High Performance Etchable RoHS Compliant Thick Film Gold Conductor Open Access

RoHS stands for Restriction of Hazardous Substances, and impacts the entire electronics industry and many electrical products as well. The original RoHS, also known as Directive 2002/95/EC, originated in the European Union in 2002 and restricts the use of six hazardous materials found in electrical and electronic products. All applicable products in the EU market since July 1, 2006 must pass RoHS compliance.

Directive 2011/65/EU was published in 2011 by the EU, which is known as RoHS-Recast or RoHS 2. RoHS 2 includes a CE-marking directive, with RoHS compliance now being required for CE marking of products. RoHS 2 also added Categories 8 and 9, and has additional compliance record keeping requirements.

Directive 2015/863 was published in 2015 by the EU, which is known as RoHS 3. RoHS 3 adds four additional restricted substances to the list of six. The four new substance are, Bis(2-Ethylhexyl)phthalate (DEHP), Benzyl butyl phthalate (BBP), Dibutyl phthalate (DBP), and Diisobutyl phthalate (DIP).

Companies wishing to export materials into Europe and Asia and/or conduct business with companies that supply to the European or Asian market will have to be able to meet the RoHS and REACH requirement. Unfortunately, it is well documented that the combination of lead oxide (PbO) and cadmium oxide (CdO) to Gold conductors provide excellent adhesion to the alumina substrates, and has been the norm for a long time in our industry in providing excellent physical properties and robust wire bond reliability. However, the demand for replacing PbO and CdO as well as some Phthalates in the traditional gold thick film conductor formulation has provided an opportunity to develop new products. As a result, a new formulation was developed using lead-free glass frit and metal oxides.

The focus of this paper is to provide information on process flexibility of this new formulation, which can be utilized either via screen-printing of fine lines (<75 microns) or in etched designs (<25 microns) while still exhibiting excellent wire bond adhesion using both gold and aluminum fine wire. The development of this formulation was as follows, first the gold powder selection was completed targeting excellent fired film density, followed by developing an organic vehicle to achieve fine line printing, and last was the selection of glass frit and oxide combination for maximum wire bond adhesion. This paper will discuss further the main material drivers, which provide the improved design flexibility including their opposing limitations.

The traditional gold powder type used in the traditional thick film gold conductor paste are typically a sphere/flake powder. Fig.1 is the SEM photograph of this type of powder.

This type of gold powder is used because it was readily available and cost effective. The fired film made with this gold powder had less potential for blistering and agglomeration due to the porous fired microstructure. Fig.2 shows the backlight density of a fired film made with this type of Au powder. These surface anomalies can lead to sporadic wire bond values. Fig.3 shows a SEM image of the gold powder used in this new paste formulation. The powder is mainly a spherical powder having mono size particles and very tight distribution. The advantages of using this powder type is it results in a denser fired film, improved line resolution and a smoother fired film surface, which provides higher and tighter wire bond distributions. Fig. 4 shows the backlight density of the conductor paste made with spherical gold powder.

This new gold conductor formulation uses a new organic vehicle formulation, which was developed to ensure excellent printing characteristics as well as good shelf life at room temperature. This new vehicle consists of Ethel Cellulose, solvents, thixatrope, and a plasticizer. The vehicle is designed to be clean burning at less than 400°C to ensure maximum densification. Fig. 5 shows the effect of vehicle on printing characteristics of a 75-microns gap, where the top photo did not hold the 75-microns spacing while the photo on the bottom is the new formulation. This combination of optimized organics and spherical Au powder easily maintained the 75-micron gap.

In this new gold conductor formulation, traditional metal oxides were used and several glass frits were tested in order to maximize wire bond adhesion to the substrate. Table 1 displays a list of some of the common glass types used in thick-film paste development. The glass frit selection took into account the fired film surface properties and wire bond performance on both bare 96% Al₂O₃ and on top of standard 850°C dielectrics.

