Technology

Gold Plating Considerations for Microwave Circuits

Dr. K. Ramachandran, Filtran Microcircuits Inc.

Introduction

The conductor layers in virtually all printed wiring boards are made of copper foil laminated onto the dielectric material. Copper is only next to silver in electrical conductivity (Fig. 1) and is readily available in foil thicknesses varying from 1/4 oz/sq ft (350 µin or 9 µm) to 2 oz/sq ft (2.8 mil or 70 µm). The desired geometries are realized by one of several well established wet chemical spray etching processes, after defining the pattern by conventional photolithography. A wide selection of liquid and dry photoresists are available for use with these etchants.

In general the edge definition of an etched circuit pattern in bare copper is primarily influenced only by the thickness of the copper foil as illustrated in Fig. 2. To a first order of approximation, an initial phototool line width of w is reduced, after etching, to (w-2t) where t is the thickness of the foil. By extremely tight control of the etching spray uniformity and the end point of etching, it is possible to reduce the undercutting to about 0.5t on either side. Copper etching is also very consistent from point to point across the panel. This makes it possible to partially compensate for the undercutting loss by adding an etch factor to the phototool, typically equal to the thickness t of the foil being etched. In practice it is possible to reproduce a line width with an absolute accuracy of (w ± 0.5t) across a 12 inch square panel. As the design frequencies increase the need for tighter tolerances becomes more and more apparent. At 10 GHz for instance, the designer would expect an absolute tolerance of ± 0.5 mil (12 µm) on a line or gap, which is within practical limits of realization. Even at lower frequencies such a pattern precision is highly desirable for implementing filters and tuned circuit elements without having to manually trim the tuning pads during assembly and testing.

Plating over Copper

While bare copper circuits can be etched to high precision, the poor corrosion properties of the metal make it undesirable for practical applications. In the presence of moisture, air, chlorine etc. bare copper is readily tarnished making it unsuitable for subsequent soldering and other assembly operations. Certain antitarnish formulations have been used with limited success to alleviate this problem. However, a proper remedy demands a protective overplating of more resistant metals such as tin, tin-lead, nickel and gold. Conventional tin-lead plating of 200-300 µm is quite satisfactory for lower frequencies in the range of 2 GHz. At higher frequencies the skin effect phenomenon severely affects the circuit performance. Most of the electrical conductivity takes place on the top layer of the circuit, called the skin depth as shown in Fig. 3. Though tin-lead satisfies the corrosion and assembly considerations it is over 10 times less conductive than copper. At 10 GHz the skin depth is in the range of 40 µin (1 µm), and the excellent electrical properties of the base copper are overshadowed by the lossy tin-lead layer.

Gold Plating

Gold is not only a very good conductor but has excellent corrosion resistance. Gold plating, though relatively expensive, is a well established technology developed over the past several decades. Apart from the higher cost, the circuit fabricator is faced with a number of challenges in producing gold plated fine line circuits on copper clad material, as elaborated in the following sections. The main problem arises out of the fact that gold cannot easily be etched in the presence of copper. Thin film gold (generally vacuum deposited) on ceramic substrates can readily be etched to yield fine circuit patterns. However, when the copper-gold combination is etched, accelerated etching takes place on copper because of electrochemical potentials, resulting in poor edge definition. Gold plating and pattern definition are in fact two different issues often having conflicting requirements.

Electroless Plating

There are a number of proprietary formulations for applying a thin coating of gold all over the copper traces of a circuit by simple immersion in a hot solution. Electroless plating does not require external electric field, and all isolated areas of copper including the edges of conductor will be uniformly plated with gold. Since the circuit is first etched in bare copper alone, the best possible resolution could be achieved. However, there are serious limitations to this method. The gold deposit is relatively thin (20-30 µin) and is of lower purity (97% typical). Such a deposit is not suitable for long term corrosion protection or for wire bonding purposes. Since the plating process occurs as a displacement reaction of copper with gold, there is some loss of the initial copper thickness. For very fine line patterns on thin copper this is undesirable. Subject to these constraints the electroless method is convenient and inexpensive.

