Introduction
Palomar Technologies offers a variety of Auxiliary wire options for our Wire/Ball bonder. Auxiliary wires are com-monly defined as an additional wire outside of a normal ball-to-stitch wire bond. Whether it be a pre-wire, generally referred to as stand-off stitch, or a post-wire, also known as a security bump, Palomar offers several options for our customer’s diverse applications.
The motivation behind the use of Auxiliary wires is the inherent weakness of the stitch bond. Due to the much small-er contact area of the stitch to bond pad, thsmall-erein lies the challenge of successfully bonding the wire to non-optimal bond pad metalizations. The decreased reliability of the stitch bond can negatively affect the tearing of the tail at the stitch bond, resulting in inconsistent free air ball sizes during flame-off. Marginal materials, risk of contamination, and non-optimal intermetallic bonding have motivated manufactures to improve stitch bond strength and reliability.
Auxiliary Wires
The earliest attempt to make improvements on the stitch bond have been with the use of a security wire or safety wire which has been an asset for decades. This is essentially a second wire with the first bond being placed on top of the stitch of the previously bonded wire. The security wire will then need to have its second bond terminated to another area or on the same bond pad if room permits. The security wire was eventually phased out due to the Industry’s contin-ually growing need of smaller packages and the ever decreasing space available for termination of the security wire. To reduce this second bond termination area, the security wire is often replaced by a singular ball bump - otherwise known as a security bump or post-wire (Fig. 1, 2). The security bump, however, is not a perfect solution as it lacks a completely homogenous interconnect with boundaries at the bump to wire interface and the bump to substrate interface [1].
Z
R (XY)
Bond 1
Stitch
Standard Wire
Security Bond
Bond 1
Post Wire Security Bump
Stitch Figure 1 - Using a security bump to further secure the stitchbond of a standard forward bonded wire.
Improve Wire Bond Capability
and Reliability through Use of
Auxiliary Wires
Author: Nicholas Miller, Palomar Applications
Engineer
Alternatively, stand-off stitch bonding is more versa-tile than using a security bump with additional bene-fits. Stand-off stitch wire bonding can be incorporated into circuit designs more efficiently, with fewer inter-connects (die-to-die bonding), increased wire strength properties, and lower loop potential for tighter over-head clearances.
Z
R (XY)
Bond 1 Stitch Stitch on Ball Bond 1Standard Wire
Reverse Stand-off Stitch
Pre Wire Ball Bump
Figure 3 - Replacing a standard forward bonded wire with a reverse stand-off stitch to lower overall height of wire with respect to the die and improve wire pull strength.
Stand-off stitch bonding involves a pre-wire or ball bump placed in the same location as a stitch prior to bonding the primary wire interconnect (Fig. 3, 4). When the primary wire is bonded, the termination of the wire is stitched on top of the previously placed ball bump. The result is a near homogeneous stitch-to-ball bond interconnect with an inherent improvement in stitch bond pull strength. An efficient application of the stand-off stitch bond technique is with reverse bonding, where the stitch-to-ball bond is placed on the die bond pad, providing an opportunity to create a lower loop profile that would normally be impossible with a standard forward wire loop profile (Fig. 4). Additionally, the wire is stronger due to a decrease in work annealing in the heat affected zone above the ball during loop formation, which is needed for low-loop applications [1,2]. While lower loop profiles are possible with forward bonded wires, it not easily accomplished without severely decreasing the pull strength of the wire due to a weakened area above the first bond.
Figure 2- Forward Bonded wire with low loop height and secured with a security bump placed over the stitch bond.
Figure 4 - (Left) Forward bonded wire with stitch bond placed on top of a pre-bonded ball bump, more commonly known as a Stand-Off Stitch Wire. (Right) Reverse bonded Stand-off Stitch Wire terminated to a die pad.
Another Auxiliary wire option offered by Palomar, for unique packages with limited bond pad pitch (BPP) and bond pad opening (BPO), is V-bonding (Fig. 5). While V-bonding is not a new concept, Palomar offers a more streamlined and easily programmed option for 2-3 wires to share the same stitch (bond 2) location and can be applied to secu-rity bump, stand-off stitch, forward bonded and reverse bonded wires. Our most common application of V-bonding is with reverse stand-off stitch bonding. A large ball bump, limited by the size of the die bond pad, can support the termination of 2-3 wires, which can reduce the need of larger diameter wires or the challenge of placing multiple wires on the same bond pad with limited space. Oftentimes, placing multiple balls or stitch bonds within a small bond area require specialized fine-pitch capillaries and smaller diameter wires. There is also a risk of damaging the bond of an adjacent wire from contact with the capillary. These types of situations also require high accuracy in the form of micron or sub-micron placement accuracy, which is where V-bonding becomes indispensable as a strong alternative.
