11 Fundamentals of Solder Alloys in 3D Packaging
333
presents the compositions of the compounds formed and indicates the formation of
Cu 3 Sn and Cu 6 Sn 5 in the as bonded micro joint. The thin Cu 3 Sn was formed between
Cu 6 Sn 5 and Cu (pillar or trace). Filler was introduced by the paste of the TCNCP
(thermal compressive non-conductive paste) process.
11.2 The Solder Alloys in Microbump
A variety of solder alloys have been commercialized and in use for the conventional
C4 solder bump and the BGA solder ball. The solder balls are discrete products
of which the compositions are adjusted by molten alloying. The alloying technologies are feasible for producing whatever the desired compositions. Accordingly, a
number of solder ball compositions have been commercialized, especially after the
patent concern of the Sn–Ag–Cu solder no longer exists. The C4 solder bump is being
produced via WLP (wafer level packaging). The WLP can adopt either stencil printing
using solder paste or electroplating. The solder in the stencil printing process is in
the form of solder powder of which the composition is generally alloyed by molten
technology and thus can feature a variety of composition. However, the composition
of the electroplated C4 solder bump and similarly the solder cap of 3D microbump
produced by electroplating is restricted by the redox potential of the elements especially when alloying is proposed. Accordingly, there are limiting compositions of
the solder cap reported. Table 11.1 lists the solder alloys reported for microbump.
It is apparent that the composition of the microbump is very much restricted. The
most frequently mentioned solders for the microbump are pure Sn or Sn–xAg. The
plating process of Sn and Sn–xAg is commercially mature in terms of plating bath
and plating conditions. Pure Sn bump is rarely used for C4 bump, while the Sn–xAg
with relatively low Ag content has been familiar to the electronic industry especially
for the ductility consideration.
Sn–Ag is eutectic system with intermetallic compound. The eutectic composition
is Sn3.5Ag. In the high Ag content region, with Ag content greater than 3.5%, the
system will form Ag 3 Sn intermetallic compound. A high fraction of the compound
in the solder tends to downgrade the ductility of the solder joint. Thus the Sn–Ag
solders being adopted are in the low Ag content range, e.g., with Sn2.5Ag or less Ag
contents. Some investigations even applied pure Sn. The Sn–Ag–Cu ternary solder in
3D IC practice is seen less mentioned than in conventional C4 bump. The Cu content
may complicate the intermetallic compound formation. The Sn–Bi solder is applied
only when very low bonding temperature, reflow or thermal compressing bonding is
needed. The incorporation of Bi may raise the concern of embrittlement.
333
presents the compositions of the compounds formed and indicates the formation of
Cu 3 Sn and Cu 6 Sn 5 in the as bonded micro joint. The thin Cu 3 Sn was formed between
Cu 6 Sn 5 and Cu (pillar or trace). Filler was introduced by the paste of the TCNCP
(thermal compressive non-conductive paste) process.
11.2 The Solder Alloys in Microbump
A variety of solder alloys have been commercialized and in use for the conventional
C4 solder bump and the BGA solder ball. The solder balls are discrete products
of which the compositions are adjusted by molten alloying. The alloying technologies are feasible for producing whatever the desired compositions. Accordingly, a
number of solder ball compositions have been commercialized, especially after the
patent concern of the Sn–Ag–Cu solder no longer exists. The C4 solder bump is being
produced via WLP (wafer level packaging). The WLP can adopt either stencil printing
using solder paste or electroplating. The solder in the stencil printing process is in
the form of solder powder of which the composition is generally alloyed by molten
technology and thus can feature a variety of composition. However, the composition
of the electroplated C4 solder bump and similarly the solder cap of 3D microbump
produced by electroplating is restricted by the redox potential of the elements especially when alloying is proposed. Accordingly, there are limiting compositions of
the solder cap reported. Table 11.1 lists the solder alloys reported for microbump.
It is apparent that the composition of the microbump is very much restricted. The
most frequently mentioned solders for the microbump are pure Sn or Sn–xAg. The
plating process of Sn and Sn–xAg is commercially mature in terms of plating bath
and plating conditions. Pure Sn bump is rarely used for C4 bump, while the Sn–xAg
with relatively low Ag content has been familiar to the electronic industry especially
for the ductility consideration.
Sn–Ag is eutectic system with intermetallic compound. The eutectic composition
is Sn3.5Ag. In the high Ag content region, with Ag content greater than 3.5%, the
system will form Ag 3 Sn intermetallic compound. A high fraction of the compound
in the solder tends to downgrade the ductility of the solder joint. Thus the Sn–Ag
solders being adopted are in the low Ag content range, e.g., with Sn2.5Ag or less Ag
contents. Some investigations even applied pure Sn. The Sn–Ag–Cu ternary solder in
3D IC practice is seen less mentioned than in conventional C4 bump. The Cu content
may complicate the intermetallic compound formation. The Sn–Bi solder is applied
only when very low bonding temperature, reflow or thermal compressing bonding is
needed. The incorporation of Bi may raise the concern of embrittlement.