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Question I am trying to determine the standard spec/guideline for which I I Stanley should be controlling temperature/humidity. They are contemplating purchasing elaborate equipment to control these variables in plant. Their concerns are ESD along with solderability (paste etc.). Any help would be appreciated. -Troy
Answer The ANSI/ESD-S20.20-1999, ESD Association Standard for the Development of an Electrostatic Discharge Control Program for - Protection of Electrical and Electronic Parts, Assemblies and Equipment (Excluding Electrically Initiated Explosive Devices) recommends a relative humidity >30% RH <70%. Humidity can have an effect on increasing corrosion on the metal pads or contacts, then causing problems with the interconnects. Usually, humidity helps to reduce the overall charge build-up in the local environment to help reduce ESD Events. MIL-STD-1695 specifically addresses relative humidity levels in the range of 30 - 70 percent in areas where electronic parts and hybrid microcircuits (MIL-STD-1695, b work areas 5 and 6) are handled or processed. MIL-HDBK-263 Standard - Relative humidity between 40 percent and 60 percent in ESD protective areas is desirable as long as it does not result in corrosion or in other detrimental effects such as PWB delamination during soldering. The ultimate range should be determined by your facilities needs, building constraints and your ESD control program. An increase in relative humidity (RH) increases the water content in the air. This increases the natural conductivity of the air which increases the natural recombination of charged molecules (neutralization). When an item becomes charged, the surface is in an imbalance charged state. An increase of natural recombination from an increase of water content in the air will increase the rate of natural neutralization on charged surfaces. (refer to addition info on humidity below) humidity: moisture content of the atmosphere, a major element of CLIMATE. Humidity measurements include absolute humidity, the mass of water vapor per unit volume of natural air; relative humidity (usually meant when the term humidity alone is used), the ratio of the actual water-vapor content of the air to its total capacity at a given temperature; and specific humidity, the mass of water vapor per unit mass of natural air. Relative humidity is usually measured by means of a HYGROMETER. The National Weather Service’s temperature-humidity index gives a single numerical value that serves as a measure of comfort (or discomfort) during warm weather. When the index is at 70, most people feel comfortable; at 75 about one half are comfortable; at 80 most are uncomfortable.Here is a link to a white paper with a table from a paper “Exploding the Humidity Half-Truth and Other Dangerous Myths”, Moss, R., EOS/ESD Technology Magazine, page 10, April 1987. As you can see from this table, a difference of 60% RH, can yield an increase by a factor of 3 to well over 40 in charge generation relative to the acquired voltage level. Controlling RH is one way to aid your program, but should not be substituted for a complete well rounded program, on setting up an ESD control program. Question: Should humidity be controlled in ESD protected areas? To what upper & lower limits? Answer: Yes, consistent with cost, it is desirable to have ESD protected areas be humidity control. Tribocharging will generate higher voltages as the humidity decreases, so higher humidity is better for ESD control. However, higher humidity will cause employee discomfort & facilitate corrosion and delamination.Per ANSI/ESD S20.20-1999, the recommended range for humidity is greater than 30% relative humidity but less than 70% [see Table 1 ESD Control Program Technical Requirements Summary].MIL-STD-1695 specifically addresses relative humidity levels in the range of 30 - 70 percent in areas where electronic parts and hybrid microcircuits (MIL-STD-1695, b work areas 5 and 6) are handled or processed. MIL-HDBK-263 Standard - Relative humidity between 40 percent and 60 percent in ESD protective areas is desirable as long as it does not result in corrosion or in other detrimental effects such as PWB delamination during soldering.The range should be determined by facility needs, building constraints and ESD sensitive products that are handled. This range should include a temperature range and be documented in the written ESD control program.As Table below shows, triboelectric charging persists even at high relative humidity. The fact remains that triboelectric charging becomes troublesome below 30% relative humidity, as shown by the high voltages attained at 20% RH in the Table 1. According to Koyler ET. Al. [1], Relative humidity values should include an associated temperature because a temperature factor is involved in surface resistivity.STATIC VOLTAGES GENERATED BY:ACTIVITY (@ 70 degrees F) 20 % RH 80% RHWalking across vinyl floor 12,000 Volts 250 VoltsWalking across synthetic carpet 35,000 Volts 1,500 VoltsArising from foam cushion 18,000 Volts 1,500 VoltsPicking up polyethylene bag 20,000 Volts 600 Voltssliding styrene box on carpet 18 ,000 Volts 1,500 VoltsRemoving Mylar tape from PC board 12,000 Volts 1,500 VoltsShrinkable film on PC board 16 ,000 Volts 3,000 VoltsTriggering vacuum solder remover 8,000 Volts 1,000 Volts Aerosol circuit freeze spray 15,000 Volts 5,000 Volts Temperature affects how the air holds moisture and is used to derive Relative Humidity (RH). The warmer the air (the more energetic the air molecules get), the