1) Your volts/mil is very high indeed. Even if it does not exceed any
(static) specifications from some kind of standard, you will find that
actual performance degrades with temperature and altitude!
2) The thinner your material the more pronounced any little material defect
becomes, making it more difficult to meet specs.
3) The specs mentioned so far apply inside the material and are quite
different at the edges or surfaces of the material. Where does the actual
failure occur?
4) With FR-4 you are not dealing with a homogenous material. Therefore you
have to consider the differences in dielectric constant for the different
materials (like the glass and the epoxy). The fieldstrength across 2
materials with the same thickness is not the same when the dielectric
constant differs. Therefore you can have very high local fieldstrenths
resulting in corona, which will cause failure over time.
To verify whether corona is present, you can place a resistor (as high as a
MOhm) in the return wire of your test set-up and using a scope to check
whether noise is present when voltage is applied, or you can listen for it
using an ultrasonic transducer.
Good luck!
Ahne Oosterhof, A-Laser, Inc
(503)641-9428
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Wanner Bernhard
Sent: Friday, December 10, 1999 7:36 AM
To: [log in to unmask]
Subject: [TN] WG: dielectric II (arghh! sorry!)
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
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