Julie Dobbins wrote:
> I am looking for information about the reliability effects of haloing on
> PWBs. I am experiencing high fallout for edge haloing that reduces the
> edge spacing by more than 50%. Please respond if you know of anywhere I
> can find information on this condition.
>
Hi Julie--
I doubt if you'll get a response to the reliability effects of printed board "edge haloing", I've never
seen anything published or reported on the subject. If you haven't, you might try "technet's" archives on
the subject, though I don't remember anything over the last couple of years.
FIRST - determine what caused haloing in the first place. Is it manufacturing processes/practices or is
it a material problem -- or both. Was it mechanically induced by a "depaneling operations", etc.
The concerns about haloing is that it may entrap contaminants (due to printed board manufacturing or
assembly operations), which due to functional use environments of high humidities may contribute to reduced
insulation resistance, or worst case CAF (cathodic/anodic filament growths).
For space or reduced atmospheric pressure applications, haloing is a contributor to long-term outgassing,
and possible contaminations due to the vapor pressure characteristics of the contaminant. If an attempt is
made to "encapsulate" the haloing by conformal coating, then a "pressure" vessel has been included in the
design, and it will depressurize with time and temperature. Also, due to reduced pressure (increased
pressure in the haloing are) the haloing may propagate due to additional delamination of the base material.
Your questions suggests that these are printed boards and not assemblies. The next concern is whether the
haloing will propagate due to the thermal abuse printed boards receive during assembly, or during handling
(flexing the printed board or assembly) especially if these printed board assemblies are placed in ATE
fixtures.
In reality, haloing is probably more cosmetic than functional most of the time. But this depends on
whether the design is double-sided or multilayer; if multilayer on layer-to-layer spacing;
conductor-to-edge spacing, at each layer; and how "gross" the haloing is - meaning what percentage it is
of the thickness of the printed board or of layer-to-layer spacing.
In the newer design document IPC-2222, for "rigid printed boards", 10.1.1 Edge Spacing states "..... the
minimum distance between conductive surfaces and the edge of the finished board, or a non-plated-through
hole, SHALL, not be less than the minimum spacing in Table 6-1 of IPC-2221 plus 0.5 mm."
So if the haloing doesn't propagate, then you could use the above requirement, and the printed board should
be functionally OK for all Classes of Product.
If they don't meet the above design requirement - then the layer-to-layer spacing and the location needs to
be determined. If per chance they are double-sided printed boards (though I doubt it), from a practical
stand point, the haloing could bridge from the edge of the printed board to the conductive patterns, and it
most probably would not be a problem for application voltages <~50Vdc. This is because there is sufficient
remaining dielectric to meet the minimum dielectric thickness requirements for 50Vdc (0.1 mm [4 millinch]).
Depending on how sever the haloing is, on some low layer count multilayers, there may be sufficient
dielectric thickness to meet design requirements - though there is a risk of halo propagation and
microcracking of the base material, both of which could contribute to latent failures.
Lastly - (though looking at you e-mail address these printed boards might be a Class 3 product
classification) depending on requirements and political philosophies, they could be repaired. Consider
using a "slitting" saw to cut down into and remove most of the haloing by forming a groove along the edge
of the printed board. Partially fill the groove (with a very small amount - almost only "wet the bottom of
the groove) with a long-cure high-flow epoxy (one edge at a time), then "stand" the board on the opposite
edge in a vacuum chamber, pull a partial vacuum to outgas the voids in the remaining haloing, after
"bubbling" stops slowly bring the vacuum chamber back to atmospheric pressure and let it cure. Remove from
the vacuum chamber and repeat the process for each of the edges. Then lastly, fill all of the grooves
with epoxy to level the edge. This will never completely fill the all of the voids caused by haloing, but
it'll eliminate contamination and may reduce propagation of the haloing. "May" is used, because initially
the remaining halo voids will be at reduced pressure, with time they may refill with a solvent or gas (air)
due to diffusion. If this happens, and then the printed board (or assembly) is subjected to thermal shock
(soldering operations) the trapped gas will expand and could induce further voiding (delamination).
If this is not acceptable, and you want/need to save the printed boards/assemblies, I know of a printed
board repair facility who can remove the haloing (all of it) and replace the base material.
Julie, hope this helps, if not and you want to discuss it further on need additional information, please
feel free to contact me direct via (not interlayer connections (;-} ) as provided below.
--
Ralph
Ralph Hersey & Associates
3885 Mills Way
Livermore, CA 94550-3319
PHN/FAX: 925.454.9805
e-mail: [log in to unmask]
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