Chapter 22 Materials Selection and Design Considerations

Figure 22.29
Photograph of a
leadframe on which
the central plate and
contact leads are
labeled. 2�.
(Leadframe supplied
by National
Semiconductor
Corporation.
Photograph by
Dennis Haynes.)
22.17 Die Bonding • W121
and die-bonding adhesive must also be thermally conductive to facilitate the dissipation
of heat generated by the IC. (3) A coefficient of thermal expansion comparable
to that of Si is highly desirable; a thermal expansion mismatch could destroy
the integrity of the bond between the IC and the central plate as a result of thermal
cycling during normal operation. (4) The leadframe material and substrate must
also adhere to the die-bonding adhesive, and the adhesive and substrate must also
be electrically conductive. (5) A secure and electrically conductive joint between
the leadframe and the connecting wires must be possible. (6) The leadframe must
be resistant to oxidation and retain its mechanical strength during any thermal cycling
that may accompany the die-bonding and encapsulation procedures. (7) The
leadframe must also withstand corrosive environments at high temperatures and
high humidities. (8) It must be possible to mass produce the leadframes economically.
Normally, they are stamped from thin metal sheets.
Several alloys have been used for the leadframe with varying degrees of success.
The most commonly used materials are copper-based alloys; the compositions,
electrical and thermal conductivities, and coefficients of thermal expansion for two
of the most popular ones (C19400 and C19500) are listed in Table 22.6. For the most
part, they satisfy the criteria listed in the preceding paragraph. Also listed in the
table are the compositions of two other alloys (Kovar and Alloy 42) that have been
used extensively in leadframes.The desirability of these latter two alloys lies in their
relatively low coefficients of thermal expansion, which are closely matched to that
of Si [i.e., 2.5 � 10 ]. However, from Table 22.6 it may also be noted that
�6 (�C) �1
both electrical and thermal conductivities for Kovar and Alloy 42 are inferior to
the conductivity values for the C19400 and C19500 alloys.
22.17 DIE BONDING
The die-bonding operation consists of attaching the IC chip to the central supporting
leadframe plate. For the copper alloys noted in Table 22.6, attachment may be made
using a gold–silicon eutectic solder; however, melting of the solder requires heating
the assembly to 500�C (900�F).
Another adhesive possibility is an epoxy bonding agent, which is normally
filled with metal particles (frequently Ag) so as to provide both a thermally and