Ionization Technology and the PCB Assembly Environment

Estimated reading time: 13 minutes

Printed circuit board (PCB) assembly processes involve semi- and non- conductive materials and electrically isolated conductors, which can generate and retain large electrostatic potential. Electrostatic discharges (ESD), if left uncontrolled, affect both yield rate and finished-product quality.

By Richard Rodrigo and John Gorczyca

Damage caused by static discharges falls into three categories:

• Damage to product, components or process tools resulting from a direct ESD event
• Surface contamination due to electrostatic attraction (ESA) of particles
• Process equipment latch-up caused by electromagnetic interference (EMI) from an ESD event.

The movement of people and materials in the PCB processing environment generates static charges, which are the result of two primary mechanisms: triboelectric and induction charging. Triboelectric charging occurs whenever two materials come in contact and then are separated. Induction charging occurs whenever a conductive object is in the presence of a charged object.

Triboelectric charging can occur between virtually any two materials including insulators, conductors and liquids. During charging, electrons transfer from one object to the other, leaving one with a net positive charge while the other obtains a negative charge. The relative charge depends on numerous factors, including the material's affinity to acquire or lose electrons, contact pressure, size of contact area, speed of separation, and surface finish. ESA of particles to a surface stems from the strength of the field emanating from the charge on the surface of an object. Electrostatic particle deposition also is attributable to particle size and its electrical charge. Three forces hold a particle at rest on a surface. These include electrostatic, electrochemical and the force of gravity (Figure 1).

Figure 1. A particle at rest on a surface is held by electrostatic, electrochemical and gravity forces.

The Value of IonizationThe purpose of ionization is to prevent the attraction due to electrostatic charge of particles to surfaces. Neutralization of such charges is effective in removing surface contaminating particles of less than 5 to 10 μm. Ionizers add or remove charges from molecules in the air, which can carry a charge and, thus, minimize the potential for damage on both insulators and conductors.

Humidity occasionally is considered an alternate charge-control method. However, it is an ineffective alternative at any practical, tolerable level of relative humidity (rh). For example, at 40 percent rh, it takes more than an hour to discharge a 1,000 V charge on an isolated aluminum conductor.

Figure 2. An ion is a molecular cluster of about 10 molecules bound by polarizing forces to a single charged oxygen or nitrogen molecule.

In areas such as clean rooms, product assembly and machine environments, air ionization is the only practical method of static control. A typical ionization system can remove 1,000 V in less than 10 seconds. Static charge neutralization via air ions depends on various complex interactions:

• Ions (Figure 2) require airflow around the target object or surface. Ions are incapable of penetrating equipment enclosures or passing through solid matter, liquid or thin film.
• An air ionizer is capable of neutralizing a charge because it produces mobile positive and negative charge carriers. Two mechanisms, conduction and exchange, permit ions to neutralize charge as follows:

1. Ion molecules are carried to the intended target in two ways, electrostatic force and airflow. Electrostatic charge attracts ions of the opposite polarity, prompting (ion) current flow and accelerating ions toward the charged object. Electron exchange then can occur, resulting in charge elimination.
2. Charged air molecules cover and surround surfaces within range of the ion emitter system. A charge on the surface will attract ions of the opposite polarity and repel those of a like polarity until charges are neutralized.

Air IonizationAir ionization complements and completes any program that intends to minimize the risk from charge sources. Grounding is the first line of defense in controlling ESD and will dissipate static very rapidly. However, in some cases, grounding is impractical or impossible, and insulators can not be grounded. In such situations, ionized air can bridge the gap between charged objects and ground potential (i.e., "conductive air" permits electron flow to or from charged objects, thus satisfying any charge imbalance). The conduction process significantly decreases discharge time.

Although ionizers are capable of delivering many benefits, including particle contamination control, ESD-sensitive device protection and process equipment lock-up reduction, before investing in ionization equipment it is important to evaluate the task it must accomplish. First, consider the discharge speed required by the dynamics of the process (i.e., the requirement for positive/negative ion balance should be determined in terms of process- or product-charge sensitivity). Next, resist the temptation to specify the means of achieving the desired performance. Instead, when specifying ionization requirements, set specifications around performance requirements. (Setting technology requirements can lead to choosing a less desirable solution, which can result in a costly mistake.)

The process should begin with answers to three questions:

• What decay time is required for a reasonable level of product or process protection?
• What offset voltage can the process accept?
• How will parameters and results be measured?

