Page 81 - Wire Bonding in Microelectronics

60 Cha pte r T h ree

Grain structure of gold ball and wire

• Top is the normal wire
grain size. +
Stable
grains
Dissipation of heat Temperature gradient Thermally
stable
zone

• The grains are larger in +
Heat
the heat affected zone. Grain affected
growth zone (HAZ)
area

EFO
• Very large grains + Very remelted
resulting from EFO- coarse zone
grains
melting of the gold wire.

Grain structure for low loop wire
(a) (b)

FIGURE 3-6 (a) Sketch of the grain structure for a gold wire before and after ball
formation, showing the heat-affected zone. (Courtesy of H. Chia, AFW, Inc.)
(b) An etched gold wire and ball revealing the actual grain structure. (Courtesy of
K&S wire.)

Some of the metallurgical problems associated with gold ball
bond formation and wire looping have long been studied [3-5]. Those
problems are (1) clubbed ball formation (ball not centered on wire);
(2) grain growth and weakness of wire above ball; (3) neckdown for-
mation and leadframe tie bar severance; and (4) wire scratching.
These do not occur often in usual situations, and most autobonders
are normally programmed to avoid them (3). Also, in some cases (2)
and (3) wire metallurgy is being improved to limit the problems. Nor-
mal maintenance usually eliminates wire surface damage (4). Never-
theless such problems do occur enough to note and Table 3-1 can be
used indicate some of the approaches for troubleshooting.
Specific troubleshooting information on wire and bond tool prob-
lems can often be obtained from the wire and bonding tool manufac-
turers. Several of these have extensive catalogues that devote part of
the space to such problems as well as to general technical explana-
tions. Often they have very useful Web sites, some are listed in fur-
ther reading at the end of the book.