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


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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
__________________________________________________________________
This message may contain information that is privileged and confidential to
LaBarge, Inc.  It is for use only by the individual or entity named above.
If you are not the intended recipient, you may not copy, use or deliver
this message to anyone.  In such event, you should destroy the message and
kindly notify the sender by reply e-mail.

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