Hi, Lee
The bond junction inside the diodes, and the glass body of these types
of diodes is very brittle and very susceptible to cracking. If conformal
coatings are used, they must be chosen based on the ability to avoid
stressing the very components they are meant to protect from moisture,
particularly glass body components. The coatings or epoxies must be
somewhat elastic to minimize stresses induced by differences in CTE or
other transmitted forces imparted from flexure. Hard epoxies and
conformal coatings have been known to crack glass body devices, even the
coatings that are relatively thin. This may be instigated by stresses
imparted on a glass diode when rigid epoxies or coatings are placed in
the gap between the PWB and a glass body diode.
Likewise, I am prohibited from using staking epoxy over glass body
diodes per J-STD-001DS, the IPC Space Addendum I think you were
referring to, for the same reasons.
There is some good information on this in IPC-HDBK-001.
Essentially, if you capture the glass body (through-hole or SMT, doesn't
matter) in an encapsulation of hard CC or epoxy, (and I am not going to
argue the choice of words here, encapsulation, coating, potting,
whatever) and the board sees flexure or CTE stresses, those stresses are
transferred directly to the glass diode, cracking it.
There have been a number of high-level failures from glass-body diodes
captured in hard epoxies. Here are some I am aware of:
www.jsc.nasa.gov/news/columbia/frr/sts-109/09_srb.pdf see page 17.
http://esmat.esa.int/Publications/Published_papers/ESA-SP-1173.pdf. See
page 75.
I am sure there are many others, as I have read them in the past. They
are all very similar. This is definitely something that should be paid
attention to.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Lee Whiteman
Sent: Tuesday, June 09, 2009 7:22 AM
To: [log in to unmask]
Subject: [TN] CONFORMAL COATING ON GLASS BODIED COMPONENTS
We have the following requirement on one of our space programs:
All glass-bodied electronic parts which are bonded, encapsulated or
conformally coated shall be sleeved with sleeving that remains resilient
over the temperature range specified for the equipment. Surface mount,
non-axially leaded, glass-bodied diodes that are conformally coated do
not require sleeving if the coating Tg is below all application and test
temperatures.
I am confident that this came from NASA STD 8739.1A as well as the IPC
Space Addendum. The question is why? Have there been documented failures
of glass-bodied electronic parts without sleeving which failed when
conformal coating was applied?
Should glass-bodied components be masked from conformal coating?
Thanks again...
Lee Whiteman, PMP
Senior Member Engineering Staff
L-3 Communications East
Telephone: (856) 338-3508
FAX: (856) 338-2906
E-Mail: [log in to unmask]
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