New Pitch for Microelectronics

CIC capillary design improves UFP bonding applications

—by Yair Alcobi, Bonding Tools, Kulicke & Soffa Industries

The consumer market demand for smaller and more complex electronic devices is driving the wire bonding industry to set new ball bonding limits. The demand for further miniaturization in microelectronic assemblies, higher density interconnections, lower cost manufacturing processes and higher reliability performance will keep pushing the set limits into ever smaller scale. In ultra fine pitch (UFP) packaging, the following process implications should be considered:

• As the ball diameter to bond pad area ratio increases, there is a need for tighter variation control for high ball placement accuracy.

• Tighter looping control becomes critical, especially when using smaller wire diameters.

• To improve first-bond adhesion, the highest shear strength and Inter-Metallic Coverage (IMC) levels must be achieved without damaging the device.

Conventional small diameter wires are unable to support the longer loop lengths necessary in today’s ultra fine pitch wire bonding applications. They exhibit increased wire sways and wire leaning, resulting in poor bonding performance. While larger wire diameters can provide better wire looping control and flexibility, they would inevitably produce a larger ball diameter than allowable by design.

The CIC Design

To permit the use of larger wires in fine pitch and UFP applications, Kulicke & Soffa Industries, Willow Grove, PA, has developed a new capillary that improves production yields and bonding performance. Designed with a distinctive inner configuration, K&S’ new CIC (Contained Inner Chamfer) capillary captures most of the additional gold generated from a free air ball during bonding in its inner chamfer, instead of allowing the gold to expand into the bond pad area during the first bond formation.

Standard capillaries deform a significant portion of the ball volume under the face of the capillary to achieve the high strength required for a ball bonding process. During the deformation under the capillary face, significant ball control is lost. As ball pad placement is very tight in UFP applications, even the smallest uncontrolled variation outside the pad can result in yield loss of the bonded device.

By capturing a significantly larger portion of the ball volume within the capillary chamfer, the CIC capillary provides better diameter control and higher strength. Very little deformation occurs under the CIC capillary face as it is controlled within the capillary chamfer. The CIC maximizes ball-on-pad accuracy, which is critical to UFP applications where bond pads are closer.

The CIC design concept is based on the optimization of the capillary inner geometry to generate the best bond for an application. It provides an optimum combination of IC angle, IC internal length, CD, and the hole diameter. As depicted in the images at the beginning of this article, note that the CIC produces a contained bond with a chamfer diameter that is slightly smaller than the first bond diameter. FAB and capillary inner chamfer volumes are almost equal.

The CIC design addresses many of the UFP technical challenges that conventional wires cannot. Significantly, the CIC improves first bond USG uniformity. As the CIC provides a bigger IC area, there is a better USG transmission to the bonded ball. In addition, better IC angle consistency improves wire bonding control and portability.

The CIC capillary also enables the use of up to twice as large a range of USG level than conventional capillary designs. A larger range provides for a more robust wire bonding process window. This provides for better process portability between bonding machines, resulting in a higher UPH per machine as well as reduced machine idle time.

The CIC also allows for a more consistent and repeatable ball bond IMC for reduced pad peeling, pad cratering and non-stick occurrences. While conventional capillaries typically provide an average coverage of 75%, the CIC extends the bond coverage to more than 80%, achieving higher shear volume for stronger overall bonds.

Test Results in UFP Applications

The CIC supports all the latest UFP processes of 50 µm bond pitch and below. While the capillary is mainly targeted at UFP applications, it can also be produced in ATLAS and TA materials for larger pitch applications. This solution has been tested and implemented on various UFP applications, both in lab tests and in mass production at customer’s facilities.

In an internal K&S lab, the CIC was compared to a conventional capillary (0.8 mil wire) in a 45 µm application using a 3 x 3 BGA test device. A Maxum bonder was used for bonding. While both the conventional and CIC designs achieved their process requirements, the CIC capillary showed a better response in terms of ball size, ball strength and inter-metallic coverage. The charts and graphs below outline the parameters and results of the lab test.

Application Specifications & Results

Machine parameters

Owing to force behavior differences as well as IC contained area differences between the conventional capillary and CIC capillary design, the stress upon the contained bond is influenced (P=force/area). Thus, the shear results using the CIC design are higher than the conventional one.

As indicated by the results, the CIC provides for better looping control and higher first-bond shear strengths, dramatically improving bonding performance over the conventional capillary.

In another laboratory comparison of a CIC capillary with a conventional capillary (0.8 mil sire) in a 50 µm application using a 3 x 3 on BGA test device, ball shear was greater, while ball size and height were slightly smaller. The charts below outline test parameters and results.

Application Specifications & Results

Machine parameters

Overall, the CIC consistently provides better process control over conventional capillaries in both fine and ultrafine pitch applications by providing higher ball shear strength and consistent first bond formation.

CIC Benefits for UFP Applications

By improving ball diameter control, the CIC capillary enables the use of larger diameter wires for UFP applications of 50 µm and below, reducing the problem of wire sweeps and short failures. To meet looping specifications in UFP applications, the wire loop profile must be smooth without S-ings or wire sway. By providing increased wire stiffness and decreased capillary hole diameters, the CIC significantly improves yields by reducing wire sways or shorting. The CIC also provides the concentricity and consistency needed for better first bond diameter bond control.

An additional benefit of the use of larger diameter wire in UFP applications is improved electrical conductivity, providing better electrical performance in the bonded device.

The CIC capillary addresses many of the technical challenges now posed by UFP applications. While processes of 50 µm pad pitch and below represent only 10% of today’s packaging applications, that number is expected to increase to more than 30% in 3 years. Although thinner wires do offer the advantage of cost reduction for simple straightforward capillary design, the CIC overcomes the problems of wire sway and leaning that occur when dealing with below 50 µm pad pitch applications. Use of larger wire diameter produces better yields, in relation to wire leaning and looping, and delivers higher first bond shear results. Its consistent, repeatable and higher percentage inter-metallic coverage reduces pad peeling, pad creating and non-stick occurrences. By providing the capability to use thicker wires, this premium process solution improves package electrical performance. The CIC is in mass production internationally and is available in various models or designs to meet 35 to 50 µm BPP applications specifications. Currently, 50 µm pitch wire bonding is in full-scale production, with 40 µm bonding in the late stages of qualification.

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