What type of plating should be used for each particular application?
Plating, including electroplating and electroless plating of metals and alloys, serves many useful functions in electronic devices: corrosion protection, diffusion barriers, conductive circuit elements, via-hole filling for semiconductor integrated circuits, through-hole connections for printed wiring boards and flexible circuits. Plating is used to fabricate passive devices on dielectric surfaces such as resistors, capacitors, inductors and to improve conductivity of metallized circuits that use thick-film conductors or frits on ceramic substrates such as molydenum, "moly-mag," tungsten and other such materials.
Plating is often used to enhance solderability. Tin, tin-lead, tin-bismuth, various silver alloys, gold and gold alloys, electroless nickel-boron and electroless nickel-phosphorus alloys are common materials for soldering. Each has advantages and disadvantages. Selection is based on the end use of the component to be soldered. For example, burn-in devices must withstand high heat excursion. Diffusion barriers are required where copper, gold or silver is used in the circuit. Diffusion of any of these metals to a service can allow oxidation that alters the desirable characteristics of the device. Even a small amount of diffusion into the plating can alter conductivity. Oxides of copper, silver, nickel and most other metals do not solder well. Copper oxide is a semiconductor that can cause noise in high-frequency circuits. Electroless nickels, electroplated nickel, cobalt and palladium can be soldered if aggressive solder fluxes can be used (RMA or certain organic acids).
Gold is often prescribed for soldering applications. However, it is well known that gold is soluble in most solders, leading to weak, dull solder joints when the level of gold contamination is high enough. If gold is used for soldering or bonding, a diffusion barrier between copper and gold must be used. The gold deposit should be thin, less than 10 micrometers. Using pulse rectifiers, gold deposits can be produced on properly prepared surfaces with little or no porosity, thus allowing the use of thinner deposits. Immersion gold deposits are commonly used over electroless nickel for soldering. Electroless gold has gained popularity because of the ability to plate isolated areas without electrical contact. Electroless gold is difficult to maintain and control to achieve consistent results.
Good diffusion barriers are electroless nickel-phosphorus, electroless nickel-boron, cobalt, and nickel electroplating comes a close second. According to AESF Project 29, electroplated cobalt and electroless cobalt-5% phosphorus and electroless nickel-8% phosphorus performed the best as diffusion barriers. Turn and Owen reported nickel-phosphorus and nickel-boron to be effective barriers after 12 hrs at 550C.
Very large-scale integrated circuits (VLSI) use multilevel circuit interconnections to provide high density and reliability in a compact structure. During fabrication, a layer of metallization is deposited on the silicon wafer and the conductors are etch defined. A layer of dielectric is then deposited and windows (via-holes) are etched through the dielectric to connect points on the metallization. The next layer of metallized conductors is then applied to form interconnections. Using this technique, the upper layer is not completely planar because of the depth of the via-holes. This problem is compounded when additional layers are required to complete all interconnections. It is important to have a planar surface topography at all stages of production, or stress, etching irregularities and serious problems in lithographic patterning can result. It is therefore essential to fill the via-holes with a conductor before metallization to produce high reliability interconnections. Vacuum deposition methods have been studied extensively. It has been found that they do not work as well as electroless nickel plating. Electroless processes are simple, low cost and easy to implement.
Ting, Paunovic and Chiu report the following process: 1-micron layer deposited by a sputtering process. Then a 1 to 1.5 micron layer of undoped oxide is deposited by an LTO process. Via-holes of 1.5 microns nominal size are formed by photolithograpy and plasma etching. The aluminum conductor is activated using a light etch to remove surface oxides, followed by DI water rinse and a 40C palladium activation solution. The wafers are rinsed and immersed in a 55C electroless nickel boron plating solution at pH 6. The plating rate was 2.8 microns per hour. For plating onto silicon surfaces, palladium activation is not necessary. Silicon is etched in a nitric acid fluoride salt-water mixture. The pH of the electroless nickel-boron solution is raised to 8, and the temperature is 55C.
Harada, et.al, reported successful via-hole filling using electroless nickel-boron on aluminum conductors patterned on silicon substrates with 600 nm thickness of phosphosilicate glass films deposited for interlayer insulation. Via-holes of 2, 3 and 4 micrometers in diameter were formed using reactive ion etching, activated with palladium chloride and plated with an electroless nickel-born solution. This resulted in a flat, smooth surface with 100% process yield.
Dishon reported that "The electroless nickel deposition process has been applied for the via filling step in the production of a thin-film multichip computer packaging module." Nickel-boron deposits were plated onto evaporated copper on a Si wafer. Cr/Cu/Cr layers were evaporated, coated with a polyimide that was coated with silicon oxide or silicon nitride, patterned and via-holes etched down to the chromium layer. Chromium was removed in hot hydrochloric acid. Activation of the copper surface was done by acid cleaning. An electroless boron-nickel strike activated the copper and electroless nickel-boron plating followed. By activating the copper surface (using the nickel strike), the side was not activated, allowing excellent planarization. Nickel-boron plating was chosen because of its good conductivity, (6 to 9 microohm/cm) lack of noise production, and it is easy to use.
Electroless copper could be used; however, the plating solution is highly alkaline and will attack aluminum, polyimides and other materials. It is difficult to achieve high levels of adhesion of electroless copper to metallic surfaces.
Flip chip devices using electroless nickel and immersion gold have gained popularity. Nickel bumps are formed through various masking techniques then over plated with immersion gold. For wire bonding to these bumps, electroless palladium is plated over the electroless nickel followed by immersion gold.