Several test parts were printed and evaluated for wire bond adhesion at room temperature and after elevated temperature aging at 150°C and 300°C for up to 144 hours. This data was used to select the best glass frit. Fig. 6 shows the SEM photographs of a poor surface which shows high roughness and pores (top) versus the new formulation which shows excellent densification with low roughness (bottom) which are both requirements for reliable wire bonding.

To further show the advantages of the new formulation, cross sections using the new spherical gold powder, the optimum glass formulation with the selection of oxides, and the new vehicle system were collected. Fig. 7 shows the cross section of the new gold formulation fired on a bare 96% Al₂O₃ substrate. The film shows little to no pores and good interfacial adhesion to the substrate.

Most individuals involved with thick film pastes and the microelectronics industry are familiar with screen printing. Screen printing can be simply described as the deposition of thick film pastes through a mesh screen in a specific design. The screen is manufactured from stainless steel mesh, but can be made from other types of materials such as polyester. Fig. 8 shows a simplified flow diagram of the screen printing process.

The screen printing process is well known and relatively a simple process. Advantages of screen-printing include the ability to produce circuits in high volumes quickly, low processing cost, and the ability to use standard processing equipment. However, screen-printing has limitations.

Line resolution is generally limited to greater than 3 mils (75 microns). Even trying to achieve 3 mil line resolution requires the use of specialized expensive screens and requires a seasoned operator. In addition to line resolution, the surface of the fine line can be rough or exhibit roping/scalloped edges. Scalloped edges will be more prominent as the line width decreases. Roping/scalloped edges and rough surfaces result in losses in RF circuits and increase the potential for shorts in the circuit. Fig. 9 shows the screen-printed resolution of the new gold formulation.

The second patterning method is the photo defining or an etching process. Like the photo sensitive process, a ground plane of thick film paste is screen printed on the desired areas of the substrate followed by the firing step prior to patterning which is a means of subtractively removing gold from the substrate. A photo resist is applied to the fired film and exposed using an ultra violet light source with a photopositive mask. The photoresist acts as the barrier to the etchant, which consists of an iodine/potassium iodide solution, and the photo resist is stripped along with Au, leaving the circuit pattern. The surface is cleaned of residue and typically re-fired to maximize adhesion. Fig. 10 shows a flow diagram of a typical photo defining process.

Of the two patterning techniques described, the best line resolution is achieved using the photo defining process. Line resolution of less than 1 mil (25 microns) is possible. The edge definition (or edge acuity) is excellent at < 2 μm and the process produces a fired film surface that is smooth and extremely dense. This produces an optimum surface for bonding wire or ribbons.

Several samples were prepared on both Al₂O₃ and IP9217 dielectric using a 400 mesh screen and were provided to Anaren Advanced Ceramic for etching. The photo in Fig.11 is a courtesy of Anaren Advanced Ceramic, showing the backlight density of the different size lines, and Fig.12 shows the SEM photo of the 1mil lines.

Additionally after etching, wire bond adhesion was checked, comparing the etched part against the as-printed part to make sure the etched film is not losing its bond strength to the substrate during the etching step. This was performed using standard 1.25 Au wire bond on both Al2O3 and IP9217 dielectric. The adhesion data will be discussed later in the adhesion strength section of the paper.

Standard 96% Al2O3 (2″×2″) test substrates were printed using a 325-mesh, 0.5 mil emulsion stainless steel screen. The screen printing parameters were adjusted to achieve 12–16 microns dried film thickness. The gold conductor was then dried at 150°C for 10 minutes in a box oven and fired in a muffle-less belt furnace at 850°C with a 10-minute peak dwell time for a total door-to-door cycle time of 42 minutes. The gold paste was printed to a fired film thickness range of 6–8 microns on the 20 mil line (see the area on the test pattern). Fig. 13 shows the test pattern used for the printing evaluation and Fig. 14 shows the printing capability of this new gold conductor.