more water moisture the air can hold. Relative Humidity, (usually meant when the term humidity alone is used), is the ratio of the actual water-vapor content of the air to its total capacity at a given temperature; and specific humidity, the mass of water vapor per unit mass of natural air. In ESD, it is the water content in the air plus the energy of these water molecules to collide with surfaces in our space (workbenches, clothes, hair, etc.) The more water content and/or energy, the more collisions and neutralization (tendencies found in warm air) versus the more sedate, less reactive water content/air molecules in colder temperatures. Therefore colder air tends to enhance the natural phenomena of triboelectric generation because it does not inhibit this process as much as warmer air. Below is attached an easier to comprehend explanation of this phenomena: Link: http://veggie.org/run/humidity.shtml Relative Humidity....Relative to What? by Steve Horstmeyer, Meteorologist The Dew Point Temperature - A Better ApproachThe ConceptHumidity is a complicated concept. Humidity refers to evaporated water substance in the air, i.e. water vapor. For years relative humidity has been what we use to inform the TV audience, but it has many pit falls and to tell you the truth almost no one understands it, including many meteorologists. Think Energy When thinking of humidity you always have to think in terms of energy. Of course this is expressed as temperature. There is only so much energy to go around and only a certain part of the thermal energy can go to do the work of evaporation. The rest goes to the other molecules in the air. Relative humidity expresses how much of the available energy has been used. A relative humidity of 50% means half the energy has been used to evaporate water from the ground, streams, lakes anywhere else it is and 50% is still available to do more evaporation. Relative to What? On a summer morning the temperature may be 75 degrees and the relative humidity 90%, a very sticky morning indeed. Without changing the amount of water vapor in the air if the temperature hits 92 degrees in the afternoon the relative humidity is 52%. Relative humidity is RELATIVE TO THE AMOUNT OF ENERGY AVAILABLE Because the amount of energy increased as the sun warmed the atmosphere the percentage of the energy used decreased, i.e. the relative humidity, all the while there was no change in the amount of vapor in the air.So when you hear someone say its feels worse than 52% relative humidity today, they do not understand the concept of RELATIVE HUMIDITY. Ninety-two degrees and 52% is a very humid afternoon. Because the concept is confusing many meteorologists are trying to get use dew point temperature. The Dew Point Temperature Dew point temperature is a measure of humidity. If you take a parcel of air, and cool it, eventually you will remove enough energy to begin to get water vapor to condense. Remember the water vapor was originally liquid water and to get it to evaporate you had to add energy. As long as it has sufficient energy it will remain vapor, but as you cool it at some point condensation will occur. The temperature where condensation begins is the dew point temperature. In terms of relative humidity, as the parcel of air is cooled, the relative humidity increases, when the relative humidity reaches 100%, you are at the dew point temperature. Dew Point vs. R.H.Unlike relative humidity if dew point increases, it is only because the amount of moisture increases. If relative changes it can be because of temperature change or moisture change, two variables leads to too many possibilities, with dew point it is strictly moisture you are tracking. Dew point can never be higher than the temperature, at saturation, i.e. 100% relative humidity the temperature and dew point are the same. So How Does Dew Point Feel? On a typical summer day the following apply: Dew Point(F)..........Perception 75+....................Extremely uncomfortable 70-74...................Very humid, quite unfomfortable 65-69...................A bit uncomfortable for most people 60-64...................Ok for most, but everyone begins to feel the humidity 55-59...................Comfortable 50-54...................Very comfortable I...................Feels like the west, very pleasant, a bit dry to some A Common Misconception One last thing if you ever hear someone say it was 90 degrees and the humidity was 90%, that has never happened in Cincinnati, (and unless the greenhouse effect goes into overdrive never will). 90 deg/90% requires a dew point of 85.5 degrees. In Cincinnati the highest ever dew point was 81 deg. for just a few minutes. In August 1995 we had four hours of 78,79,78,77 degree dew points, the highest persistent dew points I have seen in Cincinnati since working here as a meteorologist. For one hour I did see a dew point of 81, just after a thunderstorm. World Record Dew PointsHowever veterans of the Persian Gulf War know what 90/90 feels like. The Persian Gulf and Red Sea both attain sea water temperatures in the mid 90’s. That’s plenty of energy, along with the 115 degree air temperatures, to evaporate water. The dew point has been measured on the shore of Ethiopia, the area is now part of Eritrea, at 94 deg. F. The highest known dew point temperature in the world. The relative humidity with a temp of 115 and a dew point of 94 is 54% this doesn’t tell you as much as the dew point when you consider the table above. © 1995 Steven L. Horstmeyer, all rights reserved
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