Bond Wire and its Characterization at RF Frequencies

Parent Category: 2017 HFE

By Kamaljeet Singh, AV Nirmal

Abstract: This article details the role of bond wires generally employed in hybrid Microwave Integrated Circuits (MICs) and Monolithic Microwave Integrated Circuits (MMICs) for connections. Extra parasitics introduced by bond wires in the form of inductance in series with resistance is characterized accurately at selected frequencies to achieve the desired performance. Moreover, the number of bond wires, their height from substrate, frequency, and dimensions play an important role in overall circuit performance especially at high RF frequencies. Theoretical equations predicting behavior along with examples, are presented to choose the optimum number of bond wires.

Introduction

Bond wires make up an integral part of IC/MMIC packaging for making connections to other circuitry such as active devices and for input/output connections. Most RF and microwave circuits and subsystems use bond-wires to interconnect components such as lumped elements, planar transmission lines, solid state devices and integrated circuits (ICs) [1]. The main parameter of bond-wires is the associated inductance which contributes significantly at higher RF frequencies. Bond wire is considered an extra inductance in circuit level simulation. It is also employed in hybrid MICs to make connections from the innermost section of the printed inductor [2] or connecting active devices. Optimizing performance of MICs is partially accomplished by extending or trimming the bond wire connection between the components. Bond wire can be single or multiple which is chosen depending on applications. In MMIC/MIC packaging, bond wires are used to connect input and output connectors to reduce the iterative and time consuming process of making a new mechanical housing. Usage of multiple bond wires also provides redundancy in case of breakage of a single wire. Published literature generally deals with multiple bond wires with consideration for reliability and current carrying capability. This article illustrates how bond wires are modeled and provides approaches for determining the optimum number of bond wires.

Compared to ribbon, bond wire has inherent advantage of higher quality factor with the ribbon width equals to wire. Further the selection of the bond pad is of utmost important as the capacitance associated with the pad should not resonate with the inductance of the wire at the frequency of operation resulting in performance degradation. Simple lumped model incorporating the inductance value, series and radiation resistance along with stray capacitance is proposed in the article. Further incorporation of the bond wires in packages and optimum selection for meeting the RF performance is detailed in this article.

Bond Wire Concept

Bond wires primarily introduce inductance at high radio frequencies which must be taken into design consideration. The equivalence of a bond wire is the lumped inductance in series with the resistance. Multiple bond wires are also employed in some applications for reliability criteria but in case of RF it is linked with the reduction of inductance and resistance. Also, the associated inductance and resistance value of the bond wire varies with frequency. The effect of self-inductance and multiple wire mutual inductance is also significant at higher frequencies. The inductance associated with the bond wire depends on:

• Wire diameter
• Wire metal
• Length of wire
• Frequency of operation
• Thickness of wire
• Height above the substrate
• Separation between wires

Generally gold material is chosen as it has higher fusing current compared to copper and aluminium. Further current carrying capability is enhanced with the incorporation of multiple bond wires. In case of multiple bond-wires (n), the net inductance is 1/℘n times of the single wire. The main criteria are the associated radiation at higher frequency can be taken into consideration for achieving a close correlation between simulated and measured results.

As stated before, single wire can be modelled as inductance with series resistance. The radiation associated with higher frequencies is modelled as resistance G shunted with stray capacitance Cs.

Since inductance is determined by the magnetic flux external to the wire, the variation in the wire cross section dimensions has little effect. Bond wires with smaller cross-section area have larger inductance due to generation of more magnetic flux. The free space inductance of a wire of diameter d and length ℓ(um) is given as:                                                                             Note: In the present case, there is air beneath.

Considering bond wires at a height of h [3], the given equation is modified as (h, ℓ, d are in mils)

The wire resistance in series can be written as (r is radius, d is the diameter, σ is the conductivity):

R_r associated at low frequency is in pico ranges which at Ka-band increase to milli ranges value. C_p is associated with capacitance associated between multiple bond wires and generally is in pF range. Effect of higher frequency results in enhanced resistance associated with the bond wire which results in higher losses.