Typical Ion Generation MethodsThere are multiple ionization technologies and products available, each with a strength that can improve a company's yield and throughput. By understanding these characteristics and observing the differences, an assembler can be better equipped to evaluate the advantages of each technology, then choose the best suited for their application. Two major classes of technology are used for ion generation for ESD control corona and ionizing radiation. Both methods charge molecules of gas in the surrounding air to form mobile positive and negative charge carriers:

Corona Ionization. Electrically based corona technology is the most widely used ion-generation method for static charge control. Three types of commercial electrical ion-izers generally are in use and all operate on the corona principle, in which electrical ionizers generate ions by concentrating an electric field on an emitter point (Figure 3).

Figure 3. In corona ionization, ions are generated by concentrating an electric field on an emitter point.

Negative ionization occurs near the emitter point, driven by the negative power supply. Ions are generated in the plasma of the corona around the emitter as weakly bound electrons are driven from orbit and attach to a molecular cluster. The resulting negative molecule is repelled from the like-charged emitter. Positive ionization occurs in the area around the emitter point, driven by the positive power supply. The free electron is attracted back to the positive polarity emitter point in this case, the resulting positive molecular cluster accelerates away from the like-charged electric field (Figure 4).

Figure 4. In AC corona ionization, negative ions develop near the emitter point, driven by the negative power supply.

Ion-current strength is a function of applied voltage, emitter geometry and conductivity. Applied voltage duration influences ion-current strength and distance traveled especially important in a pulse direct current (DC) system where on-time of each polarity is controlled. Ion current is affected by environmental conditions such as temperature, humidity, atmospheric pressure and proximity to ground planes. Closely controlled environments such as clean rooms eliminate much of the environmental concern for ionization.

1. Alternating Current (AC) Ionizers. AC ionizers produce positive and negative ions by applying a high-voltage AC waveform at the supply frequency. Only one emitter is required to produce ions; both positive and negative ions are produced at each emitter. This "bipolar" feature is the defining characteristic of AC technology. Thus, AC systems can be located closer to objects than their pulse DC counterparts, and the time and distance between ion polarities is short. Similarly, stability is enhanced because each emitter is subjected uniformly to the differing wear patterns characteristic of positive and negative emitter electrodes. AC frequency fast cycling reduces the build up of emitter contaminates that attack electrode surfaces. Finally, AC systems produce less space charging, the fast cycle times produce a nearly continuous stream of bipolar ions, and the short time separation helps to assure rapid and complete neutralization of charges.

Figure 5. SSDC ionizers require a minimum of two emitters to generate bipolar ions.

2. Steady-state DC (SSDC). SSDC ionizers generate bipolar ions using independent positive and negative power supplies connected to dedicated emitters. These systems require a minimum of two emitters to generate bipolar ions (Figure 5). Both positive and negative power supplies operate on a continuous basis to create ions at each emitter. SSDC ionization creates a very high ion current because it produces ions of both polarities with no off-time cycle.

Properly designed systems and emitter spacing will result in a minimum of space charging and low offset voltage. Controlling the distance between dissimilar emitters can reduce recombinations of the bipolar ions. The technology is very effective when used in tabletop blowers or compressed-gas designs.

3. Pulse DC, the newest development in corona-ionization operating modes, is a more complex operating mode and is more demanding in its design, operating requirements and setup (Figure 6). As with SSDC, independent positive and negative power supplies are connected to dedicated emitters to generate bipolar ions. In this case, a square wave drive to independent power supplies is used. The pulse rate is slower than AC and performance becomes similar to SSDC when the frequency approaches 10 Hz.

Figure 6. With pulse DC ionizers, the pulse rate is slower than that of AC systems and similar to SSDC when the frequency nears 10 Hz.

Pulse frequency affects balance and total ion output. Frequency is set lower as the distance from the emitter to the target object increases, the lower frequency also being useful as air velocity in the environment decreases. The major advantage realized by pulsing positive and negative ions is optimizing the number of ions available to eliminate a static charge. The technology permits bipolar ion separation in both time and space (e.g., space reduces the chance that positive and negative ions will recombine before they reach the intended target). The technology makes it possible to use ionized air effectively in rooms with low air velocity. Electrostatic force, or the repelling of like ions, can supplement delivery normally resulting from high-velocity airflow.

Ionizing Radiation. Nuclear ionization technology has existed in commercial form since the early 1950s; early units used radium as an energy source until the mid-1960s. Today, most nuclear ionizers use polonium-210, which has a history of safe use and is utilized in many applications, particularly those suited to their attributes such as volatile atmospheres.

With soft X-ray technology, X-rays are emitted when a stream of electrons are accelerated at a target material. The resulting emission is an electromagnetic radiation of extremely short wavelength that has great penetrating power. X-ray ionizers create a hazard and require operators to be monitored for exposure levels. In most countries, a license from the national health and safety administration is required.

