Page 43 - Wire Bonding in Microelectronics

22 Cha pte r T w o

2.3 How Ultrasonic Bonds Are Made
(Empirical Description)
Small-ultrasonic welds used in microelectronic interconnections are
usually made with soft, face-centered cubic metal wires (e.g., Al, Cu,
Au) of 25 to 33 µm diameter. There is no generally accepted mathe-
matical model of the ultrasonic welding process. It is a complex
process, and the physics is not fully understood. However, many
efforts have been made to study and/or explain the process [2-1, 2-9,
2-10, 2-18 to 2-21](and modeled in Chap. 11) so an empirical descrip-
tion of the observed bonding process follows. This treatment is mostly
adapted from Harman [2-9] and Harman [2-10].
Ultrasonic Al (25 µm diameter) wedge-bond formation was stud-
ied by examining the bond foot prints left on normal Al bond pads by
bonds that did not stick. These are called bond lift-off patterns and
represent the best method of studying the early stages of bond forma-
tion. Such patterns are made by maintaining the clamping force and
the ultrasonic power constant. The normal bond time is then progres-
sively decreased below the point that the wire will adhere, and it lifts
off. The pattern in Fig. 2-10(A) results from pressing the wire against

Microwelds

0 ms 4
A B
C D

7 10

FIGURE 2-10 Wedge bond lift-off patterns. Patterns are made by maintaining
the clamping force (25 g) and ultrasonic amplitude 0.88 µm (~35 µin) constant
and progressively decreasing the weld time (optimized for 50 ms) below the
point that the wire will adhere. (A) Zero weld time (no ultrasonic energy);
(B) 4 ms weld time; (C) 7 ms weld time; and (D) 10 ms weld time.