Hi Techies, sorrysorrysorry for the mismail before! Ok, now the finished mail: I have to analyze failures of a subassemblies of our subcontractor, a well-known european hi-tech pcb-manufacturer. The failing subassemblies I've to analyze, contains transformer windings which are implemented as 14 and 16-layer pcb's. The winding shows dielectric breakdown between different winding layers (and sometimes between different windings on the same layer). This breakdown happens after some hours run-in at 120°C and 500VDC (500VDC as test voltage, failures also at lower tension at at AC). Before them, all pcb has passed successful a hi-voltage test at ambient temperature. The diel. strength for the closest distance of 65mm prepreg should be around 2.5kV, or a little bit less because IPC-2221, § 4.1.2, not 500V! May be any loss of the electric strenght of a laminate/prepreg is a function of the temperature, the hi-temp aging, and material defects (the occuring pcb temperature of 120°C is in accordance with IPC-2222, § 4.3. But this must be in a specified manner, isn't it? Okydky (so weit - so gut), now the pcb-manufacturer's statement is that we can not use the max. electrical strenght at max. temperature. And the pcb-manufacturer is not able to define a derating of the el. strenght at a given temperature. Hard to believe ... So, my question is: means IPC I can use the maximal specified electrical strenght at maximal specified temperature, or not? (If not, I would like to append some further questions..) As I have reviewed some IPC-standards, there are some "little" limitations in view of limitation of Temprature, electrical strenght, dielectric thickness, maximum operating temperature. But none of this comment means we could not use the electrical strenght at max. allowed operating temperature: Designstandards IPC-2221, Generic Standard on Printed Board Design § 4.1.2 Material Selection for Electrical Properties, refers to Table 4-1, Typical Properties of Common Dielectric Materials. Thereafter for FR-4 the electric strength amounts 39.4 x 103 V/mm2 and notice notice says : "The stated electrical strength values are commonly evaluated under test conditions with a 0.125mm core laminate thickness. These values should not be considered linear for high voltage designs with a minimum dielectric separation, i.e., less than 0.09mm." § 6.3 Electrical Clearance, refers to Table 6-1. Thereafter, the minimum spacing for internal conductors (B1) at 301-500V (peak) amounts 0.2mm. §10.1.2 Electrical Clearance, "Clearances are applicable for all levels of design complexity (A, B, C) and performances classes (1, 2, 3) (...)". IPC-2222, Sectional Design Standard for Rigrid Organic Printed Boards § 4.3 Laminate Materials, "The values in Tabele 4-1 are based on long term aging tests by UL (...) Hot spot temperatures shall not exceed the temperatures specified in Table 4-1 (...)"Table 4-1: "Clad Laminate Maximum Operating Temperatures", thereafter for FR-4 and 0.1 mm dielectric thickness the max. allowed temperature would be 120°C. For FR4 / 0.4mm diel. thickn. 130°C. § 4.3 Measurement of Dielectric Thickness, " (...) Thickness by microsection (...) taken at the closest point between metal cladings." § 4.3.2 Dielectric Thickness/Spacing, " (..) If the minimum dielectric spacing and the number of reinforcing layers are not specified, the minimum dielectric spacing is 0.09mm and the number of reinforcing layers may be selected by the supplier." § 4.3.4.3 Glass style, "A variety of glass cloth styles are available for prepregs (see IPC-EG-140). The glass cloth selection is dependent upon dielectric thickness and tolerance required, circuit filling needs, and electrical requirements of the dielectric." § 4.3.7 Laminate Material, explains the Material Code Designation and refers to Figure 4-2 for the recommended material selection process. Thanks for clarifications and all statements! Bernhard ############################################################## TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c ############################################################## To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body: To subscribe: SUBSCRIBE TECHNET <your full name> To unsubscribe: SIGNOFF TECHNET ############################################################## Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional information. If you need assistance - contact Gayatri Sardeshpande at [log in to unmask] or 847-509-9700 ext.5365 ##############################################################