Hello Ioan,
First a little history, then some comments.
Underfilling BGAs has typically been limited to fine pitch BGAs with
ball pitches of 0.8mm and finer. This was common in the late 90's to
enhance reliability of the connections, particularly for portable/hand held
products, but fell into disfavor due to the impact on the SMT process and
SMT line length increases due to the addition of the dispenser and the cure
ovens. This commonly required board level testing before the underfill
process in case rework of the fpBGA was necessary. This further confused
the line process flow.
When my team at Motorola did the first BGA manufacture (1.27mm pitch)
we studied the impact of underfilling BGAs using the underfill glob top
materials as the underfill since commercially available underfills for flip
chip were not yet available. The extension of temp cycle life was huge, but
it was found that the non-underfilled assemblies met the temp cycle life
requirements, so underfilling was not commonly practiced.
For enhanced thermal dissipation into the motherboard, we began adding
many plated through holes directly under the die, and terminating these PTHs
with solder balls. These balls were generally tied to a ground plane in the
motherboard for heat spreading. It was shown that placing this plane close
to the PCB surface and increasing the Cu thickness to 2 oz, we could get
significant improvement in thermal performance.
I have a patent with Motorola for injecting an underfill through a hole
in an IC package substrate. It works very well and provides for much faster
encapsulant flow and void elimination, so the injection process can surely
work, but dispensing along the BGA perimeter may be a more desirable process
and avoid the need for a hole in the PCB. Surely filling the air gap
between the BGA substrate and a PCB will allow better heat transfer to the
PCB. But, it is questionable how much benefit this will have at the system
level. PCBs can get 'thermally saturated' fairly quickly in a closed
system, especially when you have no air flow, the benefit of heat spreading
in the PCB without the underfill has to be carefully considered. If the BGA
substrate, and the PCB, is designed to enhance the benefit of the underfill,
it may provide enough benefit to justify the added cost. If the substrate
does not have a substantial area of Cu plane directly under the solder mask
on the bottom side, and if the PCB is not similarly designed, it is doubtful
whether you will see much benefit, especially if the BGA and the PCB are
already designed with a lot of thermal vias, thermal balls, and a heavy Cu
plane in the PCB for heat spreading.
The underfill properties are the final issue. Obviously the
encapsulant must be electrically insulative. The ability to find CTEs
(below and above Tg) that provide a good compromise between the CTE of the
solder balls and the PCB will be a challenge. The larger the BGA the more
important the issue. Normal liquid encapsulants do not have high thermal
conductivities, but they are better than air. Encapsulants with fillers
with thermal conductivities higher than fused silica are available. The
fillers commonly may be alumina, boron nitride, or aluminum nitride. These
materials typically have thermal conductivities < 1 W/m degree K. More
exotic materials with diamond fillers may also be found but the return on
the investment may be small. Even die attach adhesives, highly loaded with
silver, typically have bulk thermal conductivities less that 8 - 10 W/m
degree K, although some unique materials exist with higher values. So
electrically insulative encapsulants are likely to have thermal
conductivities below that of die attach adhesives with like fillers.
I suggest getting some thermal simulations done before embarking on
this quest to make sure adequate benefit is achieved. Good luck.
Best regards,
Leo
Leo M. Higgins III, Ph.D.
Vice President, Technical Operations
ASAT, Inc.
3755 Capital of Texas Highway, Suite 100
Austin, Texas 78704
office phone 512-383-4593
mobile 512-423-2002
[log in to unmask]
www.asat.com
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Tempea, Ioan
Sent: Tuesday, May 16, 2006 8:41 AM
To: [log in to unmask]
Subject: [TN] Underfill or else...
Hi Technet,
we have to answer a question from one of our customers: can we inject a
paste under a BGA, through a hole in the PCB, paste that would facilitate
the thermal dissipation?
I know my question is very slim, but this is what I have got for now. Does
it have anything to do with underfill? Can a sort of underfill material be
used for thermal dissipation? Or it is rather a sort of thermal compound
that has to be injected somehow?
If you are familiar with this technology, what are the materials that can be
injected, so that I do my "due diligence" before contacting the customer? We
have Camalot Gemini machines, would they be able to do this thermal paste
thing?
Thanks,
Ioan
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