1 IMAPS Long Island Symposium May 6, 1997 Design Considerations for Wire Bonding Power Hybrids Gary Silverberg & Mike McKeown Orthodyne Electronics Irvine, CA 92614-6223 Abstract This paper will discuss the package design issues that Engineers should consider when developing a large aluminum wire wedge bonding process for use in power hybrid packages. With electronic power packages getting denser and thermal management more critical, the interconnection aspects must be fully proven and have higher reliability characteristics than ever before. Wire bonding is one of the main forms of interconnection in power hybrid packages for interconnection of power die to substrate and substrate to housing. Packaging considerations for automotive and industrial power applications will be focused on. This paper will offer guidelines in terms of the various materials that may be used, dimensional considerations, and related processes that may affect wire bonding. The materials must be compatible with all the processes a power hybrid package may encounter in the assembly process as well as compatible with the available processing equipment. Therefore, working closely with equipment and material vendors from the initial design concept stage can decrease time to market and increase production yields. INTRODUCTION The key materials involved with a typical power hybrid module can be broken down into the following sections : substrate, die, housing and bonding wire. The substrate section is comprised of the material used for the substrate as well as the type of thick film and other bonding surfaces, such as aluminum clad copper bond pads. The IC die pad section reviews bond pad materials and dopants. The housing section covers lead frame surfaces in addition to the design criteria of the lead frame. The bonding wire Photo courtesy of SMP Electronics section outlines the various types of aluminum- based bonding wires that may be utilized. Other materials involved in various operations before and after wire bonding are discussed. Finally, considerations for dimensional constraints of wire bond equipment are listed. 1.0 SUBSTRATE MATERIALS The various types of base substrates used in power hybrids are alumina, BeO, AlN, and PCB. The key issue with power hybrids is to efficiently remove the heat away from the electronic package via the substrate. Beryllium oxide (BeO) and aluminum nitride (AlN) are both excellent substrate choices for heat sinking due to their high thermal conductivity [1]. Alumina, beryllium oxide, and aluminum nitride can all be used with thick film, thin film, and direct bond copper (DBC) metallizations. The rigid nature of these substrates makes them good choices for wire bonding power hybrids. Printed circuit board (PCB) substrates, such as FR4, have a significant cost advantage over the ceramic-based substrates. With this
2 IMAPS Long Island Symposium May 6, 1997 cost savings comes some difficulties for wire bonding. The organic nature of this material has a sponginess that absorbs the bond interface energy, especially for larger diameter wires. The effect is less pronounced with smaller wire diameters, but still present nevertheless. Mounting a soft substrate to a solid backing with a rigid epoxy improves the bonding. Typically the circuit traces on the PCB are copper foil with a bondable plating. This works well for small wire sizes but the foil is very thin in relationship to large wires. A 2 oz copper foil with large bond areas to prevent delamination during bonding is recommended for heavy wire bonding. An excellent alternative is to solder aluminum clad copper bond pads to the traces of these softer substrates. These boards normally see several processes prior to wire bonding, such as plating, soldering (wave or reflow) and cleaning. Like all wire bonding applications, the bonding surface must be free of contaminants. Care must be taken to remove all residue from the wire bond areas. 1.1 Thick Films Thick film inks used in hybrid devices are comprised of conductor, dielectric, and resistor pastes. The inks are screen printed onto ceramic-based substrates and then fired in a furnace. This process may be several layers thick and the resulting pattern forms the electronic circuit. Some thick films are intentionally designed for wire bonding and others are not. The most common thick films used for wire bonding are : palladium silver (PdAg), platinum silver (PtAg), and platinum palladium silver (PtPdAg). These thick films work best with a more ductile wire (higher elongation). One item to consider when using softer aluminum wire is that aluminum build up on the tool occurs more quickly than standard aluminum wire. Wire bondable thick films vary from supplier to supplier. Work closely with the thick film vendor to ensure that electrical and mechanical characteristics are fully reviewed and evaluated. One misleading thick film candidate is silver (Ag). Silver thick film bonds very well but it s affinity for oxidation and corrosion may result in long term reliability problem for aluminum wire bonding. 