Advantages of Ribbon and the Impact on the Microwave Market
Interconnecting semiconductors with ribbon rather than round wire has been popular in high frequency electronics fora long time. Surprisingly there are still countless applications bonded traditionally with round wires and ball bonders even so the electrical performance of the product could benefit from ribbon bonding technology. Reasons for this hesitation to adopt ribbon technology can be found in the machine supplier market. Fully automatic ribbon bonders with sufficient specification properties to win by cost of ownership have only been available for a few years. All those applications which could also be bonded with round wire and ball bonders were waiting for the right equipment technology to come forward even if traditional wire bonding was a compromise on the product performance.
The steep increase in the gold price and the continuous demand for gold in the electronics manufacturing market is adding significance to the choice of semiconductor interconnection. Ribbon bonding technology allows reducing the cross-section area of the gold bond while maintaining or increasing the surface area at the same time. Some high frequency electronic packages could be changed from 2 mil round wire to ½ mil x 3 mil ribbon and be produced at lower material cost (less gold volume) with the same electrical performance (same surface area per bond). The development of the gold prices can be expected to make such a change even more attractive in the future.
ADVANTAGES OF THE ELECTRICAL PROPERTIES OF RIBBON BONDS VERSUS WIRE BONDS AT HIGH FREQUENCIES
The most common reason for using ribbon bonds in high frequency applications is the so-called skin effect. Each free electron represents a negative electric charge that naturally separates from other negative electric charges, scattering the signals cannot bridge the gap to the next wire but in the GHz range the wires really start exchanging signals. Again the geometry of the ribbon helps in this case. A ribbon bonded application exchanges less crosstalk than the same application bonded with round wire [2]. This benefit will lead to better signal quality of high frequency devices and less electrical noise on the supply or ground contacts, an obvious plus for satellite or wireless technology, where bundled high frequency information carries signals of voice, radio or television transmission.
MECHANICAL AND OTHER GEOMETRICAL ADVANTAGES OF RIBBON BONDS VERSUS ROUND WIRE BONDS
Further advantages for the package design result from the ribbon’s flat top surface, which can enable stacking several ribbon bonds on top of each other. Stacked bonds are often used in high frequency applications for redundant grounding. Stacking round wires is a less robust process when the bonded wires are placed at different angles, because the round wire on top tends to “roll off” the bottom wire during deformation. The flatter ribbon avoids this complication and allows reliable bond stacking even under 45° angles. Examples from the field of high frequency electronics where this would allow improvements to the performance, design and manufacturability of existing HF circuits.
An X-strap is often desired for connecting a Schottky diode with a very high operating frequency. This design feature should create an even distribution of electrical charges and eliminate the crosstalk between the wires and their neighbourhood. 4 or even 6 equally spread loops going off the diode would be ideal, however the small and fragile bond pad on top of the diode seems to make this impossible. Where round wires would tend to “roll off” and therefore only allow manual bonding with all associated disadvantages, ribbons can be stacked at angles even in automatic mode.
The other example of a design feature demanding angled stacked bonds is a HF package in radial package design with a central common ground point. By tying all surrounding ground leads or die bonding pads to exactly the same grounding point charges cannot build up in one location of aboard. The formation of unintended capacitors by the leads can be compensated most effectively if a common grounding point is used. In a small enough package multiple grounding connections will only fit on a compound bond. Often this is done in manual wire bonding processes to allow operator intervention as the wires roll off the stack. Automatic ribbon bonding has the potential to replace such manual processes because stacking the ribbons at an angle is possible in automatic bonding.
Achieving a comparable loop geometry with round gold wire is at least challenging, with a ball bonder this is virtually impossible because loop sagging would occur at the end of the loop.
For the product, the loop stability discussed above means higher reliability because the mechanical and thermal stress that a product sees during its lifetime will apply pull forces on the loops in any direction. Superior product reliability at high acceleration explains why airborne and space applications were the forerunners in the packaging industry to demand ribbon connections over wire bonds. Some recently introduced electronic devices, such as sensors and recording boxes in automobiles and airplanes are expected to continue to function after a high speed impact. This is one reason manufacturers are adopting ribbon bonding more and more.
In some applications, the compact geometric dimensions of the deformed ribbon, which is only slightly wider than the undeformed ribbon, can help to make best use of the available bond pad area. This becomes especially clear when a single ribbon can replace multiple round wire bonds in applications with a relatively high power per I/O. In addition to the ribbon’s current-carrying capability, which can be adjusted by surface to apply this technology to combat the shadow effect. This with an application where a single ribbon replaces multiple ball-bonded round wires with the same current-carrying capability on a power LED. Bonded wires connecting power LEDs may cast a shadow from light emitted by the LED surface under a shallow angle.
The naturally low lift-off angle and profile of a typical ribbon loop greatly reduces this shadow effect compared to a group of ball-bonded wires that extend vertically above the ball.
This additional light emission increases effective LED luminosity without any change in the LED itself or the power consumption. Any active optical component which could respond to light coming in under a shallow angle can benefit from this effect as much as light emitting components. This could apply to solar cells, CCD chips, displays or laser LEDs.