Peter and other Techies,
With apologies for this very tardy response - and I won't bore you with the
why's and where fore's......and no, it wasn't because I couldn't. So, better
late than never and written at 34,000 feet - that's 10,000 meters in new
money:
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Peter Swanson
Sent: 10 December 2001 17:13
To: [log in to unmask]
Subject: Re: [TN] 2 Conformal Coating ?'s
Graham,
UV or light curing coatings:
- Solvent free
As are many thermal and moisture curing silicones. The more recent
developments in water based technology also offer a new solution to the
problem (pardon the intended pun).
- Full cure in seconds, on-line
Yes but this is a misleading statement. Conformal Coatings are intended to
be protective envelopes, conforming to the varied geometry of a printed
circuit assembly and should, ideally, be applied under as well as over,
components. This then begs the question of how to ensure the coating is
properly cured in areas where light cannot be readily applied. The method is
generally to incorporate a secondary curing mechanism thus rendering the
material more akin to a 2 part rather than:
- Single part
As I suggest above, these secondary curing agents often influence the pot
life and the shelf life of the end product. This can be a major drawback for
the smaller / lower volume electronic producers.
Virtually all modern conformal coatings are single part products, and
providing long pot and shelf life, not uncommonly around 12 months, is the
norm. This does not always apply to UV curable coatings nor indeed, to most
silicones.
- Non flammable
What do you mean by non-flammable? Are you referring to the liquid or cured
material?
Materials that are qualified / approved to such specifications as
MIL-I-46058 - IPC-CC-830 - Def Std 59/47 issue 4 - UL - etc., must meet
certain "flammability" tests in their cured state. This would not represent
any particular advantage of UV over conventional coatings.
In their liquid state, I frankly doubt that any coating material has any
greater or lesser degree of flammability. Being "resins or polymers" or
otherwise plastic materials, they have a tendency to "burn" in a rather
dramatic way, under the right conditions.
- Can meet MIL I-46058C and UL
Can meet? This is a rather curious term as it implies that many do not meet
such specifications. I am aware of many UV coatings that cannot meet the
requirements of such specifications. This is quite often in respect to their
flexibility at extremely low temperatures.
I would like to make a point here about such terms as "meets the
requirements of..." or "meets or exceeds...". Either a coating is qualified
/ approved or it is not. It costs a great deal to obtain such qualifications
/ approvals, and is one of the issues delaying the supercession of the MIL
Spec by the IPC-CC-830 - that and the issue regarding independent testing
required under the prevailing MIL spec..
- Good to excellent environmental resistance
Are you suggesting above and beyond conventional materials?
- Can be "rigid" or "flexible" (Shore D80 to Shore D40)
I am not sure that this is a stipulation within the prevailing
specifications. What benefit are you alluding to?
- Can be used on flex
Uniquely? Not in my experience.
- Can have strong fluorescence for inspection, or even be black
A requirement of the MIL Spec and Defence Standards (British) is that the
conformal coating shall not obscure markings on the board and components to
facilitate repair and rework. Whilst this is not usually a requirement for
"commercial" applications, it is fair to say that the addition of pigments
or dye's can greatly influence the performance of the coating under extreme
conditions.
The Military require that qualified coatings contain a UV tracer to aid
inspection. I think it fair to say, that the half-life of the tracer is
affected by its exposure to strong UV light during cure.
There are "black" or indeed other coloured coatings available however, the
addition of such pigments, as I said above, affect performance particularly
in respect to moisture resistance. Such pigments effectively "separate" the
coating molecules leaving space for moisture to more easily migrate in and
out of the film.
Another point - why black or coloured material? It has been my experience
that the customer is often seeking a means of "hiding" the circuit design.
This is not likely to be very successful. Clever use of chemical stripping
agents or mechanical removal of the coating can be easily effected.
They are not suitable for all applications - are better sprayed, not so good
for dipping. If sprayed (say, using PVA or Nordson type selective spray
equipment), then full cure can often be achieved just with light. Otherwise,
secondary cures for shadowed areas may be achieved with heat, anaerobic,
moisture or other mechanisms.
"Horses for courses" is an oft quoted expression - at least by me! UV
curable coatings have many applications: Fibre-optics, LCD's, some
photo-cells, most 2D applications etc.. As we built (probably?) the world's
first in-line dip and UV curing system in 1987, we learnt the hard way, the
many process issues that need to be addressed.
As an example, and as you say Peter, dip coating is not its forte. The
coating area must be separated from the curing zone, no stray light can get
to the tank and no-one should be able to view the light, or it would be the
last thing they saw. Furthermore, the assemblies have to be racked onto the
jigs / carriers face-on, so that the light can shine forth upon the face of
the assembly. This meant that the system has 1/3rd the capacity / throughput
of a conventional cure coating.
Robotic selective coating systems revolutionised the coating business -
thankfully! Yes, these coatings are much more applicable to this method but,
the time it takes to apply the product remains the same whatever the coating
type. Allowing due separation from the application zone, there is not too
much advantage to be gained by using light instead of heat.
To answer Mike Fenner's question, most pcb coatings are acrylic based, which
is a free radical cure. Expose to the correct wavelength/intensity of light
for required time and full cure is achieved. In fact, if the light is taken
away, curing stops. All the coating must be exposed to the light.