1.2 DBC Direct Bonded Copper (DBC) is a technology which directly bonds 5 to 20 mil thick copper foil to a ceramic substrate. A cupric oxide layer (CuO) is formed on the copper foil sheet. The foil is then placed on an oxide ceramic such as BeO and sent through a high temperature furnace. The eutectic temperature of the CuO is reached and the CuO flows to form a permanent bond between the copper and the substrate. The circuit pattern is then etched into the copper foil. DBC is normally used for high power applications for it s superior electrical and high thermal management properties. For wire bonding, a nickel or nickel/gold plating is usually added but the copper can be bonded to directly if it is free from oxidation. [2] 1.3 Al Clad Cu Bond Pads Aluminum clad copper bond pads are among the most widely used bonding surfaces. The aluminum surface is used to ensure an optimal bonding surface (mono-metallic system) [3]. The copper is used primarily as a proper soldering medium to the substrate. Process Engineers should consider the following issues when using aluminum clad copper bond pads: aluminum thickness, surface roughness, burrs, shape, oxidation, and the other metallization choices that are available. When working with larger diameter wire, it is suggested that you specify that the aluminum thickness of the cladding be a minimum of 2 mils. This thickness is necessary due to the risk of copper diffusing into the aluminum, delaminating the aluminum from the copper during bonding
3 IMAPS Long Island Symposium May 6, 1997 or peeling away the aluminum during destructive pull or shear testing,. When evaluating bond pads, one commonly over-looked item is the surface roughness. The rougher the surface, the less ideal the bonding due to a reduced contact area between the wire and the bond pad. Also, the rougher the surface, the more likely the possibility of trapping detrimental contaminants in the crevices that may inhibit wire bonding (such as flux residue, stamping oils, epoxies, etc.). [4] A surface finish of 36 or finer should be specified. Another critical item with bond pads is the presence of burrs caused from the stamping operation. Depending on how the bond pads were stamped, burrs will be present. Work with your supplier to ensure that the burrs are minimal and are facing the copper side (stamped down). If the burrs point up, towards the aluminum side, they may nick and damage the wire. Since aluminum oxidizes, it is best to keep the bond pads in a clean environment that is average humidity and temperature. Storage in a nitrogen box with a dessicant is optimal. Also, having the bond pads placed on tape and reel will offer the bond pads an individual pocket protected by a cover tape. If you keep this package away from direct sunlight and use it in a reasonable length of time, you should not encounter any problems. In the past few years, much talk of circular bond pads has arisen to prevent the bond pad from circular rotation that is common with square bond pads. Again, work with your supplier and see which size bond pad is needed for your power package. Lessening the amount of solder and increasing the ramp time during solder reflow will help diminish the circular rotation of bond pads. Another benefit of bonding to a bond pad over bonding directly to thick film is that the bond pad will handle current surges more efficiently due to the larger surface area. Other metallization bond pads are available and gaining popularity. Some of these include Kovar with a nickel plating and Kovar with a palladium flash over nickel plating. 2.0 I.C. BOND PADS The aluminum top metal on an IC bond pad is used to accept the aluminum wire. This mono-metallic system ensures optimum bonding. Pure aluminum bonds very well but it is subject to hillocking during temperature cycling. The aluminum will buckle from electromigration and aggregate in ridges causing fatigue failures. Copper is added to prevent electromigration. Problems may arise, however, if the copper content rises above 1.5%. The bondability degrades and in severe cases, the copper may corrode from moisture and the presence of halogens, such as chlorines, gives the bond pad a brownish appearance. 1% silicon (Si) is often added to Al bond pads to prevent back-diffusion of the silicon wafer. This back diffusion could impair the electrical characteristics of the component. The Si in the bond pad metallization can form micrometer sized Si nodules which can act as a stress riser and crack the underlying glass during bonding. This micro-fracturing will show up as electrical leakage during testing. In severe cases cratering occurs. The IC surface is exposed to many sources of contamination through wafer processing, storage, die attach, and final assembly. Small amounts of contamination could drastically effect wire bond yield. Many hybrid manufacturers plasma clean prior to bonding. Since the physical size of a die can determine it s cost, IC designers are locating bonding pads over active circuitry. The circuitry under the bond pads is fragile and may be susceptible to micro-fracturing during the bonding process. It is also true
4 IMAPS Long Island Symposium May 6, 1997 that the wafer process time directly effects cost. Applying a thicker layer of bond pad metallization increases processing time and therefore the cost. Studies have been done on bond pad thickness versus yield and the results clearly show the effect. A minimum thickness of 4 microns should be specified for large wire bonding. The combination of active area bonding and thin bond pad metallization can result in a bonding process with a very small operating window. 