RF Performance Characterization

Wire bonds are used extensively in packaging technology for chip to package or chip to RF connection bonding purposes. At higher frequency ranges, it also has some detrimental effect on RF performance. It can be modelled as a parasitic inductor in series with some resistance, if its dimension is shorter than the operating wavelength. When the RF current cycle increases and decreases in a sinusoidal fashion, the magnetic field surrounding a bond-wire expands and contracts, inducing a time varying voltage. In bond wire, inductance exists both internally (L_int) as well as externally (L_ext) and with the increase frequency the internal inductance (L_int) decreases and external inductance increases due to skin effect (L_e= L_int+ L_ext). Also the effect of mutual inductance (L_m) is to be taken into consideration for multiple bond wires. The mutual inductance is larger for narrower spacing due to the enhanced magnetic coupling whereas diameter of the wire affects series resistance.  Wire bond inductance can be represented as (S is the spacing between wires):

Ideally, bond-wires should be equally spaced, to avoid interactive coupling fields affecting the device performance. The skin-depth phenomena also is a factor. So, the dimension of the wire becomes very important. At frequencies above 6 GHz, both the substrate and mutual capacitances play an important role. To optimize the number of the bond-wires needed between RF transition and DUT (Device under Test), a parametric simulation study was performed. Figure 1 shows a generalized schematic diagram of the bond-wire connected package portion.  Figure 2 shows a graphical illustration and actual circuit photograph employing a conventional microstrip to coplanar circuitry transition.

Figure 1 • Illustration of Bond Wires with Reference Plane and the Equivalent Model.

Figure 2 • Schematic Representation and Actual Circuit with Bond Wires.

The length of the bond wires also affects the insertion loss and 1.5 dB/mm loss is observed at Ka-band frequencies. In this case d=1 mils, ℓ=10 mils and g=20 mils

At higher frequencies inductance value decreases due to skin effect phenomena thus L_int decreases. But the resistance associated with inductance increases due to enhanced surface resistance and radiation. Also, it is preferred to install an odd number of bond wires (>1) to realize optimum performance as mismatch losses are reduced considerably.

Due to the image current introduced by the ground plane, the overall inductance is reduced due to cancellation of mutual and external inductance. This is collaborated with an experiment by inserting a metallic block acting as a ground plane beneath the wire bonds as shown in Figure 3.

Figure 3. Illustration of Bond Wire with Metallic Ground Plane Insertion.

Table 1 • Losses Associated with Bond Wires at Ka-band.

As shown in Table 1, the minimum number of bond wires required is 3 for improved RF characteristics. A combination of parallel bond wires reduces the overall inductance which is summation of self and mutual inductance. The overall inductance is shown as

To further improve insertion loss, metallic block acting as a ground plane is inserted which reduces the overall inductance (changes from ~ 0.20 nH to 0.10 nH) and results in the improvement of ~1.5 dB insertion losses. This effect is due to reduction of the inductance associated with the image currents phenomena as explained above.

Summary

Modelling and characterization of bond wires using simulation and practical results is explained in this article. Simple lumped modelling of bond-wire incorporating various aspects are detailed. Bond wire plays an important role in device performance since at higher frequencies inductance and resistance associated with it causes degraded performances. It was also found that odd number of bond wires along with reduced gap height results in better RF performance.  The optimum match at Ka-band frequency is discovered resulting in better impedance match. Authors believe that this study will pave a way for more accurate prediction of RF performance at higher RF frequencies using the predicted models and parameters.

Acknowledgement: The authors wish to thank their colleagues at SCL and Deputy Director SPA,ISAC and Director, ISAC for their constant support and encouragement.

References

1. RF Measurement of Die and Packages, Scoot A Wartenberg, Artech House

2. Lumped Elements for RF and Microwave Circuits, Inder Bahl, Artech House

3. CAD for Microstrip circuits, K.C Gupta, Ramesh Garg, Chaddha, Artech House

4. Ayan Karamakar and Kamaljeet Singh”, Full-wave Analysis and Characterization of RF Package for MEMS Applications,” Microwave Review, August 2016, pp 17-22

About the Authors

Kamaljeet Singh obtained his M. Tech (Microwaves) from Delhi University in 1999 and was awarded a PhD in 2010. He joined ISRO Satellite Center, Bangalore, in 1999 where he worked on GEO-receivers. From August 2006 to February 2016 he was posted at the Semiconductor Laboratory, Chandigarh, and worked in the areas of RF-MEMS and sensors. He is presently working in the SEG group at ISAC. kamaljs@isac.gov.in,avnirmal@isac.gov.in

A  V Nirmal obtained his B.E. in Mechanical Engineering from Kerala University in 1984. He joined VSSC, Trivandrum, in 1984 and has been working at ISAC since 2004. He is credited with the production and delivery of all subsystems for the IRS and Geosat missions. He was involved in the design and development of various mechanical systems for both launch vehicles and satellites, and the establishment of test facilities. He is presently working as Group Director, Systems Engineering Group, ISRO Satellite Centre.