Page 94 - Wire Bonding in Microelectronics

Bonding W ir e Metallur gy and Characteristics 73

category. Whereas, the current carrying capacity of typical PC board
conductors may be large (several amperes), MCM conductors over
thin film dielectrics, may have small cross sections (8 to 25 µm widths
by 4 to 10 µm thickness) and can be embedded in thermally insulating
polyimide, BCB, etc. These can heat up rapidly as the current increases.
However, long before conductor burn-out occurs, the adhesive that
bonds the conductor to a PC board, or the polymer surrounding the
device conductors, may be thermally damaged and cause future reli-
ability problems. Thus, specifications are usually given in terms of
maximum permissible temperature rise rather than maximum current.
The current capacity of vias and other constrictions must also be con-
sidered, as well as solder joints which could melt. Under worse condi-
tions, the PC board or SIP insulating material could burst into flames.
There have been many studies of the safe current carrying capacities of
PC boards. See, for example, Refs. [3-36 to 3-38].
A concise summary of Cu wire advantages and problems was
given in a technical publication by Coors/Gaiser/Tool. It is included
as App. 3B. Many studies are going on to implement 1.0 mil or less
diameter wire.

Appendix 3A
A LISTING OF USEFUL ASTM∗ STANDARDS AND SPECIFICA-
TIONS ON BONDING WIRE AND BOND TESTING (All of these
were updated in 2006 and reballoted.)

(1) ASTM F 72-06, Standard Specification for Gold Wire for Semi-
conductor Lead Bonding. (Lists typical chemical dopants,
breaking loads, elongation, dimensional tolerance, metal
spool dimensions, permissible curling and twisting and in an
appendix, lists the aging characteristics—discussed above.
Note that many new Au wire dopants are proprietary and are
not given in this specification and some may be added up to
1% concentration by weight.)
(2) ASTM F 205, Standard Test Method for Measuring Diameter
of Fine Wire by Weighing. (Discusses calibration, precision
and calculations for both Au and Au, as well as for other
materials.)
(3) ASTM F 219, Standard Test Methods of Testing Fine Round
and Flat Wire for Electron Devices and Lamps. Describes meth-
ods of measuring the tensile strength and elongation, electrical
resistivity, and out of roundness, using 25.4 cm (10 in) lengths.

∗American Society for Testing Materials 100 Barr Harbor Drive, West
Conshohocken, PA 19428.