3.0 HOUSINGS Insert molded packages are very common for automotive, industrial, and power package applications. The type of resins used for injection molded packages is extremely important. PBT and PPS are two common resins used for this application. In a typical molded package a leadframe is normally molded within the plastic. The leadframe electrically connects the inner package with the outside world. The leadframe is typically wire bonded from the substrate to the leads inside the package and the outer leads are normally configured into a connector. It is important, as always, to keep the lead area that will be bonded free from contamination. Plastics and resins will out-gas when heated. Also a mold release compound is applied to the mold to aid in the removal of the finished cooled part from the mold. Both out-gassing and mold release contamination, if left on the bond areas, will degrade wire bonding. The resins must also have a coefficient of thermal expansion (CTE) that is similar to the other materials in the package. If incorrectly selected the resins will expand and contract excessively and the wire bonds in the package will be stressed to the breaking point during thermocycling. Another phenomenon know as pullback results when the resins shrink during the cooling process. If this occurs around the leadframe, the bonding areas will not be held down tightly which will result in the lead oscillating during the application of ultrasonics. The result is inconsistent bonding. When designing the leadframe for a package many factors should be taken into account including : material, geometry, support, and routing. 3.1 Plating Plating of unbondable base metals with wire bondable metals is very common in the electronics industry. The most common plating for aluminum wire bonding is nickel/ phosphorus (NiP). This is an electroless process in which approximately 100-150 micro inches of NiP is plated. Electroless plating is an autocatalytic reduction of metal from solution and is purely a chemical process. Electrolytic plating uses electric current densities through the material to attract metal ions to the surface. The plating thickness can vary due to differences in the current density throughout the material due to it s physical shape. It has been our experience that electroless plating is preferable for wire bonding. The addition of phosphorus acts as a catalyst in the electroless process [5]. The higher the phosphorus content, the harder the plating and the less wire bondable the plating becomes. A small percentage (5-10%) of phosphorus is a good compromise and will decrease the formation of nickel oxide which is very hard and impossible to bond through. In some cases, a 2 to 10 micro-inch flash of 24 carat gold is applied to eliminate the possibility of oxidation. The gold should be pure and soft without brighteners such as Thallium. The bonding process is actually breaking through the gold and the Al wire is bonding to the Ni underneath. The gold plating should not be too thick (< 40 microinches) in order to reduce the risk of Kirkendall voiding. The surface roughness should be 100 +/- 50 nanometers peak to peak. Palladium (Pd) plating as an alternative has an advantage that it can be
5 IMAPS Long Island Symposium May 6, 1997 used for both gold and aluminum wire bonding [6]. Large Al wires bonded from lead frame to substrate 3.2 Al Clad Leadframe Aluminum bonded to aluminum is the easiest and most reliable metal system for wire bonding. For large wire bonding, an aluminum clad leadframe material is recommended. The cost can be higher than plated leadframes, but the results are more consistent. Aluminum cladded materials starts with a sheet of base metal, normally copper or brass, which is skived out where the bonding regions will be. An aluminum foil is pressed into the skived-out area with tremendous force which bonds the aluminum to the base material. When the leadframe design is punched from the sheet material, the aluminum will be only at the bond areas. The copper or brass portion will be molded into the package and used for the connector. This is a benefit since aluminum wire bonds well, copper molds well and copper can be tin or solder coated at the connector. Like Al clad bond pads, the cladding thickness is important due to the risk of Al/Cu intermetallic formation. The intermetallics can form brittle areas that may eventually crack. It has been determined by cross-sectioning cladded leads that the intermetallic formation will not exceed 2 mils. Therefore, the minimum thickness for the cladding should be 2 mils. 3.3 Horizontal Leads Inside a package, horizontal leads may be supported by an underlying plastic shelf or they may be unsupported. Supporting the leads can be useful especially for large Al wires such as 15 mils or larger [7]. When using a supported lead design it is very important that pullback of the plastic either under or around the lead does not occur. A gap under the lead will act as a sounding board. Vertical oscillation may occur and the lead will hit the shelf below and bound back in a harmonic pattern. This will result in inconsistent bonding and over-deformed bonds. Mold locks or over molding of the leads can help hold the leads rigid. Overmolding is a technique where the lead is embedded in the package support and the mold covers the top edges of the lead holding the lead in place. Unsupported or cantilevered leads are also common. For structural rigidity, the leads should be at least.032 thick, <.150 long, and >.100 wide. Since these leads have no support underneath, they are not subject to the sounding board effect of supported leads. They are, however, more susceptible to resonance. If the lead resonates during bonding, energy may be lost or absorbed by the lead s movements. The resonance of a material is determined by its mass and structure. It is not easy to calculate the resonance of a structure without actually building the part and testing it. It has been shown experimentally that usually the best bonding occurs when the bond position is near the end of the lead, not toward the package wall. This seems contradictory to the idea that the lead should be as rigid as possible.
