> From: Brian O'Connell
> Sent: Tuesday, December 16, 2003 1:00 PM
> I would be very interested in yours and others'
> experience with direct NRTL engineering opinions
> on this subject.
Gladly.
For reference, I worked at UL for ten years, during which
time I evaluated a broad range of products, including
transformers (most sizes and types that UL has a standard
for), household audio, premises power distribution
equipment, spas and hot tubs, fuses (branch circuit and
supplementary protection types), energy management
equipment, attachment plugs, telephone equipment, ITE,
linear and switching power supplies, plastics, architectural
and theater dimmers, PWBs and their coatings, circuit
breakers ...
I've dealt with conformal coatings from both ends: on the
end-product evaluation side and the coating evaluation side.
Mind you, it's been several years since I've done these
evaluations directly, hence some cobwebs will need dusting
away.
> When CC is used to meet spacing requirements, it
> can be considered a component of an insulation
> system. And UL may require, for some boards, a
> Type Test based on 1446. Although it seems to be
> more typical for the application and testing of
> CC to be specified in the end-item std (e.g.,
> 60950-1, cl 1.5.2).
I can see this occurring for planar transformers that are
built up from industrial laminates (the same as any garden
variety circuit board) and intended to operate at
temperatures typical of Class B insulation systems or
higher, but not in the general case for conformal coatings.
UL's transformer standards (most, not all) have a normalized
reference to UL1446, so it is 'natural' for these standards
and the product categories they service to reference UL1446
for at least a portion of conformal coating evaluations.
Keep in mind that UL1446 would not normally apply unless an
insulation system was other than Class A.
> If the CC application is not required to meet the
> requirements of the end-item std, then its use
> must verify that it does not affect the
> electrical or mechanical properties of the end-use box.
I suppose a coating could act as a solvent (that is, the
coating's solvent) to soften the board, but by-and-large, it
would have to be a particularly brutal solvent, since most
PWBs are thermoset polymers with lots of cross-linking.
But, that's an area of materials science I'm weak in.
> Some Section Generals of FUS reports also require
> that the CC process be specified, monitored and
> subject to recurring tests. I am not certain if
> the process control applys only to usage required
> to meet spacing and di-electric withstand.
In my days at UL (four years of which included plastics and
PWBs), the FUS Procedures I set up, where a conformal
coating was relied upon to meet safety requirements,
included:
* coating manufacturer's name a grade designation
* the substrate used on (usually by ANSI type)
* the copper weight (thickness)
* any required surface preparations, priming, thermal
preconditioning, etc.
* minimum and maximum thicknesses of the coating (too thin
is ineffective, to thick might effect flammability of the
combined materials)
* cure times/temperatures/methods
* and, if need be, sample selection for follow-up testing
An item most often missed all around is that conformal
coatings are most commonly evaluated for use _directly on
the laminate surface_, without interposing solder resists or
the like. Use on top of a solder resist required tying the
manufacturer's to not only the above, but also to only those
resists evaluated in combination with the substrate and
coating. A pricey and very restrictive proposition.
Other items might be important for traceability of the
coating, if it wasn't under separate FUS.
These would normally have been placed in a special appendix
to the appendix pages, rather than the section general,
since the philosophy for section generals was they should
only contain information common to all Procedure Sections
and an appendix could point to any specific Section or
product in a Section.
(For those not familiar with UL methods, a Follow-Up
Services Procedure is an inspection document, separate from
the test report, and are established for a combination of
Applicant [bill payer], Listee, Classified or Recognized
Company [name on the product] and a collection of
manufacturers; there might be anywhere from one to hundreds
of Sections in a Procedure.)
In that time, planar transformers were uncommon, but
beginning to emerge for special applications, and I had no
direct experience with them. In the case of boards, UL1446
wasn't considered, but things can change.
> According to UL engineers that I've talked to,
> applicable UL stds for PWBs are 94, 746, 1446,
> and of course 796. But the only thing that UL796
> seems to say about CC is that (cl 10.1) it shall
> be evaluated for flammability and any mechanical
> effect on other PWB materials.
UL94 for flammability test methods and compliance criteria
UL746A for short term, small scale electrical properties
(HWI, HAI, CTI ...)
UL746B for long term, small scale electrical properties (RTI
...)
UL746E for industrial laminates (PWB substrates)
UL1446 for PWBs? Only under very specific conditions, I
suspect, but certainly not in the general case.
> While 796 has specific electrical and mechanical
> performance requirements for PWBs, the target
> seems to be the board house, not the end-use item
> mfr; which is where the CC is probably applied.
True, but a lot of the larger board houses also stuff boards
and would likely apply any conformal coatings a customer
requires. Increasingly so today, with the enormous amount
of outsourcing of manufacturing. There are also specialty
houses that only stuff boards and would likely apply
conformal coatings.
> The basic std for CCN QMJU is UL94
Careful there. UL94 is referenced, but nowhere in the Guide
Card for QMJU2 does it state that UL94 is the basic standard
used to evaluate a conformal coating. After all, UL94 is
only a flammability test specification and will never
address a coating used in lieu of electrical spacings.
> part of the
> scope says "The methods described in this
> Standard involve standard size specimens and are
> intended to be used solely to measure and
> describe the flammability properties of
> materials...". So UL94 cannot be used by itself
> for end-item application of CC.
Quite so.
> luck,
...be a lady tonight.
Regards,
Peter L. Tarver, PE
[email protected]
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