Subject: Time: 2:34 PM OFFICE MEMO RE>Volts per Mil ??? (part 1) Date: 2/27/96 Dave -- [log in to unmask] You queried, "I have a customer who has asked what is the required spacing on both innerlayers and outerlayers for 1500 VAC (~2100 VDC). Outerlayers have dryfilm soldermask covering them. I looked up IPC-D-275 and have referenced the data from Table 3-1. This customer feels these values are extremely conservative and is curious if other designers are using that criteria rigidly?" First I appoligize to all for the length of this response, without a doubt, it probably contains stuff you would love/like to live without. But with the industry going to greater applied electrical stress to insulative materials and the trend for industry design/manufacturing standards - I felt the information would be of value to the majority of you "techneters". Due to the length of this creation, it couldn't be sent in one chunk, I'm not sure if technet handles "enclosures", sor I'm sending this in two chunks, Part 1 and Part 2. HISTORY - The current Table 3-1 in the IPC's-D-275 was developed by a Task Group chaired by Charlie Jennings of Sandia National Laboratories (Albuquerque) back in the late 70's (if memory is correct). The Table is a composite of the electrical testing he performed at Sandia, and by some other member companies. In addition, there was a significant electrical testing program that was being conducted in Europe (I believe West Germany). The european effort (as I recall) covered electrical spacing, insulation resistance, current carrying capacity, and environmental testing. What the IPC's Task Group did was to perform some testing to obtain correlation with the european effort and to conduct some additional testing for a more complete test matrix. The environmental testing included various levels of cleaning and various operating environments, such as ocean front (salty atmosphere), busy street corners (vehicle exhaust and factory contamination). All-in-all, it was a good test that resulted in good test data/results. Now to you query / problem THE RULES/LAWS OF MOTHER NATURE First, the following is based on "STP" (standard atmospheric temperature and pressure). According to one of the laws of physics, gasses will ionize and breakdown according to Paschen's Law, which is a complex function relating at least the test anode geometry, the gas (density), properties of the gas itself, gas pressure, temperature, and electrical spacing; to identify a few. For brevity, we will focus on "air" at STP (~20 degrees Celsius and 760 mm Hg), and assume low humidity. The minimum breakdown voltage for air at STP is 250 Vac RMS (or 350 Vdc) and occurs at an electrical spacing (for near perfect rounded surface electrodes) of 7.6 micrometers [~0.0003 inch or 0.3 mils]. The approx. breakdown spacing for other voltages are as follows (and are the composite of several authors) and the equivalent electrical field stress in V/mil(inch): ~Vdc or ac (peak) 350 470 630 1100 1600 2700 5400 Spacing (mils) 0.3 1 2 5 10 20 50 V/mil 120 470 315 220 160 130 108 NOTE: These values are the breakdown voltage of air and should not be confused to the "corona" ionizaton potential (which is about 55 Vac, RMS / mil). The key point about the application of Paschen's Law/curve is that as long as your applied voltage is less than about 250 Vac RMS or 350 Vdc (or ac peak), as a rule-of-thumb, you should never have an electrical breakdown with an electrical spacing greater than 8 micrometers, or if the electrical field stress is less than ~100 Vdc/mil under absolutely perfect conditions. But you will/may have corona at higher voltages. IPC's IPC-D-275 Electrical Conductor Spacing Table 3-1 "Suggested" Design Requirements: The following is a conversion of the Table 3-1 into the type of printed board, applied voltage-spacing and equivalent electrical field stress in the form of "uniform voltage gradients". ________ Tabled Voltage, Vdc / Conductor Spacing, mils (inch) ___________ Type of PB for voltages = and > 100 B1 (internal conductors) 100/4 150/8 170/8 250/8 300/8 500/10 >500 Vdc/mil 25/1 19/1 21/1 31/8 38/1 50/1 10/1 B2 (uncoated external) 100/25 150/25 170/50 250/50 300/50 500/100 >500 Vdc/mil 4/1 6/1 ~3.5/1 5/1 6/1 5/1 5/1 B4 (Polymeric coated) 100/5 150/15 170/15 250/15 300/15 500/30 >500 Vdc/mil 20/1 10/1 ~11/1 ~17/1 20/1 ~17/1 ~8/1 As can be seen in the table, for B1 PB's (embedded or internal conductors) the electrical field stress is less than 50 Vdc/mil for all applications. For B2 PB's (uncoated surface conductive patterns, and altitude less than 10k ft) the electrical field stress is less than 6 Vdc/mil. And for type B4 (polymeric coated) PB's, the electrical field stress is less than 20 Vdc/mil. BASE MATERIAL REQUIREMENTS: The electrical characteristics of PB base materials are tested in two ways, one is Dielectric Breakdown (which is parallel to the base material laminations) per IPC-TM-650, Method 2.5.9 or ASTM D-299 and D-149, and the material requirements for epoxy-glass laminates is about 50 Vac/mil. The second is Dielectric Strength which is perpendicular to the laminations in the base material per IPC-TM-650, Method 2.5.6.2 or ASTM D-149 and the reqirements for epoxy-glass laminates is about 500 Vac/mil. Part 2 is another e-mailing -- sorry about that. Ralph Hersey Lawrence Livermore National Laboratory Phn: 510.422.7430 FAX: 510.424.6886 e-mail: [log in to unmask]