Polonium nuclear ionizers must be shipped under strict adherence to DOT requirements in the United States (Radioactive White I). Polonium devices require periodic annual replacement and, in most countries, the systems are regulated strictly. Possession of any nuclear material requires a qualified technician and a specific radioactive-materials site license. These requirements make alpha- and gamma-emitting or X-ray technology difficult, time consuming and costly to obtain.

In operation, alpha-emitting and X-ray ionizers use the energy from an emission to separate electrons from the orbit of oxygen and nitrogen molecules in the surrounding air. Once the electron is separated, it attaches to a nearby molecular cluster to form a negative ion, the molecular cluster losing the electron then becoming a positive ion. These emissions produce bipolar ions in equal quantities, the mix of which being very homogenous, requiring the ions to be delivered to the target object quickly.

Hundreds of millions of ion pairs are produced every minute in a new ionizer, yet most are lost due to high bipolar ion density and the resulting recombination. Since polonium-210 isotope has a half-life of 138 days; ion density is reduced by 50 percent several times a year. Early high ion density and recombination tend to compensate for the loss of ion production over the one-year useful life.

Polonium ionizers do not require a concentrated electric field to generate ions. The ions are produced very near the emitter and almost immediately are in a homogenous state. This makes them a good choice in applications that require close to the surface to be neutralized. A target object is not immersed in an electric field or a space charge that could be caused by alternate production of bipolar ions. Polonium ionizers also are safe in volatile environments because they do not require an energy source capable of igniting volatile materials.

Best Practice ApplicationsThe technologies described all have strengths and weaknesses; the task is to choose the strength necessary for the application at hand. No single technology is a panacea and none is inherently bad. Generally, more emitters deliver more even coverage and faster average decay times. High-emitter density creates higher ion density to the point where recombination rates begin to offset the benefit of additional ions.

Pulse DC and its variants are the right technology for room system installations. When these systems are set up, it is necessary to choose the correct pulse rate for the performance requirement. If low offset voltages are required, fast pulse rates approaching 10 Hz will be needed. Low offset voltage has a price, however, and the price is slower decay times (see table).

Typical ionization technology applications are the result of technology's inherent strength. Technology that results in homogeneity at proximities to the emitter, such as AC and SSDC, works well in high air velocity applications that quickly carry bipolar ions to the target object. Air speed must be fast enough to minimize recombination. Pulse DC systems allow for using slower relative air velocity or applications when separation of the bipolar ions enhance the ability of the system to retard recombination. Finally, corona ionization systems require periodic cleaning of emitters as the single most important maintenance issue for prolonging life expectancy.

Measuring Ionizer PerformanceBefore selecting an ionizer, it is important to understand the application's requirements. Ionizer performance specifications that can ensure the level of product protection required must be established, together with parameters such as a safe discharge time and offset voltage for the product and process. A procedure for periodic verification of conformance to specifications after installation should be considered.

The ESD Association has developed several widely recognized methods, practices and recommendations for ionization equipment. The standard ESD STM3.1-2000 IONIZATION describes test methods designed to permit accurate comparison of ionization equipment performance and offers equipment vendors a standard reference for published specifications. The two tests described by the standard include discharge time and offset voltage.

Discharge time is described as the time necessary for a voltage (electrostatic) to decay from an initial value to a chosen final value. Typically, the measurement is made for both positive and negative charge polarities. It generally is measured from 1,000 to 100 V but can be any initial or final voltage as required by the product or process. The offset voltage measurement is described as the peak offset voltage when measuring pulse DC ionizers. It is the maximum value observed for each polarity as the ionizer cycles between positive and negative polarities.

The test instrument recommended by the standard for taking measurements is the charge plate monitor (CPM). It consists of a 6 x 6" isolated conductive plate, voltage monitor, current-limited high-voltage power supply and a timing mechanism. The test plate has a minimum 15 pfd capacitance with a total test circuit capacitance of 20 (±2) pfd.

SummaryThe mechanisms that permit uncontrolled ESD to impact manufacturing and assembly processes negatively are well understood and documented through years of research. A mounting body of evidence has developed, from various sources, supporting ionization as a tool to increase yield and quality in the manufacture and assembly of sensitive devices. Today, leading manufacturers recognize that a properly equipped and maintained ionization program is an integral part of clean manufacturing, increased productivity and lower costs.

RICHARD RODRIGO and JOHN GORCZYCA may be contacted at SIMCO Static Control and Cleanroom Products, 2257 North Penn Rd., Hatfield, PA 19440; (215) 997-0590; Fax: (215) 997-3450; E-mail: rrodrigo@esimco.com and jgorczyca@esimco.com