Hi,
We are talking conformal coating materials and not potting or under fill,
right?
A conformal coating of a few mils is going to fail and crack long before it
rips BGAs off a board.
I have played with silicone coating, not exactly a hard or brittle
material.
Using a soft stretchy material like a silicone does not make much sense as
an under fill. Using a thick layer of a strong material that distributes
forces developed by a TCE mismatch would make sense as an under fill. That’s
why one does an under fill in the first place.
Did I get that right???
Bob Kondner
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Joyce Koo
Sent: Monday, November 30, 2009 10:38 AM
To: [log in to unmask]
Subject: Re: [TN] BGAs & Conformal Coating
Some conformal coat gets extremely hard and brittle at low temp, impact on
TC, drop, and other mechanical behavior. You do not wnat to go there.
--------------------------
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----- Original Message -----
From: TechNet <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Sent: Mon Nov 30 10:29:32 2009
Subject: Re: [TN] BGAs & Conformal Coating
I don't think modulus of elasticity matters.
I think where the material is 'fully' constrained it is the bulk modulus
that really matters and for most materials that is very high, i.e. most
materials are very incompressible. So elasticity is for an unconstrained
material which is not the situation with a thin layer under a large device.
We tend to think about linear expansions when looking at stresses on devices
not cubical expansion.
When I say I think it is because I don't know. If you can catch the
intonation on 'know' in an email.
Rex
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Robert Kondner
Sent: 30 November 2009 15:13
To: [log in to unmask]
Subject: Re: [TN] BGAs & Conformal Coating
I figure the modulus of elasticity also plays an important part?
Bob Kondner
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Rex Waygood
Sent: Monday, November 30, 2009 9:57 AM
To: [log in to unmask]
Subject: Re: [TN] BGAs & Conformal Coating
I am not a materials expert but I've always wondered if a quick glance at
the linear coefficients of expansion was inadequate to evaluate the stresses
applied by a thin layer of organic material under a large device.
At the centre of the device the edge may appear to be at 'infinity'
therefore an elemental cube of the material at the centre will experience a
linear expansion 3 times the unconstrained linear expansion. This makes the
assumption that the bulk modulus (k) of the material will be very high.
Can anyone tell me if my wonder is right or wrong?
Rex
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Douglas Pauls
Sent: 30 November 2009 13:29
To: [log in to unmask]
Subject: Re: [TN] BGAs & Conformal Coating
Good morning Juan,
I suspect that Xilinx wants you do do a thermal cycle study on solder joint
reliability when conformal coating gets under the BGA. The stresses brought
about by a mismatches in the coefficient of thermal expansion
(CTE) can rip a BGA off the board and can severely deform the balls.
Hillman and I did such a study here a few years ago. I can share a few
pictures of what happens, if Steve Gregory would be kind enough to forward
his present e-mail address for the photos. Not sure I have the right one.
Doug Pauls
"Juan T. Marugán" <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
11/26/2009 11:53 AM
Please respond to
TechNet E-Mail Forum <[log in to unmask]>; Please respond to "Juan T. Marugán"
<[log in to unmask]>
To
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cc
Subject
[TN] BGAs & Conformal Coating
Hi,
I've read in a Xilinx guide that: "Xilinx has no experience or reliability
data on flip-chip BGA packages on board after exposure to conformal coating.
It is
recommended that the end-user should characterize the board level
reliability performance of Xilinx packages before production use."
Does anybody know any infomation or study related with this topic?
Thank you.
Juan T. Marugán
Indra Sistemas SA
Espańa
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