Improving Wire Bond Strength and Reliability
Since the early days of wire bonding, engineers have been looking for ways improve the strength of the wire bond interconnect and improve its reliability. This is espcially important for industries with high realibility requirements: aerospace and defense, implantable medical device manufacturers, automotive, commercial 
optolectronics and telecommunications (RF/Wireless). For example, a heart pacemaker can't go "bad", a weapons system must hit its intended target...more industries with strict wire bond requirements are popping up all the time. Which brings us back to the point: making strong wire bonds can be the difference between success or failure, profits or loss, and even life or death. This may seem dramatic, but it is not - this is the reality of packaging microelectronics in high reliability environments.
The Security Bump and Stand-off-Stitch
The Security Wire was, for many years, the only way to achieve a stong wire bond. Now with the demand for smaller package size plus even further bond strength, new methods have been discovered. One such method is the Security Bump - it requires a smaller second bond termination site.
Stand-Off-Stitch (SOS), which is Stitch on Bump, and d
efined as 1) placement of a ball bump at one end of the wire interconnect, 2) then placing a wire with another ball at the other end of the interconnect, and 3) stitching off the wire on the previous placed ball bump. SOS has many benefits that can be incorporated into a variety of applications such as circuit design. Here one can accomplish, with Stand-off-Stitch, better wire strength, fewer interconnects (die to die bonding) and lower loops. Reverse bonding is another use of SOS (reversed bonding defined as stitch bond on bump on die bond pad). This often results in a lower loop profile than standard forward wire loop. Also, the loop is much stronger because the wire hasn't been annealed above the ball, in the Heat Affected Zone.
Why the need for Auxiliary Wires?
This paper was inspired by the many customers Palomar has served over the years and the common problems/challenges faced when presented with wire bond application needs. Using Palomar's Versatile 8000 Wire Bonder/Ball (Stud) Bumper, these methods discussed have been developed, tested and proved. Some of these problems/challenges include poorly bondable materials, non-flat bonding surfaces, odd or uncommon packaging situations or just the need for higher reliability.
Motivation for Improving Wire Bonds
- Increase Pull Strength of Stitch Bond
- Improve Failure Mode from Destructive Pull Tests (more Mid-Span and Neck breaks than Tail Breaks even with Poor Materials)
- Improve Bond Strength after High Temp Burn In
Auxiliary Wires, Security Bumps and Stand-off-Stitch were developed to overcome these challenges and provide customers with rock solid packages that can withstand even the harshest environments.
See in Action
Contact us with any questions on High Reliability Wire Bonding. If you are interested seeing Mr. Rasmussen's entire presentation on this topic, register to attend SEMICON West in associated with IMAPS Workshops "Wire Bonding Workshop", held at the San Francisco Marriot on July 14-15.