Lastly, our chain wire option links multiple wire interconnects without the need to tear at the stitch and flame-off between wire sets (Fig. 6). Chain bonding offers simple solutions for small bond pad pitch and low loop height applications. Palomar’s chain bonding technique can also be used with stand-off stitches where a ball bump can be placed under each of the chain wire’s stitches, increasing the utility of the chain wire. Figure 5- Forward bonded wires with shared Stand-Off Stitch
bonds, also known as, V-Bonded wires.
Forward vs Reverse Bonding
Forward bonding has been the standard for decades but comes with limitations. Typically, a forward bonded wire has the first bond of the wire, the ball, placed on a die bond pad and the second bond is bonded to a substrate bond pad. More often than not, the die bond pad is at a higher position than the stitch bond location which is terminated to a substrate bond pad.
Forward bonded wires are often limited by the height differential between bond locations and the proximity of the stitch bond location to the adjacent wall. Stitch bonds are not typically bonded to die bond pads or other difficult materials due to the inherently weaker stitch bond, which has a smaller contact patch when compared to a ball bond. The smaller contact area of a stitch bond can be challenging when attempting to successfully adhere wire to difficult materials, whether it be due to contamination or metallic bond compatibility (3, 4). While forward bonding is im-proved greatly with a stand-off stitch, there is still the physical limitations relevant to bond direction.
Reverse bonding offers many advantages over forward bonding. By placing the first bond on a substrate bond pad, the wire can be placed much closer to an adjacent wall with reduced risk of wire contact with the die. Reverse bonding of-fers more versatility when packages continue to shrink with ever decreasing die-to-die clearances. Previously difficult or impossible bond environments, such as deep cavities and tight spaces are now possible with reversing traditional first and second bond locations. Reverse looping forms a smooth loop that does not sag and offers the capability of low loops not generally possible with forward bonding.
However, terminating a wire to a die bond pad is challenging and has its own risks, such as, component damage and failure to successfully adhere the wire to the die bond pad. During termination of the wire to a die bond pad, the capillary must come into contact with the die face and subsequent damage to the die or other sensitive components is a risk. There is also an inherent risk of the wire, commonly 99.99% Au, not sticking to die bond pads of various metalizations, due to a small contact area. While the die bond pads consist of compatible metallic compounds for thermosonically bonding with gold, they may not be as optimal as bonding gold wire to a gold bond pad. This is not as much of an issue when bonding a ball with a much larger contact surface area, although, it does provide challenging for the much smaller stitch bond.
The off-stitch, not only solves this problem, but offers more flexibility when using reverse bonding. The stand-off-stitch bonds a ball bump prior to the primary wire interconnect. The ball bump offers the same potentially optimal bond conditions as the first bond of a traditional wire when placed on a die bond pad. The second bond of the wire is then terminated to the ball bump offering a near homogeneous connection between the wire and ball bump. With the use of the ever versatile, stand-off stitch, wire interconnects are no longer limited to traditional bond tech-niques. The termination point of wires can be placed near physical obstructions as the ball bump and stitch bond of a wire interconnect don’t require much lateral movement of the capillary.
Conclusion
The motive to improve wire bond reliability is forever pushing industry to discover and implement improvements and solutions. No matter what your package requirements may be, Palomar offers a wide variety of versatile
solutions. Through the use of auxiliary wires, you can expect increased pull strength of stitch bonds, improved failure mode from destructive pull tests, and improved bond strength after high temp burn in. Die-to-die interconnects are now simpler and easier to achieve, no longer needing a substrate interface. Loop heights can be much lower and are only limited by the final bond height of the stitch-on-ball bond. There is also a significant improvement to wire bond interconnect strength and reliability [1]. This allows for unique package solutions with closely spaced die and stacked die where a reliable and durable wire interconnect is a viable option [5].
REFERENCES
[1] Rasmussen, D. J., & Thompson, R. (2010, January). Improved Bond reliability through the use of Auxiliary Wires (Security Bumps and Stand-Off Stitch). In International Symposium on Microelectronics (Vol. 2010, No. 1, pp. 000474-000478). International Microelectronics Assembly and Packaging Society.
[2] Zhong, Z. W. (2008). Wire bonding using insulated wire and new challenges in wire bonding. Microelectronics In-ternational.
[3] Evans Jr, D. D. (2013, January). Multipurpose Wire Bonding–Bumps, Wires, Combination Interconnects, and Oper-ation Efficiency. In InternOper-ational Symposium on Microelectronics (Vol. 2013, No. 1, pp. 000324-000330). InternOper-ational Microelectronics Assembly and Packaging Society.
[4] Song, Y. K., & Bukva, V. Challenges on ENEPIG Finished PCBs: Gold Ball Bonding and Pad Metal Lift. In 2016 IPC Apex Expo Technical Conference Las Vegas, NV.
[5] Chylak, B. (2006). Advanced Ultra-Low-Loop Wire Bonds Bob Chylak, Lee Levine, Stephen Babinetz, OD Kwon Kulicke & Soffa 2101 Blair Mill Road, Willow Grove, PA 19090, USA.
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