MQFPs, LQFPs, TQFPs, and PDIPs, or virtually any IC package made by transfer
molding with epoxy mold compounds, do not have cavities. As Guy indicates,
moisture pickup up in the mold compound, as I previously described in
another note, can induce cracking if it cannot diffuse out of the bulk
before the internal pressure created during reflow exceeds the bulk material
strength or a nearby interface's adhesive strength.
Unless the mold compound has entrapped voids into which water can actually
condense most moisture induced cracking is initiated at interfaces (moisture
likes these higher stress areas) and is most commonly simply called
'delamination'. These 'delams' often propagate to a sharp corner after
which the crack may branch into the bulk mold compound and can then lead to
a crack visible on the outer surface. As a delam propagates across an
interface it can cause wire lifts off lead tips and break wires just above a
ball bond. Delam at the die attach can increase backside contact resistance
(if die is so biased) or degrade heat spreading that can lead to die
overheating causing device failure, shortened device life, or unacceptable
wire bond intermetallics and eventual problems.
The original MSL rating is totally dependent upon maintenance of the
environmental integrity of the dry-packed units unless the product is truly
MSL1 at a specified reflow temp in the preconditioning step test (235-240 OR
260 reflow). Repackaging or resealing after use of a partial lot requires
care and short term exposures outside of the dry-pack environments.
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
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-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Guy Ramsey
Sent: Tuesday, June 21, 2005 9:44 AM
To: [log in to unmask]
Subject: Re: [TN] Moisture sensitivity levels...
No, you are all wet. The sudden vaporization of moisture in plastic or epoxy
can pop the die off the carrier, pull wire bonds off the die, crack the
package and expose internal workings of the device, and more. Popcorning is
a good name. Recently, I saw a device just like the one you describe. A
ceramic with gold wire bonds, cavity filled with epoxy as an encapsulate.
The epoxy absorbed water which erupted during reflow and created a large
void in which the bond wires bent and made unwanted electrical contact. . .
with very weird symptoms.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Dehoyos, Ramon
Sent: Tuesday, June 21, 2005 9:27 AM
To: [log in to unmask]
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
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