Hi Richard!
You're right, trying to wrap my arms around this whole thing is going
to be fun.
I'm starting to learn more and more that what I thought wasn't
moisture sensitive, is...or is being labeled as such. Then I get
get components from distribution that haven't been sealed in a
vacuum bag or labeled as a MSL component.
It was that issue that raised the question by our stockroom clerk this
morning. We had one SOJ28 that came in sealed with a MSL rating of
3, and then the other unsealed SOJ28 was a MSL 1. I found that out after
checking with Cypress.
But what threw me this morning was the Tantalum capacitor. This is a solid
tantalum, a "Tantamount" as Vishay calls it. I never knew that tantalum
capacitors
were moisture sensitive. But the caps I have have a MSL 2a label on them.
They
also have a PB Free label on them. The finish is pure Sn.
After digging around Vishays web page, one thing I noticed that all the
Tantalums
that have a Sn/Pb finish on them are MSL 1, and the PB Free are 2a.
What if we don't solder using lead free solder? Do we still have to treat
them as
MSL 2a components. Further still, must I reflow at the higher temperature
that Vishay
is recommending in their profile if I'm using Sn/Pb solder paste?
Kind regards,
-Steve Gregory-
Senior Process Engineer
LaBarge Incorporated
Tulsa, Oklahoma
(918) 459-2285
(918) 459-2350 FAX
|---------+---------------------------->
| | "Stadem, Richard"|
| | <Richard.Stadem@G|
| | D-AIS.COM> |
| | Sent by: TechNet |
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| | 06/20/2005 12:12 |
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| | respond to |
| | "Stadem, Richard"|
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| To: [log in to unmask] |
| cc: |
| Subject: Re: [TN] Moisture sensitivity levels... |
>--------------------------------------------------------------------------------------------------------------|
Also, Intel has some excellent info in their website about MSD handling.
Check it out. The title is Moisture Sensitivity/Desiccant
Packaging/Handling of PSMCs.
I have also read several papers written lately that state moisture
content of plastic BGA packages is one of the main causes of warpage of
PBGAs during reflow (besides other factors). So, you could fix more than
one reliability issue with a good MSD program.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Leo Higgins
Sent: Monday, June 20, 2005 11:00 AM
To: [log in to unmask]
Subject: Re: [TN] Moisture sensitivity levels...
The MSL rating achieved by a specific IC package is dependent upon the
BOM used in the package assembly, the assembly process used, and
mechanical design aspects of the package and package elements.
BOM: The die attach adhesive, mold compound, solder mask, laminate
substrate materials, etc., all have water absorption and desorption
characteristics that greatly affect the MSL. Rapid heating may cause the
material to fail in the bulk or more commonly a separation at an
interface will occur. In general, the less water absorbed by the
specific material, the lower the risk of bulk failure or interfacial
delamination. Of course, if the material has low bulk strength or the
adhesion strength is low, failure can occur even with very low levels of
water absorption. Epoxy mold compounds will adhere better to some
polyimide die passivations than others due to the presence of adhesion
promoters in some polyimides. Die attach epoxy adheres much better to a
bare Si or GaAs die backside than it does if the backside of the die is
metalized with gold, which is common with GaAs.
Die attach epoxies and mold compounds adhere better to copper surfaces
than the silver plated regions on leadframes. This issues can greatly
affect MSL. An IC supplier may source his devices from multiple
assembly subcons that use different BOMs, but meet the IC supplier's
qual spec. Consequently devices from one subcon may meet MSL1 and
another subcon's devices may meet MSL3, so the IC supplier will normally
spec all the product at MSL3.
Assembly process: Interfaces need to be clean. Often interfaces are
activated with plasma processing before bonding or molding. Often
adhesion promoters are applied (e.g. to a leadframe). The bulk strength
and adhesion of the various polymer systems (die attach adhesives, mold
compounds, solder masks, PCB substrate resin, etc.) is greatly affected
by the time/temp cure profile. Molded packages typically require a
post-mold cure of 4-5 hours at approximately 175-180C to effect full
cross-linking and optimized adhesion at multiple IC package element
interfaces (die passivation, "bare" copper leadframe, silver ring around
the leadframe die attach pad perimeter, silver plated tips of leads on
leadframes, etc.). The proper cure profile minimizes the free volume
into which water can be adsorbed, and minimizes the presence of
unreacted epoxy side-groups to which water likes to chemically attach.
So process control can greatly affect the resulting MSL.
Mechanical aspects: A thicker package may have more mold compound so it
may provide a stronger mechanical structure (e.g. MQFP vs. TQFP). This
strength may help the package resist evaporating moisture induced stress
and avoid crack propagation from an sharp point, like the edge of the
die or the die attach pad. But the thicker mold compound cross section
may also present a greater barrier to moisture diffusion during heat up
when compared to a thinner package, so it is also possible that the
moisture may escape before a crack forms in the thinner package. A
small die on a large die attach pad may result in lower MSL than a
larger die on the same die attach pad, especially if the die attach pad
is silver plated.
So, all of these issues effect the package MSL rating.
Another issue is often neglected. After board assembly, the MSL rating
of the devices does not magically improve to MSL1. Yet is very common
for assemblers to neglect baking out an assembled board before
subjecting it to rework. This may destroy the unit being removed (if
not already defective) and neighboring components.
Best regards,
Leo
Director of Applications Engineering
ASAT, Inc.
3755 Capital of Texas Highway, Suite 100
Austin, Texas 78704
ph 512-383-4593
fx 512-383-1590
[log in to unmask]
www.asat.com
The information contained in this electronic message is
CUSTOMER/SUPPLIER PRIVILEGED AND CONFIDENTIAL INFORMATION intended only
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-----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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