Electrical Overstress (EOS), and a subset of EOS, Electrostatic Discharge (ESD), have become major sou rces of concern over the past 10-20 years. Organizations such as the EOS/ESD Association are beginning to standardize this phenomena. While manufacturers, realizing that ESD and EOS are important, specify ESD protection when purchasing equipment, the method of ESD protection is left up to the supplier. Companies which must meet the MIL- STD often do not fully understand the requirements of the standard. As a result, costly procedures and processes are often implemented that do little to address the actual problem.
Many in the electronics industry use the acronyms ESD and EOS interchangeably. However, ESD is a specific subset of EOS, and is generally considered a han dling and packaging problem. Electrical overstress ( EOS) is a broad definition encompassing many potential sou rces an d failure modes. There are two types of failures: catastrophic, which can usually be identified by testing prior to shipment, an d latent, which is a malfunction caused by electrical overstress occurrin g during normal operation. Latent electrical overstress does not cause catastrophic failure, but is severe enough to actually weaken the part, diminishing the life of the assembly.
An integrated circuit (IC) has three primary failure modes: metal burnout, junction shorts, and dielectric breakdown. All three failures are caused by excessive current in the IC, which heats the metal through resistance heating. Voltages exceeding the specific breakdown level of the gate oxide send current through the oxide, damaging metal oxide semiconductors (MOS). Any amount of current in the oxide causes sufficient heating to cause damage. This type of voltage sensitivity has resulted in “on chip” protection for most IC’s that use MOS technology.
STEADY STATE ELECTRICAL ENERGY Steady state electrical energy may be present on the soldering tip when the alternating electrical current passing through the heater couples with the tip.Transient electrical energy can be caused by both internal and power line transients. In soldering irons, this can be caused by intermittent ground or by switching power to the heater (in a conventional iron). Most conventional irons now use zero voltage switching. Zero voltage switching attempts to switch power to the h eater when the power cycle is essentially at zero voltage.Different materials brought into contact under soldering temperatures can generate substantial DC voltages of up to several millivolts, depending on the material. This voltage is of primary interest when testing soldering irons for MIL-STD compliance.ESD ESD damage is often caused by component handling and assembly during manufacture. An insulative workpiece rubbed against the cord can generate up to 5000V. Most ESD-controlled shops use items such as ground straps, conductive floors, static dissipative clothing, and ionizers to control ESD.For a more detailed description concerning a particular application of the standards, read the MIL-STD.
Resistance between the tip of the hot soldering iron and the workstation ground shall not exceed 5.0 ohms. The poten tial differen ce between the workstation ground and the tip of the hot soldering iron shall not exceed 2 mV RMS In general, MIL-STD-2000 covers EOS and is concerned primarily with steady state and transient electrical energy. By specifying a tip-to-ground resistance, MIL-STD-2000 insures a greater margin of protection against voltage transients. The 2 mV RMS is a secondary check for current leakage. Metcal systems meet both of these specifications. Recommended test methods are specified in MIL- HDBK-2000 (see Testing Methods). The 2 mV RMS is met by most soldering irons, but is considered by many to be restrictive. Most agree that damage cannot occur below 100 mV. MIL-STD-1686A deals primarily with classifications of parts and assemblies. It makes it necessary for contractors to maintain some control over the voltage level in the handling of ESDS items. MIL-STD-1686A makes reference to DOD-HDBK-263 for guidance. It is up to the contractor and their suppliers to develop the optimum method for voltage control in the handling and assembly of ESDS items.
Tip potential measurements are usually made in order to comply with the 2 mV requirement of MIL-STD-2000A, but a variety of test methods are utilized.Thermocouple effects are DC voltage. They may be eliminated by using an AC voltmeter. The MIL-STD requires a tip potential equal to or less than 2 mV RMS. MIL-HDBK-2000 specifies that this voltage be measured in a specific frequency range (50-500 Hz). A meter sensitive to DC voltages (0 Hz) or AC voltages outside this range should not be used. If a TRMS meter with an expanded frequency response is utilized (HP3400A @ 10 MHz), measurements greater than 2 mV RMS will often result. Local AM radio stations can induce 2 mV (at 1.2 MHz) or more depending on the system layout. A hand-held meter like the Fluke 8060A multimeter provides adequate frequency response, but will not pick up potential sources of error such as local radio stations. Filters can be incorporated to limit the frequency response of a given meter (see Appendix A). Be aware that Metcal Enhanced Systems utilize 10 mA DC current, which results in 1 mV DC potential at the tip. This should not affect results if proper procedures are followed.Similar precautions need to be taken when measuring tip-to-ground resistance as specified in MIL-STD-2000.
Thermocouple effects also need to be taken into consideration when performing this test. Appendix B provides a method for measuring Measurement of surface resistivity has been the subject of some discussion. Metcal tests incoming materials per ASTM D257, DC Resistance or Conductance of Insulating Materials. The reader will note that this specification is primarily used for measurement of insulating materials, but it is also the accepted standard for many manufacturers of test equipmen t. Metcal u ses a Monroe Electronics Portable Surface Resistivity Meter for conformance testing of material couponstip-to-ground resistance.
The primary concern when considering EOS/ESD protection in soldering systems is to limit steady state and transient electrical energy. Metcal offers superior protection in this area. Because Metcal systems deliver continuous current, they cannot generate switching transients. Metcal’s cartridge tip and hand cord assembly guarantee good ground path with little maintenance. This keeps the tip current leakage and tip potential at a minimum. Typical values for tip-to- ground resistance and tip potential are 0.8 ohms and 1.0mV, well within the MIL-STD specification.Metcal also offers extra protection against potential ground faults in its Enhanced Systems with an Auto- Off feature. This feature senses the DC continuity (resistance) of the output cable and handle assembly through the cartridge. If the resistance of the output circuit exceeds a preset reference level, the system is turned off. This is fail-safe protection against any ground loss.
Metcal also protects against ESD by making static dissipative (105-1012 ohms/sq surface resistivity) all surfaces that are in direct contact with soldered components static dissipative. And where possible, Metcal uses static dissipative materials with the stricter surface resistivity of 10 5 - 109 ohms/sq. This limits the opportunity for any substantial static charge build-up. However, the user concerned with ESD protection must still insure that the work surface is made of static dissipative material and is properly grounded. A static dissipative surface insures efficient, but controlled, dissipation of static charges. Dissipative surfaces will also distribute the charge over the entire surface, limiting the possibility of point charge buildup. For information on how to check static dissipative material for surface resistivity, see Testing Methods.
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