TECHNET Archives

December 1999

TechNet@IPC.ORG

Options: Use Monospaced Font
Show Text Part by Default
Show All Mail Headers

Message: [<< First] [< Prev] [Next >] [Last >>]
Topic: [<< First] [< Prev] [Next >] [Last >>]
Author: [<< First] [< Prev] [Next >] [Last >>]

Print Reply
Subject:
From:
Wanner Bernhard <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Fri, 10 Dec 1999 16:35:34 +0100
Content-Type:
text/plain
Parts/Attachments:
text/plain (88 lines)
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
##############################################################

ATOM RSS1 RSS2