Corrosion of electronic components is destructive in many ways. Loss of surface conductivity, increase of contact resistance, deterioration of the components, broken connections, soldering, brazing and wire bonding are made difficult. Failures in dielectric between metal lines due to accelerated corrosion when voltage gradients are applied. Change reported that in the absence of a voltage gradient, corrosion was only just apparent after 2,000 hours, but corrosion was observed within 50 hours with a 25v potential difference between the two conductors 0.5-mm apart.
The corrosion rate increases linearly with increasing potential differential. Selecting the right plated coating will lessen or eliminate corrosion under these circumstances. Electroless nickel phosphorus is a good protector of circuit elements. Electroless nickel, and to a lesser extent electroplated nickel and gold at a thickness to assure the elimination of porosity, serves very well. Tin could migrate under potential differences and offers much less corrosion protection.
Ceramic hybrid's and MCM-C circuits. Metallization materials such as manganese, moly-manganese, tungsten and thick-film material such as silver alloys, copper alloys, etc., all need corrosion protection. Electroless nickels offer excellent protection to all these materials. Combinations of electroless nickel-gold and electroplated nickel-gold offer high quality surfaces. However, a combination of electroplated nickel or electroless nickel-phosphorus plus electroless nickel-boron provides not only corrosion protection, but a solderable, brazable and wire bondable surface. Using electroless nickel-boron, hermetic brazed seals can be accomplished without fear of cracking or leaks.
Plated EMI shielding, although not new, is becoming essential to electronic device protection. Plated electroless copper and electroless nickel offer many advantages over conventional shielding. It can be used to plate non-conductors such as various plastics. The plated shielding has the best shielding characteristics of any of the coatings available.
Aluminum hermetic connectors require electroless nickel to provide a hard surface for the aluminum and corrosion protection. Plastic connectors are made possible by use of electroless nickel deposits to form a hard electrically conductive surface.
Printed wiring boards (printed circuits) use electroless copper for connecting one side to another. This is called the plated through-hole process. Additive circuits are also made using electroless copper. Electroless nickel has been used successfully for plated through holes and for additive circuits. The advantage of electroless nickel-boron for plated through holes is that smaller diameter holes can be successfully plated where electroless copper often will not completely plate on all surfaces, leaving voids or no connection at all. Eliminating formaldehyde, a hazardous material, is another incentive to substitute electroless nickel. Electroless nickel boron solutions produce a small amount of hydrogen. The gassing draws solution up through the holes and allows uniform deposits. Holes as small as 0.010 inch to 0.5 inch long have been plated with complete connection reliability. Electroless copper failed to connect any of the 300 holes tested. Electroless nickel serves as a good undercoat for all other plating. Plug-in fingers are enhanced in hardness and wear resistance by using electroless nickel and as undercoat for gold. Sliding contacts are made more reliable with electroless nickel undercoating.
Direct plating through holes for two-sided and multilayer printed wiring offers some advantages over the use of catalytic activators and electroless copper. One method uses conductive carbon in the holes followed by copper electroplating, thus eliminating the hazardous chemicals of electroless copper plating.
Electroplated palladium, palladium-nickel alloys and electroless palladium deposits perform as substitutes for gold plating in some applications. Combinations of palladium and electroless nickel fill other applications where wire bonding, die bonding and soldering are required. Aluminum wire can be bonded to electroless nickel boron without fear of Kirkendall voids or weak bond joints. Ultrasonic bonding with higher energy than is used for gold makes a durable, strong aluminum wire bond. Nickel cannot be thermal compression bonded using the present techniques.
Palladium, palladium nickel alloys and electroless palladium deposits are used for hybrid, DIPs and MCMs for several reasons. An oxide-free surface allows palladium to be soldered and wire bonded easily compared with nickel. A thin (0.025 to 0.05 micrometer) gold over-layer is sometimes used to enhance soldering. The solderability of palladium remains good even without the gold layer.
Tin alloys of lead, bismuth, silver and others also afford some corrosion protection to printed wiring board circuits. What happens to these plated coatings at elevated temperatures? Nickel phosphorus deposits harden considerably beginning at 300C and reach a maximum at about 385C. Oxidation takes place, and the deposit changes in volume and composition. Nickel-phosphorus intermetallic compound forms. Oxidation of both nickel and phosphorus occurs. Above 600C, migration of phosphorus takes place. Above 800C, decomposition and evaporation of phosphorus from the coating occurs. The addition of even a small amount of boron to the deposit decreases the amount of oxidation significantly. Heating nickel phosphorus in air or moist hydrogen to a temperature of 400 to 850C for 10 to 15 min removes the phosphorus from the surface of the deposit, making it much easier to solder, braze, wire bond or die bond. Ohmic contacts are made to thick film layers on ceramic semiconductors by plating electroless nickel and heat-treating.
Gold, silver or platinum thick-films are much more electrically resistant than expected from calculated values. Plating and then heat treating electroless nickel on these films enables the fabrication of stable, low-contact-resistance metal layers. Nickel boron deposits do not need the thermal excursion for die or wire bonding.
Plating metals and alloys never before possible to deposit are made possible using pulse plating rectifiers. Examples are gold iron, chromium iron, cobalt nickel iron, chromium nickel iron, nickel titanium iron and possibly others. Further control of the structure of the deposits is possible. For example, "super lattice" alloys can be produced as well as ductile amorphous alloys. Commonly plated metals benefit by pulse plating in that more uniform electrodeposits are possible for most metals as well as improved ductility and deposit leveling.