Hi, Phil,
I have been doing quite a bit of investigation into this sort of thing
myself for an application that involves random and sinusoidal vibration,
though not as high as 1500g's (0.04g2/Hz between 5Hz and 2000Hz). BGA's and
their vibration/creep fatigue problems are well known, and we have 256 pin
CBGA's 21 x 21 mm and 35 x 35mm PBGA's with 318 and 388 pins. Unsupported
(no underfill), we had joints cracking and board failures.
The board they're fitted to is FR4, and the boards are high performance,
high value, so the ability to rework them is essential. I got a few figures
for Co-efficient of Thermal Expansion to show the difference between the
die and the substrate:
a) CTE (silicon) : 2-3 ppm/deg-C
b) CTE (BT/substrate : polymide film) : 15 ppm/deg-C
c) CTE (pcb board/pad : FR4) : 15.8 ppm/deg-C
d) CTE (solderball/bump : SnPb) : 28 ppm/deg-C
As you can see there are significantly different CTE's for the various
materials involved.
I have opted to use an underfill material alone to support the BGA solder
joints. Solid potting is not an option for us. I've chosen a 'Flow' type
material as opposed to a 'No-Flow', and one that is reworkable. 'No-flow'
has its problems - because it is applied to the BGA before the BGA is
soldered to the board, it has to act as a flux as well as a filler. It
contains volatiles that are difficult to out-gas during soldering, so voids
are often formed that can suck in molten solder during the reflow process.
This detracts from the solder joints and has been known to lead to short
circuits. It is also much harder to replace a BGA using this material.
The Flow type comes in two basic forms - reworkable and non-reworkable. The
non-reworkable one has better thermal conduction properties, if that's
important, but sets hard and means you have to chizel off the BGA if you
want to remove it. The reworkable type is removable with the BGA at reflow
temperature, and any residue can be wicked of along with residual solder to
clean up the component site.
I have all sorts of bits of information on underfilling if you want to
contact me about it. We have yet to prove the success of underfilling in
practice, but a 5x improvement in BGA solder joint reliability is generally
claimed.
What I would like to find myself, if anyone knows, is a good 'Flow' type
underfill material that doesn't have to be stored at minus 40 degrees C.
Anyone know of such a thing?
Regards
Pete Duncan
ST Aerospace
DUTTON Phil
<phil.dutton@ To: [log in to unmask]
TENIX.COM> cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
Sent by: Subject: [TN] High G PBAs
TechNet
<[log in to unmask]
ORG>
04/30/01
03:50 PM
Please
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"TechNet
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Forum.";
Please
respond to
DUTTON Phil
Hello Technetters,
I am soon to design a PB for one of our customers that will be required to
withstand a shock load of approximately 1500 g's.
Does anyone have any guidelines or advice on how to achieve this?
I'm thinking of solid potting the assembly within a rigid structure as one
solution.
Components are expected to include SMD discretes and BGAs.
Of course there's the consideration of how hard the potting compound needs
to be and what it should be (epoxy, silicone elastomer), as well as what
material the board should be made of, CTE considerations between the board
-
components - compound - housing etc.
Any advice would be most welcome.
Phil Dutton C.I.D.
Senior CAD Technician
IPC Certified Interconnect Designer
Tenix Defence Systems Pty Ltd
Systems Division - Adelaide
Second Avenue, Technology Park,
Mawson Lakes. SOUTH AUSTRALIA 5095
================================
Phone (08) 8300 4400 (reception)
(08) 8300 4481 (direct)
Fax (08) 8349 7420
email [log in to unmask]
Internet Page http//www.tenix.com
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