Guest Editorial -For Plateworld.com
Don Baudrand, Don Baudrand Consulting, e-mail:firstname.lastname@example.org
Corrosion and Corrosion Control for
Metallized MCM's and Hybrid Circuits
Corrosion is defined and described for electronic devices. The results of corrosion, including oxidation, loss of contact resistance, current leakage, shorts, and diffusion changes in hybrid circuits are discussed. Diffusion of metals resulting in a nickel, copper or silver in the surface which corrode or change the surface characteristics as well as the characteristics of the body of the conductor is discussed. Corrosion minimization and prevention, diffusion barriers and corrosion protection methods are suggested.
Corrosion is the process of changing, usually, in an in undesirable fashion. We know that corroding of an anode in an electroplating solution is most often desirable, since it supplies metal to the plating solution and usually less expensive than when metal salts are used to supply metal. It is hard to think of other examples of "good corrosion". Billions of dollars are spent each year world wide replacing corroded things such as bridges building components automobiles tractors etc. No one likes to experience a computer failure and many are the result of corrosion. Failure of control devices can be life threatening, such as aircraft controls or radar, or navigation devices. You can think of many more.
Corrosion is defined as the destructive alteration of a metal by reaction with its environment. Corrosion is Nature obeying it's own rules. Entropy is the measure of the degree of disorder in a system or substance. The second law of thermodynamic has us believe the entropy is always increasing in our universe. That is, things are becoming more disorderly. In other words every thing we build tries to fall apart.
Corrosion of electronic components is destructive in many ways. Oxidation of metals is an obvious from of destruction which leads to deterioration of the component, loss of surface conductivity, contact resistance is increased. Sometimes connections are broken. Soldering, brazing, and wire bonding are made difficult or impossible.
Reactions with sulfides in the environment can be equally destructive to electronic components and devices. Although silver sulfide is conductive, it too can interfere with bonding processes. Most sulfides are non conductors and act similarly to that of metal oxides.
Chang ( 1.) describes failures in dielectric between metal lines due to accelerated corrosion when voltage gradients were applied. "With the presence of moisture and ionic impurities, metal corrosion is also accelerated with voltage gradient. In the absence of a voltage gradient, corrosion was only just apparent after 2000 hours, but corrosion was observed within 50 hours with a 25 volt potential difference between two conductors 0.5 mm apart. The metal corrosion rate was also found to increase linearly with increasing voltage differentials between the metal lines spaced at 0.5 mil in a plastic package in the range of 5-100 volts." The metal conductors used were Al- Cu- Si sputtered on silicon nitride. with 1 mil gold wire thermocompressively bonded to the Al-Cu-Si bonding pad. The failure mode was leakage of current between metal conductors. Moisture and electrolytes such as chlorides accelerate corrosion and failure.
Sarma, et al, (2,) reported failure of under hood automotive electronic devices as shorting and blistering due to"electrolytic migration" using palladium-silver conductors on dielectric materials. Moisture and the presence of electrolytes (any soluble material which when dissolved increase the conductivity of water) create the conditions for corrosion and electrolytic migration resulting in current leaks or shorts.
Diffusion of metals can lead to corrosion of electronic devices. For example, when gold is plated over copper diffusion of gold into copper and copper into gold can and usually takes place. Accelerated by heat, when copper reaches the surface of the gold plate oxidation can easily take place (unless the component is hermetically sealed). Copper oxide, in addition to causing bonding failures, and high contact resistance, can cause malfunction of the device. Copper oxide is a semiconductor, that is it passes current in one direction and not the other. Even though it is somewhat conductive in one direction, the resistance is higher than gold or non-oxidized copper. High frequency signals traveling through copper oxide can be modified to cause "noise" in the circuit or loss of signal amplitude, or rectification of the signal. Rectification means that 1/2 of the signal may be lost entirely.
