14 Fundamentals of Advanced Materials …
419
die side, two, it has to prevent the copper underneath from forming an IMC under
thermal and electrical stress, commonly called electromigration.The phenomenon of
electromigration in simple terms is the migration of atoms along the current gradient
from the cathode to anode (-ve polarity), there by depleting the substrate of copper and
causing opens. More details about electromigration and the prinicples can be found
in another chapter in this book. Typically ENEPIG surface finishes are expected to
perform well in the face of electromigration. Historically lead based solders were
used which had a naturally superioe performance against electromigration due to a −
ve z factor for Pb (details can be found in the chapter that describes electromigration
mechanism). However due to health and environment concerns, moving to lead free
solders triggered a great deal of research and development of surface finishes for
solderability and reliability of interconnections. Quite a lot of work has been done
in trying to understand the influence of surface finishes on the soldering reaction
in the case of lead free solders, the kinetics of intermetallic formation, influence
of phosphorous content on the behavior of the joint under electrical and thermal
stress, etc. More details can be found in references [24–28]; these references will
service as a guide and direct the reader to many more studies that were undertaken
on understanding the behavior of ENEPIG alloys primarily. One thing to keep in
mind is that while ENEPIG technology has been around for >40 years now, the
process of plating surface finish is definitely mature. However, the interactions with
interconnects and interconnect reliability keeps constantly evolving as the current
demand per bump increases. Hence updating oneself with progress in surface finishes
for electronic packaging is important.
Detailed discussion of all kinds of surface finishes can be found in many printed
circuits handbooks such as the one edited by Clyde Coombs. The properties of
same type of surface finish can be somewhat different among different suppliers.
The specifications of different surface finishes in Table 14.3 from DYCONEX
®
are just examples of commonly available finishes in the market. Some vertically
integrated companies sometimes have their in-house developed proprietary surface
finish compositions.
14.3.8 Summary
In this subchapter, wide varieties of base materials used for today’s organic substrates
and PWBs were discussed. The discussion focused on two things, one, the importance
of the properties of materials used in substrate fabrication and secondly a primer on
the fundamentals of chemistries used. The purpose of this discussion is to help
reader to get an idea of the type of materials used in the substrate manufacturing
process, the drivers for their evolution, the expectations from the next generation set
of materials and pointers towards selecting the right material sets for their choice of
application. The substrate material advancement enables the overall substrate and
PWB technology towards smaller, faster, and cheaper feature sets. This trend is
relevant to the entire packaging industry, 2D, 2.5D and 3D alike. It is possible that
419
die side, two, it has to prevent the copper underneath from forming an IMC under
thermal and electrical stress, commonly called electromigration.The phenomenon of
electromigration in simple terms is the migration of atoms along the current gradient
from the cathode to anode (-ve polarity), there by depleting the substrate of copper and
causing opens. More details about electromigration and the prinicples can be found
in another chapter in this book. Typically ENEPIG surface finishes are expected to
perform well in the face of electromigration. Historically lead based solders were
used which had a naturally superioe performance against electromigration due to a −
ve z factor for Pb (details can be found in the chapter that describes electromigration
mechanism). However due to health and environment concerns, moving to lead free
solders triggered a great deal of research and development of surface finishes for
solderability and reliability of interconnections. Quite a lot of work has been done
in trying to understand the influence of surface finishes on the soldering reaction
in the case of lead free solders, the kinetics of intermetallic formation, influence
of phosphorous content on the behavior of the joint under electrical and thermal
stress, etc. More details can be found in references [24–28]; these references will
service as a guide and direct the reader to many more studies that were undertaken
on understanding the behavior of ENEPIG alloys primarily. One thing to keep in
mind is that while ENEPIG technology has been around for >40 years now, the
process of plating surface finish is definitely mature. However, the interactions with
interconnects and interconnect reliability keeps constantly evolving as the current
demand per bump increases. Hence updating oneself with progress in surface finishes
for electronic packaging is important.
Detailed discussion of all kinds of surface finishes can be found in many printed
circuits handbooks such as the one edited by Clyde Coombs. The properties of
same type of surface finish can be somewhat different among different suppliers.
The specifications of different surface finishes in Table 14.3 from DYCONEX
®
are just examples of commonly available finishes in the market. Some vertically
integrated companies sometimes have their in-house developed proprietary surface
finish compositions.
14.3.8 Summary
In this subchapter, wide varieties of base materials used for today’s organic substrates
and PWBs were discussed. The discussion focused on two things, one, the importance
of the properties of materials used in substrate fabrication and secondly a primer on
the fundamentals of chemistries used. The purpose of this discussion is to help
reader to get an idea of the type of materials used in the substrate manufacturing
process, the drivers for their evolution, the expectations from the next generation set
of materials and pointers towards selecting the right material sets for their choice of
application. The substrate material advancement enables the overall substrate and
PWB technology towards smaller, faster, and cheaper feature sets. This trend is
relevant to the entire packaging industry, 2D, 2.5D and 3D alike. It is possible that