Plating Bar Approach

After the conductor pattern is etched in bare copper, electroplating of gold over all the areas electrically attached to one another is simple and obvious. However in a typical microwave circuit there will be numerous disconnected areas such as pads and stubs. It is often feasible to tie these isolated areas together by using small (~5 mil wide) interconnections added to the phototool. After the gold plating step is completed, theses "plating bars" are carefully removed to restore the normal circuit pattern. This somewhat delicate operation can be done mechanically using a knife under the microscope, or by a router bit or by laser blasting. In cases where there are numerous small isolated areas in a circuit (tuning pads for example) this approach would lose its appeal. Another limitation exists on certain dielectric materials which are prone to the phenomenon of "shadow plating". This results in an irregular build-up around the edges of the conductor, thereby destroying the fine features. For most teflon based materials this is not a problem in processing gold plated fine line patterns. The plating bar approach is the only option when the specifications require complete coverage of plating including the edges of the conductor pattern, except for very small areas where the plating bars are removed after plating.

Pattern Plating

Pattern plating is the most generalized method of circuit fabrication. A negative image of the pattern is first formed on the surface of the copper, using an appropriate photoresist. Gold is then electroplated to the required thickness all across the exposed areas of the pattern. The photoresist is stripped off in the next step, and finally the background copper etched off to define the circuit, using the plated gold as an etch resist. Though copper etchants do not attack gold, there is enhanced reaction in the vicinity of the gold-copper interface as a result of electrochemical potential between the two elements. This causes severe undercutting of the edges, and an undesirable gold foil overhang is formed as shown in Fig. 4. During subsequent cleaning and other handling of the circuit, this overhang could fold down the edges or sometimes fall off as small slivers. There is no effective way to remove the unwanted gold completely, and that has an obvious influence on the edge resolution of the finished circuit. We are investigating special chemistry and etching methods to minimize these effects. By careful control it is currently possible to limit the gold overhang to about 0.5 mil per edge.

Soldering and Reflow

The assembly of a microwave circuit involves surface mounting of components by solder reflow. Tin-lead solder normally used in electronics is not fully compatible with gold plating. Gold readily alloys with tin-lead, but when the gold concentration exceeds a few percent the alloy becomes weak during thermal cycling. This phenomenon is called gold embrittlement. As long as the gold plating thickness is small (20-30 µin) this is not considered a problem. For thickness above 100 µin, indium based alloys rather than tin-lead solder are recommended to ensure the integrity of solder joints.

Wire Bonding

When small unpackaged devices have to be mounted on a circuit, thermosonic wire/ribbon bonding is required to make electrical contacts. This is accomplished by squeezing a soft gold wire/ribbon under heat and ultrasonic energy onto a gold plated pad until a metal-to-metal bonding occurs. This generally requires pure soft gold over 100 µin thick and a temperature of 120-150°C. For soft substrates such as teflon, the bonding reliability is greatly improved if a nickel underplate of 100 µin is included in the plating structure between gold and copper. The relatively harder nickel layer provides an "anvil effect" to improve the ultrasonic coupling during wire bonding.

Gold Plating Specifications

The general requirements of gold plating for electronics is covered by the MIL G-45204C, some of the highlights of which are shown in Fig. 5. These specifications impose an additional constraint, namely, a nickel barrier between copper and gold. This is to ensure that gold/copper diffusion does not take place subsequent to plating especially when the circuit undergoes assembly and solder reflow operations above 150°C. The nickel underplate or barrier layer is specified as a minimum of 200 µin as per MIL QQ-N-290A. This thickness of nickel (quite a resistive metal) confined to the skin depth region could be very detrimental to the performance of higher frequency circuits. From our experience a nickel barrier thickness of 20-50 µin is adequate to prevent this interdiffusion problem as long as the exposure above 150°C is limited to a minute or so. The resulting compromise on the microwave performance is usually very small. For service temperatures not exceeding 100°C the gold copper diffusion plays a negligible role in the circuit performance or reliability.

Conclusions

Gold plating of high frequency microwave circuits is necessary to facilitate subsequent assembly operations without sacrificing performance. There is some conflict in the plating requirements for soldering and wire bonding. The following plating combinations are suggested as a practical compromise:

For larger scale applications, the cost of thicker gold plating should be taken into account. From a processing point of view it is quite feasible to provide 10-25 µin of gold plating everywhere except the pads for wire bonding, where a selective plating of 100 µin gold over 100 µin nickel is applied. Though this requires additional steps of photolithography, a satisfactory compromise is achieved in cost, performance and reliability.

Fig. 1. Resistivities of Conductors

Fig. 2. Undercutting of copper due to chemical etching

Fig. 3. Skin effect phenomenon

Fig. 4. Pattern plating of gold

Fig. 5. Excerpts form gold plating specs MIL-G-45204 C

Dr. K. Ramachandran, Filtran Microcircuits Inc.
Copyright © Filtran Microcircuits Inc. 1997

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