As a cost saving process, it is desirable to use silver and silver/palladium conductors in combination with gold conductors. Silver conductors are used in general for the inner layer and solderable conductors are used to print pads on top of the circuit where components will be attached. To determine the compatibility of this new gold conductor, two studies were conducted with several RoHS and REACH compliant solderable conductors to allow for use in mixed metal circuits. Table 2 shows the list of the solderable materials used in this evaluation.

In the first study the new Au formulation was printed on the bottom and the solderable conductors on top. In the second study the Au formulation was printed on top of an assortment of solderable conductors. These parts were fired at 850°C up to 5 times and the interface was evaluated for blisters, glass bleed and conductor separation. The results from the first study showed no compatibility issues with any conductors. In the second study, where the gold conductor was printed on top of the solderable conductors, there was an issue with C8728 and ET1450. There was evident blistering at the gold/silver interface. The blisters became larger after multiple firing at 850°C, but the other three conductors produced acceptable interfacial results. Therefore it is not recommended to use with the new gold conductor paste on top of C8728 or ET1450.

To test for crossover applications, the formulation was printed on top of IP9217 and IP9227 lead free multilayer dielectrics. Different drying conditions were used to dry the gold before firing. Parts were dried as low as 100°C for 10 minutes and as high as 175°C for 10 minutes. The results produced no peel off of the formulation conductor from the dielectric. Fig. 15 shows a photograph of the test pattern used to evaluate the crossover. The fired dielectric film is 10 mils wide and 45 microns thick and the gold fired film is 8 mils wide and 9 microns thick.

Al-H11 (99.99% Al, 8–13% elongation with 250 grams minimum tensile strength) wire type is used for the 5–10 mil diameter aluminum wires. Al-Si (99%Al, 1% Si, 0.5–1.5% elongation) type was used for 1.25–2.0 mil aluminum wires, and Au-HD2 wire was used for the 1.25 mil gold wire.

The minimum bond strength requirement according to Mil STD 883E METHOD 2011.7 for 1.25 mil Au, and Al wire is 3.0 to 3.5grams.

The new gold conductor formulation was printed on several substrates including both 96% Al₂O₃ and IP9217 dielectric using a 400 mesh ground plane screen to a target fired thickness of 6–8 microns. After the parts were fired, the steps in Fig. 10 were completed. The photo in Fig. 16 is a courtesy of Anaren Advanced Ceramic, which shows the new gold conductor after it was etched on IP9217 dielectric. The parts were then re-fired one more time at 850°C before wire bonding was completed using a 1.25 mil gold wire. Fig. 17 shows the initial adhesion data for both substrates, as well as the failure mode.

The paste was printed on both 96% Al₂O₃ and IP9217 dielectric using the Heraeus standard 2”×2” test pattern from Fig. 13 using a 325 mesh screen, with 0.5 mil emulsion thickness to a fired thickness of 6–8 microns. After firing, the parts were bonded with 1.25mil Au wire and placed at 150°C for up to 1000 hours. Fig. 18 shows the adhesion after the 1000 hours aging at 150°C.

The parts were also aged at 300°C for one hour to evaluate the effect on adhesion. Fig. 19 shows the aged data after one hour.

The information presented in this paper shows that the wire bond adhesion values are well above the minimum bond strength requirement for the Mil Standard 883E, and that the wire bond data for this new gold paste demonstrates the excellent reliability of this product when printed on 96% Al₂O₃, and IP9217 lead free dielectrics.

The combination of this new organic system, lead free bonding mechanism, and this spherical gold powder provides a solution of having a paste with improved processing flexibility. It fires to a dense fired film, shows excellent printable fine line definition, and can be etched for even finer lines and spaces. The data provided in this paper shows an excellent bond strength after etching on both 96% Al₂O₃ and IP9217 lead free dielectric. It is also confirmed that this new gold has good compatibility with different silver baring conductors as well as IP9217 dielectric. There is further long term reliability testing on going which will be presented during the show in October.

The authors would like to acknowledge and thank Kristin Murphy for her assistance in this study.