I'm not sure that's unacceptable...soldermask is expected to have a
dielectric strength of 500V/mil (per IPC-SM-840C), which equates to almost
20,000 V/mm. The IPC-4101 requires FR-4 to withstand 29,400V/mm for FR-4
<0.78 mm and 40,000 V when >= 0.78 mm.
Timothy Reeves
ECD Circuit Board Division
13626 South Freeman Road
Mulino, OR 97042
[log in to unmask]
(503) 829-9108 (800) 228-8198 FAX (503) 829-5482
> ----------
> From: Brian Ellis
> Sent: Friday, December 10, 1999 8:51 AM
> Subject: Re: WG: dielectric II (arghh! sorry!)
>
> Wanner
>
> I'm not an expert in the matter but your message inspires me to say three
> things.
> 1. 500 V over 65 micrometres gives just one helluva voltage gradient,
> about 8 000 V/mm,
> if I'm not mistaken. This is at the limit of the acceptable IMHO, even at
> ambient.
> 2. At 120°C, you are approaching the Tg of the laminate, where the
> electrical
> characteristics of the resin go haywire. I believe that Tg is not a sharp
> threshold,
> where you have one phase below and one phase above, but a gradual change
> over a range
> of 20 - 30°C. Furthermore, the range can be modified by the degree of cure
> (i.e. the MW
> of the crosslinked resin) and the humidity content (i.e., it is known that
> the Tg can
> drop by 20° when the resin is close to saturation, generally between 2 and
> 5% by weight
> of the resin.
> 3. A laminate is heterogeneous. Pressing a thin single prepreg to give a
> 65 um layer
> will give precious little resin above and below the silane-coated glass
> fibres.
>
> Under these circumstances, combined, I think you are approaching the
> characteristics of
> an arc welder rather than those of a test method. Pragmatically, I think
> you should not
> test or function at higher than 30 - 40°C under the Tg when you have high
> voltage
> gradients in heterogeneous insulants and that you should ensure that your
> supplier
> presses your boards to a full cure, using only first-rate materials that
> have been
> tested for a high MW polymerisation.
>
> My 2 centimes worth.
>
> Brian
> -
> Brian Ellis
> Protonique SA
> PO Box 78
> CH-1032 Romanel-sur-Lausanne, Switzerland
> Voice: +41 21-648 23 34 Fax: +41 21-648 24 11
> E-mail: [log in to unmask]
> URL: Technical and consultancy divisions:
> http://www.protonique.com
> Web services division:
> http://www.protonique.com/webserv
>
>
> Wanner Bernhard wrote:
>
> > 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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