Page 95 - Wire Bonding in Microelectronics

74 Cha pte r T h ree

(4) ASTM F 487-06, Standard Specification for Fine Aluminum-
1% Silicon Wire for Semiconductor Lead Bonding. (Most of
the same information as in F 72 above but for Al, 1% Si wire.)
(5) ASTM F 584-06, Standard Practice for Visual Inspection of
Semiconductor Lead-Bonding Wire. (This may be discontinued
but is useful as it gives manual procedures and photographs
of acceptable and unacceptably cleaned bonding wires.)

Appendix 3B
Copper Wire Bonding, a Low-Cost Solution to Gold Wire Bonding?
From Industry Newsletter & Technical Publication, Volume III,
Issue 4, July 2005. (Used with Permission of COORS TEK/GAISER
precision bonding tools.)
Introduction: Part one of this discussion covered aspects of the
ball formation; the various ways to achieve size and shape consis-
tency. In this section, the material properties in particular the hard-
ness and the impact on ball bonding will be discussed. The copper
bonding process is not simply about how to make a good free-air
ball but how to achieve good product quality and reliability.
The Facts:
Copper is a good electrical and thermal conductor, better than
gold (see Table 3B-1).
Copper wire is harder than gold (see Table 3B -1).
Copper as a raw material is cheaper and more abundant than gold
(>$0.20/oz. vs. >$800/oz).
Copper oxidizes easily, gold does not.
Copper wire bonding uses a limited number of specially designed
capillaries, gold wire does not.
Copper processing requires special hardware to prevent oxidation,
gold does not.
Copper when bonded to Aluminum pads forms thin inter-
metallic layers, Gold forms thick intermetallic layers (see gold
in Fig. 3B-1).

Typical Parameter Cu Au
−6
Resistivity (×10 ohm/cm) 1.6 2.3
Wire Hardness (HK) >64 <60
Ball Hardness (HK) >50 <39
Looping Excellent Excellent

(HK) = Knoop Hardness
TABLE 3B-1 Characteristics of Cu and Au Wires