Page 140 - Wire Bonding in Microelectronics

118 Cha pte r F o u r

bonded area will be reduced to the point that a bond pull test will
adequately evaluate the strength of a ball bond. A pull test criterion
can be based on 75% welded area (percentage of intermetallics) of the
ball. When this area becomes equivalent to the wire diameter, then
the pull test will adequately evaluate the ball-bond strength, and then
that test can be substituted for the ball-shear test. (We note here that
low adhesion of the bond pad metallization to the substrate can fur-
ther lower the pull-failure force and may result in lifted metallization;
a condition that many current specifications reject.) Such changed
pull criteria for Cu/Lo-k and fine pitch chips will be discussed in
detail in Chap. 9.
Area array bonding has recently been introduced (see Chap. 9).
Testing them is similar to testing multiple tiers of bonds in a package.
The hook can only be inserted by pulling the top layer, then going to
the next layer down. It can be further complicated by fine pitch. Usu-
ally the bond numbers are so great that it becomes impractical to pull
or shear test them, except on test structures, and then bonded only on
one layer. This method is also applicable ball shear testing of area
array ball bonds.
Visual inspection, which in the past has been used for some high-
reliability and space devices, was often carried out along with
mechanical testing. It is much more difficult or impossible to perform
on fine-pitch bonds, and also the acceptance criteria are changed.
Fine-pitch ball and wedge bonds may appear different from past
accepted requirements for courser pitch. The former have smaller
diameters and bond height, and they may be conical instead of
squashed ball shape (Fig. 4-19). Wedge bonds made with high fre-
quency US can be narrower, but still strong. Both would be rejected
under older course pitch visual inspection criteria (e.g., in MIL- STD-
883G/H and earlier editions). Such criteria has been recently (2008)
revised, and any visual criteria can now be compatible with testing
new fine pitch bonds, as well as many contemporary commercial
specifications. (See Chap. 9 for pull testing of ball bonds on Cu/Lo-k
and other fine pitch bonds.)

Appendix 4A Typical Failure Modes of the Ball-Shear
Test (Failure Mode 2 Is the Normal
Desired Test Result)