Hi Ryan!

You are correct up to a point, it depends who made the CSP. The term µBGA® 
(or Micro-BGA) is actually a trademarked name by Tessera. If it's a Tessera 
CSP, the way they package the silicon takes into account the CTE mismatches 
between the materials. You can learn more about it at: 
http://www.tessera.com/techframe.htm
There's all kinds of papers there that explain how they do that. They do say 
that alternative CSP's (other companies CSP's) that construct the package 
from a ceramic or like material, then an underfilling would be necessary.

Tessera uses elastomeric materials and polyimide film during the construction 
of the device to allow the package to absorb the flexure of dissimilar 
materials. They've licensed their technology to many, many companies...see 
below:

Contract Assembly Licensees:
 
Amkor Technology   
Advanced Semiconductor Engineering (ASE) 
ChipPAC  
ChipMOS Technologies 
Integrated Packaging Assembly (IPAC)   
Hitachi Cable 
Mitsui Hightec 
Siliconware Precison (SPIL) 
Shinko Electric Industries 
 
The following semiconductor manufacturing companies are presently supplying 
integrated circuits in µBGA® packages:

AMD 
Hitachi, Ltd. 
Hyundai Electronic 
Intel 
LG Semicon 
Samsung 
Sharp 
Infineon Technologies 
Sony 
ST Microelectronics 
Texas Instruments 
Toshiba Corporation

So before I'd go through the trouble of underfilling, I'd find out what kind 
of CSP it is...
(so far, I haven't underfilled any µBGA® yet...and I've built three different 
assemblies that have used them)

Just my thoughts,

-Steve Gregory-
 


<< Ed,
         The CTE of the plastic on micro BGA's is no longer close to that of
 FR4 when coupled with a silicon die.  Rather, it is more akin to the silicon
 die itself.  (You can imagine what would happen to the silicon die if it
 were to expand as fast as the plastic.)  This phenomenon occurs with normal
 plastic BGA's as well.  That is why you don't see many full grid array
 PBGA's.  Usually, just thermal and ground pins are placed at the center of
 the BGA where the "Distance from Neutral Point (DNP)" is close to zero.
 Otherwise, you can get an idea of how large the die is by looking at where
 balls are missing.
         On any CSP (micro BGA), the size die is very close to that of the
 package, the higher strength of the die will dominate the plastic, so the
 CTE will be close to the die.  Hence, the justification for trying to
 reinforce solder joints with an underfill.
 
 Regards
 Ryan Grant
 > -----Original Message-----
 > From: Brunker Ed [SMTP:[log in to unmask]]
 > Sent: Monday, February 21, 2000 10:23 AM
 > To:   [log in to unmask]
 > Subject:      [TN] PLASTIC MICRO-BGA
 >
 > Micro BGA underfill. Some people underfill plastic micro BGAs. The logic
 > says that plastic components reflect the CTE of FR4 substrates and thus
 > you
 > would not underfill. Yet serious companies do underfill. Why???
 > Regards
 > Edward Brunker
 > >>

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