Another consequence of diffusion is the change in electrical and mechanical properties of the device. Mutually soluble metals include: silver and gold, copper and silver, copper and gold. Whenever any of these metals are in contact with each other, diffusion will take place, often leading to destructive failure of the electronic device, even when corrosion per se is not present. It is difficult to prevent oxidation or sulfidation of silver and copper. Therefore, corrosion is usually associated with the diffusion of these metals. Barrier layers between these metals are one way to minimize or eliminate diffusion. Nickel and electroless nickel deposits are commonly used as diffusion barriers in plated components. Electroless nickel deposits have been recognized as a satisfactory barrier material. It has slow transport rate in the substrate and adjoining layer, laterally uniform in thickness and structure. They are thermodynamically stable against substrate and adjoining material and have low resistivity. (1 & 2 ) The addition of cobalt to nickel plating solutions improve the diffusion barrier (3) Palladium is also used the barrier purposes, both electroless and electroplated palladium and palladium/nickel alloys.
Corrosion minimization/prevention in multi- chip modules (MCM) and hybrid circuits.
Metallization of circuit elements can take many forms. For example, ceramic multi-chip modules (MCM-C) can use tungsten, molybdenum, Molybdenum-manganese alloys (some times called "moly-mag") as metallizing materials. These metals or alloys are usually mixed with silicon (glass) in some form and an organic binder and fired onto the ceramic to form a bonded circuit element. Silver and silver alloys copper, and alloys palladium and its alloys in organic binders, are common materials for circuits, resistors, capacitors,etc. All these metallizing conductors, except tungsten are subject to corrosion unless protected. (4) Even Tungsten will dissolve in alkaline anodic conditions. Many oxide films are protective to the metal. But corrosion can take place when there is current passing or where a corrosion cell is formed. A corrosion cell is a spot where there is a potential (voltage) due to the presence of an anode and cathode. This condition occur at dissimilar metals in contact with one another, or where oxides form passivating one area and not another adjacent area. Frequently, corrosion cells form due to alloying constituents, pits, nodules, or particles included in the plated deposits, and where a non metal protective layer starts or ends and excludes oxygen from one part of the component. ( this is known a crevice corrosion).
It is common practice to overplate electroless nickel or palladium with gold to preserve solderability. However, the overplate must be pore free to prevent oxidation (passivation) of the nickel due to the galvanic cell created be the dissimilar metals For example, steam-aged porous bold over electroless nickel or electroplated nickel will not solder easily due the passivation of the nickel.
Wire bonding aluminum wire to electroless nickel-boron deposits will not corrode easily. Aluminum wire bonded to gold corrode rapidly due to the galvanic cell created between the two metals.
Sputtered and evaporated metals used for metallizing non conductive materials have characteristics similar to electrodeposited metals. Diffusion, corrosion, electrochemical cells, impurities, nodules and porosity can cause difficulties and increase corrosion rate leading to failures. Electro and electroless plated deposits can also protect these metals and alloys if properly applied.
Corrosion, metal diffusion and/or surface imperfections just described can alter the electrical characteristics of an electronic device. This can lead to from noisy circuits to failures. When nickel plating over metallized circuits, semiconductors, via holes, on surfaces intended for electromagnetic interference (EMI) or radio frequency (RF) shielding, it is important to:Plate with sufficient thickness to provide a good diffusion barrier; (l50-200micro inches are usually sufficient for most applications)
Use clean plating solutions with sufficient filtration to remove all particles from the bath; Use good cleaning and activation pre-plate steps to insure removal of any particles from the basis material and be sure of a clean active surface;
Monitor all plating solution constituents and operating conditions so as to produce a high quality deposit.
EMI shielding of non conducting packages and enclosures is often accomplished using electroless copper followed by electroless nickel or electroplated nickel to prevent oxidation of the copper. Copper provides efficient shielding. Thickness of copper ranges from 30 to 150 micro inches (0.75 to 3.74 micro meters). the nickel layer varies from 30 to 100 micro inches (0.75 to 2.5 .