There are some coatings (a few) based on epoxy chemistry and a cationic
cure. Some cross linking will continue after the light is removed, but only
for thin sections (i.e. it doesn't propagate far), and just like the free
radical cure, all the coating must be exposed to the light.
You see, its all done with mirrors!
I refuse to be drawn into a discussion on free radicals. People who know me
will know why - I am not a chemist, but the worlds greatest parrot!!
So, an interesting technology which certainly has its place in the panoply
of coatings available for circuit protection. Whilst not a universal
panacea, they can often offer processing speed and simplicity, including
perhaps some environmental and health & safety advantages.
No argument there.
IMHO, they should not be dismissed totally out of hand by competitors to the
technology in an open forum; for one thing, this might insult the many
companies who are successfully using them!
IMHO you are correct. Unlike most suppliers, we offer a comprehensive range
of coatings including UV's.
Regards,
Peter
=========================================================
Peter Swanson [log in to unmask]
INTERTRONICS http://www.intertronics.co.uk
INTERTRONICS is dedicated to providing quality material,
consumable and equipment solutions to the electronics
manufacturing and other technology based industries, with
the highest levels of technical support and customer
service.
I hope this will be taken in the spirit in which it is offered and - if you
will pardon the pun: shed more light on the subject. I am sure that I speak
for Peter in inviting comments to the above exchange that may help engineers
decide on the best route for their product.
Regards Graham Naisbitt
[log in to unmask] <mailto:[log in to unmask]>
Concoat Limited
Alasan House, Albany Park
Camberley GU16 7PH - UK
www.concoat.co.uk <http://www.concoat.co.uk>
Phone: +44 1276 691100
Fax: +44 1276 691227
Mobile: +44 79 6858 2121
-----Original Message-----
From: Graham Naisbitt [mailto:[log in to unmask]]
Sent: 09 December 2001 19:01
To: [log in to unmask]
Subject: Re: [TN] 2 Conformal Coating ?'s
Peter,
I am semi-comatose now, so please, tell us what exactly are the advantages
of UV curable coatings?
Regards, Graham Naisbitt
[log in to unmask]
www.concoat.co.uk <http://www.concoat.co.uk>
For instant access to Product Data Sheets register on the Tech-Shot area of
http:// www.concoat.co.uk <http://www.concoat.co.uk>
Concoat Limited
Alasan House, Albany Park
CAMBERLEY GU16 7PH UK
Phone: +44 (0)1276 691100
Fax: +44 (0)1276 691227
Mobile: +44 (0)79 6858 2121
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Peter Swanson
Sent: Friday, December 07, 2001 05:51
To: [log in to unmask]
Subject: Re: [TN] 2 Conformal Coating ?'s
Graham,
You really must see someone about your delusions over UV cured coatings,
which are successfully used to coat many thousands of pcb's each year. I
know someone who has a nice sofa you can stretch out on whilst you tell him
all about it.... <g>
Suitability for UV has nothing to do with 3D v. 2D, more to do with the
complexity of the assembly geometry and the application methodology. These
will determine if a secondary cure mechanism is needed for the application.
In many instances (and with some of our customers), full cure is achieved
just with light.
Regards,
Peter
--
--------------------------------------------------------
Peter Swanson [log in to unmask]
INTERTRONICS http://www.intertronics.co.uk
INTERTRONICS is dedicated to providing quality
material, consumable and equipment solutions to the
electronics manufacturing and other technology based
industries, with the highest levels of technical
support and customer service.
-----Original Message-----
From: Graham Naisbitt [mailto:[log in to unmask]]
Sent: 06 December 2001 16:52
To: [log in to unmask]
Subject: Re: [TN] 2 Conformal Coating ?'s
Mike
You would be better to go with a conventional coating. UV is not ideal for
3D objects - works great in 2D, cos you don't have to bend light! :-)
I cannot remember what the operating environment was for your circuit - if
you would care to repeat, I can recommend something - however, I am sure
that a one part AR or UR coating from a reliable supplier will give you the
best overall results - process and operating environment.
Regards, Graham Naisbitt
[log in to unmask]
www.concoat.co.uk <http://www.concoat.co.uk>
For instant access to Product Data Sheets register on the Tech-Shot area of
http:// www.concoat.co.uk <http://www.concoat.co.uk>
Concoat Limited
Alasan House, Albany Park
CAMBERLEY GU16 7PH UK
Phone: +44 (0)1276 691100
Fax: +44 (0)1276 691227
Mobile: +44 (0)79 6858 2121
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Mike Manwell
Sent: Thursday, December 06, 2001 02:41
To: [log in to unmask]
Subject: [TN] 2 Conformal Coating ?'s
Hello,
Thanks in advance if you have taken the time to read this!
#1- Im trying different materials in a spray application. Both are cured
using UV. One material does a "secondary cure" w/ moisture. This is cool
because after 100 hours after UV cure, all material is fully cured.
The other material Im trying uses HEAT for a secondary cure, or, if
shodowed during UV cure, the material wont cure and never will. Obviously I
dont want the stuff dripping or running anywhere, but being new to this
wonderful process, just dont feel good about leaving uncured material on a
product for it's entire life. Iv been told that you can "test" the product
to see if the uncured material is causing a problem but Im not sure what
type of test that would consist of. Any insight?
#2- Has anyone out there ever installed refelctive metal plates in their UV
oven to limit shadowing?
Thanks so much,
Mike Manwell
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