6 IMAPS Long Island Symposium May 6, 1997 BAD BAD Inside Package Wall.150 Package Wall Angled first bond GOOD Mold Lock GOOD The routing of the leads as they enter the package wall can sometimes be a hidden problem. It is recommended that the leads extend at least.150 into the mold before angling. If the lead is not held rigidly in the plastic due to pullback of the plastic, the lead may flex at the joint and cause vibrations during bonding. There is not much plastic holding the lead in the direction parallel to the bond. In the example shown above, the lead is held tight within the plastic and with the addition of mold locks, the results are even better. Normally it is best to angle the first bond parallel to the lead since this should be the stiffest axis. However, in cases where the design is marginal, bonding first bond at an angle may yield better results. 3.4 Vertical Leads Vertical pins must be rigid for consistent bonding. The attachment point of the pin must be tight and the length of the pin should not exceed it s diameter. The top of the pin must be flat without burrs. When inserting metal pins into plastic housings or connector bodies, it is important to put features such as knurls on the pins very close to the surface of the plastic to assure a tight grip near the bond area. 4.0 WIRE CHOICES There are many choices available to the end user in terms of aluminum based wires. The most common wire type used in industry is the 99.99% aluminum wire (also known as four-nines ). It is used for wire bonding to die, lead frames, aluminum inlay, bond pads, etc. A second type of wire, though not as popular, is the five-nines, 99.999% pure aluminum. Some end users prefer this wire since it is able to bond easier to thick films and hard-to-bond-to surfaces. This very soft wire may help the bonding process in the early stage but consider the long-term effects that may be detrimental. Without the small quantities of impurities found in 99.99% Al wire that pin the Al grain boundaries in the wire, the grains in 99.999% Al wire will grow during temperature cycling. If the grains grow to a size approaching the diameter of the wire, the wire will weaken and possibly break along the enlarged boundary. Thorough thermocyle testing of the product is strongly recommended. For wire bonding to hard surfaces, such as those with a thin layer of nickel oxide, or if a harder type wire is preferred, there is the 0.5% magnesium aluminum wire. Magnesium aluminum wire diameters above 8 mil could crater thin metallization devices, such as power die. A wire composition that is gaining acceptance is corrosion resistant aluminum wire. One of the key dopants used in this wire is nickel. However, there are many other proprietary dopants that are added to give this wire special strength to withstand high temperature and high pressure for long periods of time. It is used in automotive under-the-hood applications as well as some industrial units. 5.0 MATERIALS & PROCESS RELATED ISSUES 5.1 Silicone Gel Silicone gels are often used for corrosion protection of electronic circuits [8]. It is interesting to note that silicone gel is not a moisture seal. Its mechanism of corrosion protection is the attachment of the ends of the long silicone molecules to the surface of the circuit. If dense silicone molecules are attached everywhere on the circuit surface, no water can collect, and no large size corrosive ions of chlorine, sodium, or other corrosives can penetrate.