These known good practice will minimize or eliminate corrosion problems in the majority of electronic devices.
Why won't these known good practices solve all the problems? What about the rest of the devices?
Some devices operate at elevated temperatures. Diffusion rate is increased due to heat. Thicker deposits may be required. Pure nickel electroplate may not be good enough. Electroless nickel provides a somewhat better barrier, but at a little higher electrical resistance. Electroless nickel-boron deposits have much lower electrical resistance.
Approximate resistivity of various metals
|Pure electrodeposited nickel||
|Nickel phosphorus 7-9% P||
|Nickel phosphorus 9.5-10.5%P||
|Nickel phosphorus 4-4%||
|Nickel Boron 0.5-1% B||
|Nickel Boron 1.5-3% B||
Approximate numbers are given for the resistivity since alloys vary and impurities in deposited metals can alter the values somewhat. It is interesting to note that nickel is more conductive than 60:40 tin-lead solder. Other solder alloys of tin lead may have different resistivities, however, separately, tin has about 11 micro-ohm-cm as does lead.
Diffusion of metals and in some cases non metals can lead to changes in resistivity, or accelerated corrosion due to a greater potential between layers or other metals which may be in contact with the temperature altered materials. Most metals oxidize in the presence of oxygen from the air. But there are other sources of oxygen. Under the influence of an anodic condition, water can liberate oxygen at the anode and hydrogen at the cathode. Water in the form of moisture or vapors can under these conditions contribute oxygen. Thus if a steel component is plated and heated iron can migrate to the surface, oxidize and thus interfere with contacts, soldering, brazing wire bonding and most any type of bonding method. Copper as an under layer diffuses readily into silver, gold, nickel etc. Oxidation of copper at the surface interferes with most all types of bonding. There is a change in contact resistance in addition to changes in volume resistance and again copper oxide at the surface is a semiconductor, causing noise in high frequency circuits. Careful selection of diffusion barriers can minimize these problems. Plated Palladium, nickel, electroless nickel, nickel cobalt alloy (up to 40% cobalt can be used). (4, 5, 6, ) Sputtered or evaporated chromium, palladium, and platinum. Most refractory metals serve as diffusion barriers as well, such as tungsten and vanadium. Some rare earth metals are being used via sputtering or chemical vapor deposition methods.
Avoid unnecessary high heat excursions where ever possible. (7.) Heating treating electroless nickel deposits result in changes in composition and characteristics of the deposit. For example, heating electroless nickel phosphorus and nickel boron deposits to 350400 degrees C for 1/2 hour will cause the formation of nickel phosphorus, or boron intennetallic compound resulting in hardening the deposit, changing its structure from amorphous to somewhat crystalline. The deposit becomes magnetic and the specific resistance changes. Heating electroless nickel deposits to higher temperatures result in further changes. Above 600 degrees C, migration of phosphorus to the surface and formation of phosphorus oxide takes place. At 800 to 1000 degrees C nickel phosphorus oxidized about 100 times faster than pure nickel. Heating nickel phosphorus in moist hydrogen results in removal of phosphorus from the surface making it more easily solderable and bondable. Electroless nickel boron deposits act differently. Even a small amount of Boron in the deposit will inhibit (but not prevent) oxidation. (8.)
The melting point of 11% phosphorus nickel alloy is 880 degrees C., while nickel boron melts at 1455 degrees C.
Protecting completed devices.
Encapsulation, hermetic sealing, coating with potting compounds are some of the methods used to protect electronic devices.
Silicone gels and epoxy coating and potting materials are widely used to protect components from moisture and corrosive materials. New materials appear more frequently than a few years age. Keeping abreast of new developments in circuit protection is time well spent.
Electro and electroless plating of alloys can improve performance if the right coatings are selected for the application. Extreme care must be exercised in minimizing the exposure of the completed device to moisture using the right plating selection and the best sealing materials of process for the application.
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