, but they are. They will crack during reflow if not baked out
properly. Of course, this all depends on the individual component.
But in general, I have never had a problem with any component popcorning
or otherwise failing, nor have I seen any problems with solderability,
if parts are baked at 105C for 6 to 8 hours. But I could have been
lucky.
An ideal situation for handling MSDs: skip the receiving inspection and
use a certified supplier. Have the parts shipped directly to the
assembly line. Have the parts remain in original packaging stored inside
a dessicator dry box (not a nitrogen drybox) such as a McDry. Open the
parts as part of the line setup when the parts are about to be run, and
immediately return the balance of the parts to the McDry cabinet,
re-sealed with their original dessicant if the Moisture Indicator Card
indicates they are still dry.
Don't forget to teach your IP/IC types the details of MSD handling. For
example, if a circuit board fails test and a component needs to be
replaced, make sure the indidual replacement component (if it is an MSD)
remains in drypack when it is issued to the rework area. It cannot sit
around for 2 or 3 days in >40% RH before it is finally reworked onto the
assembly. Components reworked with direct application of hot air or hot
gas are even more susceptible to popcorning than those that go through a
reflow process.
I recommend dessicator boxes, such as the McDry. They have a controlled
and verifiable rate of dessication, don't require nitrogen (expensive
proposition to have several leaky nitrogen boxes situated at different
points in the factory), and they actually will dessicate (dry) the
components. Correct me if I am wrong, but I believe nitrogen boxes
simply provide a nitrogen atmosphere that will prevent additional
moisture from being absorbed.
Remember, I said I found this to be a good means of protection from
thermal expansion at normal reflow temperatures. I have no idea what
safe moisture levels are for lead-free component processing, where the
reflow temperature can be much higher. But with the advent of Pb-free
solder processes, a good MSD program becomes much more important.
Also, I noted that most components, after baking, will regain nearly 90%
of their moisture weight within 12 hours, if left open at 40% RH.
Constant protection from moisture (proper storage) is the key.
And have fun trying to train operators, engineers, inventory controllers
and planners, and others who handle MSDs.
Its a very dry subject, heh, heh.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stephen Gregory
Sent: Monday, June 20, 2005 10:26 AM
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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