7 IMAPS Long Island Symposium May 6, 1997 The viscosity and thickness of the silicone gel application is important to wire bonds. The gel should be of low viscosity during dispensing and remain soft after curing. If the gel is thick after curing, the gel could damage wire bonds during drop tests or from its expansion and contraction during thermocycling. Thin soft gel applied properly will protect circuits from corrosion and be safe for wire bonds. 5.2 Epoxy & Adhesives Many electronic substrates are attached to backplates by an epoxy or adhesive. The area directly below the wire bonds should be as close to void-free as possible. This will ensure optimum transfer of ultrasonics. When curing either the epoxy or adhesive, it is critical to have proper exhaust of the volatiles from the heating chamber so that they will not be deposited on the wire bonding locations. Compliant adhesives may absorb ultrasonic energy and create inconsistent bonding conditions. 5.3 Cleaning Over the last few years, the cleaning industry has been through (and still continuing) major changes with equipment and cleaning solvents. The end user will have to find which cleaning system is right for their application. However, remember that any residue left on the wire bondable surface after cleaning will inhibit wire bonding. Many of the residues are unable to be detected unless very high magnification microscopes and surface analysis equipment is used. 5.4 Storage of Parts With today s latest manufacturing techniques, the lot sizes are getting smaller and smaller and many companies now use a lot size of one. This just-in-time type of manufacturing is beneficial since it dictates that there is very little chance of items waiting to be processed. In batch mode manufacturing, significant quantities of parts may wait long periods of time to be processed. Dust, foreign particles, and other items that drift in the air may settle on electronic substrates and may interfere with wire bonding (as well as other manufacturing steps). It is best to either quickly process batch quantities, or store them in a secure environment until processing time. 6.0 DIMENSIONAL REQUIREMENTS When designing a hybrid package, it is important to consider the requirements of the processing equipment. In the case of wire bonders, there are clearance requirements for the bond tool to reach inside a package. There are also requirements on how the wiring diagram is laid out. Designing with these restraints will increase the manufacturability of your product. Deep access capability of the Orthodyne M-360C Clearances in front of and behind the bond tool should be considered as well as the step back for cutting and breaking the wire after second bond. After the second bond is made, the bond head must move back to cut, break, and form the wire under the tool to be ready for the next first bond. Structures behind the last bond or the bond pad itself should be positioned accordingly to avoid any interference. Bond to bond spacing is determined by the wire size. Contact your
8 IMAPS Long Island Symposium May 6, 1997 wire bonder manufacturer for specific clearance guidelines. 7.0 CONCLUSIONS There are many design issues that should be considered when developing a wire bonding process for use in power hybrid packages. It is important to take each issue and address it individually and see how it affects your application. The earlier that the design issues are handled, the less trouble one will have on the production floor. BIBLIOGRAPHY 1. Al Krum, Designing Power Hybrid Packages, Hybrid Circuit Technology, March, 1989. 2. Dr. R.F. David, Manufacturing Power Hybrid Circuits, Electronic Packaging & Production, p. 52, March 1996. 3. George G. Harman, Wire Bonding in Microelectronics, International Society for Hybrid Microelectronics, p. 75, 1991. 4. Simon Thomas and Howard M. Berg, Micro-Corrosion of Al-Cu Bonding Pads, IEEE Transactions, Vol. CHMT-10, No. 2, pps. 252-257, June 1987. 5. Gerald A. Laitinen, Troubleshooting Electroless Nickel, Allied-Kelite Products Division of the Richardson Company, Des Plaines, IL. 8.John W. Balde, The Effectiveness of Silicone Gels for Corrosion Prevention of Silicon Circuits, IEEE Transactions, Vol. 14, No. 2, pps. 352-365, June, 1991. BIOGRAPHY Gary Silverberg is the National Sales Manager for Orthodyne Electronics. Prior to his current position he spent 5 years developing Hybrid processes at Hewlett Packard and 8 years at Hughes Aircraft as Bonder Applications Manager, Bonder Sales Manager, and Welding Product Manager. Gary may be reached at Orthodyne Electronics in Irvine, CA at (714) 660-0440 or via email at sales.orthodyne.com. Mike McKeown is a Sales Engineer with Orthodyne Electronics. Mike spent 10 years working for Standard Motor Products involved in the design and manufacture of automotive electronic modules, with an emphasis in wire bonding. He then worked for Semiconductor Packaging Materials, a wire manufacturer, as an Applications Engineer. Mike may be reached at the Orthodyne Electronics Eastern Regional Office at (516) 739-2690 or email at mike.mckeown@orthodyne.com. ACKNOWLEDGMENTS The authors of this paper would like to thank Mike Smith of Orthodyne Electronics and Jim Hundley for their major contributions to this paper. 6. Donald C. Abbott, Richard M. Brook, Neil McLelland, John S. Wiley, Palladium as a Lead Finish for Surface Mount Integrated Circuit Packages, IEEE Transactions, Vol. 14, No. 3, pps. 567-572, September, 1991. 7. M.E. Webster, J.A. Hearn, R.W. Bibby, Developing Interconnect and Connector Technologies for a Hybrid Engine Control Module, ISHM Proceedings,pps. 462-467, 1994.